6l6 in VHT Special 6

[kleuck]

Change the cathode resistor for a 270 or 300 ohms one (3 or 5 watt) and plug a 8 ohm cab in the 16 ohms ouptut (or a 4 ohms cab in the 8 ohms output) and you can probably get 8 or 10 watts from this amp with a 6L6.

I will say more precisely :
The OT is a 9K one according to VHT, and a friend mesured the power with a (stock) 6V6 at 3,7 watts clean.
With a good Mazda, it's pretty louder.
And when you try to tune a 6L6 with the 340 volts B+, you find a sweet spot with a 4K5 OT.
Easily achived by connecting a 8 ohms speaker int the 16 ohms ouptut for example, without replacing the OT.
I don't know what is the power but it's much more powerfull with a 6L6WXT or a 6P3S on the (now) 4K5 OT.
And with a better bias (270 ohms instead of 470) it will probably be a great 10 watter (i'll try this in a few days when i get time)

[MWJB]

I would have reservations about reducing cathode resistor value, whilst using a 6L6 for 2 reasons...

Increased current draw on the B+ rail, over & above the increase you will see simply from fitting a 6L6

And subsequent lower B+ voltage, this will go some way to negating, or even reducing power output.

[kleuck]

In this amp, the PT, as long as the OT are way beefier that is needed for a single 6V6, some folks use EL34 end even KT77 !
I won't do that permanently, but 6L6 do not eat lot of current, and i already tested the 6L6/4K5 trick for hours, playing LOUD, the PT was not even warmer than the enclosure.

[MWJB]

SE 6V6 amps, like Champs usually do have pretty bulletproof transformers, but when you rebias that 6L6 it will draw lots of current, either way, plate voltage will drop & this will reduce power, certainly not give a significant increase.

[kleuck]

We'll see,
I did not make pictures of the PT specifically, but i did with the OT (and it's the same with the PT actually : huge for a 5 watter)



The little tranny is a VJ one

[Steve Conner]

Oh, yay, I have one of those little trannies that I bought off Ebay for a few bucks. I knew it was from some Chinese 3 watter, but wasn't sure which one. Now I guess I know it's from a Valve Jr. head.

The transformers certainly look like they could handle a 6L6. But beware, it's not unknown for single-ended amps to draw less current cranked than they do at idle. Leaving it idling may be a harder test.

[kleuck]

Anyway, i will test, i just received this morning my parts (did not have suitable switches left)

And i expect the total idle current to be in the 55 mA range (60 max) , where 45 to 50 mA are flowing trough the 6V6 in the stock amp anyway

[kleuck]

Mods "6L6"
The amp is really powerful with a higher bias (did not measured the idle current yet, so don't know if it's perfect) i think 8 or 10 watts.
With the stock 470 ohms, the amp was already more powerful, but was laking lows, a little harsh when overdriven, and overall, a little "tiny sounding"
With a 280 ohms, it sounds huge.




Heater referenced to 6V6/6L6 cathode


Tonestack & other mods :


Schematic :

[kleuck]

So idle current trough the tube is 64 mA, 4 are from G2, so the anode sits at 60 mA, which means 18 watts dissipation and 90% for a 6P3S (20 watts)
Perfect, and POWERFUL.
Interestingly enough, a 6L6WXT doesn't sound as good or even loud as the 6P3S now.

[kleuck]

I let the amp "on" in pentode mode for 18 hours now, and th PT is "hot", ie less than the PT of my EL84 PP (1974X style) after 2 hours.
So, i can say that the VHT can easily stand a 6P3S.

[Steve Conner]

Good stuff.

The 6P3S is kind of halfway between a 6V6 and a 6L6GC.

Probably better for this application than a real 6L6GC, or a 6L6WXT which is probably a Russian 6P3SE anyway.

[MWJB]

Steve wrote: "Probably better for this application than a real 6L6GC, or a 6L6WXT which is probably a Russian 6P3SE anyway."

Yes, a 6P3S...with a different glass bottle....a different plate structure...., or probably it's a different tube? Do we assume that all Russian 6L6/5881 variants are 6P3S? Even the ones that sound nothing like a tube marked as 6P3S? The SED 6L6GC, Sovtek 5881WXT & 6L6EH all appear to have a similar plate structure, all different to a 6P3S, & they typically sound different to each other...maybe they're designed that way, maybe cherry picked after the event. They certainly dissipate more than the 20W Kleuck quoted for the 6P3S (though not much more for the SED, nt nearly as much as the near indestructible 5881WXT). Perhaps I'm a pedant (well, OK, you got me, there's no "perhaps about it" :-)) but I don't recall ever seeing a 5881/6L6 actually marked up as a 6P3S. Maybe they are all 6P3S, but if the construction varies that much between them, is it worth lumping them all together under that same distinction?

Kleuck, you seem to have recorded a 30vdc drop at B+2 (no comparable figures for B+ & B+1) with the 289ohm cathode resistor & 6P3S, so how much more clean WRMS do you get with the 6P3S compared to the 6V6?

[kleuck]

I'm not equipped to do true rms measures, so i don't know, the amp sounds like a 15 watter now, that's all i can say.
There's less distortion too (makes sense) i have to push the volume more to get the same overdrive i like (say 15 hour, where i was at 13 before) and the sound of the overdrive is more glassy and articulate than with a 6V6 wich sounds thicker.
As for 6P3S, and russian "6L6" in general, the problem seems to be that they are very often marked as "6L6GC" or "5881" when trey are not at all equivalent.
6P3S is close to a true 6L6GB or GT, and 6P3S-E seems to be close to a 5881, and there's no vintage russian tube close to 6L6GC as far as i know.
I'll try to get 20 watts power dissipation (so something like 70mA of total current on the cathode), as you said, the 6P3S can probably stand this power, and they are cheap ; i don't expect to hear a difference in power, but perhaps in sound ?
Then replace R26 with a 7H choke.

[kleuck]

Update



Not sure of the voltages, my DMM died, and i had to use an old crappy one, not really precise.
But the 6P3S should dissipate 19 watts now.
I modified the boost switch for more volume (82K vs 68K) and the tonestack for more mids & volume (15K vs 10K vs 6k8 originally) and lows (82k vs 100k)
Lowered the V1b cathode cap : 1µf vs 2,2 vs 4,7 originally.

[kleuck]

Update

The PIO was not that great in the tone-stack (mushy lows) so replaced by "film"
Doubled the cathode cap from the first triode (22µf) with a 22nf film too.
Better lows and clean brightnesss now