Things that are out to make me look stupid.

The Marshall TSL 601. This one definitely didn’t like me.

Having said that, I’ve never been very keen on the amp, and although I wax lyrical about Marshall’s parts department and their wonderful Imagebank, I don’t like some of the more recent design ideas. ‘Recent’ in my time scale (Old Gits’ calendar) stems from around the Norman conquest….of….erm….Norman(?).

The major problem (unless you take out a second mortgage) with amps built in the last, say, 25 years is that they have PCB’s AND VALVES. The valve base and soldered contacts associated are quite often situated on the PCB (I won’t be doing this on our ‘Most Wonderful amp………blah, blah). You will have noticed that a valve gets hot…..(Hmmm….the clever stuff eh?). What happens over a period of a few years is that the soldered contacts on the valve bases melt off the tin content of the solder, and it’s that stuff that is the main conductor of the solder alloy. There are different alloys these days but the effects are very similar. The tin content also provides a certain flexibility, and when that is gone, the solder joints crack due to vibration.

Anyway, to get back to the point, which was me being bitchy about a TSL601.

An interesting symptom came up, which I should have latched onto quicker (hence the ‘things that are out to make me look stupid’ bit.) In this case it was because I was being stupid. Most ECC83 valves designed as a preamp have an anode voltage which might be anywhere between, say, 100+ volts and 200+ volts. All the preamp valves in this amp had 400+ volts on the anodes. What this means is that the ground to the cathodes of all the preamp valves has a fault. No it didn’t. So all these valves were not working; they were taking zero current. A quick check across the cathode resistor (usually around say 820 ohms to 1K+) should show a voltage of around a volt or thereabouts. They didn’t. They were as silent as my dog, Rex, who has been dead thirty-five years.

This called for serious action. I have this four by two plank in the workshop that I batter my head with. The philosophy behind this is that the plank of wood will think it out faster than me. It worked. The plank said, ‘Are the preamp valves hot?’

I said “Duh!”.

I then looked at a schematic, which is a good idea to do BEFORE you’ve wasted an hour guessing.

The preamp heater supply in the TSL601 is rectified dc, and the bridge rectifier had blown.

I cannot rightfully claim a macaroon on this one…..BUT….the Redoubtable Dave (in the absence of a macaroon stash in his local supermart) brought me a box of very nice shortbread biscuits. So that was alright.

Thank you for saving my life yet again Dave.

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