trim pots

a DIY, modified Mullard 5-20 monoblock design

trim pots

Postby audioray » Fri Jan 06, 2006 9:53 am

hello shannon
happy holiday
finally got back to the eiclones. i was playing around with the wireing
J4--#5 to metal case grd. got rid of most buzzzz sound.
now lost adjustment with trim pot for right el34 tube it reads 700ma. pot does nunthing. the left trim im getting 550ma. seems ok. main setting is -50v.
resistors r13---r14---r19---r20---look ok. do these control the trim pots,could it be a bad trim pot. i see no burn spots.
lot of new didg. cameras in family, pict. soon. im a print guy
i allso get noise from coax cable from back of panel to j1-1. i do have the new fix in place for the grd.
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trim pot

Postby audioray » Fri Jan 06, 2006 12:01 pm

shannon my mistake on the trim pot, turning wrong way
thank you
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pin striped wonders

Postby Shannon Parks » Sat Jan 07, 2006 7:55 am

Hi AudioRay!

With your painting and machinist skills, it will be great to see a pic of these.

OK, so no bias prob. And you just did the strap to the chassis and eliminated most of the buzz? So now when you short the input at the board you are quiet, but when you add a length of input wire from the RCA input to the board, you pick up noise - correct?

My method usually involves trying to keep the input wiring as short as possible, as the high input impedance of the tube amp makes this a magic line for all sorts of unpleasant stuff to want to 'jump on'. I've read conficting things regarding shielded input wire. Anyone have a can't-miss solution on input wiring (with readily available wire)?

Shannon
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Postby TomMcNally » Sat Jan 07, 2006 10:07 am

This is what works for me - I know most of you guys don't put a potentiometer on the inputs - but it serves a couple of purposes in my mind.

1) you can kill the input quickly and easily when changing cables
2) you can lower the input level from your preamp
3) you can use it as a volume control !

Image

Note the isolated ground bus at the top of the pic. The only ground point to the chassis for the audio circuitry is at the RCA jacks (they look insulated - but there are no washers on the front panel) The pot is right there, so there is a 100K load to ground. Then always run the input leads against the chassis and away from anything else.

Another point - if you are connecting up to a system with cable TV connected to it anywhere, or a PC - expect some ground loop hum. It's hard to get rid of without a transformer (yuck)

Those mysterious white specks in the picture are invisible to the naked eye.

That amp is a "Darling" I built the other day for a friend who needed something small. http://mcnally.cc/amps.htm for some more pics.
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Available Wire?

Postby Thermion » Sat Jan 07, 2006 7:36 pm

separks wrote: Anyone have a can't-miss solution on input wiring (with readily available wire)?

Shannon


I'm not sure about a can't miss solution, but I've had good results using a two conductor shielded cable. I like to keep the audio circuit isolated from the chassis and use insulated RCA's. The idea behind using a shielded cable is that it just becomes an extension of the interconnect. There are several choices of two conductor shielded cable from Cardas and DH labs sold by the Parts Connexion. Unfortunately, Welborne Labs doesn't seem to carry these any more.

JT
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Postby EWBrown » Mon Jan 09, 2006 7:26 am

A "trick" for getting rid of cable TV induced hum is to use some form of RF isolation transformer between the cable box (or the "raw" cable" and the VCR or TV tuner. This breaks the ground path between the outdoor cable and your equipment. The cable has its own ground potential, and it can be a real "bear" otherwise to solve ground loop problems.

The TV cable, hung on the poles outdoors, is in relatively close proximity to the High Voltage main AC power lines, which can carry between 2400 and 12,000 VAC, before it gets stepped down to 120 / 240 at the "pole pig" transformers, which feed the house power. Needless to say, AC hum and noise can be induced onto the cable's outer conductor, and even the ground block at the point of entry to the house, will not eliminate this completely.

I ended up using two 75/300 ohm "baluns" wired back to back, which works well provided that the 300 ohm and 75 ohm sides are isolated.

Also, the balun from inside a TV or VCR tuner can be used, if the windings are isolated.

/ed B in NH
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