Does the Phase Driver affect amp bias (voltage)?

So I got myself an F-50, and thought it was just a bit noisy so I might as well replace the STR-440s with a pair SED Winged-C power tubes. However, according to my tech, even the coldest bias in this amp would measure well over 50ma, and cause the SED =C=s to wear out prematurely. That's something like twice Boogie's spec for bias. (Yes, I had the bias adjusted in spite of the prevailing wisdom at Mesa/Boogie.)

I'm wondering if swapping the phase driver with a 12AT7 (or even a 12AY7) would further reduce the amount of voltage hitting the power tubes, thus effectively lowering the bias and making it compatible with the Winged C.

Does this make any kind of sense to anyone, or am I just ig'nunt? Or is the whole point of using a tube like the Winged C to get the amp running hotter?

Your PI tube has nothing to do with your power tube bias.

I don't understand what your tech is talking about. If the amp is safely running Mesa STR440's it will run a set of =C= selected for a Mesa bias range.

What do you mean you had your bias adjusted? If an adjustable pot was installed you can run just about any average 6L6GC.

Are you sure this tech is not hosing you?

Dom

domct203 said:

Your PI tube has nothing to do with your power tube bias.

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This is absolutely correct (in case you need a second opinion).

Screw the tech, man, if you do have an adjustable bias pot, get in there and turn it down and you should be fine.

Although I do agree with domct203, if it can run 440s it can definitely run =C='s, those are some tough tubes.

Jackie/Don/domct -- many thanks for the replies.

No, the amp is not adjustable, and I totally trust my tech.

Before the F-50, I acquired a broken Roland Bolt-60 (a Boogie-inspired SS/tube 80s hybrid) which I brought to his shop. He took it apart right in front of me and got it working within a few minutes by replacing a missing resistor. Also, he told me that the bias range for BOLT-60 was EXTREMELY narrow, so he set it up for adjustable bias. He even sold me some used Groove Tubes laying around his shop so I wouldn't have to pay for new ones. This was all basically the tech accommodating me, not him leading me on.

The reason I mention all this background is because I initially used the BOLT-60 to test the Winged Cs before I put them in the F-50. The measurements (ma) I took with my tester were 50~200% higher than what was the retailer wrote on the box (abt 32 ma). I now understand that these Winged Cs would not damage the amp, but after both me and my tech measured 45~75ma, then the concern is that the tubes themselves would wear down faster than desirable, since these tubes are abt $80/pair.

Incidentally, I also used the bias-modded Roland Bolt-60 to test the difference in bias between a 12AT7 and 12AY7 phase-inverter (NONE.) According to my tech, this would only stop the F-50's clean green channel from crunching. So I tried a 12AT7 in the F-50, and found that with the gain dimed, I had to push the input with my compressor to get it to breakup even a little. Right now the question I'm facing is really, do I optimize the F-50 for early breakup and crunch or high headroom and maximum clean volume?

Ideally, I'd like it to do both depending on the tubes I put in, so maybe adjustable bias is the way to go. Or should I just have him restore the amp to Boogie specs?

"even the coldest bias in this amp would measure well over 50ma"


is that for a pair of tubes? then each would be about 25ma or so?

If you have him put it back to boogie spec, just get =C='s from a good source and make sure they get a set that will match your specific amp. I think members like Doug's tubes, among others...

scott

It may come as a surprise, but the outside pair of STR440 tubes in my ED run at 47ma. As far as I know none of the ED guys are complaining about short tube life.

I now have =C= tubes in. I asked for Mesa Yellow grade and the tubes I got are even hotter at 53ma. The SED winged C is a very high quality tube and should have a good and long life even these high bias levels. The best part is, they sound great 8)

zodiac272 said:

"even the coldest bias in this amp would measure well over 50ma"


is that for a pair of tubes? then each would be about 25ma or so?

If you have him put it back to boogie spec, just get =C='s from a good source and make sure they get a set that will match your specific amp. I think members like Doug's tubes, among others...

scott

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Thanks, Scott. Each tube measured well over 50ma at Boogie's factory spec bias, and my tech said couldn't get it below 38~40 ma ( I don't remember exactly). According to him, Boogie bias is abt 24~28ma per tube. Is that where the 50ma minimum comes from?

Actually, I did order from Doug's Tubes. He answered all my questions, and even offered to take the tubes back if they were really incompatible.

J.J, thanks for the measurements! I still really want to hear the Winged Cs, but I don't want to fall in love with the sound, only to have to replace them in six months like I do girlfriends. :wink:

I've been running the =C= tubes for about just under 3 months so far and the original STR440's were in the amp for about 4 months so I guess the test of time is ongoing for my tubes. The SED tubes are very heavy duty though. If you look at the STR440 and =C= side by side you can easily see the difference in build quality. The STR440 look somewhat thrown together in comparision.

In terms of sound, they are a good step up from the STR440. I still like the STR440, nothing wrong with how they sound, but the =C= has better clarity, better top end (great harmonics) and is less gritty. They seem to retain the sound of my guitar better. I'd go as far as to say they colour the tone much less than the other tubes. Overdrive is great and kind of smooth, again retaining my guitars tone. This is with my Electra dyne, but the general traits should be quite similar in other amps.

Electra-Dyne looks like a cool rig, but it's twice as much power than I'll ever need (at twice the price!).

Actually, I also got a pair of Svetlana's from Doug's Tubes ($40) which are in the F-50 now. These seem to me like the economy-class Winged Cs. They sound much cleaner and balanced than the STR-440s, and a bit sharper on crunch and lead sounds. At least with the clean channel, I seem to get a wider response from the tone controls, and can get away with a less treble on the dial. Overall, I'm very pleased with them when I'm playing jazz with my GB-10, but at a blues session last night I missed warm fuzzy crunch of the Chinese tubes.

Still not ready to give up on the Winged Cs that everyone here is raving about, I checked the data sheet. http://site.tubedepot.com/pdf/sv-6l6gc.pdf
However, it only gives zero-signal plate current (a.k.a. bias?) and maximum-signal plate voltage (beyond which the tubes distort or explode?) values for triode and tetrode mode, but the F-50 operates in pentode mode! WTF?

The ED has a 45 watt mode (using only the outside pair of tubes) which is great for blues, so it can get dirty at sensible volume. Really the ED solved my problems of wanting Fender and Marshall tones in one amp nicely. I really stuggled with the cost :shock: After I made the mistake of trying one it took about a year before I could get one.

Anyway, the data sheet does give an example of a pentode mode running in AB1 mode. The AB1 really just means it is running in extended class A which is what the ED does. If you drop the bias back it will drop into class B operation sooner in terms of volume level. No signal current is the idle bias current. Each tube would be at 58ma by my calcs.

I did miss that crunch from the Chinese tubes too, but I still like the =c= overdrive better. Have not heard the Svetlanas my self, but I bet they do sound great.

Thanks, I see that now! It seems both my tech and I missed this.

So can assume that:

1) This amp is class AB because the tubes are running in pentode mode?

2) As long as the sum of the plate dissipation measurements from each tube is the less than the maximum plate dissipation specified on the datasheet, there's no danger of damaging the tubes? (And/or should the amp's bias be adjusted to get 58ma from each tube?)

3) If there is no measurable risk of overheating the plates and blowing the tubes, than the amp is not at risk of being damaged either?

Let me know if this is correct, because I can't wait to plug in the Cs and crank'em up!

1. No not really, although that is normal. Getting technical, a 6L6 is a tetrode and not a pentode anyway :wink: There are a number of operation parameters the decide what class a given circuit would run in.

2. The 58ma is not a figure to run by. The data sheet just gives an example of how you could use that tube and the results you would expect. That example is leaning more to a hifi design...

58ma is pretty extreme and your F50 was probably not designed to operate there. Thats not so say it won't, but you have to look at the amplifiers power supply, output transformer and where the extra heat generated goes to determine if it is safe. Also, the tone may not be what you want. People generally bias a guitar amp to their tone tastes.

3. much as above.

I'd drop the tubes in and see where the bias sits. If its less than 40ma enjoy, if its more get your tech to adjust it back. Tube life will be reduced by running at the limit, but i'd still expect 1000 hours or more which is an hour a day of play for 3 years. The data sheet doesn't make any particular claim on life, but those tubes are made to very high standards and thats why they cost more than most of the other brands.