Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on October 21, 2014, 11:07:05 am
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I've some ECC82 tubes, they are rarely used in guitar amps (maybe, sometime, as PI or in the send of FXLoop)
To find an use I can use it also in CF configuration, but I was wondering if is possible to use it in other collocations on a guitar amp circuit
as V1 or V2 in Cascode configuration or simply as Anode Follower putting two in series to be used as something like a single triode of a 12ax7 tube
Often I've read that each gain stage adds harmonics so my idea has this thing on the basis
K
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Hi K,
I used it to drive tonestacks with great results. It was plate-driven and I had to adjust TS caps to taste. Basically a 5879 gain stage - gain control - 1/2 ECC82 - Tonestack + vol. - 1/2 ECC82 - PI - PA
I need to find the schem, but if you are interested, pm me.
Best regards
R.
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Find an old CONN organ in the garbage and you'll have 36 of them.
Some folks use them in reverb driver position in a Fender amp which can increase the usable range of the "reverb" control because they have much less gain than a 12AT7.
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I like them in the D'Lator type active FX loops & sometimes use them there.
With respect, Tubenit
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Often I've read that each gain stage adds harmonics ...
Only if you're driving the tube hard enough to push it beyond clean amplification (or mis-biasing enough to ensure there is little clean amplification).
Seems like a good tube to deliver moderate current in a preamp (a reverb driver might be a decent application), to act as a unity-gain mixer (though the relative levels of the individual signals need to be pretty close; closer than what you might offset with a 12AX7 as a unity-gain mixer), etc.
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If you cascade enough of them you can get as much distortion as you like, but be prepared to tweak everything.
Some examples of use, AX84 blues preamp:
http://www.ax84.com/static/corepreamps/Blues/AX84_Blues_Preamp_Schematic.pdf (http://ax84.com/media/ax84_m276.gif)
Doug's Firefly, which uses a 12AU7 as the output stage:
http://ax84.com/media/ax84_m276.gif (http://ax84.com/media/ax84_m276.gif)
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Many thanks to ALL you friends
@ Merlin
is this the Blues preamp that you mean ?
http://www.mrdwab.com/john/Blues-pre.jpg (http://www.mrdwab.com/john/Blues-pre.jpg)
on the two link you posted there is the same schematic (both are FireFly rev.3)
Franco
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I'm spending some time on research on old amp schematics
I discovered that Univox used the 12au7 in different positions on their amps
a pair attracted my attention, one has a 12au7 tube as driver and recovery of the TS (as Rzenc told :thumbsup: )
The other was in a position at which I've think sometime but was never sure if convenient (may be in the 1236, that is a Bass amp, it was), I mean V1
(http://rudn2.nodevice.com/preview/big/373/373444-1.jpg)
(http://elektrotanya.com/PREVIEWS/63463243/23432455/univox/univox_1236.pdf_1.png)
on the 1236 there is also a 12au7 after the PI before the Power Tube
I think the purpose was to drive at the max the 6L6GC tubes, is this correct ?
K
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> purpose was to drive at the max the 6L6GC
6L6 are not hard to drive. Almost any reasonable driver can over-drive them.
And the U-1075 would need more drive than the 1268 because of higher 6L6 voltages.
It's a cross-word puzzle.
Put in a tube. Not enough gain. Put in another tube. Too much gain. Throw in a tone-stack. Or a mixer. Less gain. You move things around and change them until you have a reasonable gain-structure from input to output. You also want a reasonably minimum cost, which suggests dual-triodes, but you have to get the right halves in the best places.
The U-1075 has a lot of inputs, so has a lossy mixer, and has to start with high-gain preamps and individual gain controls. After the mixer the level is too weak to go to a tonestack, but too much gain added may overload. So a low-gain stage. Then tonestack. The output stage is lifted direct from Fenders, needs pretty strong input, so need a little gain after the tonestack. While two low-gain stages seems complex, they are just one dual-tube. This also allows a 2.2K+0.3uFd cathode network, twice, to provide a lift and shelf in the midrange, a classic guitar EQ.
The 1263 looks like two hi-gain stages before the tone/volume network but the NFB around these stages means a very clean input with moderately high gain. All the tone/volume loss is in the middle, and then it needs fairly high gain to the output. The 1263 is unusual for a guitar amp in that all the gain happens in just two NFB sections. It might be very low coloration before over-load.
Also I am sure the guy who designed the Japan-made UniVoxes had the same problem as everybody here. He just wanted to try something different each time.
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I like your guidance on those old circuits if well explained there is a lot to learn there
As always Many Many Thanks PRR
Franco