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#1
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Small Fully Integrated Amplifier
This is something I built in 1968. Finally wrote it up in 2005. Go to-
www.audioXpress.com & click on the tab Web-exclusive Articles. There you will find the PDF 'A Tiny Vintage Garage Amp'. Ya, I know, you could do better!! Cheers to all, John |
#2
Posted to rec.audio.tubes
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Small Fully Integrated Amplifier
On Mar 11, 2:27*am, John L Stewart John.L.Stewart.
wrote: This is something I built in 1968. Finally wrote it up in 2005. Go to- www.audioXpress.com & click on the tab Web-exclusive Articles. There you will find the PDF 'A Tiny Vintage Garage Amp'. Ya, I know, you could do better!! Cheers to all, John -- John L Stewart Not bad John and its like many guitar practice amps. The 6AQ5 is harder to find now than EL84 which probably has a mushier over drive sound than 6AQ6. Maybe consider adding variable NFB. And where's the reverb tank and speaker? Golden Tone amps in Oz used to make a guitar amp with a single EL84 in the 1960s, but no reverb, but with 12" speaker. Because its PT was more than large enough I was able to change it tp PP with two 6V6. It had a very primitve and thumpy going tremelo, so I put in tremolo from Fender with neon and LDR under wrap, much better. Not much NFB used, THD below clipp don't matter, sounded OK for small venues or practice. Lat week I repaired another Golden Tone PP with PP 6AQ5. No FB at all. Input was 6AU6 in pentode, high gain, and power amp driver was 12AX7, one input gain, one concertina inverter. Tone was controlled by a cap and pot from the first audio gain 12AX7 to ground. The rising speaker impedance without NFB ensures a bright sound, and this can be cut with the tone cut control. No need to boost bass, just cut treble and turn up volume. Not much deep bass anyhow because the cabinet is small, 6mm plywood. and the back panel has long been lost. There were two inputs feeding together for the input 6AU6 so a singer and piano-accordian could liven up an Italian wedding. Sound quality was controlled by red wine. But Geloso made a special little baby amp head for which a speaker in box was needed. http://www.flickr.com/photos/25988513@N08/2504502894/ I rescued one at a rubbish tip in 1994, moments before a bulldozer ran over it. I repaired the arcing OPT and rewired it and sold it for $350 to a bloke who had another and they much suited his music classes at the school where he taught. He's now Dr Zot. http://www.zot.com.au/vintage..html Patrick Turner. |
#3
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Did an amp many years ago with +ve current FB from the OPT Secondary. Used a piece of resistance wire to pickoff the current sample. Interesting to watch the scope while driving the resonance of the speaker load as the +FB was varied. Too much & then instabilty. But once setup with the users speaker it worked well for years. For a very complete compendium of guitar amp schematics try The Tube Amp Book, 4th Edition by Aspen Pitman. A very good resource indeed. Besides the schemas there are lots of photos. My copy is about 10 years old. The editors keep adding to it so it may be into the 5th Edition. Objectives & circuitry usually not hifi. Not what the users want. Oddly, The McIntosh schema is included. I think Pitman referes to it as the 'King of Tube Amps'. I would have to agree with that. And the Williamson schema is shown as a late entry. Cheers, John |
#4
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Hey Flipper, I like your use of low cost tubes that few can figure out an application for such as your 45B5s & the 20EZ7. And the pie tins are a nice touch. Used to think about using pie tins but the way things are going they may be no longer available. And they if are, will the thin metal be still strong enough? You reminded me of a guy in Texas who built amplifiers commercially back about 15 years ago, name of Alvin Bryant. He was written up by Joe Roberts of the now definct magazine Sound Practices. Al favoured PP 300Bs & lots of iron. He was a chain smoker & I think now quite dead. But I guess he had a lot of fun anyway! I did buy a couple of 6AW8s a few years ago for my stash, figuring to try a low power amp as you have. Yours looks great, nice to see it can be done. I did a couple of projects similar to your 13EM7 a few years ago for publication in AudioXpress magazine. The first was a 6EA7/6EM7 connected as a FB pair & got very good results. The next was simply a 6LU8, again a FB pair on the same chassis as above by changing the socket. That way it could run SET or by a simple reconnexion run SEUL. Both results very good. I document all my stuff as proof, unlike a lot of the stuff that gets published. I tried that amp as a FB pair, pentode connected. The 6LU8 pentode connected is not a great audio tube at all. Never meant to be but interesting just the same. Modified several 40s radios to FB pairs as well. Audio is not my main thing. Rather hitech sales. Been with HP, R&S, Field Aviation, Etc. Before that U of T Physics Research & Ferranti Packard 3-phase power station transformers. It has all been great fun! Cheers, John |
#5
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My wife tells me they are still available here, so next time at that store I will take a look. Should be OK for experimental work. What drawing software are you using? Looks like good stuff. Cheers, John |
#6
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Quote:
Oldies Music Last edited by Vance_Iam : April 4th 11 at 04:19 PM |
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