rob aylestone
Moderator
Lots of new members join to sort a single problem, and this guy hasn’t been around since 2019, so sadly wont read the reply. It’s easy to miss the fact you’re replying to a long dead topic. Welcome, by the way!
Amp reverb noise - tube problem?
- Thread starter dainbramage
- Start date
I would advise against using a 12AT7 (ECC81) instead of the AX7 (ECC83) in that position because the bias resistor is the wrong value for that valve and it will draw excessive anode current. I don't have the schematic but if one triode section uses a small transformer you might burn that out.
That said, it is unusual to find the 12AX7 as reverb driver/recovery valve, it often IS a 12AT7 but properly biased!
The 12AX7 is not as a rule microphonic (it is after all an AUDIO/hi fi valve whereas the AT7 is definitely not! It was designed for RADAR and 'scopes)
See if you can find a TAD 7025 HG, a superior ECC83.
JUST! see you Rob. No matter the facts remain true.
Dave.
That said, it is unusual to find the 12AX7 as reverb driver/recovery valve, it often IS a 12AT7 but properly biased!
The 12AX7 is not as a rule microphonic (it is after all an AUDIO/hi fi valve whereas the AT7 is definitely not! It was designed for RADAR and 'scopes)
See if you can find a TAD 7025 HG, a superior ECC83.
JUST! see you Rob. No matter the facts remain true.
Dave.
rob aylestone
Moderator
When you go back through this topic, it is actually useful stuff - because as each year goes on, valves get more sort of 'magic' and mysterious. Nowadays, we're looking at swapping identical valves to find the 'best' sound, while years back, you swapped them to find a working one. I remember an early job where the youngsters got the valve testing job - huge box on one side, three on the other labelled Good, OK and U/S, which then we had to stop using to describe unserviceable items - perhaps the first example of a word becoming 'offensive'. One American engineer took great exception to the term, and in fairness, we kind of got it.
I suppose the point here is that swapping valves of the same class does give you differences. Maybe we did lose something with absolute identical performance between solid state components. Maybe this drove the folk who have moved the valve 'magic' to things like capacitors?
I suppose the point here is that swapping valves of the same class does give you differences. Maybe we did lose something with absolute identical performance between solid state components. Maybe this drove the folk who have moved the valve 'magic' to things like capacitors?
Personally I take these "night and day" claims for differences between valves of the same type number with a huge pinch of salt. Valves are made to the same precision as watches because they are expected to work in equipment just a well as a replacement. Yes, critical stages, a field OP valve say, some adjustment to the amplitude (height) and linearity would be needed but that is because the old valve had lost emission and the pots tweaked during the life of the telly.
I was once asked to build perhaps a unique rig. One that allowed a double triode (83) to be A/B tested against another brand instantly. We were swapping the whole stage over with relays whilst keeping the heaters running. Result?
Yes, some top guitarists ears claimed a tiny difference between brands, especially when pushed hard but it was agreed that no one else would hear those difference in practice.
Whenever one reads of such "obvious" differences between valves it is always a totally subjective judgement. Never do we get the two sets of DC condition, any changes in the AC gains nor any before and after recordings. And "double blind" tests? The claimants would not know one from a spanner.
I did sit and test over 100 83s but I was MEASURING for hum and hiss. Also microphony but that had to be a subjective test. I.e. I flicked the valve and harckened unto it!
BTW, Solid state device, i.e. transistor are MUCH more variable in their characteristics it is only DC and AC NFB that makes them work!
Dave.
