Fuchs EL34 bias question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kingsxman, Feb 24, 2012.

  1. kingsxman

    kingsxman Silver Supporting Member

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    I've got a Fuchs Music man mod that I have out of the chassis (to add a gain boost). I got the gain boost working (adding the relay) and thought I'd check the bias on the amp while I had it out. (no external bias points).
    The amp has JJ EL34 tubes in it. So I measure from pin 8 of the power tube to ground (as I've read to do and as Andy mentions in a video) and I get .020 v. I've biased amps before so I know that this is 20ma. From what I've read....EL34's shoudl be around 40 ma. I brought them up to 40 ma but one tube is stuck at 5 ma. This could explain the weak clean tone I'm getting.

    Questions: Is 40ma the proper current level to bias JJ EL34's?
    What do you suppose is wrong with the one tube that wont bias? (I'm going to swap it and see if the problem follows the tube or not).
     
  2. dazco

    dazco Member

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    Yeah, 40 is about right for a plate voltage of around 435v. I assume there are 1 ohm resistors from the cathode to ground, and if so i would say either the tune is bad or the resistor is. Swap tunes in thier sockets and see if the low voltage follows the tube. If so, you have a bad tube.
    By the way, as i understand it the screen current can be subtracted from what you get. So if you see 40 mA, you are actually 5 Ma under the 70% max rule. By the way, 70% isn't a hard fast rule, but i find at low volume like bedroom volumes it sounds better and is plenty safe. Stage volumes it may be best to stay a little lower and the tone shouldn't suffer.
     
  3. michael stuart 57

    michael stuart 57 Member

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    40 ma is ballpark use your ears

    probably new tube time buy a matched pair and keep the good one for a spare
     
  4. Fuchsaudio

    Fuchsaudio Member

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    NO 40-MA !!! In a Music Man (which has 700 volt plates and 350 volt screens) you want 20 ma per tube. This is especially true with todays tubes, which cannot take the voltages of a good Sylvania/Philips from days gone by. Each tube should be 20 ma per tube in high power setting. If you have a low/high switch (we've done that on a number of MM's, and not to be confused with a full/half power switch) then there would be a second single trimmer on the chassis for adjusting the .040 setting, after you get things at .020 at high power, you trim the bias in low power to be about 40 ma per tube with that pot, since the plates are 400 and screens 200 in low power mode...

    If you have a tube not letting you bias it, move it to another socket. If the problem moves, the tube is bad. If the same problem occurs in that socket, then the screen resistor may have popped, the the offending tube (which killed it) should be replaced.... Please check with me on the care and feeding of "my babies" :)
     
  5. dazco

    dazco Member

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    Wait....you have 700v on those?!!! What El34's do you put in em?
     
  6. kingsxman

    kingsxman Silver Supporting Member

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    Thanks Andy. I was hoping you'd chime in since its after "business hours" and I wanted to get the cover on this thing this weekend.

    Yeah, I was a little worried as it seemed like this was a long way for these tubes to be off. I only ran it for about 15 minutes at 40 ma. So hopefully no damage done.

    I did move the tube to a new location and after I moved it all is ok. Not sure what was up with that but maybe my sockets need retentioning or something.

    Hey, how long do you let the amp warm up before you set your bias?

    (BTW, I LOVE the fact that you have the individual bias adjustments for each tube.).
     
  7. Science Amps

    Science Amps Member

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    I think any new production tubes will take that kind of plate voltage if the screen voltage is kept well below max. Power pentodes (especially modern ones) are prone to screen failure in most guitar amps because of over voltage, over dissipation, and poorly aligned screen grids. I can imagine the screens are pretty happy at 350vdc. Of course the bias still has to be right as Andy noted.

    (25 watt max dissipation / 700vdc plate voltage) * 60% = 21mA
     
  8. Fuchsaudio

    Fuchsaudio Member

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    I've used JJ's and Sovteks various branded tubes. It's luck of the draw today, regardless of the voltage it seems. Technically, the tube manual says an EL-34 is a 1000 volt tube.... :) For awhile, I was getting 20/30% failures at start-up or burn-in from JJ. Sovteks were more reliable, but I didn't care for the clanky tone. Recently (the last year or so) the JJ's have been usually within 3-5 ma out of a box of 20, without paying for matching. We have separate bias pots because I like that and it eliminates the need for matched tubes.

    Every company (so it seems) runs hot and cold on quality. That being said: we've loaded lots of Music Man's with JJ's and had no issues. We burn (cycled) 4 hours on and 4 hours off for a few days before we ship, and that shakes out the funky tubes usually. Sometimes the amps come in with original Sylvania/Philips and they are still doing 90% of the power output with 30 years on them !!

    I set my bias, let it run for 20 minutes to an hour and do other things. I come back to it and rebias again, and then move on. I also verify the line voltage is 120 or so for reference. No sense setting for proper bias at 110, and them run the amp on 120 or 125....
     
  9. kwd1987

    kwd1987 Member

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    For the sake of a future user searching for help...

    I went round and round thinking something was wrong with my amp because it didnt' do what I kept hearing much better sound from examples online.

    Fuchs Music Man Mod EL34

    Two visits later to a tech, there WERE some issues but after they were resolved, I was still left lacking. I couldn't get a low volume and it sound good. The bass notes in overdrive were farty and nasty sounding. There was a metallic rasp to the overdrive in general. I had to set my OD trimmer super low to make it bearable.

    I went back and forth with Andy as he tried desperately to help me figure it out from afar.

    Finally, I aggregated all of the good advice from all of the different threads on these amps.

    Here were my steps to that finally got me there.

    Speakers:

    I ordered two ET65s in a semi-open back cab.

    I made a reverse polarity cable for the speakers. Made a slight difference (noticeable more when super loud).

    I set the send on the series loop at 9:30 and the return at 12:00

    Tubes: I started with JJs in V1/V2 and JJ EL34s. Sounded dark and muddy. After trying everything other than tubes suggested and not getting there, I tried this lineup and it was like freaking magic. FWIW, I also ordered some NOS Sylvanias but this ended up better...

    TAD EL34B - STR power tubes
    V1 is a Ruby Tubes High Grade
    V2 is a Tung Sol
    V3 is a Ruby Tubes graded JJ
    V4 is a microphonic RCA Black Plate from Mike @ KCANOS

    It seems a high gain nice tube in V1 is better than in V2. The V2 needs to be a nice bright tube like the Tung Sol, at least for my amp!

    I set bias for EL34s right at 20ma

    I set the PI where it sounded good and balanced, didn't use scope, probably should...

    Now I can set my OD trimmer up to 50% and get into some serious heavy rock, thick OD tones that stay clear all the way down the volume knob to conversational levels.

    I could play this amp at midnight if I turned the bass off so it didn't rattle the walls haha.
     

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