Runaway plate current in a new Princeton Reverb build

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ToobAmps
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Runaway plate current in a new Princeton Reverb build

Post by ToobAmps »

Hey all,
Long time lurker, but never had to post before. Heck, I learned so much from this site that I always had my questions answered. But this issue has me stumped.
I recently completed an AA1164 Princeton Reverb clone. Everything tested fine at start up. Plate voltages are within spec (450 VDC on both), Negative bias voltage is good (-35.5 VDC). Pulling about 15mA on the inside power tube and a little closer to 18 mA on the outside power tube. Voltages down the ral are within 2 VDC of spec.
So far, so good. But...
I always monitor plate current as I start my new builds up, and it was a good thing I did in this case. After about 10 minutes, I heat an audible crack/snap sound through the speaker, then the bias voltage dives to anywhere from -15 VDC to -17 VDC (and of course, the plate current goes through the roof). As soon as I noticed I powered the amp down.
I've tried new tubes, different tubes, and re-flowed all the solder connections, checked for loose wires, connections, etc. Problem still there,
It seems as though a component gets warm and then freaks out.
So, to check this hypothesis, I dug out all of my multimeters (4 in total) and placed them at various locations:
1 at each bias voltage on each tube (pin 5)
1 at the bias point coming from the tremolo intensity pot (prior to the 220K resistors)
1 at the diode on the bias board (on the negative side)
And fired up the amp again.
When the bias voltage took a dive (or, more properly rose?), the negative voltages were different between the diode (-37 VDC) and the wiper of the tremolo intensity pot (-17 VDC). After powering down the amp, I noticed a loose-ish ground connection on the tremolo intensity pot. I use a bus ground soldered to the back of the pots...I know, that could open a can of worms on the topic of grounding schemes, but...it's pretty darn quiet until it nosedives.
So, do I need to change the pot?
I'm going to have to order the pot anyway, so before I over think this issue (and drive myself nuts yanking and replacing components), I'm going to ask the forum.
Thanks in advance!!
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martin manning
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Re: Runaway plate current in a new Princeton Reverb build

Post by martin manning »

You might have damaged the intensity pot by soldering to it. Maybe try tacking in a couple of 120k resistors in place of the pot and see if it's stable.
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pompeiisneaks
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Re: Runaway plate current in a new Princeton Reverb build

Post by pompeiisneaks »

Yeah what martin says makes sense to me, I've had brand new alpha pots go bad on me due to me putting too much heat on the when soldering for too long (It was like my 3rd build and I'd done something wrong with the wiring and had to resdolder/desolder as well as attempting poorly to solder to the back of the pot without roughing it up etc) They can go if they get a bit to beat up in the hook up process.

~Phil
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ToobAmps
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Re: Runaway plate current in a new Princeton Reverb build

Post by ToobAmps »

Thanks for the tip Martin and Pompeii!
It will give me something to do instead of over thinking and screwing something else up in the process.
So, if i am understanding this correctly, basically tack two 120K resistors in series, solder the bias lead to one end, join to the board at their junction, and solder the last wire to the other side of the resistors in series?
I'll give this a go and report back.
Thanks!
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martin manning
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Re: Runaway plate current in a new Princeton Reverb build

Post by martin manning »

Yes, basically making a fixed value divider, like the pot set at “noon”.
ToobAmps
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Re: Runaway plate current in a new Princeton Reverb build

Post by ToobAmps »

Well, no dice.
I noticed that the inside tube's bias voltage dove about -7 VDC more than the outside tube (-17 VDC versus -24 VDC) while the -37 VDC was constant coming from the diode.. I'm going to see if it follows the tube or the socket. I suspect it follows the socket. But, maybe the tube smoked...
Nevertheless, I'm stumped...I thought for sure it was either the pot or the diode.
Dang.
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martin manning
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Re: Runaway plate current in a new Princeton Reverb build

Post by martin manning »

Perhaps a leaky coupling cap on the hot side?
ToobAmps
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Re: Runaway plate current in a new Princeton Reverb build

Post by ToobAmps »

So I installed a brand new set of matched 6V6 tubes and powered up. The plate current went to 30mA on each and I powered down immediately. I must have smoked one or both sets of the test tubes.
I'm going to install an adjustable grid bias in it. I'll need to order a 10K pot.
I just don't want to risk anymore tubes.
After that's installed I'll report back. And if it is still an issue pursue other possible remedies.
Thanks for all of your input Martin!
tubeswell
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Re: Runaway plate current in a new Princeton Reverb build

Post by tubeswell »

What Martin said - test the coupling caps for DC leakage - and test the grid pins (Pin#5) on each of the output tube sockets to verify you have stable negative VDC (within the ballpark desired range)- before you go putting any more output tubes in.
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Re: Runaway plate current in a new Princeton Reverb build

Post by Stevem »

If you do not have a stable bias voltage of atleast -17 on pin 5 of each socket then do not install a tube!
A issue like this is also a very good reason to put in place a in line 100ma fast blow fuse in the center tap of the OT so you don't fry it!
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Not screaming like the passengers in his car!😊

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ToobAmps
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Re: Runaway plate current in a new Princeton Reverb build

Post by ToobAmps »

Thanks for all of the input guys.
I lifted each of the coupling caps and measured no leakage. Set it aside and waited for the 10K pot to arrive. I installed the adjustable bias mod and now it is rock solid stable. Left the amp on for over a half hour and still solid. It must have been a cold solder joint that I missed somewhere (like three times)?
I don't know, but it seems to be fine now. Sounds great, and still does after "putting it through the paces."
Nevertheless, +1 on the 100mA fuse! Doesn't hurt, and better safe than sorry.

But I do have another question if you gents would be so kind as to entertain it:
The customer wants to use 6L6GCs in the amp. I used a PT that has 120mA on the HV secondary and an OT with 6.6K primary impedance. The amp sounds pretty good at about 40mA PC with 420 VDC on the plates for about 17 watts PD. Is it safe to run (for gigs) at this level or should I tell him we'll need a higher mA rating on the PT to be safe?
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pompeiisneaks
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Re: Runaway plate current in a new Princeton Reverb build

Post by pompeiisneaks »

6k 'should' work for 6L6's, but I'm guessing that's an extra because the normal 6V6 princetons want more like 8-10kk. also though the heaters go up to .9 amps from .45 for 6V6 so make sure the heater winding has the current handling on the PT. You'll also need to adjust the bias so it can give different negative voltages for the tubes. plate current on the 6V6 is usually 34ish mA but the plate on 6L6 is 116 (on the datasheets I reviewed, hopefully I didn't make a mistake).

Thus you'll need four things 'right' for the 6L6's instead of using 6V6:
1. correctly tuned bias settings
2. enough heater current
3. enough b+ current.
4. the right impedance

Sounds like you've addressed 3 and 4, but not clear on 1 and 2.

~Phil
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tubeswell
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Re: Runaway plate current in a new Princeton Reverb build

Post by tubeswell »

ToobAmps wrote: Sun Feb 25, 2018 5:57 pm ...
The customer wants to use 6L6GCs in the amp. I used a PT that has 120mA on the HV secondary and an OT with 6.6K primary impedance. The amp sounds pretty good at about 40mA PC with 420 VDC on the plates for about 17 watts PD. Is it safe to run (for gigs) at this level or should I tell him we'll need a higher mA rating on the PT to be safe?
To run 2 x 6L6s, you want a PT with at least 150mA (preferably 180mA or 200mA) on the HT winding, as well as allowing 0.9A per tube filament on the heater winding. But the OT also needs to be power-rated for 6L6s - so thats anything from about 35W upwards. If you put 6L6s through a Deluxe Reverb OT, they will burn it out more quickly.
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ToobAmps
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Re: Runaway plate current in a new Princeton Reverb build

Post by ToobAmps »

That's what I was afraid of: needing more HV on the PT secondary. The PT I'm using is the one found at this link: https://www.tubesandmore.com/products/t ... 0-v-120-ma
It's rated for 660 V CT @ 120mA; 6.3 V @ 3A; and, 5 V @ 3A. I think that's enough heater current...

The OT I'm using is the Hammond 1760H, found here: http://www.hammondmfg.com/pdf/EDB1760H.pdf
It's only rated for 20W, 6.6K primary impedance...it sounds as though this might be a problem?

If I biased the tubes on the cooler side, would that help? While waiting for parts, could it be played without too much fear?
Thanks a million fellas!
ToobAmps
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Re: Runaway plate current in a new Princeton Reverb build

Post by ToobAmps »

ToobAmps wrote: Sun Feb 25, 2018 6:55 pm That's what I was afraid of: needing more HV on the PT secondary. The PT I'm using is the one found at this link: https://www.tubesandmore.com/products/t ... 0-v-120-ma
It's rated for 660 V CT @ 120mA; 6.3 V @ 3A; and, 5 V @ 3A. I think that's enough heater current...

The OT I'm using is the Hammond 1760H, found here: http://www.hammondmfg.com/pdf/EDB1760H.pdf
It's only rated for 20W, 6.6K primary impedance...it sounds as though this might be a problem?

If I biased the tubes on the cooler side, would that help? While waiting for parts, could it be played without too much fear?
Thanks a million fellas!
I meant HV current...
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