KT66/KT77/EL34 compatability in Marshall Vintage Modern

[jordan86]

EDIT: reloaded a less compressed schematic that is more readable.

Doing some research for a buddy's Marshall - 50W Vintage Modern. He's not a techie. His amp runs two KT66's, but we were discussing possibly running KT77 or EL34 (more cost effective).  Attached schematic for this specific amp but just wanted to check with you guys that my thinking is correct. I've been looking over the JJ data sheets specifically but I assume pinouts are same across all brands (always will double check though).

A few quick questions...

1. From a pinout standpoint, KT66 and KT77 are directly interchangeable (I understand the heater current demand is ever so slightly increased with KT77).
2. Also regarding pinout, KT77's could go anywhere an EL34 would be used since Pin 1 is not used on KT77
3. To put an EL34 in a normal KT66/77 placement, Pin 1 would either need to be tied to ground or pin 8. This assumes pin 1 is not being "borrowed" to tie in another grid resistor (ala Fender) and that the PT can supply the extra heater current.

Side note: I think the pin number in this schematic may be wrong? Shows K as pin 7 and heaters as pins 2 and 6? Or maybe I am reading it wrong?

Full resolution schematic is here. http://musiciansroadhouse.com/download/file.php?id=3067&sid=c1081d40b07c85fe5fad793c18119f2d

[jordan86]

Man...no love here. Attaching data sheets if anyone cares as to which specific tubes I am looking at. I think I've answered my own questions though.

- 1 & 2 both true.
- 3 is also true. Seeing as this amp is fixed bias, the cathode is tied to ground already so connecting to pin 8 is same as ground.
- I believe the pin numbers on the Marshall schematic are indeed incorrect. Unless they just have some funny British numbering system :) Will reference the schematic numbers to the actual PCB and sockets if/when I get to open it up.

If there's any concerns you see, or just a thumbs up, I'd appreciate it.

[sluckey]

The pin numbers are wrong. But you already knew that.