[moboman]

I bought a used super 70, and there are diodes instead of the rectifier tube. Also the transformer hums and i was wondering if Van Alstine had an approved way of dampening it.





Transformer:


What should I do about the diodes and what should I do about the transformer hum?

Thanks,
David

[avahifi]

This was a special transformer we had built for us years ago when there were no reliable 5AR4 rectifier tubes available.  You can't use the original power transformer with a diode bridge because this causes the B+ voltage to run 50 volts too high.  We had a special transformer made then with a lower high voltage output to accommodate the more efficient diode bridge. Soon good 5AR4 rectifier tubes came back again and the need for the special transformer was eliminated.  We only had one run of them build and they are long gone now.

The hum is probably a iron laminate in the core working a little loose over the years.  The units with the special low voltage transformer were just fine in general, so this transformer hum is a rare but annoying issue. There is not much that can be done about it.  If it is in good working order and you can live with the mechanical hum from the transformer, the just leave it alone as is, don't replace the diodes.  If they have failed the unit would blow fuses immediately.

There are good replacement power transformers available now from Dynakitparts and Triode and more.  However, you will need to re-install the 8-pin tube socket and wire back in the 5AR4 tube rectifier wiring to use one of these.

I suppose this is an example of why (on my web site) I advise my clients to ask me about used AVA equipment they are thinking of buying on the open market before spending the money.

Regards,

Frank Van Alstine

[Tom Alverson]

I bought a used super 70, and there are diodes instead of the rectifier tube. Also the transformer hums and i was wondering if Van Alstine had an approved way of dampening it.



What should I do about the diodes and what should I do about the transformer hum?

Thanks,
David

David:

Maybe the hum is due to some DC on the AC supply?  I'm still planning on stopping over soon and I think I have an isolation transformer somewhere that we could use to test that theory.  Or you could try plugging into a different AC outlet where the load may be more balanced (and not have some DC superimposed on the AC).

Tom

[rlee8394]

Moboman,

The diodes are probably fine. I would make sure that the hardware on the transformer itself was tight. Then I would try to isolate the transformer from the chassis with rubber grommets. Place a grommet between the transformer and chassis, and another between the mounting nut and chassis on the under side. You may want to use a large flat washer between the nut and grommet on the under side of the chassis. You can also buy transformer isolation mounts from DynaKitParts for $6.00 a set. Go here for info:

http://www.dynakitparts.com/store/product.aspx?id=255

While isolating the transformer from the chassis won't eliminate the transformer hum itself, it will keep it from radiating to the chassis, making it a bit less noticeable.

Ron

[moboman]

Thanks for the replies.

Frank, I know you advise contacting you about used AVA equipment but this was from a local guy and I bought it for not much more than a stock st-70. I figured even if it didnt work, it would still be a deal. The amp works perfectly(other than the hum) and there is no hum in the speakers, just dead silence.

Thanks again for the upgrade to my super pas, it sounds incredible.

Tom, I've had the amp plugged in a couple places and it's the same hum. I have a separate circuit even that it was the only thing on and it hummed.

The surroundings are much more comfortable since the last time you were over as well as the equipment being newer.

Ron, thanks for the link.

I'll probably just live with the bit of hum since the transformer was designed for there to be diodes. I thought the diodes were installed by someone after it was upgraded and before I bought it so I thought I'd post and ask about both because I knew the transformer was Frank's.

Thanks again everyone.

[rlee8394]

moboman,

What type of update did Frank do to your Super PAS? Did you get the buffered line card?

Ron

[moboman]

Ron,
He replaced the Omega buffers with insight buffers and put in a new quad of 12ax7 jj tubes and a new 12x4 tube as well.

[rlee8394]

I have a Super PAS 3 as well. Thinking about having Frank install the Insight+ buffers. Do you think it's worth the $300? I would think so. I'm going to use it as the preamp for my second system. It's currently feeding my Insight 170.

Ron

[moboman]

I use it with my Mosfet 120C and now this super70.

The new buffers over the old ones were worth the 300.