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Need some input from users of this Power transformer.

What do your voltages look like? I scratch built a 70 using Shannons board a couple years ago, and all the voltages where through the roof!

The 6.3 winding was 7
The 5v winding was almost 6
The 360 winding was 400v!!!!!!!

It worked ok but I couldn't resolve this issue. I was afraid I was ruining the tube heaters so in the closet it has gone for 2 years now. I'm ready to build a 12B4 preamp now, and need to resolve this issue.

On some of the forums it was stated that the old Dyna tranny expected 115 Mains and I had 120, so that was the issue, and I should use a Variac.

Well thats doesn't seem right because no one else seems to have to do that? The Dynaparts owner blamed it on the my mains too. I think is that this tranny is defective and I will have to shell out another hundred for a transformer, and partially rebuild this amp.

Is my thinking right here or have I missed something? Waht could cause such high voltages other than a bad transformer? My mains is only 2% high, but the secondary voltages are over 10%

Hi ,

Are you measuring the voltages with all the tubes in there sockets and warmed up or tubes out of sockets .The fillament voltages will be high with the tubes out of the sockets , what type of meter are you using it might not be acurate if its a cheapie .If the voltage is indeed high a resistor can be placed in series with the fillament windings .Wouldn't worry about the B+ unless its over the max of the filter caps .

Mark

Tubes are in when testing, and Im using a fluke RMS meter. The heaters look extremely bright! Too bright.

Yeah I've been told I can use resistors, but it just seems like a kludge to the real issue. I'm hope to hear some reports from other users of this clone tranny. I'm curious if they are all like this or did I get a dud. I was one of the very first customers, and was worried about buying these trannies with no feedback. This fear may have been correct.

Hi ,

Are you using EL34's or 5881 (6L6's) , a tube with a lower fillament current draw will cause high fillament voltage .

I guess which way to procede depends on wether it hurts you pride or pocket book more . Personly I would rectify and filter the 7 volts and regulate it back to 6.3 DC over a new transformer .

Mark

What's your line voltage?

Many US transformers were designed for 117V to compensate for the inferiority of the power grid way back when. Nowadays, 120 to 126V is common on household lines.

Cheers!

Geek wrote:What's your line voltage?

Many US transformers were designed for 117V to compensate for the inferiority of the power grid way back when. Nowadays, 120 to 126V is common on household lines.

Cheers!

My Mains are 120v which is 3 volts higher than 117. I may be mistaken about how this works, but I don't see how that could explain over 10% of over voltage?

Hi,
If you had a problem with our power transformer you should have contacted me again and not waited (2) years. Most high voltages on the secondary are a result of a higher primary voltage...Our PA 060 PT is wound for a 120 vac primary and not 117 vac like the originals.

Here's what I will do...Contact me via Email or phone and I will issue a RMA # ...Return this unit prepaid shipping and I will test this unit. If defective, I will replace this at no charge to you with the exception of shipping cost...which will be at your expense.

Kevin @ Dynakit

Wmacky,

I agree with Kevin. If you measured your voltages and felt they were off for some reason after you first built your ST-70 you should have contacted him IMMEDIATELY and I KNOW he would have replaced your transformer. I have dealt with Kevin at Dynakitparts for about 4 years and he stands behind the items he sells. His offer above to STILL replace your transformer even after 2 years (which is probably 1 year beyond its original warranty) shows you the kind of guy he is.

Bob Latino

Bob01605 wrote:Wmacky,

I agree with Kevin. If you measured your voltages and felt they were off for some reason after you first built your ST-70 you should have contacted him IMMEDIATELY and I KNOW he would have replaced your transformer. I have dealt with Kevin at Dynakitparts for about 4 years and he stands behind the items he sells. His offer above to STILL replace your transformer even after 2 years (which is probably 1 year beyond its original warranty) shows you the kind of guy he is.

Bob Latino

Bob, I did talk to kevin, but never got back with him after still having the issue. Perhaps I didn't want to accept I had a bad unit, and would have to get out the soldering iron again after completing a long build.

BTW, I'm really liking that ST-120! I little too much really...

Dynakitguy wrote:Hi,
If you had a problem with our power transformer you should have contacted me again and not waited (2) years. Most high voltages on the secondary are a result of a higher primary voltage...Our PA 060 PT is wound for a 120 vac primary and not 117 vac like the originals.

Here's what I will do...Contact me via Email or phone and I will issue a RMA # ...Return this unit prepaid shipping and I will test this unit. If defective, I will replace this at no charge to you with the exception of shipping cost...which will be at your expense.

Kevin @ Dynakit

Kevin,

Thank you, that is a very nice offer.

Before I do that I will pull the amp out, and test the voltages again including my Mains. (It's been a long time now). Anything else I should do or check? If the mains is high, the Secondary voltages should only be off by the same percentage, is that correct?

I would have called you back again, but I was new to tubes at that time, and unsure what the problem was. I had asked for help on various forums, but never got any clear answers. There were lots of comments about variacs, old designs for 115v Mains, using resistors, but no one ever said your transformer is bad?

I hope I didn't rattle your cage, as I too had nothing but great service, and help when I did the order. I have no issue with you or Dynakits just with this tranny. I posted only to see if they were all like this, or if I got was unlucky enough to get a defective unit.

William

For anyone that likes amp pics, here the amp in question. I'm still pretty proud of the gal.

William,

The chassis you are using. Is it a Triode chassis ? The thing I can't understand is why the lettering "RIGHT" and "LEFT" are reversed from what they should be. On a Dynaco ST-70 the RIGHT channel is on the RIGHT and the LEFT channel is on the LEFT when you face the front of the amp. On your chassis they are the other way around ? Strange ?

I like your choice of the ElectroHarmonix "fat boys" output tubes. I have a set of those on one of my ST-70's - nice top end on those tubes. I think you have a 5U4 rectifier in there. If you use a 5AR4 in place of the 5U4 you will get a higher B+ voltage and possibly a couple of more watts out of the amp. Also, a 5U4 is directly heated and a 5AR4 is indirectly heated. The use of a 5AR4 will allow the amp to warm up at a slower rate.

Bob

See a stock ST-70 photo below and your photo below that shows the left and right differences ...

Bob,

Yes you caught that labeling oddity! Tha chassis is from a ebay seller from 2 years ago that did a run of these. The rectifier in the pic is a 5U4. It was given to be by a forum member for testing as the new 5AR4 blew on first power up. I now have a replacement back in. As for the fat boys, I bought then on looks alone. I thouhgt the amp looked good with them 8)

You might be able to bring the voltage down a little bit with a CL90 Thermistor in series with the primary of the transformer.

I agree, Kevin has given you a very generous offer 2 years after your original purchase.

Sal

If I wanted to bring my 125V line down to just under 115V, I'd go to Radio Shack and buy the 12V, 3A transformer. Then I'd make a bucking autoformer out of it. You basically wire the primary and the secondary together in series, and put the 125V line across the ends. Then pick up ~114V off the primary. Make sure the whole thing is assembled inside some kind of grounded or double-insulated box, since there's no isolation between primary and secondary. You ought to check the output voltage too, since if they are wired in one phase it bucks, and in the other phase it boosts.

This thread discusses it pretty well:
http://www.diyaudio.com/forums/showthre ... did=133843