ST-70 no bias and blowing fuses

(This is also posted on Audiokarma)

I'm starting a new thread so I don't hijack another thread and my symptoms might be bit different.

I recently acquired a nice ST-70, plugged it in and it worked well. I wanted to 'bring it up to date,' replacing the rectifier, electrolytic caps, input jacks and bias test jacks. Did that and now its blowing fuses and bias is 0 volts.

I've gone back and rechecked solder joints, checked resistor values in the bias circuit path, etc. Checked voltages at several points along the path:
input voltage: 120V AC
filament voltage: 6.85 V AC
AC side of new 1N007 diode: 50 V AC
DC side: 33 V DC
input to bias pots: 13.3 V DC
at eyelets 6 & 21: ~ 7 V DC
at eyelets 22,23 1 & 2: -.03 to -.20 V DC
at pin 8 of the EL-34s: 0 V DC
B+ BTW is ~495 V DC
Measurements were made with both qa VTVM and a DVM and are fairly close to each other.
Inputs were shorted during measurements.

I'm stuck. Any ideas?

Ray.

The first thing that I see is that the DC side of the bias rectifier (that you replaced) should be -65 VDC. The voltage rating of your 'new' diode should be at least 500V. I would suggest a 1N4007 (1A @ 1000V) in any case.

Keep in mind that a filtered rectifier (which this is) should deliver about 1.4 x the input voltage not considering voltage drop across the rectifier. A 3-stack selenium rectifier has about a 5V voltage drop, making perfect sense at 50 VAC in. With a silicon diode, you should be getting on the order of 70 V.

Best of Luck With It!

Did you replace the bias caps as well? I suspect you did, but if so, check your polarity.

As it stands now, I suspect that your diode is toast, and you might be lucky and that is all it is.

In the future, take pictures beforehand. Make sure that all your wiring 'makes sense' and is correct. It is _very_ easy to ground a B+ wire to the 4-section filter cap, clearances are not huge.

Check polarity orientation of the newly installed bias rectifier diode.  If you have a positive voltage then it is installed backward.  Diode band (cathode) should connect toward the transformer side.

Those familiar with electronics can get so "positive voltage-oriented" that building a negative power supply can be counter-intuitive!

Progress!!!
The diode was backwards (1N4007) and I replaced the caps I had installed with 50uF 100V
electrolytics. The ones I had originally installed were in correctly but... I now have -71.5 VDC on the DC side of the diode and about +2.2 VDC at the bias jacks with the pots all the way down. The 10k resistors have been replaced with 5.1K and that change got the bias to the 2.2 level from around 4-5 VDC.

I'll try leaving the amp turned on for a while and see if it still blows fuses using a 3 Amp quick blow fuse rather than slow blow. We'll see what happens.

Thanks for all the suggestions and assistance - I really appreciate it.

Its probably been 50+ years since I worked with or troubleshot analogue tube circuits.  Basic electronics training in the Navy went from simple analogue tube amps to simple transistor
amps to saturation or cutoff, ie 1s and 0s in about 2-3 weeks.  Then it was computers.

Ray.

If your 3A fuse gets past the turn-on surge, you should be fine for the duration. If it fails an hour later, there is definitely still something going on. An OEM (Factory spec.) 70 is rated at 175 watts, or about 1.5A these days at full output. So a 3A fuse should hold indefinitely - again, once it gets past the surge.

Best of luck with it. As I am sure you know there are a LOT of after-market opportunities to modify/upgrade this amp.

All fixed (for now)!! Checked the resistance of the bias pots and they are 10K as they should be. Concerned about the bias voltage, I changed to 5.1K resistors in the bias supply, still to high. Ended up with a 10k in parallel with the 5.1k on the ground side and set the bias to ~1.4 volts. The amp is happy and I'm happy!
Still has a 3amp non-slo-blow and it will stay there for a while. now to enjoy some vinyl.

Ray.