$40 Amp Mods

[borge]

i just got this amp [hows the pilot light ]for $40 nz it has 2xEF41, 1xEZ40 1xEL41 two volumes, a tone and two inputs

ive been told that since each input is controlled by each volume that both inputs use 1 EF41 each and if it could be rewired so it has one input using both EF41s

i would get alot more gain.

does anybody know about this sort of mod or reccomend a good amp forum?

[lovekraft]

Got a schematic, or even a brand name? A cursory search shows the EF81 listed by Mullard as a "...variable mu RF pentode for use as an RF or IF amplifier" . Why not try a clean boost in front of it rather than attempting to mod an unknown amp? BTW, unless there are obvious signs of replacement, you're probably way past due for replacing all the electrolytic caps, especially the power supply filter caps.

[borge]

no schematic and no brand name.

im guessing its home made judging by the build of the cabinet and lack of any sort of label asides from the hand written notes on the chassis (tube names,tube socket pin #s ect)

i tried a few boost and dist pedals and they sounded good ive decided against modding it im thinking about turning it into a head and making a decent 2x12 cab (the 8" alnico distorts bass badly)

good idea?

bad idea?

[mushy the shroom]

Using it as a head sounds like a good idea (if I were you I would go so far as to construct a new casing for it). You probably know this, but make sure you match the ohms for the cabinet and head. Otherwise, you would have some serious problems.

[thedoctor]

Looks to me like that amp is made for 75 volt output. Be careful there as each cab will need to be hooked thru a line transformer. That may not be what the speaker-mounted xfmr is for but it sure looks like it.

[lovekraft]

Yeah, that scares me as well - it may be a constant voltage transformer, or it may be tha the speaker was designed to run directly from the plate of the output tube (some post-war speakers were evidently set up that way - it was a bit before my time, so I don't know much about it). Either way, you won't be able to simply hook up an 8 ohm speaker and go. If it was me, I'd find one of those true vintage amp freaks and put it on him for a couple of hundred, as-is.

[thedoctor]

Okay, I blew that picture up and it has two unbalanced inputs. One for guitar, one for mic. The lead going to the speaker-mounted transformer is put on the speaker with masking tape and is run out of the chassis through and old Romex clamp. It won't be tube-direct but it idoes require line xfmrs for each speaker. Look at the size of that power transformer on the chassis! WOW! If it was done right, the geetar input should already be fairly high gain. Could mess with some power supply resistors and the coupling caps but I think LK has the right approach. Probly worth more to someone as-is than it would be as a cab driver. Although, now that I think about it, Chicago has it some LOUSY cab drivers!

[borge]

so im probably better off selling it?

what if i said i wanted to build a tube amp from scratch one day, is it worth keeping for parts?

if its not since the speaker is stuffed and the cabinet is pretty bad looking aswell

(2 of the red plastic dials were broken in the mail) would making it a nice looking head get me a better price?

and is there anyway around needing a xfmrs for EACH speaker?

[thedoctor]

You don't need a line xfmr for each SPEAKER, you need one for each CAB. If you can still get an impedence reading on the original speaker's voice coil, or read it on the back of the speaker, you can run ONE cab of equal impedence with the xfmr you have. Add another xfmr for each additional cab or to run a new internal speaker.

Almost forgot, I wouldn't plug that thing in until you put a 3-wire power cord on it to ground the metal chassis.

2nd edit. The mind is the second to go. DIYaudio is a good amp forum but it is HIGHLY structured toward security and anti-rant/rage. The search function on this (PG) forum will also give you some great links.

[borge]

you can run ONE cab of equal impedence with the xfmr you have. Add another xfmr for each additional cab or to run a new internal speaker.

Almost forgot, I wouldn't plug that thing in until you put a 3-wire power cord on it to ground the metal chassis.

←

im not sure if im understanding you right, so i could power ONE cab with the current xfmr but i need another for an additional cab or a new internal speaker.

i understand why i need another xfmr for an ADDITIONAL cab but not why i would need another for a new internal speaker unless you mean a new internal speaker in addition to the one cab using the xfmr i have?

i guess you couldnt see it in the pics but the power cord is 3 wires with one grounded, one to the power xfmr and one to the tone/onoff pot.

thanks for the help everyone.

[thedoctor]

OK, to clarify, the xfmr you now have will run one speaker or cab of the correct impedence. Anything else you want to hook up will need it's own xfmr. And it is ok if they run a little hot. In the 50s, these were responsible for several wall and cieling fires in schools and churches. Poor installation, I guess. Like screwing them to a piece of wood lath and covering them with insulation.

[borge]

oh ok sweet,

i think i will make it a head and and make a 2x12 cab.

then i can replace the head when i see one i like.

are many of the parts in the amp likely to be useful in a scratch built amp?

[thedoctor]

No.