Fender Bassman 70 noise

[phantombox]

Hi.
I'm servicing a Fender Bassman 70. It had several issues, all are fixed now but one:
There is an erratic, none-constant noise (like a really old and scratched vinyl record) with volume on channel 1 and 2 all the way down and master-volume up. Actually, its almost the same if master-vol is at 3 or 10.
There are 2 brand new MESA 6L6-GT tubes installed, preamp tubes were checked with tube tester, I already checked for any cold solder points, but the noise is still there.
Anyone had the same issue before?

Phantombox

[Jazz P Bass]

Check the .01uf cap that is before the MV pot.
It is a blocking cap.
It has to stop the 280Vdc on the plate of V2 pin 6.
There should not be any Vdc on the MV pot itself.
While you are at it check all of the high voltage nodes for Vac ripple (more specifically, lack of Vac ripple)

[Enzo]

A noisy amp? Yes, all of us, many times. Will it be teh same cause amp to amp? No.

Isolate the problem. Got a scope? Got a signal tracer? In any case, pull the PI tube, 12AT7 V4. Noise go or stay? If it stays, then the noise is in the power tubes - unlikely but possible.

PI tube out kills the noise? Put it back and pull V2, the tube that feeds the MV and V4. Noise go or stay?

That tells us before/after in at least those stages.

Use the controls. If the MV turned to zero kills the noise, then the noise is coming from before it.

You replaced the power tubes, but you checked the preamp tubes with a tube tester. Hate to tell you this, but tube testers pretty much tell you that a tube lights up, and emits electrons. It is a very rare tube testeer that can tell you a tube is noisy. You need to substitute other preamp tubes that you know work. You can do them one at a time, no need for a "whole set." Borrow them from another amp.

Plate resistors are a common source of noise in amps, when they go noisy. Once you isolate the source of the noise to a stage, they would be prime suspects.

[phantombox]

This amp seems to be modified in a way that the original schematic doesn't apply anymore: bunch of cables replaced, cap values changed, signal path altered, dead spiders added, etc...
I will see where the problem is...

[phantombox]

I pulled out all preamp tubes, one by one. When I got to V3 (normal channel) the noise disappeared. Using the tube from V3 in a different socket didn't make the noise come back, so its not the tube but the circuitry on V3. Now I have to check the components and/or signal path ...

[phantombox]

The amp still sound like a popcorn machine. There are 2 resistors 100K 1W that supply V+ to pin1 and pin6 of V3. I measure 285V on pin6 but 437V on pin1 when according to the schematic there should be around 280V. I guess that is what's wrong with the amp, but all circuitry is according to the schematic, as far as I can tell.
Any ideas?

[Tom Phillips]

...I measure 285V on pin6 but 437V on pin1 when according to the schematic there should be around 280V. I guess that is what's wrong with the amp, but all circuitry is according to the schematic, as far as I can tell.
Any ideas?

I this case they most likely problem is an open in the circuit that is preventing the expected current from flowing through the resistor. Therefore, there is little or no voltage drop across the resistor.
I'd do the following
1) First try a different tube
2) Check the connection from pin three and trace it through the 1,500 Ohm resistor to ground. Verify the resistance from pin 3 to ground with the amp turned off and the caps discharged.
3) Make sure that both heaters in the tube are on. You should see two glowing heaters, one heater in each side of the tube. A bad solder connection at the tube socket can interrupt the heater power and turn off that section of the tube. Could also be a bad tube socket.

Tom

[phantombox]

Pin 3 connected to ground through a 68K resistor (I didn't pay much attention to this detail 'cause there was no crackling before). After I replaced this R with 1.5K, V on pin 1 dramatically dropped to 292V (maybe I should have done this before...).
All contacts on valve socket seem to be OK: I applied some alcohol to pins, inserted tube several times, moved it around by hand and attached cables on other side also moved so they do make contact.
I also inserted another tube I had lying around but the amp is still "popcorning"...
Is it possible to tell if both filaments are glowing by just looking at the tube?
BTW: the tube I used was a Philips Miniwatt ECC83 NOS in 1A condition

[Justin Thomas]

Is the "popcorning" the same noise as in your first post? If so, check the 220K & 470K (in your amp) resistors that mix the two channels before entering V2. My BF had a random frying/popping noise. Checked/changed every resistor in the amp, one at a time. Those were the last two to be fixed, noise is 95% gone, which is fine with me. If it's not the plate load resistors, I go straight for those now. They carry signal and get voltage, so... takes a few minutes & is cheap. HTH,

Justin

PS stop bragging!

[Tom Phillips]

...Is it possible to tell if both filaments are glowing by just looking at the tube?...

Yes. You can see a small dot of an orange glow at the top and bottom of each side of the tube. Yours is working because the tube is drawing current.

[phantombox]

"Popcorning" is still the same. I lifted one side of the 220Ks and the 470K of the circuit and measured. R is slightly lower than expected (10% to 15%). I will change these on Monday.

[g1]

I'm getting the idea that what you are calling V3 is not. If there was a 68K there instead of 1.5K I don't think it could have worked right. V3 is a designation on the schematic that does not have to match up with the physical layout. Sometimes they do, but it's not always the case.
Also, your troubleshooting process is using the assumption that pulling V3 stopped the noise, if it's not really V3, then that changes everything.
However, I see you mentioned there are lots of mods, so I could be wrong.
Best to follow the circuitry from the preamp tubes and find out which V is what.

[phantombox]

Let me see if I get this right: according to the schematic that "Jazz P Bass" (fender_bassman-70_sch.pdf) posted and that I already had before, V1 is the preamp tube for "Bass Instrument", V3 for "Normal", V2 is the one that only uses one triode before the master volume and V4 is the 12AT7 driving V5 and V6. That's how I described my troubleshooting process up to now.

[Enzo]

Keep this in mind: your amp is making a random noise, it is not a steady tone like feedback, nor is it some continuous distortion. So it won't be caused by an off value part. A 220k resistor that has become 330k might change the tone or the amp or reduce the gain of a stage, but that won;t cau8se pops and crackles. A cap might measure 1uf instead of 10uf, and that might affect bottom end response, but not make pops and crackles.

Point is then that you probably won;t find the bad one because it has drifted in value, perhaps every resistor in there has drifted. If the bad one has drifted, replacing it will cure the problem if it is at fault, but the resistance itself is not what makes the change.

V3 does have some 68k to pin 2, or at least to its grid on one side.?

Lifting the 470k mixsing resistor is a way to measure it, but while it is lifted, power up the amp to see if that kills your noise. I suspect it does, as anything V3 related comes through it. SImilarly, you can lift something like a coupling cap to isolate a section.

[g1]

Let me see if I get this right: according to the schematic that "Jazz P Bass" (fender_bassman-70_sch.pdf) posted and that I already had before, V1 is the preamp tube for "Bass Instrument", V3 for "Normal", V2 is the one that only uses one triode before the master volume and V4 is the 12AT7 driving V5 and V6. That's how I described my troubleshooting process up to now.

I pulled out all preamp tubes, one by one. When I got to V3 (normal channel) the noise disappeared. Using the tube from V3 in a different socket didn't make the noise come back, so its not the tube but the circuitry on V3. Now I have to check the components and/or signal path ...

Sorry if what I said didn't make much sense. What I meant was we expect tubes to be called V1 and so on in order across the chassis, but that is not always the case. But if V2 is only using one triode, it should be pretty clear looking at the wiring. The 68K was obviously wrong as the tube was not conducting and now it is.
What I don't get is why pulling V2 did not affect the noise.
V2 is after V3. If the noise is in the V3 area, pulling V2 should have at least caused the noise level to drop, even if the noise still bled through the 220K.