Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Speakz on December 15, 2016, 11:08:17 am
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Wondering if anyone can point me toward directions to remove tremolo and switch to long tail PI on Hoffman Princeton Clone?
Thanks for any help you can give.
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Look at a Princeton Reverb preamp, tone stack and reverb section married to a Deluxe Reverb LTPI and power section minus the tremolo.
In other words, Princeton Reverb up to the phase invertor & at the phase invertor, Deluxe Reverb on ..........
With respect, Tubenit
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Do you want to modify an existing amp? Or are you just planning to build one with these changes?
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Thank you Tubenit I am looking at that and attempting to break it down:
1. In term of the PI I am following from the .01uf cap (to the left of .022) to where the wire would "normally" go to PIN 1 of the the speed pot and bringing that to the 220k PI resistor (to the right of the other 220k)?
2. Then I would begin modifying the configuration to be the same as the AB763 single channel Wiring V4 of the PR in similar fashion to V5 of the Ab763?
Sluckey: I am modifying my existing PR clone. Which is stock to the hoffman board except for a 25w Deluxe OT. I'm looking for more of the AB763 type tone and also will add a mid pot in place of the tremolo pot.
Thank you both
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Sluckey: I am modifying my existing PR clone. Which is stock to the hoffman board except for a 25w Deluxe OT. I'm looking for more of the AB763 type tone and also will add a mid pot in place of the tremolo pot.
This is a modified Hoffman PR board. The trem has been removed and the ab763 LTP PI has been put in. If you used the Hoffman board then this should be straightforward. I used the existing turrets so you don't have to add any. I didn't show the MID pot since that doesn't affect the board.
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Steve,
That was a remarkably thoughtful and gracious gift putting that together! You have contributed so much to this forum and it's members (including me). I am truly grateful for all your help to others and myself. THANKS!
Very nicely done. Your layout drawings are stellar.
With respect, Jeff
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Sluckey and Tubennit Thank you both! :worthy1:
This is extremely helpful and I can learn a bit more by comparing layouts to see the individual stages . I will check in again later once I get a chance to look over more thoroughly.
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....... I can learn a bit more by comparing layouts to see the individual stages .
Layouts are for finding where the parts are in the chassis, the circuit schematic is what you study/compare to see the individual stages.
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Sluckey: I am modifying my existing PR clone. Which is stock to the hoffman board except for a 25w Deluxe OT. I'm looking for more of the AB763 type tone and also will add a mid pot in place of the tremolo pot.
This is a modified Hoffman PR board. The trem has been removed and the ab763 LTP PI has been put in. If you used the Hoffman board then this should be straightforward. I used the existing turrets so you don't have to add any. I didn't show the MID pot since that doesn't affect the board.
Sluckey,
That was really cool to modify the layout quickly. I have read that the latest version of Visio is real buggy. What version are you using? That is a nice skill to have.
I was considering building the PR no Trem. I don't know what this tonal difference between the ab763 and the PR would be, so I'll start with the Hoffman PR.
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I've tried to draw an editable SCH schematic to match Sluckey's excellent VISIO layout conversion. Hopefully, this matches well but double check the schematic with the layout.
Steve, if you see this and find an error, feel free to edit this post with a corrected schematic.
Comparing this schematic with a Deluxe Reverb, I am thinking this will have considerably more gain since the cap going into the LTPI is .02 compared to .001 on the DR. And there is not a 220k dropping resistor to ground either prior to the LTPI as on Sluckey's one channel DR.
Speakz, if you have not seen Sluckey's excellent Visio schematic and layout of his one channel Deluxe Reverb without vibrato, please check out his website. Everything he does with amps is just superb!
With respect, Tubenit
EDIT by sluckey... revised schematic
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I'm using Visio 2010. Doug used Visio 2007 to create the original PR layout.
I would expect the PT and AB763 Deluxe Reverb to sound very similar. The only circuit difference is the phase inverter. Of course the DR has bigger iron which IMO will have a bigger tonal effect than the differences in phase inverters.
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Jeff, your schematic looks fine. I also suspect the gain may be higher than a typical AB763 and may need to be tamed with a simple voltage divider just prior to the PI input. And the value of those two 18K resistors for nodes C and D may need tweaking a bit too.
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Thanks Sluckey! I appreciate the extra eyes on it. I'm thinking the voltage divider and B+ resistors tweaked will be needed also.
Jeff
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Here's a revised pdf with layout and schematic. Tremolo removed, Mid and Master added, and Cathodyne PI replaced with LTP PI. The Master control will tame any extra gain or allow the preamp to overdrive the power amp. The Mid and Master pots also fill up the front panel holes vacated when the tremolo was removed.
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> * Princeton Deluxe Reverb no vibrato.gif
100K on V3a drawn wrong?
> more gain since the cap going into the LTPI is .02 compared to .001
Coupling caps don't "add gain". They cut bass. 0.02u will cut less bass. If you innocently calculate a 1Meg load, the 0.001u looks short-bass (170hz). In fact that node of the LTP is boot-strapped to much more than 1meg loading. The 0.001u is good below 90Hz. An exact calculation is tricky, best to just try different caps after you pick your *speaker*. (Small or extra-large speakers may need some bass-cut to reduce slap or boom.)
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Thanks PRR, I corrected schematic. With respect, Tubenit