#124 started

[norburybrook]

Ok,

256 and 252 so closer.

the hum is still very bad.

I get a really strong motorboating when I check the voltage on pin 1 also does the same when I check on the board at the 110k resistor and then the bias supply eyelet at the capacitor


M

[martin manning]

I think the PI is ok now, but make sure the PI node filter ground is good, as well as the screen node filter ground. They are joined on the PS board and ground at the power tube cathodes.

[norburybrook]

sorry martin you've lost me there

where are the PI node and the screen node?






M

[larsvictor]

Marcus, he asks you to check the common connection from the negative pole of the two 20uF/500V filter caps to ground. The PI node and screen node are the positive poles of these caps.

[martin manning]

Yes B+2 and B+3. Note in the layout the negative terminals of those caps are tied and grounded at the power tube cathodes. In the PS board I drew the PI filter is grounded with the preamp filters a la later ODS so they are not tied together. I don't know if that is an issue, but in any case make sure they are both securely grounded.

[norburybrook]

thanks Lars


there's good continuity between ground and all the power board connections to ground.

there's good continuity between the B+ nodes from the power board to the pre amp board.


I'm wondering if the general loud hum is in fact constant motorboating and I've got my OT primaries the wrong way round? can that be a symptom or is it always the loud howl?


M


M

[norburybrook]

Yes B+2 and B+3. Note in the layout the negative terminals of those caps are tied and grounded at the power tube cathodes. In the PS board I drew the PI filter is grounded with the preamp filters a la later ODS so they are not tied together. I don't know if that is an issue, but in any case make sure they are both securely grounded.

yes I can see that now martin.

Should I tie them together?


they're both independently showing good ground connections.

[martin manning]

Swapping the primaries is worth a shot. Leave the grounds as they are for now. Grounding the PI filter with the preamp filters works too. You would have to disconnect the PI filter ground from the preamp filters and connect it to the screen filter ground.

[larsvictor]

Looked at your pictures. The two F&T 22µF/500V Caps near the power tubes. Their positive poles should be connected by the 2k7 Xicon 5W resistor. Their negative poles should be tied together. Are they ?

[larsvictor]

I'm wondering if the general loud hum is in fact constant motorboating and I've got my OT primaries the wrong way round? can that be a symptom or is it always the loud howl?

Swapping the primaries is worth a shot

Maybe. You may just desolder the yellow NFB wire from the speaker output. That is the fast way to demask positive feedback.

[martin manning]

Yes good point, Larsvictor. Marcus, what you surely don't want is for the PI filter to be grounded at both locations (input jack and power tube cathodes).

[norburybrook]

OK,

I've moved the grounds to where the should be on the schematic. So B+3,4,5 are now grounded at the input, and B+2 is now grounded at the Output cathodes, and B+1 is grounded with the main CT's from the PT

I've removed the NFB.

noise is still there, it might even be louder.


One thing, I noticed.

if I turn the treble pot to 10 the noise gets a lot quieter.



what does this tell us?


M

[martin manning]

So we can conclude that your OT primaries are connected correctly and you can reconnect the NFB. The increased noise at low Treble setting makes me suspect some problem in the tone stack or PAB relay wiring. Do you still have low volume in OD mode?

[larsvictor]

AFAIR you have another (D-style) amp. If it has an "effect loop" (preamp out / poweramp in) you could cross check the #124 to exclude that at least a part of the hum is derived in the preamp section (preamp #124 patched to poweramp other D-style amp)

@lars, the heaters have a center tap so no need to add the resistors.

Is the CT properly grounded ? Measuring from pin4/5 or 9 to ground should show a very low resistance.

Lars

[norburybrook]

Morning lars

yes the center tap is fine it's showing 0.2R to ground from pins 4/5


I did have the same thought about using the send return into another amp.


Last thing I did last night was to move the grounds from the power board to their new positions.

I switched on this morning and the hum now is volume dependent. Also I now have no sound at all when I plug a guitar in. When I plug in a guitar the hum stops but I have no sound at all.

yesterday I had some sound and the hum was independent of volume.

????????????

where to start...again....

I'll use my other amp to check/pre power sections first however it would seem it's a pre amp issue.

is there anyway I can bypass certain sections i'e tonestack/relays with a jumper cable?


marcus


p.s. pre amp out from my BM into return on the #124 sounds great (except for hum) so it's definitely pre amp issue. correspondingly there's no sound from the pre out into return on BM