Ampeg B15N Cathode R

[Smokebreak]

I've got one of these here I'm fixing for a friend. One of the tubes was fried and a new pair fired everything up just fine. Cathode resistor on 6L6s is stock 250r, and it had drifted to 275. I put the correct value of 250r in, and it's still only dissipating 17W or so each tube, Amp looks like it has been maintained and modded(diodes floating off the 5U4 sockets, dropping string changed), but everything looks good and sounds good.
HT off 5U4 is 350VDC, which seems low for 6L6 and schematics I've seen, but I'm wondering if this is by design? Just trying to figure out if I should leave it be or start dropping the Rk to get closer to 30W, or start looking for other issues..
Thanks, J

[martin manning]

There are a number of B-15-N circuits... do you know which one it is? Date of manufacture? Cathode bias and 5U4 says early 60's

[Smokebreak]

I'm not really sure. It has 5u4 so I'm thinking '68? The schematic is under the chassis, but it's barely readable. What I can read, I see that some values have been changed. I think I figured it out though. The PT only has a few markings on it, but one of them is "120V" Then I saw the mounting holes for the previous PT. So I guess it was replaced at some point for lower voltage, which would explain why someone was making a SS rectifier on the 5U4 socket. I'll drop the cathode resistor, though it sounds really good and warm and dirty.

[Leo_Gnardo]

So I guess it was replaced at some point for lower voltage, which would explain why someone was making a SS rectifier on the 5U4 socket. I'll drop the cathode resistor, though it sounds really good and warm and dirty.

Replacing that drifted cathode resistor with 250R will increase your bias current and probably add a couple watts too.

It could have been that the original PT went bad - many do in B15's - and the replacement just happened to be a little less power. For instance instead of paying the absurd price for a replacement B15 transformer, I go all mad scientist, melt out the old one, replace with a PT intended for Vibrolux Reverb, accept the small loss of power (5-10% just a snip, repack the shell with transformer, beeswax & dry white sand instead of tar, repaint and save at least $100. It's an ugly job but my Scottish genes like saving money even if it's for someone else.

You could go back to solid-state substitute rectifier and get the maximum power, or 5AR4/GZ34 and split the difference.

[Smokebreak]

Haha! Well, that should have been my first clue, the shell was missing
I'm quite unfamiliar with these amps.
Yea, going down to 250r spec bumped it up a couple watts , so why not go down to 200r and get more dissipation, or am I going to hit some point of diminishing returns?

[Leo_Gnardo]

Haha! Well, that should have been my first clue, the shell was missing
I'm quite unfamiliar with these amps.
Yea, going down to 250r spec bumped it up a couple watts , so why not go down to 200r and get more dissipation, or am I going to hit some point of diminishing returns?

I'd go up with the B+, with a 5AR4/GZ34 or solid snake silicon octal plugin (just the rectifiers, no added resistors as with Weber's "copper cap" units). You could have a rectifier for every occasion. Even use a 5Y3 for less power, mo grind, when you need that.

Missing shell: cheap or lazy. Yes I've done it a couple times for customers who are dirt poor.

[Firestorm]

Haha! Well, that should have been my first clue, the shell was missing
I'm quite unfamiliar with these amps.
Yea, going down to 250r spec bumped it up a couple watts , so why not go down to 200r and get more dissipation, or am I going to hit some point of diminishing returns?

I'd go up with the B+, with a 5AR4/GZ34 or solid snake silicon octal plugin (just the rectifiers, no added resistors as with Weber's "copper cap" units). You could have a rectifier for every occasion. Even use a 5Y3 for less power, mo grind, when you need that.

Missing shell: cheap or lazy. Yes I've done it a couple times for customers who are dirt poor.

Yeah, you're not going to get any more power with further fiddling of the cathode resistor. The closer you take it toward Class A, the less power you'll have. Maybe try fixed bias. You'll pick up, what, 25 volts in plate to cathode potential. Switchable fixed/cathode bias would give you the best of both worlds. If you want it really loud though, you need a new PT.

[Smokebreak]

Ok thanks guys.