• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Look what Santa brought!

Hi all,

I have just completed my second amp after about 200 hrs of work!

It is stereo amp based on the Mark III KT88 monoblock with 6AN8 drivers, a couple of NOS Mullard GZ34 rectifier tubes and some Genelex 'Gold Lion KT88 tubes. Used new components throughout with sprague orange drop caps.

All hand made apart from the SDS Caps board. I have also added (this is going to drive a stake into the heart of the purists, sorry) digital bias meters for both channels - don't crucify me for this one....

The chassis is aluminum with the base and transformer covers black nickel plated and the top is plated with 18ct Gold.

It sounds great, but to be totally honest maybe not quite as good as my first project (with EL34 output tubes), seems t be a bit less punchy with the bass - or maybe I was expecting too much - I really couldn't fault my first amp for sound.

It also has one small problem that I don't seem to be able to solve. With no input signal, when I turn the volume pot, there is a very slight 'crackling' through one of the channels as the pot rotates, especially at high volume. The channel is silent even at full volume when the pot is stationary and sounds fine when playing sound. I have changed the pot but the problem remains so I think that the issue is downstream of the pot and only in one channel!? It doesn't cause a problem when listening to the amp but I am a perfectionist at heart, so any ideas will be acted upon.

Enjoy the pictures and thanks again for the advice along the way.

Cheers,

Rob
 

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Second build

Congrats Rob!!
Your second build is even better than your first (Followed your first!)
The gold plating is very tasteful & elegant.....the black nickel sets off the gold superbly!
Now the question...do you have a place in the home for it?
I see audio sprouting thruout the house......all different decor to match(?) the amps....??
You didn't want to string speaker wire in all the rooms now did you!................Sooo you have two now...and how many rooms are left?
_____________________________________Rick...........
 
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
 
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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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.
 
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
 
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