• 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.

ST35 voltages WAY too high!

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Hey I just got my scratch built ST35 up and running. I've got a PA774 repro power tranny and hammond 1650F's on the outputs.
I am using the stock power supply with 2x1n4007 and a little more capacitance. 100uf-50ohm-100uf-6.8k-22uf.

At the plates, I am reading 440v! My bias voltage is also 16v. :hot: I've heard of voltages being a little high in these things, but only on the order of 380v or so. What should I do? Also, the power transformer buzzes pretty nastily.. I'm thinking I might have had less problems with a freakin hammond :( I even tried tightening up the laminations but it didn't help too much.

There is also an audible buzz through the speakers when it is on...it is definatly power line induced because as soon as you unplug the amp it stops.

Anyways, anyone else find these voltages pretty nasty for an EL84? I understand I could go with 7189's, but I'd prefer to use my quad of mullard's for now..
Im pretty sure the heaters are around 7v , a lot of my amps do that if they have low resistance windings ( IE arent loaded easily.) I will measure tonight to see. My line voltage is always around 123-124v which makes working with 117v transformers no fun :( . I got my pa 774 from triode electronics..its a new transformer and I ASKED AROUND and everyones opinion was that it was redesigned for modern primary voltages...APPARENTLY not. I have had this transformer for about a year ( slow project) so can't send it back :whazzat:

That's pretty high for mains. I guess the question is how to get it down then. You could adjust R34 and R35 if you're not using a choke. If you are using a choke you could implant a resistor inline to possibly lower voltage, or more dramatically, use a choke which greater resistance. It looks like you could adjust the heater voltage as well by increasing the value of R36 and R37. I'm just guessing, so you may want to confirm this.

Alternatively you could do something like what I did myself. I have a 115v mains power transformer, but my mains was at about 119 which had the voltage up a bit high. So I brought it down by dropping in a 1A isoloation transformer which when loaded caused the mains to sag down to 115V. Probably not very graceful, but it was cheap and I can pull it out of the circuit during the summer when the mains comes down to 117V.
our line voltage has been 121-122v for as long as I can remember, but more recently it has been hovering around 123-124v. I built a bucking reducer with taps for 110v and 117v ( cool trick with a 12.6v CT transformer, 6.3v winding acts for the 117 and full 12.6 is for 110.)

I kind of want to avoid HAVING to use such items so it would be nice if I could just get the apropriate voltage from the xfrmr. As it sits now however, I am thinking I am going to be eating a transformer and getting something more like 240-0-240 from hammond's 370 line. Their 270FX (275-0-275) gives me 405v on 120 mains, but drops to 360-380ish on 117v power. FX does not have the current capability.. I don't think...does it?):confused:

I am seriously considering El-cheapo at this point, especially if it is sonically superior. I can get by with 10-15 UL watts easily.
ThSpeakerDude88 said:
our line voltage has been 121-122v for as long as I can remember, but more recently it has been hovering around 123-124v...

I've measured my line voltage several times (dozens) over the past two years and the lowest it has ever been is 123V. I'm using a true RMS meter. The highest I've seen has been slightly over 125V and the average seems to be 124. I'm in the far west 'burbs of Chicago. Your numbers seem to match pretty well with what I've seen.
Im in indy, so its run from the ancient IPL plant. I think we really need a thread of line voltages, because these seem to be causing a lot of trouble with our vintage AND new gear since most companies seem to only want to produce transformers for 117v operation. This causes them to be noisy, run hot, and put out too much voltage for our poor amps to handle. You wouldn't think 5-8 volts would make that much of a difference but it does.

So does anyone know where I can get a NEW PA774 that really is rated for 120v operation??
123 ish is the typical line voltage around here.

I use a variac on some of my older ham equipment to set the correct line voltage (this is also helpful for service as you can bring devices up slowly), but, as noted above, a small bucking transformer is another easy and effective way to reduce line input voltage.

There should be lots of diagrams online as to how to wire the buck; they are pretty commonly utilized with old equipment.

I was planning to use an Edcor XPWR104 power transformer for my Diytube ST35. You can ask them if they have something similar with 120V primaries. They do custom work, but there may be an extra fee.

Send me an e-mail if you're interested in selling your PA774.

With regard to El-Cheapo, try holding out for Tubelab.com's latest creation. I've got a 35 6aq5's and I'm holding out to see what will happen there before I choose a circuit.
Wow, that is high. I would unhook the PA774 and double check the voltages before rectification to verify that it is not the problem. If it checks out OK then I would double check wiring, maybe you have a bad center tap ground. Then again it could be a bad winding all together. Either way a 60v difference is a LOT! One other thing is to verify you meter is correct.
I'm pretty sure my meter is correct, its a medium priced radio shack digital meter, better than most cheapies at least. I have two other cheaper meaters but they give me around the same results, give or take a volt or two. I am going to disconnect the transformer today and check its winding resistance.
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