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Old 27th December 2007, 03:58 AM   #11
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Default DC on the pot

Hmmm, sounds like DC on the pot may be a problem but strange that it is only in one channel? Now I have 3 questions -

How do I test for it ? (I have a scope). Where does it come from and most importantly, how do I get rid of it, keeping in mind that I cannot physically move the pot.

I used shielded cable on the input side, would have thought that this would have eliminated it.

Anyhow, thanks for the very nice words so far.

Cheers,

Rob
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Old 27th December 2007, 04:07 AM   #12
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The most common cause is a slightly leaky coupling capacitor. I know you used all new orange drops, but one could still be leaky. A scope will measure DC assuming it's not ancient. If a cap is feeding that control, just try a new cap and see what happens.

Victor
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Old 27th December 2007, 04:14 AM   #13
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Absolutely beautiful. I know I haven't had 200 hours of free time since I read your post describing the completion of your ST70. I'm wondering how you've managed to fit it in.

That's a whole lot of amp there. It must weigh at least 65 pounds. Fantastic work.
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Old 27th December 2007, 04:20 AM   #14
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Thanks Victor,

The pot is simply a duel gang 100K log pot placed just after the input jacks.

I am not 100% sure where i should be measuring to find this DC current (ie between which points) also, I presume that the coupling cap to which you are referring is the 0.1uf cap as seen on the diagram?

Thanks for the help, I could ignore this but it just irritates me!

Cheers,

Rob
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Old 27th December 2007, 04:27 AM   #15
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I don't see the pot in the schematic you have attached. Keep in mind that it is not a particularly good idea to have a pot directly after the input socket as it will change the input impedance and the frequency roll-off point as the volume is adjusted.
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Old 27th December 2007, 04:36 AM   #16
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No not that cap. I didn't know what your circuit was until now. If it isn't coming from whatever is connected to the input, it could be leakage from the tube itself or something in that area of the input. This is assuming it really is DC on the control. You have to measure the input to the control and the wiper with a DC voltmeter or your scope. Then compare it to the good channel. I'm guessing the wiper goes to the grid. Conceivably it could even be a defective/noisy potentiometer.
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Old 27th December 2007, 04:44 AM   #17
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Put 1k grid stopper directly to input valve's grid pin. If problem still there, add 0.1uF Orange drop to the Log pot's wiper output (central pin). Listen and tell us if it still crackles.
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Old 27th December 2007, 05:05 AM   #18
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Nice. I am so jealous... My end goal is to make my own amp, but I'm at the very start. Still waiting for my first multimeter to arrive in the mail. lol...
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Old 27th December 2007, 06:17 AM   #19
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Hi Tv Bower, well I thrive on very little sleep, so lots of late nights after the family has gone to bed. Heavy alright - too heavy but at least it's not going to get stolen!

Richard, thanks for the kind words - my wife still won't let it into the habitable part of the house, but she is beginning to show signs of weakness since a good friend admonished her and told her that it was "a work of art" however, in the meantime it stays with me in the listening room/workshop

Fauxpas, I was in the same boat one year ago. With the help of this forum you can definitely achieve your goal. If you search my posts, I described how I completed my first project from start to finish and also included who supplied what. Good luck, it's great fun.

Hi there Dan, I realise that the pot is a bit of a compromise but I wanted to build an amp that hooks straight into a CD player without a preamp. I did it this way for my first project (following some advice from the forum) and it seemed to work so well that I just replicated it for this amp.

Thanks for the advice Victor, I will swap the inputs around and change the tubes to see if it helps. Will also check for DC on the pot. Failing that, I will then go with Salas's advice and try the resistor and the cap.

Cheers,

Rob
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Old 27th December 2007, 06:41 AM   #20
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Default Damn you motorola

Hey thanks Victor,

I did the bleeding obvious and changed input tubes - did not do this first up because I assumed the problem HAD to be in the construction.

Swapped the 40 year old motorola tubes for a couple of RCA tubes, hey presto, crackle gone! Bad tube I guess. The sound is better too. Thank you RCA.

So now, I can put the base on, wait for the family to go to bed and have a few drinks, listen to the music and watch the tubes glow

Will look forward to seeing all you folks with my next project, which is going to be (shock horror) a replica fender guitar amp for my son.

Have a great New Year folks - catch you soon and thanks again for the help.

Rob
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