disturbance when turning the pot

[Leevi]

I'm repairing a Dynacord Twen and I wonder a raspy tone which comes
when I start to turn the vol pot (it occurs in the first little sector only). Normally the reason is a
a bad coupling cap before the pot or the pot but I have have changed both. For more information
see the schematic below.

http://www.kolumbus.fi/risto.kivioja/RikstoneAmps/TwenII_cir.pdf

/Leevi

[JayB]

I'm repairing a Dynacord Twen and I wonder a raspy tone which comes
when I start to turn the vol pot (it occurs in the first little sector only). Normally the reason is a
a bad coupling cap before the pot or the pot but I have have changed both. For more information
see the schematic below.

http://www.kolumbus.fi/risto.kivioja/RikstoneAmps/TwenII_cir.pdf

/Leevi

It's probably a slight ground loop or the interaction between the tubes biasing for the next stage.

On second thought, did you check out C3 in that schematic?

[tubesornothing]

Sounds like DC on the pot.  Measure either side with a DC voltmeter.  Should be really small.  Replace C2 and C3 if there is DC on the pot.  Its most likely C2, as this has higher voltage behind it.

[OldHouseScott]

Maybe try something in the range of 22k - 100k as a grid stopper coming off of the volume pot wiper. I don't really see how C2 or C3 could contribute to the noise. I'm pretty sure that's not a connection between the right side of C2 and the grid of stage 2.

[JayB]

It has to do with the unbypassed cathodes. I just remembered. I have the same situation with a home built amp and every time I remove the cathodes, I have that same symptom as Leevi has. Sometimes, it will hum just at that first bit of the pot. I have yet to figure out why it does that but I have found if I add a small 1k resistor in series with the ground lug of the pot to ground, it goes away.

[Leevi]

OldHouseScott
I added a 47K grid stopper and it helped. Thanks!
/Leevi

[Leevi]

JayB,
That might work as well but can you get the volume really off
because you have the 1K resistor there?
/Leevi

[tubesornothing]

I don't really see how C2 or C3 could contribute to the noise.

If the coupling cap is leaking, you will get DC on the pot and it will sound scratchy.

[Leevi]

I didn't change C2 or C3 and can not give any statement about that.
On the other hand in the original schematic there is not any grid stopper so
the grid stopper is probably a work around for this problem.
/Leevi

[tubeswell]

C1 is the relevant cap to check if you have scratchiness on the volume pot. .

C2 and C3 are art of the active-tone/presence control circuit, and changing them won't affect scratchiness on the vol control.

So lift the 'tail' end of C1 from the circuit and clip your V-meter on to it and fire up the amp. If there is leakage there you will see DC on the cap.

[tubesornothing]

My Finish is not that good, I was looking at the wrong pot - yes C1 is the "synder"

[Leevi]

Ok,
BTW the language is not Finnish but German.
Lautstaerke = Volume
Klang = Tone
/Leevi

[tubeswell]

I studied German for a couple of years in high school, but you don't need to know that to tell what is what on that schematic.  Klang is a neat word tho'. I'm definitely gonna label my next tone control "Klang"

[tubesornothing]

I studied German for a couple of years in high school, but you don't need to know that to tell what is what on that schematic. 

 

[JayB]

JayB,
That might work as well but can you get the volume really off
because you have the 1K resistor there?
/Leevi

No. It takes just that unusable beginning out of the pot, where it's scratchy. You can use a lower resistor value and it should work the same. A grid blocker would do the same, but just block some highs also.

I'm really thinking it's got to do with the bias of that stage. The volume pot is effectively the grid leak resistor. At the low end of the dial, it's allowing it to pull a -voltage to the grid causing the scratchiness. I bet it's a ground loop between the cathode and grid.

Clip in a couple cathode bypass caps real quick.

[tubeswell]

I studied German for a couple of years in high school, but you don't need to know that to tell what is what on that schematic.

 

Just pulling yer chain