The Amp Garage

Hi Sluckey. I don't want to ruffle feathers here and this sort of discussion always seems to but you have posted a reasonable point. I certainly would not disagree with you per se but I'm afraid I'm unusual in issues like this. The 70% rule is a rule of thumb, nothing more. It is somebody's idea of ...

You are absolutely right Bepone, nothing wrong with the old fashioned method of metering the voltage, it's just a little bit fiddly to do it by probing around the base of the valves and if you do make a slip... With the anode voltage being around there, there is always the chance of a mistake causin...

Hi. Don't know if anyone has gone down this route this far before. We know that the early SL100s need valve socketed bias monitors to set bias current. It's simple to do that but I always find that approach pretty bothersome. The usual recommendation is to add 1R resistors to the cathode which is gr...

Just paralleling another 10uF to the originals was what I had in mind Martin, just as you suggest. I mentioned 22uF just as a reflex response to use E12 values. It's by far the easiest and quickest way and is completely reversible, always a consideration in a classic amp like this. I know what you m...

I've now rebuilt the bias circuit with a new 25k Cermet trimpot and a small 2k2 offset resistor to keep it towards the lower edge. The original problem was definitely one valve which went out in a big way. Blagging a set of valves from one of my other amps has stabilised things and means I am back o...

Of course, you're absolutely right, the meter itself is a high impedance load which will show any leakage as a voltage. A senior moment. Thanks for the metaphorical face slap Sluckey. Apologies for doubting you PDF64.

Thanks guys there is some great advice in there which raises some interesting points. Dropping the bias current when putting in a different set of valves is of course the sensible way to do it. Did I? We-e-e-ell, mea culpa, nope! :oops: Even though I knew I should I just hit the "Go" button and hope...

Hi guys. I have had this amp since 1970 so I know its history intimately. A fair way back now it had a mains Tx transplant with a Marshall supplied replacement, (Coca Cola and HT don't mix too well). It has also had a recap only a couple of years ago. All looks as it should inside. There are no bulg...