B+ low plate voltage, dropping resistors

[diddymix]

Hi guys,

Just wanted to ask any advice with regard to the B+ line and troubleshooting a triode with much lower plate voltage than it should read. Its very annoying but I have checked the plate resistor and connections, the cathode resistors, the grounds, swapping tubes... but this one triode position V2B still reads lower.

Basically my preamps are reading

V1A 195V
V1B 198V

V2A 200V
V2B 150V - This one. (Should be around 203V)

V3A 215V
V3B (CF no plate resistor) 345V

PI - A 290V
B 300V

Just wondering what on earth could cause V2B to be so low.. Can this happen with the dropping resistors, with there not being enough resistance between stages?? Really scratching my head with this. Thanks for reading

[diddymix]

Hi guys,

Just wanted to ask any advice with regard to the B+ line and troubleshooting a triode with much lower plate voltage than it should read. Its very annoying but I have checked the plate resistor and connections, the cathode resistors, the grounds, swapping tubes... but this one triode position V2B still reads lower.

Basically my preamps are reading

V1A 195V
V1B 198V

V2A 200V
V2B 150V - This one. (Should be around 203V)

V3A 215V
V3B (CF no plate resistor) 345V

PI - A 290V
B 300V

Just wondering what on earth could cause V2B to be so low.. Can this happen with the dropping resistors, with there not being enough resistance between stages?? Really scratching my head with this. Thanks for reading

[Stevem]

What's the Cathode voltage on V2A and V2B?
Do you have a schematic of the circuit around V2 that you can share?

[martin manning]

I have no idea what circuit you're looking at, but it's likely that triode is drawing more current than it should be. You can calculate the current using the plate or cathode resistor value and voltage drop to confirm. Perhaps the coupling cap from the previous stage is leaky? What is the voltage on the grid?

[diddymix]

Hey guys, well its a DSL401.. but I copied the Dumble 102 schematic on here (for V1 and V2)... V3 Ive used as a different clean channel on a switch.

So what I did this morning was just get get rid of all the dropping resistors and connect up all the string with a copper wire to see where Im at with just the tubes. So it looks like this

plate resistor - cathode resistor - plate voltage - cathode voltage-

V1A - 220K 3.3k 256 2.3
V1B - 150K 2.2k 260 2.2
V2A - 220K 3.3k 256 2.3
V2B - 150K 2.2k 195 1.2
V3A - 100K 1.8k 285 2.2
V3B (CF) none 100k 420 285

So that cathode voltage sits at 1.2... so is relatively lower.. Thanks the gent above So do think this says theres a problem with a connection?

Thanks Martin, oh right so the grid actual coupling cap could affect this and over pull the current?? Ill check this..

[DimenBrun]

Current thru v2b is actually lower than the others based on the Vk and rK. Is the supply voltage the same on all the sections? You could try disconnecting the coupling cap feeding V2b....but if it was leaking you should have more current. The other thought I have right now is oscillation on that stage. You should draw up a schematic based on tracing out what you have wired up here. There has got to be an error there somewhere.
Good luck

[martin manning]

Something seems to be wrong with V2B's plate resistor. All give a calculated supply voltage of approx 410 except V2B.