Has anyone worked on a Guytron?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by telewacker, Mar 18, 2005.


  1. telewacker

    telewacker Supporting Member

    Messages:
    2,268
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    I have the GT100 head and I'm almost where I want to be with it. I want to modify the master tone stack to voice it more like a Fender, less like a Vox, and remove some fizziness from the EL84 section. I don't need, and can't afford, the FV model upgrade...I'm a one-amp-one-guitar kind of player, and if I can just make these hopefully minor changes I'll be satisfied. I love the response of the amp and its ability to cover any gig dynamic. It sounds great with my Tele now that I have a cab and speakers that I like.

    I called Guytron and spoke to somebody about my issues with the amp. Apparently Guy is very busy, so I don't know if he has the time to deal with it. I'd rather have it worked on locally so I can minimize downtime and also be able to play test it to get the sound just right.

    Has anyone here been inside one of these? Seen the schematic?

    Thanks, Paul
     
  2. renico00

    renico00 Member

    Messages:
    238
    Joined:
    Jul 24, 2002
    After reviewing the schem, the tonestack is much closer to a typical Marshall than a Vox. It would be cake for any tech to switch out some values to match a typical Fender stack. With regards to the fizzines of the EL84, the entire thing revolves around the slaving of that EL84 power section. I have never played one of these, heard or even seen one in person. So, I'm not sure what kind of fizziness you're referring to. Maybe someone else can comment on that. Good luck.

    Jeff
     
  3. Mark Robinson

    Mark Robinson Member

    Messages:
    6,888
    Joined:
    Sep 9, 2002
    Location:
    Redondo Beach, California
    First go to the Guytron site and download the manual. Near the end you'll find two important pages, the block diagram, which tells you which number tube does what, and the tube chart, which tells you which tube is which.

    With these in hand, you can try some tube swapping and see if that helps. One really critical tube in these things is V3. It's way deep in there, near the front panel, third from the right as you look in from the back. This tube drives the EL-34 section. A 12AX7 is probably in there. You might try a 5751, or even a 12AU7 in there, this clear things up and reduces compression a bit. A warning though, a lower gain tube in there will also take away some of the Hiwatt type zing in the clean channel if you run that pretty high.

    V4, sits between the two smaller transformers, it is for the master tone section. If you have a weak or worn-out tube in there you are really sunk.

    Also, make sure you have a straight pair of robust EL-84 tubes in there. When they wear down, the amp sounds like crap.
     
  4. telewacker

    telewacker Supporting Member

    Messages:
    2,268
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    I have new JJ EL84s in there, and have got good results with GT12AX7Ms in V1 and V2. I tried a Mullard CV4004 and really couldn't hear a difference (whereas there is a noticable improvement with the vintage Mullard in my Fenders). I have a 5751 on the way, so I'll try that in the EL84 phase splitter postion. I'll check V4 too.

    The website recommends against NOS EL84s. Any idea why?

    I'm getting the amp more dialed in with each gig...I think a minor tweak to the master tone stack will cinch the deal.
     
  5. Mark Robinson

    Mark Robinson Member

    Messages:
    6,888
    Joined:
    Sep 9, 2002
    Location:
    Redondo Beach, California
    On the El-84 situation. The amp is very hard on those tubes. So it would be a shame to kill off some Mullards or whatever. I change the El-84 pair every 6 months, or more frequently when use is high. The JJ's sound pretty good initially but don't hold up very long. The Sovteks sound a little harder, not as blended, but they stay tighter, longer. I usually run the JJ's and change them often.
     
  6. telewacker

    telewacker Supporting Member

    Messages:
    2,268
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA

    What about the mil spec 7189s? Do you think they would hold up longer? I bought some NOS GEs on Ebay for a good price.

    The amp is used about 4 hours a day, on average.
     
  7. telewacker

    telewacker Supporting Member

    Messages:
    2,268
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    One other question; have you experimented with different tubes in the V4 position, and if so can you describe the results? Or is it mostly a matter of having a fresh generic tube in that position?
     
  8. Mark Robinson

    Mark Robinson Member

    Messages:
    6,888
    Joined:
    Sep 9, 2002
    Location:
    Redondo Beach, California
    Don't know much about the 7189 but it might have a slightly different pin-out and idle draw, I've never tried them. Call Guy about that.

    As for V-4, I got started on messing with that cause I was trying some new, supposedly pretty great 12AX7 tubes. So I swap a few into the amp. Suddenly I've got this hum going on. Dug around and found it to be the V-4 tube. Since it sits between those two transformers, it needs to have a certain type of filament or something to avoid the hum.

    I have probably 50 12AX7 tubes, accumulated since the late '70s. So I just started fiddling. What you want is a tube with nice open frequency response. I'm trying to remember, it might be spiral filaments you need, to eliminate the field hum. Wish I could remember. But if you start swapping stuff around and you get some hum, look to that tube.

    Right now I've got some selected Ruby Chinese 12AX7 in there, a couple Electroharmonix selected Russian tubes, NOS Philips 5751 in the V-3, an RCA black plate 12AX7 for V-1 and a CV4004 in V-2. I've tried pretty much everything that's currently produced. I buy most recently from folks who will select for current as well as gain, since the Guytron will stink up the room if there are low current producing preamp tubes in there. It's a long signal path with lots of gain stages.
     
  9. telewacker

    telewacker Supporting Member

    Messages:
    2,268
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    Well I'm getting closer. I tried the 5751 in V3 and found that I did indeed lose too much zing. I found a Ruby 12AX7 that works well in that slot, and a Mesa 12AX7 that seems to be sounding good in V4. My CV4004 sounded too loose and unfocused in V1, good in V2, but the new GT Mullard copies sounded better in both positions. I tried a balanced JJ 12AX7 in the EL34 phase splitter position and that seemed to add a bit of clarity, though the change was very subtle.

    I have Sovtek EL34s in there now...what do you recommend? Is it worth it to get NOS? I'm still looking to enhance the mids a little, though I expect that modifying the tone stack will be the final solution to that. Do you have a schematic?
    I'll call Guytron again I guess but lately they seem somewhat unavailable.

    Thanks for all the useful information!

    Paul
     
  10. Mark Robinson

    Mark Robinson Member

    Messages:
    6,888
    Joined:
    Sep 9, 2002
    Location:
    Redondo Beach, California
    Svetlana, now "Winged C" the originals, not the Mike Mathews re-branded Sovteks. I've also run JJ Tesla. I had a very slight preference for the Svetlanas once when I felt flush and ordered a quartet of each. Most of the sound of this amp is in the pre section and whatever you call the intermediary EL-84 section. I do tend to try to run the master pretty high, however, to extend the clean channel headroom.
     
  11. dfarrar

    dfarrar Silver Supporting Member

    Messages:
    270
    Joined:
    Sep 12, 2004
    Location:
    Marietta Ohio
    Great info guys ...thanks for this thread!! I seem to have lost a little power with my GT-100...It just doesnt thump like a 100 watt head should?? I run the master at about 12:00 and from that point on it doesnt get louder just more compressed. Is that typical of these?
     
  12. Mark Robinson

    Mark Robinson Member

    Messages:
    6,888
    Joined:
    Sep 9, 2002
    Location:
    Redondo Beach, California
    Especially if they are JJ's. I love the JJ's FWIW, but they don't seem to hold up in my amp. I have the Weber biasing probe/meter and run the thing right on Guys numbers. But hte JJ tubes will go crappy after a lot of hard use. I've revived my amp by swapping them. I buy quartets. Then swap in a pair. If I'm questioning my sound, I'll quickly swap, see if theres a big difference, when there is, I bail on the old ones.

    Also I've found that the amp is a bit sensitive to low voltage. We were having power fluctuations in our neighborhood recently, lots of flickering lights, dimming lights. The power folks came around, copped to the problem, corrosion, we're literally 100 yards from the Pacific. Now the amp is roaring along again.
     
  13. dfarrar

    dfarrar Silver Supporting Member

    Messages:
    270
    Joined:
    Sep 12, 2004
    Location:
    Marietta Ohio
    Thanks for the info Mark, but I just put put a matched set of JJ's from Guy in there a week ago. The amp definitely sounds better but I just dont seem to have much headroom. Could it be the EL-34's? Thanks again!
     
  14. telewacker

    telewacker Supporting Member

    Messages:
    2,268
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    Either of you guys have trouble getting Guytron to communicate with you? I've left phone messages and emails and I've been totally ignored. I need a schematic and I've asked about modding the amp to reduce the EL84 fizz a bit and change the tone stack to be a more pronounced in the mid range...bacically the 2nd position focus and third voicing on the FV model. What's the deal?
     
  15. dfarrar

    dfarrar Silver Supporting Member

    Messages:
    270
    Joined:
    Sep 12, 2004
    Location:
    Marietta Ohio
    telewacker- I havent had any problems at all. Matter of fact Guy has always replied to my e mails promptly. Maybe things are just busy for them at the present time. Hope you get the info you seek!
     
  16. telewacker

    telewacker Supporting Member

    Messages:
    2,268
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    Hopefully that's all it is. I'm admittedly getting a little frustrated. I really like this amp, if I can just tweak it a little I think I'll be happy with it. The FV model is out of my fiscal range at this time.
     

Share This Page