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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 7th February 2019, 12:37 PM   #31
Terry Demol is offline Terry Demol  Australia
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Quote:
Originally Posted by nauta View Post
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.
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.
Very true Enzo.

I'm constantly having to replace cathode resistors in AC30's to bring them down from *way over max plate dissipation.

And they are by no means the only offenders.

That's guitar amps.
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Old 7th February 2019, 10:03 PM   #32
PRR is offline PRR  United States
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Quote:
Originally Posted by aut0m4tic View Post
...noticed that a lot of the old organ and console amps ran the screen within a few tens of volts of the plate....
SO??

We do not use True Tetrodes! For anything! Rip those pages out of your books!

A Pentode (includes "Beam Tetrodes" which are truly pentodes) can swing the plate WAY below Vg2, to maybe 20% of Vg2, without any funny effects.

The relative potentials of Plate and G2 are NOT directly meaningful.

Vg2 directly limits the Maximum Plate Current. Look at the attached 7027 curves.

Vg2 = 150V -- 75mA
Vg2 = 300V -- 210mA
Vg2 = 450V -- 385mA

Output power is voltage times current. If you have big tubes and wish to run a low main supply voltage, you normally raise Vg2 as high as is convenient (or until something melts).

You "could" run a 300V plate supply and a 450V G2 supply... but that would be inconvenient.

You sure can run a 600V plate supply and a 300V G2 supply. See 6550 8417 KT88 100W suggestions. But G2 current varies a lot, and it is normally inconvenient to derive a well-regulated G2 supply from a much higher plate supply. (Solid-state rectifiers made this more convenient.)

In a clean-sheet amp-only design, it could be "optimum" to design a low Mug2 tube to get large current at low Vg2 and low Vg1 bias and drive. But that moves the inconvenience to the power supply's Vg2 supply. In RF work where high impedance loads are practical, you do see 700V on plates, 200V on G2, and a separate Vg2 rectifier/supply.

In Audio, we dislike really high impedance loads (cuts bandwidth). And also separate G2 supplies. There is a class of Audio Power tubes which are designed to work well with Vg2 near or slightly less than plate supply. This includes all our 6V6 6L6 ELx4 the KTs and others.

So yes: Vg2 near (even above) plate voltage is a perfectly respectable thing to do.
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File Type: gif 7027-Vg2-mA.gif (25.7 KB, 56 views)
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Old 8th February 2019, 12:52 AM   #33
aut0m4tic is offline aut0m4tic  United States
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Wow! I sincerely appreciate the knowledge shared on this forum.

PRR
Thanks for that! It's like the clouds parted and the sun was let in. I appreciate your time and examples. Screens make more sense now.

My test bed oscillator is doing great in the kilohertz region... one day it will just amplify! Interesting how screen current rises when it oscillates.
I even took it back to cathode bias, and screen supply from b+ no smps just resistors, as I thought it was switching noise.
Going to take a few days to get some study time in before I heat the amp back up.
Thanks for all the help. At some point I'll have this thing going, and be able to adjust it to textbook values, and see how it sounds.
I'll start with a simple volume only, then single tone control, and if I can get that to work without it wanting to transmit low frequency morse code, I'll go for a TMB and master volume.
Cory

Last edited by aut0m4tic; 8th February 2019 at 01:17 AM.
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Old 8th February 2019, 08:13 AM   #34
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by PRR View Post
A pentode (includes "Beam Tetrodes" which are truly pentodes) can swing the plate WAY below Vg2, to maybe 20% of Vg2, without any funny effects.
Very good point, as this is just the most advanteous propriety of screen grid valves!

Quote:
You sure can run a 600V plate supply and a 300V G2 supply. See 6550 8417 KT88 100W suggestions.
We could also add 807, LS-50, GU-50, EL152, e.g.
Best regards!
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Old 9th February 2019, 12:56 AM   #35
PRR is offline PRR  United States
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me> ...swing the plate WAY below Vg2
KP> ... this is just the most advanteous propriety of screen grid valves!

Watch your language.

"Screen grid valves" includes True Tetrodes. Which go goofy when Vg2<Va.

The "propriety" that I think you mean comes from G3, Suppressor Grid (or side-electrodes).(*) This discourages splash-back electrons from Plate going to Screen, the "problem" with True Tetrodes.

(*) The electrons do not care if the potential minimum between G2 and Plate is induced by a loose "grid" or by side-plates. But they look very different on the patent drawing, why RCA _never_ said the P-word around the 6L6. The two techniques have other tendencies, but there are many tricks and "same" tubes have been built both ways.
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Old 9th February 2019, 02:21 AM   #36
aut0m4tic is offline aut0m4tic  United States
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After several hours of reading (mostly the Radiotron Designers Handbook) I realized that I needed to look at my power supply. Sure enough, I hadn't decoupled my preamp from my screen supply. I added a 27k resistor, and a 10uf cap and she plays a lot nicer now.

I finally broke out the oscilloscope, and also found that the coupling capacitors between the phase inverter and the 6v6's weren't letting much signal past. I guess I should verify all the components that I reuse, sure would have saved some head scratching.

Now that I know the transformers and everything will work, I just need to cleanup the grounding scheme to quiet down a little hum, and I'll be ready to experiment with a tone stack. After that, I'll see if I can add back the smps and bias supply, and see if the operating listed in the manual sounds different (better or worse being too subjective). Then I can start to answer the original question, with some firsthand experience.

I still cant believe the knowledge gathered and shared here. It allows a "dumb ole lineman" as we call ourselves, to learn how and where to look for info to make decisions instead of just guesses!

Cory
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Old 11th February 2019, 08:16 PM   #37
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by PRR View Post
The HIGH-voltage Fenders come in the late 1960s, when 99% of tubes were made in about 3 plants. Leo may have had a gentleman's understanding that all the tubes he was likely to get, from whatever factory (all the brands horse-traded) would be made of 400V stuff...
Fender reissued some of their 1960's amps, and kept the original power transformer specs for authenticity in some cases. Meantime, the AC line voltage in North America has climbed considerably since 1960.

Combine the too-high 1960's voltages with original-spec transformers and higher contemporary line voltage, and the results are ridiculous. The Fender schematic for my '65 Princeton Reverb reissue shows 440 volts DC on the 6V6 anodes.

The 6V6 screens are fed from node "Z" on the power supply. The power supply schematic shows 425 volts DC at point "Z".

The result is that most of these amps end up using JJ 6V6S output valves, pretty much the only current-production "6V6" that are beefy enough to tolerate the abusive screen voltage. Unfortunately, the JJ 6V6S doesn't sound much like a real 6V6 tube, tending to be too clean and sterile sounding.

If you look at the attached images, it would seem the JJ "6V6S" is identical in internal construction to their JJ 7591. Original Tung Sol datasheets show the two valves to have been very different (with quite different distortion specs), but who's to argue about it today? JJ can slap any label on any tube they want (probably some random Russian tube that had adequate ratings), as long as the pinout is correct.


-Gnobuddy
Attached Images
File Type: jpg JJ_6V6S.jpg (148.5 KB, 30 views)
File Type: jpg JJ_7591.jpg (70.1 KB, 29 views)
File Type: png 65_PRRI_Voltages.png (211.4 KB, 29 views)
Attached Files
File Type: pdf Tung_Sol_6V6.pdf (173.6 KB, 2 views)
File Type: pdf Tung_Sol_7591.pdf (95.4 KB, 3 views)
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