New Build, voltages very high.

[mumford]

Finished building my first design the other night, thanks for the help on the schematic. It has SS rectifier, 2 gain stages, a DC coupled cathode follower, FMV tone stack into a 5e3 power section, with octal tubes in the preamp. I'm using a mag components tweed deluxe power transformer.

Firing it up using a light bulb limiter and no tubes resulted in voltages about 200vdc higher than I expected. Is that normal for no load? I don't get what could make the voltages that high.

V1 plate 426
V2 (DC coupled Cathode Follower) plate 515, grid 220
V3 plate 473
PI (cathodyne) plate 496, grid 46
Both 6V6s screens 537, plates 542
OT B+ 542

Power supply nodes:
post rectifier, pre sag resistor 544
node 1 534 (expected 360)
node 2 533 (expected 320)
node 3 512 (expected 200)
node 4 510 (expected just under 200)

I am not able to measure any resistance between the OT primaries and secondaries, I am assuming it is bad (it was salvaged, and I didn't test it.)

The heaters are all 3.2vac.

Any idea why the DC voltages are so high?

[David Root]

Are you using a bridge rectifier (4 diodes) with the PT center tap NOT connected to ground??

If not, then you are 60V high just without the 5Y3GT tube rectifier. That would take your B+1 down to about 475 without the other tubes in. What is the AC spec on the power transformer high voltage secondary? Without tubes in the voltages should be significantly higher than the schematic.

Did you measure "no resistance" between the two ends of the OT primary and separately between the two ends of the secondary. What t=do you mean by "no resistance"? 0.00 ohms or infinite ohms? This is not clear.

[Phil_S]

First, the bulb limiter is meant to protect your build in the event of a short. If the bulb is still dim, you are done using it. You will never get good readings as long as you have a bulb in series with the a/c line feed.

Second, your voltage readings will sink like a stone when you put in the tubes.

Third, you are quite literally playing with fire if any of those voltages are above the voltage rating for the caps. On that basis alone, I'd put in the tube rectifier and see where it goes, and if readings are still high, I would not allow the circuit to stay hot for longer than what's needed to get the voltage readings.

If, once you get a reading with the rectifier in place and all seems reasonable, put the power tubes in. Once you have the load of the power tubes, provided the PT rating is appropriate to your build, I think you will see voltages come much closer to expectation.

[mumford]

Are you using a bridge rectifier (4 diodes) with the PT center tap NOT connected to ground??

No, I’m using two 1N4007s on each secondary with the CT grounded.

What is the AC spec on the power transformer high voltage secondary? Without tubes in the voltages should be significantly higher than the schematic.

335v a side.

Did you measure "no resistance" between the two ends of the OT primary and separately between the two ends of the secondary. What t=do you mean by "no resistance"? 0.00 ohms or infinite ohms? This is not clear.

I measured between the primaries, and they measured 120 ohms and 142 ohms, respectively from the B+ to the 6V6 plates. I tried measuring from the B+ and 6V6 plates to the speaker outs and got infinite ohms.

Once you have the load of the power tubes, provided the PT rating is appropriate to your build, I think you will see voltages come much closer to expectation.

In the Duncan PSU everything worked out ok, voltage and current-wise.

[Phil_S]

I tried measuring from the B+ and 6V6 plates to the speaker outs and got infinite ohms.

I dunno what you are doing, but I think you are trying to see if the primary and the secondary are connected to each other somehow? The speaker out is the secondary. It has no connection at all to the 6V6 plates -- that's the primary. I think infinite on your meter confirms they are not connected, as it should be. If it were 0 or nearly 0, that indicates a short, which means your OT is toast.

[mumford]

Thanks, Phil. OT might be ok after all.

What led me to believe I should be seeing something was this sentence from the GEOfex Amp Debugging Page: "Finally, test for the resistance from primary to secondary. A low value, under a few K indicates an internal short, and a dead transformer." I was reading this to mean that I should see something over a few k.

[mumford]

With tubes installed, the light bulb limiter dims for maybe 3-4 seconds, then glows brightly. Would a bad OT cause the light bulb limiter to light up brightly? (With no tubes, it stays dim.) Using the "tap test" from the Jack Darr book, I get no sound at all, including when I tap directly on the OT primaries. (I tapped lightly on all the plates and the 6V6 screens with a long screwdriver.)

I've checked my wiring and it matches my schem, grounds have continuity, etc. I can see the heaters going in the 6V6s, although none of the tubes get too warm (I'm assuming because of the limiter). I have voltages where I should without tubes, and none where I shouldn't. (I haven't tested voltages with the tubes in and limiter plugged in.)

[David Root]

Take the bulb limiter out, put all the tubes in and then check it out. It sounds like there is nothing wrong, except you have a higher voltage PT than the circuit wants because you are using an SS rectifier instead of a 5Y3GT.

With a 335-0-335 PT and SS rectifier, at idle you should see about 410-420V B+1 at the rectifier DC out. A classic 5E3 is looking for about 365V. That's the difference the 5Y3GT rectifier makes.

If that doesn't happen, check your wiring on that DC coupled CF.

[ToneMerc]

Take the bulb limiter out, put all the tubes in and then check it out. It sounds like there is nothing wrong, except you have a higher voltage PT than the circuit wants because you are using an SS rectifier instead of a 5Y3GT.

I agree and that PT can't be a 355V secondary either. Something is still not right, there's 542V B+ under load and I bet it's at least 560V unloaded. Which makes that PT secondary around 425V@120, those documentated voltages don't lie.

TM

[mumford]

I fired it up with tubes, and the voltages were where I expected them, more or less. Still no signal, though. I need to dig out the other dual-6V6 OT I have and try that. Seems like it has to be that or my PI tube is bad (I only have one on hand--6SL7).

[ToneMerc]

which PT did you use?

[Structo]

In my experience, the bulb limiter should glow brightly for a second as the caps charge then go to a low glow.
If it stays bright that means there is excessive current flowing.

It also depends what wattage light bulb you use.

On 120vac mains, a 25w light bulb limits the current to just under 250ma.

I generally fire up an amp with a 25w bulb with no tubes, then a 60w with tubes.
If that checks out then I plug the amp into the wall.

[mumford]

I am using a Magnetic Components 40-18021, their basic 5e3 PT.

I should probably update the thread title, since the voltages appear to be ok with the tubes in.

[drz400]

You need to put some tubes in
I should have read your last post first LOL

[mumford]

My first problem was that my heaters were pulling 2.1 amps, and the PT is rated for 1.65. I wired the first two preamp tubes to the 5v line just to test things, and they are all heating now. I bought a 12v radio shack transformer that I will run in series for those two tubes.

The OT is bad. I built an audio probe and ran my looper into it. Signal everywhere there should be, going in to the OT. Luckily a mag components OT is only about $30.

Honestly, I'm feeling great about things right now. Voltages are good, sound is getting where it should be. I thought the OT might be bad from the start, so it's not a huge bummer. My first design actually works!