Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on December 15, 2010, 12:03:05 pm
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I just built a cathode biased Princeton reverb and the trem will not work.The rest of the amp sounds great but the trem is DOA.
I replaced the caps just for good measure and the voltage is fluctuating nicely,meaning the tube is oscillating.I have built many princeton circuits but this one is the only one I have cathode biased.
It is running hot for current at 16 watts as I haven't fine tuned the cathode resistor yet.I'm wondering if that has something to do with it?The fixed bias versions don't like to work if they are biased too hot.
Any insight would be welcomed.
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Put a big ole resistor under the cathodes and it will probably start working. Then fine tune it. The trem will be affected by the bias regardless of whether its fixed or cathode biased.
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Huge resistor,still no trem. Hmm...are you sure this should work without fixed bias?
This is the first cathode biased PR I've done.I just grounded the left leg of the intensity pot and added a R/C at the tube cathodes.
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Just a thought. Try it without the cathode capacitor.
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Interesting thought,but what possible difference would that make?
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I was thinking that just like on power supply the capacitor evens the wave.
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I just grounded the left leg of the intensity pot and added a R/C at the tube cathodes.
Should work. Just keep diddling. Here are a couple circuits that are about the same as what you're doing...
http://www.el34world.com/charts/Schematics/files/gibson/GA-16T.pdf
http://www.el34world.com/charts/Schematics/files/gibson/GA-18T.pdf
http://www.el34world.com/charts/Schematics/files/gibson/GA-19RVT.pdf
And I've seen a 6L6 cathode biased amp using this same circuit, but don't recall which amp.
Removing the cathode cap should reduce the bias and maybe allow the trem to work.
Are you sure the oscillator is working? Do you have a scope or an analog meter you can monitor the oscillator plate with?
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The plate is fluctuating normally but still no trem.Hmmm...
I see those schematics and by all rights it should be working.Hmmm....
Tunnel vision maybe.Amp sounds great otherwise.Gonna try the PI grid stopper too.
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The plate is fluctuating normally but still no trem.
Good; but then the trem "signal" is getting lost somewhere. I would trace backwards, connection by connection, starting at the power tube grids. A volt meter (analog would be cool) should show the low frequency sweep of the trem's VDC.
Note that if you're feeding trem to 6L6's (instead of 6V6's), as we know you are wont to do, then standard Princeton trem voltage will be anemic. One solution is to reduce the the 1M series resistor feeding the intensity pot to 500K or less.
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This one is 6v6. I'm using a 250k intensity pot.
I have an analog meter and scope.I'll try tracing the signal with the meter and see whhere it gets lost.
Thanks for all the suggestions guys!
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Ok just for fun I hit the min/max function on my digital meter and at the tube grids the voltage fluctuated from -3.8 to +3.8v Now I am not sure what it should be,so that's all I have to go on.
So it's oscillating but not enough.I think a resistor change might be what this needs because of the cathode biasing.Or I could just pop in a fixed bias board and be done with it.Naw,I'm gonna try a resistor first.
The grids are fluctuating and the bias resistor is fluctuating too,obviously not quite enough.Those Gibson amps sure have a nice trem,maybe that's the ticket for this one too.We shall see........ :dontknow:
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Changed the resistor from 300k all the way to 2.2meg and no worky.
I am not going to go as far as changing the circuit to a Gibson,so I'm throwing in the towel and making it fixed bias.
I admit it,I'm a quitter!
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Ok,correct me if I'm wrong here.To get this trem to work and actually 'vary' the bias of the power tubes,it has to have quite a swing of voltages.Now if you look at the PR voltages,the oscillator half of the 12AX7 has 260v on the plate.Now all those Gibson amp have about 100v.That may be the clue I need here,I do want to understand this
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Here's how I re-enabled trem in an el84 Traynor -- originally fixed bias -- which someone else previously modded to cathode bias. The trem works great. I think it should work for 6V6's: http://www.el34world.com/Forum/index.php?topic=6675.0
Note that my 1M intensity pot is tamed by a 1M paralled resistor = 500K. But these are nominal values, I think the actual value was 780K. So your 250K pot may be bleeding your trem voltage to ground. After all, it was getting another dose of fixed bias voltage before the cathode bias conversion.
I agree that your initial trem voltage seems low. Check the power tube cathode voltage @ idle: probably about +16VDC. Shoot to deliver toward 1/2 that in trem voltage @ the power tube grids: maybe -7VDC -to- +7VDC. Per the above link watch for redplating not only at idle but also under strong signal conditions.
Also checkout: http://www.lynx.bc.ca/~jc/tremoloCircuits.html and http://www.freewebs.com/valvewizard1/trem1.html
The question remains, why is your initial trem voltage so low? Many trem tubes are run @ 300 - 400 VDC on the plate, even though this exceeds the tube's rating. Lowering the voltage to 100 may not help.
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So you have a 100k-300kish resistor to ground (from the bottom of the 6V6 grid load resistors) where the trem is connected?
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I decided to just convert it back to fixed bias because I don't want the amp to start looking like a pin cushion.
I will do another one and design the trem around cathode bias like the Gibson amps.
I'm using a Traynor YCV40 power transformer in this amp with a full wave bridge rectifier.It provides 409v on the plates and 385v cathode to plate.The amp sounds awesome and has a Celestion Blue in it.I'm using a Deluxe Reverb OT as well.I'll post some pictures after it's all done.It's in a Tweed covered cabinet with top mounted controls.I just have to shellac it and get the last bit of wiring done so the new owner can pick it up sunday.It'll be sad to see her go;it looks great.