Fender Bassman Screaming (not good)

[Jocomo]

Regarding my 1964 Fender Bassman amp (6g6-B)...after a blown fuse and a 6L6 tube that burned up, I put two new JJ 6L6 tubes in, turned the power on, let it warm up for 5 minutes and then took it off standby. The amp emitted an ear piercing, loud, high pitch sound so I turned it off immediately.
I've eliminated all tubes and any input item (like a guitar) from this problem. I can go into more detail of what I did and what has happened later if needed. Right now I'm thinking this problem is being caused by a resistor that looks burnt. It is the resistor that goes from pin 4 to pin 6 on the power tube socket. On the schematic it is supposed to be a 470, 1 watt resister. The "burnt" one is coming in at 411. On the other power tube socket, the same resistor is reading 464. Also, there is a resistor that goes from pin 5 to pin 1 on both tube sockets. This resistor does not seem to be on the schematic (although I'm not that great at reading schematics). This resistor is measuring 1570 on one socket (the one with the "burnt" resistor) and 1516 on the other socket. My question is this. Do you think the burnt resistor that is measuring 411 could be the cause of this amp screaming like a banshee when I put new power tubes in and powered it up/took it off standby? Again, I am 99 percent sure all my tubes are good, the volume is down all the way and there is no input to the amp.

[Jazz P Bass]

The 470 ohm/ 1 watt resistors are the screen resistors.
If they go open, that tube will not work.
Although the 'burnt' one could be injecting noise, I have not come across that.

The resistor from pin 5 to pin 1 is the bias grid resistor.
The factory simply used pin 1 as a convenient solder point.
There is not any internal connection from pin 1 to anything on a 6L6.

[g1]

It seems more likely that the squealing is an oscillation that is causing the resistor to burn.
Has any wiring at the output transformer been touched recently?

[Enzo]

If a resistor looks burnt, it is, and it doesn't matter what it measures. replace it with a fresh 470 ohm resistor. resistors have "tolerance" meaning they do not have to be exact. A 10% tolerance means it can be off 10% and still be OK. That would be 47 ohms for a 470 ohm resistor. 5% are also common, and that means the value wants to be within 5% or about 24 ohms. So 464 ohms is just fine. Unless it looks burnt. I tend to doubt the resistor caused the noise. it is usually a sign of a power tube that shorted out.

The 1500 ohm resistors are probably just fine, they are well within any tolerance specs. Those are what we call grid stoppers. They are not on the 6G6b drawing, but if you look at the AB165, AA864, or other more recent versions, they are there.
http://bmamps.com/Schematics/fender/...a270_schem.pdf

Did this amp used to work for you, and only now has this problem since the tube died? or did you acquire the amp blown and it could have been doing this before the tube failed?

[Leo_Gnardo]

If a resistor looks burnt, it is, and it doesn't matter what it measures. replace it with a fresh 470 ohm resistor. resistors have "tolerance" meaning they do not have to be exact. A 10% tolerance means it can be off 10% and still be OK. That would be 47 ohms for a 470 ohm resistor. 5% are also common, and that means the value wants to be within 5% or about 24 ohms. So 464 ohms is just fine. Unless it looks burnt. I tend to doubt the resistor caused the noise. it is usually a sign of a power tube that shorted out.

What Enzo said ^ ^ ^ . Failing tubes are often the source of awful noises. Also failing filter capacitors. In an amp that age, if hi voltage and bias supply caps are original, they really should be replaced with fresh ones. To complicate matters a bit, filter caps that were replaced 20 - 30 or more years ago are looking at their end of service life and should be replaced. If there's a preamp section where two cathodes share a common resistor/capacitor combination, a failure of that cap can result in loud obnoxious noises too.

Also a first on your part dear new MEF member, a post I could read from across the room with my glasses off. How'dya do that?

[Jocomo]

Thanks for all the responses! Here is some more information. I've had the amp for about 25 years and it has worked beautifully many, many times. I used it for regular electric guitar and play it through a Marshall 4 x 12. Usually, I use a cabinet that's 8 ohms but last Sunday, when the fuse blew, I was using a cabinet that was 4 ohms. I had the amp in the shop about 15 years ago and I don't remember the issue - might have just been getting new tubes and bias adjustment. Last year, I turned the amp on without the standby on and I think the sudden jolt to the power tubes blew a tube (but I'm not sure why the tube went bad). I replaced the tubes with new Groove Tube 6L6s and it worked fine. I've played through it about 5 times since that incident and even turned it up all the way to show my friends how freaking loud this amp is (note: I don't use this amp for my regular practice and rehearsals for reasons I won't go into). 3 days ago, I set it up and played through it for a short while because I planned to used it in the following day in a band rehearsal. I played through it for about 10 minutes just to make sure it was ready to go for the next day. I did not move it or change anything. The next day, when I took it off standby, it blew a fuse. Later, when I had time, I took it out of the cabinet and gave it a look over. I did not see anything unusual (but I'm not an expert). I took out the power tubes, replaced the fuse and powered it up. It did not blow the fuse. I concluded it was a bad tube that had shorted, just as someone had commented earlier. I figured, if it was a bad power supply, it would have blown the fuse with the power tubes removed.

OK, here's the part where I probably did something stupid. I powered the amp down, put the same tubes back in and, when I took it off standby, one tube started burning up, looking like a small blue thunderstorm inside and making a bad sound. As fast as I could, I turned the amp off. I went up to my local shop and got two JJ 6L6s, put those in, powered it up, left in on standby for 6 minutes - everything seemed fine until I took it off standby and the amp shrieked so loud, I think I woke up the neighbors - a very high pitched scream - she was very unhappy!
Of course I powered off the amp as soon as possible and did some research. The volume was all the way down on both channels and there was no input into the amp. Also, I replaced all 4 of pre-amp tubes with groove tube 12AX7s, which is what I've used before. So, I really don't think it's a microphoic or bad pre-amp tube. Sounds like you don't think the resisters referenced earlier are causing his ocillation but I don't know. The wiring at the output transformer has not been messed with - I can test the output transformer by measuring the resistance across the coils but I'm not sure what values to look for. Any suggestions are welcome.

[Jocomo]

oh, just read Enzo's - my eyes are bad so I use the bigger font. Looks like I may have to replace a lot of parts - I'll keep testing various parts of the amp. My meter does not measure capacitance so if I really want to take this journey and start replacing the caps (which I agree, probably need it) I'll have get some better equipment. Thanks again.

[g1]

Don't put that bad power tube in again, but try with the old preamp tubes.

edit: never mind that, I thought you changed the preamp tubes before it squealed.

[Jocomo]

One of you kind souls gave me a link to a schematic of a later model Bassman that has the second resistor across the tube socket. I was looking at that schematic and I noticed that the newer schematic calls for a 12AT7 tube in the position nearest to the power tubes. I found an old 12AT7 tube, put it in place of the 12AX7 I had in that position and tried the amp (I did not there think there was any difference between the 12AT7 and the 12AX7 except for a minor gain differential.

IT WORKED! No screaming. I did not think the pre-amp tubes were the problem because I had switched all of them out for Groove tube 12Ax7s earlier in the analysis.

Now, let me just say, that before I tried that little experiment with the 12AT7, I was probing around in inside the chassis, tracing wires to and from the power tube sockets, to just get the wiring straight in my head and to see if I could see anything unusual.

One more thing. I was so glad the amp worked, that I played guitar loudly for hours until I was starving for food. I went out to get food, came back and played some more - there was another round of high pitch noise but for some reason I turned the "Presence" control all the way up and it went away! I have no idea what's going on.