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X-101-C Transformer Temp after New Tubes

MadPik

MadPik

New Member
Greetings! Y'all were super helpful at assisting me with getting my X-101-C back up and running and I have enjoyed many hours of happy listening since then. Anyway, since I knew nothing of the life or health of the original tubes (3 vintage Fisher branded and 1 vintage GE) I caught a sale picked up a matched set of the Tung-Sol 7591a reissues. I rebiased the amp to 42V after swapping them in.

Since then my power and output transformers will heat up to the 150-160F range after an hour or so of listening. They don't seem to get any hotter than that, however, this is warmer than they ran with the original tubes. Line voltage at my home is fairly steady at 120V and I haven't seen it higher than 122V.

I'm at work at the moment but my plan when I have a moment is to:

  1. Check the bias again to see if anything may have changed now that the new tubes have a decent amount of playtime.
  2. Swap the old tubes back in, re-bias, and double-check the transformers do in fact run cooler.

Is this something I should be concerned about? Are other changes needed to accommodate the newer tubes?

Thanks!
 
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Some of the new manufacture tubes bias differently than the old manufacture tubes. It will be more accurate if you put 10 ohm resistors in the cathode circuits of all 4 output tubes and then measure/calculate the actual currents through them - rather than simply bias'ing them to a particular voltage. I'm guessing, but maybe 34ma is a good target. With the new tubes you could be drawing significantly more than the optimal current value and that will shorten the life of the tubes and put stress on the power transformer and output transformers.

Others will chime in to fine tune or correct what I'm saying.
 
Tim; Even tho he can install 10Ω resistors in the cathode circuit, BIAS is CATHODE BIAS, not FIXED. Different way of setting bias.

MADPIK; Spec Voltage on the BIAS CIRCUIT IS 40V, no more. Otherwise you'll be overdriving the 7591's and they tend not to be nice then. Don't go lower than 40V or you'll have 4 or 5 grandkids before the 12ax7 heaters in the BIAS circuit heat up enough to conduct as they are running @ 40/4 = 10Veach. With the Bias @ 40V the temps should come down a little, but 155-160*F is ok, while on the high end of the range. If you're mech inclined, read up on AUDIODON's (RIP) X-101-C build up by Dave G. I'm thinking your's would be a good candidate for EFB.
 
Larry,
Yep, cathode bias. I hear you on the heater voltages for cathode bias amps that use the front end 12AX7 tubes as bias resistors. In several Scott amps I've been working on, heater voltage per tube is below 10V and the old stock front end tubes work fine. The 222B that is playing as I type cathode biases the 6BQ5 output tubes at 38.5VDC; that's 9.6V per 12AX7 and it warms up to play music in about 45 seconds. Lately, though, I've seen that some new production 12AX7's like the Sovtek 12AX7LPS can be unhappy at very low voltages, but they also seem to do well enough at or above 9.5V per tube. I worked on a Scott 272 recently that had 6 12AX7's running at about 7.5VDC per tube, and that played havoc with the Sovteks (low output and higher distortion). I brought up the supply voltage to about 9.5V per tube and they worked fine at this level. Has your experience been different?
Dave
 
Pretty much the same as your's. I try and keep domestic 12ax7's in my consoles, and cathode biased stuff especially units that hold hostage 4 tubes for BIAS regulation. The Russkie stuff goes in the 400's and 800c's.
 
So I had time to readjust my bias back to 40v and have over 10hrs of playtime since them. After about an hour the temps on all the transformers are between 155-160F (measured with my infrared thermometer) and don't seem to increase from there. Aside from the transformers running a bit warmer than others I've heard or, I have no buzzing or other issues so I think I'm just going to sit back and enjoy the music. Thanks again for the advice everyone!
 
With or without a case on the "C"? You can take a 6" personal fan and run on low, blowing in the general direction from about 18-24" away. Enhanced air movement will often reduce temps a few. Don't blow air directly on the tubes tho.
 
I've been considering a fan set up. The temps are with it in the case but with the metal cover/baffle off. It is currently powering a pair of McIntosh XR-14s which aren't the most efficient speakers...
 
Take it out of the case, temporarily, run it and check temps again. If no discernable difference (<5-10*F drop) then a fan setup is warranted to keep them in line.

Is there a shield panel between the output tubes and the transformers like on the receivers? I'm wondering if the output tubes(Tung-Sol Re-Issues) are radiantly heating the transformers unduely. And also if you put 10Ω cathode resistors, measure the mv across each 10Ω resistor. Calculate dissipation for each one. They may be running hotter (bias wise) than EH's or domestic old stock tubes.
 
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Just curious,
I've had PT get kind of warm. I never saw OPTs get to 150 (or near it). I suppose if output tubes are very close they could radiate a bunch of heat at the output iron. Am I wrong?
 
OPTs create little to no heat of their own. They sink it from heat producing elements around them.

Dave
 
larryderouin said:
With or without a case on the "C"? You can take a 6" personal fan and run on low, blowing in the general direction from about 18-24" away. Enhanced air movement will often reduce temps a few. Don't blow air directly on the tubes tho.
Click to expand...
Would blowing the air directly at the tube cause more wear on them due to the constant temperature variations?
 
I'd bet not.
The tube is a controlled environment.
Small envelope very tight tolerance. Sealed. Air wouldn't be changing internal temps. Just helping external temps dissipate.
(Inside the tube temp would remain constant.)
 
In my experience, Some large sized tubes (Rectumfriers and output tubes for oneor Two) don't like having a DIRECT path airflow from a FAN, as they are/get very hot, and the glass envelope tends to be thinner in places than smaller tubes. The glass gets a cool spot (or hot spot) which can lead to disproportionate expansion/contraction (ie: crack) of the envelope, loss of vacuum, and your wallet empties. If you're gonna use a fan, blow the air a little higher than the top of the tubes to pull hot air (convection) away from the tubes. Or if the units are in a cabinet, have the fan(s) pulling out the top, with passive vents down low. Enhance the natural convection.
 
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