• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Toasted PW tranformer?

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Tosted PW tranformer

I finally was able to do a bench test of a Tubelab SimpleSE that I've been working on for quite a while. In any case it sounds very nice! All for 10 minutes then I decided to flip the standby switch. The 5A fuse in my variac blew and likewise the 3A fuse in the amp's chassis. I replaced the fuses and this time I used a killawatt meter in front of the variac to measure current. The Hammond 274BX was buzzing loudly and drawing about 100watts at only 40 volts ac on the variac. I think the resistance on the 274's primaries was less than an ohm I think.

I think I shorted the primary my PW transformer. :(

Aside from making a doorstop out of the 274 is there anything I can do? Repair? Core exchange?
Strange that the transformer started buzzing after you threw the standby switch...

Are you using solid state or tube rectifiers? With the power off and tube rectifier removed, use an ohm meter to check the wiring on the standby circuit and make sure you're not accidentally shorting a secondary to ground.

-- josé k.
Joined 2004
Paid Member
Post a schematic of how you wired your power supply, and double check to make sure you really wired it that way.

Next disconnect everything except the primary and check to see whether or not the transformer is actually bad or you have a shorted component elsewhere..

How about rectification - tube or ss?

Snubber network on secondary of pt, switch or ?

Does your standby switch the transformer CT or something else?

Conventional switches have been known to short to chassis when used on the HV dc side of power supplies - particularly as the secondary winding creates an inductive kick when suddenly unloaded - that energy wants to go somewhere and usually what happens is the voltage rises as the field collapses until it arcs over somewhere that is unless you have made provisions for this issue.
Joined 2004
Paid Member
Note that although the primary winding resistance is relatively low, your measurement does not indicated that the primary is shorted. (DCR is usually low on primaries.) Most likely you have a wiring or design error as I mentioned above. You may get lucky or not.. (Dead short somewhere?)
Thanks everyone. I pulled the rectifier tube out and still the same problem. A lot of current. Then I disconnected the high voltage secondary and tried again. The transformer was pulling about 25 watts and the tube heaters were glowing happily. No buzzing. I'm embarrased I made a quick assumption about the PW tranny.

There appears to be a short somewhere on the PCB. I do have the rectifier diodes on the PCB although I'm not using them. Nothing appears to be charred or melted.

I'll try to get a photo posted soon.
Many folks have the board switchable between solid state and tube rectification with a switch so it shouldn't be a problem. By not shorting the terminals for choosing rectification the board should default to tube rectification.

I'm thinking I should pull the solid state diodes out, but they don't seem to be shorted according to my multimeter. I'll need to take a closer look.

Check out tubelab.com for more details.

I've also got a high-rez link to a photo that should make it easy to see the wiring.
Hi whitelabrat:

Remove the FRED diodes and try the amp again using the tube rectifier. There have been some issues with IXYS diodes blowing when the standby switch is flipped. They often test OK. The amp will work without them. They are only used in the SS rectifier mode, but when one dies it makes the transformer really unhappy. Unless you ran the transformer with a dead doide and no fuse for a long while it will be fine.

You should have a fuse in the circuit between the line input and the power transformer. 2 amp in 120 volt countries, 1 amp in 240 volt world.

I am 1200 miles from home, and missed the start of this thread. Just got DSL installed here, like 10 minutes ago.
I had a very similar problem with my SimpleSE. After removing the solid state diodes, I was still using the standby switch with a JJ 5AR4 and eventually fried the rectifier tube as well. My solution was to remove the solid state diodes and also remove the standby switch. No problems since. Here is a link to a very similar problem with an EL84 PP design. Similar standby switch implementation with similar failures. http://www.diytube.com/phpBB2/viewtopic.php?t=493

Hope you don't have any more problems. Apart from this, my SimpleSE has been great!

I saw the same post for the Diytube ST35 board. I just got one recently and realized that the standby switch issue applies to the SimpleSE as well. I'll be using the ST35 board for a project to salvage an old PP EL84 amp.

I've done away with the switch and the diodes on the SimpleSE and I've had reliable operation for just about a week now.

Also another thing I've discovered. I've gone around and done a sampling of the resistance of the primary side of several power transformers. They all seem to be less than 2 ohms.
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