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6V6 in triode connect mode

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Dear all,

Pls, see attachment, RCA manual 6V6GTA. They said that in triode connection mode, at 250V plate voltage, plate resistance is 1960ohms.

Have you ever tryed this one before? Or could you suggest schematic to operate this mode. How about SRPP 12AT7 for driver?

Thanks in advance

Cominup
 

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> RCA manual 6V6GTA. They said that in triode connection mode, at 250V plate voltage, plate resistance is 1960ohms.

Any pentode can be a triode. But in a pentode, the screen grid sucks big current even when plate voltage is low. So a pentode is designed for a high Mu (G1 to G2), 10 or 20, for good power sensitivity. In a triode, current falls when plate voltage falls. You want a low Mu, like 5, to reduce this effect. 6V6 Mu is 10. So it is OK but hardly great as a triode power amp.

For 1935, this was very good performance. It might still be pretty good for 6V 0.45A heater power. It won't knock your socks off.

An externally hosted image should be here but it was not working when we last tested it.


I've used the 10 Watt plate rating, which really applies to TV V-sweep duty. SET duty is tough too, but perhaps less likely to gross abuse than cheap TV work, so at your discretion you could run 12, 14, even 18W in your 6V6 (they are tougher than the book says).

Two possible operating conditions:

B+ = 350V
Iq = 28mA
Vpk = 210V
Ipk = 28mA
load = 7.5K
power = 2.8 Watts
grid = 0V/-30V/-100V
THD = gross
maybe 1.4W with listenable THD at 320V, 30mA, -25V bias

B+ = 240V
Iq = 40mA
90V pk
22mA pk
load = 4.1K
power = 0.9 Watts
grid 0V/-14V/-33V
THD ~ 10%

Ah: RCA's 1940s datasheet gives suggested values for triode operation, somewhat better than my estimates because they push the 14W rating for audio triode operation.

> How about SRPP 12AT7 for driver?

Huh? It only needs 15V-20V of drive, with 250V-300V of supply voltage available. No reason to muck it up with complicated drivers. For zero-feedback operation, a 6SN7/12AU7 is plenty of gain. If you want damping better than 2, use 6SL7/12AT7 and take some NFB around everything.
 
6V6 in Ultralinear

No reason why you can't use UL for 6V6 - Va max 315V and Vg2 max 285V - I'd say its damn near ideal.

There is a fellow here in OZ whose been designing and building his own tube amps for 30+ years. He claims his favourite amp is 8 off 6V6 UL connected per channel (4 pairs). On my estimates he probably gets about 40 Watts.

I'm a big UL fan - check out IM distortion figures for Triode, UL, Pentode of just about any output pentode data sheet and guess which is best - UL everytime. This is telling me that UL is the most linear mode to run a Pentode. Also easier to drive than Triode Mode (low input capacitance).

Cheers,
Ian
 
I’m thinking of getting a 6v6 version of a Marshall Plexi built withd a triode/pentode switch. Would the tone of a 6v6 in triode be crappy or just really quiet? If I could get an amp built that has a substantial volume drop for practice without the tone being awful or completely lacking dynamics that would be awesome.
 
Triode mode switch attenuates volume very little so it misses its main job and to boot amp loses balls, bite.

Quite disappointing.

Tiny Terror does it the proper way: ówer tube is always in pentode mode and to boot: no NFB spopitput is wild and good, and to drop from 15W to 7W they reduce +V by switching different PT taps.
 
JMFahey, what do you mean, when you say: "to boot amp loses balls, bite" and: "no NFB spopitput is wild and good"

I am confused, please explain

The EL34 dates back to at least 1949 and the EL84 dates back to 1954, both are Philips (Mullard) types. I am pretty sure neither was "designed" for UL operation, otherwise old Philips databooks would surely have mentioned that. In Philips databooks are no mentions of UL-operation conditions, only pentode and triode operation.

Alan Blumlein invented "distributed loading" (patented in 1937, 1938 in the US), Hafler/Kereos investigated and propagated UL-operation which is a special case of Blumleins distributed loading.
 
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