Jansen 12 Sixty head repair

[MikeC]

Hey all, I have an old Jansen 12 Sixty head that I'd like to repair. I have a schematic drawn out by a guy called Tom Henderson and the first thing I notice is that my amp has 5W/485 ohm "plate resistors" soldered to pin 3 of each of the 4 power tube sockets. The other end of the resistor goes to the OT primary. Usually, pin 3 connects straight to the OT primary. One has seriously smoked (see pic below). These resistors are not shown on my schematic. Any thoughts or ideas on why they are there and indeed if they should be?

Jansen 12 Sixty Plate Resistors.jpg (3.89 MiB) Viewed 2981 times

Jansen 12-Sixty Schematic.jpg (265.92 KiB) Viewed 2965 times

[RectifiedAmps]

Do they look original? Your photo suggests as much. I haven’t seen this before but I’ll check to see if I have any 12-sixty photos in my archive. All I can think is that they were there to hobble the output maybe? Strange…

[MikeC]

Yeah they do look original. Thanks.

[sizzlingbadger]

Check the OT resistances, does it look normal for 6L6's ? Maybe they used a different OT and the resistors are there to match the impedance, and would reduce the power output too.

[MikeC]

Good idea - I'll see if I can find out what the typical resistance of the primary would be. I have no way to measure the impedance.

[Jay]

Is it a 6L6 or 6L6GC?
When doing the caculations according to one datasheet from ampage.org, those four 2W resistors are undersized for the GC version and way under for the other. If you must keep them, change to 5W.

[RectifiedAmps]

Looks like the earliest Vox AC10s and AC15s had resistors between the anodes and OT primaries as well:

Vox AC15 '59.pdf

(202.13 KiB) Downloaded 156 times

Must’ve been a ‘60s thing

[sizzlingbadger]

Plate Stoppers for stability were a thing.... but they were usually <= 100 ohms.

[MikeC]

Is it a 6L6 or 6L6GC?
When doing the caculations according to one datasheet from ampage.org, those four 2W resistors are undersized for the GC version and way under for the other. If you must keep them, change to 5W.

The installed tubes are 7027A - same as 6L6 I believe but some pin-out differences. The pins with the "differences" are not connected on the valve base so no worries there. The resistors are much bigger than 2W resistors - I guessed 5W but don't actually know.

[MikeC]

Plate Stoppers for stability were a thing.... but they were usually <= 100 ohms.

Yeah, it's strange. When I power it up I may do so without the plate resistors and see what voltages I have.

[MikeC]

Looks like the earliest Vox AC10s and AC15s had resistors between the anodes and OT primaries as well:
Vox AC15 '59.pdf

Must’ve been a ‘60s thing

Again, interesting, but 100ohm (not 485ohm). I was really hoping to find out how Mr Jansen designed it (Beverley Bruce & Goldie that is)!

[Anvil Amps Alan]

I am currently working on a 12 100 which has been seriously molested internally.
Do you have any more gunshots of your chassis which you could share?

[Slowy]

Is it a 6L6 or 6L6GC?
When doing the caculations according to one datasheet from ampage.org, those four 2W resistors are undersized for the GC version and way under for the other. If you must keep them, change to 5W.

The installed tubes are 7027A - same as 6L6 I believe but some pin-out differences. The pins with the "differences" are not connected on the valve base so no worries there. The resistors are much bigger than 2W resistors - I guessed 5W but don't actually know.

The pins are very different. Here's what happens when you don't know this.

IMGP2728.JPG (1.25 MiB) Viewed 2439 times

[MikeC]

I am currently working on a 12 100 which has been seriously molested internally.
Do you have any more gunshots of your chassis which you could share?

No sorry I don't as I sold the chassis to a member here "Optical". He may well be able to assist.