Guytron amp - bias and tube pulling questions

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1/5/2000 3:19 AM

Tarrie

Guytron amp - bias and tube pulling questions I bought a Guytron last summer. I'm pretty happy with it so far, but I've got some questions that Guy really hasn't answered (He's a great person to do business with, but He's busy and has his own ideas how things should be). I take no offense at this, but I was wondering if someone could enlighten me. I emailed Guy and asked him for a recommendation for biasing the amp. He wrote back and told me he installed one ohm resistors right on the board for this purpose, and said I should go for 30 mils. Biasing was a snap, but I wrote back and asked him why 30. A bit low? He said that's the way he designed it. Can someone explain this to a beginner? I thought El34 tubes pretty much liked to see between 35 and 40 ma, and that's that. Am I missing something? I left the tubes at 31-33 ma. I don't want to try to go higher if it's not designed to in some way I don't understand.     The other question (which gets asked a lot I guess) that I asked Guy is if I could pull two output tubes with two remaining. He said people used to do that so they could get output tube distortion at lower volumes, etc. His uses EL84 tube distortion which are then put through the EL34 power stage. He was rather adamant about it, so I didn't press the issue. The truth of the matter is that I love the cleaner part of the amp, don't need 100 watts by any means, and would like to try some other EL34 tubes, and would rather not have to buy four of them, especially if I buy some really nice ones (Mullards, maybe?). So here is my second question to all of you in the know (actually a two part question). Can I simply pull two outer or inner pair and rebias those remaining tubes? Anything else have to be done? And more importantly, do you think it's a good idea to pull of four Sovtek to replace with two better EL34s? Thanks to anyone who can shed light for my dim eyes.     Tarrie

 

1/5/2000 4:58 AM

Bruce

No, power tubes don't just "like" a particular current level but they do work and sound better when drawing enough current at some given plate voltage, that sets the tube in a specific range of idle power...(watts).   Example: 520vdc on the plates with 470vdc on the screens of a pair of push pull Svetlana EL34s @3K4ohms, might sound better to me with the idle current at about 17 watts.   17w/520v = abt 33ma.   Depending on the actual plate voltage, 30ma at idle, per tube and if measured by a cathode resistor, might be a little cold but still OK.   The cathode resistor will also be measuring the 3ma-4ma screen current too so the actual plate current will be a few ma less then what you measured. Safer though.   Perhaps Guytron feels 30ma is safer for his warranty, or, the PT, tubes, or, the plate voltage is such, that the 30ma per tube is about the right current x B+ for somewhere around 55% to 70% of the tube's ratings and works very   well there.   IMO, most normal players can't hear much, if any, difference between 30ma and 34ma, even though they think they can or think they should be able to, so I'd go a little colder and save the $$ in wear and tear of a new amp.     Yes, you can run two tubes (one on each side of the output trannie) if you double the speaker load to the output.   Never use tubes 1 and 2 or 3 and 4.   However, 1 and 3, 2 and 4, 1 and 4 or 2 and 3 or OK.   The load on the power supply will be less with only two tubes and the plate voltage to those two power tubes will now be higher, so you'll have to set the bias with a little less current to get the same 17 watts.   I don't know what speaker cabinet you have, but, if you double the speaker load, by, as an example, running a 4 ohm load on a 2 ohm tap (I dont think there is one), or, run an 8 ohm load on the 4 ohm tap, or rewire a 4 ohm 2x12 cabinet for 16 ohms and run it on the 8 ohm tap, you get the output back very close in primary zed as with four tubes. Etc.     Bruce

 

1/5/2000 5:29 AM
Tarrie	Bruce, thanks so much for the reply. Guy wrote that he designed the circuit so that 30 ma would be in the middle of the acceptable range. I can only guess that he meant that he did, indeed adjust the plate voltage as you indicated. I guess the only problem I would have in pulling tubes is knowing where to set the new bias. How MUCH less current would be the question. Thanks again.     Tarrie

 

1/5/2000 7:06 AM

Bill

quote: "I guess the only problem I would have in pulling tubes is knowing where to set the new bias. How MUCH less current would be the question."     You should check out Randall Aiken's site, he has some good articles on biasing.     http://www.aikenamps.com     The general method is to look up the max plate dissipation for the tube in question. An EL34 is 25 Watts. Mulitply the max dissipation by 0.7 or 0.6 or 0.5 or .....     For example, 25 Watts * 0.65 = 16.25   This is what you are going to set the bias current to give you. Measure the plate voltage with only 2 tubes in (wait a minute for things to warm up). Then use the equation P=V*I .     P = 16.25W   V = value you meausred   and I = P/V     Enjoy.

 

1/5/2000 1:47 PM
Graywater	Hey Tarrie,     I know nothing about your amp in particular - was just checking out the thread, and noticed your "quote:" "His[sic] uses EL84 tube distortion which are[sic] then put through the EL34 power stage."     What does this mean? Almost sounds like an EL84 driver stage pushing the EL34s, but somehow I don't think so.     GW

 

1/5/2000 2:49 PM
steve m	GW,     According to what I've heard, I believe that this amp uses an SE EL84 into an OT. The 100w EL34 power amp boosts this signal, IIRC.     Steve

 

1/5/2000 9:15 PM
Carlo	OK here's a dumb question...   What's "IIRC"?     Carlo

 

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