problem with Bluesbreaker voltages - help...

[HTH]

ok, I've taken some voltage measurements and compared them against the voltage chart in the (Doyle) 'History Of Marshall' book - the PI has something funny going on with the grid and cathode voltages...

Pin1 269v
Pin2 109v
Pin3 167v
Pin4 6.8v
Pin5 6.8v
Pin6 266v
Pin7 117v
Pin8 167v
Pin9 6.8v

anyone got ideas what's wrong?

[HTH]

bump!

sorry to be pushy, but I could really do with some help here - I'm stumped

[stokes]

I dont have a schem for a Bluesbreaker,but the voltages that are on the grids and cathodes are developed by the cathode resistor and fed to the grids via the 1meg resistors.I would start by checking these components.Or maybe try changing the tube,could be an internal short in the tube itself.

[HTH]

hey Stokes,

the voltages on the PI should be around 220v - 270v on the plates and around 40v - 50v on the cathode.

I'm seeing in the region of 170v ON THE CATHODES and it goes up above 300v with the NFB taken out. I've tried changing the tubes, oh - and the voltages drop down in the mV range with the PI tube removed.

I've checked the resistor values and they all seem fine - caps are ok too, no leakage.

All of the other voltages in the amp seem fine, just the voltages around the LTP.

This is driving me nuts, anyone?

[Rick Erickson]

Sounds like the PI Cathode has no ground reference except through the NFB resistor. Check the Presence pot and its interconnecting leads.

RE

[HTH]

Sounds like the PI Cathode has no ground reference except through the NFB resistor. Check the Presence pot and its interconnecting leads.

RE

hi Rick,

I've just checked the presence pot and everything is fine there - solid ground connection.

any other ideas?

[Rick Erickson]

Could be a leaky coupling cap - pull the caps on both grids of the PI and see if the cathode voltage changes. I'm thinking the far side cap.,(non-input side) usually a .1uF 250V could be leaky, especially if it's a mono-ceramic type. Other than that - make sure that all resistors are the correct value and that they are hooked up per drawing, no broken traces, solder bridges etc..
RE

[HTH]

Could be a leaky coupling cap - pull the caps on both grids of the PI and see if the cathode voltage changes. I'm thinking the far side cap.,(non-input side) usually a .1uF 250V could be leaky, especially if it's a mono-ceramic type. Other than that - make sure that all resistors are the correct value and that they are hooked up per drawing, no broken traces, solder bridges etc..
RE

Rick,

I'm pulling the PCB (again) tomorrow and I'll take those coupling caps out from either side of the PI.

They're new 'AF' brand polypropylene caps that I've never had problems with before, but I suppose you can always get a dud straight out the box. The resistors are the correct value too.

It's getting late here so I'm gonna get some shut-eye, I'll take another look at it tomorrow night and report back with findings.

Thanks for the help so far, much appreciated.

... Ian

[Enzo]

You checked the ground connection to the presence pot, but did you check the pot itself for open condition? The original suggestion was the PI cathode to ground path might be open except for the feedback. Look at the schematic and add up all the resistances from cathode there to ground. Now measure ohms to grounf from the cathode. Similar? Follow down the path to ground and measure resistance at each step. Disconnect the NFB if necessary to prevent confusing readings.

[HTH]

You checked the ground connection to the presence pot, but did you check the pot itself for open condition? The original suggestion was the PI cathode to ground path might be open except for the feedback. Look at the schematic and add up all the resistances from cathode there to ground. Now measure ohms to grounf from the cathode. Similar? Follow down the path to ground and measure resistance at each step. Disconnect the NFB if necessary to prevent confusing readings.

this got me thinking - the presence pot was wired was with the feedback wire from the pcb going into the top of the pot, the wiper and bottom lugs were wired together and there was then a cap going to ground.

I've replaced this setup by grounding the 'bottom' of the pot and connecting the cap between the wiper and ground. Voltage all look fine now.

Gonna pop a 'test' set a 6L6s in there now and hope it's ok.

[HTH]

ok, panic over - feel free to give me a virtual kick up the arse.

the problem was with the PI wiring - I should've guessed since the cap on that pot wasn't stock (doh!!!)

anyway, it works now - ain't got the feedback connected though. hope the hum quietens down once the feedback is connected up again (it's a tad noisy).

the resistors in the PI all measure fine and the voltages are where I'd expect them to be (they were seeing a little over 300v at one point). do you think the PI tube could be damaged from that high voltage? (surely the cathode on a 12AX7 isn't rated for those kind of voltages (?)

[Enzo]

Viltages on the tube apply to those ACROSS the tube, NOT absolute voltages with respect to ground. If you put 600 volts on the plate of a 12AX7 and 500 on the cathode, there is only 100 volts on that tube.

The tube is fine, I'd bet my lunch money.

[Rick Erickson]

I've replaced this setup by grounding the 'bottom' of the pot and connecting the cap between the wiper and ground. Voltage all look fine now.

Make sure the Presence pot is a 5K then, not a 22K. The wiring you described before you made the change was usually for a 22K pot and included a 4.7K resistor mounted at the pot and run to the ground buss to ground the Cathode. It had the advantage of blocking DC across the pot so it didn't "scratch" as it was turned. I personally prefer the scratchy pot circuit, because "real" Marshalls have scratchy Presence pots.

RE

[HTH]

I've replaced this setup by grounding the 'bottom' of the pot and connecting the cap between the wiper and ground. Voltage all look fine now.

Make sure the Presence pot is a 5K then, not a 22K. The wiring you described before you made the change was usually for a 22K pot and included a 4.7K resistor mounted at the pot and run to the ground buss to ground the Cathode. It had the advantage of blocking DC across the pot so it didn't "scratch" as it was turned. I personally prefer the scratchy pot circuit, because "real" Marshalls have scratchy Presence pots.

RE

Enzo, thanks for the clarification on the voltage accross the tube. I really must learn this stuff properly one day. Once I've finished this cisco ccna course I'll dive a bit deeper (maybe take a night class)

Rick, the presence pot is 5k (measured it out of circuit). It does scratch a little like you said, but it's not as bad as many Marshalls I've come accross.

One final question... the standby switches always pop on these bluesbreakers - can I connect a cap between the top and bottom lugs of the switch to kill the pop? (something like 0.22uF at 400v ????)

Also, I've moved the grid wires round a little - V2 seems VERY sensitive to positioning, I'm gonna put some shielded wire there and see if I can improve matters a little. Only thing is though, theres no real ground point at that point on the chassis - I seem to recall some JMP Marshalls that used the plate as a tie point for the shield but I'm sure this isn't the best of ideas really. anyone?

[Enzo]

You can run a lead over to a ground point. Remember you are not grounding something in the signal circuit, you are only grounding a shield.

What is the voltage across the standby switch when it is OFF? If you put a cap there, it better bave a higher voltage rating than what you find there.

Pops are not necessarily from the switch arcing. The tubes are hot, and when teh switch is flipped there is a large current surge as caps fill up and voltages rise quickly in the circuits. The tubes amplify that into a pop.