Nasty distortion in decaying notes, Fender Princeton clone, when vol is up high

[mikepukmel]

For people who don't own a scope are there any cheap alternatives that you would recommend, perhaps something that would plug into a USB port? Just wondering...

Hmmm... any way to safely cut voltage down to 5vdc? Would something like this work?


http://www.piccircuit.com/shop/pic-d...RoCtz0QAvD_BwE

Steve A.

P.S. I wonder if builder experimented with lead dress which can often contribute to oscillations as Olive Oyl used to say...

P.P.S. Could you post link to thread or group at Facebook, Mike?

Here it is Steve:
https://www.facebook.com/groups/1576...?ref=bookmarks

You have to scroll way down in one of the sub-threads in the post.

[Chuck H]

You guys nailed it! He pulled the reverb tube and the problem went away. Looks like an oscillation problem. Dang the parasites! *** Chuck, the cathodyne works differently, of course, but something in particular about the cathodyne?

I can't specify about what may be happening because I've never tested for it and I'm not SGM But I do know that cathodynes act hinky when overdriven (The Valve Wizard site has a good write up) and THAT particular distortion is one I've associated with the higher current of too much LF.

I'd be interested to know, does the distortion occur with the reverb tube in and the reverb turned down? Because I've experienced odd distortions (not dissimilar) when a reverb tank is unplugged but the reverb knob is not at zero. I'm wondering if the load on the reverb transformer is too high and causing a mild version of the open load sound.?.

[John G]

That's great, some progress.
A couple of things. First reinstall the reverb driver tube and fit a 500pF to 1nf cap (greater or equal to 1KV ) across the driver tube from anodes to cathodes. Boogie use 750pF.
Try your tests gain.
Secondly I usually do the Paul C and Stokes mod, but on the Paul C mod I replace the 2M2 resistor for a 3M9. I also install a 330Kohm resistor to the grid of the phase driver tube.
You could reduce the phase- o/p tube grid caps with a smaller value say 22nF.
The trick is to only do enough to subdue the ratty distortion artefacts and not stifle and reduce the dynamics of the amp.
John

[Steve A.]

Not always. Improper phase of NFB sometimes it just causes very slight oscillation or other strange issues. I was surprised to find this out as I was always taught it would be blatant as well. Now I probably suspect it too often, but it's easy enough to check, and has solved some fairly odd issues.

I guess it would depend on the NFB circuit. In a stock BF/SF amp it is almost a dead short (820R?) so the effect is instantaneous in my experience.

Thanks!

Steve A.

[Steve A.]

Here it is Steve:
https://www.facebook.com/groups/1576...?ref=bookmarks

You have to scroll way down in one of the sub-threads in the post.

I guess I need to belong to the group to view that link. What is the group's name?

Thanks!

Steve A.

[mikepukmel]

Hi Steve, sorry 11 days late (never liked teh Facebook link thing). The link to the group is
https://www.facebook.com/groups/1576...?ref=bookmarks
the name is
"Fender Amp Clone Club"
I just noticed that on top it says "secret group", don't know what that means. I see Juan is in there. I just scrolled through posts for 20 minutes and can't find the one I book marked. I had been following it for weeks.

[Steve A.]

Hi Steve, sorry 11 days late (never liked teh Facebook link thing). The link to the group is
https://www.facebook.com/groups/1576...?ref=bookmarks
the name is
"Fender Amp Clone Club"
I just noticed that on top it says "secret group", don't know what that means. I see Juan is in there. I just scrolled through posts for 20 minutes and can't find the one I book marked. I had been following it for weeks.

Yes, Facebook certainly does not meet the minimum requirements for a BBS style user forum like MEF or TGP. FB can handle millions of users but doesn't know how to index threads and posts.

Steve A.

[mikepukmel]

its very odd that the group was taken 'secret'. Just a plain, ordinary "fender amp clone club" thing. When I joined, it wasn't secret. I think the admin's didn't have time to approve etc, so they took it private.

I tried like anything to get the guy to join MEF, and ask here, but he didn't do that for some reason. REALLLLYYYY long thread on this topic, then it just disappeared. Bummer too, since it had a lot of really great info about a home made, custom Princeton type build. When I see something odd, like the wierd sound artifact, I love to keep tabs on the thread, and see how it was resolved, and sometimes copy the whole thread, or relevant parts to a directory on my machine, so I can find it later if need be. Nothing like learning from others "journey".

The guy had a custom built amp, with lots of odd mix and match parts, really interesting looking thing. He went through lots of debugging odd crackling, hum etc and a lot of it was a bad solder joint, or bad cap or some wires that needed re-routing.

Im certainly, admittedly no Michaelangelo of amp wiring, but the wiring in that amp, if you looked at threads where people complain of oscillation, looked like it would be oscillating itself so hard that the neighborhood dogs would be circling his house when it was on. But it seemed to play fine, except for a few 'artifacts'.

My main reason for following and trying to help with resolution is education. So, Im no master amp wiring guy, but what kinds of things cause problems, vs are just ugly? So, for this one, I'll never know.

[mikepukmel]

I guess it would depend on the NFB circuit. In a stock BF/SF amp it is almost a dead short (820R?) so the effect is instantaneous in my experience.

Thanks!

Steve A.

Steve, as far as I could tell, from looking at photos, and looking up part values by markings, it looked like a 'stock' build attempt. I think the transformers might not have been exact princeton replacements. So, if we change the NFB resistors, it could cause these odd sonic artifacts?

[Enzo]

It is just a guitar amp, anything can cause sonic artifacts.