6V6 (other than JJ) that can take high plate dissipations

[HaroldBrooks]

I recently got an amp with two coin based G&E 6V6s and I love the sound of this amp. The amp with its stock 200 ohm cathode resistor (measured 219ohms) had a very high plate dissipation ( 15.5 watts on one tube, 13 watts on the other). Much to my amazement, there was no red plating at all, even after running it for close to two hours.

My Questions :

1.) I was following what I though was the correct method (Output Transformer Resistance Method) of measuring bias current through the output transformer, that is to measure the resistance of both sides to the center tap (~227 ohms in both), and then the voltage drop from both sides to CT, and divide the voltage by ohms to get the amps. Then measure the plate voltage and multiply by amps to get watts. I noticed the voltage measurement at the plate was roughly the same as at the OT lead. Is this a correct method, or am I doing something critically wrong ?

2.) I swapped tube sockets and the big 15.5 watts followed the tube. I also noticed that particular tube was slightly micro-phonic, but not red plating in any way, even when I stepped up the voltage a bit with my Variac.

3.) Other known good NOS tubes did red-plate a little when installed in this amp.

4.) I purchased another coin based G&E 6V6, and this one also draws big watts just like the other tube, but it is not micro-phonic in anyway, and again does not even have a hint of red-plating.

5.) I like the way this amp sounds with the high plate dissipations, but I did bump up the cathode bypass resistor to 250ohms, just to bring everything down a little bit ( so I can sleep ).

6.) It seems like these coin-based 6v6 tubes can take huge watts without red-plating, is this generally the case ? Are there other older style NOS tubes like this ? (I know about the JJs already).

I want to keep the amp exactly as it is, but instead of searching only for the somewhat rare and purportedly temperamental coin-based 6V6, I would like another vintage tube option if it's available.

Thank for your input and help !

[Helmholtz]

The tubes only "see" the voltage between plate and cathode, so you need to subtract the cathode voltage from the plate voltage to calculate dissipation.
Also the (rather high) DCR of the OT should make plate voltages some 7V lower than plate supply (OT center tap).
What are your measured/calculated plate currents? What is plate supply voltage? Schematic?

[HaroldBrooks]

The tubes only "see" the voltage between plate and cathode, so you need to subtract the cathode voltage from the plate voltage to calculate dissipation.

Thanks Helmholtz. I realize that was a mistake.

But shouldn't the net calculated dissipation should be fairly close anyway since the cathode voltage is very low (plate voltage around 350vdc) ?

[Helmholtz]

Please see my edit above.

[HaroldBrooks]

Here's the schematic : http://schems.com/bmampscom/valco/Na..._Tremotone.jpg

I measured the hot tube voltage drop as 9.98vdc and the plate voltage right around 350vdc (fluctuating a bit) and the OT at 227 ohms (warm but amp off for a minute or so .

9.98/227 = .044 amps x 350vdc = 15.4 watts. If I subtracted the voltage drop (350 - 9.98 ~ 340vdc) it would work out to 14.96 watts I believe, and still at the point that would red-plate most 6v6 tubes.

[Dave H]

Is 350V the plate voltage or B+ voltage?
I think you've subtracted the OT voltage drop from the plate voltage but it's the cathode voltage which should be subtracted as you need to know the cathode to plate voltage.
The cathode voltage will be about 20V. If 350V is the plate voltage the plate dissipation will be 0.044 x (350-20) = 14.5W

[HaroldBrooks]

Ok. You are correct, I need to measure the cathode voltage drop !

I think that's what Helmholz was saying in the first place, and I didn't get it and thought it was the same as the OT drop, it's not.

Yes indeed, 350vdc is the pin 3 plate voltage, but if I took 20v off from 350 we would have (330/350 the ratio of difference) x 15.4 = 14.5 watts of dissipation for that hot tube. Closer to the design maximum I see, but still rather high.

Still, I would like to know how a 6V6 tube can handle above 14 watts, and still not red plate, and if there are other older tubes that have been recognized to take the same abuse. Maybe it's something else inherent in that coin-base tube that I am not understanding. I now have three of those tubes and none red-plate, so I think it's a trend.

[Leo_Gnardo]

We had a similar question at The Amp Garage a couple years ago, concerning an amp that really stresses the bejeezus out of 4 x 6V6, the Jim Kelley amp. And its Kitty Hawk clone. Jim Kelley himself popped in and suggested NOS RCA and Sylvania as tubes that would put up with the voltage and not break down. I know, you're not putting 500V on yours but I thought I'd better mention it anyway. From time to time I find old RCA, Sylvania or GE 6V6's, likely OEM installs, still plugging away in old amps.

[g1]

The misunderstanding is in the assumption that a tube must red-plate as soon as you exceed the max. plate dissipation spec.
This may or may not be the case, not any kind of rule.

[HaroldBrooks]

The misunderstanding is in the assumption that a tube must red-plate as soon as you exceed the max. plate dissipation spec.
This may or may not be the case, not any kind of rule.

Yes, I think that's my problem, assuming automatic red plating past 12w or so.

But still I've seen this now in the coin-based tubes, they all handle high watts. All my other RCA, Sylvania, etc... tubes red-plate at around 12w, and many were bought as NOS and look brand new.

I do have one other set of G&E 6V6s that I intentionally put to the test, and they too can handle big plate dissipation, but I don't think they are quite as tough as these late Coin-based versions and they are the only pair out of a couple dozen I now own.

Also, I've seen 6973 tubes go fine at close to 17 watts ! even though the spec sheet says 12 watts max plate dissipation. But that's an entirely different tube, so I shouldn't muddy the waters...

[mozz]

Filament voltage? I ask because........................if that voltage is high, other voltages will be high, and......................................people have a tendency to always bump up the cathode resistor to get things back to diss limits.

But, and this i think i am learning from here, keep the cathode resistor low to keep your sound you like, and find ways to reduce the HV to bring the diss back to limits.

But also, i could be way wrong and the engineers way back then knew the tubes would run at 14-15w all day, so they designed with that in mind. The days of over building stuff, tubes for instance, made to last above and beyond what we told them, quality and the customer got more than he paid for. Happy customer, return customer.

[Tony Bones]

All real 6V6GT tubes will handle 14W. Early datasheets list the Design Center rating as 12W, but later datasheets show the Design Maximum as 14W for the same tube. And that's just the plates; the screens are good for another 2.2W. If you measure cathode current, then you're measuring both plate and screen current together.

I'd feel OK putting just about any 6V6GT in that amp, except maybe the rally early coke bottle shape versions.

[J M Fahey]

Maybe a Russian tube can be an option.
In general they made tubes at military factories and meeting military specs, so they tend to be overbuilt.

Many models are NOT exact clones of American types, and resellers try to get the most buck for the bang; AFAIK one Russian type , sold as a "weak 6L6" (which would be counterintuitive) is actually a very overbuilt 6V6 equivalent.

Search for it and compare specs, you might be surprised.

[Justin Thomas]

Hey Juan,

Would that "weak 6L6" also have a .45A heater? What's the popular name of that tube? I might be interested in some...

Justin

[J M Fahey]

Donīt remember the exact details.
Only remember reading (maybe in this very forum) that XXX brand 6L6(yyy letters behind it or even no letters at all) was actually ZZZ (insert Ciryllic code here) which was for all purposes a VERY GOOD 6V6 type, so good that it reached the lower limits of 6L6 some true specs and could get away with the impersonation.

I commented that here so maybe somebody would chime in and say "hey!! you are talking about xxx brand 6L6!!!!"