SUPPORT FORUMS FOR ZKITS >> ZKIT 1 Support forum >> volume pot question

Message started by babgrip on 01/03/10 at 14:15:18

Title: volume pot question
Post by babgrip on 01/03/10 at 14:15:18

I recently bought bought some one else's z-kit project. bad idea so far.

My question   is it possible to bypass volume pot i belevive it has failed . amp squeals if you touch pot and output tubes seem to be getting excessive voltage. this happens with out any input signal or speakers hooked up. previous owner had attempted to hook up volume pot,rca inputs and binding post outboard ,he did not use any sheilded wire and amp had terrible distortion beyond halfway on pot rotation , and very loud hum that varied with pot movement.i installed everything back on  pcb . amp sounded good at full rotation on pot but any thing less and right channel would drop out. hum still present i tried adding ground from board to ac  ground terminal on plug and added ground to pot body this  seems to have made things worse may have been wrong thing to do  as board is mounted on wood not metal. may have overheated pot . hope i have not fried whole thing.would like to be able to keep working on it with out having to wait for new pot to come in mail

Title: Re: volume pot question
Post by dank on 01/03/10 at 18:51:05

Sure, you can bypass the volume pot...but thats the same as "full rotation" which you said worked.  Just leave it at full and continue working.  You can also use two single pots rather than the dual ganged one that comes with the kit, then you can adjust the volume independently for each channel.  You can even use Radio Shack pots if you have to have it today,  100K-Ohm Audio-Taper Potentiometer  Model: 271-1722  Catalog #: 271-1722.

You need to check one thing that would make the amp sound ok at low volumes but distort at anything over 1/2.  Be sure that pin 6 of eack output tube (shown as a n/c on the schematic) is connected to pin 3 of the same tube.  (this is an internal connection on some output tubes but not on others...if you are using 6p15p's and don't have this connection wired on the board for each output tube (or have the CCE mod installed)  then you will get distortion at anything over about 1/2 watt of output.  I believe later Zkit boards have these connections made in foil and earlier ones don't.

Be sure to read the "hum problem" thread in this support group too.


Title: Re: volume pot question
Post by babgrip on 01/03/10 at 19:42:08

Dan thanks for your reply  My main concern is that if you were to even toch the pot things go crazy so for saftey's sake i need to replace or bypass.I have read the hum thread and which is why i tried the grounding route as suggested by Steve. Hopeing bad pot is cause of hum issue .(what is considerd normal hum level) it is much higher than my FI-X 2A3 and an Ideal Innovations PP w/KT88's that i had in the past. The kit is only 2 months old so i would imagine that it has the newer board. I will post any thing that i learn.

Title: Re: volume pot question
Post by dank on 01/04/10 at 00:12:26

If you remove the source, the hum should go away - even at maximum volume setting.  Now if your volume pot is damaged (open), I can see how that might cause problems by leaving the grid of the input tube open with no resistor to ground.  I have always felt that 100k is a little too high here, and have used 50k replacement pots in my Zkit1 (I'm using two so I can control each channel).

If you remove the source (because this will short the source to ground) and temporarly solder ("tack") a small wire across all 6 pins of the pot you should not have any hum.  If you still have hum, you will need to find the problem before continuing as it does not involve the volume control.  Be sure to remove that wire before plugging any source back in, and (as always) be sure to power things down before doing any soldering.


Title: Re: volume pot question
Post by Alan H on 01/04/10 at 11:41:10

I have my speaker connections and pot off-board. I tried shielded and unshielded wires and in my environment/amp-case/audio-chain it made absolutely no difference.

The night/day event for me (as per the hum thread) was adding two lengths of solid core copper directly from each RCA shield at the input to the pot connections closest to the word 'taper' on the board. (a la Dank/DLL).

Also - I presume you've retouched all the solder points.

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