[ksan]

 
I am one of those who had humming noise with 6SN7 new tubes on Darkvoice 336SE headphone amp. After reading many threads including suggestions for Mod I had hard time to believe Darkvoice would design and manufacture any amp that would not work well with driver tube (6SN7).  I didn't feel like modifying the amp even it's simple to do.  I preferred finding a solution that not requiring modification because I believed that any modification to the amp is going to change the design intend of the product and it could also lead to sound signature change or else.
I contacted Darkvoice and they provided me with a simple suggestion:  Aging the new tube.
 
By following their instruction my 6SN7 tubes are now quiet and sounding really sweet.
The aging process time varies depending on how loud the humming noise you have on individual tube.  One of my 6SN7 took 120 continued burn-in hours to get rid of the humming noise completely. Below is what I suggest you to try before doing any mod to your amp:
 
  1. There are total of two stock tubes in Darkvoice 336SE Headphone AMP (back tube: 6N5P  |  front tube: 6N8P).
  2. Remove back tube-6N5P( leave the socket empty).
  3. Keep 6SN7 tube (the new tube that has humming noise) in its socket.
  4. Turn on the Headphone AMP and leave it on continuously for 48 hours (for burn-in the new 6SN7 tube).
  5. After 48 hours put the back tube 6N5P back to its socket and listen to it.
  6. Continue burn-in process If humming noise is still persist, until it's gone.
 

 

[Lorspeaker]

just a  caution-note to the above post ....i tried it....
and when i tried to put back the backtube,
there was arching n popping sound,
for a few secs i tot my amp or the backtube was burnt.
some discharge going on i guess.
so be careful pls.

 

[scottosan]

If the noise is in fact from the heaters, I will try to DC elevate the heaters. This involves using 2 resistors and a cap from the B+ DC rectified power supply to the center tap of the heaters. I should have mine next week.  By installing a cathode bypass cap, you are modifying the gain and boosting frequencies,right?.

 

[scottosan]

One more thing. Why is one leg of the heaters going to ground? I would.expect that with DC heaters but not AC.

 

[doco]

welp just ordered two 220uf caps
http://www.ebay.com/itm/171263489815?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
 
will report back how it does for me. i've got a noisy hum on my rca vt-231 grey glass and sylvania vt-231

 

[DefQon]

The other method is fully regulate the heater bridge from the 6AS7 to 6SN7 to complete DC rather then AC but given the tight space inside, it ain't easy.

 

[doco]

  The other method is fully regulate the heater bridge from the 6AS7 to 6SN7 to complete DC rather then AC but given the tight space inside, it ain't easy.

 i might end up asking an acquaintance of my dad about that. he's a longtime electrician and used to run a radio tube shop before solid state immediately took over.
 
also does the voltage on the capacitors truly not matter for the fitz mod? i'm just double-checking since i only read one post about it and that was on page 1 of this thread.

 

[DefQon]

That was just an idea to ensure better measurement, not sure if it will get rid of the hum from random 6SN7's, the heater is 6vac across both tubes, 6.3vac/dc is the max according to spec sheet. The fitz mod seems to be a cap acting as a blocker but there is no vdc B+ in the front end, only till it runs back to the output stage with the 2 pairs of 3 10uf film caps on each channel of the pcb board, 70-75vdc for them, power filter caps are about 120vdc to 190vdc.

 

[doco]

i interpret that as a "no, it doesn't matter what voltage is on the capacitor you use for the fitz mod". am i right? (lol). the caps i ordered arrives on thursday but won't get installed until saturday night.

 

[doco]

it's been almost two weeks since the caps were soldered. absolutely zero noise with any 6sn7 i've put in. it's been said many times but i also recommend anyone to do this if they encounter horrible noise or hums with their old age 6sn7s that were bought. the only very slight noise i can hear is from the 5998 i have. my other 6AS7 tubes don't make noise.

 

[DefQon]

Good to hear. Any cap over 16v is fine as there is only about 6-7vac going through the wiring that the caps are soldered to.

 

[Claritas]

I am experiencing hum as well with some tubes. Usually, it goes away after a while but not always. So I'm thinking of doing this mod.

What would be a higher quality part or firm to use, if I'm already going to the trouble?

I might also want to replace the volume pot for less volume with more gradations. I'd greatly appreciate any knowledgeable input.

 

[DefQon]

I am experiencing hum as well with some tubes. Usually, it goes away after a while but not always. So I'm thinking of doing this mod.

What would be a higher quality part or firm to use, if I'm already going to the trouble?

I might also want to replace the volume pot for less volume with more gradations. I'd greatly appreciate any knowledgeable input.

The only pots that will fit is the ALP's range.

 

[PretentiousFood]

Since the caps won't see much voltage, this would be a really good place to throw in some polymer caps instead of the usual electrolytics. Something like this would do nicely.

http://ca.mouser.com/Search/m_ProductDetail.aspx?Panasonic/16SEPC220MD/&qs=sGAEpiMZZMvR1wrj203KOKNYKQR8GUMBcxAf8dpS/mk=

The advantage over most electrolytics would be much lower ESR. They're a lot closer to film caps in that regard.

Provided that the amp doesn't use feedback, you can (and arguably should) use a very big capacitor. Electra-Print does that to improve intermediation distortion, and over sizing the cap will make it perform more like a black gate. I've tried increasing it a lot in a vintage speaker amp though, and the tubes started to oscillate. It would be a good idea to check the output with a scope.

 

[Baldeagle58]

I can confirm that removing the back tube and leaving the unit on for 48 hours (no signal, just turned on) will make the 6SN7 (front tube) go totally silent. Kind of a pain but the way to go.
This amp is an outstanding bargain so it's well worth taking the time.
Silence is golden as they say.