KMD GV60D hi/lo switch

[mhuss]

This amp is a solid state amp with two 6L6GC output tubes. One interesting wrinkle: the driver transistors feed the output tube cathodes, so they're in a common grid configuration.

Anyway, it has a Hi / Standby / Lo switch. When in Lo, it puts about 500 volts on the plates. When in high is puts 750v on the plates! Call me timid, but isn't that a bit much for (particularly today's) 6L6GC tubes? The common emitter driver transistors can definitely drive the cathode to near zero volts.

I don't recall ever seeing common grid specs in an audio output tube spec sheet...

[Enzo]

Same circuit as Musicman and Peavey VTX series amps, and others.

What is the idle voltage on the cathodes? 90v?

Note your screens are substantially lower voltage than the plates.

And no, it doesn't scare me for the 6L6s.

[vintagekiki]

Safe operation with Ua = 600 - 750V is possible when Ug2 = 1/2 - 2/3 % of the anode voltage value, and the bias voltage is set to amplifier operate in class B. A similar power amp configuration is seen at the Dynacord older amplifier (Gigant, G1000 ...)

A constant current source with transistors in the cathodes of the output tubes provide a kind of auto bias so tubes that do not need to be matched.

About Musicman

http://www.pacair.com/mmamps3/downloads-and-resources
http://www.pacair.com/mmamps3/node/1487
http://www.pacair.com/mmamps3/forum/2
https://el34world.com/Forum/index.php?topic=15910.0
https://steelguitarforum.com/Forum11/HTML/004357.html

https://irationaudio.com/2015/05/03/musicman-2100-rd/ ¹⁾

[Helmholtz]

A similar power amp configuration is seen at the Dynacord older amplifier (Gigant, G1000 ...)

True regarding high plate voltage, but these old German amps with plate supplies of around 800V all employed the EL34, which has a plate voltage limit of 800V (2kV at zero current). These designs were based on an EL34 datasheet example (B+ = 800V, Vs = 400V, Ip = 25mA, Raa = 11k for 2 tubes, plate idle dissipation 80% - looks more like class AB to me).

The 6L6GC is specified only up to 500V.
So here we have out-of-spec operation.

[vintagekiki]

http://www.tungsol.com/tungsol/html/faqs11.html
_{What are Class A, Class B and Class AB amplifiers?}

The most notable Class B guitar amplifiers are those made by Music Man. These employed a cathode driven circuit using driver transistors to provide the signal to the output tubes. These amplifiers have an output of 75 watts with two output tubes and 150 watts with four output tubes, where a Class AB amplifier would be rated at 50 watts for two output tubes and 100 watts when using the same type of tubes.

[mhuss]

Back to my initial concern, 6L6GCs made today are not exactly as rugged as the STR 6L6's made back in the day that MusicMan was selling these amps...

[glebert]

I've always heard the Russian tubes can take higher voltages. I have coin base 5881 tubes in my GV60. It had 6p3s tubes in it before, which I think are essentially the same tube.

[mhuss]

Good suggestion, thanks.

[g1]

I don't see any bias info on that schematic. It would be very easy to bias this unit in a conventional manner which will be extremely hard on the power tubes.
The music man configuration is pretty much the same and biases the power tubes around 6mA each. Yes, that is 6 milliamps. Somewhere in the vicinity of 10% dissipation by rough calculation.

[mhuss]

Sounds about right for 750 anode volts. Thanks.

[vintagekiki]

I don't see any bias info on that schematic.

http://www.pacair.com/mmamps3/node/1487
https://music-electronics-forum.com/filedata/fetch?id=842071

[mhuss]

Perfect, thank you!

[vintagekiki]

I've always heard the Russian tubes can take higher voltages. I have coin base 5881 tubes in my GV60. It had 6p3s tubes in it before, which I think are essentially the same tube.

https://www.ampvalves.co.uk/5881-6l6gc-valve-differences/

It all depends on what the maximum plate dissipation is designed for and whether it is intended for commercial or professional use.
The original metal version was rated for 19 watts dissipation while the later 6L6GC is usually rated for 30 watts.


https://drtube.com/en/library/tube-datasheets

[pdf64]

https://www.ampvalves.co.uk/5881-6l6gc-valve-differences/

It all depends on what the maximum plate dissipation is designed for and whether it is intended for commercial or professional use.
The original metal version was rated for 19 watts dissipation while the later 6L6GC is usually rated for 30 watts.


https://drtube.com/en/library/tube-datasheets

Articles such as the above, which make direct comparisons between design centre and design max plate ratings, are a bad info source in my view.
RCA AN-174 explains the difference http://www.one-electron.com/Archives...on%20Tubes.pdf

[vintagekiki]

useful link

http://www.one-electron.com/Archives...-AppNotes.html

[mhuss]

Further compounding the confusion is that many tubes sold as, say, 5881, are really Russian tubes with "similar" characteristics.

Also, who knows how closely to the original specs the current manufacturers are keeping, and how much variation they allow? It's not as though there's an independent testing body that can certify them.