Dpn't trust fender's Back Panel Bias Point Values

[darrellcheng]

A word about Fender's Bias values. I own a fender pro tube twin amp that shipped with sovtek 5881's. These tubes have redplated when set to a 40ma bias which is the recommended bias value on the amp. Do use a bias calculator when dealing with some of these newer amps! 40ma is fine for 6l6gc's, but not 5881, which are 23 watt tubes. for an el34, (25 watt tube) we have 33.33 at 60% and 38.89 at 70%. The junky sovtek 5881's definitely want to see something less than 40ma per tube, 80ma per pair. I know some tubes can handle 70-80%, but these were the stock tubes

[Merlinb]

Assuming the B+ is a typical Fender value of 460V or something, that's less than 19W dissipation; doesn't sound too scary.
Do the stock 5881s actually show any signs of discomfort?

[Steve Conner]

Those Sovtek 5881s have a fairly good reputation. They're a rebadged Russian military grade tube, originally known as the 6Pi3S-E, or a bunch of other codes, depending on who did the conversion from Cyrillic to Roman letters.

Other forum members have tested them in the past, and they took quite a lot of abuse before redplating. I've personally run them at 40mA per tube and 475V B+.

[MWJB]

Ditto what Steve says. The Sovtek 5881 probably only got that designation because it was reasonably close to a 6L6 but clean & military sourced.

40mA is probably on the hot side even for most current production 6L6GC, most of which are 25W tubes (17.5W idle dissipation in fixed bias at 70%).

FYI the manual states recommended bias is 60mA at each test point (30mA per tube). Fine for any 5881 or 6L6. You shouldn't need to run most 6L6 Fenders at as much as 70% dissipation to get them sounding right.

[darrellcheng]

Originally Posted by Merlinb

Assuming the B+ is a typical Fender value of 460V or something, that's less than 19W dissipation; doesn't sound too scary.
Do the stock 5881s actually show any signs of discomfort?

Yes at 38ma I have redplating on all four tubes. Bringing it to 60%, all was good. The manual for my amp states 80 per pair. I see that many of you have good experience with the 5881's. I have also read of bad experiences like mine.

[Jag]

Did you mesaure plate voltage. Bias current means nothing on it's own.

[darrellcheng]

yes i had 450 at the plates

[MWJB]

Jag wrote: "Did you mesaure plate voltage. Bias current means nothing on it's own." - Well, yes and no. If we had no idea whatsoever of plate voltage (forgetting for a moment that Fender have put recommended idle current in the manual) then, yes, current alone would be meaningless. However, as we know we're dealing with a current production, fixed bias, 6L6 Fender it's safe to assume that plate voltage is substantially over 400v and very unlikely exceeding 525v (pretty well the highest voltage you'll see in a normally biased Fender). If we biased to 30mA then that's >12W per tube at >400v And 15.75W at 525v.

The amp would still sound OK in both examples, it would be a matter of personal taste whether you wanted to bump up current by a few mA in the lower voltage example. So I would say go in at 30, if a few more mA transforms the amp...well, that's how you're going to run it anyway, irrespective of the numbers.

[Jag]

Yes, I agree. It was a "Quick Reply" but, you're right, setting bias current to a "safe" level will be okay.

However, he mentioned setting the bias to 40ma and I would not suggest doing that without knowing the plate voltage. That's 10ma per tube more than Fender spec and the thread header referred to Fender Specs. This is what I meant.

However, at 450v, 40ma shouldn't be killing the tubes. I just thought, considering the issue, it would be prudent to be sure what plate voltage we were dealing with.

[darrellcheng]

Originally Posted by Jag

However, he mentioned setting the bias to 40ma and I would not suggest doing that without knowing the plate voltage. That's 10ma per tube more than Fender spec and the thread header referred to Fender Specs. This is what I meant.
At 450v, 40ma shouldn't be killing the tubes.

I think you misread one of my posts. The specs directly on the bias points on the back of the amp states 80ma for each pair. At 450v and 40ma, it is killing the tubes, maybe they were a weak set.
I was suggesting that one should disregard these values and use the proper method using plate voltage and a bias calculator, which gave me a lower current draw and the tubes were fine and everything was great and spectacular.

I was on a break in the middle of rehearsal and I have not checked the bias for awhile so I decided to pull out the MM and trust the fender specs, since I wasn't going to pull the chassis right there. 5 mins into practice, all 4 redplate. Then I troubleshooted.

I don't know what specs you are referring to, im referring to the ones that are silk screened on the black plastic faceplate sticker on the back of the amplifier right under the bias points and pots.

[Jag]

We're all on the same page so, no sweat.

Since I don't know what amp model we've been talking about, I went with MWJB's post. IIRC, 30ma was spec for the last Fender with test points that I worked on.

Now that I think about it, though, I think the Red Knob Twin spec says 40ma. Apologies for causing confusion.