Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on November 06, 2012, 05:30:57 pm
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I was always having trouble getting a good deep trem from a cathode biased Princeton Reverb and Sluckey had suggested just moving the one wire from the old bias supply to ground.It doesn't work well.( no offence to sluckey at all,we are blessed to have him.it just requires more tweaking)
So I revisited it on an amp I had built and simply wired the trem exactly like a Traynor Guitarmate,the cathode biased one before they had reverb.
It works flawlessly and has double the intensity on tap as a normal PR does.No bias issues at all either.
Killer,killer good trem! I do back off the idle bias a bit from most cathode biased amps to be safe with the swings.Wow! Never going back the Fender way,ever!
Edit: my spelling was attrocious!
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Phsyco, can you point us to the schematic you worked off of?? Thanks
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I just built a little Ampeg Jet, 6V6 P/P cat biased with power tube trem. I started with some JJ 6V6s and the trem was not as strong as I'd like. Then I put some NOS Sylvanias in it and the trem came alive. Different tubes made a big difference in this case. Might make a diff in your Princeton too.
http://www.el34world.com/charts/Schematics/files/ampeg/j12b_jet.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/j12b_jet.pdf)
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Traynor YGM2 in Doug's schematic library.Pay special attention to the way the pots are wired and where the 1meg resistors go in the trem circuit.Only 1/2 a 12AX7 used for the oscillator.
I have 375v on the plates of the JJ 6V6's.I'm using a 500 ohm cathode resistor bypassed by a 22uf/150v cap.(i had it on hand).
It sounds and works fabulous.And as an added bonus,when you lift the cathode resistor it has an old school throatiness like an old Supro or National.
The bias still makes a difference for the bias-vary trem but once you get it right it's marvelous!
Sluckey:thanks for pointing me in the right direction in the first place!
One more thing: I went through a couple of PI/oscillator tubes and settled on an EH12AX7 that worked perfectly;yes that tube has to be really strong too!
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... simply wired the trem exactly like a Traynor Guitamate ... It works flawlessly and has double the intensity on tap as a normal PR does.
I've never really looked at this before, but I notice something now.
Look at the Princeton's trem circuit, and the output from the plate. The trem signal goes to a 1MΩ and 0.02uF to ground, with a 0.1uF cap at the junction of these connecting to the Intensity pot.
It looks like the Guitarmate doesn't have the equivalent of the 1M and 0.02uF, but passes the trem output to a 200pF (?) cap and on to the Intensity pot.
The first 1M and 0.02uF looks like it will act as a voltage divider in the Princeton, and at 6Hz, the 0.02uF cap looks like ~1.6MΩ. So the trem signal gets cut by a bit more than half there, even before the Intensity pot cuts it further.
If the cap I'm guessing is 200pF is really 200pF, then it cuts the trem massively in conjunction with the Intensity pot; if it's really 200uF, there is almost no reduction, and that would explain why the trem seems like it's at least double that of the Princeton.
So what value cap did you use leading into the Intensity pot? That might solve the puzzle for everyone.
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Traynor YGM2 in Doug's schematic library
That's wired exactly like I was suggesting for a cathode biased 6V6 amp. Same as the Ampeg J12B Jet.
I have 375v on the plates of the JJ 6V6's.I'm using a 500 ohm cathode resistor bypassed by a 22uf/150v cap.(i had it on hand).
It sounds and works fabulous.And as an added bonus,when you lift the cathode resistor it has an old school throatiness like an old Supro or National.
Hmm, I would have thought that when you lift the cathode resistor it would sound very quiet. :wink:
but passes the trem output to a 200pF (?) cap and on to the Intensity pot.
I see what appears to be a .1µF @ 200V. A .1 makes a lot of sense. 200pF makes no sense. 200V is questionable though. The YGM1 uses a .2µF for that cap.
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I used a .15uf cap I had laying around. There are some interesting differences that are hard to see at first glance.I used a 3meg speed pot instead of the Traynor's 4meg and it works fine.
The speed and depth are beautiful.