AC15 RI red plating, no audio

[wcrankshaw]

I just got finished swapping a lot of the caps for SoZos, adding a choke in place of R6, switching C5 to a 50uF/50V, removing R4 all together, and lowering C2 to 16uF/450V, and plugging in a GZ34. Now I have no audio, and the EL84s are red plating. However, there is the typical oscillation from the trem passing through the speakers. Any ideas on this one? Thanks in advance.

[ThermionicScott]

Well, the GZ34 is going to raise voltages a bunch over the Sovtek "5Y3". That plus the screen-voltage increase from the choke will increase the current draw of the EL84s quite a bit. Perhaps one of your fancy new coupling caps is a dud, and that's why no signal is coming through. Can you hear any hum from the output stage?

Not to lecture you , but speaking as a software engineer, it's often best to implement one or two changes, test to make sure the thing still works, then implement others, rather than trying to change everything at once.

- Scott

[wcrankshaw]

There is some hum along with the oscillation. For what it's worth the 5Y3 was a JAN/Phillips - not sure if that's a true 5Y3 or not. Is there something specific that I can do to limit the current to the EL84's without messing with the tone?

There's a possibility I've got a cold solder somewhere in the new components. That would be enough to kill audio right?

[ThermionicScott]

There is some hum along with the oscillation. For what it's worth the 5Y3 was a JAN/Phillips - not sure if that's a true 5Y3 or not. Is there something specific that I can do to limit the current to the EL84's without messing with the tone?

Okay, I skimmed your post too quickly -- if we've got output, that's a good start! I would go back to the 5Y3 until you've got the front end sorted out. (BTW, a JAN/Philips is definitely a "true 5Y3" -- I assumed that the AC15 came with a Sovtek "5Y3" which is directly-heated and kicks out more voltage than a real one.)

There's a possibility I've got a cold solder somewhere in the new components. That would be enough to kill audio right?

Yep! A simple way to start would just be to reflow all the joints you worked on, add a little solder to each, and make sure they're shiny when they've cooled.

If you still want to push the envelope after fixing the signal path, I'd work up the rectifier chain gradually. Maybe try a 5R4 rectifier or the Sovtek 5Y3 I mentioned earlier next, as it'll kick up the voltages somewhat, but not as drastically as the 5AR4. To keep the tubes from melting at these higher voltages, you'll need to increase the value of the cathode resistor. Go too far, though, and you'll get crossover distortion at high output levels. I think you'll be alright if you size the resistor so the tubes dissipate 12-14 watts at idle.

Also, good luck!

- Scott

[wcrankshaw]

Thanks for the info. Just to clarify something: I removed the 470ohm cathode resistor that is in series with C5. C5 was then changed to the 50uF/50V. These changes were under the recommendation of someone who does this stuff a lot in LA (on these specific amps), but I don't want to post his name so as to promote misinformation if there was a miscommunication between him and me. You can probably assume who it was. So, by removing R4 it appears that the only cathode resistor on the EL84s is a 100R/5W hooked to both cathodes. Is this a big problem and allowing way too much current through?

[ThermionicScott]

Thanks for the info. Just to clarify something: I removed the 470ohm cathode resistor that is in series with C5. C5 was then changed to the 50uF/50V. These changes were under the recommendation of someone who does this stuff a lot in LA (on these specific amps), but I don't want to post his name so as to promote misinformation if there was a miscommunication between him and me. You can probably assume who it was.

I don't know who it is, but it's a harmless mod by itself -- basically removes the tone-shaping that the AC15 reissue has at that point.

So, by removing R4 it appears that the only cathode resistor on the EL84s is a 100R/5W hooked to both cathodes. Is this a big problem and allowing way too much current through?

The capacitor (C5) blocks DC, so R4 wasn't "helping" with the cathode current. It's that 100-ohm resistor, which is too low in value to begin with! I'd swap in a 130- or 140-ohm resistor and remeasure the dissipation.

Just out of curiosity, what plate and cathode voltages are you getting?

- Scott

[wcrankshaw]

Unfortunately I'm nowhere near the amp right now so now voltages, but I want to say they were upper 300s.

So...major palm to the forehead on this one. I was re-reading emails about this, and i was supposed to change the 100R/5W to a 130R/5W... That solves that I guess.

Now I just have to figure out the signal path point of death.

Thanks for all the help, and helping me figure out my own DOHH! moment.

[ThermionicScott]

Upper 300's, yikes. That's about the point where you start considering fixed-bias for those poor EL84s.

There is a mod out there using a Zener diode in the cathode circuit to prevent the bias from shifting too far. EDIT: It's the "Chuck H" mod: http://music-electronics-forum.com/t10540/

Glad to be of help... hope that amp comes through in the end!

- Scott

[Jag]

Replacing the 100R bias resistor with a 130 or 140 will bring the plate voltage down a bit too.

Remember to subtract the voltage across the cathode resistor from your plate to ground voltage to get the voltage the plate actually "sees".

[Chuck H]

Woah now. I've come up with a few nifty circuits (but apparently nothing cool enough for a spot on "The Blue Guitar" site, which I've been bucking for ) but this one isn't mine.

Unfortunately I can't find the site where I got the idea from. This guy also had a zener circuit for creating an adjustable bias for cathode biased amps by regulating voltage to the grid via the cathodes standing voltage. Very cool. If I find it I'll post the site.

It WAS my idea to incorporate it with the "Paul Ruby mod" for maximum effect. And the combination works brilliantly. But the PR mod by itself outshines my little addition IMHO.

Chuck

[wcrankshaw]

UPDATE - I fixed all the messed up traces with a conductive pen (those traces basically desolved when replacing several of the components). So, now I've got sweet sweet tone, but here's the rub. One EL84 is red plating. I've got 130ohm across the cathodes of the power tubes. I'm a little confused as to why only one is getting so much current, and I've swapped one for the other and the red plating still occurs in the same socket. Any ideas?

[ThermionicScott]

Woah now. I've come up with a few nifty circuits (but apparently nothing cool enough for a spot on "The Blue Guitar" site, which I've been bucking for ) but this one isn't mine.

Unfortunately I can't find the site where I got the idea from. This guy also had a zener circuit for creating an adjustable bias for cathode biased amps by regulating voltage to the grid via the cathodes standing voltage. Very cool. If I find it I'll post the site.

It WAS my idea to incorporate it with the "Paul Ruby mod" for maximum effect. And the combination works brilliantly. But the PR mod by itself outshines my little addition IMHO.

Chuck

Ah, okay. I appreciate the humility!

UPDATE - I fixed all the messed up traces with a conductive pen (those traces basically desolved when replacing several of the components). So, now I've got sweet sweet tone, but here's the rub. One EL84 is red plating. I've got 130ohm across the cathodes of the power tubes. I'm a little confused as to why only one is getting so much current, and I've swapped one for the other and the red plating still occurs in the same socket. Any ideas?

How are the plate and screen voltages on each socket? Try cleaning the sockets with some DeOxit -- perhaps one of the control-grid connections is dirty and allows that tube to runaway a little.

- Scott

[wcrankshaw]

Plate is 255-260V on both, and pin 2 (screen) is 39V.

Also, I swapped some other tubes I had in there and both turned red. I guess it's happening to both, but is just a little sporadic as to which ones get red hot.