Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: moonbird on June 19, 2011, 02:26:01 pm

Title: Preamp VVR bypass tips
Post by: moonbird on June 19, 2011, 02:26:01 pm

Hello all -

I am trying to add a Hall VVR to a 5e3 clone i have. From what I read, the best way is to isolate the preamp from vvr control by bypassing the preamp B+ BEFORE it hits the vvr.

Once I got to thinking about it -- I had some questions --

1. Obviously the dropping resistor to the preamp needs to be increased. The common sense answer is to sum the total DCR from dropping resistors and any chokes that were before the preamp and add that sum to the dropping resistor normally in front of that power supply node. This would be a place to start I guess - might have to do some adjustments from there. Is this the method or am I missing something?? Any hints??

2. Because much of the normal smoothing is removed before the preamp using this technique, has anyone run into hum (ripple) problems on the preamp tube signal. Should a node be added to provide a dedicated reservoir cap for the preamp and more smoothing??

Any and all experience and tips very much appreciated! To whom it applies -- happy Dad's day!!

Title: Re: Preamp VVR bypass tips
Post by: TIMBO on June 20, 2011, 01:14:14 am

Hi moonbird, The VVR is a useful mod and i have added to my silver jubilee and also to my next build. I have added a schem to what you are looking for. As for the filtering there are different ways to do it so this post might help, look at the very bottom at the 18w schems and they will give you some ideas http://www.el34world.com/Forum/index.php?topic=6899.0 (http://www.el34world.com/Forum/index.php?topic=6899.0) Good Luck

Title: Re: Preamp VVR bypass tips
Post by: moonbird on June 20, 2011, 05:29:03 am

Thx Timbo - seems to be the definitive guide. I tried a search for vvr before posting, but for some reason did not hit this one. Thx again.