JJ 6V6S @ 290V / 49mA cathode biased .. 100% plate dissipation. Okay?

[thehoj]

I've got a little Push-pull cathode biased 6V6 amp I've put together, don't really have a schematic for it, it sounds perfect for my needs, but I'm running the JJ 6V6's at 290V (not too high I know) but @ 49mA.
I know the max plate dissipation for these tubes is 14w, and so 14/290 = 48mA. Is it feasible to think that these tubes will be okay running at 100% plate dissipation for hours at a time?
I haven't noticed any red plating or anything, it just sounds so good I really don't want to touch anything with it!

Also, am I correct in stating that this amp is running at full Class A?

[jazbo8]

You are pushing the limit of the tubes, but if they do not red plate, then perhaps the JJ's can take it (the tube life is probably reduced though). It is in A1 mode alright.

Jaz

[loudthud]

The determination of Class A or not Class A hindges on the impedance of the output transformer and the load line it creates. An easily observable test is to look at the cathode current waveform just below clipping with an oscilloscope. But that question leans towards academic nit picking. A tube rated at 14W does not suddenly red plate at 15W. If you like the sound, go with it. If the tubes only last a couple of weeks, you might want to dial it back some.

[thehoj]

Thanks for the info guys.

I'll see how the amp does. If the tubes only last a very short while, I'll adjust.

I'm using a 10W Hammond 125D OT, Using lugs 2&6 for 8000ohm @ 15ohm voice coil load. I'm measuring 290V exactly at the Plates, and dropping a bit with 470ohm resistors before the screens - not sure the exact voltage at the screens, but slightly less I guess. My only other options with this OT are 5600ohm or 10,800ohm @ 15ohm voice coil load, so it seemed like the best option. It sounds good to me, in any case.

[Leo_Gnardo]

I'm running the JJ 6V6's at 290V (not too high I know) but @ 49mA.
I know the max plate dissipation for these tubes is 14w, and so 14/290 = 48mA. Is it feasible to think that these tubes will be okay running at 100% plate dissipation for hours at a time?
I haven't noticed any red plating or anything, it just sounds so good I really don't want to touch anything with it!

Those JJ 6V6 are pretty tough. I think you'll have no problem. You could swap in a cathode R maybe 20% higher resistance to dial down the Ip if it worries you enough. And if it seems to lose something in the tone, just go back to the original value.

One indirect way of determining class A is measure the B+ with no signal applied, then again at clip. If the B+ doesn't sag with output at the clipping point, you can pretty well guess it's class A. (I've seen some old Gibson amps where the B+ actually rises at clip, but just a little, maybe 2%.)

[thehoj]

Those JJ 6V6 are pretty tough. I think you'll have no problem. You could swap in a cathode R maybe 20% higher resistance to dial down the Ip if it worries you enough. And if it seems to lose something in the tone, just go back to the original value.

One indirect way of determining class A is measure the B+ with no signal applied, then again at clip. If the B+ doesn't sag with output at the clipping point, you can pretty well guess it's class A. (I've seen some old Gibson amps where the B+ actually rises at clip, but just a little, maybe 2%.)

Cool little trick.. I will definitely be trying that out. Just need the opportunity.. lol. Even at 10-ish watts this thing is freaking loud.. Gotta wait for the wife to leave the house.. haha.

[jazbo8]

Darn it! Made a mistake in my earlier post, when I said the amp operated in A1 mode, I was looking at the wrong graph JJ's datasheet only shows the triode curves which is odd since the 6V6S is a beam pentode... So the amp actually operates in AB mode. Also, when you said bias current of 49mA, that was for a single tube, right? Anyway, 10k load is what the datasheet recommends, but 8k is not too far off, so you should be fine either way.

[thehoj]

Darn it! Made a mistake in my earlier post, when I said the amp operated in A1 mode, I was looking at the wrong graph JJ's datasheet only shows the triode curves which is odd since the 6V6S is a beam pentode... So the amp actually operates in AB mode. Also, when you said bias current of 49mA, that was for a single tube, right? Anyway, 10k load is what the datasheet recommends, but 8k is not too far off, so you should be fine either way.

I have 1ohm (0.5% tolerance) resistors from cathode to ground on each 6V6. Specifically I'm measuring 49mV over either resistor.. So probably not exactly 49mA on each tube, but pretty close.

I wish I knew how to read those graphs they provide on the datasheets.. I can never seem to figure them out though. What datasheet are you looking at for info?

[eschertron]

I've got a little Push-pull cathode biased 6V6 amp I've put together, don't really have a schematic for it, it sounds perfect for my needs, but I'm running the JJ 6V6's at 290V (not too high I know) but @ 49mA.
I know the max plate dissipation for these tubes is 14w, and so 14/290 = 48mA. Is it feasible to think that these tubes will be okay running at 100% plate dissipation for hours at a time?
I haven't noticed any red plating or anything, it just sounds so good I really don't want to touch anything with it!

Also, am I correct in stating that this amp is running at full Class A?

FWIW I have a champion 600 RI, single 6V6 in SEA mode. Fender's schematics have voltages listed, and I have confirmed by my own measurements, that show the 6V6 is idling at about 16W. As long as you have resistors to keep grid 1 and 2 from melting, you should be OK... I think.

[frus]

I have 1ohm (0.5% tolerance) resistors from cathode to ground on each 6V6.

how so? you've said it's cathode biased?

[jazbo8]

JJ's 6V6S datasheet.

Could you please post a schematic? As frus said, the 1ohm resistors are only for current sensing, they are not for cathode-bias. I think your bias is around -15V or so, but it would be good to confirm with the actual reading.

Jaz

[eschertron]

JJ's 6V6S datasheet.

Could you please post a schematic? As frus said, the 1ohm resistors are only for current sensing, they are not for cathode-bias. I think your bias is around -15V or so, but it would be good to confirm with the actual reading.

Jaz

Or measure from grid (pin 5) to ground (or cathode, pin 8) for the grid bias voltage, as you've said you don't have a decent schematic. You are correct in calculating the cathode current using the cathode resistor voltage drop, but as jazbo8 and frus posted, the power section won't turn out to be class A.

[thehoj]

Alrighty, here's the schematic that is closest.

View image: 6 V6 Squeaker

I guess my 1ohm resistor is between cathode and cathode resistor.. Maybe I'm getting a totally inaccurate reading then.
Also my cathode resistor is a 100ohm connected to a 1K Rheostat so that I'm able to adjust the bias.. Ideally the rheostat would have been a lower value, but I had the 1K, so I figured why not use it.

I had previously set the amp up with 6K6 power tubes and had the rheostat turned up a bit so that I was measuring 27mV over the 1ohm resistors. I was also using some zener diodes to reduce the B+ for the amp. When I put the 6V6's in I took the zener's out figuring that I could deal with the increased B+, and when I first turned the amp on it was measuring like 60mV over the 1ohm resistors, I turned the rheostat as far as I could down and that brought the voltage over the 1ohm resistors down to 49mV.. And now that I'm giving this some more thought that doesn't really make a lot of sense. I turned the rheostat all the way down to reduce the bias current, but if the rheostat was basically out of the circuit that would mean I have just the 100ohm cathode resistor, and that should make the bias current higher.. If it's cranked all the way on, it's like 1.1K, which seems like it'd be way too high..

I'm not by the amp right now so I can't confirm exactly what's going on. I'll have to do some more measurements later.

[jazbo8]

For some reason, I can't see the schematic image file. Anyway, now we know what you have for the bias circuit. You should be able to dial in the bias with the rheostat, the JJ datasheet suggests Ia=35mA per tube (screen voltage at 250V), so between that and where you are now is probably a good compromise.

[thehoj]

For some reason, I can't see the schematic image file. Anyway, now we know what you have for the bias circuit. You should be able to dial in the bias with the rheostat, the JJ datasheet suggests Ia=35mA per tube (screen voltage at 250V), so between that and where you are now is probably a good compromise.

There.. Does that show up for you?

I ended up putting one of those zeners in to bring my B+ down to 280V, now I'm able to get my bias current lower.. Currently running (measuring 38mV over the 1ohm resistors) and it still sounds really good, so I'll probably just run like this for now. Although I'm curious if I'm getting an accurate measurement of my current.. One thing I noticed is that the mV reading I was getting over the 1ohm resistor jumped anywhere between 38mV and 50mV while I was playing.. Is this normal for cathode biased amp?