Scratchy volume pot on TAD DIY deluxe reverb

[kiffou78]

Hi,

I've build a fender deluxe reverb amp clone and I've got a problem with scratchy volume pots (both channel), channel 2 is more scratchy than channel 1.
Schematic is here : http://www.griesamps.com/sitebuilder...b763_schem.gif

I've swap preamp tubes between channels 1 and 2 : no change
I've cleaned pots with some cleaning contact spray : no change

I've mesured voltage at the central pin of channel 2 volume pot for each volume level position (bass pot to 1, and treble pot to 1) :
Volume
Level Voltage (mV)
1 0
2 15
3 37
4 55
5 70
6 90
7 125
8 190 (max)
9 120
10 10

I think that DC voltage in pot can be the reason of scratchy pot so I've tried to add a coupling capacitor (0.1µF) between output of tone stack (point where central pin of volume pot and bright capacitor are soldered) and tube grid (see schematic). This solves the problem of scratchy pot without changing amp sound. Then I've mesured the grid voltage relativ to ground : 1.5V

Do you think those voltage value are normal ?

Do you think solution I've found is right ?

By the way, it seems that I've found a solution but I still don't understand why with the stock schematic I've got a scratchy pot (I've never seen any ab763 circuit with those coupling capacitor I've added), can you help me to understand deeply the problem ?


Thank you very mutch

[DF96]

Have you changed the valves containing the second triode in each channel? Excessive grid current could be the problem. Are the valves new? If so, this might settle down after a few hours running as the getter soaks up any remaining gas. As you suspect, it is not a good idea to put DC through a volume pot. A design which does this is not "deluxe" in any engineering sense of the term!

When you put a capacitor in, did you add a grid leak resistor to ground? If not, this explains the 1.5V on the grid.

[kiffou78]

Ok, this is the mod I've tried (clearer with picture) : Picasa Web Albums - aurelien - ScratchyPot

Have you changed the valves containing the second triode in each channel?

No, I've just swap both channel tube. Channel 2 still more scratchy than channel 1 even after swaping. (channel 1 vol pot voltage is at maximum 15mV while channel 2 vol pot voltage is at max 190mV as I wrote)

Are the valves new?

Yes they are TAD valves. I'll try with 60's philipps valve

When you put a capacitor in, did you add a grid leak resistor to ground? If not, this explains the 1.5V on the grid

No, no resistor to ground. By the way somebody tells me in another forum :
"Your solution is bad (if I understand it correctly) because it leaves the grid with no DC reference to ground."
So what do you think if I add a resistor as shown in this picture : Picasa Web Albums - aurelien - ScratchyPot
What would be the value of resistor ?


Same people told me that "It sounds like one of the caps in your tone stack is leaky.". So I'll try also this test to check if problem stills there : Picasa Web Albums - aurelien - ScratchyPot


I'll tell you what
Thank you very much

[kavermei]

Originally Posted by kiffou78

What would be the value of resistor ?

Try 1M.

Kenneth

[DF96]

Yes, 1M would be fine. Or 470K or whatever you have handy, the value is not too critical.

You might have leaky capacitors, but if they are new then this is unlikely.

Could you confirm the voltage reading for position 10: if it really does go down at the top end then it is not a leaky capacitor but grid current from the valve. I have heard that some modern valves don't have such a good vacuum when they leave the factory as this takes extra time in pumping, so the manufacturer just relies on the getter soaking up the remainder once it gets hot. Also the metal may not be as well degassed as NOS valves. When this amp was first made valves were made properly and had low grid current so they could get away with saving a few pence on components.

You may find that an NOS valve may give trouble if it has been unused for many years, but again it should improve after a few hours. This is because some air will leak in and it will take a while for the getter to deal with it.

[kiffou78]

Yes I confim that voltage decrise from Vol 7 to Vol 10, mesured with oscilloscope and 10x probe.


Yes valves are new TAD 12ax7
Ok now I've tried to change valve for :
- a almost new (lets say 40hours old) JJTesal => same problem, same voltage
- an old 60's 12ax7 philipps miniwatt, not NOS for sure cause I got it from an old reel2reel tape I found on a trash .... => same problem, same voltage
- a cheap chinese 12ax7 almost new (40hours) => same problem, same voltage.

An ohter test with canal 1 preamp took off : same problem, same voltage

Note : voltage is little bit higher today : 210mV at maximum level (that is to say vol pot to 8)

Thats all valves I got.

So it seems not to be valves....


Next step : Unsolder tone stack capacitors one by one and try each time scratchy vol pot to try to identify which one would be the leaking one. Capacitor into tone stack are : two orange drop and one sivler mica...

I tell you what

Thanks a lot

[kiffou78]

Ok,

Next step is done : all tone stack capacitor have been unsoldered and vol pot still scratchy with same mesured voltage.

So I deduce that tone stack capacitor are not guilty.

On the other hand, valve seems not to be guilty neither since I've tried many valves.


I'm lost

[kiffou78]

Me again.

Before soldering back tone stack capacitors, I've made an other try :
Take off the valve
Then : no connection to tone stack and no leaking from valve since their is no valve

And ..... : No more scratchy sound for sure since their is no valve, ... but still have 190mV in my Vol pot !

The only thing I see :ceramic valve socket is bad

Next try :
I leave tone stack unsoldered, no valve into socket and I unsolder the wire from VolPot Center Pin to valve grid.
=> At this point, no more voltage into vol pot (I was expecting this one), but voltage at grid PIN (no valve inside) is 15V !


Next try :
I leave tone stack unsoldered, no valve into socket and I unsolder the wire from VolPot Center Pin to valve grid (same as before) AND I unsoldered the Plate High Voltage PIN
=> At this point still no voltage on vol pot for sure and PIN 7 (grid pin) of valve socket is at 0V


Last try :
From previous try, I solder back vol pot to grid PIN 7 (still plate PIN high tension unsoldered)
=> No voltage on volume POT



So I think socket are bad or something like leaking.... What do you think ?



If you don't understand my tries, I can post picture of my tries

[kiffou78]

Ok, got the solution, every thing is allright.

When I first solder wires on valves socket, I used some soldering cream which I 've not cleaned. Indeed, this kind of cream is current conductor. After having cleaned this cream with solvent (acetone), everything is OK !

Beginner mistake I guess, this is my first DIY tube amp ever.


Thanks for all your advices !

[DF96]

I never thought of that. I deduced that it was current coming from the valve grid because of the way the voltage changed - a leaky capacitor would make the voltage go up at the top, not down again. I'm glad you found the problem, just a few 100M of leakage is enough to do this.

I once had a similar problem on an FM tuner I built from a kit. It was noisy. Eventually I traced it to some soldering flux left bridging two PCB tracks. One of these was around +12V and the other was gate 2 of the MOSFET RF amplifier, which was a high impedance point. When the flux had absorbed enough moisture from the air it started conducting - a few weeks after I had built and tested it!

By the way, ordinary multicore solder should be fine, no need to use anything else.