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cyclecamper 24th January 2013 12:12 AM

single-ended parafeed guitar amp
Anyone ever try making a parallel-output single-ended class-A parafeed guitar amp, where you use a big expensive coupling capacitor to block the DC to a conventional output transformer instead of a really big really heavy really expensive typical SE output transformer with a gap?

I'm considering making something like a Champ without tone controls but with a switchable extra gain stage, super-quiet preamp with DC heaters and an adjustable input network, and a parallel single-ended output section with grid chokes and plate chokes. That's getting popular with the SET HI-FI guys, and it seems to me it would make a great guitar amp too. It might require some careful 'tuning" of the output with dampers so the output caps and output transformers don't have any ringing but it might make a cool amp with enough power for a usable clean tone and also more of the tube breakup instead of trans saturation when it get dirty.

djgibson51 24th January 2013 07:27 AM

Keep talking. I'm interested.

Tesla88 24th January 2013 08:36 AM

Do you mean a choke loaded Parafeed (also a current source instead of choke) SE?
Intresting , for low power amp , to reduce costs of output transformer ...
What made me think is about power distortion - break up , we're used to the classic power section distortion , wonder if a parafeed sound better or worse ! Try it :)

cyclecamper 24th January 2013 08:05 PM

Only two ways to find out how it sounds when overdriven. The easier might be to buy a similar hi-fi kit and abuse it with some mods and a guitar preamp.

Tubelab_com 24th January 2013 08:22 PM


The easier might be to buy a similar hi-fi kit and abuse it with some mods and a guitar preamp.
Has the Big Dumb Blonde One ever plugged a guitar preamp into his 300B SE amp???? You better believe it!

I have tested every amp I design by plugging in my old ADA MP-1 into it cranking it up to 11 and cutting loose. I do not have a parafeed amp, but I have abused several DHT and iDHT SE amps. I find that an SSE with a smallish OPT sounds quite like a champ, especially with a wimpy rectifier (5Y3) and small caps in the power supply. A TSE with a 300B in it doesn't sound much different from an SSE with an EL34 when cranked to 11. The big difference in these two amps is when turned down or run right at the edge. The 300B doesn't transition into distortion as easilly as an EL34 in pentode. The "edge" is more blurry with the 300B, it is a good sound for blues.

The OPT makes the biggest difference, with the Hammond 125CSE being my favorite for a small SE guitar amp that is going to be cranked, or driven with pedals. The 10 pound Edcor CXSE.....far too clean, and really heavy on bass, too much bass for a small speaker. It is a good combination for an acoustic or rythym player using a full range speaker. Even a bass guitar sounded real clean and sharp.

cyclecamper 25th January 2013 03:15 AM

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Changing subject a little, the 6V6 Champ I have is from a kit by STF electronics. I expected the power transformer to sag like an authentically small reproduction of a cheap amp at the low end of the Fender line, but with them being a Hammond distributorship franchisee I was hoping for a large single-ended output transformer perhaps bigger than the power transformer. But the output transformer is truly teensie. This is certainly not one of your monster tube amps, but would you recommend an output transformer upgrade for the little Champ? The good part is that little transformers at this end of the scale are pretty cheap, so an upgrade is not painful in the wallet.

I'd better stop dreaming and prioritize my projects.

JMFahey 25th January 2013 07:32 AM

I'd believe that needing a big f*ck*ng inductor (rated for proper Class A current, which will be considerable) which will be heavy and large anyway, need gap, etc., plus a "good" although somewhat smaller than usual output transformer, will end up costing *more* than getting the normal OT which goes there.
And very much doubt the sound will "improve", considering it's a guitar amp.

Tubelab_com 25th January 2013 03:23 PM

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This is certainly not one of your monster tube amps, but would you recommend an output transformer upgrade for the little Champ?
I bought a pair of these about 10 years ago:


I can't read the sticker on your transformer, but it may be the same one. I tried them but they are still on the shelf, so I must not have found anything that I liked them in.

The little OPT will saturate when tested into a dummy load, but saturation was not seen on the scope when driving a small speaker. This is because the impedance of a smallish (5 to 8 inch) 8 ohm guitar speaker goes way up as resonance is approached, and resonance is up into the guitars normal fundamental frequency range, 82 Hz and up. The little OPT may indeed saturate if you try to drive a 12 or 15 inch speaker with a Champ.

I used the Hammond 125CSE OPT's in many of my "champs". The 125CSE is about twice the size, maybe bigger. It has a fatter sound with more bass. The 125CSE does not saturate at any power level seen from a 6V6 that is not glowing like a light bulb. I also like the 125CSE because you can intentionally mismatch it for a variety of sounds.

Your power transformer is a Hammond, but I can't read the sticker. I used an Allied 6K56VG in some of my early Champ clones that used the 6V6. It is made by Hammond but cheaper, and similar to the 270EX. B+ is about 300 to 320 volts depending on rectifier choice.

I started making Turbo Champs using an Allied 6K7VG power transformer that resulted in 430 volts of B+. The output tubes were EL34, 6L6GC or KT88. I made several of these about 15 years ago. Most used the 125CSE OPT. Each one was different, and each sounded different. I made one with a KT88 and a pair of 6X9 inch car speakers. I thought it sounded terrible, but one of my daughters friends that played rythym guitar with an ES335 just loved it, so I gave it to him.

The OPT makes a big difference in a small amp with a small speaker where speaker resonance is "in band". The combination of speaker / OPT / cabinet will affect the tone more than anything else. Parafeed????? No one has tried it, so no one can say. It may sound bad to one player, and be loved by another. There is only one way to tell......

I got started in the whole tube amp business when I was about 12 years old by making Champ clones from discarded TV's and radios. I had discovered a formula for an amp that cranked using a 6SJ7 driving a 6BQ6 with a vertical output transformer from a TV for an OPT. That was nearly 50 years ago. Much later in life my daughter started playing guitar, and was in the high school band. I made her an amp, then her friends wanted one, that led to........

Old turbo champ schematic. I drew this from memory about a year after I built them all. It may not be totally accurate, and each amp I made was different. The schematic is most like the last one I made.

I had acquired about 25 old HP tube type audio oscillators that were scrapped for the metal value. I got an idea to use the parts for small guitar amps. Most did use the 125CSE OPT, but a few were P-P and more powerful. Many of the Turbo Champs were made using parts from disassembled HP audio oscillators. The power transformers, power supply choke, and many of the resistors and caps were sourced from the HP's. In fact I started by making a guitar amp "head" out of the HP oscillator itself. The oscillator circuit already had a P-P amp using a pair of 6K6's. The OPT had a 600 ohm secondary, so it was replaced with a P-P OPT stolen from a console stereo.

djgibson51 25th January 2013 11:22 PM

The Australian made Mudlark uses inductors from fluorescent light circuits as affordable choke loads in a hifi EL34 amp.

JMFahey 25th January 2013 11:34 PM

Well, maybe, but Fluo Reactances were never meant for DC, so ......
I bet that their AC inductance drops like a brick in vacuum once important DC starts passing through them.
Which ends out being a Lower Frequencies killer.

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