Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Nov Fri 22, 2019 2:41 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 24 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Bell & Howell push pull 25C5 amp
PostPosted: Jan Sun 24, 2016 5:59 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
Topic title originally was about the projector the amp was in.

I have one that the church I go to is throwing away.

I have no use for it as is, but saw that it does have a push pull 25C5 amp.

I am going to get the amp out of it and the motor as well along with any other parts I can use.

where can I find the service manual? only need it so I know how to connect the motor and amp up separately from the projector.


Last edited by Tube Radio on Feb Fri 12, 2016 5:33 am, edited 2 times in total.

Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Jan Sun 24, 2016 10:17 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4276
Location: Boston, MA USA
Not sure you want to bother. That amplifier is only good for 4 watts output and is hot-chassis. If you must salvage it, just follow the wiring from the power cord and interlock switch. There are no power transformers in that projector, everything is line-operated.

Perhaps better to keep it together as a projector and use it if the worm gear has not yet cracked.

More information here: http://www.16mmfilmtalk.com .

-David


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Jan Mon 25, 2016 12:13 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
it does work, but I nor anyone at the church has a use for it.

the output power is no big issue.

I would use a triad N-68X isolation transformer with it.

I did find a service manual, but it is missing pages 62-116 according to the table of contents listed in the service manual.

Of course I can buy the service manual, but since I am only needing the amplifier schematic, that would be a waste of my money.

I can make up a schematic by studying the circuit board which is what it looks like I may have to do.

Don't yet know if I will actually use the amp for anything or if I will just salvage it for what usable parts I can get.


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Wed 03, 2016 5:16 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
figured out the amp. for 120 Vac line voltage and doing away with one 25C5 that is not part of the amp (looks like it drives a coil much like is done with the erase head in reel to reels) I will need a 302 ohm 7 watt resistor to drop the necessary voltage. my plan is to to test the amp as is and see if it is usable. if the amp is usable then I will buy a Triad N-68X isolation transformer.

that will put out 126Vac unloaded up to at least 1/2 its rated load.

unloaded B+ will be around 178Vac and may settle to around 160 Vdc which is too much for the 35C5 tubes.

my plan is to power the amp up just to see what B+ is with the amp being fed 120Vac. the tube heaters have a separate power wire. there was a power resistor near the amp which I did save. I'll check it's value and see if it is large enough to drop the line voltage low enough for the tubes. will leave the other 35C5 in for now.

if B+ is around 140 Vdc at 120 Vac then how should I go about reducing the 126Vac down to 120Vac and reducing the voltage to the heaters.

some time tomorrow perhaps I'll try to draw up a schematic and post it here.

the output transformer looks like it has a feedback winding. don't know what effect that has on the sound quality though.

looking at the board, a schematic will be pretty hard to draw up.

EDIT:

Did some calculations and for a 150mA filament string the 100 ohm resistor that was mounted on the projector chassis drops 15 volts so that is what dropped the necessary voltage for the heater string.

Will fire up the amp this afternoon using my RCA WP-25A isolation transformer.

Already had the amp powered up when I was checking the projector before disassembly so no point in trying to do a slow power up. The amp seemed to work far as hearing the dirty volume control when I rotated it and I didn't notice any hum initially, but the smaller speaker may have made the 60Hz hum hard to hear.

I am tempted to re-stuff the filter caps, but most likely I will just replace them provided they are indeed bad. Nothing real special about this amp to want to keep it looking right cosmetically' although if the cap can had a cardboard tube on it I would re-stuff as that would be easier to do. Remove the can slightly above the base (gives something for the tube to attach to) then discard it. Install the new caps then glue the cardboard tube in place.


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Wed 03, 2016 7:38 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4276
Location: Boston, MA USA
Amplifier schematics here:

http://www.film-tech.com/warehouse/manuals/BH500.pdf

-David


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Wed 03, 2016 8:09 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
Thanks so much

Odd how all the online manuals for the particular projector model the amp came from all omit the pages with schematics and the parts breakdowns. It could be all whoever scanned that manual had.

Looking at the schematic it looks like this might be a rather good quality amp.

The other 25C5 drives an oscillator which powered the exciter lamp.

For now I will test it as is, but I may remove the resistor feeding B+ to the exciter oscillator 25C5.

The problem will be that this requires a 16 ohm speaker.

I do have the original speaker so it is possible to use that with the amp, but if I wanted to use a larger speaker I would have to either buy a vintage 16 ohm speaker or I wonder if one of those Hammond 600 ohm to 4/8 ohm transformers would work feeding the 8 ohm tap on it to the 16 ohm output then using an 8 ohm speaker on the 4 ohm winding to ground.

I would then use a Triad N-68X isolation transformer to power the amp.

Maybe perhaps I can use the original speaker and mount it all in a box and use it as an audio signal tracer.

Now all I need to do is extract the schematic, clean it up a bit, keep an original copy then make a schematic showing what I have done to the amp.

EDIT:

Great news :D

I tried the amp and it actually sounds quite good through the original speaker.

Running come calculations it looks like I may need to increase the value of two coupling caps.

The input coupling cap is reducing the bass response too much so I will make it .01uF like the others just to see if the improvement in bass response is good enough.

Given this is not a HI-FI amp I am not trying for 20Hz bass response. I figure on having the bass starting to roll off at around 40-50 Hz which is where I prefer the bass response to roll off at when it isn't a HI-FI amp.

The other coupling cap I may have to increase in value is the one from the preamp to the 500K volume control. The total resistance includes the feedback circuit which is a three position switch also used as the tone control. It has 223.9K of resistance to ground which includes a 3.9K cathode resistor of the first 12AX7 stage and is in parallel with the 500K volume control which gives a total resistance of 154.65K.

First I will feed a signal in at the volume control just to see what the low frequency response of the output transformer is before I go increasing the value of the two coupling caps.

Now for an :oops:

If anyone has anything with Callins electrolytic caps in it don't bother trying to power it up without replacing them.

I had the cathode bypass cap for the output tubes blow its top completely off. Causality one 25C5. Noticed the filament was no longer glowing then I saw a very small crack in the side of the tube.

Before powering the amp up again I will need to go ahead and substitute a resistor for the 25C5.

The electrolytic can cap itself does not get warm, but I may go ahead and replace it anyways.

For what may be around a 5 watt amp (150Vdc B+) boy does it get loud with its original speaker :shock:


Last edited by Tube Radio on Feb Wed 03, 2016 10:53 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Wed 03, 2016 10:30 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4276
Location: Boston, MA USA
B&H published separate service and parts manuals. The amplifier schematic would sometimes be found in the parts manual, which is the case with this projector.

The amplifier will work fine with an 8 ohm speaker.

-David


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Wed 03, 2016 10:55 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
I'll give it a try with an 8 ohm speaker, although I will try the original speaker in a cardboard box for testing purposes and if it sounds good enough I may just mount it and the amp in an open back wood box and use that.


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Wed 03, 2016 11:24 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4276
Location: Boston, MA USA
Use a bass-reflex enclosure of the same volume as the projector case with a port the same size as the one on the rear of the projector.

-David


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Thu 04, 2016 1:20 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
I'm pretty sure the case itself was not even remotely close to being air tight or ported. best I can remember any opening at the rear of the case would have been close to the size of the speaker opening.

Normally I would replace all resistors, but I may just test these and only replace those that are out of tolerance since this is an early pc board.

EDIT:

Just did a calculation and to take up 25 volts extra I will need a 167 ohm 3.75 watt resistor.

Does that sound about right?


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Thu 04, 2016 7:12 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4276
Location: Boston, MA USA
I'm telling you how the projector was originally designed. The case was not airtight, nor does it need to be. The port does double-duty as the ventilation grill for the tubes. It is on the back of the case (opposite the speaker) and looks like another speaker grill. Many of the better table-model and portable phonographs of the 1950s and 60s were designed the same way.

I have been working with these projectors since 1966. I currently have a 552T, similar to yours except with the better, newer, 25 watt solid-state amplifier.

-David


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Thu 04, 2016 8:22 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
Oh ok. Didn't know the case didn't have to be airtight.

Does that grill also serve as the air inlet for the blower that cools the projection lamp?

Also I need to drop 44.8Vac in the heater string in order to get the 75.2Vac the heater string requires.

When I had the other 25C5 installed the 100 ohm resistor was correct.

What value resistor would I need to reduce the voltage?

I can either leave the 100 ohm resistor installed and just use another resistor or I can use a single resistor.

I could also use a capacitor, but I'm not sure what its value needs to be.

I have several film caps for speaker crossovers I can try.


I suppose the original speaker is quite efficient as it has a rather large alnico magnet on it.

I'm thinking of either leaving the volume control connected where it is and adding a 500K audio taper pot to the input or moving the volume control to the input and just installing a 500K resistor where the pot was.

I replaced most of the resistors. A few I don't have and will need to order. Will also need to order the main filter cap.


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Thu 04, 2016 11:56 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4276
Location: Boston, MA USA
Yes, the blower draws air through the rear grill. The air is drawn past the tubes and helps keep them cool when the projector is running.

I think if I were trying to re-purpose this amplifier like you are I would rewire the heaters to a conventional 150 mA series string, and substitute 50C5s for the 25C5s. That way the heater string would add to 124 volts and no series resistance at all would be needed. Should be easy to do, may be necessary to cut a couple of traces and add a couple of jumpers to the PC board.

If I were more ambitious, instead of adding an isolation transformer I might get a power transformer with HV and heater windings, rewire the heaters to 6.3V parallel, and substitute 6AQ5s for the 25C5s. More work, however.

-David


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Fri 05, 2016 3:30 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
The heater string was already a conventional series string.

I had thought of using a pair of 50C5 tubes as that would be real close to the 126Vac the Triad N-68X puts out, but that would be pointless as I need to drop the voltage down to 120Vac anyways for the B+ supply so that the 25C5 tubes don't exceed their 150Vdc plate rating.

I did find a solution to the voltage issue. I tried a 100V 2.5A diode and that dropped the voltage too much. I then added a 22uF 450V electrolytic cap after the diode and that raised the voltage to 83Vdc. Tried a 10 ohm resistor after the filter cap and that didn't drop the voltage enough. Proceeded to insert the 10 ohm resistor between the diode and filter cap and that was just about right. I needed 75.2Vac for the heater string. I have 77.2Vdc going to the heater string.

I haven't dealt much with the 25C5 and had forgotten just how hot they run.

Doing a frequency response test with a 12" 8 ohm speaker the amp does produce output down to at least 40Hz

For now I have the original speaker mounted in a small cardboard box and it sounds quite decent. The bass response of course isn't as good as it was with the larger speaker, but for now that is ok.

Turning the tone control to bass does help some.

I still need to connect the amp up to my scope so I can see how a sinewave looks down around 40Hz. If the sinewave still looks like a sinewave at between 40-50 Hz then I may find a decent vintage 16 ohm 12" speaker or I could use two 8 ohm 12" speakers in series.

I can either use just the single 16 ohm or dual 8 ohm speakers or I could build a crossover, use the 12" speaker for the frequencies below 200Hz then use the original 16 ohm speaker for frequencies above 200Hz. That said I don't really want to be stuck with the size of open back cabinet the 12" speaker will require. I may try to find a decent 8" speaker and use that as the cabinet can be much smaller.

I temporarily reinstalled the metal bottom cover of the amp until I get back to finishing up the amp. I still have a few resistors to replace.

I have yet to test the tubes, but given how the projector looked like it wasn't used very often I'm sure the tubes are still good, but will eventually test them anyways.

Whenever I decide on a speaker or speakers and I get a cabinet I will use a fan to help cool the 35C5 tubes as they are so hot they even cause the output transformer to get hot from one 35C5 being right next to the transformer.

So far I am well pleased with this amp.



Thanks so much for all your help.



EDIT:

Just had a thought.

I wonder what impedance the feedback winding of the output transformer is and if it can drive a speaker or if it is even a good idea to try?


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Fri 05, 2016 7:29 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4276
Location: Boston, MA USA
The series heater string is 300mA. With two 25C5s and two 12AX7s you will need to drop 60 volts which is a big 18 watts of purely wasted heat. If you rewire the 12AX7s for 12 volts each and substitute 50C5s you will have to drop no voltage at all and will run much cooler.

-David


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Fri 05, 2016 8:13 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
Don't know how I missed that the 25C5 is 300mA :oops:

I had thought of 50C5 tubes which would work better with the 126Vac output of the Triad N-68X, but I would then have to figure out what resistance I need in the B+ circuit to get B+ back down to what it should be as it will be a bit higher with the power supply being fed 126Vac.

That would also require cutting some traces and rewiring.

For now I may leave it as is given it works so good.

I'll do a frequency response test using my oscope this afternoon just to see how good the bass response will be.

I'm shooting for a clean sinewave that starts being reduced in amplitude at somewhere between 40-50Hz. If the sinewave is not clean (doesn't look like a sinewave anymore) at that frequency then I'll know the transformer is not adequate.

I then will have to decide if I want Edcor to build a transformer or if the feedback winding is the same impedance as the speaker winding maybe a P-T291 output transformer will work.

I am considering moving the volume control to the input then I may use a 500K panel mount pot with screwdriver adjust where the volume control originally connected. That way I won't overload the input and can adjust the gain.

The way it is now I get a little buzzing.

I failed to save the knobs as I was not thinking. I did find a knob for the volume control, but will need one for the tone control.

I may try to figure out he switch arrangement and get a regular panel mount rotary switch then I can use a regular 500K pot for the volume control and a properly sized knob.

That would require the use of shielded wire to go to the switch and I don't know how well the circuit would like it given that switch is part of a feedback circuit. Looks like it is a 2 pole three position switch and I may have one or I may have a similar switch that the wafers can be mounted on.


Maybe a year or two ago I would have most likely passed on this amp, but after hearing how well a push pull 35L6 conversion of a Hallicrafters S-41W went I sort of fell in love with the series string type of output tubes.

I find it interesting that one datasheet for the 25C5 says for use singly or in push pull, but only lists operating conditions for single ended use.

I now know why the 25C5 tubes were so hot and bright. I calculated everything based on a series string current of 150mA thinking the 12AX7 tubes were wired for 12 volts. :oops: I actually need 62.6Vac for the heater string.

EDIT:

Reducing the voltage to the heater string was easier than I thought.

Eliminated the cap and resistor and with just the diode I get 65.8Vac which s close enough to 62.6Vac.

Something tells me that the Triad N-68X is not enough.

Did some calculations and I come up with approximately 122mA of B+ current.

Tried moving the volume control to the input and got feedback so I rewired it as it was.

I do notice a hum when I turn up the volume control. Not going to troubleshoot it until I have replaced the filter caps. It doesn't sound like plain 60Hz hum though. It sounds more like the hum one gets when using an un-shielded wire where a shielded wire should be used.

Looks like I will have to add a separate level control at the input. That will actually work out better that way and make it easier to mount the amp. The tone control will rarely be adjusted and the volume control will not be used as a volume control so the control doesn't need to be readily accessible from the front panel.

Just did a test and when I unplugged the power cord from the isolation transformer the hum went away so the problem
could be where the common is connected. I should move it to where the filter caps ground and see if that stops the hum.

Another option is to build an external 150Vdc 500mA power supply then use a resistor to drop the heater voltage down to what it should be. That way only DC voltage will be going to the amp.

EDIT 2:

Tried several ground points and still got hum so I will be looking at an external 150Vdc supply.

That won't be hard to make as I can use the next higher power Triad isolation transformer then use a bridge rectifier along with a large value filter cap.

EDIT 3:

I did a frequency response test on the output transformer and all was good until I got down to 54Hz. The sinewave started looking non-sinusoidal and got worse the lower in frequency I went.

That said I will be looking for an output transformer.

Here's what I know about the original transformer.

At 400Hz.

1 Vac 16 ohm output

1.142Vac feedback winding.

16.24 Vac full output transformer primary winding

That gives me a 16.24:1 turns ratio for the primary to 16 ohm secondary.

So to get the impedance ratio I take the square of the turns ratio which is

16.24 X 16.24 = 263.7 for an impedance ratio of 263.7:1

The impedance is then 16 X 263.7 = 4.219K. A transformer with 4K primary impedance will work.

Now the other secondary.

1.412 X 1.412 = 1.304 for an impedance ratio of 1.304:1

The impedance is then 16 X 1.304 = 20.86 ohms.

A single 16 ohm winding should work.

I used this website to help with the transformer calculations http://www.radioremembered.org/outimp.htm

The P-T291 transformer will only work if I use a 4 or 8 ohm speaker, but that will require lowering the value of the feedback resistor. In order to use a 16 ohm speaker the primary impedance will be 8K which won't be right for this circuit.

Edcor has a transformer that will work, but it is flat down to 20Hz, although I can alter the coupling cap values to cause the amp to roll off the bass where I want it.

https://www.edcorusa.com/cxpp25-ms-4k

Nice thing about that transformer is it has taps for 4 8 and 16 ohm speakers.

I can get a 60 watt 4.2K transformer, but I don't think 200 ohms difference in impedance will much matter.

https://www.edcorusa.com/cxpp60-ms-4_2k

Did a test of disconnecting one end of the feedback winding then grounding the negative speaker wire to the B- line and taking the feedback from the positive speaker wire. Didn't notice any real change in the sound so a single winding is all I need.

I suppose that separate feedback winding was only used to keep the speaker winding completely isolated from the AC line given this is a transformerless amp

This may very well be the best sounding series string tube amp I've ever heard.

Looks like real quality went into the amp and its circuitry which is noticeable in the audio quality.

I'll go ahead and order the output transformer then when it comes in I will install it and see if the currently installed coupling caps will roll off the bass where I want it. If so then I'll just order the rest of the resistors and maybe upgrade the coupling caps to film caps unless there is no real need to.

I'm almost tempted to leave the amp as is including the speaker in the cardboard box so I can amaze people that something so small can go so loud :mrgreen:

Thinking about it this would make a great amp for an RCA 45 player that doesn't have a built in amp.


Top
 Profile  
 
 Post subject: Re: Bell & Howell 550 Specialist Autoload Filmosound project
PostPosted: Feb Mon 08, 2016 4:47 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
Here's a thought.

This 6" X 9" speaker is 32 ohms

http://www.goldmine-elec-products.com/p ... ber=G21087

So maybe I could put two in parallel for 16 ohms.

That might provide decent bass response, but I don't know how efficient the speaker is.

Since it is listed as an automotive speaker, unless it is older then it may not be very efficient, but I've never really heard of a more modern automotive speaker rated at 32 ohms.

Working up a power supply I found that a Triad N-68X transformer puts out 120Vac for an input of 120Vac when powering the amp.

The buzz seems to be reduced some, but not all the way gone.

I am using the one in my Halicrafters S-41W to test the amp with. Will connect it to the amp tomorrow and see how well the transformer does before ordering one.

Looks like I found an output transformer that may work. It came from a 15 watt stereo push pull 6BQ5 amp.

it has a ratio of 32:1

32 X 32 = 1,024

1,024 X 4 = 4,096

That is close enough so I may try that transformer tomorrow then try one of my Isophon 8" speakers as it is 4 ohms.

If it works good enough then I'll leave it as is. If it does not then I'll buy that Edcor transformer.

EDIT:

The transformer does work.

While looking for a chassis to mount the amp on I found the chassis from a smaller German radio. I finished stripping it then mounted everything to it.

I then thought how perfect as I still have the cabinet and the original speaker. I tried the speaker in a cardboard box (around 6" X 9" speaker) and it sounds decent enough.

I will use the radio's original volume control to adjust the volume and the amp's volume control will set the gain.

I've got an external power supply in the works so the heater string is operated on dc to eliminate the slight buzz from the AC heater current.

Here's the schematic

Image

Found the speaker magnet felt a little warm and putting my scope on the output confirmed a 75KHz oscillation.

I tried a 220pF cap in place of the 100pF cap between the plate of the first section of the second 12AX7 and ground. That helped a lot, but not quite enough. I then installed a 300pF cap across the output transformer primary and the oscillation is completely gone. I'm thinking that's why there is a .22uF cap across the speaker terminals on the amp board. It probably killed the oscillation when used with the original output transformer and 16 ohm speaker.

The speaker sounds great in the radio's cabinet.

I don't know how well the 1.3meg volume control will work feeding a stage with a 500K grid resistor. If it don't work well I will just cut one lead to one of the parallel 1 meg resistors. Any idea if

I'd like to reinstall the dial glass, but I'm going to have to figure out if it is possible with where the volume control may have to be mounted.

That said I could just order a long shaft 500K variable resistor and cut it to fit.

Image

So far I am well pleased with the sound quality.

Doing a frequency response test I get good low frequency response down to around 30Hz, but that was with the speaker connected. I should test it again using a fixed 4 ohm resistor, but 'm pretty sure it will still be good. The sinewave no longer is distorted looking at frequencies under 55Hz. Suppose what is helping there is the fact I am using an output transformer meant for push pull 6BQ5 tubes and the peak audio voltage output I suppose is a good bit less than it would be with a pair of 6BQ5 tubes.

The radio originally had a couple plastic pieces to throw sound from the rear of the speaker out the sides of the radio. If I can find those two pieces I may re-install them or I may try to use a couple 16 ohm speakers from an AKAI reel to reel and mount them there. That would be 2.66 ohms impedance. When testing the output transformer I connected two 8" 4 ohm speakers to it and it seemed to not affect the amp at all so maybe that will work.

Once the two power transformers come in I just have to mount them and the associated power supply components and I can putt he chassis in the radio cabinet.

I do think I'll replace the main filter cap can, but I gotta figure out how to remove it without further cracking the board.

Latest schematic revision

The unmarked resistor in the heater string circuit should be two 330 ohm 5 watt resistors in parallel.

Image


Top
 Profile  
 
 Post subject: Re: Bell & Howell push pull 25C5 amp
PostPosted: Feb Mon 22, 2016 8:28 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
Good news.

I got the 50C5 tubes in so I went ahead and worked on the amp getting it ready.

I found an easy way to rewire it to have the 12AX7 tubes wired for 12 volts. Cut off pins 9 and 5 feed a wire through the pc board hole for pin 5 and solder it to pin 5 then run it over to the pc board hole for pin 9 and solder it.

I will mount the isolation transformer, bridge rectifier and filter cap today.

I removed the original can cap from the pc board and for now mounted it to the radio chassis and will use it until my next Mouser order which should be Thursday when I order parts for a Philco 118 I am working on.

I put some 5 minute epoxy on both sides of the crack on the pc board.

The 500K volume control I ordered from Mark Oppat I wound up using for the Philco since the volume knob fit its shaft perfectly so I'll need to get another control. Think I may have a 1 meg somewhere. If so I can just remove one of the 1 meg parallel grid resistors for the first 12AX7 stage so the impedance will then be 1 meg.

Haven't decided on a power switch yet, but if I do use one it will be mounted where the original tuning knob shaft was.

Given I am using a bridge rectifier I will have to run the tube heaters on DC which should eliminate the buzz, but I will need to figure out what value of resistor I will need.

Also given the AC output of the isolation transformer is 126Vac unloaded to at least 260mA (approximate current draw of a Hallicrafters S-41W with push pull 35L6 audio) I will need to increase the value of the 5 ohm resistor to drop the B+ down to 150Vdc.

I am thinking of possibly re-installing the dial glass from the radio and I could use a zener to drop the needed voltage and install a pilot light across it to light the dial or I could use a string of 1N4007 diodes like I did for the pilot lamp in my S-41W.

EDIT:

Got the amp finished and it works good. The buzz is completely gone. All that is left is a slight hum like one might hear when using a cable not shielded very well feeding a high gain input.

Looks like I may need to find the correct output transformer as the one I'm using is dropping 10 Vdc and as I recall when it was used with the push pull 6BQ5 amp the voltage drop was a bit less. Don't know how much 10Vdc B+ difference will affect the power output, but I'm sure it isn't much. Also as it was originally the 25C5 tubes were run right at its 150Vdc design center plate rating and now for the 50C5 tubes it is 139.4Vdc on one output tube plate for an input of 120Vac.

Heater string voltage is 125.1Vdc and has 292mVac ripple.

Wound up using a 100 ohm resistor and an 82 ohm resistor.

Had 1.415 volts of ripple, but added a 22uF cap to ground after the 82 ohm resistor and another 22uF cap to ground after the 100 ohm resistor.

Main B+ is 152.5Vdc. B+ after the 5 ohm resistor is 152.1Vdc. Ripple is 673mVac

I do notice that the maximum volume level gets louder after the amp has been on a bit. I didn't check the 12AX7 tubes so maybe one of them is going bad.

EDIT 2:

Made the mistake of leaving the amp on last night and when I got home about 10 minutes ago the amp was still working properly.

I was going to rebuild the amp on the radio chassis, but for now I think I'll leave it as it is since it works so good.


Top
 Profile  
 
 Post subject: Re: Bell & Howell push pull 25C5 amp
PostPosted: Oct Mon 09, 2017 5:39 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 407
Location: Olympia, Washington
Bell & Howell started making "hot chassis" amplifiers for their projectors back around 1949. They were looking for options for projector customers who wanted lighter weight and lower priced projectors. So you had a choice of amplifiers in projector models. You could order a model with the conventional power transformer amplifier (higher priced of course) or you could order a model with a hot chassis (no power transformer) amplifier at a lower price.

The amplifier you have been working with was designed for the 500 series projectors and introduced in 1962. One major drawback is that the amplifier pc board was mounted vertically in the projector and it had the tubes mounted horizontally. To top that off, the amplifier was mounted right directly underneath the projector drive motor. I used to service lots of these amplifiers and 99% of the time the machines were brought in for service, it was for a "noisy" amplifier. It was always bad output tubes. The tubes would go microphonic because of the high vibration of the motor.

Oh yes, if you have the original speaker jack still wired to the amplifier, it's a Switchcraft S-250 connector (smaller than a conventional 1/4" jack).

By the way, all of your Photobucket pictures are no longer working.


Top
 Profile  
 
 Post subject: Re: Bell & Howell push pull 25C5 amp
PostPosted: Oct Mon 09, 2017 9:25 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20354
Location: Warner Robins, GA
Yes photobuket changed their terms to where only payikg customers could third party link.

Haven't really done much with the amp in a year.


Top
 Profile  
 
Post New Topic Post Reply  [ 24 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 5 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB