Hoffman Princeton Reverb tremolo issue

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Hi all,

I'm helping a friend who just built a Hoffman board Princeton Reverb. The amp is more or less playing OK except there's no tremolo. I don't have this circuit on-hand and am trying to guide him troubleshooting over the Internet. We're not getting far. I have a question:
First off, the bias circuit is supplying steady negative voltage to the 6V6 grids, currently around -37. So no tremolo oscillation, no surprise. I had him do a bunch of things, check voltages on cathode and plate, etc, apart from swapping out V4 tube and checking to make sure wire feeding foot jack was not shorted to ground (it is not).
One thing I haven't considered until a few minutes ago is a problem he had early early in startup. To start up, I had him adjust bias to max negative, so the tubes would be running cool as possible when he turned it on for first time. He got crazy high numbers of negative voltage out of the bias diode (about -400) and we traced that back to a bad ground connection on his 47uf bias cap. Once he established ground there, all was good and the bias pot range is about -25 to -45, where it should be. I had him replace that bias filter cap as it saw a lot more voltage than rated. Dropped in tubes, started it up, and playing, apart from tremolo. The amp is currently dissipating around 65 percent, but we had it down as low as 55 percent, still no tremolo.
To my question:
I am wondering if that initial extreme voltage through the bias circuit into the tremolo circuit may have fried out the 22uf/50V cap in the tremolo circuit? I wouldn't think so because there's a .1 cap between it and the bias circuit, so that should have stopped the DC from hitting it. It's hard to adequately trace out the circuit as I'm going on photos and his answers to my questions, and do not have the amp in front of me. I don't want to keep spitting a million suggestions at him and sending him off on red herrings which will confuse things more. But in folks' opinion here, could that initial high negative voltage have hurt things downstream in the tremolo circuit?

Thanks!

[sluckey]

But in folks' opinion here, could that initial high negative voltage have hurt things downstream in the tremolo circuit?

no

[Diverted]

Didn't think so, but wanted to confirm. Back to tracing. Thanks!

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Thanks again Sluckey.
We still have not been able to get to the bottom of the lack of tremolo. He's getting no oscillating negative voltage out of the intensity pot. I have helped him trace out the circuit five or six times, check for issues including the circuit being grounded at the switch or elsewhere (it's not, according to what he tells me) and even pulled the board to see if he had an under-board connection miswired. He doesn't. He can't find anything amiss.

So this afternoon I'm going to go over as much as possible with him on the phone. I'll be using a Princeton Reverb clone I just built for another friend a few months ago, that is operating properly. We are going to try to go over every voltage and resistance in my tremolo circuit and see if we can find places where his numbers do not more or less line up.

When I get these numbers I really could use some help interpreting some of them, if I do find inconsistencies. Would it be OK to post them here and get some interpretation help? I'd appreciate it!

Before we talk, I am asking him again to check DC voltage out of rectifier, output tube plate and cathode voltages, negative bias voltage out of intensity pot with pot turned up and also turned down, voltages on V4A pins 1 and 3, as well as resistances to ground for all pins on V4A.
Then we will probably go through every part of the circuit and test DC, AC voltages and resistances at every point.

Anything else I should be asking for?

Thank you for any help you can provide!

[sluckey]

The DC voltage on V4 pin 1 will be fluctuating wildly if the oscillator is working. This just appears as random numbers on a digital meter.

Steady plate voltage indicates the oscillator is not working. If this is the case, then make sure the cathode cap is not installed backwards and the cathode resistance is 3.3K. Insure the grid resistance is 1M. Measure these resistances with one probe directly on the socket and the other probe connected to chassis. Insure every resistor in the V4A circuit is the right value. Insure every cap is the right value. Insure there is no wiring error.

If you find the oscillator is working, then check the wiring and components between V4-1 and the intensity pot. Also try changing the bias pot to see if you have trem.

If still no joy, have your friend join the forum and reply to this thread. We need hi-rez pics. Maybe some more eyes can spot something. Have you actually seen any pics of this amp?

[Diverted]

I have, but they're not great. I'll try to get some better pix.
Last I checked his plates seem to be fluctuating properly, but I'll confirm all this this afternoon. We did find an issue in which the bypass cap off the cathode was shorted to ground (pin 3 shorting out). I had him replace that cap to be safe and we started again. Still no dice.
Also had him confirm bypass resistor was good; it read around 3.4K so is fine.
We're going to go through it and I'll look to see if there are any wild discrepancies.
Thank you for the help. I'll tell him to post something for you folks here. I appreciate it,

Ted

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OK, not much further along but possibly a few clues.
I went through all the numbers. It appears that the oscillator is functioning properly.
The power tubes are biased at about 56 percent, with around 390 volts on the plates and around 17ma of current each.
We did this:
Checked all connections, all appear correct. Double checked all resistors. Ran resistances from grid pin 2 (1.05m) and cathode pin 3 (3.3K) on V4a. They're correct.
The plate voltage bounces around quite a bit on both plates, and at the wiper of the intensity pot, with intensity turned up. With it turned down it's more steady. So oscillator appears to be workign.
One clue he gave me was that he can hear a faint tremolo pulsing when powering down the amp.
I thought this might be an issue of something eating up the oscillation (a short to ground?) so I had him read for continuity at every junction in the amp. We found no shorts to ground anywhere, identical to the amp I have with me on the bench for reference.

At this point, it appears that the circuit is wired correctly. Still, no love. He does not have another bias pot to toss in, but it appears to be working correctly as it properly changes the negative voltage headed to the power tubes when you turn it up or down.

Given that he can hear a faint pulsing when amp powers down and caps discharge, any recommendations? I had him replace the 1m cap (R63) between intensity pot and V4a plate with a lower value (470K). He noticed no significant change.

I am going to have him email some better resolution/clarity photos.

Thank you for the help!

I have lots of more numbers we took while going over the circuit, if there is anything you don't see here. I can probably provide it.

Ted

[sluckey]

Crank the bias pot until you have 25mV on the cathodes of the 6V6s. Got trem now?

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Thanks on that tip. Unfortunately the report is no change with bias cranked up to around 69-70 percent dissipation.
I am going to get a big batch of photos in the morning and pore over them to see if I can spot something. Definitely a stumper at this point.

[Diverted]

My friend found the issue after we concentrated on the tremolo circuit endlessly: 22K feedback resistor wasn't attached to speaker signal lead, so no feedback loop. Glad he fixed it, but surprised that was it. The Princeton I was using to help him go through his amp has a feedback on/off switch, and tremolo works fine regardless. Thanks for the help.

[Willabe]

My friend found the issue after we concentrated on the tremolo circuit endlessly: 22K feedback resistor wasn't attached to speaker signal lead, so no feedback loop. Glad he fixed it, but surprised that was it. The Princeton I was using to help him go through his amp has a feedback on/off switch, and tremolo works fine regardless. Thanks for the help.

There's no 22K in the -FB loop, it's a 2.7K. There is a 22K in the -bias circuit. 

And the -FB loop has nothing to do with the trem circuit. That's why your amp works with or without the -FB loop.

[sluckey]

I suspect your work is not done yet.   

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Thanks everyone.
I know, the last post sounded a bit off. That's the info I was given by my friend, with no photos to sort things out.
I told him I thought it was odd that putting the feedback loop in was killing the tremolo because they're not corrected. My guess is he inadvertently made some other change while noticing/fixing this one. Anyway, he's happy with it, amp's playing as it should now and voltages check out as far as I'm told. So all's well that ends well.

Thanks for the help.