Blues Jr problem

[12AX7]

The amp has an issue where as you turn the first volume up at some point in the rotation it begins to whistle. As you turn up more it gets worse. The frequency also changes as you turn it up and the tone controls change the frequency as does the fat switch. I have swapped out nearly every cap in the signal path and even the bias cap and the 4 big filters in the PSU. At one point i cut the connection after the volume and it stopped. But if i cut the connection between the MASTER and the PI it still does it ! It's very very low of course because the signal path is cut. But somehow it's still getting to the speaker so i keep thinking it's in the PSU since the bottleneck between the pre and PA were cut. But like i said i tried all the caps in the PSU. I'm beginning to think it's not a cap. Tubes are fine. they were the 1st thing i swapped out. Any ideas?

[Zipslack]

Have you scoped it?  You might want to check the BillM audio page about PI oscillation here  http://billmaudio.com/wp/?page_id=115
Try repositioning the cables first, and then try the capacitor addition.  Since it recently started doing it, I would start checking component values (especially the higher-power resistors) to see if they have drifted.

[stratele52]

+1 Zipslack

Check also resitor on Phase Inverter or the negative feedback circuit from the speaker.

[12AX7]

thanks 4 the replies. That6s not the problem however. The video clearly shows it's not anything like that. It's also not the ribbon cables, as i have had them every which way while trying things with the board in, then hanging out of the chassis. This is like a flute like tone from a digital keyboard that can change pitch with the volume and tone knobs. Similar to that voice effect they use that Cher used on that song years ago. That "do u believe in love" song. As you turn the knobs you can get that sort of quick pitch drop. Hard to explain. I'll see if i can record it, tho the PC where it is may not be able to record.

[Zipslack]

I'm still thinking oscillation, but maybe not PI...possibly radiated since it still gets to the speaker with MV disconnected.  Possibly grounding issue somewhere?

Also, check this...http://www.geofex.com/ampdbug/squeal.htm

[12AX7]

It's not tubes. I DID however just think of the perfect analogy as to how it sounds. Imagine a digital keyboard with a pitch control. One of those sticks that you push left or right to raise or lower the pitch. Now get a flute type patch and hold a note and while it's playing push the stick up or down to raise or lower it. Sounds just like that and when you turn the volume control it does what the stick does.

[plexi50]

Did you try a different set of power tubes? Are the power section screen resistors and grid resistors suspect?
Also are all solder connections on the pots and jacks good

[Zipslack]

Doh, almost forgot.  I was just fixing a problem on a Blues Deville that's also common to Blues Jr - check the solder joints on the power tube sockets!  There is a lot of heat radiating up from the tubes and the solder joints WILL go bad (crack, come loose) - you can get some weird effects from bad solder joints - popping, crackling, whistling, or no power to the tube at all.

[phsyconoodler]

Preamp tube microphonics? Also,did you have the output jack out recently and reverse the output transformer wires?
  We need more info.Did this happen suddenly or after you did some repairs/mods to the amp?

[12AX7]

Did you try a different set of power tubes?

Yes, like i said i replaced them all

Are the power section screen resistors and grid resistors suspect?

I haven't checked those but i will.

Also are all solder connections on the pots and jacks good

Yes, as far as i could tell and i check the whole board with magnifier light
   

Doh, almost forgot.  I was just fixing a problem on a Blues Deville that's also common to Blues Jr - check the solder joints on the power tube sockets!  There is a lot of heat radiating up from the tubes and the solder joints WILL go bad (crack, come loose) - you can get some weird effects from bad solder joints - popping, crackling, whistling, or no power to the tube at all.

I already did that and i know about blues jr's and that problem. Very common on most every one to some degree in fact. All the pins cept the n/c ones i scraped the traces and re-soldered them so the solder flows further onto the traces. They're all good now. Plus i re-tensioned the pins which is also a issue with these. (worse sockets i have come across)
   

Also,did you have the output jack out recently and reverse the output transformer wires?
 We need more info.Did this happen suddenly or after you did some repairs/mods to the amp?

No to all.


EDIT: turns out it was one of the filter caps, one of the 2 i DIDN'T replace. (because i didn't have enough) Stole a couple from another amp and problem solved. Those i had clipped and that didn't work, so i guess one of them was open otherwise clipping them would have solved it.

[phsyconoodler]

Ok,so it WAS after you did some work on it.

[stratele52]

Ok,so it WAS after you did some work on it.

+1

[12AX7]

Ok,so it WAS after you did some work on it.

No, thats stuff i did trying to solve this issue. .

[phsyconoodler]

Ah,then the cap was the original issue and you isolated it by changing the rest but ran out of caps.
 Good repair!