ST-70 failing DBT with power tube installed

I am new to the hobby and am trying to restore a ST-70 as my first project. The amp had been stored in a basement for decades, showing signs pitting and mildew on internal wiring. The original owners plugged it in to a 120 VAC wall socket, blowing the fuse and damaging the rectifier (only one filament side lights). That's where I start.

So far I have replaced the can capacitor, bias supply caps, and swapped out the selenium (plus any resistors in those areas). I got new fuses and a new 5AR4 rectifier tube.

With all tubes removed, the dim bulb tester (DBT) remains dark when supplying 120 VAC from my variac. However the power transformer seems to be supplying higher voltage than it should. I measure 400 VAC from Red to Red/Yellow. My understanding is that this value should be closer to 350 VAC.

Installing only the rectifier doesn't change the above measurement, and the DC reads about 530 V.

Now when I install the power tubes (and 8 Ohm speaker load), the DBT lights up. I assume this means I have a short somewhere. And here is where I could use advice! What are some sensible next steps?

Does it sound like my power transformer might be bad?
Should I try to get my hands on another quad of EL 34s to see if it's a tube short?
Should I desolder and pull the output transformers to test there?

What is the wattage of the lamp you're using in the DBT?

-Pat

Cubdriver wrote:What is the wattage of the lamp you're using in the DBT?

-Pat

150 Watt

It's entirely possible that I'm misinterpreting the DBT! This is my first project and my assumption that it would only light in the event of a short may be incorrect.

Ok. How bright does the lamp get? An ST-70 draws about 175-190 W, so current draw from the AC line is on the order of 1.5 A. A 150 W lamp by itself will draw 1.25 A. I guess the key to start with is the brightness of the lamp, because that'll help determine how much current is passing.

-Pat

botto wrote:It's entirely possible that I'm misinterpreting the DBT! This is my first project and my assumption that it would only light in the event of a short may be incorrect.

I haven't used one on an ST-70, but it would likely remain at least partially lit given the current the amplifier draws.

-Pat

It's tough to estimate. The 150w bulb is around 30-50% brightness I would say.

I should probably try plugging into the variac directly and raise the knob until the transformer output voltage hits 350 VAC. Put the fuse to the test.

Apologies if this was all just my misunderstanding of the effect of the DBT.

botto wrote:It's tough to estimate. The 150w bulb is around 30-50% brightness I would say.

I should probably try plugging into the variac directly and raise the knob until the transformer output voltage hits 350 VAC. Put the fuse to the test.

Apologies if this was all just my misunderstanding of the effect of the DBT.

50% brightness might be about right given what the amp would draw. Do you have an actual ammeter you could put in series with the AC mains as you bring it up on the variac? That way you can monitor the current draw and stop if it gets excessive. A variac and series ammeter are what I typically use to bring things up initially; keep an eye on it and stop if current draw seems to be rising too much. Monitor the line voltage going into the amp as well; stop at 120.

-Pat

I only have a clamp type. A series one sounds like a good idea. I could put one together.

Update: I powered the amp up to 117 VAC from the variac and let it warm up while I checked bias. The bias test point were reading low, at around 1V with the pots at full CW. I still need to replace the driver board caps, so maybe that's the culprit.

Then one of the EL34s started sparking, causing loud pops in the speaker! So, no more power until I replace the remaining capacitors.

Thanks for the guidance so far.

Hopefully you didn't smoke the EL34. I'd definitely make replacing the coupling caps a priority given that it spent many years in a damp basement - they're likely resistors at this point I'd imagine they're leaking so badly.

Good luck with it, and keep us posted on your progress.

-Pat

Thanks! New caps are in and the left side reached 1.56V bias. The right side bias is still very low. And, after about 5 minutes, the EL34 that has sparked previously started glowing blue. It got brighter and then the sparking and popping occurred again.

I'm not totally certain of the measurement, but it looked like the bias voltage started climbing just before the fireworks.

Sounds like that EL34 is likely toast.

Try and find someone in your town who can test the tubes for you . It seems to me you have one EL34 with low plate impedance that is running away as soon as it warms up .
Have you checked filament voltage ?

Filament voltage from the transformer measured correct from what I recall (I don't have my notes on hand today).

I think you're right about the tube. I'm going to try to borrow a simple tester and a spare good tube from a friend this week.