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This post follows on from the linked post.
To help mitigate the hum problem, 10R (5W) bypassed by a 220nF capacitor has been placed at the 0V ground chassis connection. It was unclear whether this has made any difference when using my test set-up (i.e. different speakers and amp location).
I got "hands on" - literally - to check something... When turned on without a source connected, the amp squeals. When I move my hand to within ~ 10cm (4 inches) of the amp, the noise stops. If I then touch the RHS input tube, it squeals (but not if I touch the LHS input tube). If I touch the chassis, it squeals at a higher pitch. However, if I am standing away from the amp with it squealing, then touch the LHS RCA return, the amps stops squealing (but not if I touch the RHS RCA return). Note, when connected to a source, it is quiet for an amp with AC filaments.
The input wiring looks to be a bit of a mess and uses an unshielded signal wire (which runs close to the output transformer) to the grid cap. I will rewire the input this week. Is there anything else I should check before I put the amp back together?
Cheers,
91.
P.S. The next amp I build will be easier to work on!
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Follow Ups:
A squealing amp is usually an oscillation. So, you should rule that out first following the good suggestions made by the inmates.
If I am building my own amp I always run a bus bar with at least 12 gauge bare copper grounded at one of the power trans mounting bolts. From this point all the connections should be based on most current first to least current last on the bus bar. Sometimes I ground the input and output jacks on their own ground and sometimes I ground on the bus bar. I have not been able to detect any difference between doing it either way. good luck, Dak
> > A squealing amp is usually an oscillation. So, you should rule that out first following the good suggestions made by the inmates. < <
Cool - that is what I am planning. Let's see where that gets me.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
If rewiring the input does not heal the squeal, I will change my ground point for the input stage, try grid-stopppers on the 2A3s, and consider changing from what node and to where I make my chassis connection.
I am curious how others' wire/ground their pentode input stages. I run a multiple star config - each stage has its own local 0V ground point (NOT connected to chassis), with all local ground points connected to the PS reservoir cap -ve; the chassis connection is made from this point (now via 10R with 0.22 uF bypass). The pentode input stage has its 0V ground point at the screen bypass cap -ve, a technique sometimes used to minimise noise in pentode *output* stages. I am wondering if the more traditional method of grounding the input stage to the PS smoothing cap -ve would be a better idea?
Also, do builders using buss or multiple star grounding prefer to make the chassis connection at the RCA, near the reservoir cap, or somewhere else? Maybe one of those "ask 10 people, get 10 different answers" type questions? For my next build (monoblocks), I'm considering making the chassis connection at the RCA.
Thanks again.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
hey 91
Tidy up the internal connections with some shielded cables, ground the shield on one side only. Make sure you have grid stops on all grids. Sounds like parasitic oscillations...
Have fun
Stuben
Is it possible that the secondary wires off the output tranny is in wrong polarity? This is in respects to the global feedback network.
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