looking for some ideas. rebuilding heathkit w-3m amps.

[kyleaudio]

i just won a couple heathkit w-3m amplifiers on ebay from two different sellers (they havent gotten here yet), right now i am on deployment with the uss ronald reagan aircraft carrier and being at sea for a over a month at a time with not much work to do, i needed a little project to keep me sane. since they came from two different sellers i'm pretty sure i'm looking at basicly rebuilding both of them, replacing some resistors, all capacitors, suck out and replace all of the old solder and replace the tubes on one to match them up. this will be my first time doing this much work to a tube amp, the most i've done is just a little tinkering here and there with my sherwood S5500III.

that being said, i havnt really found much info on these amps on the web and i havnt found any info about rebuilding them. so i'm here for advice. does anyone have any suggestions as far as upgrades, modifications, changes in the circuit, and which old components should be replaced with newer and better parts? have any of you already rebuild one? how did it come out? any help would be great.

the manual can be found here with the best schematic i've found for it.

thanks, kyle.

[Bandersnatch]

hey-Hey!!!,
I'd suggest upping the voltage on the amp-chassis B+ capacitors to 500V. Digikey's snap-in Panasonic will be good there, and you can up the capacitance w/o endangering the tube rectifier.

As to the circuit, I'd suggest cleaning the amps, restoring them and *THEN* worry about modification. On that front, there are many ways to skin the cat, yes?
cheers,
Douglas

[kyleaudio]

good idea, safety upgrade!

but as far as mods, i dought that both amps will be the same inside, one of them looks like it had a re-cap done (they used 450V caps) and i want to match them up for a dual mono setup, might as well upgrade the parts that i'm changing while i'm at it.

another thing of concern, the manual states on first startup that there will be background noise and this indicates normal operation. i wonder how much noise there is and if i can silence that noise? i guess i'll have to wait till they get here.

[kyleaudio]

well i finally got my amps a while ago and i've been slowly rebuilding them as my mail comes in (military mail takes forever!) and after seeing how many people looked at this thread without posting, it seems like there is some interest in these amps. so far i've completely gutted one amp and power supply, cleaned them up and installed all new wire, resistors, and caps (except for the signal caps, the new ones havnt come in the mail yet). even tho i am using an old sony radio speaker in a cardboard and duct tape encloser, it sounds really good, very clean sound with no background noise that i can hear. but i do have a couple problems.

first problem, when i replaced the capacitors in the power supply, the voltages went up, where the voltage should be 440vdc +/- 20% with a line voltage of 105-125 (my ships voltage is at 120vac), after changing the caps i get 466vdc. a bit high. with the old caps the voltage was arround 450vdc. so why did the voltage go up? i replaced the 20uf caps with 22uf and the 50uf with 68uf panasonic ecg caps. this raise in voltage is causing the two 47K 2W matched resistors to get VERY hot, the blue ceramic coating on them has browned a bit. also the bias current went up, the heathkit manual says that bias should be between 40 and 60 mA, before i changed the power supply caps it was arround 61mA, a bit high, but not too bad. now it reads 74mA so why the increase? did i miss something?

the other problem that i have is when i turn the amp on, after a few seconds i get the "motorboat" sound for about a second and for about 5 minuites when it is playing music, the quieter bass gets a bit of static. it could be the signal caps, but i wont really be able to find out untill i get the new ones.

anyways here are some pictures from when i got them and after some cleaning, i'll post some more later.

[kyleaudio]

more pics

[Boris_The_Blade]

Those 47K resistors in Williamson designs are known to get quite hot. It's probably best to change each 47K, 2W resistor to two 100K, 2W resistors in parallel for more power dissipation abilities. 466VDC for the B+ is close enough to 440 for no problems, they probably measured theirs (on page 18) with 110-115VAC on the input. Nice set of amps there!, I didn't know you could work on this kind of hobby stuff at sea.

[kyleaudio]

yea, i was thinking of replacing those resistors with either higher watt resistors or two 100k as you suggested, i just wasnt sure if there was something wrong or if it is just supposed to be that way. but they dont really care about working on hobbies here as long as i have enough room to put it away. i'm an IC (interior communications electrician) so we always have some sort of electronic gear being ripped appart in here anyways.

[kyleaudio]

thought you guys might like my cardboard speaker enclosure, lol.

[DigitalJunkie]

Do you have equalizing resistors in parallel with the series connected caps?
In this picture it appears that the caps are connected in series,but I don't see any resistors:
http://www.diyaudio.com/forums/attac...0&d=1252786047

[kyleaudio]

no, no resistors on those caps, but there werent any before either, nor are there any resistors in the schematic.