Blues jr III plate V issue

[daz]

The tone is just bad like one tube was bad so i swapped the tubes but same thing. Checked the plates and one was fine the other when i tried to read it the meter goes haywire and locks up. Turning it off and back on sets it right again but every time i check the plate on that one side same thing happens. Remove the tube, same thing happens Pull the OT's primary connector off the board and check the voltage at the OT and it's fine. Theres nothing i can see in the schematic that could be in the path that could be an issue except that diode, i guess a "flyback" diode? And it reads the same as the good one on the other side. i thought maybe the OT was screwed till i checked it unplugged from the board and saw it reading correctly. What am i missing?

by the way, the meter is 600VDC

Ahhhh....seems a screen resistor is bad....

[Enzo]

Yes, sometimes my meter freaks out on a plate reading. It isn't the voltage, because the same voltage measures OK elsewhere in the amp. I write it off to interaction, and probably something to do with the winding inductance. SOmetimes reversing the meter leads helps. But your issue won;t be output plate voltage anyway.

You found a screen resistor, hope that is all it is. But certainly from your complaint, a missing side of the PI comes to mind, like an open plate resistor there, or connection.


FLyback diodes can go open, and you will never know it. If they go short, your fuses blow.

[Mick Bailey]

Let us know if the screen resistor sorts it. Parasitic oscillation immediately sprang to mind. whenever I touch a meter probe on a plate or screen and get a really strange voltage, or a noise that starts or stops (and the amp sounds poor) this is usually the cause. Usually.

[g1]

I've found this whole series of Fender amps (with the ribbon cables) very finicky in terms of where you hold your hands/probes. It can cause odd meter readings or hum issues.

[daz]

It was indeed the screen R. I don't understand why they would use a 1/2 watter for that. Seriously, how much more for a say 3 watt or something. Geez...charge an extra 25 cents for the freakin' amp !

[Enzo]

A half watt 100 ohm resistor needs 70ma of current to sit at half a watt. That would be a LOT of screen current for that EL84.

Screen resistors burn up when the tube fails. Note the screen resistors here are specified as flame proofs.

COnsider there is 307v on the screen node. If you short that to ground through the resistor, it would take a lot more than 3w not to burn up. And then some other thing would burn up instead.

[daz]

A half watt 100 ohm resistor needs 70ma of current to sit at half a watt. That would be a LOT of screen current for that EL84.

Screen resistors burn up when the tube fails. Note the screen resistors here are specified as flame proofs.

COnsider there is 307v on the screen node. If you short that to ground through the resistor, it would take a lot more than 3w not to burn up. And then some other thing would burn up instead.

Thanks, i was thinking it must be something like that. i suppose they did that because they bias them so hot ?

[Enzo]

They do that because tubes fail, I don't care how hot you bias them, they fail. Tell me any EL84 amp that never lost a power tube?

[Chuck H]

They do that because tubes fail, I don't care how hot you bias them, they fail. Tell me any EL84 amp that never lost a power tube?

True that! EL84's are the bitches of guitar amp design. No other type seems to be so consistently abused so far outside their ideal parameters. And they're cheap! So... Smoke 'em if you got 'em

[Leo_Gnardo]

I've found this whole series of Fender amps (with the ribbon cables) very finicky in terms of where you hold your hands/probes. It can cause odd meter readings or hum issues.

Watch the output on your scope while probing or moving ribbons, or output transformer primary leads. You'll see the amp burst into ultrasonic oscillation. Sometimes just a snivet riding the test waveform, other times full tilt rail-to-rail bat music. Mostly in Blues Juniors. Can't be good for the tubes. Once the amp is stabilized, sounds so much better. How many amp owners and hack techs "throw" premium-priced transformers in plus other parts besides, trying but never succeeding to solve what can be fixed with a scope and a chopstick.

Fender's board layout isn't much help either. Especially in the Blues Jr & Pro Jr, some not-very-smart choices made with output section components and how close they're stuck together. One of my customer's amps started a fire back there. What a PIA to fix. Would make sense to put those screen grid limiter R's as well as control grid stopper R's right on the tube sockets, and fly wires to them from the board.

Half-watt CC 100R's just perfect for EL84. Supposed to fuse if the tube shorts. Sometimes it works... Just like the 1W CC 470's in the larger Fenders from 60's - 70's.