Single Ended Amp Higher Volume Distortion

[Tapsnap]

I need help identifying the source of a weird distortion. It occurs at high volumes only and it sounds like a blown speaker. The speaker is fine.  In fact, I swapped it out for another and got the same sound. It’s a single ended amp and I have uploaded the schematic and the photos of what I have.
 
I have gone through the entire schematic and checked all connections and they seem fine. I checked the voltages and there are some problems. The voltage on the preamp plate shown as 73v, actually reads 122v. The voltage on the preamp plate marked 85v, actually reads 139v. The cathode resistor R6 reads 0.87v, not 1.1v as it should  and the cathode resistor R7, reads  1.17v, instead of 1.94v.  All other voltages seem to be right on or pretty close.

At first I thought it was the plate resistors, but I checked them and they read 100K. I had added two 68pF caps to the plate resistors, going to ground,  which I had seen on another schematic that takes out any super high frequencies. I disconnected those, but still had the same problem.

I had heard  about something  called motorboating which I checked out on line and the sound is not quite the same as mine. Still the video talked about the power transformer not being powerful enough to supply the circuit. I thought my back to back 3amp Radio Shack transformers would be plenty powerful enough but now I’m wondering if they are, because I forgot to account for the 6.3 volt heaters.
Any thoughts?
P.S   I know the layout plan is sloppy and there are some missing grounds at the jacks.

[sluckey]

What are the voltages at C1, C3, and C4? What does the filament voltage read across pins 4 and 5 of the EL84?

[Tapsnap]

C1 is 278v
C2 is 259v
C4 is 197v

[Tapsnap]

I'm not getting any reading across pins 4 and 5

[sluckey]

I'm not getting any reading across pins 4 and 5

If there really is no voltage then the filament will not be lit, the tube will be cold, and the amp will make no sound.

[PRR]

> The voltage on the preamp plate marked 85v, actually reads 139v.

Those small-tube voltages do not look right.

Further: (197V-85V)/100k is 1.12mA in the plate leg, while 1.94V/2.2k is 0.88mA in the cathode leg, but they should be =equal= (within 0.001mA). Meter loading may be some of that, but it looks dubious.

I asked the idiot what they should be, and it could not find those numbers. By interpolation, those readings imply a tube of Mu near 35, not the 20 60 100 we get in the AU AT AX series tubes. So they could have been taken with a 12AY7. Or taken under the influence...

Your readings seem plausible and workable. There's enough headroom in the driver to slap the EL84 senseless, and controllably clean as desired.

[JB]

There's enough headroom in the driver to slap the EL84 senseless, and controllably clean as desired.

Too much possibly?  There's no grid stopper on that EL84, blocking distortion when turned up too loud?

[Tapsnap]

Sorry. The voltage between 4 and 5 on the EL34 reads 6.6v.
But here's something to add to the mystery, the tube I had in the preamp socket was a 12AX7A and I swapped it out for an ECC83S and the sound is much worse. In fact, the noise is more of a pronounced BRRRRRRR at all volumes. I swapped that one for two other ECC83S's and the same BRRRRR. When I change back to the 12AX7A the noise only occurs at higher volumes. Of course the 12AX7A and the ECC83S is essentially the same tube but there does seem to be a substantial difference in the distortion it creates.

[Tapsnap]

Holy moly, I just realized my mistake, I’ve been using a 12AX7 when I should have been using a 12AT7. Duh!!

[Tapsnap]

I just replaced the preamp tube with a 12AT7 and it's still doing the BRRRRR thing at higher volumes. Its an old tube from an old piece of equipment, so I'm going to order an new one and I will probably order a 12AY7 at the same time. Any other thoughts?

[Colas LeGrippa]

You need a power tube cathode bypass cap to solve your prob. Try a 50 uF 50V.
Put a 12ax7 instead of the 12at7 and swap the 4m7 grid leak for a 1 m.

[Colas LeGrippa]

In a petfect world a choke (5h  ) would greatly improvre your build. You would remove the 470 phm resistance in the power supply and hook up the choke right in there




Colas

[Colas LeGrippa]

If you hook ip a 12ax7, use 1k5 as cathode resistors the first one bypassed with a 25uF cap, the second one unbypassed

[Tapsnap]

Colas, l’ll try all that tonight. I happen to have a 4 Henry choke. That I could try out too. I’ll let you know how it goes.

[Tapsnap]

So Colas, I made those changes and it fixed the problem. Only now I'm getting more hum. I didn't install the choke because there was not place to fit it and I installed a 22uf cap instead of a 50uf cap because that's all I had. I am posting the schematic with the changes and the voltages as they are now. The new voltages are in blue. Do these voltages look right to you?

[Tapsnap]

Ahh, I realize I drew that capacitor in the wrong spot. It should be on the cathode, not the grid.