Looking for some help

[PRR]

What do you hear with your ear in the speaker?

Nothing!? Then suspect the speaker jack (plug, wire, speaker). A "live" output stage will always have some small whisp of sound, even if all else is FUBAR.

No, you can't have "too much" voltage on the tubes. And your values look OK. If they were twice as high, it might work "great" but not for very long. "Dead silent" is something else.

[dude]

What do you hear with your ear in the speaker?

Nothing!? Then suspect the speaker jack (plug, wire, speaker). A "live" output stage will always have some small whisp of sound, even if all else is FUBAR.

No, you can't have "too much" voltage on the tubes. And your values look OK. If they were twice as high, it might work "great" but not for very long. "Dead silent" is something else.

Good thought, the 4 ohm spk wire broke off the jack. Fixing the amp the output jacks were not secure and too much twisting. Soldered back the the wire, amp plays and seems fine. It's too late to check it out tonight. I guess I got caught up in thinking of a more complicated problem and turned out to be something very simple that even a rookie would have checked first. Thank you.


Most of the Fender style SE amps don't have that much voltage on the preamp tube, I don't want premature wear on the tubes, 7591A are not cheap. Just leave it or reduce?


al

[sluckey]

Your voltages are fine. I'm running 470V B+ on my push/pull 7591 amp.

[dude]

Your voltages are fine. I'm running 470V B+ on my push/pull 7591 amp.

I cleaned up all the wiring and the Amp plays very loud, clean and distortion sounds pretty good. After playing hard for 15 minutes I checked all voltages, higher than I first reported.


First node is 404v (highest in the amp) after the 100 ohm, 394v, after the 4K7, 352v.  To tremolo 320v through the 68K (actually a 68K and a 4.7K in series for 72.7K.


At the 7591 grids, 352v through the 68K to 12AX7 (pin 7) 325v and pin 1 240v.  100 ohm biasing resistor on the cathode of 7591  8.7v across the 100 ohm. Plate voltage on 7591 385v. If my math is correct the dissipation of that 7591 is about 32 watts minus a little for the screens. High, maybe up the valve of the 100 ohm resistor to 150 ohm?  I've never had an amp with such high voltage on the first stage 12AX7 either, that's more than last post if OK? fine, I'll leave it that high. But I think I'm cooking my 7591...?


Just want to fine tune the 7591 bias, 8.7v low?  And the tremolo is extremely thick and too fast even at 2, the knob is on/off  slow to fast. Past 5 is useless just too thick, deep and to fast.


I appreciate your help and learned a lot, thank you.  I don't know if the 320v to the tremolo is too much , I do know I can raise the last cap value in the tremolo to slow down the speed...?


These voltages are after the amp was played for half an hour.


al

[sluckey]

Just want to fine tune the 7591 bias, 8.7v low?

That 7591 is cooking at over 30 watts! I'd replace that 100Ω/5W with a 250Ω/5W or maybe even 500Ω/5W.

And the tremolo is extremely thick and too fast even at 2, the knob is on/off  slow to fast. Past 5 is useless just too thick, deep and to fast.

Change that .01 cap to a .02 cap.  Also, there's a 220K resistor on the terminal strip next to the trem tube. If still not slow enough, change that to 1M. Maybe one or both of those two things will get it slow enough for you.

[dude]

With a 300 ohm bias R, I got the dissipation to 20 watts, that's not subtracting the screens so I'm probably at 19 watt, the max. As  I change each bias R up, the plate voltages changes, goes up but increases the cathode voltage. Seems I read that's normal. 


Probably would need a 320 ohm to get about 18 watts. Don't have anything. Using an old stock Sylvania 7591A


I think with a class A amp, dissipation can be at the max and you're ok...? I know I can get a little anal, I'm a cabinet maker and 1/16" makes a big difference with a cabinet, I can't deliver a cab with a small gap.


At one time I was thinking I'd run a small batch of head cabs to fit the same chassis, maybe a chassis Doug sells. I was thinking a kit one glues up themselves. Saves a lot of shipping. No tolex, just wood, dovetails and almost any kind of wood. The thing is I have to make at least ten as it takes too long by the piece.


19 watts ok?  if not, I'll see if I can string two together for around 320/350 ohm This is A class, right?

[PRR]

> caught up in thinking of a more complicated problem and turned out to be something very simple that even a rookie would have checked

It takes experience to check the simple things first.

My first paid service, as a "rookie", I had half the studio torn apart no luck, Stan came in and plugged-in the turntable wire. Arg, embarrassed

The "experience" does come in when you must QUICKLY sort the easy possibilities FAST. But even when you have all week to investigate, start simple.

[PRR]

7591 is a hi-gain 6L6. 500V is safe forever. Even at higher price, I would not over-fret somewhat higher voltages.

[dude]

Amp's back in the cab, sounds great. Bias is set to 18watts dissipation, left the high voltage as is with the new PT. Ended up with three .02 uf's in the trem circuit, works well. Tone knob works well.


Just wanted to thank PRR, Sluckey and 2deaf, especially a thanks to Sluckey for drawing the schematic.


al