Is a Standby Switch Important on a Guitar Amp?

[randyc]

Addendum to above comments: I'm getting close to completing my write-up on the design of the vacuum tube output stage (the most challenging part of guitar amplifier design, if one can really apply the term "challenging". Stay tuned if interested ... if not, just keep on copying

(NOT aimed at anyone specific, more to the entire class of those that replicate mistakes and misconceptions perpetuated over the years regarding devices that are universally misunderstood.)

cheers,
randyc

PS: the design example that I intend to present does NOT use one of the few tubes always used in the tens of thousands of audio amplifiers produced up to the present.

[Archie]

My Laney LC15R doesn't have a standby switch.

[lpdeluxe]

Randy, I e-mailed you the schematics from my BJ manual.

[randyc]

John, they're downloading now, will get back to you on what I find

[randyc]

I looked at LPD's Fender Blues Junior schematic. There is no timing circuit to prevent high voltage being applied before the tube is "warmed up". I DID find something to support my theory that the problem with applying high voltage before proper temperature is achieved lies with the screen grid and not with other theories commonly discussed.

Before getting to that point, however, I made some notes on various amplifiers, regarding amplifier, output tubes used, output power, plate voltages, screen voltages and how voltages were measured.


Low power amplifiers
Fender Champ, 6V6GT (1), 6W, 400V, 350V, measured under signal drive

Medium power amplifiers
Fender Princeton, 6V6GT (2), 12W, 420V, 415V, schematic data
Fender Blues Jr, EL34 (2), 15W, 328V, 307V, schematic data
Ampeg Reverberocket, 7868 (2), 18W, 360V, 350V, measured no signal
Epiphone 25*, EL34 (2), 22W, 400V, 396V, measured no signal
Fender Deluxe Rev*, 6V6GT (2), 22W, 415V, 415V, schematic data

High power amplifiers
Fender Bassman*, 6L6GC (2), 50W, 425V, 425V, schematic data
Fender Showman*, 6L6GC (4), 85W, 450V, 443V, schematic data
Fender Twin Rev*, 6L6GC (4), 85W, 460V, 458V, schematic data


* standby switch included


Of the two newest amplifiers, the Epiphone Galaxie 25 and the Fender Blues Junior, the Epiphone has a standby switch, to allow leaving high voltage disconnected until tube has reached operating temperature. The Fender Blues Junior has NO standby switch but the circuit DOES include reverse polarity diodes on each output plate so that the screen grid can NEVER be at a higher potential than the plate.


Out of the eight push-pull, higher power amplifiers, only two do not have provisions to prevent screen grid failure by the application of high voltage before the tube reaches operational temperature, the Fender Princeton and the Ampeg Reverberocket. There's no particular consistency that I can see; these amplifiers are operating at roughly similar plate/screen voltages as the remainder of the amplifiers.


I think that the relationship of screen grid to the remainder of the tube operating conditions is still not completely understood. My earlier suspicion, seemingly - at least partly - confirmed by the notes posted above from web research on the screen grid, are that the standby switch is a required feature of enhanced tube reliability, although not necessarily for the reasons that many people believe.

P.S. In the previous post, several mentions were made regarding tubes oscillating when screen grid voltages were too high, compared to plate voltage. It may be worth mentioning that my Ampeg Reverberocket - the only medium power amplifier without a standby switch - oscillates when first turned on if there is no input signal. This is a fairly recent phenomenom, occurring in the last couple of years. I imagine that it has something to do with the aging of the tubes. Anyway, I always have to turn volume control all the way "down" before switching amplifier "on".

[lpdeluxe]

small correction: the Epi and BJ have EL84 power tubes.

[randyc]

Yep, I can go back five years in one of the other jazz forums where I'm a member, and find posts of people correcting me for that exact typo - I just seem to like the "3" better !

[TieDyedDevil]

TDD: As a rule, I prefer to discuss things like this privately, rather than complicating possible misunderstanding with public exposure (which might consequently encourage a participant to adopt "attitude"). However, attempting to send you a PM, a forum response states that you have chosen not to receive private messages from the forum, so ...

I have several problems with what you posted (above). I don't mind when my errors are noted and I'm told of them - how else is one to learn to be more careful ? But NOT in the disrespectful, patronizing manner that you used. There's a long-established standard of professional courtesy between colleagues - even when major disagreements are experienced -that you chose to disregard.

I hope that this doesn't occur again. I'm going to assume that impulsiveness generated your "gotcha" post rather than deliberate rudeness. I wouldn't feel comfortable exchanging "in kind" comments so please give me no reason to do so.

Thanks,
randyc

My sincerest apologies, Randy. I intended no disrespect. Intention, however, is not always conveyed in writing. The tone I was trying (and obviously, failed) to adopt was one of gentle, good-natured chiding of a valued colleague's error. Again, I'm sorry for having offended you.

In the future, please feel free to contact me via email, which I prefer to forum PMs. My email contact information is and has been available via my forum profile since I joined this forum.

[randyc]

Apology accepted and reason for misunderstanding now obvious, thanks. Try sending yourself a PM and see what's wrong with the settings - or better still, I'll try to send you a test message.

cheers,
randyc

[randyc]

OK, e-mail is operational, PM doesn't work, will implement your suggestion in the future, thanks.

randyc