Out of my depth with Stromberg-Carlson AU-34 rebuild!

[Kerrang]

Hello everyone! As my thread title tells you, I need help. And having read some of the threads in this forum, I think the signal-to-noise ratio is more than acceptable, so here goes...

Previous experience:

(1) Ampmaker SE-5a 5W British overdrive amp built from kit, worked first time, followed instructions, did not kill myself. I understand how to drain caps Lovely amp BTW!

(2) Bought a JTM-45, replaced 6L6 with KT66, biased to taste, no problems, still alive, crowd chants "Angus!"

(3) Bought a Stromberg Carlson AU-34. Condition unknown...preamps 6SJ7 -> 6SF5 -> 6N7 driver inverter, missing rectifier (5U4G), 2x 6L6 push pull cathode biased, one of the 6L6 shattered - white deposit inside head, cracks down side of glass...

(i) replaced all capacitors - replaced the big cans, and figured why not change them all as many smaller caps were vintage and may have passed their best-before date

(ii) checked resistors...suspect a lot of the low wattage (all?) are carbon comp, the high power 10W ones are ceramic wirewound. Values seemed pretty good, nothing wildly out of tolerance

(iii) decided to change the spec 5U4G rectifier for a 5AR4 (GZ34). Reasoning was follows:

(a) 5U4s are directly heated. They have no time delay before they slam the circuit with high voltage. This amp has no standby, 5AR4 GZ34 warms up slowly.
(b) The 5U4 draws 3.0 amps of 5vac filament current. The 5AR4/GZ34 only requires 1.9 amps - ok i hope this means i am cutting my PT some slack, perhaps useful as i want to run KT66 instead of 6L6 once i get it debugged

Observations:

(a) With no tubes (or valves as we say over here installed
Voltages are high. Expected that with the 5AR4 GZ34. Although I now see 530VDC at pins 2,8 of the rectifier. spec is 415 VDC. PT runs at 117VAC, outputs 400VAC. Hope to myself that putting the tubes in will put some load on the circuit and drop the voltage?? (yes i am learning, and yes i am fairly ignorant of how electricity actually behaves, so please correct any flaws in my understanding, I am here to learn

(b) insert valves. I had a spare pair of 6L6 kicking around. Objective is to measure B+ voltages at various points as it makes its way from the rectifier past the power tubes to the preamps tubes. Voltages slightly higher than spec, but too ignorant to worry about that right now

(c) I think (and it was so fast i could be wrong) there was a white flash in one of the power tubes. And the fuse blew. Fitted new fuse, repeated test. No flash. But fuse blew again. I have a speaker load plugged in.

(d) Realise i have been a doofus and soldered a wire wrongly. I think it would have meant the preamp stage was getting too much B+

(e) Fix wiring. Try again. 1 of the 6L6 looks like its red-plating. Fuse blows.

(f) Think maybe I damaged one or both of the 6L6. Go to parts bin, find 2 healthy EL34. What the heck, I never liked EL34s anyway.

(g) Switch on. Hey looking good. EL34s look healthy. Quick! Must measure voltages!

Voltage measured across cathode resistor 32V
Actual resistance of cathode resistor 477R
Number of power tubes sharing the cathode resistor 2

So far so good...

Then I put clip my multimeter leads to pins 3 and 8 of one of the 6L6 and power on
I want to measure the DC voltage measured between plate and cathode. (Actual Plate Voltage) Octal - pins 3 and 8. Yes I have been to the weber bias calculator

Voltage rises fairly slowly as the rectifier warms up, gets up to 430VDC, wavers around there for several seconds, then huh?! it starts to fall, keeps falling, then there is visible arcing (audible buzz) in the rectifier quickly followed by (sounds rather like a match being struck alight) the fuse going pffffft and all black inside

Now i am stuck!!!

Possibilities that occur to me:

- I have a dodgy rectifier...maybe, it was in the JTM45 i bought and I replaced all the tubes before I fired it up. Maybe i have damaged it somehow?
- Maybe the 5AR4 GZ34 higher voltages disagree with this amp, but my gut (not my brain) tell me thats not it. I could try a 5V4, which I understand would lower the voltage a bit...or just buy
- My wiring is still wrong...I have checked and rechecked I think its ok

Any other causes are outside my abilities right now, please send help. What tests - if any - can I do with the preamp tubes and driver installed but no power tubes?

Cheers!
Gareth

[Enzo]

Hi, welcome to the forum.

My first reactiion is: don;t overthink it, and don;t try to outhink yourself. For example replacing the 5U4. Stromberg built amps for many many years, and I don;t think all of a sudden the common 5U4 tube becomes a problem. You reason the 5U4 us direct heated so it will instantly make B+. HOW it is heated is not the issue. Direct or indirect, it still has to warm up. It is only when a tube cathode surface hits a certain temperature that it emits electrons and allows current to flow. And that includes direct heated cathodes.

And even if it did, a small amp like this will not be troubles by a fast application of B+ voltage. The amp doesn;t have a standby switch because it doesn;t need one. IN a guitar amp, the standbu is there to silence the amp, not to protect tubes. it is ther so you can turn the amp function off during breaks without having to wait for the tubes to warm up when you return. A little PA amp like the S-C was more likely expected to just run all day.


And lowering the current draw on the 5v winding wil not leave those amperes for the 6v winding. Yes, you would lower the overall flux level in the core a little, but we don;t generally come cloes to that limit anyway. But the wire size and resistance is a limiting factor, and backing off on the 5v won;t help you if your 6v is not up to larger heater load.

As you found, without the tubes, the B+ will soar. Tubeless voltage readings are pretty meaningless, other than to determine if voltage is present or not. But it is where we start. You changed out the filter caps, good, they probably needed it. Now with the rectifier ONLY and no other tubes, you get over 500v. OK for now. Check that that 500+ volts gets to BOTH pin 3 and 4 of BOTH power tube sockets. Now even more important, check pin 5 of each, there should be zero volts DC there. Any positive voltage would be the reason for red plating and would be caused by a leaky coupling cap almost certainly.

EL34 is NOT a drop in replacement for 6L6, they are wired different. Their bias needs are different. I am going to guess that if you look inside, pin 1 of the 6L6 sockets is not wired to pin 8. They may even have used pin 1 as a handy soldering point for other components. EL34s MUST have pin 1 wired to pin 8. (Or one of a couple other options) Not doing that may explain funny operation.

Why not try the intended 5U4?

Arcing inside a tube is usually a death warrant. Sometimes ist is just a sign of excessive load - like a shorting tube downstream. But with the other tubes out, does it sit pretty and make a steady voltage?

A suggestion. Nothing wrong with measuring pin 3 to 8 directly on teh power tube, but if you clip one lead to ground, then measure pin 3 then pin 8, all it takes is subtraction in your head to get the number, and that way you don;t have to probe two hot pins at the same time, and it means only one hand has to be in the chassis.


You also could have had a transformer arc over internally or some other rare symptom.