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Struggling with an ST-70

goldswimmerb

goldswimmerb

Failed out of engineering, but look at me now!
Subscriber
Hey, looking for some help here as this thing is whooping my butt...

I have an ST-70 I am trying to get up and running properly, from what I've been told it worked just fine for a while then one day started "flashing then popping a fuse".... I've been going through and checking everything but I can't find anything major that would be causing this.

Basically, it'll turn on, run for a few minutes, then once the bias tops about 1-1.2v the rectifier tube will Arc and the fuse will pop.

I've tested with several rectifier tubes (as that was my initial suspicion) and they all exhibit the same behavior.

I've also tried different output tubes and get the same result each time. It also doesn't seem to want to act up on the variac

From my observations it would seem that:
The power transformer is of the new variety as the lamination stack seems taller.
The main filter is original, but tested fine.
Bias selenium has been swapped to a proper diode
The two 50uf caps tested off and were replaced.
The 10k ohm resistors test fine on the meter.
Power supply resistors are a bit off (around 10%) but I don't see a tolerance band.
The 15.6 ohm bias resistors test fine on the meter.
The 1k ohm grid Resistors test a bit high (10% or so).
The tube sockets have been reflowed.
Bias potentiometers tested for dead spots.

The resistors on the PC-3 board all were replaced at some point, looks like they did match the ones that needed to be matched with metal films.
The capacitors on that board have been replaced.


Starting to pull out my hair here...
 
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GordonW said:
How was the main filter cap tested? These can test OK on a multimeter, and still break down under full voltage.

Regards,
Gordon.
Click to expand...

So it was tested with an LCR meter, and since I know they can break down under load I tried bypassing the two sections that had the most AC ripple temporarily with no changes...
 
Shadowdog said:
View attachment 2094857 One mod that you can do that will help the overworked GZ34/5AR4 last without hurting the sonics is the ST-70 diode mod. It is mentioned on with a pic. on the tubes4hifi site.

The newer faster UF4007 may make a sonic improvement for you for a few cents more.

http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod
Click to expand...

Ill definitely try that, though I want to be able to get this thing working without modifying that first as the issue seems to be something surging/pulling way too much current... So Far its Arced: Whatever rectifier was in there, a known good big bottle style RCA 5AR4, a new Sovtek Tube (yeah new ones are bad), and some random one I had laying around with no markings...
 
Don’t use J.J. Gz34’s . Sovteks have worked well for me but nothing beats the old ones i.e Mullard.
 
goldswimmerb said:
Ill definitely try that, though I want to be able to get this thing working without modifying that first as the issue seems to be something surging/pulling way too much current... So Far its Arced: Whatever rectifier was in there, a known good big bottle style RCA 5AR4, a new Sovtek Tube (yeah new ones are bad), and some random one I had laying around with no markings...
Click to expand...
After it arcs thou... Is it really good anymore? (I don't know the answer to that question)
 
billyz said:
If using New production gz34’s definitely do the diode mod to the socket.
Click to expand...

Im pretty sure the issue is something outside of the 5AR4 pulling a ton of current and causing that arc before it pops the fuse, but then again I am a novice with tube gear.
 
Divide and conquer. Remove power tubes, power up if arcing you have a problem in the power supply most likely. If all good install power tubes to just one side and check for arcing. If no , then swap to the other side. and so on. Hopefully not an output transformer.
 
I agree, don't use a JJ GZ34. I had one flash and short out. Have you tried another rectifier tube? The one that is flashing may very well be shorted.
 
Lavane said:
I agree, don't use a JJ GZ34. I had one flash and short out. Have you tried another rectifier tube? The one that is flashing may very well be shorted.
Click to expand...

As per my earlier post, I've tried about 4 different ones...
 
One thing is you said you got things as close as they can value wise except for some you had to up since today's values are a little different. What is the value coming off the cap right after the rectifier as if being to high can cause arcs.
 
tonyg88 said:
One thing is you said you got things as close as they can value wise except for some you had to up since today's values are a little different. What is the value coming off the cap right after the rectifier as if being to high can cause arcs.
Click to expand...

I Put the original filter back in the circuit after swapping out didn't change anything.
 
What I also noticed is that there is a hum and a bit more hiss in the left channel than the right. Right is quiet, left is audible... This is why I hate cleaning up other people's work...
 
I've used a few JJ's, and I had one flash and blow a fuse when I short-cycled it. Since then, it's been fine for over a year in my ST-70. I'd not recommend them, but they should be usable. My guess is that they are acting as an early indicator that something else is amiss. Perhaps a stronger rectifier would last longer, but I'd not subject a nice 5AR4 to an amp that's prone to blow them, until that issue is rectified. ;)
I'd follow Billyz's recommendation to remove the power tubes, etc. and powering on to check the power supply.

Edit: note that the amp has been fine with a new JJ rectifier. I never tried the one after it flashed; they were cheap enough that I bought three at the time I was restoring my ST-70. At least they were only $17 each! I should put the questionable on my tester to see what it shows.
Side note: I'd always recommend using a set of cheap but functional tubes when working on and testing an amp, assuming the main tubes are valuable. I'd hate to damage one out of a nice quad of vintage tubes while resolving amp issues.
 
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Why dont you start with :
-remove el34 ( JJ EL34 works very good in this amp btw) , and GZ34. Power on, measure the DC at pin 5
oh all EL34 sockets. They should measure -30 to -45 Volt, adjustable with the bias adjustment pot.
power off
- if bias is ok on all 4 , install rectifier and power on while measuring plate voltage ( pin 3 ) on power sockets.
it should come close to 500Volts, if not you have a bad capacitor.Don't linger too long only until
voltage does not increase anymore. Power off, wait at least untill voltage has been reduced below 100
volt.
- if DC voltages is ok, measure resistance between power tube catodes and chassies ( pin 8 ) it should
be 15.6 ohm .
- install EL34 on one side, power on and observe ( and adjust bias to 1.5 volt) Does it stay alive
and is the DC voltages ok ( you can measure on the other sides tube sockets)
power off
- move the tubes to the other side, repeat procedure.
power off
if succeeded so far i expect the amp to work. Reasons not to work is that the powertubes
have been destroyed by overcurrent.
Note that all power on with EL34 installed must have a speaker connected ( or power resistor but
i would prefer a speaker is any noise here is an indication of other problems)
 
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