I am working on this Amp and it looks like I need to add a Bucking transformer before I am totally done as well as I need one Cap I don't have, 33X500V,
Anyway I don't have a schematic and that is OK,
But I am wondering what the output on this amp is.
Thanks for any information you may have.
Peter
I am pretty much all done with the exception of a 33@500V cap,
I added a Radio Shack 3 Amp. 120/12V transformer to buck 12 Volts to bring incoming voltage to an acceptable 108 Vac here at my house bringing the Pin out voltages to just below spec, ( Tranny below deck)
Tonight I monitored the set over a 3 hours time frame and it ran nice, Main transformer hit 121* and just sat there,, This has a good clean tone and plenty powerful for my usages.
Now all I need is the 33@500 cap and 4 rubber feet that are missing for the base plate and I will be totally done.
Peter

This is a post-war PA amplifier that Sams should cover.

As for the cap, you can series-stack a pair to replace it.

Larry

Rauland model 1835
Tube line up
Five, 6SF5s
Three 6SC7s
Two 6L6s
One, 6X5
One 5U4
Twelve tubes in all.
I believe it was made in 1948.
Playing pretty good now with only One Lytic. not changed out,
While it does not get warm I still don't trust it nor do I trust the 6X5 but that is a separate issue,
Tag on back says, 130 VAC, Well with my puny 120 VAC here I am on the high side for the filaments, So In goes the bucking trans, I figure since it's a Radio Shack one with center tap all I need is the tapped side and drop it 6 to 8 VAC and I should be fine.
Where your you expect the B+ to be on something like this?
Peter

Sams: https://www.samswebsite.com/en/photofac ... /id/129567 (Sams #60-17).

With two 6L6 tubes and no power driver tube in an amp of this age, the 6L6's are running in class AB1. You can expect 25 watts out of this unit. The B+ for the 6L6 plates and screens should be in the vicinity of 350 volts (360 max by the specs).

I would expect the B+ for the small-signal tubes to run around 250 volts max. Without the schematic, this is only a guess, but I suspect the 6X5 is there to supply B+ to the small-signal tubes, thus leaving the 5U4 free to service the output stage only. On the other hand, if this is a fixed-bias output stage, the 6X5 may be in service as a bias rectifier.

In any case, don't fret about the 6X5. If it is volted within its limits and is not redlined by design, as it was in certain Zenith receivers, it should cause no more trouble than any other rectifier tube. It will cause even less trouble if they gave it an independent and floating heater winding with no reference to ground, thus removing the single greatest cause of heater-cathode shorts in such rectifiers.

Good-looking job so far. Keep up the good work.

Larry

close enough
http://www.installer.com/tech/watts.html

I found a copy of the Sams Photofact (copyright April 1949) in a three-volume Sams set, "Postwar Audio Amplifiers and Associated Equipment." I'm in no position to scan and forward it (and I advise you to get hold of your own copy ASAP), but I can answer most of your original questions now.

NOTE: All voltage measurements in this Photofact were taken with an AC line voltage of 117.

The 6X5 rectifier is used to send B+ to the 6L6 screens (rated 350v at the screens) and the small-signal tubes. It gets its voltage from taps on the same power transformer winding supplying the 5U4, which serves the output plates only. B+ at the 6L6 plates is rated at 485v.

The output stage is a cathode-bias Class AB1 type. The 6L6 cathodes should show 24v above ground, thus, control grid bias is - 24v.

The 6-volt tube heaters (including the 6X5) are referenced, through a center tap on the A+ winding, to the cathodes of the output tubes, thus providing the tube heaters with a 24-volt positive bias (to decrease hum). The 6X5 cathode voltage is rated at 350v, putting it 326v above the heater. Maximum heater-cathode voltage for a 6X5 (by RCA manual RC-16, 1950) is 450v, so the 6X5 heater-cathode voltage in this case is below published redline.

The output transformer is tapped at 4, 8, 16, 250 and 500 ohms. A feedback loop is returned from the 8-ohm tap, via an 8200-ohm resistor, to the cathode circuit of 6SF5 #5, the phono preamp. It is thence referenced to the cathode circuit of the two 6SC7 mixer tubes. The Photofact doesn't specify output wattage, but I'll stick to my ballpark guess of 25 watts, especially since you're dealing with cathode bias and not fixed bias here.

NOTE: With line voltage at 117, voltage across 6.3-volt heater pins measured 6.6.

The single six-prong Jones-type socket on the rear apron is "for remote control unit or shorting plug."

The fuse is rated at 3 amps. Dial lights are specified as Type 44.

Your bucking transformer is a GOOD idea with this unit. I'd be very leery of running it at any more than 110 volts. The voltage ratings at 117 AC make me uncomfortable, especially the 6L6 plate and screen voltages, which I'd expect to see in a 6L6GC unit, but not in a unit designed for any earlier 6L6 versions, which weren't rated so highly as the GC.

To help conserve the output tubes, I'd keep a 5U4G in this amp, instead of a 5U4GB, because the 5U4G has a higher voltage drop.

Hope this helps, and continued good luck with your project.

Larry

Larry, Thank you for the information !
Can you look for something for me?
WIll you look for the 3 Lytic Values in the 450-500 range on this set,
I am thinking the Lytics are not original to the set as to one can has 3, 10 @ 450s tied together and the last post 40 @ 450 tied independently.
I would like to get it right,
Thank you for the additional information!
Peter
And Yes, Bucking transformer in,
When Lytics in, More pics,
Nice and roomy on this Amp, Makes for good aesthetics on the restoration!
P.

Glad to. I'll take care of it when I get home from work.

Larry

OK, Peter, here we go.

The 4-element filter can cap (C-14 on the schematic) is original to the set as wired, with the triple 10-uf sections tied together, and the single 40-uf section tied independently. The 40-uf section comes off the cathode of the 6X5 (first filter), and a 4700-ohm resistor separates it from the triple 10-uf sections (second filter).

You can replace that can with a single 40-uf section and a single 30-uf section, both at 450v. I'd guess they used that particular cap combination of one forty and three tens because they couldn't find a dual cap with a 40-uf section and a 30-uf section at the time of manufacture.

On the other hand, they could have simply been using up a batch of war surplus caps picked up on the cheap. It was common for postwar manufacturers to buy huge lots of surplus parts dirt cheap, and use them up in any possible way, orthodox or otherwise.

The other, shorter filter can cap (C-13 on the schematic) is the 30-uf, 500-volt unit tied to the filament/cathode of the 5U4G.

If you haven't yet found a replacement, a couple of identical caps at 60 uf, 450v apiece, wired in series, will serve. I'd parallel each of those caps with a 330K, 1 watt load balancing resistor (like the caps, the resistors should be matched as closely as possible).

If you must find 60-uf units and can't, don't aim lower, but don't aim any higher than you can help. If that filter value is too high, the 5U4G will arc over due to excess peak plate current.

Same with the 6X5...ESPECIALLY the 6X5. I'd keep every filter cap on that secondary B+ rail strictly by the book, the first filter especially.

Hope this helps.

Larry

Agree with all. You can make a really nice Guitar Head from this amp, and the cage construction is practical for such purpose. You may have to rip out the microphone preamp circuits and rip off a Fender circuit (or whatever you like) for that purpose. Good speaker array this can rip!

Of course, restored to it's original purpose, great PA amp for small/medium venue like a church, or whatever.

Looks like this one was all original then,
I went with a 47 for the 40 and a 33 for the 30, both at 450 V, I hope close enough,
As far as the 500V goes I just ran out of the 33 @ 500 I had and need more,.....
Now with some help I would like to increase the bass a tad, Anyone care to help me in choosing a cap value off the bass circuit before I go and start patching a few different values in place.
Thanks all for your help !
I will leave the Amp as stock as possible with the exception of the bass cal.
Now If I could only find a 33@500 V shipped to me in the next couple days I would be really happy!
Peter

Two PM's sent.

Larry

In order to increase bass response you must increase the size of all audio coupling capacitors between the tube stages.

To make sure the capacitors are large enough in value follow this formula

1/(6.28*grid resistor in ohms*coupling capacitor in farads)

OK, Pete, here we go again.

This amp takes the “minimalist” approach, being as simple and straightforward as it can possibly be, with a rock-bottom minimum of parts.

There are only a few coupling caps that you could or should change. The four 6SF5 mike preamp stages discharge through .02uf caps (suitable for voice) into the 6SC7 mixer stages. Replacing these with .1uf caps should reduce the bass rolloff threshold of the mike channels below the level of audibility. The downside is that it may increase hum.

Unfortunately, these are the only coupling caps worth changing. The 6SF5 phono preamp stage shares cathode and plate circuitry with the 6SC7 microphone channel mixer stages (thus giving the phono channel significantly less gain than the microphone channels). The feedback loop from the output transformer secondary ties into the cathode circuitry here.

These five paralleled stages (6SF5 phono preamp, two 6SC7 twin stages) couple into the phase inverter through C21, a .1uf cap that should be let alone; there is nothing practical to be gained by increasing it.

The phase inverter is of the floating paraphase kind, and it couples into the 6L6 output stages through .05uf coupling caps. Since these should only begin to roll off bass response around 30 Hz or so, there isn’t really much to be gained by replacing them with anything larger, and I’d be inclined to leave them be.

Your real problem here is the bass/treble control system. These aren’t true emphasis and de-emphasis controls (with a “flat” response at the center of the controls). They are simple shunt and bypass controls (with a “flat” response when they are open all the way).

The bass control is nothing more than a .001uf cap (to pass treble) in parallel with a 2-meg pot (to pass bass and everything else). The treble control is only an old-fashioned shunt-type “tone control,” with a .05uf cap in series with a 500K pot tied to ground.

A true emphasis/de-emphasis control system (which is what you really want) would involve a significant loss of overall gain unless an extra stage of amplification is inserted to compensate for the loss. The only convenient place in this amp to insert such a stage is at C21, the .1uf cap coupling the mixers to the phase inverter. Unfortunately, this would place the extra stage inside the feedback loop, which would be problematic and perhaps unfeasible.

PS: See George H. Christ, Tubes and Circuits (New York: Gernsback, 1960), pp. 93-100, for a proper clinic on resistance-coupled amplifier analysis.

Hope this helps, and hope the amp is doing well.

Larry

Larry, I went and installed a .1 cap in place of the .02 cap and split a signal to two mic inputs and can't hear the difference between the .1 and the .02,
I then went and played all kinds of music through the set with some good speakers and still I could not detect any difference, So I went and soldered the .02 back in leaving the set basically electrically stock. I added new pics to the first post of the before and after undercarriage,
I am pretty happy with the Amp, I had it cranked up with a pair of 100 watt monitor speakers and I was able to go all the way with nearly no distortion, I was surprised it could get that loud and still be exceptionally clear, I am impressed,
Thanks for all the help ! !
Now it's nearly time to start the 50 Watt Ampro theatre Amp.
Peter

Ok here's what you should do to see what the frequency response of the amp is. Use an audio generator on each input and an oscope on the speaker output across an 8 ohm load. You simply sweep the generator from I'd say about 200 Hz down to 20 Hz and note where the sinewave starts to reduce in voltage. Try the same using the .1 uF caps in place of the .02 uF caps and note where the sinewave starts to reduce in voltage. Also follow the same procedure only swapping out the .05 uF caps on the 6L6 grids with .1 uF caps. Whichever value caps will get you down to at least 30 Hz while still producing a proper sinewave are the coupling caps you should leave in circuit.

That said if you go below 60 Hz and the sinewave starts to distort then your transformer cannot handle low frequencies very good and you should then leave it as is or install a better transformer.

Concerning the tone controls you can either leave them full CW or just disconnect them and their associated components making sure to short across the capacitor that is part of the bass control.

Glad to oblige.

If you're content with things as they stand now, better to leave well enough alone and move on to the Ampro, which will likely be a tougher nut to crack. The experience you gained here should serve you in good stead.

Keep us posted.

Larry

People are amazed at how a 25 watt tube PA system is loud enough to peel the paint off the walls while sounding clear. Try that with a 25 watt transistor amp and there is no comparison. I had a DJ who thought my 60 watt tube Bogen system sounded better than his 1000 watt solid-state system, I'm sure it does.
Don

When are you going to tackle the Ampro?

Larry

And once again I see the reality of having back up tubes to test equipment with,
While the tubes I had in the Cardwell all tested good in my Precision as well as me TV7 tester they were still an issue,
One of the 3 6SL7s bought an issue to the amp and one of the 6l6s carried some noise, I really need to learn how to use the oscilloscope as well to try and keep a better stock of tubes to try and swap them out.
So in the end the work I did was OK, Just a couple tube issues that my tester didn't pics up but could be seen on a scope.
OH well, He only charged me $20 for time and a couple buck per tube,
I haven't tried it indoors yet, It is a bit too heavy to carry around with the recent surgery, So I will have the wife carry it soon.
Thanks all.
The Ampro if a BIG project and I will get to it soon.
Gotta remember it is 44 LBS all day and night and tossing it about is not on the menu for a few more weeks.
Peter