amps good to go!

it was just the PI - one of the 3 channels doesnt work and the reverb tank connection is bad but the client is satisfied with a reverb-less two channel amp (one of the 3 is just a bright version) so im letting it go -

thanks, guys!

(ps: the 'ghost voltage' issue was my lightbulb limiter acting funny! gotta figure that out....maybe a bad connection somewhere)

as for the drought - yeah its starting to feel more and more real - interestingly enough australia came up a few times after i read your post, mainly as a model for what we can do/what we have to look forward to. im curious to see how regulations trickle down to us 'regular' folk. the big question here is who should bear the responsibility, joe schmo or 'Big Ag,' which consumes 80% of our water, supposedly. so far doesnt seem like the state is going to crack down on agriculture - a lot of people are talking about almonds (1 gallon of water to grow 1 almond!), most of which arent even sold here.....but as far as real change, things are pretty vague at this point....

replaced the PNPs

have about ~160V now on each driver collector

getting ~110Vp-p on the tube grids -

next step, see if these tubes are any good!

thanks, Roly!

i live East of San Francisco -

i replaced the PI drivers and the voltages look better, but they are not balanced. one of the driver's collectors gives me 170V (the one hooked to the 100k resistor) and the other one (to the 82k) closer to 270V. the bases are at 10ish volts and the emitters are about 0.6V below that so thats okay. i thought the different collector resistors were there to balance the voltages out - the ~100V difference seems a bit large to me - am i right in that assumption?

im going to replace the PNP transistors under the drivers (level shifters? what are they there for??) and see if that helps -

on a separate note, i did some snooping around the PI and found that:

its clear from the underside of the board that the driver transistors were replaced - they are MJE 3439 - which seems like a suitable replacement BUT whoever put them in did not orient the b/c/e terminals correctly - i did some tests with the DMM on diode range and they have the base and the collector switched. probably took a guess and guessed wrong!! so no wonder the voltage is bad. im assuming they are probably fried because of that error? i found some replacements at the electronics shop near me so ill get those and put them in and see if the voltages look better -

this amp is huge and cumbersome and a pain to take apart but i think its *mostly* working

i didnt do a road test but just took some measurements for now -

just a few questions (refer to attached schematic):

1) i get +300V at the PI supply, but on the transistor end of the load resistors i get ~10V everywhere i meausure - any of the transistor leads that arent at ground give me about 10.25V. one or more of the transistors bad probably? ive dealt with and fixed a bunch of these hybrid peaveys and almost ALWAYS the PI transistors have to be replaced.....

2) im getting a weird looking ripple at all points on the amp when measured with my scope. kind of a large, jagged wave. even when i touch the scope to ground its there. about 100Vp-p!! what the.... i thought it may have been death cap related so i removed those caps but that only seemed to make it worse......any ideas?

3) related to that, what do the two caps on the power cord side of the PT actually do?

good lesson in diagnostic-ing - thats for sure

anyway, tubedepot was kind enough to send me a new pair - ill pop those in when they get here and im 99% certain things will be just peachy -

cool, thanks Roly -

i think this amps good to go - louder sounding and all cleaned up -

hm now you have me all paranoid about ripple current -

would a physically large, high voltage 47uF have a high ripple tolerance generally? or does it really depend cap to cap and better to just err on the side of caution?

i attached a datasheet for one i have - a 47uF / 350V - its 16x20 mm - but there is no ripple current on the datasheet (in general ive found that parameter hard to find......)

PS its 30V over a 250 resistor at idle so 120 mA of idle current....

well i popped in the red-plater and one of the 'old' tubes (the good one) and it biased up fine and gave me about 40 watts....

not a model sinewave, but they are different tubes (the 'old' tube is a 6L6-GC and the red-plater is a 5881).

man why didnt i just try a tube swap to begin with instead of this wild-goose chase??

now im just wondering if i should try and get tubedepot to send me a pair of tubes that are actually properly in spec!!!

i figured that much when i did the dissipation calculations myself; ~40mA a tube didnt seem that crazy as it was still within 70% dissipation.

what puzzled me was just that the bias range didnt seem right - what's the point of the bias pot if we're already at 70% dissipation at max negative bias? i guess to account for crappy valve tolerances....

i guess ill give the 'new' tubes a go at fullish power for a while and see what happens. (and i'll check the original bad guys)

QuoteAnyway, gas in there, lots of it it seems, it's gassy and a dead loss except mounted on a bit of wood as an exhibit.

so thats a for sure bad tube? i guess its not white-white, but its certainly bright, as bright as the heaters, and neither healthy electron blue nor heater orange....

and on a similar note - if neither of the original tubes shows any internal signs of a short and seems okay in the amp, are they usable? remember one of them was red-plating pretty bad. once a tube red-plates it is gone-zo? or not necessarily?

i tested R66 in circuit and its reading 1.1 ohms so 1 ohm.

i checked CR3 totally out of circuit and im getting a 0.548 forward voltage drop.

screen resistors are OK.

ill dbl check R66 and the diode -

as far as the flybacks, i was thinking if werent as they should be theyd be showing up as a high-resistance in paralell with the OPT and therefore making current draw funky....

update: dbl checked R66 and diode - both seem good. opened them and used my own current detectors (same idea with a 1 ohm on the cathodes) and still reading upwards of 45 mA per tube when bias is turned all the way down to -57V.

not sure what to try next....

okay update update:

frustrated by the situation, i thought id try some old tubes that were pulled from another amp. popped them in and the amp biased up totally right! and i got it to give me ~40-45 watts! only issue is, one of the 'old' tubes started to kind of glow a whitish bright as i gave it power - not red - but a white glow from within the tube plates. bad, right? thats probably why i had it laying around in my 'used' tube pile. anyway i tried a bunch of swap combinations and determined that if i put in either of the 'new' tubes, i couldnt bias it right. this pair of 'old' tubes did the trick but im wary of the magic glowing one.

wtf?

should i ask tubedepot to send me another pair? sounds like maybe i got some duds (this has happened before with preamp tubes, but now with pwer tubes). i got JJ 6L6GCs - any thoughts?

k replaced the tube sockets - hum dinger resistors - got some new tubes -

things seem OK but im not happy with the bias situation and i cant quite figure it out

first of all, ive attached the actual schematic (its a different version than the first schematic i posted)

at maximum negative bias, about -57V, im still getting 80mV at the test point - too hot! says i should have 60mV there - and doesnt make sense that these 6L6s would be passing so much current with a bias voltage that low....these are new tubes so i dont think its them. opened the PI coupling caps - no difference - checked the diode in parallel with the bias test point cathode resistor - its all good. no ripple or anything weird in the bias supply - other voltages look good - (477 on the plates)

(one thing that might be funny is that the flyback diodes are measuring 1.6V on a diode meter test - they are R3000's - shouldnt i be getting a 3V forward voltage drop? or should i not trust my meter to be precise? im thinking if i had a bad shorted tube  it could have damaged the diodes (this amp had a blown fuse when i got it in) - at least enough to break down the semiconductive junction and now making it difficult for the tubes to conduct properly?)

any thoughts?

hm i see very funny serves me right

hard to figure out all the parameters for a cap i already have - but i looked at all the similar ones (size/temperature/voltage rating) and my 220uF guy should be alright. the 47uF would not have been!

hmmm i see i see -  good to know i was totally unaware of those constraints!

would a 220 uF 50V cap work better? its still small physically.....

where could i find the ripple current limit or calculate it for a modern cap?

hmm yaya that makes a lot of sense

well im replacing the socket and 'hum dinger' resistors =

we'll see what happens!