Leo and his 420V 6V6

[35L6]

RCA states design maximum for 6V6 at 350V plate and 315 screen . Leo ran them at around 400V with screens not far behind . Did he have some design trick to protect them or did he just get lucky ?

[Helmholtz]

RCA states design maximum for 6V6 at 350V plate and 315 screen . Leo ran them at around 400V with screens not far behind . Did he have some design trick to protect them or did he just get lucky ?

I guess he just tried and got away with the 6V6s he used.
Maybe he reassured with his suppliers.
Also at full output voltages might be close to datasheet limits.

[pdf64]

Yes, as anode dissipation is reduced from max, the voltage that the anode can tolerate increases, eg see p6 https://tubedata.altanatubes.com.br/...010/e/EL84.pdf
so 550V anode voltage limit at 0 anode current.
And that’s a design centre limit, design max would be even higher, maybe 600V.

[35L6]

It's posible that RCA was being consevative and they could just take it . I do wonder how much B+ dropped and if it was in Leo's calculation . I know some of you out there have some of these amps and if you could tell me how well that 400V holds up and if they actually put out 20W clean ?

[Helmholtz]

Yes, as anode dissipation is reduced from max, the voltage that the anode can tolerate increases, eg see p6 https://tubedata.altanatubes.com.br/...010/e/EL84.pdf
so 550V anode voltage limit at 0 anode current.
And that’s a design centre limit, design max would be even higher, maybe 600V.

I understand the Vao and Vg2o limits only apply in cutoff ( the "o" meaning zero current) .
It is essential that a tube can stand (2*B+) - Vsat because that's the plate voltage it sees during cutoff with a transformer load.

[Helmholtz]

It's posible that RCA was being consevative and they could just take it . I do wonder how much B+ dropped and if it was in Leo's calculation . I know some of you out there have some of these amps and if you could tell me how well that 400V holds up and if they actually put out 20W clean ?

I can confirm that my 80s Super Champ puts out almost 22W before clipping with a B+ of 413V at idle.
I'm using NOS Groove Tubes 6V6GTAs (probably Sylvania made) which seem to hold up well.
Nevertheless I decided to lower B+ to 380V by setting the mains voltage selector to 240V instead of 220V (nominal mains voltage here is 230V).
This lowers output to around 18W. Sound difference is insignificant.
I have no experience with current production 6V6s.

[g1]

Don't forget that Leo's next project was MusicMan where they ran the 6L6 plates in the later amps at 700V.

[Tom Phillips]

It's posible that RCA was being consevative and they could just take it . I do wonder how much B+ dropped and if it was in Leo's calculation . I know some of you out there have some of these amps and if you could tell me how well that 400V holds up and if they actually put out 20W clean ?

Below are recent actual measurements that I took after a full checkout / restoration of a 1963 Deluxe Reverb Amp. The line voltage was maintained at 120V during the measurements. The values recorded from top to bottom are:
1) The transformer HV secondary winding AC voltage
2) The B+ at the OT center Tap
3) The AC line current draw
4) The RMS power output driving an 8 Ohm test load


Cheers,
Tom

[pdf64]

I understand the Vao and Vg2o limits only apply in cutoff ( the "o" meaning zero current) .
It is essential that a tube can stand (2*B+) - Vsat because that's the plate voltage it sees during cutoff with a transformer load.

I’ve got no reference to back this up, but I envision a sliding scale between anode voltage and anode dissipation.
The sliding scale wouldn’t necessarily be straight line linear.
That would allow the typical saggy HT class AB amp idling over the anode voltage limit, provided the full load HT voltage sagged into the acceptable range.

[Helmholtz]

I’ve got no reference to back this up, but I envision a sliding scale between anode voltage and anode dissipation.
The sliding scale wouldn’t necessarily be straight line linear.
That would allow the typical saggy HT class AB amp idling over the anode voltage limit, provided the full load HT voltage sagged into the acceptable range.

I dunno.
Could be, but that's pure speculation.
When I see an absolute voltage limit, I don't expect any interdependence with other conditions.
Without knowing the possible failure modes it's hard to develop an opinion here.

[35L6]

19.7 is close enough to 20 for me ! The evidence is in favor of the tubes were tough , he tried it , it worked , so he did it again . If we are going to speculate , maybe he raised voltage to offset sag or was just chasing 20W . We'll never know .

[Tom Phillips]

19.7 is close enough to 20 for me...

Yep. Also keep in mind that I only posted one sample. I've measured Deluxe Reverb Amps that spanned the range from 16W to 22W. After their amp was repaired / refurbished, the owners were all satisfied because of the overall tone and no one commented on the loudness of their amp compared to other Deluxe Reverbs.

[pdf64]

I dunno.
Could be, but that's pure speculation.
When I see an absolute voltage limit, I don't expect any interdependence with other conditions.
Without knowing the possible failure modes it's hard to develop an opinion here.

I think that as Philips publish 300V push pull conditions, it’s inherent that with signal, as one anode’s voltage falls as it draws more current, the other’s must rise as it draws less current.
Yes at some point it’ll result in cut off, but it’ll be well above 300V by then.
Of course these are instantaneous levels rather than the average continuous levels that the valve info is based on (I’m assuming again ).

[g1]

No one wants to touch the MusicMan thing?
That's 200V above the supposed maximum limit. Which several manufacturers continuously ignored so I'm not really sure what the 'maximum' means anyway ("should not be exceeded under the worst probably conditions")..
Fender, Traynor, Ampeg and various others all did it on a regular basis. Their schematics were published showing them exceeding the specs yet the tube manufacturers still warrantied the tubes. Radio Shack gave lifetime warranty on tubes knowing the kind of circuits they could go into.
Field failures due to design problems would have broke these amp companies if it were an issue (warranty claims).

One thing evident is that for all these examples they were biased quite conservatively. I'll agree with pdf64 about the sliding scale involving anode voltage and dissipation.
The mythical '70%' target bias or even 60 at times is where you seem to start getting into trouble with high plate voltages.
We can talk about datasheets or we can talk about actual real world examples. They did it and it worked fine, how did they do it?

[35L6]

OK I'll bite . The 700V thing did involve some design tricks . That cathode drive circuit is pretty slick . Have you been in a room with one ? Those things are LOUD !