Fender Vibroverb clone Vibrato issue

[Minnievirginny]

Whilst that is a perfectly feasable scenario, no, I had no reason to mess with what was (and now once again is) a fabulously sweet sounding amp.

In fact I was discussing the point with my girlfirend this morning and saying that of all the components likely to fail, I would have put a potentiometer as least likely.

I have now completed work on the Twin Reverb which is also sounding great. I just need to find somewhere to test it full throttle but we have decent neighbours.......

Thanks again for your thoughts as I doubt whether I would have got round to swapping out that damned pot (which, incidentally, looks like it has just come off the production line) until a bit further down the line as it fell in to the 'swapping out parts even though they test OK' catagory.

Cheers

Larry

[Minnievirginny]

I must add as a P.S. that my girlfriend is, understandably, fascinated by my conversations about the workings of amplifiers - what girl wouldn't be?

[pdf64]

…
I used the Biasing Tube Amplifier Calculator @ https://www.amplifiedparts.com/tech-art ... calculator
This suggests that for a plate voltage of 446 volts and a 6L6GC tube a current of 47ma will yield a 70% bias. Have I missed something or misunderstood?
…

I think all these online bias calculators are doing a disservice, by failing to distinguish / differentiate between design centre and design max rating systems.
The design max system was introduced in 1958, and was typically used for North American valve info published after then. It (artificiality) inflated anode limits, making the exact same valve type appear beefier than it was under the design centre system.
The apparently 30W anodes of 6L6GC really aren’t 20% more beefy than the 25W anodes of EL34. Rather it’s just a design max - design centre thing, with the actual absolute max limit of both types probably being about 35W.
For a given valve type, 60% of its design max limit is probably equivalent to 70% of its design centre limit.

Explanation of the 3 rating systems, from RCA RC30

6V6GTA design centre 12W 1956
https://frank.pocnet.net/sheets/127/6/6V6GTA.pdf
6V6GTA design max 14W 1963
https://frank.pocnet.net/sheets/135/6/6V6GTA.pdf
Design max system rationale RCA AN174
https://www.one-electron.com/Archives/R ... 0Tubes.pdf

[Minnievirginny]

Thanks pdf64 - most interesting

[Reeltarded]

I must add as a P.S. that my girlfriend is, understandably, fascinated by my conversations about the workings of amplifiers - what girl wouldn't be?

Easier to name the interested one. Your girlfriend.

[martin manning]

I think all these online bias calculators are doing a disservice, by failing to distinguish / differentiate between design centre and design max rating systems.

Agreed. When Pa max ratings for different tube types are included, they always seem to be inconsistent.

The design max system was introduced in 1958, and was typically used for North American valve info published after then. It (artificiality) inflated anode limits, making the exact same valve type appear beefier than it was under the design centre system. ...
...
Design max system rationale RCA AN174
https://www.one-electron.com/Archives/R ... 0Tubes.pdf

In the linked RCA Application Note, the original JETEC publication (May, 1957) is included. There is nothing artificial about the design max rating system. It is actually more transparent with respect to the capabilities of the device. The problem occurs when people are unaware of the difference and how it applies to what they are doing, be it equipment design or field service.

The apparently 30W anodes of 6L6GC really aren’t 20% more beefy than the 25W anodes of EL34. Rather it’s just a design max - design centre thing, with the actual absolute max limit of both types probably being about 35W.

Based on the 6V6GT example, there was 14/12-1 or 17% of margin from design center to design max. If you were to apply that to the EL34 (25W Pa max design center), it would land at 29.2 for design max vs. the 6L6GC's 30W. But who knows what margins were actually included in the 25W EL34 design center rating for manufacturing and operating environment variation?