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 Post subject: Stereo tube amp on a budget.
PostPosted: Nov Sun 05, 2006 6:01 am 
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While I am waiting for parts for my 6L6 tube amp project, I decided to build another tube amp on a budget. This amp should pump a whopping 1.1 watt per channel.

How this came about, I was thumbing through my tube manual and came across the 35HB8 tube. This tube is almost ideal, it has a preamp triode and power amp all in one tube. I figured if I use 2 38HB8s and 1 50DC4, the filaments would add to 120 volts. Since this is transformerless, I will use a polarized power cord and a isolation transformer for safety.

Here are a few photos, its almost finished, I started this 2 days ago, probably have about 3 hours working on it. All I need to do is add RCA jacks, linecord and power switch. You will notice a 35W4 instead of a 50DC4, I am waiting for that in the mail, I should really check my tube stock..... Total cost, less than $20.00!

I used 70v pa distribution audio transformers, which has a 2500 and 5000 ohm tap, tube prefers 3500 ohms. Transformer is rated for 4 watts and 20-20000 hz. Lets see if that holds true. I am not looking to emulate a Scott amp, just a kick around mini amp.

Not sure if I should put a volume control. Details of how it sounds and schematic in a few days.

Regards,
Sal Brisindi

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 Post subject:
PostPosted: Nov Sun 05, 2006 1:26 pm 
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Cute. With efficient speakers, 1 watt will drive you out of the room. I see you don't like to cut transformer leads short either.

I would add the volume control, unless you're only going to be using this with sources that already have controls.

The 4 watt rating and freq response are presumably for zero DC through the winding.


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 Post subject:
PostPosted: Nov Sun 05, 2006 2:20 pm 
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Sal, it's a nice amp, but, with out bass and treble controls, I wouldn't expect any kick butt sound. I suspect you will probably get pretty much flat response.
Amd. as for the volume control, If you add a tapped control, and the correct value capacitor-resistor networks, it will make it a loucness control, and you will have bass even at low volume. Nice amp, though.
:D

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 Post subject:
PostPosted: Nov Sun 05, 2006 3:15 pm 
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Thanks for the compliment Alan and Bill,
I didn't cut the transformer wires yet as I want to see which tap will sound the best, I can cut most of them now as my choice will be between 2 of them.

As far as the volume control, I figured if I connected it to a portable cd or mp3 player, I can control the volume through them.

This amp is so small, you can fit it in the palm of your hand!

Regards,
Sal Brisindi

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 Post subject:
PostPosted: Nov Sun 05, 2006 4:09 pm 
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Bill Cahill wrote:
Sal, it's a nice amp, but, with out bass and treble controls, I wouldn't expect any kick butt sound. I suspect you will probably get pretty much flat response.


Actually, truly high-end systems don't have tone controls because they mess up the phase response.

The tone controls on my preamp are bypassed with the built-in switch and I almost never use them.

Tom


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 Post subject:
PostPosted: Nov Sun 05, 2006 4:46 pm 
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Tone controls are intentional distortion....

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 Post subject:
PostPosted: Nov Mon 06, 2006 2:37 am 
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I just completed my budget tube amp and the results are in, it sounds like crap! The good news its not the audio transformers fault (P.A. distribution transformers) as I substituted a real audio transformer with the same results. Now I have to play with some component values to see if I can make it sound good. Since there is a preamp stage, I think I will scope it to see where the problem lies. I will keep you all updated. This amp measures 6" wide by 4" deep so it is noce and compact.

I decided to use a 1/8" stereo earphone connector instead of RCA phono connectors since I have an adapter cable if I need it.

I will keep you all updated on this. Here are some more photos.

Regards,
Sal Brisindi

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 Post subject:
PostPosted: Nov Mon 06, 2006 4:38 am 
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Sal, If you will e mail me your schematic of the way your amp is now, I can draw some simple mods that may help the fidelity, and performance. But, I need the schematic to know how you have it wired. If you need my e mail, please pm me. Thanks. Bill Cahill

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 Post subject:
PostPosted: Nov Mon 06, 2006 5:08 am 
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Bill,
I will draw a schematic exactly the way I built it tomorrow after I get home from work. I pretty much copied the schematic from the Sylvania tube manual that shows 1 channel. The only difference was I used a 100 ohm resistor from the cathode to ground instead of 150 ohms.

I did make some headway as it was oscillating and put a capacitor across the primary of the audio transformer but the volume is not loud enough.

There is also a hum but I didn't add any more capacitance to the power supply yet to see if it will go away.

I will email you tomorrow.

Thanks,
Sal

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 Post subject:
PostPosted: Nov Mon 06, 2006 4:39 pm 
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Hi Sal,
Did you rotate a tube socket or find two Sylvania tubes with matched labels?

-Ed


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 Post subject:
PostPosted: Nov Mon 06, 2006 11:55 pm 
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Hi Ed,
I have 3 of these tubes so I chose 2 tubes with the Sylvania lables facing the front.. :)

Regards,
Sal Brisindi

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 Post subject:
PostPosted: Nov Tue 07, 2006 2:23 am 
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Does anything get past you guys???

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 Post subject:
PostPosted: Nov Tue 07, 2006 5:32 am 
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You shouldn't reduce the value of the cathode resistor in the output stage from the original specs, because it sets the bias. Going from 150 ohms to 100 ohms is a very drastic change in that position in the circuit.

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 Post subject:
PostPosted: Nov Tue 07, 2006 9:31 pm 
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Mr. Detrola wrote:
You shouldn't reduce the value of the cathode resistor in the output stage from the original specs, because it sets the bias. Going from 150 ohms to 100 ohms is a very drastic change in that position in the circuit.


Dennis

The 150 ohms was for a single tube. Sal says the 100 ohms is for 2 tubes. More current, higher voltage drop. He might even have to reduce the resistance a little bit more. I would rather use individual 150 ohm resistors for each channel. The common cathode resistor may mean more channel crosstalk, depending on the cathode bypass capacitor.

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 Post subject:
PostPosted: Nov Tue 07, 2006 9:56 pm 
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You are not likely to get channel crssstalk with that. I have factory amps that have the two cathodes tied together.
Further, I don't even like the design he used, and offered sme simple mods. He is getting hum caused by two things. Not enough filtering, and the way the filaments are wired.
Next: he has squeling rpblems due to wiring problems in his design. He needs to shorten leads, etc...
Next: Again, he needs a volume control. Too much signal is getting through, adding immesedly to the distortaion.
He also has several safety issues, which makes this amp a major shock hazzard.
Also, The caps he experimented with to attempt to get better tone, are the wrong way to go. He needs to add a tone composition capacitor of a fairly low value, such as .033 on the primary of the output transformer. I have found that when you simply put it accross the primary, it cuts the treble. However, it you tie the tone cap. from the plate of the output to the cathode, you well get better bass response without affecting the treble appreciably. This feature was used in several radios produced by Admiral, and RCA in the 40's. Again, experimentation is required to use best value, without causing performance issues. Thsi amp is NOT a ghetto blaster. It is a simple, cheap stereo amplifier.

Another safety worry I have, is due to occasional shorts, including in rectifier, there is no safety measure in that design. I opt for what radio mannufacturers did until they decided to just get cheap. That is put a low value half watt resistor between the hot side of the line, and plate of the rectifier. Thsi can be a value of anywhere from 10 ohms to 22 ohms. The idea is not to cut the voltage, it's merely to put a safety device in in case of a short.

Now,as for the filtering: That is also not a good design. Not enough filtering, for one, and they connected the preamp to the same filter as the output tube. There just isn't enough filtering. Period. He needs to add a filter, and resistor to network, and to experiment with cap values on the electrolytics to get the least amount of hum. There is hardly enough filtering as it is.

These are my personal opinions as I have gathered from what was done over the years by the manufacturers. I will also mention here that the better playing ac dc sets were in following with many of these features. I may have mad one mistake in a resistance wattage value, however, as that may cause an overheating. I opt for staying with a 1 watt resistor on cathode of output tubes.

Thank you. Bill Cahill

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 Post subject:
PostPosted: Nov Tue 07, 2006 10:23 pm 
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So, where is this schematic??

Dave


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 Post subject:
PostPosted: Nov Wed 08, 2006 12:45 am 
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Brian McAllister wrote:
Mr. Detrola wrote:
You shouldn't reduce the value of the cathode resistor in the output stage from the original specs, because it sets the bias. Going from 150 ohms to 100 ohms is a very drastic change in that position in the circuit.


Dennis

The 150 ohms was for a single tube. Sal says the 100 ohms is for 2 tubes. More current, higher voltage drop. He might even have to reduce the resistance a little bit more. I would rather use individual 150 ohm resistors for each channel. The common cathode resistor may mean more channel crosstalk, depending on the cathode bypass capacitor.



I can't find where he says that 100 ohms was for 2 tubes sharing a common resistor?? In any event I was going by his photos, and they show a separate 100 ohm resistor bypassed by an electrolytic from pin 8 (cathode) of each 35HB8 socket to common.

I agree that I would rather see individual resistors, although some manufacturers chose to use one common resistor.



The circuit as pictured can't possibly work well, as Bill pointed out it's lacking some important features. The circuit found in the tube manual was never intended to be a very good quality (definitely far from high fidelity), only an example of how the tube could be used. IIRC the 35HB8 was designed for inexpensive portable stereos in a simple 3 tube amplifier with input from a medium-output ceramic or crystal cartridge. It was probably driving a speaker in the 4" to 6" range that didn't have any bass response.

One could certainly tweak the circuit to sound better as Bill has suggested. It all depends on what the intended use is and what kind of speakers will be used on it. One watt into a very high efficiency speaker is loud, I have tried it with a transistor radio having output in the milliwatts. But if the unit doesn't have good response it's only loud.

It looks really nice, and there's still room for adding a volume control and other needed components. I think it will make a great headphone amplifier, or be good for background music with efficient speakers. However, I would like to see the isolation transformer be a permanent part of the amplifier rather than external to it, but there isn't room on the chassis for that.

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 Post subject:
PostPosted: Nov Wed 08, 2006 2:00 am 
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Mr. Detrola wrote:
Brian McAllister wrote:
Mr. Detrola wrote:
You shouldn't reduce the value of the cathode resistor in the output stage from the original specs, because it sets the bias. Going from 150 ohms to 100 ohms is a very drastic change in that position in the circuit.


Dennis

The 150 ohms was for a single tube. Sal says the 100 ohms is for 2 tubes. More current, higher voltage drop. He might even have to reduce the resistance a little bit more. I would rather use individual 150 ohm resistors for each channel. The common cathode resistor may mean more channel crosstalk, depending on the cathode bypass capacitor.



I can't find where he says that 100 ohms was for 2 tubes sharing a common resistor?? In any event I was going by his photos, and they show a separate 100 ohm resistor bypassed by an electrolytic from pin 8 (cathode) of each 35HB8 socket to common.



You are right. I read more into his statement than was there and did not really look at how it was wired.

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 Post subject:
PostPosted: Nov Wed 08, 2006 4:27 am 
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Bill,
I received your email with the schematic, thank you for your time on this. For those who mentioned it, I put a 100 ohm cathode resistor for each tube, I did not share 1 resistor. The reason why I used a 100 ohm resistor is because I had that in my parts stock. I did try a 150 ohm (1/2 watt) for testing purposes and it sounded a little better. As far as the power supply goes, I worked on it last night and added a larger capacitor which did quiet the hum down a bit. I would have used the larger value first but I couldn't find them in my dungeon...errr I mean basement until last night. I will wire a 1 amp fuse for safety plus I know there is no isolation. This was to be a learning experience for the 6L6 tube amp.

I should have practiced what I read about wiring the tube filament correctly but I said how bad can this be, well, the hum was pretty bad so I will follow Bills idea.

I guess I should remove the neon light in the middle and put a volume control also. I figured I didn't need one as I was going to plug in a cd player and control the volume from there.

I just wanted to have some fun building a quick and dirty cheap tube amp with parts I had in stock, it won't be a daily driver, just a simple mini tube amp for fun. For serious listening I have 3 tube amps for that.

I know this amp won't knock the plaster off the walls.

To see the schematic go to http://www.tuberadios.com/temp/35HB8ampsmall.jpg

I will post Bill's version with his permission.

Regards,
Sal Brisindi[/code]

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 Post subject:
PostPosted: Nov Wed 08, 2006 2:01 pm 
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Try eliminating that bypass capacitor on the cathode of the output stage. You'll lose some gain, but you'll gain some degeneration across that cathode resistor and possibly get better fidelity.

If you need the gain, then use a smaller capacitor... 100uF is too large. I'd be more comfortable with maybe 2uF.

Tom


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