A Marshall JCM900 100 Watt Combo  arrived in the workshop.  This version is the 4102  It was very poorly. The first issues seen were : 10 amp fuse in the plug!! Low power output and VERY  loud mains hum Pots noisy Amp plays a dying gasp when amp switched off Pops / crackles and very low volume. The valves were tested on an AVO MkII and CT160 tester.  The preamp valves were down to a lot […]
Note: We now have a new, superior solution to Marshall JCM2000 bias drift issues. Find the details here. The early (1997-2003) Marshall JCM2000 DSL50, DSL100, TSL100 and TSL122 amps are often troubled by an unstable bias (drifting bias). These are nice amps and I have one myself This can be difficult to diagnose correctly, as it can take half an hour or more of testing and/or playing before this Marshall JCM200 bias drift shows up.
Here are Things learned about Tube Amps in 2018 . They are in no particular order. Perhaps some useful info about your amp. I thought I would share them before they faded out of my head Loudness Your ears respond to sound like a compressor.  So when you ears hear the sound from a 10 watt amp and a 100 watt amp, you don’t hear an amp that is ten times louder.  Your ears hear
We have broadened our range of EL34‘s in our store. But which one is best for my amp? This is a handy quick guide to EL34’s We now have :- JJ EL34 JJ EL4 II JJ EL34L EH EL34 Tung Sol EL34B We offer all the above in matched pairs and quads. The EL34 was one of the last additions to the audio valve market and it was introduced by Mullard in the early 1950’s.
Which EL34 ? We explain differences on selected types Read More »
A Marshall JCM900 amp arrived.  The JCM900 combo is basically a 100 watt twin 12 inch speaker combo and is a true Marshall stack in a matchbox. Nice and easy to transport.  When new these amps  supplied in two variants, with one having 4x EL34’s in the output and the other variant having slightly lower power with 4x 5881’s. This amp was dead. Zero output and a lovely hum. Inside the amp proudly displayed
Note: We now have a new, superior solution to Marshall JCM2000 bias drift issues. Find the details here. I have now worked on two Marshall JCM2000 Â amps and both are around 10 years old. Both had very similar faults. The fault on both was that the amps sounded “rubbish” and both were cooking the EL34 output valves. Instead of running at about 45 mA of bias current the lowest adjustment you could achieve was over