Fender '59 Bassman LTD (RI) OT troubles?

[tubeswell]

Got a friend's '59 RI LTD on the bench with 'sounding funny' (distorted) troubles and lowish volume. The output tubes haven't been changed in a couple of years and it gets gigged regularly. I decided to check a few basics before replacing any tubes. (As it happens - he didn't have a proper 6.3V lamp in there - He had a 28V lamp ?)

So - anyhow back to the OT, I get a B+ of around 460, but upon checking the idle voltage across the OT primary, the Brown half (to the CT) measures 43.5R and reads DCV drop of 1.07V with the power on, but the Blue half (to the CT) measures 46.5R and reads a DC drop of 0.33V. Shorted primary turn? Whaddaya reckon? (All the speaker voice coils have the right continuity)

(BTW - anything else that possibly could've been fried on the way (besides the OT)? - it seems to be running okay apart from the distorted sound/lowish volume)

[Tom Phillips]

My thoughts.
Based on just the data you posted it's not conclusive that the OT is bad.
The resistance difference in the two haves seems within normal tolerance to me.
The voltage drop difference could be because the bias current flowing through each tube is way different such as 24.6ma on the brown side and 7 ma on the blue side. Have you checked that?

[tubeswell]

Will check that thanks Tom. BTW from my Rmeasurements, it seems like the single 1R bias resistor (R48) on the main board in this amp is connected to both cathodes (can't see without getting behind the board, and the '59 Bassman layout and manual is obviously not correct for the layout in the LTD version)

[Enzo]

And have you checked the health of the screen resistors? Are they all within tolerance? Is there good screen voltage on all?

And if you haven't replaced those power tubes, have you at least swapped places with them to see if one tube is just a poor performer?

The speaker impedance can't reflect back to the primary and affect DC current there. Your DC resistances are about the same. I sure as heck won;t worry about a 3 ohm difference. SO it appears one tube is conducting a lot harder than the other.

Shouldn't affect power tube idle currents, but weak crappy sound in recent model Fenders often is due to an open plate resistor in the PI stage. It leaves one side of th push pull without signal. Easy to look for missing plate voltage on pins 1 and 6.

[tubeswell]

Both screen resistors look okay and measure pretty damn perfect. PI plate resistors measure pretty damn perfect as well. I'll chuck in a new set of output tubes and see

[Tom Phillips]

… it seems like the single 1R bias resistor (R48) on the main board in this amp is connected to both cathodes (can't see without getting behind the board

If that’s the case then you can plug in only one power tube at a time and measure the voltage to calculate the bias current. Use the same socket for each measurement. Then you will know if you have vastly different or a bad tube.

Both screen resistors look okay and measure pretty damn perfect. PI plate resistors measure pretty damn perfect as well….

Even though the resistors check out it’s good to also measure the voltage at the tube socket to make sure that it is correct on all pins of the power tube and the PI. This rules out broken wires and bad solder joints etc.

[tubeswell]

It was the same 6L6 plate socket pin causing the measurement problem after I swapped the tubes over, so...

with new OT and new pair 6L6 (6P3Se) Plates = 458 and 459, Screens = 458 and 458, tuned to 30mA per tube.

FWIW PI Plates 279 and 287, PI cathode(s) 33.3, PI Grid(s) 22.1 (seems awfully cold?)

(and I don't fancy the sound of the RI Jensens on board, although there's nothing 'wrong' with them. They just sound harsh)

[Enzo]

Before you let it go, look close at ALL the 6L6 socket pins for solder cracks. I routinely have to resolder those.

You have 33v on the PI cathode, and "22v" on the PI grids. I bet those readings are taken to ground. COnnect your meter between cathode and grid directly, and measure the voltage difference. I bet it is more like a volt or two tops. That would be your meter impedance interacting to make that 22v reading - circuit loading. SO if there is a volt between them, then the voltage at the grid is more like 32v, you just can't directly measure it with a meter.

I could be wrong, let me know.

[tubeswell]

All the socket pins check out fine, but I re-flowed them all just in case (Thanks for the tip Enzo). I must say that they use really thin wire on these things. Even the heater wire wire gauge looks to be on the small side.