5E7 bandmaster trouble, the squel

[jezzbo]

Hi everyone,

So some time ago I had trouble with rebuilding my friends 5E7 bandmaster clone. Thanks to you guys I was able to fix it.
This is the link to the original tread.
https://tubeamparchive.com/viewtopic.ph ... hlight=5e7

Today I connected it to some speakers.
It sounds really nice but there was a big hum (groundloop, found it fixed it) and the amp begins squealing when the treble knob is at the first 1/4 turn and the last 1/4 turn. Inbetween (neer halfway) the amp is fine.

Any idea why this happens?
Besides this, the amp sounds great and has relativly low noise.

Thanks for helping.

Jesse

[sliberty]

Sounds like the lead dress issue I recently resolved in one of my builds. I shielded and relocated the input wire, and I relocated the wire that feeds the volume pot. These specifics may not be relevant in your amp, but I would recommend that you start chop sticking the longer, low signal level wires and see what you find out.

[jezzbo]

Will try that, thanks.

[jezzbo]

chopstick the wires didn't make a difference.
But I disconnected the negative feedback wire. (connection from 4 Ohm speakeroutput to presence pot) En the noise from the treblepot was gone!

The OT 2Ohm tap is connected to one speaker outputjack. (white wire in the pix).
The OT 4Ohm tap is connected to the other speaker outputjack. (green wire in the pix).

1. I did not use (had any) switching outputjacks. Should I?
2. Anything wrong that you can see in the pix?

Here is a link to some pix.

http://users.telenet.be/jezzbro/5E7%20g ... ve%20bias/

Thank you for helping me.

[Tillydog]

...I disconnected the negative feedback wire. (connection from 4 Ohm speakeroutput to presence pot) En the noise from the treblepot was gone!...

It is likely to be the routing of the wire to the presence pot that is causing the problem in that case.

Try to re-arrange the wiring so that the wire from the output to the presence pot doesn't pass near any of the preamp components or wiring, and keep it tight to the chassis. (This might mean that it has to take a longer route.)

Andy

[Structo]

These amps have a 2 ohm output.
Did you use an actual Bandmaster OT?
So three 8 ohm speakers in parallel is 2.6 ohms.

Try moving the neg feedback to the 2 ohm output.

Or:

Are you sure you have the phasing correct?
In other words, usually when we get squealing and the feedback is removed and it stops, it means the primary wires on the OT are reversed causing positive feedback.

Since you most likely cut the primary wires to fit the amp, you can reverse the two wires coming from the phase inverter to accomplish the same thing.
At the two 220K resistors.

[jezzbo]

Ok, I just connected the negative feedback wire to the 2Ohm tap.
Now there is only microphonic feedback when the treblepot is on 0.

Stil have to check phasing.

Thanks guys.

[jezzbo]

Are you sure you have the phasing correct?
In other words, usually when we get squealing and the feedback is removed and it stops, it means the primary wires on the OT are reversed causing positive feedback.

Problem solved! This was indeed the problem.
I learned a lot. No more squealing pigs in this amp

A big thank you to all of you for helping me.

Jesse

[Structo]

Great!
Good job on tracking it down.