New JJ 6v6S very odd issue

[pjd3]

So good news.

AP was great about taking the pair back and sending me a new pair matched and burned in.

They were solid as can be and biased up like a champ. And they sound pretty darn good as well. Yes, a bit different than the NOS Sylvanias. Hard to describe but, maybe a little more present and edgy, but in a good way.

What I noticed was that with plate around 435 vdc and cathode current @ 20-21mA, I needed -43 volts on the grids as opposed to around -39 for the NOS Sylvanias for the same bias/emission setting. The pair were within a milli-amp of each other but probably do to the OT primary DCR difference (41 and 46 ohms). My NOS's had drifted around 4 milli amps apart but still sounded good.

So, very happy with it all and very appreciative of the service I received from Tanner at AP and will be happy to get tubes from there again. They went above and beyond.

Its nice when a little tube story ends well

Thanks,

PJD3

[TUBEDUDE]

I always shift sockets with the output tubes to get the closest match. Love those Sylvania shiny black plate 6V6's.
(I don't use JJ power tubes ever).

[pjd3]

Hey TubeDude, thanks.

Before the gig this weekend I did open the amp up and switch the tubes to look at cathode current. It brought them about a milli-amp further apart than the one milli-amp difference that it was so, that was a good thing to do and I just put them back as they were.

I decided to look at screen current in order to maybe get more honed in on actual plate current/emissions.

What I found surprised me and I had to wonder if I was doing this right. My calculations came out to 0.5 milliamps. I was expecting to see at least a couple of milliamps eaten up by the screen. I believe I was reading .25 volts across the 510 ohm screen resistor giving about .0005 A as a result. I just biased the JJ's with 435vdc @ 20-21mA's which was close to 70% of 13W. (Since many call the 6V6 a 12 watt tube but many tote the JJ's as being more like a 14W, I just decided to go in the middle with 13 as the max emission. What the heck.

Was I suppose to be inputting some nominal level voltage sine wave when reading the screen resistor voltage? Inquiring (and somewhat baffled) minds would love to hear anything about that.

Thanks everyone,
Best,
PJD3

[martin manning]

Measure screen current (via screen resistor voltage drop) with no signal, same as you do when setting idle bias. That level seems low, typically it's ~5-10% of Ia per your expectation. JJ lists 6V6S Pa max at 14W on their data sheet.

[pjd3]

Yes Martin,

I thought that was impossibly low, at least according to my expectations based on everything I've heard discussed about 6V6 screen current. I even remember making sure the meter was on dcv and not acv.

So this perplexes me a bit. I'll likely go back into the amp and try this again, see if there was anything I missed first time around.
I'm nearly certain after biasing up the amp, I simply made dc voltage measurements across each screen resistor and noted that they were both very close - somewhere around .25 volts.

Yup, I'll have to do this again to see whats up in there.

Thank you Martin.

Best,
PJD3

[mhuss]

I've read that JJ 6V6 tubes are closer to 6L6 in internal construction. This would explain why they bias a bit different than NOS 6V6. This is also likely why they seem to handle higher plate voltages than true 6V6 tubes.

I like the TADs for current manufacturer.

[TUBEDUDE]

Also why they don't sound the same.

[martin manning]

I've read that JJ 6V6 tubes are closer to 6L6 in internal construction. This would explain why they bias a bit different than NOS 6V6. This is also likely why they seem to handle higher plate voltages than true 6V6 tubes.

The plate structure is larger than 6V6GT, but electrically they are pretty close. Here is a trace from some time ago on top of GE curves.

[pdf64]

… many call the 6V6 a 12 watt tube …

I think that confusion / misunderstanding about 6V6 anode dissipation limits arises from a combination of factors.
Firstly the 6L6 got developed and beefed up over the years of production such that its limit rose about 40% by the GC version, perhaps nearly 60% if the 7581A is included.
Secondly the rating system used by most manufacturers (not Philips / Philips valve types) changed from (more conservative) design centre, to design max. The latter was adopted in 1958, so valve info written after that had anode dissipation limits increase around 15%, as if by magic!

Here’s RC19 published 1959, p236 notes all 6V6 versions (metal case to GT) as a 12W anode limit http://tubebooks.org/tubedata/RC19.pdf
That’ll be RCA’s last use of the design centre system for their valve types.

Whereas RC20 published 1960, p258 notes all 6V6 versions metal case to GT as having a 14W anode limit, using design max values http://tubebooks.org/tubedata/RC20.pdf

Here’s RCA AN174, which introduces and justifies the design max system, and explains how to use it https://www.one-electron.com/Archives/R ... 0Tubes.pdf

Here’s a handy overview of the differences between the different valve rating systems. From that it may be read that any real 6V6 should be capable of accommodating at least 14W anode dissipation.

[pjd3]

Thanks for all the added input and info.

At this point in my "tube amp build career, only 2 amps so far", I'm not feeling quite confident enough to trust my instincts when biasing, fully. So, for now, I try to go with what I'd call, "my closest rendition of what may be a design center/average based on the specific tube type". So, until I develop a bit more I shoot for the median of what I've seen as a bit too much, and not quite enough. Its served me well so far, my tone is good and satisfying to myself and to others it seems, and my tubes have lasted for a reasonable amount of time, years even. So playing it safe has payed off so far. But, it will be nice to get a little more experience under my belt and know what I want and what is electrically (and financially) reasonable. That's why I picked 70% of 13 watts with the JJ's. That just seemed like a fair compromise between data sheet specs and what a large number of people seem to be doing to arrive at good tone with some reasonable longevity. .

Thanks everyone, (and who knows, maybe I'll grow a new pair soon and do up "70% of 14 watts". I've just been afraid that should I do so, civilization might cease to exist as we've know it.

Best,
Phil

[martin manning]

6V6S data sheet from JJ specifies a "Limiting Value" of 14W Pa in pentode mode. They give no clue as to what "system" they are using for that rating, maybe one of their own invention. I wonder if any of the current production tubes are rigorously observing the old standards?

[pdf64]

… who knows, maybe I'll grow a new pair soon and do up "70% of 14 watts"…

It’s probably ok to do that provided you’re in control of (or at least monitoring) the key variable here, mains voltage.

If you’re playing out, and not using a mains voltage monitor or controller, my reading of AN174 is that when setting bias using a design max limit, the mains feed to the amp should be variaced up to its upper allowable limit (129V in the US I think?).

[pjd3]

Thanks for that input.

I did occasionally look at mains voltage, but probably not enough. The B+ on this amp (Sluckeys 20 watt Plexi) does or can vary from 430 to 436vdc at "random" times. I just assumed that was fluctuation of AC primary voltage.

A note, at one point my B + was down to around 430 and I wondered why when the AC input was a usual 122-123 or so. Then I noticed that I had the space heater on low (6 amps) and plugged into the same strip as the amp. The moment I shut the heater off the B+ increased right up to 136-138 at one point. That's when I learned the impact of local loads effecting the amps behavior. That slightly concerned me, and raised the question of "where should I actually bias this amp from"? I decided that I"m probably alright as I tend to shoot for moderate biases and not too hot. But, it might be a good idea to meter up the AC input and just watch the amp throughout the coarse of a day, and see where voltages go based on AC input. I expect a reasonably predictable relationship between AC input and B+ bases on the PT HV winding ratio would be easy to determine (as long as there isn't some other not so apparent element involved).

Thanks again for your insights, always trying to move them into my understanding of all this.

Best,
PJD3