50 watt... sound has excessive swirl as note decays

[garytube]

Hi everyone,
I got the amp put together, and it seems to work great at all volumes, but at max volume, I'm hearing the "swirl" sound as the note decays, and to me it seems excessive for a Marshall style amp. Also when I lower my guitar volume a touch while playing(amp still cranked), the swirl is quite noticable, moreso on particular low notes/ low E string. This happens on either channel with bass control at various positions. Is this inherant in this circuit? I had a fender tweed bassman reissue that didn't do this... hmm....could it be that I need to recheck the bias or need new power tubes? The tubes that are in the amp already have some mileage on them (winged C EL-34).
Some other details about the amp:
1. I biased the tubes at 40mA with 434V= 17.5 watts
2. I'm using a 100K negative fedback resistor on the 4 ohm tap, but that was the only change I made from the '68 lead spec.

Any tweeks or changes that I could do to lessen the "swirl"?

[Roe]

try a different driver tube in v3 and see if the problem disappears

[garytube]

Thanks for the reply! I just tried a couple different tubes in V3. It does swirl less with a phillips 5751, but is still noticable. I also tried a chinese 12AX7, Sovtek 12AX7 Triad 12AT7, RCA 12AT7. The Chinese tube in V3 makes the most swirl...

[suede]

One thing to try that has nothing to do with the amp. Make sure you guitar is in tune. Especially when the amp is pushed hard.

[flemingmras]

Where is the amp biased at?

[garytube]

One tube is 38mA and the other was about 40mA and the B+ DC voltage was approx. 435 v. Should I try changing the bias? I'm think I'm going to take it out of the cabinet right now and tinker with it a bit. I have some new groove tubes (Mullard EL34) that I might try again, but I think I need to drop the 47K resistor to 33K because I could get them over 27-28mA.

[flemingmras]

Well your bias is right at the max...which isn't a bad thing as long as you're not redplating. But you could try turning it down and see how it sounds.

Also, which choke are you using?

[garytube]

The choke is C1999 (whatever that is)... its the one that came standard with my kit. I didn't see any redplating. Bias being at max... maximum being 70% (hopefully not 100%!!)?
I did put in the Groove Tubes EL34M's, and had to put a 100k resistor parallel with the 47K to get these to bias right. I put them at:
V1= 38mA
V2= 40mA
B+= 432VDC
I have the amp over at the practice space right now, so I'm unable to check it out. But, when I tried it briefly at full tilt a couple days ago, it seemed to have less swirl, and more punch.

[flemingmras]

The choke is C1999 (whatever that is)... its the one that came standard with my kit. I didn't see any redplating. Bias being at max... maximum being 70% (hopefully not 100%!!)?
I did put in the Groove Tubes EL34M's, and had to put a 100k resistor parallel with the 47K to get these to bias right. I put them at:
V1= 38mA
V2= 40mA
B+= 432VDC
I have the amp over at the practice space right now, so I'm unable to check it out. But, when I tried it briefly at full tilt a couple days ago, it seemed to have less swirl, and more punch.

If you've got Sozo caps in there they do require a break-in time as they form.

After about 20-25 hours you'll start to hear them "sweeten up". Full break in is after I believe 100 hours if I remember right.