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

[fred]

This is my first post on this forum.

I am an amateur amp builder (built two amps total). Good clean fun albeit a bit dangerous.

I really learn a lot reading posts from this forum, and normally a search function turns up the information I am looking for
but I am a bit stumped with what to do here.

I am hoping some of you can help me out.

I'm having two issues with my new 5F4 (tweed super) build that I believe are related (if not really the same problem).
It's an otherwise good build I think, rather quiet, no hum (when no instrument is plugged in).
The amp has a JJ 12AY7 in V1,
2 Chinese made 12AX7s in V2 and V3,
Sovtek 5881WXT in V4, V5.
Electroharmonix 5U4GB rectifier.
Using my Variac to get wall voltage to 120, I get very good voltage readings (nothing weird).
The tubes are biased at 39 mA (about 60% dissipation) right now.

Issue 1:
Guitar plugged in any of the 4 inputs.
Turn the channel volume to any amount.
If Presence is at any 9 (2:00 or so) or less, then all is fine...I can turn treble and/or bass all the way up and amp functions normally.
If presence gets past 9, then as Treble passes 10 the signal cuts out. If Treble is below 10 or so, the amp functions normally.

Issue 2:
Guitar can be plugged in or not.
Turn all knobs (minus the Presence) full to 12. Then...
as I sweep Presence of 0 to 6-ish, all is fine with the amp.
As I slowly increase Presence past 6 you begin to hear a slight pulsating sound (at a regular frequency - like a drum beat).
The pulsating sound gets louder (but doesn't really increase in frequency) as Presence increases.

Has anyone had or know what the issue(s) is/are?

Is this parasitic oscillation/blocking distortion?

I put 1.5K grid stoppers to pin 5 on each power tube.
I also put 470 Ohms screen resistors in.

Can anyone tell me what's going on? and how I can fix it?


Fred

[Leo_Gnardo]

Possibly what you're seeing is ultrasonic oscillation due to the feedback polarity being reverse what is should be. Quick experiment: try reversing the plate leads to your output tubes, then see what happens. It will get better, or worse. If better, then leave it that way. If worse, then the chase is on!

[fred]

Possibly what you're seeing is ultrasonic oscillation due to the feedback polarity being reverse what is should be. Quick experiment: try reversing the plate leads to your output tubes, then see what happens. It will get better, or worse. If better, then leave it that way. If worse, then the chase is on!

Thanks Leo. I will have to try that tomorrow as if I get it wrong and the amp makes screeching/motorboat noises it might piss the Mrs. off a bit.
Actually, the amp sounds lovely until you start cranking the wrong things to 12.

[Justin Thomas]

The thing with thenegative feedback & switching plate leads, it doesn't <ALWAYS> result in horrible loud sxreeching if it's wrong. That's a worse case scenario. It can cause anything from mild volume dropouts to wonky controls, a thuddy attack on bass notes, brittle high end... No way to predict it. But you'll know ic you try switching them cuz it'll sound either better or worse...

Justin

[fred]

The thing with thenegative feedback & switching plate leads, it doesn't <ALWAYS> result in horrible loud sxreeching if it's wrong. That's a worse case scenario. It can cause anything from mild volume dropouts to wonky controls, a thuddy attack on bass notes, brittle high end... No way to predict it. But you'll know ic you try switching them cuz it'll sound either better or worse...

Justin

I'm going to be thrilled if this solves the issues! Thanks Justin.

[Chuck H]

Tip of the day...

Don't snip and trim the OT leads any more than necessary when you swap them. Just in case you find you need to swap them back. Otherwise you could find yourself short. Just twist them together if there's extra lead length in the swapped arrangement. That way you won't have to redo it for appearances if that's the fix.

Someone may chime in and say that disconnecting the feedback loop is the easier way to check the polarity. And it is, but since that also disconnects the presence control, and your presence control is an actuator for some of the symptoms I think it's better to keep that circuit operational for evaluation.

[fred]

Ok all. I swapped blue and brown OT primaries at power tube plates and powered up.
With volumes all they way down I get I really high piercing frequency.
I'm going to swap them back.

@ChuckH: Yes, very wise advice and I already learned that lesson with a power transformer - I snipped wires too soon and oops.

[Chuck H]

Ok all. I swapped blue and brown OT primaries at power tube plates and powered up.
With volumes all they way down I get I really high piercing frequency.
I'm going to swap them back.

Clearly there's another issue causing the instability. If you followed the usual hardware layout (transformer, tube, jacks and control location, etc.) it could be a lead dress or grounding issue. Less likely a wiring error of the decoupling/filter caps in the power supply, but still possible. Do you have the ability to post photos? That might help diagnose lead dress possibilities.

[fred]

I can try to post photos. What would you like to see?

[Chuck H]

I can try to post photos. What would you like to see?

Clear shots of the wire routing at whatever angles are necessary to show that. We understand that not ALL the wiring is going to be visible, but we often get photos where half of the wiring is obscured by a chassis lip over the control panel or some other aspect of the photo angle. Thanks for playing along. I hope you don't have any issues posting photos. We've had to implement some stringencies here WRT new and lower post count members due to some past issues. If you have any trouble don't trouble to look for a work around, just let us know. We'll make efforts to open your forum privileges with the Boss and then let you know when to try again. Maybe a non issue.

[fred]

Not sure if this is going to work...

[Chuck H]

That's great for closeups And these could be important. Can we please get a backed off shot so we can trace where everything is coming and going from?

[fred]

Let'a try this... by the way ont he top of the chassis the order of the transformers is PT --> Choke --> OT.

Running out of picture spce. I had to remove a photo from my last post to post this:
It's a rats nest of wires in tweed chassis I will try to post backed off shots of the tube socket side and the control panel side tomorrow.

[loudthud]

What are you using for an output transformer ? If it has multiple secondary taps, are you using the 4 Ohm tap ? The 56K feedback resistor is not appropriate if you are using the 8 or 16 Ohm tap. Use 82K for an 8 Ohm tap, 100K or 120K for 16 Ohm tap. Use 39K on the outside chance you are using a 2 Ohm tap.

[fred]

What are you using for an output transformer ? If it has multiple secondary taps, are you using the 4 Ohm tap ? The 56K feedback resistor is not appropriate if you are using the 8 or 16 Ohm tap. Use 82K for an 8 Ohm tap, 100K or 120K for 16 Ohm tap. Use 39K on the outside chance you are using a 2 Ohm tap.

For OT I'm using Allen's TO35M and its 4 Ohm tap. This transformer has 4 and 8 Ohm taps. My 4 Ohm load is coming from 2x 8 Ohm speakers wired in parallel.