Fender Blue Junior III "tinnitus" Issue

An attempt to calm a Blues Junior III issue with a very sensitive front end tube - the first 12AX7 - seems to have a tendency to oscillate and cause acoustic feedback from mechanical vibrations in the cabinet apparently at high frequencies and was described at "tinnitus" (a very high frequency ringing sound). I tried numerous brands of vintage 12AX7 tubes and settled on a vintage military version of 12AX7 by Tung Sol.

[@ElPasoTubeAmps]

I ended up leaving the RCA branded Mullard tube in the amplifier and the musician was very happy with it.

[@johntempleton6368]

I've worked on many Blues Junior amps and have a couple of observations. Part of the problem is the design of the amp. If you were to tap on the ribbon cable you'd get almost the same results as tapping on the tube. Using a tube in V1 that has a large plate structure makes the problem worse because the grid structure is also larger. There are a couple of recently introduced tubes that will tame this problem quite a bit. The first is the JJ E83CC. It has a dedicated frame around which the grid is wound and this greatly reduces the noise caused by mechanical vibration. The other is the Electro Harmonics 7025. I'm not sure what tricks they used but it also has greatly reduced noise due to mechanical vibration. Another thing to do with those amps is make sure the ribbon cable doesn't contact the rear panel when it's installed. The board mounted sockets and use of very stiff ribbon cable all contribute to the transmission of vibration.

[@ElPasoTubeAmps]

Makes a lot of sense what you say. I have notified the musician to come pick it up but I may take another look at it now, especially about those grey ribbon cables possibly touching the back of the cabinet. Never thought about the physical size of the triode structure. We know 12AT7's have small triodes and usually are much quieter but I always attributed that just to the gain of the tube. The JJ tube that is original has smaller triode sections in it and it seems just barely more microphonic than the one I left in it. Thanks for the information. I always learn from you guys...

[@diabolicalartificer]

Is the microphony an issue when in use? Would a damped valve socket or shielded can type socket help? If the front end is prone to parasitic oscillation too maybe the design needs tweaking to reduce gain at HF, and or larger grid stoppers, especially if it's a known issue on these amplifiers.

[@ElPasoTubeAmps]

I am trying to not modify the circuit. For one thing, I have found that these types of circuits work with different gain tubes like maybe changing the 12AX7 to a 12AT7 or others of that family of tubes. I remember years ago there were these rubber O'rings that were sold that was supposed to help. Never tried them as it seemed like snake-oil to me. I have to admit, I did wrap a big rubber band around one of the tubes to see if would help. I couldn't tell any difference. Personally, I think this tube should be shielded and I like the idea of larger grid-stoppers if the problem continues. Thanks for your thoughts.

[@EJP286CRSKW]

I tried the O-rings. Absolute snake oil, as I more or less expected. Stabilizing the glass doesn't stabilize the grid.

[@ali2ndmail]

Can’t help wondering if there is something else contributing to the microphone as well as the tube, as all tubes you put in v1 seemed to have it to some extent.

[@ElPasoTubeAmps]

From my experience with guitar amplifiers, they are all pretty sensitive to having that first 12AX7 tapped on with a hard object like a screwdriver.

[@ronfrey5327]

I have a 50 watt bassman its a rebuild but its not getting sound I think it might be jumper related would like to send it out im near Dallas how can I get ahold of you id like to call you about it. Id just mail chassis part if I could have you go through it see if you can get sound and bias it. But its basically a rebuilt 68 bassman, Is that something you might help me with if I mailed it out ?

[@pappydaddy7447]

Thanks for this video from one of your fans.

[@bretthibbs6083]

When you were tapping the tube it sounds like it was coming from the reverb tank I'm wondering if the tank was picking up that sound and I know that you were tapping the tube but I thought that the reverb tank has some kind of connection to the preamp section

[@ElPasoTubeAmps]

I agree, I could hear the reverb tank adding to it but there was also plenty of acoustic feedback coming from the tube as we could hear the difference when changing tubes. I should have set the reverb to off.

[@1oldtexan922]

Had a similar issue with one I worked on years ago. Found a microphonic cap in the first stage amp.

[@ElPasoTubeAmps]

I haven't thought about microphonic caps. If this doesn't resolve the issue for the musician, I will look into that. Thank you.

[@pomonabill220]

Interesting that the different tubes had different microphonic responsive sounds when tapped. I have seen the tube "flareups" when first powered but assumed that the tubes were made that way, and didn't know that helped with shortening the warm up times. I will remember that in the future though. Nice to know that ... thanks!

[@ElPasoTubeAmps]

Hi Bill, Faster warmup is the most reasonable explanation I have gotten for these vintage European tubes and their flashing filaments. It has been suggested it was to help "de-gas" the tube but I think I remember reading that was debunked. Maybe it also helped distinguish their premium audio tubes from the American made tubes - who knows - but ultimately, it backfired in a big way and many people think they are bad and likely throw them away. Goes to show how a good-idea sometimes doesn't work out so well. I am trying to save as many as I can by making mention of it.

[@SirLoinMagroin]

It is not necessary to turn the amp off when swapping the V1. You are cycling the tubes unnecessarily. Won't hurt a thing. The Blues Jr's EL-84's are notorious for being over biased hot at the factory and also add to the noise. Not to mention heat damage and short power tube life. Fender has never fixed this. Even in the series IV version. Virtually every amp tech has dealt with this flaw on this model. Their filter caps are also prone to early failure. Any good amp tech can cool the bias, and easily bullet proof these amps with a few quality components and a couple of mods to make them last. Otherwise, they end up ruining the board from heat damage in two or three years of regular use.

[@jutukka]

I have noticed that usully if the amp is turned on through a NTC or light bulb reducing the PT primary circuit startup current peak, those tube filaments don't flash like that. Of course, no surprise. I usually always use that kind of soft start if possible, hopefully to give power supply capacitors, fuses and diodes easier life as well.

[@ElPasoTubeAmps]

I definitely like soft-start circuits also but I try not to modify circuits for my customers. A few years ago when I did more of this kind of work after retiring, a number of people brought me their vintage amplifiers that had been modified and wanted them back to original. Strange how when an amplifier first comes out there are so many ideas to make it better and then 40-50 years later, everyone wants them back to original. 🙂 The European tubes flashing the filament is normal and healthy - they are designed that way. I have known this for 60 years but since having a YT channel I have learned that many people think they are defective and throw them away and it is not a new thing. It seems many people have thought them defective for a very long time. Oh well... Thanks for your comments.

[@jjsant3250]

Maybe a microphonic component in the area? Seems strange that they are all that loud.

[@ElPasoTubeAmps]

You are the second one that has mentioned other components as microphonic. I will look into that if the musician says the problem continues. Thanks...

[@gordonwelcher9598]

You could place a tiny speaker next to the tube, connect it to a signal generator and manually sweep the frequency. Maybe you would see where it resonates.

[@ElPasoTubeAmps]

That is a neat idea. I will try it. I knew I would learn something from you guys. Thanks...

[@Antony_Jenner]

Not the best amp Fender made, but hey if you like it so be it!

[@jimcatanzaro7808]

I hand wire board for these instead of the cheap pcb

[@ElPasoTubeAmps]

Years ago I had a difficult problem with one like this - not this same model - but the problem was that the grey interconnecting cables are solid wire and stiff and a wire had broken right at the PCB. Very hard to see and find. I determined than it was best to try to not move these PC boards around as any/all of those interconnect cables could easily break. Nothing like point-to-point wiring. Thanks for your comments.

[@jimcatanzaro7808]

Ya the pcb boards are not ment to be gigged and tossed around in a van and bad under hot rod cold conditions if left in a bedroom or living room they will last a long time But stored in a garage or a basement not a good idea and that’s where these usually go

[@vadenk4433]

Junk those IC caps while you're in there. Those things are garbage

[@MrAletube]

GT tubes are all crap