Lowering the gain on my Carlsbro TC 60 combo, help needed

jonnyburgo

I have the reissue combo, it is a brilliant sounding amp on the clean channel, lovely cleans with tons of low end thump, However the drive channel is not so great, can do a nice crunch with the gain wound down low but gets a bit fizzy after that. So, if I replace V1 with a 12 at7 or 12 au7 will this help to make it a more useable channel?

it has 4 12ax7's and 2 El34's

chugington

Hi Jonny - you may want to try as a start a 5751 tube in place of the V1 12ax7 if you can find one - I've had great results with that tube. It has a gain factor of 70% compared with a 12ax7. 

impmann

I have a Groove Tubes 5751 that I tried in my Hot Rod Deluxe - I didn't like it, tbh.

Its brand new in its box and ran up once - yours for a tenner delivered if you want to give it a try. :-)

ecc83

Wait for ICBM to opine before ad hoc bottle swapping!

Dave.

jonnyburgo

@impmann what didnt you like about it? did it lower the gain?

impmann

It did lower the gain. But it affected the tone and I didn't like it - maybe it was a result of the gain being lower. In the HRD, it also created a slightly fuzzy decay to the notes when set squeeky clean - again, not sure if thats because the overall gain was lower or because of the design of the HRD.

Hope that helps, mate.

ICBM

I don't like 5751s, all the ones I've tried have had a middy, 'smooth' type of sound. The gain is not that much lower than a 12AX7 either.

I also not that keen on 12AT7s in some 'gain' positions, they can sound a bit bright and 'cold'. (Although not always.) 12AU7s sound very nice but have a rather large gain reduction, which can be too much. 12AY7s often sound the best, to me, and have less gain reduction than the AU. But you need to be careful with all these types - they all draw more current than the 12AX7, which can risk damaging the plate resistor in some (fairly rare) cases, and the 12AT and AY7 have too low a cathode voltage rating to be used in a cathode-follower position - the 12AU7 is OK.

You probably don't want to change V1 - I don't have a schematic for the modern TC60, but it's likely that V1 will be shared between both channels. At a guess V2 might be for the overdrive channel only, V3 might be the tone stack driver and/or something to do with the FX loop (and may be a cathode-follower) and V4 will almost certainly be the phase inverter. These are just guesses though! It's also possible V2 could be a cathode-follower, so I wouldn't just try a 12AT7 or AY7 in that.

Probably the best, safest, least-cost option to start with - without the schematic, anyway - would be to get a 12AU7 and try it in the V2 and V3 positions and see what effect it has on both channels. You could also try it in V1 or even V4, but expect it to affect the clean channel too.

jonnyburgo

Thanks for that @impmann and @ICBM would I need to rebias? 

impmann

No as this is a swap out of pre-amp valves.

DartmoorHedgehog

jonnyburgo said:

would I need to rebias? 

No.  Preamp valves are always self-biasing.  (or at least none that I've ever heard of, although I suppose it would be technically possible to have an adjustable-bias preamp...  would it? @ICBM )

EDIT: Too slow

ICBM

There are a very few preamp valves which are run fixed-bias - all the ones I know of as phase inverters - but none are adjustable. There's no theoretical reason they can't be, just that it's not necessary.

All the ones in almost all other amps are cathode-biased. That's actually why the lower-gain valves draw more current.

BowynMadley

I love 5751s! It's a personal preference thing I think. For instance I don't like jj pre amp tubes whereas most people do. I've got a set of tad 5751s in my amp (v1, v2 and reverb) and they sound much more rounded and less harsh. Give 5751s a try the worst that can happen is that you don't like them.
Bo

ICBM

Ha, I hate JJs too .

4114Effects

@jonnyburgo I have a spare EHX 12ay7 you can try, fully working but it had a slight hum in my amp, but you may have better luck in yours. At worst you can see what it sounds like for free and buy a new one if you like the sound.

Pm me your address and I'll pop it in the post to you. A fellow forum member kindly sent me a valve for free recently so I'm just paying it forward. :-)

jonnyburgo

Wow thanks! PM'd