Question for amp techs Can I replace a 12ax7 with a ecc81?

jonnyburgo

Got a superchamp xd with a rattly 12ax7, what effect would swapping it for a ecc81 have or is it a no no?

ICBM

It will reduce the gain in the power section slightly (basically meaning you need to set the volume control a bit higher for the same actual volume) and open up the tone a bit - clearer with more bass and treble, less midrange.

It should be safe as far as I can tell from the schematic. There are a few Fender amps where this substitution sometimes causes trouble, but they use 1/2W plate resistors - this amp has a 1W so it should be fine.

ESchap

I think it will be "safe" to try it.  But you'll definitely lose gain.  Personally I don't like 12AT7's (ECC81) in tone influencing positions other than as reverb driver or phase inverter. To me they have a sort of metallic ring to them.  But others like them.  So you've nothing to lose.

This might help with the gain thing ...

ESchap

well at least the advice is consistent!  must have been typing while you were posting @ICBM

ICBM

I agree with them often sounding 'metallic' 'cold' or 'sterile' in early gain stages - not always, but they can. They're usually very nice in LTP phase inverters, giving a cleaner and bigger sound - that's why Fender switched from 12AX7 to 12AT7 at the start of the BF era.

This amp is a bit more complicated in that the two stages are used as an intermediate gain stage and a cathodyne phase inverter rather than a LTP inverter, but at a guess it will still sound OK. No harm in trying it, anyway.

In some modern Fender amps substituting a 12AT7 or 12AY7 for the V1 12AX7 can blow the plate resistor, because the lower-gain valves draw slightly more current - but that's because Fender use a 1/2W resistor here. This amp has a 1W resistor, so it should be OK.

Cirrus

Is ecc81 @ecc83 's younger brother?

ecc83

Cirrus said:

Is ecc81 @ecc83 's younger brother?

 

"Out of the mouths............" The 81 is an "RF" triode, originally found in TVs and scopes as a sync switcher. The 83 was I think developed primarily for audio and probably came later, you certainly never saw them in tellies! I have only tried a handful of ECC81s, in front ends and they have been as miccy as frick! No doubt where the "metallic" sound comes from?

 

As ICBM says, be careful what you swop. The reverb driver in many amps is an 82, almost anything else will quickly die. Same for the OPv driver in Series Ones and the bigger HTs. An 83 will not last long and will sound shit anyway.

The HT pedals necessarily use pretty wee anode resistors so don't swop out the ECC83 (bit of a PITA to do anyway for the uninitiated so I guess not many have tried?)

 

MOST amp designers knew what they were at you know!

 

 

 

Forgot! That list of gain figures? Note that even in the best optimized circuit you will only get 75% or so of that. The figures are "mu" NOT Vout over Vin.

 

Dave. (why is typing here so sluggish? Not so on sos and HR forums.)