magnecord CM6K chassis hack.

[DummyLoad]

early in the year, a friend brought over a magnecord CM6K amp chassis that he wanted to convert to a deluxe type amp. after noting he had limited space and was minus a 9pin socket, he abandoned the idea and farmed the transformers leaving me the chassis to dispose of. fast-forward to june, during a cleaning of the cluttered mess that is my lab, i decided that it was too nice of a chunk of steel to simply toss as it is still in decent shape. what can i say: i'm a pack-rat.

with just 2 small bottles holes & two 3 big bottle holes, with a fender tremolux PT fitting the PT cutout to work with, some more part research revealed that a hammond 1620A OT would fit in the OT mount requiring that just 2 OT mounting holes need be elongated to fit properly. OK, so now i had the foundation of a 30-40W guitar amp.

i needed to be able to build the preamp without adding a third bottle, but need enough gain to drive a treble/bass tone stack - see chassis pics, and push a pair of big bottles that would make 30-40W, so obviously the 6V6 were out the question. more research. since i have a PT that makes about 420-450VDC depending on rectifier, and a 6.6K UL OT that's rated at 20W in full fidelity, i considered 6L6GB/C. they will deliver ~25W auto biased with 400V and 6.6K Ra-a load, but the mullard EL34 datasheet is what caught my attention: 35W with 430V with 6K Ra-a in UL mode running autobias. perfect! that's the output stage.

https://tubedata.wernull.com/sheets/129/e/EL34.pdf  - see pg. D2: 43% UL condition.

now to drive the EL34 to full power with just two 9pin bottles. i initially thought 2 6AN8 would definitely do it, but they're odd-ball tubes and would be kind of a bitch to wire in the limited space. so after much deliberation and with part count being of primary concern, i decided the first stage would be a pentode - an EF86 driving a tone stack feeding a gain stage then to a cathodyne for the PI. the gain stage/cathodyne structure would give me the most gain with PI function using just one 12AX7. it wouldn't be a marshall 2204 or trainwreck gain animal, but should do well enough in the clean-gritty arena.

attached is what i formulated. i have it built on a breadboard and was pleasantly surprised as to how well EF86 pentode driven bass/treble TS works. BTW, the tone stack was plagiarized from frank filipse's (sp?) electron tube pages URL.

chassis pics here:  https://www.dropbox.com/sh/90ppvqjmpycwjnt/AAC0CE16OnIO7HIGyK6LEVJCa?dl=0

--pete


EDIT: revised plans in reply # 5.

   

[davidwpack]

That sounds awesome! KT66 may be an option too?

[bmccowan]

Nice project. I admire the planning you put into it, as I am typically impatient and just start cutting, drilling, and soldering.
BTW, I like the sound of a pentode > PI > power tubes. The early Gibson circuits with 6SJ7s and later 5879s are not far off from your circuit. I hope you keep posting on this.

[PRR]

> with just 2 small bottles holes & two 3 big bottle holes, ...i needed to be able to build the preamp without adding a third bottle, but need enough gain to drive a treble/bass tone stack

My first thought was to Plagiarize The Classics. Say Fender *face with 2nd channel and trem/reverb omitted. Two cascade volt-amps is plenty gain in one small hole. Driver another small hole. Two big holes to stuff and bias as desired. And your empty-state rectifier, if you must.

The ~~440V 6k loading condition can be designed to work with "any" large octal (not 6V6) without change. Tie pin 1 to 8 with a 470k resistor. EL34 seemed to run fine that way, and now-rare 7027 will just warm the resistor. 6550 base will float to Vk harmlessly.

[DummyLoad]

My first thought was to Plagiarize The Classics. Say Fender *face

thanks for the suggestion PRR, but i don't want nother fender amp, or copies of one (house full of them now). i should have stated that in the initial post. i DO like your plan with the multi bottle approach - it would be a novel touch. don't want to fool with fixed bias.

--pete

[DummyLoad]

bummer! sylvania GZ34 arced with 50uF filter. downrated to 32uF filters + 16uF in place of the 50uF plate + 32uF for preamps.

new plans attached.


--pete