DSL - Master Volume Mod.. Buzz/Hum

[diddymix]

Hello People :)

I was hoping I could get some guidance and expertise on something I am trying to accomplish with my DSL401 amplifier. I'm not an technician by any means on tube amps but can replace and change things one at a time.. but, so far so good. I have done some mods to the V1 preamp section and am pleased with the results.. and now have tried a modification to the master volume control.
I've basically moved the master volume from its old position VR12, to before the phase inverter input cap C93 (22nf). I have replaced where the pot was.. with 2 x 220K resistors from the bias source.. that feed each pair of EL84 output grids. I also connected the PI outputs to each pair of grids so it all connects up correctly.. The new master volume pot is an A1M value however.. The pot is grounded to where the PI tail resistor is grounded.
Anyways the result is really impressive and much more what I was looking for, cleaner overall and more open with real punch.. The only thing is.. now there is an annoying buzz when I turn the volume up. Just with the top of the chassis open with an ear close it sounds like its coming from the EL84's... Also now the volume control is waaaayyyy more sensitive, almost like I barely turn it up to a 1/4 rotation CW.. and the amps freaking loud.
Could anyone help me with what they think could be going on?? I'm not a tech but my feeling is it could be to do with the bias value and the 220k resistors?? Could it be something to do with the fact Ive made it a fixed bias value but I have not modified the bias circuitry?? Before the mod, the bias source actually flowed through the old volume pot in its old position VR12 (A200K). Could I be right in that surrounding bias supply circuitry was designed for this kind of setup?

 There are a few other changes Ive made around the output stage... but the amp was not buzzing like this before I made this master volume move. So I'm not sure if they make much difference.. I really want to keep this change the amps tone is so much better but the buzz is really annoying!!!!

Please any thoughts you may have on this are much appreciated!!

Kind regards :)

[shooter]

the "word pic" you painted confused me, maybe a sketch?

but the buzz is really annoying

my guess, you are letting VDC onto the 84's grids, either because the coupling caps got overheated in your mod, or it's now wired to add DC when you turn up MV

[sluckey]

The bias doesn't know any difference in whether it is fed through a 200K pot or a 220K resistor to get to the grid. Maybe you made a mistake when replacing the dual pots with 220K resistors? SHow us a pic of your actual work.

[diddymix]

The bias doesn't know any difference in whether it is fed through a 200K pot or a 220K resistor to get to the grid. Maybe you made a mistake when replacing the dual pots with 220K resistors? SHow us a pic of your actual work.

Hi Sluckey, thanks and shooter. OK i did a drawing of what it now looks like, sorry I dont have a camera to take a pic of the component changes etc.. I've carefully checked the PCB and continuity tested the route from the PI and everythings connected right. So basically I removed the two series resistors on the DSL schematic (R115, R116).. changed the couplers to 0.1uf,  I also changed the grid stoppers up to 34K and the screens to 220R.
These changes before the MV mod worked with no buzz.. its since directly hooking up the bias via the 2 x 220K resistors and moving the volume pot to Pre PI.. that its appeared.
 I read somewhere that the bias feed resistance + the grid stops... that there is a total limit one should not exceed for the tube type.. do you guys think it could be this? I'm thinking that maybe its this or that the value of the grid resistor is perhaps not ideal for the bias circuitry?.. A guy on amp garage pointed out that the 220K value is what one would hear with the (old position) master volume on max.. Thoughts appreciated!

[sluckey]

Very curious, why did you change the grid stoppers to 34K? And the screen resistors to 220Ω?

If you had no buzz before the mv mod, but you have buzz after the mv mod, then reverse the mod. You just may find what is buzzing in the process.

[shooter]

moving the volume pot to Pre PI.. that its appeared.

I would replace the pot and coupling cap feeding the pi.

[diddymix]

Very curious, why did you change the grid stoppers to 34K? And the screen resistors to 220Ω?

If you had no buzz before the mv mod, but you have buzz after the mv mod, then reverse the mod. You just may find what is buzzing in the process.

Hey Sluckey thanks, well I read online that upping EL84 grid stoppers to a fairly high value can soften some of the glassy brash high end that can occur in bad designs.. It does actually help a little, but not a huge amount.. As for the screens it was someone stating that upping them can prolong tube life.. i did try it to 1K before but the feel got a little bit 'squishy' which i didnt like so put it to a happy compromise... not sure if Ive made much difference there again..
I did try reversing the mod and its simply no buzz. So leaves me thinking its in my configuration around the output section.. aghhh its driving me nuts., it really does sound much better Pre PI!!

[diddymix]

moving the volume pot to Pre PI.. that its appeared.

I would replace the pot and coupling cap feeding the pi.

Hi again shooter, what dya reckon?? perhaps smaller pot eg. A500K and a smaller cap (10nf)??

[shooter]

what dya reckon

you're Tshootin not gigin so anything in the junk drawer that's good.  trying too eliminate "the last thing I did" fault 

[Ritchie200]

Should you be going direct to ground with that pot? 


Jim

[sluckey]

This project is really two mods in one. Neither one should affect the other.

     #1. Replace the dual MV pots with 220K grid leak resistors.

     #2. Install pre PI MV.

Either of these mods could cause buzzes or noise. You need to determine which mod is causing the buzz.

I suggest you completely restore the amp to it's original configuration. Then turn the dual mv up all the way. Evaluate the sound. Listen closely for any buzzes, noises, etc., to establish a baseline. Then do mod #1 only. Compare the sound to the baseline. If sounds buzzy then try to determine why before doing mod #2. Once you are satisfied with mod #1 results, then proceed with mod #2. Compare sound with the baseline. If sounds buzzy then try to determine why.

It's unfortunate you don't have a camera. Some pics of your actual work could provide some clues. Your description of the mods sounds pretty straightforward, but it's possible that what you actually did does not line up precisely with your description. It's very easy to make a mistake when dealing with a pcb.

[jojokeo]

Depending on what's been moved and where - there's a good chance that shielding the wiring will help if it's not already? PPIMV's are noisy and need shielding. The "new master" very well could need shielded wire depending where it is and the wiring lengths?