New EX 6V6s are drawing quite a bit more current than old RCA 6V6s

[BrianS]

Hello everyone!  I have just completed a cap job on a customer's old Gibson GA-19 RVT amp (Falcon).  Everything went pretty smooth...except for slicing my finger open while improvising a new "clamp" to hold in the new caps that replaced the old can cap!

Anyway, I do not have a tube tester, so I suggested that the customer buy a new set of output tubes in case his old ones were wearing out.  I figure it's always good to have an extra set of power tubes on hand anyway, especially since the ones in the amp seem to be quite old.

So I installed the new tubes, plugged in and gave her a play...sounded great!  Very warm toned, not "Fendery" at all, but still very nice.  Cranked up to 10 it had a very nice overdriven tone...crunchy and smooth, not mushy or squishy at all.

When I checked the bias, I was running at around 37ma of current...pretty darn hot!!  I had not changed any values of any parts...cathode resistor/capacitor look original.  So, I decide to plug the old RCAs in.  They also sound darn nice, maybe a little cleaner tone.  I take my measurements and find that these are running at a more acceptable 31ma of current.

I guess I'm wondering if I should change anything here?  What I'm thinking of doing is telling my customer to keep the RCAs in the amp and keep the EX tubes as backups.  According to the info provided by Mr. Hoffman, the EX tubes should be able to take this kind of current no problem...right?

Tonally, everything seems good, so...opinions?

Thanks!

Brian Stewart
Red Wing, MN

[sluckey]

Sounds like the new tubes are just stronger. Kinda what I'd expect.

[HotBluePlates]

The Gibson schematic shows 18v across a 270Ω resistor.

18v / 270Ω = 66mA   ->   33mA per tube.

37mA => 12% high
31mA => 7% low.

Call it +/-10% either way, so I don't think you have an issue, unless you're seeing redplating.

[alerich]

I have the same experience with an old set of Sylvania 6V6 tubes I have. If I install a new set of any current production tubes and then swap in the Sylvania tubes the current drops equally for both tubes. I have always attributed it to older tubes made in a different era under different conditions as opposed to them just being worn out since they are worn out almost exactly equally, if that were true. They sound great - I just hesitate to run them in higher B+ applications.

[BrianS]

Thanks for the responses.

Actual measurement on the cathode resistor is 290Ω, and I measured about 22V across it, and 298 volts on the plates...with the EX tubes in.

With the RCAs in there, I got 19V across the resistor and 305V on the plates.

No redplating, and the tone is good.

Thanks again for the reassurance.  It's been a long time since I've done a much electronics work, and I'm getting back into it.  It sure is nice having resources like this board that we didn't have 20 years ago!

Brian