MK IV Blowing Power Transformers

I have rebuilt two Dynaco MK IV amps, and the one using the original transformers is running great. However, the second is giving me problems. I reused the original output transformer, but installed a new PA-135 power transformer. After playing it for a week this new PA-135 failed (shorted secondary). I replaced it with another new PA-135 and also added a CL-90 thermistor. After 10 days of (heavy) use it seems that this transformer has also failed with what I think is a shorted primary (3.7 ohms across the primary leads). Also, the thermistor is fried. The power transformer did get pretty hot to the touch. Not so hot that I couldn't keep my hand on it, but it was borderline too hot. The output transformer is original/old, as well as the 7199 and EL34s. The rectifier tube, choke, can capacitor and driver board are all new. Every resistance value I compare to the working amp approximately matches except for the pt primary resistance. Is there anything other than the power transformer being faulty that might cause this, e.g. bad output transformer, bad tube(s)?

Thanks for any help/ideas!

One thing you should do is measure your house line voltage throughout the day. Mine often is up around 125VAC. You can use a suitably large variac to reduce the incoming VAC to your stereo to around 115-118 VAC. Will keep most older gear much happier and run cooler. Not sure if this could have been your problem but it's good insurance.

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Yes - using the Dynakitpart pt and 2A fuses. Given that the amp plays great under moderately heavy use for over a week before the pt fails, I tend to believe that it has to be something that is transient? So, maybe over-voltage is a somewhat likely possibility even though the old amp keeps going - maybe it happens that this old pt is super robust? In any case, I just ordered an inexpensive watt meter.

At this point there is no way to check the voltages, as the amp is blowing fuses with no tubes installed. I am thinking that I should install another pt, new tubes, and maybe (another) new can capacitor and even a new output transformer, and then carefully check all voltages. After my previous fix, I was certain that all was well when I fired up the amp (first through a light bulb) and everything worked like a champ. Next time I will check everything.

obviously something is WAY wrong if you are blowing fuses even with the tubes removed.
But since you've got two identical amps, you should be looking to find any difference between the two.

For sure it is way wrong as the pt is blown, but my problem is figuring out what is causing this (for the second time). After my rebuild the amp played fine for a week before the (new) pt transformer failed with a shorted secondary. Then, I replaced the pt with another new one and again the amp played fine for over a week before it again failed with a shorted primary (< 4 ohms across the windings). When I compare resistance reading to my working amp with old (original?) transformers everything matches. The leads on the output transformer are pretty bad, and it may be that it was a big mistake not to replace that as well as the power transformer. I am also regretting not getting new matching EL34s, as I understand that bad tubes can cause a pt failure? Finally, I am also thinking about bad power and have ordered a 1KVA variac with isolation.

Any thoughts or suggestions?

you possibly have a bad output transformer, maybe with an internal short that is intermittent
if you add individual cathode resistors on the EL34 then you can check the bias on each one rather than the pair.
Disconnect the link between the two output tubes (pins 1 & 8 on each tube are connected, and then connected to the other tube, so break the connection between the two tubes)
then replace the 13.5 ohm resistor with a 10 ohm resistor on each tube to ground. You will be able to connect a meter across the 10 ohm resistor and should read between 0.40 and 0.50v. (the original dynaco setting would result in 0.57v on each tube, which is a little high). The reading on each tube should be within 5% of each other, if not, get a matched pair of tubes.

If you can keep your hand on the transformer for more than five seconds--it's not running very hot in my experience. If you have a Harbor Freight store nearby I suggest you buy one of their IR thermometers--they go on sale for about $10. I use it to scan the components on boards being debugged and heat sink temperatures. I found a faulty bias pot this way.

I bought one of the last Mark IVs that Sound Valves sold from a guy who bought late 70s, early 80s and had the same thing happen with both units after I built them. I dont remember who hot the PA-135s got but wondered if I got a bad batch of these from the end of the production cycle. I subsequently bought a set of Hammonds which look to match the specs of the PA-135s very closely so am going to try and sub those and see if it works. Keep you posted.

Scott

Interesting. One would think that there would be a good way to protect the pt in addition to the 2A line fuse?!

The same thing happened to me. The problem was that the filter cap was bad.
Good Luck.

and it is working well. New pt, can cap (this one rated for 600V), output transformer, and rectifier diode mod. I also did the rectifier diode mod on the original, plus I put new tubes in both of them and biased them both at 1.08vdc. I am using a variac to supply around 111vac to both amps, with a kill-a-watt indicating that the new one is drawing a steady .71 amps, and the old one .75 amps. Neither pt is getting very hot, with the new one getting much less hot than the last one that blew. Playing them for many hours a day going on five days, and so far so good - beautiful sounding amps:

http://woosfamily.net/dennis/plane/