[himynameiskiera]

The 6080 goes in the back as a power tube.

 

[TravAndAlex]

Couldn't resist a Darkvoice 336SE and the following tubes for a combo deal:
 
Drivers: 

Sylvania 6SN7GT
Raytheon (Japanese calibration with Hitachi) 6SN7GTB tall bottle
Raytheon 6SN7GTB short bottle
Raytheon 6SN7GTA Short bottle
CBS 6SN7GTB short bottle
RCA 6SN7GTB silver print
RCA 6SN7GTB orange print
Channel Master (Hitachi?) 6SN7GTB tall bottle
Sylvania made 6SN7GTB
 
Output tubes:

Nanjing 6AS7G (the light show special, very low hours)
Russian 6AS7G - both tubes are huge
CBS 6AS7GA - extra tall triple mica
Sylvania 6080WB
Raytheon 6080WA
RCA 6080WA
RCA 6AS7G grey plate (2)
Sylvania 6AS7G black plate 
 
Anyone have any suggestions of where to start with tube rolling?  What combos might be the best?  The number of options has me just about overwhelmed.

Also, my headphone collection is:
 
HiFiMan HE-400s (22 Ohm)
Symphones v7's (? Ohm)
Grado 125i (with wooden cups) (32 Ohm)
V-Moda XS (28.5 Ohm)
Meze 99 Classics (32 Ohm)
 
All are low impedance cans, which are not the first choice for this amp (but as they say, I got what I got).  Suggestions on what tubes may be best or what headphones might play the nicest?  My understanding is that use with low impedance headphones may decrease the bass (increased rolloff) and may cause more audible hum.  I have ordered 220 uF Nichicon capacitors to do the Fitz mod if needed.  Will the Fitz mod also improve the bass rolloff or just reduce hum?
 
Really looking forward to trying it out (my experience so far is with a hybrid tube/o2 amp).  Just trying to read through all the posts to figure out where to start.  

 

[himynameiskiera]

With low impedance, swap out the output caps. I replaced them today with new mundorf 100uf e series to reduce the bass cut off. Even for my hd 650 and hd 600, the bass is far deeper and is generally more defined. The chinese stock caps are terrible quality. 
next thing to be installed are some clarity cap bypasses and then a new volume pot.

 

[TravAndAlex]

Ok... so there's hope?!?  

Can you elaborate?  How many are required?  How did you decide on 100uF?  Down to what impedance headphones will this support/improve.  
 
Are there drawbacks from doing this mod?

I'm excited about this if it is as easy as swapping in some new caps!

 

[himynameiskiera]

I'm using a single 100uf for each channel but the absolute ideal for something say, 32 ohms would be a 220uf cap or even something higher. I chose 100 uf simple because I am using my hd 650 and hd 600 as primary cans and I wanted to keep the bass sounding more like it did at 30uf but with the added quality of the mundorfs. My grados also sound better than they did at 30uf per channel.
 
I've noticed no drawbacks at the moment other than inconvenience as my pcb was a little too short. I think the amp is slightly more neutral now so if you like a very warm sound then this may change it. The clarity cap bypasses will be added tomorrow and should change the sound some more.

 

[TravAndAlex]

What are you using for the clarity cap bypass?

 

[himynameiskiera]

1uf. I didn't choose this value, rather it was recommended over on the polk audio forum. I'll report back on how they sound.
I don't think the bypass is vital as I'm listening with just the two mundorfs installed and it sounds far better than at stock.

 

[TravAndAlex]

Looking at a Canadian parts place.  This sound right:  

MUNDORF-71881	QTY	ADD
220µF / 23VAC-63VDC, MLytic® Bi-Polar AC (ECAP63-Raw), 16mmD x 39mmH

Available in 220, 270, 330, 370, 400...  400 is on serious sale right now.  Would that be too high?

 

[himynameiskiera]

I don't know for sure but when I looked at other people doing the mod, they choose the 100vdc caps. I don't honestly know if it will make too much of a difference. But the uf rating of 220 should be absolutely perfect. I've seen people use two 100uf per channel so the 220uf will be ideal. Also, some revisions of the PCB are shorter and so when I installed mine I had to use a funny angle to solder.

 

[TravAndAlex]

Dang... I didn't notice that the 220's I was looking at were rated 63V DC.  Do you know what the original output capacitors are rated for voltage (is it marked on them by any chance)?  I wonder what the lowest rated voltage I can use is.  I've read the threads here and all over... no one states the original voltage that I could find.
 
Can you post a link to the actual capacitors that you used?  I can't seem to find anything larger than the 100uF caps with the 100VDC rating.


Thanks for your help!  

 

[himynameiskiera]

The original caps say 250v on them so the voltage shouldn't make too much of a difference. 
 
http://www.hificollective.co.uk/catalog/ecap100110-100uf-100vdc-mundorf-ecap-electrolytic-p-4007.html
 
These are the ones I used. I think you could actually just solder two of the 100uf per channel if it came to it.

 

[TravAndAlex]

Cool.  Thanks.  I think I'll see if I can squeeze 3 per channel in there with a Mundorf F Cap 1uF / 450V, Mcap EVO Oil as a bypass cap.  That should help out my low z cans.

HA!  I thought I had read every post in every chain on this amp... but I just found this gem which includes required voltages for the caps:
 
"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.
 
Any cap over 16v is fine as there is only about 6-7vac going through the wiring that the caps are soldered to."
 
 

SUMMARY FOR ANYONE WHO HAS GOT THIS FAR:
 
For the original Fitz mod (reduce or eliminate tube hum), two 220uF capacitors rated 16v+ is required.  I will be using:  Nichicon-Elec_Caps-Series FG 220uf / 25v D(10 mm) x H(16 mm)

To change the output capacitors to support low impedance headphones, 100v+ non polarized (bipolar) capacitors are required rated between 200-400 uF for each channel.  I will be using:  Mundorf E Cap 100uf / 35VAC-100VDC, MLytic Bi-Polar AC (ECAP100-Raw), 18mmD x 39mmH.  I will use 2-3 in parallel for each channel (depending on space). 
 
The recommendation is to also use a film capacitor rated at 1 uF also in parallel for each channel as a bypass capacitor to improve high frequency playback.  I will be using: Mundorf F Cap 1uF / 450V, Mcap EVO Oil 25mm W x 16mm L.  This may present a challenge due to space as film capacitors tend to be large - and these are short and fat.
 
 
Thought I'd save someone else the trouble of deciding what they need if they want to do the recommended mods to the Darkvoice 336SE to support low impedance headphones.  I'll report back when it's all arrived, installed and tested.  From everything I've read, it does no harm to replace the above capacitors, and sound quality (low bass reproduction) is greatly improved with these fairly simple modifications.
 

 

[TravAndAlex]

Darkvoice arrived 2 days ago and the new caps were waiting for it.  Very tight to work on and to fit 4 new caps per channel in there, but stacked into a little pyramid, the 3 100uF caps will just fit under the bottom cover.  
 

 
Initially (unmodified), the sound quality was less than stellar with my HiFiMan 400S and Symphones.  The best sounding (and not ideal) was my Meze 99 Classics.  After replacement, the other headphones sounded much better, and bass response seemed equal to my solid state amp.  

So far so good... these are initial impressions only.  YMMV.

 

[TravAndAlex]

One question for the experts out there:  

Since in stock form, an OTL amp is supposed to be an ideal match with a high z headphone (such as the Beyerdynamic 880 600ohm), will the replacement of the output capacitors with significantly larger values negatively impact the performance with the high impedance headphone?

 

[himynameiskiera]

Not at all. All you're doing is increasing the absolute limit rather than the sustained limit. The values of 100uf allow fewer frequencies to be cut off, for high impedance cans, less uf will be drawn.