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Author Topic: 12AU7 Hybrid power amp - Evil_Food  (Read 4176 times)

joecool85

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12AU7 Hybrid power amp - Evil_Food
« on: November 07, 2011, 09:15:00 AM »

**Note**

ORIGINALLY POSTED BY user Evil_Food

Something strange was happening with the forum software and this thread wasn't viewable so I had to re-make it.

______________________
Hi guys,

I was browsing Schematic Heaven and I found a Musicman PP power amp design that uses hybrid cascodes consisting of transistor on the bottom and a valve on the top.
I started analysing the design and decided to make my own low power version with a 12AU7 double triode.



The output transformer is a Ģ12 3.5W Oxford Electrical Products Audio Transformer http://uk.rs-online.com/web/p/products/2106475/?searchTerm=3.5w+audio+transformer&relevancy-data=636F3D3226696E3D4931384E44656661756C74266C753D656E266D6D3D6D61746368616C6C7061727469616C26706D3D5E285C772B5B2D5C2E5C732F5D292B285C772B293F2426706F3D3926736E3D592673743D4B4559574F52445F4D554C54495F414C5048415F4E554D45524943267573743D332E357720617564696F207472616E73666F726D65722673633D592677633D4E4F4E4526

If you click on the "Range Overview" tab, there's a picture with the turns on each tap on each side of the transformer. The datasheet says 16k as maximum impedance, but if you do the calculations you can get other values.

What I think might be more interesting is the power supply. It uses a Cockcroft-Walton voltage multiplier to step up and rectify the voltage from 24VAC to ~280V DC. My main reason to do it with a multiplier was to save some money, as I already had a 2x12V power transformer. This way I can heat the tube with 12Vac and provide +/-12V DC.



What I liked in the voltage multiplier was one of its shortcomings - poor regulation. The power supply has high output impedance and when the amplifier enters class B operation, there's a voltage sag on the HT rail.

I haven't looked with an oscilloscope, but the DVM shows about 30V sag whit "harder playing".

I haven't got a preamp for it yet, but I quite like the sound when it's overdriven on its own.

I haven't got a camera or anything I can make a sound clip with, but next week I'll ask a friend (who's actually a musician) to bring along something to record what it sounds like.

If anyone is interested in the voltage multiplier power supply I can post the equations needed to design one.
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Pilot1

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Re: 12AU7 Hybrid power amp - Evil_Food
« Reply #1 on: December 05, 2011, 08:00:12 AM »

very clever,
but why common grid output?
guitar needs 100Hz - 8kHz bandwidth
CG is very wideband... ::)
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J M Fahey

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Re: 12AU7 Hybrid power amp - Evil_Food
« Reply #2 on: December 05, 2011, 10:04:58 AM »

Hi Pilot 1.
Yes, you're right, Guitar does not *need*much above 6 or 8 KHz, which wonīt be reproduced by the speaker anyway, but bandwidth does not hurt either.
Any popular Power Chip , from TDA2003 to TDA7294 easily meets 50KHz or more bandwidth, so no intrinsic problem with that.
The schematic posted above follows the very clever Music Man ideas (search http://www.music-man.com/techinfo/old-amps.html)
As an example:www.music-man.com/techinfo_old/old_amps/2165-rd_&_2100-rd.pdf
They could achieve incredible feats:
1) slamming brutal 700V on plates (650V on 6L6) without burning them; in fact theyīre known for their reliability.
2) Pulling 65 REAL watts from a pair of tubes, up tp 130W from a quad.
3) no tube biasing or matching necessary, the SS side of the circuit takes care of that.
4) they could drive those tubes fully with a simple dual Op Amp and a couple mid power transistors
5) Loud, clean sound, way beyond what, say, a Twin could provide (the reference amp up to that point).
6) not surprisingly, it was a "real" Leo Fender product and concept ... although he couldnīt say so for legal reasons.

Leo, you are my man !!  :dbtu: :dbtu: :dbtu:
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Loudthud

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Re: 12AU7 Hybrid power amp - Evil_Food
« Reply #3 on: December 05, 2011, 01:21:59 PM »

The bias seems to be set by Q3 and RV1. Probably not a big deal but that part of the circuit could be improved for better temperature tracking.

It is my understanding that Leo was not involved in the electrical design of these amps.
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J M Fahey

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Re: 12AU7 Hybrid power amp - Evil_Food
« Reply #4 on: December 05, 2011, 05:43:49 PM »

Quote
It is my understanding that Leo was not involved in the electrical design of these amps.
Agree.
Donīt think even the earlier ones were 100% his work either, besides the first, simplest ones (which practically came straight out of datasheets and tube manuals anyway).
I see him more as a *concept* and *sound* genius than anything else, not forgetting his marketing and organizational genius too.
He was one of those rare, one of a kind guys, who can build a team of geniuses, each in his own area, and have them work together towards a common goal (his own).
He didnīt play guitar either; yet ..... ?
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