Quieting an old beast

[tubeswell]

Did you short the presence pot with a wire? (i.e.: ground the NFB - and make the LTP tail grounded at the same time), just to rule it out?

[black_labb]

didnt completely short it, just took it out of circuit. I'll do that when I get back from work.

Thanks for helping me on this invisible problem

[black_labb]

Didnt fix it Tubeswell.

I know that the issue is within the LTP PI, most likely the input. I've isolated the PI/power amp so I know it is not coming from the pre. I've found that probing either grid of the LTP with the multimeter reduces the buzz a huge amount, though I have checked continuity to ground.

Right now I have eliminated the negative feedback loop by grounding it. as per tubeswell's suggestion.

I've replaced the components making up the LTP (though I haven't replaced the 100k plate resistor, it does measure 105k which should be fine).

I have tried many different tubes of course

I've redone the ground scheme.

The voltages on both of the triodes of the LTP are sitting at P:G:K 155:15:35V +-2V

could the coupling caps be causing the issue in any way? I wouldnt think so as the voltages are as expected. Same situation with the thought of a bad connection in tube pins.

I'm sure I've overlooked something otherwise I wouldnt be having this issue. any wild ideas?

[Steve Conner]

Probing the grids of a LTPI causes a big shift in the DC voltage. (The grid voltage should be nearer 33 than the 15 you measured.) This probably lowers the gain and that's why the buzz is reduced. If you want to measure the grid voltage, do it relative to the cathode.

But, maybe it is capacitive coupling from something. Does the buzz change when you move your hand near it, or cover up the open bottom of the chassis with a metal sheet?

Is it heater hum? Try temporarily running the PI tube heaters off a battery, or elevating the heater string to stop them emitting. Or try a different tube, the amount of hum from heater emission varies widely between tubes.

If none of that gives a result, probably ground loops, leaky tagboards or similar black magic

[black_labb]

Thanks steve.

I dont think its heater hum, its very buzzy and not a sin wave by any means. I've tried different tubes including some with spiral filaments as well.

I doubt it is ground loops as what I've done to it should have this fixed it or atleast improved things noticeably. by adjusting the treble control I can reduce the buzz so it is on the input of the LTP.

The leaky circuitboard idea seems pretty likely at this point, they weren't of the highest quality.


I'm thinking I may just completely rebuild the PI and poweramp side of things. It's alot of work but would have been quicker than the troubleshooting I've been doing.

[black_labb]

Eliminated the hum there by rebuilding it onto tagboards instead of old nasty circuitboard (not all circuitboard is nasty, but this stuff is) .
Still have some hum in the preamp though, no rest for the wicked

[tubeswell]

Grounding like R.G.'s article?

[black_labb]

The preamp is also on a board. I've grounded the board to the star ground but I'm not looking to rebuild the preamp as well (though it would be a suitable amp for modding).

[tubeswell]

Methinks that the residual hum is still most likely grounding related then (if you have a preamp circuit in there which you haven't changed)

[black_labb]

I think so as well. I'll do a bit of work on the grounding and see if I can separate the preamp stages into a couple ground connections. I'm also considering completely rebuilding the preamp as well but at a later date.

I'm looking to sell this amp and keep the same amp that is in better condition in terms of hum ect, but I'm thinking I may sell the other one and keep this one. I have a buyer lined up who wants the amp once the hum is reduced. maybe I'll let him decide which one he prefers. their both pretty much the same to me aside from the preamps, and even they are pretty similar.

[black_labb]

I've just done a few things on the other amp (same output design and very similar preamp) when I noticed that the circuitboard material is different. I think the circuitboard material is that bad that it causes alot of capacitance between the traces creating serious hum. The other board has identical traces on the poweramp board, identical grounding scheme ect ect ect and is very quiet. The amp in question needed the poweramp rebuilt and probably the preamp as well to get it quiet.
The other explanation is that the humming amp uses a voltage doubler that creates higher peak current which makes grounding issues show up much more than they would otherwise.