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Thinking about an Avatar 2x12 8 ohm cab with a Eminence Redfang and Wizard installed....
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Be careful!!!!! The maths shows around 47 mA of quiescent current on EL34 cathode resistors (47ohm)... that's around 2.2V, not 22!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! The point is not quite visible on the 1965 schematic! But I think it does in the 1976 schemo. for better working, please take a look at the EL34 manufacturer's datasheet, where it should say the quiescent current for that voltage.Originally posted by JC@ View PostComment
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It's OK, that one already got picked up by the man with the amp.Originally posted by slideman82
For me that's one of the best bits of getting a "new" old amp, just being able to dig in and have a play without worrying too much about how much time you spend on it. I especially like it when people say "it doesn't sound very old for what it is".Originally posted by TD_MaddenComment
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While I'm waiting for my new toy to warm up so I can recheck the bias...
With 470 V on the plates 47 mA bias current you're dissipating about 22 W a tube, which isn't terrible for a quasi class A amp (I only hope RG doesn't read these threads because I'll get castigated for that one!) I generally use the OT primary resistance to measure bias current, so that I'm only looking at the current through the plate. That way I can work out roughly what plate dissipation is.
For a coolish Marshall EL34 bias I run 15 watts, haven't yet got a clue what
6L6 tubes are going to want...better go and see if those plates are glowing yet!
LiamComment
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By the way, a word of warning about the NOS Tesla EL34s....apparently the center key isn't very well-defined, making it pretty easy to insert the tube incorrectly. Mike Kropotkin includes a warning notice with these tubes....putting one of these tubes in wrong can QUICKLY redplate it!
Ask me how I know this and how fast I can unplug an amp!!! No harm done this time, but a real scare, especially when the tubes are 80 bucks each!
I did bias to 2.2vdc across the 47r, but I'm used to seeing bias portrayed in a more standard way...like in ma. How can I connect in order to see that type of display on my Fluke?
In that regard, even set at 2.2v, the smell of "old amp" is really obvious; hope I'm not on the verge of an OT meltdown! Another good reason to see the milliamps!Comment
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If you think it's getting really hot, bias it at a minor voltage! Also, listen to the sound, if it is sounding ok for that bias voltageOriginally posted by TD_Madden View PostComment
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I love cathode resistors for that, but you have to do a bit of math. Just use current = volts / ohms. 2.2 V / 47 ohms = 0.047 A , which is 47 mA.I did bias to 2.2vdc across the 47r, but I'm used to seeing bias portrayed in a more standard way...like in ma. How can I connect in order to see that type of display on my Fluke?
If you want to read directly in mA put a 1 ohm resistor in series with the 47 ohm cathode resistor. Then if you measure dc volts across the 1 ohm with your Fluke you get the current. I.E. 0.033 V is 0.033 A, or 33 mA.
HTH
LiamComment
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Another tip! I don't know why AC50's factory schemo affirms 47 mA of quiescent current... EL34 has to be set around 39, I've read it in many places... If it gets hot, do the maths and re bias it! Check on http://www.aikenamps.com/Why70percent.htmlOriginally posted by TD_Madden View Post
I'm interested on this, because I want to build myself an AC50!Comment
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Mike Kropotkin sent me a note stressing that the NOS Teslas should be biased for 15-16 watts MAXIMUM each.....
At 47mA, they were putting out almost 24....and their maximum is 25.....pretty dangerous to the plates!Comment
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Whoa, steady there Slideman, plate dissipation depends on the plate voltage as well as the current. Power = current x voltage. 39 mA would kill either of my Marshalls' tubes in no time as they have quite high plate voltage.EL34 has to be set around 39, I've read it in many places...
I've been using 50 - 70 % of max plate dissipation for idle since I read that excellent Aiken article years back, but some amps were designed to be run quite hot. You really do need to use your ears as well as a meter to find what works.Comment
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just rebiased and the Teslas are idling at around 30mA each at 499-500v...
that's about right for 15 watts each. Perfect.
Now to find and remove the residual hum.....Comment
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Just been doing this on my new toy. First place to look is generally the first preamp tube IME. On my Marshall BBRI today it was heater wires too close to the plate wire on pin 1. Moved them so they crossed at right angles and were as far apart as possible, and it's pretty quiet now.
(You know the sort of day when the "no-name" NOS ECC83s you had been saving for a rainy day turn out to be 60s Mullards
Just had one of those)
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yes, this thing had a couple "Silvertone" 12AX7s...made in Holland.Comment
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