zambiland wrote:I would add the midrange knob. Around 15 years ago I found a blackface Bandmaster for $25 that became the vehicle for my amp experiments and I added the midrange (big success!), a presence control (another big success!) and a Torres effects loop mod kit (huge failure and much anger at Torres who sent me the wrong kit and refused to honor the implied warranty of fitness for a particular purpose and refused to help me in any way to get it to work right. I was so pissed, as it cost me the first channel of my amp. Avoid that guy like the plague).
The odd noise you are hearing sounds like bad tubes to me. It also could be bias problems. It could also be a rubbing voice coil in the speaker. It sounds like this amp is due for an overhaul anyway. New tubes, new filter caps (and maybe new 25/25 caps in the preamp section), new plate resistors, etc. should clean the amp up greatly. You might also want to convert the bias circuit to blackface specs and beef up the bias cap a bit. I did that on my Showman along with extra filter caps and it gave the amp a lot more headroom (always a good thing in Jerry-land!).
zambiland wrote:Two things I would check. 1 is the speakers. You might not like the tone of those speakers as they break up.
The 2nds is the bias. It sounds like you might have the amp biased a little cold. You get more crossover distortion as the amp gets cranked, which sounds like harshness as the amp breaks up. Bias is not an absolute number but a range where you make tradeoffs for a warmer, louder, punchier tone for tube life. I tend to bias at about 75-80% of static dissipation. It's been a while though, these days I usually just guitar through an SMS Classic into the power amp section of an Eden WTX-500 into a JBL. It's pretty much my dream tone.
As far as extra filter caps, I mean adding some extra capacitance. I added 200mf of caps to my Showman, which basically acts like a reservoir of instant power for transients. Since I played bass through it a lot, it's a great mod for that.
tigerstrat wrote:I would rethink having the ecc81/12at7 as your 1st gain stage (V2); in my exp it's very important to have a high quality 7025/ecc83/12ax7 in that spot. If you want less gain in V2, try a 5751, not a 12at7. OTOH, I have found that a well-balanced 12ax7 can make a decent PI (V6). Go ahead and pull V1 if Normal channel is unused, and V5 can probably go too.
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