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Topic: One output tube hotter |
Jonathan Shacklock
From:
London, UK
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Posted 7 Jun 2012 5:05 am |
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| I've recently replaced the output tubes in my Fender 75 with matched/tested 6L6's. This amp has a hum balance pot and a bias balance pot, not an adjustable bias. I've noticed one of the 6L6's feels hotter to the touch than the other, even after I balance the two trim pots for minimum hum (which is I think how to set them up). If I swap the tubes it's always the inside tube that runs hotter. They seem to both glow the same amount. Should I be concerned? The sound is fine, a big improvement on the old tubes. |
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Jack Stoner
From:
Kansas City, MO
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Posted 7 Jun 2012 5:45 am
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The Bias adjustment pot is not really a hum balance pot. In many, one side of the grid voltage is fixed and whether its right or wrong I always set the other side (the adjustable side) to the same voltage as the fixed.
If the plates are glowing red there is a problem, and the first place to look is the grid voltages. |
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Jonathan Shacklock
From:
London, UK
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Posted 7 Jun 2012 6:11 am
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Hi Jack, it's not a typical bias adjustment pot, it's a balance pot for matching the output tubes. I've bought a matched pair and set the pot in the middle (where the idle noise is least). Do I need a bias tool to set this type of pot?. There is also a separate hum balance pot.
The plates aren't glowing red but there is a noticeable difference in the heat from each tube. |
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Bill Hatcher
From:
Atlanta Ga. USA
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Posted 7 Jun 2012 7:09 am
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that is not the most ideal adjustment. the hum balance is for the filiment voltage. you just turn it till the hum is at its lowest point. has nothing to do with the bias as you know.
now for the bias balance. if you have a nice matched pair then when it so called "balances" you still may not be running those at an optimum level. i hate those bias balance things. thats why i never buy fender amps that dont have the blackface bias set up where the bias adjusts on both sides of the push pull set up and adjusts both tubes the same up or down depending on which way you turn the pot.
you need to at least measure the MA draw per side after you adjust the bias. two 6l6 amps...i think about 60ma per side is ideal. 4 6l6 amps...120ma per side.
i dont know if you can use the shunt method on your amp to measure the current draw or if you have a meter that reads DCMA. thats the way i do it on my older fenders.
bottom line...yes you need something to measure the current draw, or you wont know if the amp is biased correctly or not.
as for the temp...unless the tube plate is glowing red, that is not a good way to judge how the bias is set up on the amp. here is a link to the shunt method and others. good luck
http://www.duncanamps.com/technical/lvbias.html |
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Tim Marcus
From:
San Francisco, CA
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Posted 7 Jun 2012 7:31 am
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| Bill Hatcher wrote: |
about 60ma per side is ideal. 4 6l6 amps...120ma per side.
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I think your numbers may be slightly high - each tube should draw somewhere between 25-35mA depending on how you like it, and how long you want them to last.
new production tubes tend to draw more current than the originals they are copies of
in order to get the most out of your new set of tubes, I think you should take the amp to a proper tech to set the bias (which will involve replacing the bias resistor) or have it converted to an adjustable grid bias design like the blackface fenders had
the good news is this is all very easy routine stuff for any competent tech.
one may be hotter to the touch because it is next to the power transformer. There may not be as much air around it. You can always mount a little fan behind the tubes (like a computer fan) to keep them cool but the noise may bother you
_________________
Milkmansound.com |
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Bill Hatcher
From:
Atlanta Ga. USA
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Posted 7 Jun 2012 8:25 am
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| sorry about that. i couldnt remember if it was 60/120ma total divided by two or not. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 7 Jun 2012 8:43 am
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| Tim Marcus wrote: |
one may be hotter to the touch because it is next to the power transformer. There may not be as much air around it. You can always mount a little fan behind the tubes (like a computer fan) to keep them cool but the noise may bother you |
With matched tubes any palpable heat difference will be due to bias difference... you won't be feeling the heat difference from the transformer, the tubes cook much harder than that.
And setting the bias balance control in the middle is no guarantee of balance... you have to pull the finals out and measure the negative bias voltage at both grid pins while adjusting the pot. This should get both tubes the same heat, maybe...
Notice the low-power switch gets involved...
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/fender_75_schem.pdf
This amp is not quite so easy to 'blackface' as other amps... make sure your tech sees this. |
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Jonathan Shacklock
From:
London, UK
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Posted 7 Jun 2012 8:47 am
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| Great replies gentlemen, thank you. The low power switch, of course... I will report back... |
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Jonathan Shacklock
From:
London, UK
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Posted 7 Jun 2012 1:47 pm
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OK, it's not the low power switch or proximity to a transformer. After a few minutes idling on high power (w/master vol on 10) the inner tube (V6) is too hot to touch on top (just gave myself a blister on my 'm' picking finger) while the V7 tube is significantly cooler - you could leave a hand on it indefinitely. No heat coming from the transformers, no visible glowing from the plates with the lights turned off. The tube seemed to be hotter on the top (opposite end to the pins) inside edge, roughly lining up with pin 5. Starting to think this is more than a bias issue but I guess it's back to my tech.  |
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Tim Marcus
From:
San Francisco, CA
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Posted 7 Jun 2012 2:02 pm
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keep in mind that new production tubes aren't manufactured very well. They vary a lot even within the same batch.
A matched set won't stay matched for long. Its very common for them to drift within the first few hours of use.
when ordering new prod. tubes, its never a bad idea to pay a little more and make sure they are hand tested and selected and come with a warranty. Its for this exact reason that I only use Ruby tubes if I am building something with new production.
_________________
Milkmansound.com |
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Jonathan Shacklock
From:
London, UK
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Posted 7 Jun 2012 2:59 pm
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| I bought these Svetlanas from http://www.watfordvalves.com which if you believe the hype run more checks on the tubes "than any other facility in the world", and I paid about as much as you can pay for them. When I swap over the tubes the heat stays with the V6 so I don't suspect faulty tubes just yet. The amp just came back from my tech who checked the bias resistor with the old TAD's, apparently the bias was fine but he measured the output at below 40w. He tried some used svetlanas and measured 60w. So I bought the same model winged c's and noticed more volume and better tone (than the TAD's). I was hoping not to have to go back and have the bias checked again since he said it was in the right range. |
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Jack Stoner
From:
Kansas City, MO
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Posted 7 Jun 2012 3:19 pm
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| The Bias or the Bias balance can change with different tubes, even tubes from the same brand. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 7 Jun 2012 4:47 pm
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I think you have a leaky coupling cap at V5, or that one of the plate resistors in the phase inverter or output section has drifted in value.
I'd replace all the coupling caps, just as a matter of principal (it's good maintenance) and then do resistance chechs on everything in the V5-V7 area.  |
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Joseph Meditz
From:
Sierra Vista, AZ
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Posted 7 Jun 2012 6:54 pm
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Hi Jonathan,
What I would do is measure the bias. Measure the bias when you first turn on the amp and adjust the pot to make them the same for both power tubes. If you can't make them the same and the bias on the tube that gets hotter is more positive than the good side, then a leaky coupling cap out of V5 may be the culprit. If so, disconnect the coupling caps and see if you can adjust the bias.
On the other hand, if you can set the bias the same on both tubes, then monitor the bias for about 20 minutes. As the amp warms up you should see the hotter tube's bias go more positive because the additional heat is stressing the cap.
This time consuming process is something a tech is not inclined to do. My guess is that the bias was probably OK when the tech measured it and the amp was cold.
To answer your original question, yes, you should be concerned because, when it gets worse, you will toast your output transformer. A high voltage fuse might protect it ... if the amp had one  |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 7 Jun 2012 6:57 pm
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Really at this point, with out a voltmeter or bias probe, you're just guessing. If you're not experienced in high-voltage techerry I'd defer to your local tube wizard, unless you've got one of these (bias probe):
http://taweber.powweb.com/biasrite/br_page.htm |
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Jonathan Shacklock
From:
London, UK
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Posted 8 Jun 2012 1:15 am
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I really appreciate your replies everyone. I don't have a bias probe and I'm going to have to go back to my tech with this. I think the warranty on the tubes specifies having a qualified tech install and bias anyway. This is getting expensive.
I put the old TAD 6L6GC's in this morning, guess what, no discernible temperature difference. This may explain why my tech didn't find a problem (although he did test with winged c's also). |
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