Fender SRRI Can't Bias Hot Enough

[stratomaster]

Short Version: Even with the bias pot cranked, I can't get the tubes to bias hotter than around 25mA.

Long Version: Before I knew enough to bias tubes on my own, I took my SRRI into a shop for a repair. It was busting fuses everytime I turned it on. The shop, as I have come to learn, is questionable at best. They fixed the issue and installed a new set of tubes in it while they were at it (at my request).

Since then, I had a tube blow (about 2 years later, so I hope it was just a bad tube and not related something else wrong with the amp) and I bought a bias tester w/ a 1ohm resistor in it to read mA as mV. I bought it on the cheap, so I don't really know what precision the resistor is, but reading resistance from lead to lead on the tester gives me around 1.4 ohms.

That having been said, I can get the tubes to bias to a reading of around 34mV on my multimeter, but no hotter. Plugging into Ohm's Law, that gives me less than 25mA of current through the tube.

Could their repair have robbed me of more power tube current? If so, how do I correct it? Thanks

[Mark Black]

Out of curiosity what does your meter read (resistance) when the test leads are shorted together? I'm just wondering if the ".4" part of the 1.4 ohm reading is just resistance of the test leads and maybe you actually do have close to 1 ohm in the bias probe. If the 34mV (equaling 34mA) reading turned out to be correct you are probably pretty close or a little hot, depending on the plate voltage.

Aside from that the repair shop may have changed the bias range to accommodate the power tubes they stocked or to reduce the chance of biasing too hot by a DIY'er. I'm not overly familiar with the re-issue amps, but in the older ones that would be accomplished by changing the resistor which was tacked onto the case of the bias pot. That resistor might be elsewhere on the re-issue amps. If this is the correct schematic: http://www.fender.com/support/amp_sc..._Schematic.pdf
Then you would be looking for R59, which apparently should be a 27K. Bigger = colder range, smaller = hotter.

Could also be simply that the tubes you just got are on the low side of "normal" gain range. The ones we get in the shop these days are all over the place...

By the way what tubes did you get for replacements?

[stratomaster]

I had them install JJ tubes. It was a full kit that I bought online from eurotubes. I tested the original 5881s it came with and the Ruby tubes that are currently in the amp (after one of the JJs blew). They all test around the same, so I'm guessing they must've futz'd with the amp some. I doubt the shop replaced the resistors with DIY bias people in mind. Trust me, they are not that benevolent.

Good point on the leads being strung together. They were 0.3 ohms when shorted. So I am, in fact around the advertised 34mA of current. That is pretty good, but is it not odd that I can't go hotter? I will give my amp the once-over with that schematic in hand when I get a chance. Thanks.

[acorkos]

R59 is on the main board in the area near the treble pot. Stock 27k would have stripes red-violet-orange.

[Mark Black]

And actually thinking about the fact it is a PCB-based amp I doubt if any shop would take the trouble to remove the PCB for installation of a different resistor unless they had some other reason to get in there.

For what it's worth 5881's usually run colder than JJ 6L6's, especially if they've had some hours on them.