Blues Deluxe Low B+ voltage crackling distorting

[freeformfx]

Hey guys been awhile since I've been on the forum but I'm baffled and in need of help bad.
I have an original Blues Deluxe that's served me well for twenty years now and the other day after playing a little low volume bass through it it went all bad on me.
The B+ voltage is only reading 370v and the power tubes are pulling nearly no bias current ( 4mA is as high as I can set them) I've installed adjustable bias previously. Curiously the preamp voltages seem ok since at the first 22uf filter I'm still getting 370v there too which is what the schem calls for there. Then drops down to 330v also as it should. I've measured the secondary AC voltage coming off the power transformer thinking it may have given out and it reads 335v across the winding which is what the spec sheet says is correct here. The fact that the tubes are pulling no current but the B+ is already so low is baffling. Is there something I'm missing at the beginning of the PS that could be dropping the B+ so drastically that's failed or shorted?
I orded 4 new 100uf 350v FandT filters thinking I may just try replacing them as the ones in there from a previous cab job are of unknown origin, just caps I had in my supply
FWIW I did have no output at all at one point and found Q1 J111 fet shorted to ground I pulled it and put in a socket and temporarily placed a J201 until the part arrives but that's not causing my problems the amps sounds the same with or without it in the circuit

any ideas are very much appreciated guys, Thanks. I don't wanna have to give up on the old horse and buy a new reissue one. It is my main amp

[freeformfx]

btw with the output tubes pulled the B+ still only reads 407v. COULD it still be the PT anyway? I'd hate to have to buy one for nothing.
I could list all the things I've tried like resoldering tube sockets etc...but I'll wait for a response.

[Enzo]

The transformer is always the last thing on the list.

Is that low voltage full of ripple?

[DRH1958]

Plus if is is the PT, it seems all the B+ voltages would be off, not just the B+ for the power tubes. Did you try reading the screen grid voltage? Also, 370v on the plate is still enough to get them conducting. How about bias voltage? Should be about -45v. You should check pins 4 and 5 for these two. It sounds like it has to be those main filter caps like you think and are replacing. Especially if you get around 425v on the screen and -45 for bias. In fact, check these two at the source and directly on the pins of the tubes to rule out resistor trouble.

[freeformfx]

Plus if is is the PT, it seems all the B+ voltages would be off, not just the B+ for the power tubes.
Yes this is what I thought also
Yes the screen grids are at the proportionate voltage to the plates of the output tubes one volt less than and the bias voltage is there I think it's like -51v. I should mention that I did put in a 22uf 25v cap in the bias section cause it's all I had but I don't think it's given out yet. I know it should be a 63v cap
In doing some more testing around the B+ I did find that I have 88vAC at the positive lead of C34 (the main filter) is this normal I don't remember ever seeing that before but I might be a little rusty ?
thanks for any ideas please keep em coming cause I'm stumped unless it's in the filters

[freeformfx]

I also tried jumping a 70uf 350v filter I had across C34 but it totally shunted the entire output for some reason. All those caps measure within reason with my capacitance meter (in circuit) sorry I don't have a functioning scope before anyone asks. Just a nice meter and homemade signal tracer (listening amp) right now
No Enzo I didn't measure any noteable AC along the B+,Z,Y and X voltages except for the node going in at that first filter cap, 88vAC And the distortion/crackle does have a sort of oscillating ticking sound in it when the string is struck harder

[mozwell]

First, replace that 22u 25V cap, it needs to be at least 63V as per the schematic, a 25V cap may last, but it also may explode. Replace this before you power up again.

Second, measure all the power supply nodes, measure DC and AC ripple & report findings here.
If the amp is 20 years old, it "may" need new power supply caps, but lets measure before we start shotgunning parts....
Don't forget to measure DC and AC ripple in the bias supply and the low voltage +/-32.8V and +/-15V supplies

Third, have a good look over soldering on capacitors in the power supply & check the connections to the choke

Last, check all the resistors in the power supply, are the solder connections ok, have they drifted in value

Once we check all of the above, we can start to look further

[gbono]

Like Enzo said: "Is that low voltage full of ripple?"

I assume you measured B+ without tubes installed?

With only 4mA flowing - are you sure the grid bias is correct or modification isn't the issue?

Look for cracked solder joints on PCB/tube socket, plate resistors check out?

[freeformfx]

plate resistors have all checked out ok and or been replaced if drifted, the grid bias is pretty much close to the schem at I believe -51v
I'll check for AC all the way down the line again and report back, good idea. But what about that 88vAC at that first PS node?

[freeformfx]

as they go the ps voltages are 370 , 368, 370 and 330 then the 51v supply(strange I don't get 32.8v as on the schem) and I get 17v at the low voltage supplies.
As far as AC ripple i get nothing higher than .05v anywhere except for at the first B+ cap positive node where I get anything from 13vAC or 88vAC.
Which I then pulled that first series cap and checked it out of circuit and can't get a reading higher than .12nanofarads on the capacitance meter and a resistance of 35Mohms to infinity. Me thinks that cap is open or something and hopefully the causing of all the issues. I don't have one to replace it yet, at least one that fits.

[Enzo]

Well, that first filter is part of the B+ supply, so if you get 88vAC there, then that is a problem right there.

[freeformfx]

so you're saying then it sounds like that filter cap in my situation? would that explain the lack of bias too?
and would the bad filter in that position cause the 88vAC to show up there? sorry so many questions but as I said I'm rusty I had a bad brain injury a few years ago. some stuff starting to come back to me.

[g1]

Yes a bad filter cap would give you more AC at that node, like the 88VAC you are seeing.
Not sure what you mean about lack of bias? The schematic I have shows 38VAC form the transformer bias winding, is that the 32.8 you referred to?
Then there should be around -50VDC for bias voltage. It must be negative. Do you have that?

[Jazz P Bass]

Look at the schematic.
There are two capacitors there on the B+ node. C34 & C35.

The bias winding is totally separate from the B+ winding.

[gbono]

Which I then pulled that first series cap and checked it out of circuit and can't get a reading higher than .12nanofarads on the capacitance meter and a resistance of 35Mohms to infinity. Me thinks that cap is open or something and hopefully the causing of all the issues. I don't have one to replace it yet, at least one that fits.

Sounds like your problem - C34 or 35? You can solder another capacitor (must be same working voltage) across the suspect cap to see if you found the issue.