|
Home
/ FAQ
/ News Classifieds / Events |
Audio Asylum Thread Printer |
Get a view of an entire thread on one page |
66.32.67.236
A friend asked me to take a look at his Fender Super Reverb because he'd been having a problem with a tube going red hot. He told me it was the socket and not the tube, but I think he may ultimately have been mistaken.
The output tubes were Philips ECG 7581A types. They tested fine at EICO tube tester voltages, but performance in the amp was something else. They'd run fine for a few minutes, then one would start to go into grid emission as measured by a voltage drop across the 1.5k grid stop resistor. It was rapidly increasing before I shut the amp down.
So, I swapped in a pair of old GE 6L6GCs from the spare tube box: perfectly stable bias.
Clue: ONE of the output tube coupling caps was replaced at some point. Why the tech didn't replace both as a pair, I don't know.
What I also wonder about is why Fender did things like use a 220k grid resistor with fixed bias--over twice the maximum on the tube's datasheet. Isn't that asking for grid emission and bias creep problems?
David
> > What I also wonder about is why Fender did things like use a 220k grid resistor with fixed bias--over twice the maximum on the tube's datasheet. Isn't that asking for grid emission and bias creep problems? > >
Almost all fixed-bias guitar amps use over-spec grid resistors... makes the output stage easier to drive. You can get away with it if you bias them on the cold side.
Switch the tubes between the two sockets and see if the same one goes ballistic, or if the same socket goes ballistic...
Do both 7581A start going ballistic, together? Or just one tube? You may want to check the BIAS POWER supply for consistent negative voltage in the range in the schemo.Also, 7581A usually biased hotter and put out more watts than 6L6GC. Maybe the extra current/voltage needed is revealing a parasitic problem, that the stock 6L6GC wouldn't reveal? A WAG.
Edits: 07/03/09
"Do all four 7581A start going ballistic, together? Or just one tube? You may want to check the BIAS POWER supply for consistent negative voltage in the range in the schemo."
The Super Reverb has only a pair of output tubes, not four. Of the 7581As, both showed gradually increasing grid emission, but one went a lot faster than the other. Since I'm not a total moron, a stable negative bias supply was one of the first things I checked :-)
"Also, 7581A usually biased hotter and put out more watts than 6L6GC. Mabe the extra current needed is revealing a parasitic problem, that the stock 6L6GC wouldn't reveal?"
I biased all the various pairs of tubes I used at the same level, around 35mA per tube. Yes, you should be able to run 7581As slightly harder, but in this situation, that would just have made them go out of control faster.
David
T, when I asked about the bias you set the amp to, I was trying to figure out why the 7581A's were bad, but the 6L6GC you replaced were ok.If one tube is starting to go ballistic first, it's not uncommon for its push-pull partner (and since there are only two tubes, all power tubes :-) .... to shows signs of too much current. This is due to the excess draw by the first tube. This one is prolly the bad tube.
The 7851A can test fine and when in the amp & powered up, be no good. If the 6L6GC you replaced in the amp work fine at volume for a good length of time, then prolly need to look for new pair of 7581A's. If you want to keep that tone.
You may want to eyeball the resistors on the power tube sockets for damage or out-of-tolerance values.
Edits: 07/03/09
I will certainly check the resistors on the sockets. I have seen them burned completely open on other Fender amps.
What I think I'll do is offer to sell my friend the pair of used/good GE 6L6GCs I have. They match well, and look *exactly* the same as some GE 7581As on eBay at the moment. They also showed the least sign of even incipient grid emission of any pair of tubes I tried in the amp. I'll let him play the amp and see if he likes them. I'm not a guitar player myself, and I have no amps that use a pair of 6L6s. The Leslie amps I have require a matched set of four. I have two other GE 6L6GCs, but one of them is extremely microphonic, as I found when I installed them in the Leslie. It set up an interesting howl.
David
They sit in a pretty hot enviroment most of the time (during use, of course). Use same wattage rating resistors, nothing fancy, as you want them to act as "fuses" for the power tubes*.* (I know that you prolly already know that, but Fender INSISTS on the disclaimer). :^)
Edits: 07/04/09 07/04/09 07/04/09 07/04/09 07/04/09 07/04/09 07/04/09
T, since you've worked on Fenders, ever see a pink ceramic based STR 415 (6L6GC) or STR 416 (6CA7). I believe both were special issues the 415 for Fender's Evil Twin and the STR 416 for the Mesa Boogie Mark III. I'd love to get a pair or two of each, NOS.
I have to talk to old guitar amp nuts who actually remember these tough, great sounding tubes. I remember someone actually put nearly 700 VDC on those STR 416 and they hardly glowed. Remember the blue and red glass Tesla E34L's?
I've heard of those pink-based tubes, but I've never seen one.
I'm just starting to work on Fender amps. Some of the designs seem pretty hard on the miniature tubes, running them at excessive plate voltages, etc... I guess these were designed back when tubes were cheap.
Yeah, but a lot of amps run plates at high voltage. I remember working on a CJ monoblock that ran a hefty voltage on the 6CG7. Gotta be sure that the cathode resistor/bypass caps are within tolerance, so the current draw is ok. And coupling caps are ok.
BTW... if you drop the plate voltage, sometimes the great headroom of the amp kinda goes away. As well as the crunch in the overdrive. Anyhow, need for quiet gain stage tubes (esp the first stage) is a must. I like BP RCA 7025 or Euro (TFK or Philips made) ECC83 in that position. They give nice overdrive and stay quiet, despite the crazy abuse.
Post a Followup:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2009, Audio Asylum, all rights reserved ]
[ Go Green ]
[ Take5 ]