ST70 diode warning

Peter C, you have the correct diagram and I am connected exactly as pictured.

I’m feeling a bit guilty about all the time you guys are wringing your hands over my small problem. But I appreciate the quick education.

I believe in retrospect that some sort of voltage spike or impedance excursion occurred as a result of my failing to turn on the speaker’s LF amp before powering the ST-70. That is not to say that there may not be another issue with my ST-70, as I did purchase it used. It biases easily, sounds great on the B&Ws and was professionally assembled by Triode USA. It sports all JJ tubes.

It would be convenient to find a smoking gun here because you may notice the B&Ws have an “Auto - Off” switch; the Auto position is intended to keep you from having to reach down near the floor or remember to turn the amp’s main on and off switch. It senses a signal and powers the amp automatically and after a modest period of silence powers the amp off (standby). I am now hesitant to use it.

The last open item for me is how to rewire the amp for 16 ohm use. The manufacturer is having an apparent staffing issue possibly COVID related. I thought perhaps I could just swap the 16 ohm tap for the 8 or 4, but the 16 ohm tap is wired to the input board. An entry in the manual says it is for feedback. If anyone else has this amp and knows the answer I’d be much obliged for some advice. It will be at least another couple weeks before the Altecs are ready for prime time.

Peter H, I assume the 100 ohm resistor would apply to the 16 ohm tap when I go that route as well? And any particular wattage rating or resistor type?

Thanks again, all.
Jeff

If a safety resistor is connected to the 16 ohm tap i suggest 150 - 200 ohm 5w
Mount free from isolated wires and other components. Wirewound is good as they usually have
heatresistent porcelain body.

As for the blown fuse, ST70 is hard on the rectifier tube. Even harder if it is power cycled or
the AC glitches. Try to wait a minute before reapplying power.

jeffjmr wrote:Hi all,

Old audiophile with first tube amp. Recent factory build ST70 by a VTA competitor (sorry, got it on auction at a decent price.)

So far so good. Love the sound, and the ability to fiddle with it (and some electronic fiddling experience). I am in the middle of restoring a pair of Altec VOTT A5Xs with the intent to drive them with the ST70.

I have read almost this entire forum, and am soaking up all the knowledge I can muster, and am thankful to Bob and all else who share. I will attempt to return the favor as opportunity arises.

My concern is, somewhere along the line I read a post from someone who was not a Dynaco fan, who complained that he had no time for an amp with drivers that could be blown up at any moment by the failure of the single phase inverting diode!

Yikes! That is a pretty sobering thought. Since I have yet to perform the famed yellow sheet diode mod, I can only find one diode in my amp and that is on the power supply capacitor board.

Any idea if this is the diode he refers to, and is it that critical? If so, is there a more robust diode besides a 1N4007  called for here? Or perhaps wiring two in series as insurance?

I have parts on order to install a CL-80 and the yellow sheet rectifier diodes mod, and a GFI that interrupts on power fails with manual reset. Any other mods suggested? By the way, I am going with the more robust DGP30-E3/54 diodes instead of 1N4007 unless there is some reason I shouldn’t.

Thanks in advance for any advice.
Jeff

Could you be thinking of the "Selenium Rectifier" that is usually replaced by a diode to prevent this problem?

I am not sure which component the poster was referring to. If there is a single passive component which failure would toast the output tubes I’d like to know.

Jeff