Newbie question: Some issues with my Tweed Super (5F4) build.

[fred]

Here's some additional information I didn't think to mention in the OP...

1. This is not a kit build. I followed Mojotone's schematic and layout for the most part but included the grid stoppers and screen resistors which Mojotone doesn't include.

2. I cut the circuit board just over 13 inches in length, you can see it extend beyond the pilot light. Therefore the wires leading to control grid a just a tad longer than usual. I read in Gerald Weber's book that this my introduce oscillation in Fender amps...not sure.

3. I dropped one of the Sovtek 5881WXT. I didn't worry about it at the time because I was told these are built very solid (like a tank)... is it likely microphonic now? Preamp tubes and PI have all been tested recently.

[Chuck H]

What you have is classic instability due to parasitic cross talk between circuits. What we typically call parasitic oscillation. Any circuit leads in close enough proximity to each other will swap info somewhat relative to that closeness and their aspect to each other. Similar things can happen when ground leads are shared. If this cross talk is in like phase you have a positive feedback loop that is invisible, but real. Since your problem seems to manifest with the presence control, but doesn't seem very dependent on the volume control, I'll say that your power amp is "talking to" your later preamp stages. Now the bad news...

Look at the layout drawing for the 5f4 below. Notice that leads are short and routed for minimal looping and crossing. Not at all bunched together. I think you have entirely too much lead length on a lot of the circuits and the best solution is to re wire the amp as much like the layout drawing as possible. Including lifting the board to access those leads underneath. That would be the best solution.

Other solutions would be almost as difficult to affect because it would mean hunting down and mitigating just those cross talks that are happening. Which is difficult. Maybe even including the addition of bleeder capacitors that are considered a mo-jo no-no

Where is the unused tap for the OT secondary routed? It should be kept as far from the preamp as possible. You can also try routing any NFB and presence circuit leads toward the power amp end and intentionally away from the preamp. But that's as much as I would try before breaking down and doing a proper lead dress.

JM2C and sorry I suppose.

[Leo_Gnardo]

I dropped one of the Sovtek 5881WXT. I didn't worry about it at the time because I was told these are built very solid (like a tank)... is it likely microphonic now?

Yes those are tough tubes. It's never a good idea to drop any tube. Remember what's taught in parachuting school, the fall never hurts anybody. It's the sudden stop at the end... In any case, if the tube works, it's OK, doubt it's the cause of your presence control misbehavior.

Preamp tubes and PI have all been tested recently.

Most testers tell us very little about whether a tube is suited for audio use. Nothing about hum, noise, microphonics, gain etc. Using a test voltage that's a fraction of what the tube will really see in use, they test for short circuits, and whether the tube has any emission (do it pass current?) and that's just about all. Unless you have a very fancy tube tester, that's all you get.

[fred]

@ChuckH: very thoughtful post. Thank you. It is very true, we should keep signal wires as short as possible and, simultaneously, as far away from high voltage (magnetic fields) as possible. It really is a complicated version of a traveling salesman problem: not only have the salesman traverse each location exactly once but also avoid getting run over in the process. I was planning to do all this anyway, just a bigger job. Not an hour or two either...but worthwhile.

[fred]

Yes those are tough tubes. It's never a good idea to drop any tube. Remember what's taught in parachuting school, the fall never hurts anybody. It's the sudden stop at the end... In any case, if the tube works, it's OK, doubt it's the cause of your presence control misbehavior.



Most testers tell us very little about whether a tube is suited for audio use. Nothing about hum, noise, microphonics, gain etc. Using a test voltage that's a fraction of what the tube will really see in use, they test for short circuits, and whether the tube has any emission (do it pass current?) and that's just about all. Unless you have a very fancy tube tester, that's all you get.

Indeed. I need to put the tubes in a working amp and crank the $h!+ out of it (when the Mrs isn't home that is).

[Leo_Gnardo]

Indeed. I need to put the tubes in a working amp and crank the $h!+ out of it (when the Mrs isn't home that is).

That's the best test there is: find out what your tubes do "in real life."

Another clue: when your missus likes the sound, you have something very special indeed, you will know you hit the mark! I'm sure some other MEFsters will concur.

[fred]

Thank you all for your tremendous help!
Both issues are now gone, problem solved! The amp is FANTASTIC!!
As I was shortening signal path wires I came across a couple of cold solder joints.
I will post some pictures and if I can get quality audio I'll try to post that, too.

Very happy!

[nosaj]

Thank you all for your tremendous help!
Both issues are now gone, problem solved! The amp is FANTASTIC!!
As I was shortening signal path wires I came across a couple of cold solder joints.
I will post some pictures and if I can get quality audio I'll try to post that, too.

Very happy!

Stick around , there is so much more to learn. BTW where are you located?

nosaj

[fred]

It will be some time before I can supply decent audio since my house has a ton of interference.
Here are the latest gut shots.

[fred]

Wonderful MEF members, this amp has an undeniable jump blues/rockabilly tone to it.
I'm going to leave this (crappy piece of playing) here for you to enjoy (?):

https://soundcloud.com/user-669964122/rockabilly