Marshal Major

[CJ]

Got an old one on the bench, couple of blown KT 88's

these new tubes say they can handle it but the 600 V plate supply eventually causes problems like bias drift,

don't know the year, and it has a slightly different circuit to the ones in the book,

it is no fun to haul around,

this must be the Joe Walsh signature model,

 

[CJ]

silicone sleeving on a lot of the wires, lucky i did not have to undo the harness,'

note the chassis mounted 5 watt screen resistors.

 

[CJ]

somebody installed a master volume ,

the two tubes that went bad were mounted in belton tube sockets that somebody changed out, the inner two tubes were on original ceramic, so we took the hint and installed two ceramics on the outside pair,

 

[CJ]

i told the guy to put 4 of these in there but he wouldn't bite.

EHX rusky KT88's go for 350 a quad due to the war so why not?

6550's do not cut it in this amp,

 

[CJ]

here is another 1967 schemo,

note mistake on second stage cathode, 47K s/b 470

and PI grid rersistors s/b 270 K not 270

this amp had 56 K plate resistors on the PI instead of 68 K, with 100 K f/b resistor instead of 47 K, 1 K 5w screen resistors instead of 250 5 w, 50/50 cap on stage 2 cathode instead of 25 uf, 0.22 uf coupling caps to the pwr tubes instead of .047 uf

 

[Whoops]

somebody installed a master volume ,

the two tubes that went bad were mounted in belton tube sockets that somebody changed out

Were those Belton Micalex?
If so those are pretty good sockets, I doubt Micalex sockets were the problem, but I understand not taking chances and changing the sockets

 

[CJ]

yeah i think so>

 

[Brian Roth]

So, what make and models of NEW (not NOS) ceramics are still available?

Bri

 

[CJ]

i think it is mostly all offshore now, but it has been alright so far,

just go to evil bay and search ceramic tube socket, i get mine from a guy in florida.

watch the diameters, there are two sizes, but top mount bottom mount does not matter because you can uncrimp the plates and flip them over if you make a mistake in ordering,

 

[Whoops]

yeah i think so>

View attachment 105387

Yes those are Micalex, they are pretty good, most DIY amp people prefer those to ceramic sockets.
I’m sure those were not the culprits for the tube failure, but if the new ones work great and give more confidence that’s a win

 

[Whoops]

So, what make and models of NEW (not NOS) ceramics are still available?

Bri

I don’t know the makers but there’s a lot of offering for ceramic sockets in different models, check it out here:

https://www.banzaimusic.com/8-pin-Octal-Sockets/

 

[danfrank]

Yeah porcelain octals are what this amp needs, the cheap Chinese ceramic sockets won't do so great. I like searching epay for old Amphenols.
As for which current production KT88s to use, I've had good luck with the JJs, they are built like tanks but the transconductance is a bit on the low side for KT88s. They do hold up well with the voltages in these amps.
I've worked a lot on the Marshall PIG Majors (3 knob Major), the predecessor to the more common 6 knob Major... These 3 knob Majors have 720+ volts on the output plates, which is really pushing the octal sockets to their limits. The TT21 or TT22 tubes would probably have been a better choice for the "Major" amps.
Nice amp on your bench, BTW!

 

[CJ]

720 jeeezus, i want hazzard pay for those amps,

anything over 600 and electricity starts to disobey the laws of physics especially AC,

 

[heatwalk]

720 jeeezus, i want hazzard pay for those amps

The SVT Surcharge? Last one in guy wanted to keep 2 prong... Gotta keep that all original shock mojo

So, what make and models of NEW (not NOS) ceramics are still available?

Bri

https://www.tubesandmore.com/search/node/ceramic/?categories=Vacuum%20Tube%20Accessories