A Bias Question

Dogbreath

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I just put in a new matched pair of Electro Harmonix 6V6's in my Deluxe Reverb Reissue. I got out my Bias Probe and saw that the two new tubes were running cool at a little over 15 ma. I believe that I biased the pair of JJ's that I was replacing at 25 ma. What I found was that I could only get the new tubes up to just under 23 ma. I went ahead and set them at 20 ma. The amp sounds outstanding, but what I'm wondering is if it's normal not to be able to set the bias higher than the current upper limit of 23 ma depending on the particular pair of tubes? Are these just cooler running tubes?
 

Dogbreath

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When I speak to the guy who does my biasing, he always mentions it being done on a % dissipation, not just a nominal figure.
I understand this. The Bias Probe gives the final figure that the math will also arrive at. I think I already have my answer. I believe my output transformer is failing. I’ll be taking it to a tech as soon as I can find the closest and best one to me. I’m getting a power drop out that tube replacement doesn’t take care of.
 

CB91710

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What's your voltage?
Pulling more current will tend to pull the voltage down, which impacts wattage, and % of max wattage is what you're shooting for.
At 350v (plate to cathode, not plate to ground) you're OK at 24-28ma
At 400v, you don't want to go over 24ma

It's a game of balance

You're wanting to be between 60 and 70% max dissipation (14w for Class AB), or 8.5 to 9.8 watts... call it absolute max 10w for a round number on the 6V6GT
But a straight 6V6 (non-GT) specs out at 12w, so your current draw needs to be much lower.
 
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Dogbreath

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What's your voltage?
Pulling more current will tend to pull the voltage down, which impacts wattage, and % of max wattage is what you're shooting for.
At 350v (plate to cathode, not plate to ground) you're OK at 24-28ma
At 400v, you don't want to go over 24ma

It's a game of balance

You're wanting to be between 60 and 70% max dissipation (14w for Class AB), or 8.5 to 9.8 watts... call it absolute max 10w for a round number on the 6V6GT
But a straight 6V6 (non-GT) specs out at 12w, so your current draw needs to be much lower.
The issue is that I’ve been experiencing abrupt power drop. It will return and then drop out until I shut it down. I suspected a faulty power tube, but when I biased in a new pair, the highest the bias pot would go is 23ma. I plugged straight in tonight without any effects or tuner. I got the same drop out after about fifteen minutes. The amp is eleven years old and I’m outside of my electrical experience. It’s time to hunt down a tech. There are three that are 1 1/2 to two hours away. Time for a road trip. If it’s too expensive it’s time for a new amp.
 

Dogbreath

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My 1974 silver face Champ Is still among the living, and it sounds as good or better than my other amps. What an amazing piece of amp magic.
 

Eigen

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Regarding your first question, yes some tubes are just cooler than others and will often not get into optimum bias on amps thst have a narrow bias circuit. I've had several pairs of JJ EL34's like this... Would have had to modify the bias circuit to get them to bias, no go on these amps for me. Ended up using them in another amp with a much wider bias range, sounded perfectly fine.

Regarding the drop in power, do you mean the amp just goes quiet while playing or what is happening?
 

Dogbreath

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Regarding your first question, yes some tubes are just cooler than others and will often not get into optimum bias on amps thst have a narrow bias circuit. I've had several pairs of JJ EL34's like this... Would have had to modify the bias circuit to get them to bias, no go on these amps for me. Ended up using them in another amp with a much wider bias range, sounded perfectly fine.

Regarding the drop in power, do you mean the amp just goes quiet while playing or what is happening?
After fifteen to thirty minutes of play time I hear a very short and mild crackle sound and then the volume drops by about one third to one half. Bass and treble also decrease. If all will come back for three or four minutes then drop out again. I’ve changed and biased two new matched pairs of power tubes with no change.I can come back to the amp the next day and the problem repeats in the same way.
 

Dogbreath

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Regarding your first question, yes some tubes are just cooler than others and will often not get into optimum bias on amps thst have a narrow bias circuit. I've had several pairs of JJ EL34's like this... Would have had to modify the bias circuit to get them to bias, no go on these amps for me. Ended up using them in another amp with a much wider bias range, sounded perfectly fine.

Regarding the drop in power, do you mean the amp just goes quiet while playing or what is happening?
Thank you for validating that it’s normal to just have a pair of cool tubes.
 

ultra80096

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Have you ruled out a possible pre-amp tube? I just experienced this same drop out issue, messed with various new-used power tubes & bias, just to find a V2 12AX7 was actually the culprit. I didn't have the bias numbers issue though. That was steady.
 

Eigen

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Could be something a simple as contact corrosion on the plugs (if you've got contact cleaner, might be worth going to town on the plugs) , issue with the electrolytics or a bad solder contact that manifests as the amp heats up etc.

What makes you think it was a faulty OT? If it sounds nice and loud until the fault appears, I would think the OT should be ok.
 

Dogbreath

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Thanks everyone. With your guidance I changed out V2 and V4 and cleaned some plugs. PROBLEM SOLVED. Both of these tubes were relatively new and did not look problematic. I was wrong. The amp is good as new. Thanks again.
 

zdoggie

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I'll change tubes ,and clean or inspect prongs on tubes ,but when it comes to sticking my hands around 600v it time to pay a qualified tech My dad was a ham radio guy ,amateur extra , 2000wats rms and he built the whole thing him self
and I got warnings in a regular basis about the danger and to this day I distance myself from high voltage .

zdog
 

CB91710

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I'll change tubes ,and clean or inspect prongs on tubes ,but when it comes to sticking my hands around 600v it time to pay a qualified tech My dad was a ham radio guy ,amateur extra , 2000wats rms and he built the whole thing him self
and I got warnings in a regular basis about the danger and to this day I distance myself from high voltage .
I still have scars from the holes in my right palm and left index finger inside knuckle from a ~2-gigawatt-second hit that I took from the 700v radio amplifier power supply in 1990.

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20210723_155432.jpg
 

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