This is a fairly common problem and can happen to any amp, regardless of what topology it uses, especially when it ages somewhat and gets used fairly often. This little amp is vacuum, so it has somewhat higher voltages than those found in most sand amps. Higher voltages stress components to a somewhat larger degree, even if good quality and correctly specced parts are used. So I suspected maybe a coupling capacitor or two might have gone leaky, which usually results in crackling when adjusting volume controls, or for that matter, any potentiometer within an amplifier. This is then a story of what I do when testing and repairing an amplifier with noisy controls. This applies equally to Hi-Fi or guitar or even PA amplifiers. By PA I mean the types that you normally cant understand at the railway station or airport. Note the authentic vintage dust that can be purchased on Ebay
. Yes, there are some shonky ebay operators.So when examining the underside of the amp, I noticed that the four cathode bypass capacitors on the cathodes of the output valves had popped their corks. By that I mean, for some reason they had overheated, and thats generally caused by over voltage, or being too close to a heat source. The last is not the reason, as I spend a lot of construction time ensuring that nothing can cause this. i.e. no component that generates heat is anywhere near electrolytic capacitors.
Said capacitors were RIFA 100 uF @ 25 volt units made in Sweden, cost an arm and a leg and fail very rarely. The bias voltage "normally " found on a 6BW6 (or 6V6 in a nine pin jar ) is around 18 volts, give or take. These have been removed in the pic, as they were leaking electrolyte all over. It can be corrosive, and is defiantely not healthy to handle, although not as bad as some old electrolytics which contain PCB.
( PCB = Poly Chlorinated BiPhenols which is now banned the world over because it DOES cause cancer in humans ).
Yes, read that again and remember it.
New electrolytics will be fitted. These are different to what was origionally in place, but are close enough and no sound change will happen. New caps are Siemans low leakage long life 68 uF 40 volt units. In fact its getting quite hard to source modern RT series caps. RT means they have a tail out both ends of the cap suitable for point to point wiring, rather than RB series which are designed for printed circuit board mounting, and have both legs out one end of the cap.
Also they are uselessly short and dont fit back where the origional cap was removed.
I also did some clean up of my shoddy work when I built the amp origionally.
Most notably the DC heater supply for heating the valves in the integrated phono stage. It was cobbled together for testing, and as it worked marvelously well I ledt it thus, but was a bit shamed when I saw it again.
In the photo Stereo4 right on the back panel facing the viewer is the end on view of two 10,000 uF 16 volt capacitors, and two five watt wirewound resistors. One of those resistors was "hanging " on full length lead length, as it was just dagged in there to see what heater voltage I obtained by using it. So after testing for 15 or 18 years its finaly been fitted in its final position.
More tomorrow.
Joe
