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Thread: 6l6 in VHT Special 6

  1. #1
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    6l6 in VHT Special 6

    Hi,
    I was wondering what mods i would have to do to run a 6l6 in my vht special 6 and get the full 20 some watts from it. OT replacement? Bias? ect..
    http://www.vhtamp.com/pdf/VHT_Specia...ic_5-17-10.pdf schematic

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    That looks like a well thought out amp.
    Triode, Pentode switch.
    Half power, full power switch.
    How many whats do you measure right now with the 6V6?

  3. #3
    Senior Member ThermionicScott's Avatar
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    20 watts from a single 6L6 is pretty optimistic. 10-12 is more like it.

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    ok. So what do i have to do? Just swap in a 6l6? or do i have to change something else?

  5. #5
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    It doesn't make anymore power with a 6L6 plugged in and the 6V6 sounds the best anyhow.
    They make about 4 watts with the 6V6.
    Yeah, you could rip the two transformers out and rebuild it with big ass class A iron for a couple hundred dollars so you could run the 6L6 and get maybe +10 watts out of it. Seems like a waste of money to me.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  6. #6
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    ok then. Are there any mods you suggest for this amp? Also, couldn't i do something like the Weber 5E3P Proluxe?
    Last edited by kev1n; 12-24-2010 at 03:58 AM.

  7. #7
    Noodle of Reality Steve Conner's Avatar
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    Well, what is it about the amp you're not happy with and want to change?
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    I would like more clean headroom and i dont like the way the eq is voiced, i only use it bypassed.

  9. #9
    Noodle of Reality Steve Conner's Avatar
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    Probably best to sell it and get a bigger amp, then. The mods probably won't make you happy.

    Edit: I see "Mod friendly" is actually in VHT's advertising blurb, so I guess you'll feel cheated if you don't get any suggestions for mods. Try removing C12 and see if you like the effect of the tone control any better. If not, put it back in and remove C11. Or, try one of the huge variety of mods for the Epiphone Valve Jr, this is practically the same amp.
    Last edited by Steve Conner; 12-24-2010 at 01:21 PM.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  10. #10
    Supporting Member Jazz P Bass's Avatar
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    You also could try a speaker cabinet with more "paper" than that single 10" speaker.

  11. #11
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    Thanks. I think im going to try new tubes and a weber sig 10s and see how that sounds.

  12. #12
    Senior Member ThermionicScott's Avatar
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    A more efficient speaker would be my "mod" of choice. What does the VHT Special come with?

    - Scott

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    it comes with a vht speaker which is supposed to be inefficient on purpose to be quieter for recording.

  14. #14
    Senior Member ThermionicScott's Avatar
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    Well, there you go. Pop in an efficient speaker and those 4 watts will be much louder.

    - Scott

  15. #15
    Senior Member kleuck's Avatar
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    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.
    Last edited by kleuck; 02-16-2011 at 10:41 PM.

  16. #16
    Senior Member kleuck's Avatar
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    Quote Originally Posted by kleuck View Post
    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)

  17. #17
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    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.

  18. #18
    Senior Member kleuck's Avatar
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    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.

  19. #19
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    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.

  20. #20
    Senior Member kleuck's Avatar
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    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

  21. #21
    Noodle of Reality Steve Conner's Avatar
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    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.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  22. #22
    Senior Member kleuck's Avatar
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    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
    Last edited by kleuck; 02-23-2011 at 09:01 PM.

  23. #23
    Senior Member kleuck's Avatar
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    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 :
    Last edited by kleuck; 02-28-2011 at 12:14 PM. Reason: translation

  24. #24
    Senior Member kleuck's Avatar
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    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.
    Last edited by kleuck; 03-01-2011 at 12:17 AM.

  25. #25
    Senior Member kleuck's Avatar
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    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.

  26. #26
    Noodle of Reality Steve Conner's Avatar
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    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.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  27. #27
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    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?

  28. #28
    Senior Member kleuck's Avatar
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    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.
    Last edited by kleuck; 03-01-2011 at 02:37 PM.

  29. #29
    Senior Member kleuck's Avatar
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    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 : 1f vs 2,2 vs 4,7 originally.

  30. #30
    Senior Member kleuck's Avatar
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    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 (22f) with a 22nf film too.
    Better lows and clean brightnesss now




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    Senior Member kleuck's Avatar
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    Senior Member kleuck's Avatar
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  33. #33
    Senior Member kleuck's Avatar
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    Compresseur
    Telecaster micros Tonerider Alnico III, VHT Special 6 'British mod), volume 14 h tone 12 h en clean mode ; cab loaded with an Eminence Ramrod ; tubes 6V6 Mazda & 12AX7 Siemens "smoothplate"
    Sound files are tests of my compressor, so first amp only, then comp 10 o'clock, 12 o'clok, full tilt.

    Deuxime first without comp, then comp 12 o'clock.

  34. #34
    Senior Member NorCalTuna's Avatar
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    My buddy has one of these! It lives at my house and we gig with it to local coffee shops and such. I think it's a neat amp but I didn't like the tone control either. The boost is horrible! Why they didn't buckle another cap over the treble cap (unless randy owns a patent on this?) and get some smoother tone stack bypass action? Anyhow... I noticed that their design is essentially a champ with two tone stacks- the one knob one, and one typical t/m/b one fixed at the factory through resistors. Epic! Who knows whats next?

    250K and 250K Concentric Pot | Allparts.com

    It's not an audio taper but it'll keep me from drilling holes; I'll have the thing wired up tonight : ^) If anybody knows where to get a better pot, please indulge me!

    I may also add a little NFB and a larger power tube cathode bypass cap but we'll see what the night holds.
    Last edited by NorCalTuna; 03-26-2011 at 07:45 AM. Reason: Forgot the article "a!"

  35. #35
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    Quote Originally Posted by NorCalTuna View Post
    ...The boost is horrible! Why they didn't buckle another cap over the treble cap (unless randy owns a patent on this?) and get some smoother tone stack bypass action?

    250K and 250K Concentric Pot | Allparts.com

    It's not an audio taper but it'll keep me from drilling holes; I'll have the thing wired up tonight : ^)
    I'm not familiar with this mod- description/schematic/links to previous threads would be appreciated.

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