5E3 build, no clean tones, possibly oscillation?

[buchhla]

Hi, I have started this build over a year ago, didn't work out of the box, got busy, then we moved etc, but trying to pick it up again now.

I am using a Hoffman turret-board layout, but reusing a PT and OT from a magnavox console radio that also used a 6V6 PP arrangement.  I get sound out of it, but there is no clean at all and with both volumes to maybe 1, it just sounds bad, hard to describe it.

PT voltages are slightly high, but the way I am measure dissipation on the power tubes, it looks low.

using my scope, the pre-amp section looks clean.  Injecting a 500mv signal in, I get  clean amplified sine waves almost to full volume and then get some clipping, but normal and symmetrical.  if I look at the output of the power stage. I see this:

With just a 8ohm dummy load on it, if I turn either volume up past about 2, I can actually hear an oscillation from the circuit, and it matched the frequency of the injected signal.

Any ideas on what steps to take from here?  As you can see, when I build the chassis out of wood, but I did foil line everything.  Could that be related to the issue, or am I missing something else stupid?   

Thanks!

[Leevi]

The wood chassis is definitely one part of the root cause. The circuit is now very sensitive for  the outer disturbances that will be amplified.


/Leevi

[glass54]

Hi buchhla
(+1 with Leevi)
But more important, do I see your input sockets wired to pot housing and finally terminate on "chassis" near transformer and main Power Amp earth and Heater 100R resistors? This will cause you some headaches.
I assume your 10.4Volts on 250R 10W should be placed on the other side of resistor.
I would inject 50mV RMS into input socket, as 500mV is a tad high for preamp, possibly explains your gross distortion above "2" on Volume Control?
Kind regards
Mirek

[brewdude]

If memory serves me, that looks like the distortion common to overdriven cathodyne PI that Merlin described in his book. Try a large grid stopper (470k-1M) on the cathodyne triode.

[pdf64]

That is a great example of what extreme crossover distortion looks like in cathode bias p-p amps.

According to the reported VDC, there’s something desperately wrong with the screen grid supply, ie over 200V dropped across the 470ohm screen grid current limiter resistors!
My guess is that 470k have been used.

[sluckey]

I agree with pdf64. Pic shows 470K but should be 470Ω.

[buchhla]

Thanks everyone, I will take a look at this tomorrow!  I can't believe I missed the resistor value, duh!  That would also make sense as to why I am seeing clean signal through the preamp stages and my voltages are off.

Cheers!

[glass54]

Well spotted Sluckey  (and PDF)
I'm impressed. Just shows you the benefit of a fresh set of eyes ie someone else having a look.
Kind regards
Mirek

[buchhla]

FYI, that was the issue.  Just swapped out those resistors for 470, not 470k and the amp works great.  I do have a little hum/noise which I think will be inherent to not being in a full shielded chassis, but outside of that she sounds just like I was hoping!

Thanks again!

If people are curious, this is what my non-traditional build looks like.