Most likely it is a microphonic tube. The usual culprit is V1. That's the tube on the far right when looking at the back of the combo amp. Replace it with a known good tube and see if the noise goes away. If it doesn't then do the same, right to left, with the rest of the preamp tubes one at a time until you find the bad one. If that doesn't do the trick then the next thing to replace is the power tubes. They have to be replaced as a set. Use ONLY JJ 6V6's in this amp.
Another way to test for microphonic tubes is to LIGHTLY tap on them with a pencil and see if they ring. This test isn't 100% but it's a fairly good indicator.
Hi guys Thanks so much for the inf. I have replaced the power tubes as I happened to have a matched quad set of the JJ 6V6's and am now making my way through the preamp tubes. I am confused though as the list of preamp tubes on the Dr Z site puts a JJ ECC81 in V3 but the diagram in the amp shows V3 as the only 12AT7.
Does anyone have the tube layout for the 20/40 and / or confirm the recommended tubes for each position please? I would also be interested in what each tube did in the amp?
No discrepancy there. An ECC81 is a 12AT7. Just different nomenclature for different sides of the ocean.
The tube functions are: V1 & V2 - First and Second gain stage. V3 - Reverb tank driver. (That's why this position is a 12AT7. A 12AX7/ECC83 doesn't supply enough current to drive that transducer) V4 - Reverb recovery. V5 - Vibrato. V6 - Phase inverter/power tube driver.
OK, I have changed all Power Tubes to JJ6V6's all pre amp tubes and all leads and whilst there is some slight improvement (?) I am still getting a real mains hum with some added high pitched sizzle when compared to say my SRZ. Any more ideas around connections I should check / clean etc before I take it to an amp tech? Or is the additional noise expected because of the extra Reverb and Vibrato circuits and am I just being unreasonable?
There will be some as thats the nature of tube amps. Also make sure you get some distance away from it if you have a guitar in hand. Sometimes getting too close with a guitar plugged can cause extra hum.
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Mains hum and hiss are most likely coming from different sources. If the 60 Hz. ( Oops. I meant 50 Hz. ) hum is coming from the amp without anything plugged in you probably have a filter cap with a bad connection. If you don't have experience working on tube amps you should let a tech fix that because they can be dangerous.
If you are plugged in and standing too close to the amp the guitar might be picking up noise from the amp's transformer. That's normal.
If the tube changes didn't fix the hiss then you need to have a tech take a listen. It's hard to diagnose without actually hearing it.
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roscoenyc: I have never in my life experienced tube rattle. That said, I've only been playing electric guitar for 49 years.
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capofirstfret: I've experienced tube rattle in two different amps that I've had. In both cases I bought the rubber rings from Eurotubes and it fixed it for me.
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