135W Fender Twin is slowly burning up phase inverter tubes after a recap.

[cgoodson]

I know the first response might be I got a cap backwards but I have checked it with the schematic and everything looks right to me. Anyhow, my friend bought this amp a few weeks ago and brought it to me to look it over. It still had the original filter caps so I decided to replace all of them with equivalent caps however I did substitute a 100uf where there was a 70. The reverb was also really weak sounding and had been modded to have a dwell control which I disconnected and put things back stock, (that didn't help, both the tremolo and reverb are still weak sounding). I also replaced a few caps and the 100K resistors on the board and neatened up some sloppy work done by someone previously.

When I was done the amp was getting a tiny bit of distortion which went away after I put a new phase inverter tube in. I played through it for a couple hours and it sounded great so I let my buddy take it home. Then he played though it about 5 hours and it worked fine but blew a fuse after. He replaced the fuse and used it a couple more hours and then the phase inverter tube went bad.

Now I've got the amp back looking it over. I replaced the bad tube then pulled out the chassis to check some voltages and when I turned on the amp on there was a bright flash in one of the power tubes and the fuse blew. Now I just have power tubes pulled and have been checking voltages to the schematic, everywhere I've checked seems to be within a reasonable range, maybe slightly high. I've rechecked my work over and over and I'm just not sure what's going on with it, any help would be greatly appreciated.

[Jazz P Bass]

If all the voltages look good, then put in new power tubes.

[Twist]

You may have two unrelated issues. Even new tubes can go bad quickly. The output tube that flashed is probably trash now. Unplug the amp, drain off the high voltage, then take a small metal scribe and make sure all the receivers inside the output tube sockets are tight. The negative bias voltage may be at pin 5 but the socket is too loose to make contact with the tube pin. I'm assuming you have a method for setting the bias. As for the PI tube, I've had many EH & JJ tubes go bad right out of the box.

[pdf64]

Those 135 watt UL amps are very hard on the output tubes; a 'perfect storm' of high B+, UL and stiff supply. They were designed around the Sylvania STR387, a super beefed up version of 6L6GC, current production tubes don't stand a chance if the amp gets cranked. At least increase the screen grid resistors to 1k 5W and keep it clean. Pete.

[cgoodson]

Thanks for the replies everyone, I really appreciate the help. I have worked on a good few amps over the years but my knowledge is limited so any help means a lot. I have to say I've been lucky to have mainly worked on older Fenders which are very neat inside, almost art IMO, this amp looks like a rats nest! I wonder what bonehead made the call to stop worrying about getting the layout right and to stop using good wire??
The voltage on the phase inverter is around 310 that's 10 volts over the max for a 12at7, if the tube is of poor quality maybe that would shorten it's life. The schematic says it should be at 280V, so just to get it within the proper range I'm going to bump up the resistance a little there and see how that does. Also, I went ahead and tightened up the power tube sockets. The reverb will have to wait until I can get some power tubes, after looking that part of the circuit over I think the problem is likely to be the tube, cable or tank but I guess I will see.

I told my buddy about these amps being hard on output tubes, now he's considering getting it going and getting rid of it. What I was wondering is if there is possibly a substitute for the 6l6's that might last better or could I somehow mod it to be easier on the tubes? I found a website selling tubes called "TAD GE 6L6GC STR Fat Bottle", supposed to be a replica of the STR387, anyone have any experience with them? The website is - Tubes

[Prairie Dawg]

TAD are fancy assed Chinese tubes. If I'm buying Chinese I'll pay the market price and not for the label. I have had good results with Ruby 6L6GC-STRs whick *look* like STR387s. Whether they *are* as good is problematic, but I have put half a dozen quads in various high voltage amps over the past year or so and no complaints so far-Mesa, Ampeg V4 and V22, and Fender Twins.

[JoeM]

The voltage on the phase inverter is around 310 that's 10 volts over the max for a 12at7, if the tube is of poor quality maybe that would shorten it's life. The schematic says it should be at 280V, so just to get it within the proper range I'm going to bump up the resistance a little there and see how that does.]

What is the cathode voltage sitting at in the PI? It's elevated due to the tail resistor etc, so the effective plate voltage is less than the 310 you're measuring.

[cgoodson]

What is the cathode voltage sitting at in the PI? It's elevated due to the tail resistor etc, so the effective plate voltage is less than the 310 you're measuring.

The cathode is showing about 66V, the schematic says it should be 110V. I would assume something isn't right there but I always thought the voltages shown on a schematic are actually what your supposed to get at that spot. Is 66V on the cathode and 310V on the plate ok?

[jmaf]

Hi cgoodson, it is unlikely the new capacitor is to blame, adding 30uF will not increase voltages, it will only improve your power supply, by adding a little more work load on the rectifier.

My guess is that amp was made for 115 V and you're running it at 127VAC, so all your voltages are a bit high. 310 is high for the PI but lots of 12au7's can stand that. As others mentioned, tubes can fail immediately after coming out of the box, especially when running at their absolute maximum ratings.

Check all your contacts and solders, socket pins and so on. Slightly rock the tubes and mess around the wiring using a chopstick to listen for pops and noises, you'll likely find some cold solder or loose socket pins.

[kg]

sovtek 5881s are ridiculously hardy tubes. at least, they USED to be. i've tortured the shit out of those suckers and they just keep going.

[MWJB]

Is the PI tube a 12AT7?

You effectively have 254v on the plates of the PI tube, as you deduct cathode from plate voltage to determine what the tube actually sees.

What voltage drop do you getacross the 470ohm cathode resistor and is that 66v at both pins 3 & 8 of the PI tube? Does the amp have 22K PI tail resistor (this may have been changed, throwing off voltages)?

You say "voltages are OK", but if the amp is failing, they aren't. Please list dc voltages for power & preamp tubes.

[jmaf]

sovtek 5881s are ridiculously hardy tubes. at least, they USED to be. i've tortured the shit out of those suckers and they just keep going.

They are as hard as they come, indeed. But I've found their sound to be too "clinical" and dry, unlike the 6L6's typical warmth.

[kg]

They are as hard as they come, indeed. But I've found their sound to be too "clinical" and dry, unlike the 6L6's typical warmth.

agreed. however it might be possible to counteract some of their sonic shortcomings with some circuit tuning?

if i had an amp that i needed great reliability from and it got a lot of use, i'd seriously consider it. they're just so cheap and tough.

[jmaf]

agreed. however it might be possible to counteract some of their sonic shortcomings with some circuit tuning?

if i had an amp that i needed great reliability from and it got a lot of use, i'd seriously consider it. they're just so cheap and tough.

I don't know of a power amp modification to warm up the tubes(accidental pun), except maybe adding small resistors in series with the plates to cause sag.

This time last year I invested almost a hundred bucks into a pair of SED for an amp that ran on 450V DC plates, 385 V DC screens, nothing out of this world. They started arcing within hours....tubes do that sometimes, but when you pay a high price for good tubes, you sorta deposit more expectations on them...

With the 5881's I've never seen one go bad...never.... The WXT+'s are very sturdy as well, and cheap as they come.

[Gtr_tech]

Yep, I'd go with those wafer base Sovwreck "5881s". They'll take whatever you throw at them. They don't sound the greatest, but neither does a UL fender.....for guitar anyway. Prolly be fine for bass hooked into a bass cab. Those tubes are great as pass regulators in pwr supplies too.....