Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: worth on March 05, 2011, 02:50:43 am

Title: cathode biased Princeton Reverb trem ?
Post by: worth on March 05, 2011, 02:50:43 am

What changes , if any , do I need to make to the trem circuit of a Princeton Revrb , if I change from fixed to cathode bias ?

Title: Re: cathode biased Princeton Reverb trem ?
Post by: tubenit on March 05, 2011, 05:30:07 am

http://www.el34world.com/charts/Schematics1.php

Take a look at the Vox  Pacemaker & Cambridge

Also look at the Gibson Scout  &  Devon tremolo

If you still need help, just repost & I can redraw a PR that is cathode biased to show you.

with respect, Tubenit

Title: Re: cathode biased Princeton Reverb trem ?
Post by: tubenit on March 05, 2011, 08:21:04 am

Compare this to the Vox Cambridge.  You could try this with your PR cathode biased. This is probably how I would attempt it. I changed one of the .01 caps out to a .02 for a slower vibrato on the original '76 PR I owned.

With respect, Tubenit

Title: Re: cathode biased Princeton Reverb trem ?
Post by: tubeswell on March 05, 2011, 12:34:14 pm

The way tubenit has drawn it will work. I recently did a bias-wiggle trem on the output stage in a cathode-biased SE amp, and I found that the trem depth control isn't nearly as responsive as similar circuits in fixed-bias amps. You have to turn the depth pot to about 2/3 before you start to notice any shimmy because (from what I think? - correct me if I'm wrong) the AC from the trem output is 'fighting' the natural tendency of the cathode bias resistor to 'auto-bias' the output stage.

But I still liked the result - FWIW here is a soundbyte link of the trem from the 'thread of the build' for the amp in question:

http://music-electronics-forum.com/attachments/12710d1297150553-super-ac4-g10-6l6-5u4g-lo-ht-trem.mp3

and the schematic:

http://music-electronics-forum.com/attachments/12712d1297150735-super-se-ac4-v4-.pdf

Title: Re: cathode biased Princeton Reverb trem ?
Post by: Fresh_Start on March 05, 2011, 01:34:35 pm

If you want a foot switch for the tremolo, I would move its location and have it short the wiper of the Intensity pot to ground.

Cheers,

Chip

Title: Re: cathode biased Princeton Reverb trem ?
Post by: worth on March 06, 2011, 08:36:58 am

Thanks for the replies , but I may end up staying with fixed bias. Take a look at this Ceriatone "Prinzetone".... people seem to like this trem circuit more than the PR. trem.

Title: Re: cathode biased Princeton Reverb trem ?
Post by: sluckey on March 06, 2011, 08:44:27 am

It's the same except for the bias pot. BTW, that's a plain Princeton rather than Princeton Reverb.

Title: Re: cathode biased Princeton Reverb trem ?
Post by: worth on March 06, 2011, 09:00:45 am

Yeah.. it's a Princeton , but does it matter as far as the trem circuit goes ? I may be wrong , but the "Prinzetone" trem DOES look different to me, ( the 220k resistors in series ? ).

Title: Re: cathode biased Princeton Reverb trem ?
Post by: Fresh_Start on March 06, 2011, 10:38:29 am

Please use the Search function.  There have been many discussions about the Princeton Reverb bias vary tremolo circuit in the past year.  These include several possible "enhancements".  There also are older discussions of bias vary tremolo in the Archives section.

Title: Re: cathode biased Princeton Reverb trem ?
Post by: bluesbear on March 06, 2011, 10:59:00 am

I can't see any reason why you couldn't just run the trem like a VibroChamp. It would have to be just before the PI. If I'm wrong, someone please say so before I try it myself sometime!
Dave

Title: Re: cathode biased Princeton Reverb trem ?
Post by: worth on March 06, 2011, 11:42:56 am

"fresh start"... why presume that I don't use the search function?! I ALWAYS do before asking advice . Thanks for your guidance though . Bluesbear , I believe a Champ uses both halves of a 12ax7 for the trem .. A Princeton uses only a half.

Title: Re: cathode biased Princeton Reverb trem ?
Post by: sluckey on March 06, 2011, 06:19:15 pm

Quote

but the "Prinzetone" trem DOES look different to me

I looked more closely at the Prinztone layout and the Princeton AA964 layout. You're right. There are considerable differences, not just in the trem circuit. I think you would have to say the Prinzetone is Princeton inspired, but definitely not an attempt to duplicate.

Title: Re: cathode biased Princeton Reverb trem ?
Post by: bluesbear on March 06, 2011, 10:59:19 pm

Right, sorry. I was musing over the problem rather than thinking about what you're actually dealing with. Guess that won't work.
Dave

Title: Re: cathode biased Princeton Reverb trem ?
Post by: Fresh_Start on March 07, 2011, 07:58:52 am

Quote from: worth on March 06, 2011, 11:42:56 am

"fresh start"... why presume that I don't use the search function?! I ALWAYS do before asking advice . Thanks for your guidance though . Bluesbear , I believe a Champ uses both halves of a 12ax7 for the trem .. A Princeton uses only a half.

My apologies.  With a relatively low post count and a very short, general question I made the mistaken assumption that you had not done a SEARCH for "Princeton"and "tremolo". 

Here's one thread I was thinking about:
http://www.el34world.com/Forum/index.php?topic=9255.msg83833;topicseen#msg83833
Using one tube triode plus a MOSFET buffer might get you where you want to be.

This is the Archive thread I had in mind:
http://www.el34world.com/Forum/index.php?topic=604.msg1695;topicseen#msg1695

Cheers,

Chip