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ST-70 fuses blowing .... ehhhh ....

Is it me or does it appear the diode is grounded to the chassis, I have this amp as well and used a small terminal strip at the location of the old selenium part. On mine it bridges the diode to the bias circuit across the terminal, but isn't grounded at that point. Hard to tell from the pic though...
 
UPDATE:

I replaced the bias capacitors, resistors, and rectifier diode. No more fuses blown!

BAD NEWS:
Very weak signal coming out, and I'm barely getting voltage out of the bias ... fixed one thing, broke another.
 
petercapo said:
From the photo, it does look like the diode is grounded to the chassis. If so, it needs to be corrected. But, a small terminal strip might be hiding under the diode...
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No, the diode had a small terminal underneath it. I did end up replacing the diode and terminal with a fresh one.

New terminal with bias capacitors and resistors.

I found the grounding from terminal 4 was not actually soldered on, so that problem is fixed.

NEW PROBLEM: Little to no bias voltage is coming out at the terminal, and the signal is really weak. Not sure what's going on there, but looks like the voltage coming off the quad cap is a bit high (515V or so).
 
I know it has probably been stated a thousand times before on other posts and forums, but please be cautious when taking measurements on live amplifiers. The best advise I ever heard regarding this is to keep one hand in your back pocket when probing around lethal voltages. I generally attach the black probe to a ground point so I only need the use of one hand to do the probing. For what it's worth...
 
WHen you say no bias voltage, do you mean at the test points on the front or at pin 5 (grid) of the EL34? Very different meanings. If you mean at the test point on the front, that means low current flow. Thats either too much negative voltage at pin 5, or low voltage somewhere else. I'd measure voltages at the quad cap and compare to what the manual has to say about it. Low B+ makes for low current flow.
 
gadget73 said:
WHen you say no bias voltage, do you mean at the test points on the front or at pin 5 (grid) of the EL34? Very different meanings. If you mean at the test point on the front, that means low current flow. Thats either too much negative voltage at pin 5, or low voltage somewhere else. I'd measure voltages at the quad cap and compare to what the manual has to say about it. Low B+ makes for low current flow.
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Clarification: Bias on the front biasing point, not on the output tubes.

I did measure voltage at the quad, and it's a bit high at it, 515 or so at section 2 and 3. The first section is at 345, dead on. I'll probably get home tonight at write down the voltage test points. ... it's frustratingly close to working.

To clarify for the group, with regards to my current state of knowledge: Yes, I'm confident in my ability to be safe measuring whatever, and soldering wherever. And I understand the functions of each component. So don't worry, I won't kill myself.

However, I am most certainly short on knowledge when it comes to this circuit, and overall electronic theory. ... I had some schooling quite a few years back, but I'm pretty sure I was stoned the entire time during the program, and hence my memory of the particulars is hazy ... :|. And I'm lacking a variac, so there's a limit. Which is why I'm trying to defer to the forum before starting an arduous task of finding someone who will work on a tube amp.
 
ok, so if you have reasonable voltage at the quad cap, check what you have at pin 5 of the EL34. If its considerably too negative and the bias adjust pots won't increase it enough to get in spec, you may need to increase the value of the resistor between the two bias filter caps to compensate. Probably around -30 is reasonable.

If thats good, check the plate and screen voltages directly at the EL34 socket. Screen is pin 4, plate is pin 3. It should read pretty nearly the same as the high voltage sections on the quad cap, so ~500v. If things were loaded that voltage will be lower, but mostly we're looking to make sure voltage is getting to where it goes.

if the plate, screen, and grid voltages are basically normal, and the tubes are actually heating up, about the only thing left is the output tubes are just dead.


you did hook the heater circuit back up and the tubes are lit, right?
 
gadget73 said:
ok, so if you have reasonable voltage at the quad cap, check what you have at pin 5 of the EL34. If its considerably too negative and the bias adjust pots won't increase it enough to get in spec, you may need to increase the value of the resistor between the two bias filter caps to compensate. Probably around -30 is reasonable.

If thats good, check the plate and screen voltages directly at the EL34 socket. Screen is pin 4, plate is pin 3. It should read pretty nearly the same as the high voltage sections on the quad cap, so ~500v. If things were loaded that voltage will be lower, but mostly we're looking to make sure voltage is getting to where it goes.

if the plate, screen, and grid voltages are basically normal, and the tubes are actually heating up, about the only thing left is the output tubes are just dead.


you did hook the heater circuit back up and the tubes are lit, right?
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I'll take some readings hopefully tonight and write them down. All tubes appear stable, heater is hooked up, and all glow like how they should. No huge issues other than essentially no bias. I hooked up speakers -- sounds like only half of the signal is getting through, if that makes sense. When I turn up the volume at the preamp, there's some crackle, and still only partial signal output.
 
UPDATE:

0 DC volts at pins 4 and 6 of the rectifier tube. But it gets proper readings with AC multimeter setting. Hope that’s ok.

EL readings:
Pin 1:0
2: 1v
3: 500v
4: 500v
5: -60v
6: -60v
7: 1v
8: 0v

I’m assuming this is a PT problem. The voltage reads 7 v dc on pins 2 and 7 on the right channel. Is this a correct assumption?
 
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Pins 2 & 8 should have 5 VAC. Pins 4 & 6 should have the high voltage in AC volts. You should see half the AC voltage between pins 4 and chassis, and 6 and chassis.

Only four pins are used for the rectification: 2, 4, 6, and 8. Pins 3, 5, and 7 are just tie points and have nothing to do with the 5AR4 tube.

Now with the meter set for high DC volts, there should be 350VDC or more at pin 8 measured with the black probe on the chassis (ground or center tap) and the red probe on pin 8.
 
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-60v at the grids of the EL34 sockets is the problem. That should be in the -30 to -40v range. Do the bias adjust pots have any effect? If they do, I'd try increasing the 10k resistor between the caps to something larger.

If no change, check the 10k from the second cap to ground. If that is open or just too high in value, you'll get no or limited adjustment range and overly negative bias voltages. You can pop the 5ar4 out for testing this stuff. No point in having high voltage present when messing with the bias supply.
 
gadget73 said:
-60v at the grids of the EL34 sockets is the problem. That should be in the -30 to -40v range. Do the bias adjust pots have any effect? If they do, I'd try increasing the 10k resistor between the caps to something larger.

If no change, check the 10k from the second cap to ground. If that is open or just too high in value, you'll get no or limited adjustment range and overly negative bias voltages. You can pop the 5ar4 out for testing this stuff. No point in having high voltage present when messing with the bias supply.
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I don't believe the pots are changing anything.

Brand new resistors in the bias circuit. However, I just found out that one of my .02 capacitors has a broken leg, one of the 2 .02’s that grounds the heater.

Perhaps that’s the culprit?
 
that won't affect anything in the bias circuit.

Sounds like the resistor from the second cap to ground isn't actually grounded if the pots aren't changing anything.
 
gadget73 said:
that won't affect anything in the bias circuit.

Sounds like the resistor from the second cap to ground isn't actually grounded if the pots aren't changing anything.
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Dammit it’s fixed!!!

guess what it was? Forgot the jumper cable from terminals 1 to 6.

I’m indebted to this forum for the amazing help. Thanks much. Cheers!
 
i never realized they had a jumper. I would have assumed it just ran to a grounded lug on the terminal strip directly. I've never owned one.

Either way, good deal. How does it sound now?
 
It may seems silly.
I know many of us over the years building gas grills or IKEA furniture have set aside the instructions.
Often when people open threads on Dynaco gear, someone (or more than one) chimes in to download the assembly manual and go through each step until they end.
In this case, it likely would have got you to the problem.
In any event, I'm glad you resolved the issue. It's really a very good amp. Enjoy!
 
gadget73 said:
i never realized they had a jumper. I would have assumed it just ran to a grounded lug on the terminal strip directly. I've never owned one.

Either way, good deal. How does it sound now?
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Sounds amazing, however only ran it for a few minutes. .... I'm assuming the ceramic disc capacitor with the broken leg on terminal 6 grounds the heater component of the transformer? If so, I believe the PT will overheat, if I'm not mistaken.

Hence I'll need a new disc cap ... but other than that, the amp sounds amazing. ... soundstage is really present.
 
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