Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on February 07, 2011, 03:18:49 pm

Title: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on February 07, 2011, 03:18:49 pm

Hey guys I have a cathode biased amp with VVR on the pwr tubes and the PI. The pre-amp is like a single channel 6G16 with reverb. I cant use the 6G16 bias vary tremelo, so Im looking for a easy way to install tremelo using the spare 12ax7 available on the chassis.  I was looking at the Gibson GA2-RVT schematic http://www.el34world.com/charts/Schematics/files/gibson/GA-2RVT.pdf
The trem appears to wiggle the B+ rail?   Im thinking of installing it on the pre-amp B+ rail only, so as to not interfear with the VVR.  What do you guys think? Is there an easyer way?  Any suggestions are much appreciated.  Punky

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: Dave on February 07, 2011, 03:39:01 pm

Vibrochamp tremolo. Its about the best sounding tremolo I an think of (in my opinion) is not interested in what your power tubes are doing.

Dave

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: sluckey on February 07, 2011, 03:43:19 pm

This trem does not wiggle the B+ rail. The trem signal leaves the wiper of the depth pot and goes to cathode follower V2, whose cathode is also coupled to the cathode of preamp V1. So the trem wiggles the bias of V1 to modulate (wiggle) the guitar signal being amplified by V1.

Since you are not VVR-ing the B+ to the preamp, it should work fine.

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on February 07, 2011, 06:29:54 pm

Thanks Dave, Did you add this trem to a push pull amp?  I would guess the intensity pot would be scratchy with DC voltage on it?  Punky
http://www.el34world.com/charts/Schematics/files/fender/champ_vibro_aa764_schem.pdf

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on February 07, 2011, 06:36:11 pm

Thanks Sluckey, I didnt see the second half of V2 as a cathode follower. Do they still make 250k and 500k reverse audio pots?  Can I use the standard audio pots and wire them backwards? I`ll turn those with my left hand. Ha Ha.
Punky

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: sluckey on February 07, 2011, 07:08:34 pm

The popular Marshall 1974 18Watter trem is another good candidate that uses common parts.

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: Dave on February 07, 2011, 08:19:25 pm

http://www.triple-t.no/torevibe/

That is a link to our Tore friend added the VibroChamp trem to a standalone reverb. It should give you a good reference to start with.

Dave

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on February 08, 2011, 11:17:48 am

Thanks for the ideas guys.  WOW!!! Im impressed with the Torevibe`s tremelo in that sound clip. The Vibrochamp looks to have the simplest circuit, I will give that a go first and let you know how I make out.    Punky

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on February 08, 2011, 08:01:41 pm

Well I did the vibrochamp trem circuit. Very strong trem, go past 9 o clock with the intensity( 25KL) and its too much. The 3MRA  speed control works fine.  I also noticed when I stopped playing and even with the vol, bass, treb, and reverb controls off , I could hear thud thud thud throught the speaker in time with the trem speed. I played the amp for a while messing with the trem controls and all of a sudden the trem got week and the thud thud thud got louder. I had to turn the intensity control way up to get the signal to modulate instead of the thud thud thud. Any ideas what happened and what to check for?  The ungrounded leg of intensity pot is connected the cathode of the triode just before the PI. The amp is a single channel 6G16. Thanks for any help you can give me.  It may be a couple of days before I get a chance to tinker with it again
http://www.el34world.com/Forum/index.php?action=dlattach;topic=5348.0;attach=6102

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: tubeswell on February 09, 2011, 10:54:18 am

Hi punkykatt

Could be the trem started to induce an unwanted oscillation of another sort. Maybe try changing the voltage divider at the output of the trem circuit so that the trem voltage swing is lower?

Also I would think that 'injecting' the trem into an earlier stage in the (pre-)amp will be 'noisier' than if it was injected at the output stage (because the interim tube noise on the low-voltage part of the trem swing gets amplified more.)

Or, you could run the trem to the grid load resistors for the output stage (the way its done in a Gibson GA17RVT). This will wiggle the output tube grid voltage. This method does work and can be made to make the trem sound good enough, albeit I concede that it doesn't work as well as it does on a fixed bias output stage tho' (perhaps because the trem voltage wiggle is 'fighting' the functioning of the cathode resistor of the output stage?), so you generally have to turn the depth pot up a lot more to induce the wiggle.

FWIW here is a clip of the trem (on a low-depth setting) on a recently-completed cathode-biased SE amp that I made which uses that (output driven trem) set-up I described above.

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

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on February 10, 2011, 06:20:11 pm

 :smiley: Sucess!!!  I had all afternoon to mess with the trem. After redoing all grounds and joint connections, rechecking resistors for spec etc. I did a voltage check and found the PI and power tube plates oscilating with the trem speed. The power supply upstream was not oscilating.  Luckly there were some unused turrets on the board so I could move the 68k cathode resister  connections  of the trem circuit further away from the PI circuit turrets.  That took about 90% of the thud thud thud out. Then I installed a 25uf/25v bypass cap across the 1.5k cathode resister the 25KL pot ungrounded leg is attached to. BINGO!!! quiet as a mouse.  I am really impressed with this Trem.  It sounds just like the Tore revibe demo in the above sound clip that Dave posted. (by any chance , does anyone know the name of the song that is played at the end of that demo?)  The tremelo and reverb in this amp is the best I have ever heard.  
    
 " Dave" I cant thank you enough for recomending the Vibrochamp trem.  Punky :blob1:

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: Dave on February 10, 2011, 09:45:35 pm

It is my pleasure to have helped. I hope you enjoy it.

Dave

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: TubeStake on March 24, 2011, 12:19:30 am

Quote

name of the song that is played at the end of that demo?

It's "Harlem Nocturne" Google Danny Gatton's version to hear that tune slayed with a Tele.

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on March 24, 2011, 08:14:23 pm

Thanks TubeStake.  My Bass Player ended up knowing the title of that tune, just last night we polished up the Ventures version of Harlem Nocturne. Danny Gatton is a monster on guitar. :worthy1:

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: Tone Junkie on March 24, 2011, 09:10:08 pm

Punkykatt if thats your picture on your moniker I have to tell you ,you have a twin out there I crack up everytime I see your picture because Im staring at my cousin Wayne Mathews from Oregon. If I had you guys side by side everyone would swear you were twins.
Thanks Bill

Title: Re: Add tremelo to a cathode bias amp with VVR
Post by: punkykatt on March 26, 2011, 01:01:15 pm

Yes Bill, thats my mug. That shot is from three years ago and Im even uglyer now.  I have been told many times here in Florida and in Connecticut that I have a double someone knows. One time someone even called out my real name and as soon as I started talking they new they made mistake. Punky