no clue here... how can i identify a suitable transformer for an old kay 704 amp?

[pinkjimiphoton]

i got it. went down tonite, reinstalled the IT and took a leap of faith after checking voltages (which are indeed all still on lsd25) and popped in the new output transistors.
fired right up. figures.

thank you ALL for the help and advice to get this sorted out.

i did check the transistors with my meter, but i got some weird results, which i assume were leakage from the germanium. the UPPER transistor had what i believe to be a short between e and c, tho i didn't write down what was what.

this thing has about twice as much balls as it did, and a horrible germanium distortion that makes my pants too dang tite.

tomorrow i'll finish buttoning her up. thanks to everyone again for helping me thru this.
you guys rock

peace
jimi

[J M Fahey]

Now we DESERVE some MP3

[pinkjimiphoton]

gimme a little bit, i'll try and shoot a couple minutes of video of it in action and post the link here.
ir's a sweet little amp,i cut my teeth on it's big brother, the 30 watt galaxy 8. loved it full blast.

used to pummel my whole town with that thing

[pinkjimiphoton]

thanks guys, here ya go, some video... first with a 63 kay speed demon guitar, then my 66 gibson firebird.

nice little amp, couldn't have done it without you

https://www.youtube.com/watch?featur...&v=HlDj4AvOkGU

[The Dude]

That little amp sounds way better than I would have expected. Nice work! (and even a little Creedence for the Dude. )

[pinkjimiphoton]

That little amp sounds way better than I would have expected. Nice work! (and even a little Creedence for the Dude. )

the bigger ones sounded just like that, but loud as ....

my original vanguard galaxy 708 was 30 watts into a 16 ohm speaker. i used to run it, pegged, thru 2x12 jbl's at 4 ohms... no wonder it blew up... but damn, it was loud, it was almost as loud
as my bud's vt22.

sad there's so few of 'em left.

thanks for the help, the dude.

peace!!

[pinkjimiphoton]

you guys ain't gonna believe this one. fired her up, was working great. figured i'd (stupidly) change the input caps to three different chiklets, .01, .039 and .1.

the amp was working. now it's freakin dead again. wtf!!!!!!!!!!!!!!!!!!!!!!

back to the bench it goes.

when i first turn it on, i can hear it hum... but then it fades away. can't get any signal thru. same symptoms as last time. i wonder if i fried the new transistors?

or... it would seem more likely a bad solder connection maybe? i mean, it was working great until i changed them caps from ceramic to chiklets.

voltages are weird again, too. ay yi yi, makes your head hurt

[The Dude]

Changing those caps should not have caused anything to malfunction. Don't second guess it. It's a broken amp just like any other and the same troubleshooting methods apply. If you think you fried the new output transistors, check them for shorts. You say "voltages are weird". How so? Can you post the voltages that are "weird"?

[Jazz P Bass]

Which begs the question: "What output transistor replacements did you install & where did you get them?"

2N554 PNP Germanium Power Transistor

[pinkjimiphoton]

hi guys... i made an audio probe with a biggish orange drop grafted to a guitar cord, and then trapped some random noise in an old akaii headrush to use as a signal gen into the amp. messed around checking voltages. the 22k resistor, i think it's r4 on the schematic was showing around 26 volts on the 3.3k side, and nothing on the node going to c of q1 and b of q2. looking at the schematic and tracing along and using the probe, i found i had signal to the c of q1 if i disconnected it from the base of q2, the signal was present all the way to the b of q1, and was louder on the c, so i knew that stage is amplifying. there's a wire connecting c of q1 and b of q2, and disconnecting that cured the short i had been seeing, and proved the first stage was amplifying. the second stage there's really nothing there, the transistor has a small e resistor and other than that is hooked directly to the transformer and the input to the trem. so i disconnected that, and replaced the 2n591 with an ac128, i looked up an "equivalent" cross ref and got the expected nte 102 or 103. so i checked the data sheet for the original transistor, and the ac was close enough for rock and roll. i've got it working on the bench while i type this, gonna go try and reconnect the tremolo circuit and see if she still flys.
once i get it running, i'll try and write down voltages, etc

thanks guys... stay tuned

[pinkjimiphoton]

ok, definitely q2 was damaged too. the new transistor has a bit higher gain, so the little amp really smokes now. holy cow.

i re-did the input caps as i'd planned, worked out nicely. kept .01 for input 1, went .039 for input 2, and .1 for input 3. should be able to cover a bit more ground with it now.
believe it or not i think i learned a lot in this thread, and some of the info was key to understanding what was going on.
this thing is laid out almost like a circle within a circle inside, so some of it's a real headscratcher to look at.

almost wondering how it would sound with a higher gain transistor in q1, too. but i don't think i'm gonna fix it any more if it isn't broken.
the tone control is a lot better, and the whole amp is much clearer and louder, while still keeping that nice ge grit.

have to post another video of it now.

thanks guys!!

[pinkjimiphoton]

Which begs the question: "What output transistor replacements did you install & where did you get them?"

2N554 PNP Germanium Power Transistor

i bought a nos matched pair on ebay. i don't think it was necessarily the power transistors. when i tested them with my meter, i think the leakage inherent in them misled me to think
they were shorted. but i think i was looking at the wrong place. i had been looking at the output, but it was actually the driver stage for the interstage transformer.
weird thing, as predicted, outside the circuit it read fine. <smh>
EDIT:
i just looked, thought it was ebay, but no, was talonix. they work great.

[pinkjimiphoton]

i want to end this thread with a sincere thank you to the three guys who helped me thru this, J M Fahey, The Dude and Jazz P Bass for leading me thru, asking the right questions
to get my mind working, and offering the right advice for me to be able to figure it out. i am indebted, and will pay it forward.

thank you my friends!!

will post vid of the finished amp when i get a chance. i gotta get ready for a gig tonite (hard, cuz i injured my spinal cord recently)... the show must go on...

peace and thanks
pink jimi

[pinkjimiphoton]

the things screamin now. i wrote down some of the voltages. i didn't bother with the tremolo part of the circuit cuz it just modulates away..

so the basic circuit, sans trem works out like this:
(all with meter leads reversed so these are negative voltages, amp volume and tone on full, speaker connected, no input)
tr1
e 0
b .14
c .42

tr2
e .30
b .42
c 19.68

tr3 (upper)
e 16.6
b 16.7
c 32

tr3 (lower)
e 0
b .1
c 16.6

can i ask another dumb question? could i quiet some of the noise down in this thing if i went with a full wave rectifier and some actual power supply filtering in this? it sounds great, but it is kinda noisy, like an old vox where ya hear the subharmonix from the power supply.
i can definitely live with it. and i don't wanna eff this thing up TOO bad.

thanks again...

[The Dude]

Does the noise go away when you turn the volume down? I suspect some of the problem might be that, because of the input arrangement (3 inputs basically paralleled with only caps separating them), they could not use switching/shorting jacks. On a modern amp when nothing is plugged into an input, the jack shorts tip to sleeve and mutes the input. Not possible in your arrangement because shorting any one input will NEAR short all of them (capacitor isolation only). Think of it as having a guitar cord plugged into the amp with no guitar on the other end. It's going to be noisy. If the amp were mine, I'd probably unwire 2 of the inputs and install a switching jack in place of the one that remains. Of course, if you have some need for multiple inputs, forget that.