Which transformer for small practice tube SE amp - diyAudio
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Old 31st May 2012, 05:52 PM   #1
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Default Which transformer for small practice tube SE amp

I want to build a small <5 watts practice amp for my nephew but I've never built a guitar amp before. I have built tube stereo amps.

I have a few tubes that may work: 6AQ5, 25L6, 12BK5. I will most likely use the 6AQ5 in pentode mode and use a 12ax7 as pre. For the speaker I am either going to use a 12 inch CTS all paper full range driver or a blaupunkt 7x11 all paper driver.

I am not a musician and have no clue what to look for regarding the sound/distortion of the amp. It appears to me that for some of these practice amps it is good to go into the distortion region so it can emulate the larger amps at higher volumes, providing a similar tone/effects at low volumes.

I have a few record player SE transfomers that were used with 25l6 and 50EH5's. These are small transformers. I also have some midsize Triads, larger Tamradios from a Sony r2r and some chunky edcors.

Are the small ones better for this application?

Alfredo
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Old 31st May 2012, 06:29 PM   #2
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On another forum this one was suggested.

Transformer, Output, 8 W, Single Ended | Antique Electronic Supply LLC

But you probably have what you need already. I have the three tubes you listed but have not used the 12BK5 yet. You should test the impedance ratio of the transformers to determine what tubes to use with them. If you can take a low voltage AC source, you could use a 6V or less transformer or the like, feed it into the secondary of the output transformer and measure the voltage on the primary. Be careful though, the voltage gets stepped up and can give you a good shock. Take the ratio of the two voltages and divide the smaller into the larger. this will give you the turns ratio. Multiply this number by itself and then by the speaker impedance you want to use.

Offhand the 6AQ5's want to see from 5k ohms to 10k. The 25L6, around 2k to 4k, the 12BK5's, 6-10k. It depends on the voltage you run them.

You can do a simple Champ kind of circuit or add another preamp tube and get a bit more complex. On the size of transformer, you can go a little smaller than you would with full range music due to less of a demand in the bass end but excess iron does not hurt.
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Old 31st May 2012, 06:41 PM   #3
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What will you be using for an output transformer? You really need to match the output tube and transformer with a power transformer that will provide a suitable B+. A 6AQ5 really wants a higher B+ than a 25L6.
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Old 31st May 2012, 06:43 PM   #4
Cassiel is online now Cassiel  Libya
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Quote:
Are the small ones better for this application?

From my own experience I would say no. The best sounding Champs have a larger output transformer than the no so great sounding ones. With only 5 Watts you will get into distortion pretty soon regardless of the OT you choose, having said that, a tiny output transformer will saturate earlier and easier but to my taste that type of distortion simply doesn't sound good. If you can't test them because you can't play guitar it's a hard choice. Size doesn't tell the whole story, it may be possible a small one (not ridiculously small) sounds the best. You really never know until you try. But if you have to make a blind choice (deaf in this case) go for the larger one.
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Old 31st May 2012, 07:01 PM   #5
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Thanks guys,

Yes, not playing is a serious handicap. I have a Guitar Center near my house, somebody there may be able to test it and give me an opinion. I would have to stick to the 6aq5 and the transformers that give me close to 5k on the primary.
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Old 31st May 2012, 07:06 PM   #6
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There are lots of great transformer choices around. 6AQ5's actually sound fantastic in champ style circuits.

There are lots of other choices, I just mentioned the ones below as I have personally played with them.

I have bought from a few SE's MPS a several times
SE OUTPUT XFMRS - Musical Power Supplies Inc.- Quality Tube Amplifier Parts You CAN Afford

Edcor is another great choice. Last time I put together a SE 6AQ5 Champ I used the Fender Blackface aa764 circuit with the XSE15-8-5K output
EDCOR - XSE15-8-5K


Though a bit bigger than an original, I found that it gave the amp a bigger feel as well as smoother breakup.
I used one of MPS's PT's ( PT275.2) a little on the overkill side but very reliable and budget priced.
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Old 31st May 2012, 07:06 PM   #7
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You could consider going octal and using 6V6 rather then 6AQ5.

Old 6V6's are pretty common and can generally handle abuse in guitar amp situations better then the 6AQ5.

Great old transformers are found in the vicinity of great old 6V6 tubes and can be had locally for cheap or free.

Last edited by Alvis; 31st May 2012 at 07:08 PM.
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Old 31st May 2012, 08:10 PM   #8
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Quote:
Originally Posted by Alvis View Post
You could consider going octal and using 6V6 rather then 6AQ5.

Old 6V6's are pretty common and can generally handle abuse in guitar amp situations better then the 6AQ5.

Great old transformers are found in the vicinity of great old 6V6 tubes and can be had locally for cheap or free.
But considering he already has the iron and tubes and that 6AQ5's are dirt cheap and plentiful why bother? As long as the voltage is more or less within the limits of the 6AQ5 the tube will take whatever abuse a 6V6 will take. The question is, what for the front end?

Sounds good to me.

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Old 31st May 2012, 08:32 PM   #9
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Yes, I want to use what I have on hand. I have plenty of larger tubes but my nephew wants something he can play at relatively low volumes inside the house. I may build him two, one with the 12 incher and another with the oval. I would try too keep the profile of the oval speaker box as small as possible so it can be truly portable.

They have a band Home | Can You Canoe and they already have all kinds of big powerful gear.

I was planning to use a circuit similar to the Champ and try to modify the EQ to add a Mid Pot as well (Nephew asked for one). Maybe I do one with just Bass/Treble and another one with the three.

I see the Champ circuit has no Choke on the power supply and the filtering is very straight forward, is it relatively silent without one?

Last edited by avincenty; 31st May 2012 at 08:43 PM.
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Old 31st May 2012, 10:26 PM   #10
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Sign up for the Trinity forum and get a schematic for the Tramp. Probably could not do much better.

Trinity Amps - Trinity Tramp Overview
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