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More noises on my 212 HD 130 with Audio Sample

Hello everyone,

I play a 1970s 212 HD 130 with the 12ax7 phase inverter.

I posted a few months ago about an intermittent static issue I was getting. I seemed to resolve that issue by re-flowing the solder connections on a few suspicious looking spots.

The amp has been playing relatively well since then. However, I have recently noticed a new sound creeping up every now and then. It's pretty annoying. It's kind of like static or fizz that breaks through the normal signal. The static I was getting before showed up regardless of whether I was playing or not. This static seems only to appear when I play.

Follow this link to hear a sample I recorded:

Noise can be heard from about 3 seconds to 10 seconds.
At 35 seconds I switch from low power to high power.
Noise can be heard again from around 41 seconds to 52 seconds; 1:36 seconds to 1:38 seconds; and at other times til the clip ends.

I would love to hear from those with more experience if you've ever heard a noise like this before. It may help save me a little time trying to diagnose it myself.

My guess for now may be a bias issue. I notice that my bias reading fluctuates from .50 down to .492 and up to .530 sometimes. Is it normal to see the bias fluctuate like that?

I have materials here to create a signal tracer. That may be my next adventure.

Any other thoughts or suggestions would be appreciated! Thank you!



I couldn't hear any fuzz or noise, but my ears are old and may need waxing. The bias voltage fluctuations looked OK (5%) for an un-regulated power supply. When was the last time it was cleaned and serviced? How old are the power supply caps? Maybe look for more bad solder joints and clean the tube sockets. Love the fat reverb BTW. -mgriffin


Thank you for the feedback!

Thank you for the feedback!

The noise I'm referring to appears at the beginning of the attack of my guitar signal. It's a fizzly, ripping, static like sound that quickly disappears. It almost sounds like a sparkler or the fizzy sound a roman candle makes when it goes off.

I had it serviced last Fall, but I wasn't happy with how it sounded when it came back. Buzzy, hummy, and the tubes seemed to run really hot and produce an overheated smell. I decided to use this amp as a way to learn more about amp repair and maintenance. I've done quite a bit to it the last six months or so.

It has new power supply capacitors, filter capacitors, and electrolytic capacitors as of January 2020. I replaced the bias potentiometer, the 620 ohm resistors and diodes around it (the board looked a little cooked in that area). New tubes as of March (matched quad of JJ E34Ls). I put in new LM307H opamps on the main board for channel 2. I replaced the Zener diodes on the main board. I've replaced the capacitors on the driver board. Replaced a couple capacitors on the tube sockets. A few other things too. This thing was pretty beat up when I got it and has made all kinds of weird noises every now and then. It's been fun to breathe new life into it, but it's caused a few headaches as well.

So, with all that having been done, it may just be a bad solder connection somewhere.

It might also be my guitar's shielding! Playing through it tonight, it sounds pretty good, but I have one guitar that builds up static. I noticed the sound when playing through that guitar.

Thanks for the feedback regarding the fluctuation in the bias reading. I'll keep that in mind going forward.


Quick update: I determined

Quick update: I determined that this noise is (most likely) NOT a guitar shielding or static-build up issue. Played with two well-shielded guitars tonight and heard the same noise again.



Have you run the amp through a different speaker? -mgriffin



I played through a different set of speakers and the noise didn't show up. The noise is sometimes pretty sporadic, so that test may not really determine anything. Also, I'm playing through a new set of speakers in the amp, so I don't think the speakers are the culprit.

Other updates:
- I reflowed the solder connections on the driver board. After doing this, the noise has still appeared from time to time.
- I've been able to "catch" the sound coming through channel 2 rather than through channel 1.
- The tremolo and reverb footswitch rca jacks are suspect. The reverb jack makes lots of noise if I jostle the footswitch input, even to the point of cutting the reverb completely out. Bad RCA jack?
- The tremolo rate will change on its own after I work on the amp. I have to adjust the internal trim pot to get back to normal.

Don't know if those last two issues have anything to do with the original issue, but thought I'd add it to the list of observations!

The amp sounds great like 95% of the time. It's that last 5% that drives me crazy. Hopefully I'm not chasing ghosts!


Quick update on the reverb:

Quick update on the reverb: It's a bad footswitch connection, not a bad rca jack.


Good work.

Good work. I think you've figured it out. How does it sound on Channel 2 with the foot switch disconnected? -mgriffin


Plays like a dream

Played it the last fifteen minutes or so with no issues. Plays like a dream. Clean, overdriven, and fuzzed out. I'm hoping the footswitch is indeed the culprit. If it's not, I'll be back! Thanks for the guidance, mgriffin!


Back to the drawing board

Playing this afternoon without the footswitch and the sound returned. I was playing with an overdrive and delay pedal through channel 2 when it started. I quickly switched over to channel 1, but could not duplicate the noise through channel 1.

Time for a good tech? Any recommendations?


Input Jack?

Sorry you're still chasing a gremlin. Are the Channel 2 input jacks in good shape? IE good spring tension, clean and free of rust, good solder joints, mounting nuts tight? Does the noise happen on both inputs of Channel 2? -mgriffin


The Gremlin

Thanks for the suggestion. The noise appears through both inputs in channel 2. I checked out both input jacks on channel 2 (and those on channel 1 for good measure). To my eye, they appeared to be fine. I scrubbed them good with some De-oxit as well. Put it all back together, fired it up, sounded good for a while and then the sound reappeared.

So, I threw in the towel, called an amp repair tech in the closest big city and plan to drop it off tomorrow. 1 to 2 week turnaround time, so I hope to have an update for you then.

Thanks again, mgriffin, for the helpful suggestions. Stay tuned for more.


2475-130 Biasing problem

Hello Forum:
Troubleshooting a 2475-130 amplifier w/solid-state driver transistors, and found one of four tubes shorted out. Replaced all four power tubes (6CA7 JJ quad matched) and still unable to bias amp. Replaced the high voltage flyback diodes, and both 470pF HV caps, and now able to bias up the amplifier (25mV across each 3.9 Ohm Resistors). I'm now getting a white noise "hiss" from the amplifier, and it increases in volume when I turn up the channel volume controls. Set master volume at "5", and adjust each channel. Noise increases or decreases with channel volume. Also, tried channel volumes set a zero, and reverb and vibrato off. Increase master volume - noise apparently increases with the master volume. So without a signal tracer, seems that the white noise is coming from the power section of the amp. The op-amp is an RC4558, and not the 1458. Can i use a NE5532 low noise op-amp,and see if in fact the noise is coming from here. All comments and suggestions are thankful.


Why not?

You can try it. It won't hurt the amp. Don't be surprised if it doesn't solve the hiss issue. Is the hiss loud with a guitar plugged in? Is the Treble cranked up to 10 and the Bright switch ON? Try shorting the Input jack and see if the hiss goes down. Try removing IC7 (if in a socket) and see if the hiss goes away. If yes, the output stage is not the source of the hiss. Put IC7 back in and work back through the pre-amp stage. Remove IC2 and repeat. Let us know what you find. Good luck. -mgriffin

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