Please help with Eico ST 70 power supply transformer

hugh811

Active Member
Hi everyone,

I am working on the recap of an Eico ST 70 tube amplifier. The amp is in working order. It sounds good to my taste, even there is a slight 60hz and 120hz noise. That's why I have checked through the power supply section, filter caps et cetera. I measured the DC voltage at Pin #8 of the Rectifier tube GZ34. The result is approximately 460 VDC. According to the schematics, it should be only 440 VDC. Others points in the power section also checked, and the result was also approximately 20 VDC higher than stated on the schematics.

Rectifier tube changed, speakers connected or not, the voltmeter always reads 460 VDC. The power transformer seems to be in good working order, because i turn it on for a few hours without problem, no excessive heat.

Could you please help me to identify the cause of this problem ? I will try to attach later the schematics. The original file is too large to upload on this forum.

Thank you for your help !
Hughie
 
Hi Hugh,

Have you checked your input line voltage? Do you have a variac? It would bring down the B+ if the line voltage is too high.

Dean
 
What brand of tube is reading high? I had an ARS 5AR4/GZ34 that produced B+ above 500VDC @ 121VAC line voltage.

Does anyone use a 5U4 in these amps? It would seem that the PT would handle it.
 
Does the 440v. B+ have ripple on it ? That would cause the meter to read high. You mentioned there was some hum. I'd change the power supply filter capacitors and go from there.
 
I think Deano nailed it. That was designed with 115 (maybe 117) as an input voltage. You're likely seeing 123 or so (pretty common now). If your primary is high your secondary will be high. For long term longevity you should consider a bucking transformer. Running a little low is better than running a little high. You tubes and your PT will thank you.
 
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