another one from a friend -

works - speakers are shot (cone-wise) but working - while he's getting those fixed up or getting new ones im trying to see if anything inside needs work

its all pretty old and worn/rusted (see pics) - i think the amp's from 1948!!

its got those cool tubes with the plate tap on top!

two primary concerns (as youll see in the pics):

1) someone put in a new power cable and just thought to leave the safety ground hanging!!!

2) the leads coming out of the OT are kind of weird - each side of the winding comes out as a two, unstranded, wound together, exposed copper wires....these have some green oxidation on the ends right before the terminal where they hook up to the speaker wires - thoughts? i tried to look up this OT - triad 1746 - but not much luck....whats up with these exposed leads? thicker for wattage handling? kind of a small OT, too....anyway, just curious if i should clean them/cut off the green part/replace the OT.....

http://dropcanvas.com/#7SH56kJ26C2G6C

im thinking more for vocal or other preamp - not for a guitar line out

i built a champ a while ago and my buddy made me a cab for it!

sounds great - i want to put a line out on it for recording -

anyone have a definitive method??

PS its a 5F1

what do you mean, "aeroboard"?

forgot to update this:

everything was working fine - heat tests yielded no results - it was all good

UNTIL the owner came by and i was proudly showing to him how everything was fine and then - the problem came back!!! low volume weird hum etc etc. i was a little bit embarassed because i had the guy over to return the amp but also relieved that the problem reoccured before i gave it back to him.

anyway, i opened it again took a closer look and found that one of the zeners in the power supply - there are two that regulate the +/-16V rails - was shorted. it didnt look back but the area underneath it on the PCB was a bit charred/brownish looking and it measured short. phew!!!

replaced that and the amp was all good - sounded even better than it had been -

my thought is: the junction in the zener was working but breaking down - once the amp was running for a bit the zener got hot enough from holding that voltage steady and the junction stopped working properly - maybe finally it broke down for good and measured shorted even when the amp was still pretty cold.......plausible?!

haha whoa those are some crazy techniques!!

well i cleaned the power tube pins/sockets and ran it for an hour into my dummy load and then played it some and the problem has not reappeared.

i might try the blowdryer idea for a minute and see what happens....

hey!

long time no post -

fixing a music man tube/ss hybrid for a buddy

works fine sounds great for about 1 hour and then starts to lose volume and completely crap out, making some weird sounds on the way!

any ideas where to start with something like this? seems to be heat related...whats a good way to diagnoze/troubleshoot problems like that?

im guessing its the power amp or PI, because once it starts to crap out it doesnt matter which channel you plug into -

thanks again for the help, roly!

just to update you guys - the vibrato worked fine...once i realized that the switch - which in this amp is an RCA jack - was wired to be off. there MUST have been a shorting jack in it or something, because once i shorted it internally, the tremolo worked. oh yeah, and once i put in the original opto-coupler, as well. the one i got from tube- depot didnt work!

the tube was also an ecc81 instead of an 83 and the vibrato definitely sounded better with an 83 in there.

so its all good, and got a little recapping in the process!

hm alright i made some progress on my own

the person who gave me the amp claimed the vibrato had been working fine and there had never been a switch. upon closer inspection, the switch that turns the vibrato 'on' was OPEN, so the vibrato was definitely OFF. once i closed it, i started to get a tiny vibrato effect, but not quite. THEN i realized that the grids of the vibrato tube - the LFO + neon driver - were directly connected. ??. i got that back to stock and its working but not that 'intense' -

i guess i need to go through it and see whats up....the bypass caps on both cathodes were 35 uF - someones been in here and fudged around, i think!! im also starting to doubt that the vibrato ever worked.......was my buddy fibbing?!

amps fine but vibratos not working....

thought it was the LDR/optocoupler so i replaced that but no dice

so far the only issue i can see is that both cathodes of are at 0 V, instead of 2 and 13DVC.

but ive never really troubleshot this circuit before - good place to start?

its not actually a dual showman, but it seems to be pretty much exactly this schematic:

http://www.thevintagesound.com/ffg/schem/twin_reverb_sf_100_schem.jpg

edit: ive got -44V at the grids of the vibrato tube - is that right?

thanks!!

phew thanks, Roly!

ive been reading richard kuehnel's power amp book and i was interpreting his explanation of operating class a little incorrectly, i guess. now i understand the distinction between A and AB and its relationship to bias much better - so then let me take a step back and ask about the kay amp:

looks like its set up to make a bit less power than id like

1) because of the unbypassed cathode bias
2) and the lowish supply voltage
3) which together are giving me a smaller cutoff to saturation swing
4) and some cathode current feedback which is even further reducing the grid voltage during high current operation

in otherwords, if i wanted more power out of the amp id have to

1) mod it to fixed bias
2) raise the supply voltage
3) find a different DC operating point
4) get a different amp

?

but along these lines, how much P-P voltage should the PI be giving them? normal AB style, double grid bias, so like 60V P-P? (i also see the lower 6L6 is fed off the screen grid of the 7199  - not familiar with this kind of PI, any notable differences?)

QuoteAre the output bottles wired in parallel as Single-Ended, or is it Push-Pull with a PI?

push-pull - but cant it be class A push-pull? i thought the main distinction was just where the control grid was biased (not how the tubes are set up) - around cutoff for AB and in between cutoff and saturation for A. i figured if the grid is only 30 volts below the cathode, as opposed to like -40 to -50 volts, then the power tubes are conducting fully even when idle and its class A...

on that datasheet, where it says "max signal power output," is that for one tube or two? and is that in RMS watts or Peak? it says 24.5 watts under cathode bias and 50 watts under fixed bias. it says "values are for 2 tubes" up top - does that mean two tubes in push-pull cathode bias are only making 25 watts together? (would that be because of cathode feedback/degeneration)? there is no cathode bypass cap - see schematic attached.

the op bottles are new - i havent run any real tests or anything - i was just making grand plans.

B+ ~ 400V
Cathode ~ 30V

got this amp the other day because i was intrigued by the cabinet design - its a 1x15 but with a grill on each side and the controls and stuff on the traditional 'rear' to encourage you to face the speaker butt out - and thought itd sound good to use together with my sound city concord -

recapped everything but the main can and its sounding good but its a little quieter than i expected - took another look - didnt realize it was a class A design!

i was thinking itd be an interesting and fun learning experience/experiment to modify it to run AB instead. ive looked into tapping off a negative bias supply (http://music-electronics-forum.com/t2545/ - thats a useful thread!) and am deciding between the ampeg style capacitive divider and this simpler one: http://music-electronics-forum.com/attachments/547d1175892694-kscan_0001.gif. what kind of diode is appropriate for this - just any ole high voltage silicon style 1N400x?

but before i get into that i want to make sure im not missing something -

1) i cant find any info on the specific output transformer - if its meant to run two 6L6's Class A, should it be able to handle AB operation?
2) and a follow up to that: because of the greater range of input voltage of to AB designs, will the amp actually be making more power - larger voltage swing?
3) is this a stupid idea?

okay okay youre right i was just getting impatient!

banged around in there and determined it was the modular connectors - resoldered all those connections - and glad i did - in the process saw that the main lead going from the tip of the input jack was not even soldered on anymore just hanging on because the wire was wrapped around! - but still making a crackly/pop until i resolderd the connectors going to the graphic EQ. now it seems all good.

just old joints, i guess!