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Old 6th April 2012, 07:03 AM   #1
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Default Forgive me for I have sinned. Help with Volume Pot!

I have been following the on-line thread for Tube Newbies for some time in an effort to wet my theoretical feet. I've read a lot of reference books (Basic Radio Course John T. Frye GERNSBACK PUBLISHING) and the like and even picked up a thing or one, I think

I've been admonished by many to simply set about doing and building but I have to admit I don't feel comfortable or particularly competent with much of this.

So a friend suggested utilizing an existing salvaged working two channel SE EL84 amplifier with a 12AX7 gain stage and 6CA4 tube rectifier as a means of getting my feet wet and studying the actual goings-on of a simple tube audio amplifier in action.

So now I want to take this amplifier and add one set of RCA inputs and a stereo volume pot so I can do some listening.

Adding the RCAs won't be a problem but sorting out how to hook up a stereo volume pot is.

From what I can gather I need a dual gang stereo log pot of 100K.

The wiring of the pot according to my research would be (shaft facing towards me with gang terminals at the top

1) Far right terminals inner and outer gangs: L&R Input Signal and L&R Output Signal grounds
2) Center terminals inner and outer gangs: Output Signal + leads L&R
3) Far left terminals inner and outer gangs: Input Signal + leads L&R

But where exactly do I solder the wires from the 3 terminals of each the gangs to exactly?

Lest I be accused of being lazy and not DImyselfing, or of fearing the inevitable OMG you did what! I tried valiantly to connect the pot to the amplifier but without success ie:connecting the center terminals of the gangs directly to 12AX7 pins #3 and #8 (circled in orange) OUTPUT SIGNAL and the far left terminals of the gangs to pins #2 and #6 (circled in green) INPUT SIGNAL

The pot ground connections seemed self-explanatory. I took wires from the gang ground terminals to the input audio ground and another pair to the speaker output grounds

As for the input signal connections I took wires from the gang input + terminals to the audio signal L&R + input

Several variations on the same connection theme later and I was no closer than when I started. Audio Signal? Yes! Audio Output? Yes! But maximum output and hugely distorted! A little smoke from the pot as well...

Went back to the books and tripped over some talk about no DC on pot wipers etc, paralleling resistors etc.

OK. for those who might be patient enough to assist and tolerant enough to forgive my foolhardy foray I'm all ears!
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Last edited by devilsindetails; 6th April 2012 at 07:10 AM.
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Old 6th April 2012, 07:13 AM   #2
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Hi,
Smoke on pot? Bit concerning. Measure the dc voltage between wiper and ground. For safety reasons put a capacitor between input terminal and pot input terminal and try again.
Your problem sounds to me a high dc content on your signal source. What signal source do you use anyway? Can you post the schematics?

Hope this will help you a bit.

Stef.
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Old 6th April 2012, 07:20 AM   #3
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Pins 3 and 8 are the cathodes, you want pins 2 and 7 which are the grids. Signal grounds should not be at the speaker terminals, but at the input tube (12AX7) grounds.
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Old 6th April 2012, 07:28 AM   #4
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Quote:
Originally Posted by Stefkorn View Post
Hi,
Smoke on pot? Bit concerning. Measure the dc voltage between wiper and ground. For safety reasons put a capacitor between input terminal and pot input terminal and try again.
Your problem sounds to me a high dc content on your signal source. What signal source do you use anyway? Can you post the schematics?

Hope this will help you a bit.

Stef.
Thanks. I used a CD player for signal source. Sorry no schematic. Should I simply forget the volume pot right now and see what kind of output signal I get SANS volume pot?
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Old 6th April 2012, 07:33 AM   #5
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Quote:
Originally Posted by cogsncogs View Post
Pins 3 and 8 are the cathodes, you want pins 2 and 7 which are the grids. Signal grounds should not be at the speaker terminals, but at the input tube (12AX7) grounds.
OK so pins 2 & 7 are for signal input and 3 & 8 are for amplified signal output Yes?
Thanks for the advice on the grounds. You are talking about both input and ouput signal grounds to 12AX7 grounds?
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Last edited by devilsindetails; 6th April 2012 at 07:49 AM.
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Old 6th April 2012, 07:42 AM   #6
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I would connect a capacitor say 100nF between signal source and valve grid before i go any further. Your CD player may have not a dc de-coupled output signal.
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Old 6th April 2012, 07:43 AM   #7
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Quote:
Originally Posted by devilsindetails View Post
Thanks. I used a CD player for signal source. Sorry no schematic. Should I simply forget the volume pot right now and see what kind of output signal I get SANS volume pot?
Does your CD player have an output level control? If so that may work satisfactorily. If it does not, it will be extremely loud and distorted.

Looking down at the pot with the shaft pointing towards you:

RI RO G
LI LO G

Where as:
RI=Right Input
RO=Right Output
G=Ground

LI=Left Input
LO=Left Output
G=Ground

Quote:
Originally Posted by devilsindetails View Post
Yes You are right of course....I can't even count the pins correctly...they were connected to 2 & 7. Thanks for the advice on the grounds. You are talking about ouput signal grounds correct?
The grounds from the RCA jacks and the output signal grounds should be connected to the same terminal on the pot, then to the grounds located at the input tube socket where the 2.2k resistors (Red, Red, Red) connect to ground.

Last edited by cogsncogs; 6th April 2012 at 07:56 AM. Reason: Changed brown to red, duh.
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Old 6th April 2012, 07:50 AM   #8
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Another thing: use screened wiring for the input signal. I was looking closer at your image, the heater wire of the 12AX7 crosses the input wire to the grid. I can't see a grid stopper resistor on the grids of the 12AX7 either. Just connect a 1MOhm resisror from grid to ground as close as possible to the tube base.
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Old 6th April 2012, 07:52 AM   #9
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If the pot "smoked" then you may have inadvertently connected it to a voltage source. Hopefully the pot is still good but it may not be as you may have fried it, sorry! Be careful as there are a few hundred volts on some pins of the input tube socket, namely pin 1 and pin 6, which are the plates (anodes). 4, 5 and 9 are the filament connections.
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Old 6th April 2012, 08:04 AM   #10
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Thank-you sir! Yes my inexperience and unfamiliarity are fully exposed! I will have a go at it again using your advice using a NOS spare pot on hand. Alas, no output level control on my CD player.
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