Pin 3 on the right is settling around 422v... I have replaced the 30uf (D) and the 20UF (C) sections of the cap.
Bias is fine at 1.56v. Same with the Left...
my B+ seems really low.
Struggling with an ST-70
- Thread starter goldswimmerb
- Start date
Pin 3 on the right is settling around 422v... I have replaced the 30uf (D) and the 20UF (C) sections of the cap.
Bias is fine at 1.56v. Same with the Left...
my B+ seems really low.
Ok, you are at the "el34 removed" GZ34 installed" test ? Yes then 422V is way to low.
Is the can cap cool ? Any noise ?
How
Just to understand, was 422 Volt measured with all EL34 removed ? If so, 422 volt is to low
Did you then continue and install EL34 finding that you can adjust bias to 1.56 V ? Also
on the other side ( but only tubes in one side at a time )?
Is the can cap still cool ? Any noise ?
422V was with tubes only installed in one side. When tubes are installed left side that channel has more hum than when the same tubes are installed in the right.
The transformer seems to have a lot of vibration to it, but not very much mechanical hum.
Can cap is cool, two sections have been bypassed. I also did throw in the diode mod on the rectifier just in case.
I'm suspicious of that too. It seems to be a later revision (if that was a thing) as it is marked differently from the two other ST-70s I have here. Definitely original due to the wiring.
When I remove all the tubes, except the rectifier I get about 512v on B+
I suspect the transformer.
Remove the GZ34 and measure the AC at pin 4 and pin 6 in the GZ34 socket. The manual says 360V.
Hold it here, you said that the diod mod was made, in that case you wont find AC
at pin 4 & 6 you will have to open the amp and measure at the transformer windings.
At the same time examine the diode mod and see that it is made ok and that the
diodes are working and connected the right way.
but that is probably with all tubes installed.
Could someone with access to a ST70 report what voltage to expect with EL34 + GZ34 removed ?
I did the diodes only after I was still having issues just since I figured it *may* help if the tube is the issue.
billyz
Super Member
I was suspicious of the output transformer. Which is rare , it is probably something else. You know for certain the power tubes are good. It would only take one bad one. You have to divide the circuit up and try to test each section . Like only powering up one side. Swap the good side tubes to the bad side . Etc . This is what a working set of used up tubes are good for , so you don’t crater a good set while diagnosing.
whoaru99
Epic Member
I seem to recall some conversation/debate if this really helps the rectifier or if it's actually more like protection for something else (transformer??) in case the rectifier arcs/shorts.
Has that ever been bottomed out?
Yeah, I've been using a working set of sovtek EL34's that I replaced from my main ST-70. It seems that with either side having to power outputs the B+ falls significantly. B+ with no tubes is 512 or so V. B+ with just 7199s is about 480v, B+ with just one set of outputs and a single 7199 is 422v. Either something is wrong with the power transformer (or choke), causing it to be unable to sustain current or the last guy working on this seriously messed up something...
General knowledge. If loaded PSU has 430volt an unloaded of this type might give up to 500Volt. I also asked
for someone to verify this with a working st70.
Pio1980
AK Member
Both, particularly if the plates are paralleled after the diodes.
Manual has B+ listed. Not to mention, the listed voltages in the manual are assuming 117v if i recall, so it should actually be higher than that.
One of the pin3 measurments was with powertubes removed.
Wound up figuring it out by inspecting the wiring very carefully (the colors have faded some). At some point a previous owner had swapped a Blue/white and green/white cable (as they look the same unless you look closely). So anyway I had a Plate and screen pin flipped on one channel, swapped that back to how it was supposed to be and everything is working as it should. Also replaced the rectifier tube since the old one was absolutely shot.
Very minor transformer hum is the only thing I notice now and it's expected on an ST-70 with the original transformer running at what the line voltage is here.