Tips for rebuilding this amp?

[Ty_Bower]

You'll probably find B+ rises further still when you reduce the current through the EL84s.

I found a 100 ohm, 5 watt part in the toolbox, so I put that in. I think it probably need something more like the 120~150 you suggested, but I didn't have anything like that on hand.

I took some more careful measurements this time. The right tube is running 329V at the plate. B+ is 341V. The OPT measures 302 ohm. This puts the idle current at 39.7 mA, correct?

The left tube is 325V at the plate. B+ is the same. OPT is 288 ohm. Idle current is 55.6 mA? By the way, the left tube is still showing signs of distress.

I imagine I need to get something bigger than 100 ohms in there for the shared cathode resistor, but I am concerned that the mismatch in the tubes appears to have become worse instead of better. Last time I guessed they were idling at 40mA / 52mA. Now the right tube seems the same, but the left has gotten worse? I must be missing something...

 

[Ty_Bower]

Where's the "I'm stupid" icon?

I figured for all the times I've taken the tubes in and out of this little amp, chances are I would have swapped left for right by now. Nope. This time I deliberately made sure I put the left tube in the right socket, and vice versa. Sure enough, the faint glow moved to the other side.

Boy do I feel dumb.

I guess I either need to get a pair of tubes that match, or I need to build separate biasing circuits for each side. Where I can buy a couple of sixty cent resistors without spending $5 to ship them?

 

[Ty_Bower]

Well, I've got a pile of these Russian 6n14n tubes, so I decided to start swapping them in and out and try to measure them. I quickly realized that any absolute measurements were impossible due to the shared cathode arrangement. All I could tell was that this tube pulled more current than that tube. Anyway, after a while I was able to find a pair that were pretty close. They're at least within 10% of each other. At the time I estimated one was pulling 45mA while the other was 47mA. This was with a plate voltage of ~327V and probably around 9V (?) at the cathode.

With this pair installed, both tubes had a faint, gentle glow to the plates. I'm guessing they were both being overdriven, about 14~15 watts plate dissipation on a tube rated for 12 watts. At least they were relatively balanced.

Today I took another step to try to determine what to do next. I checked the line voltage - it's about 124 VAC. For some reason, I seem to remember it was only 110 when I was a kid. I guess everyone suffers inflation. I borrowed a Variac and plugged in the amp. Turning down the line voltage to about 112 VAC, things seem to look much better. I can't say I see the plates glowing any more. Plate voltages are down to just a hair under 300V. Idle currents are about 40mA, give or take a milliamp or two. One tube is a half watt over 12, the other is a half watt under.

So, short of keeping it plugged into the Variac all the time (which really isn't an option), what do I do next? As an interesting side note, the power transformer now runs cool enough to keep your hand on it indefinitely. Previously, it was hot enough that you could touch it for 4 seconds, but not 8.

 

[Eli Duttman]

So, short of keeping it plugged into the Variac all the time (which really isn't an option), what do I do next?

Replace the 82 Ohm Resistor between the 6CA4's cathode and the 1st filter cap. with a 100 Ohm part. If that's not enough extra resistance to lower the B+ sufficiently, try a higher value. An inexpensive inductively wound part will give you a tad more filtering. $1.79 buys "Rat Shack" Catalog # 271-135, which is 2 pieces 100 Ohms/10 W.

 

[rdf]

How about lowering the screen voltage instead, replace the 2100 ohm PS resistor? With almost half a meg in the 12ax7a plate I can't see it much affecting the front end.