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Tube Guitar Amp - Running Output Tubes in Spec?
Tube Guitar Amp - Running Output Tubes in Spec?
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Old 4th February 2019, 07:12 PM   #1
BJosephs is offline BJosephs  United States
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Default Tube Guitar Amp - Running Output Tubes in Spec?

Given how many threads there are about how guitar amps run output tubes outside their recommended conditions (excessive b+, higher idle dissipations, high screen voltage), Iím wondering has anyone here made or found an example of an amp that actually runs the output tubes to recommmended conditions? Iíve been planning to make a 15w AB push pull (el84 or 6V6) with a HIFI OT that I have (Hammond 1650E). The PT I bought was a repurposed before I got to test it out but another is being freed up from a project that will give 285v B+. It seems 285v with an 8k load is actually a recommended operating point for both tube types. Am I about to find out that all the magic comes from torturing my tubes? ☠️

In case anyone is curious the initial inspiration for a wide band OT was the fact that I play mostly metal which benefits from a cleaner, flatter power section. I wanted to take that to itís conclusion but at the time Iíd still planned on using a more historically accurate (read: abusive) set of operating conditions.
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Old 4th February 2019, 10:06 PM   #2
Enzo is offline Enzo  United States
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The specs in the tube manual are for table radios. They run the tubes nice and politely so the tubes have long life and less distortion. None of that is a guitar amp goal.

I can't think offhand of any like you describe, but a lot of that is because I never pay attention to that. I might start by exploring older Ampeg designs. Mr Hull hated distortion.

A lot of things like high screen voltages happen simply because they can get away with it. Why go to extra effort if the tubes will withstand it?

Imagine your mom's sedan. She drives it nicely to the store and follows the rules. Now imagine you want to drag race her car on the weekend. You will surely exceed the recommended operating conditions. Well, guitar amps are the drag racing version of table radios.
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Old 4th February 2019, 10:56 PM   #3
nauta is offline nauta  Canada
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Hi

Anyone who thinks that guitar amps use the tubes out of spec do not know about tubes or tube safety rules, so in this regard, nearly ALL musical instrument amps run the tubes within ratings. They have to, or there would be a lot of damaged OTs.

The only abuse you see occasionally is screen voltages being exceeded, as in the Marshall 2-375W amps from the 1980s. High screen-stops save those tubes but otherwise all within spec.

Data sheets are filled with hi-fi applications that in general DO NOT reflect the capability of the tube.
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Old 5th February 2019, 07:44 AM   #4
Enzo is offline Enzo  United States
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I respectfully disagree. Leo Fender ran 6V6s in amps like the Deluxe at 100v over the spec for decades. Clearly tubes will withstand conditions far in excess of the spec sheets.
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Old 5th February 2019, 11:24 AM   #5
BJosephs is offline BJosephs  United States
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Iím sure every el84 amp schematic Iíve ever looked at ran the the plates well beyond where the X axis on the plate curves ends. And Iím not just talking about classic ďCommercialĒ designs - lots of people recommend idling el84s at (or slightly above) their max dissipation because they can. It seems there are plenty of examples of at least one parameter being violated which is really all I was getting at.
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Old 5th February 2019, 05:23 PM   #6
nauta is offline nauta  Canada
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hey
Does no one here know about tube data? It changed in the 50s or so but many data books continue to print out of date info, and even tube suppliers like EH continue printing wrong data.

6V6 and EL-84 both have anemic-looking ratings if you look at the bullet points, but when you dig deeper, especially for 6V6, you see it was used in circuits with 1500V (flyback not DC). Not use if EL-84 was used in similar circuits but there were articles about it from years ago where the designers stated its quite high voltage limits.

Fender NEVER operated a tube outside of its specs.
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Old 5th February 2019, 07:01 PM   #7
BJosephs is offline BJosephs  United States
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Quote:
Originally Posted by nauta View Post
Does no one here know about tube data?
I guess I'm not sure what you think you are contributing to this thread. I am trying to have a discussion regarding running guitar amp output stages in recommended conditions and, by extension, within all specified limits. You seem to be having a different discussion by asserting that the specs themselves are wrong and overly conservative - which may very well be true - but has no bearing on my original question:

Quote:
has anyone here made or found an example of an amp that actually runs the output tubes to recommended conditions?
I know why they were operated outside of the the limits and I know that many amps are just fine running that way (and that there are more than a few tube eaters out there too!). I want to compare notes with others who have ended up building amps that didn't push the limits and learn if they were happy with the results.
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Old 5th February 2019, 10:12 PM   #8
Printer2 is offline Printer2  Canada
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Lectrolab R400, 280V plate, 210V screen on a 6V6.
Matchless DC30, 345V, 325, EL84.
Vox AC15, 310V, EL84
Vox AC30, 355, EL84

Some early Tweed Fender amps were lower voltage.
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Old 6th February 2019, 02:28 AM   #9
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by Enzo View Post
I respectfully disagree. Leo Fender ran 6V6s in amps like the Deluxe at 100v over the spec for decades. Clearly tubes will withstand conditions far in excess of the spec sheets.
For a while.

But this is where the "tubes wear out" and "tubes are unreliable" memes come from. Especially when coupled with built-to-spec (maybe) new stock valves.
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Old 6th February 2019, 03:33 AM   #10
JMFahey is online now JMFahey  Argentina
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Stan, a VERY experienced Guitar Amp Tech, moved to St Petersburg, Russia.

He was amazed to find hardworking Pros (think "Uncle Vanya and His Accordion" type musicians) who have 10 gigs a week all year long, whose amplifiers had 30 y.o. original 6L6 in their sockets, still working very well.

He was very intrigued, until he found they were all run "by the book", say 360V +V, 250V screens, datasheet recommended idle current and dissipation, etc.

Yes, "meager 25 to 35W RMS" from a single pair ... so what?
More than enough in a living room, party hall or even outside, weather permitting it.

So I guess this answers at least part of your question.
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