Reipair of 1938 wurlitzer amp - Page 1
Forum section: electronics / repair

author: lilshawn
author_rank: Regular Contributor
timestamp: November 30, 2017, 03:19:31 pm
content: I'm in the process of trying to source parts for an old Wurlitzer amp. It appears it's rectification tube is totally gonzo due to the main B+ filter cap(s) having failed. Not a problem since these tubes are still available.My issue is with the main filter capacitor. This particular capacitor is some kind of potted mess with a common "negative" leg and a series of varying capacity outputs. 2 of the lower uF outputs measure absolutely no capacitance at all and the 20uF, only 0.3uF.The schematic shows 4uF, 20uF, 8uF for C15, C16 and C17I'm wondering now, what should I be replacing these with? I mean obviously I can just replace them with individual capacitors and tie one leg all together.There seems to be the required capacities available as "motor start" capacitor blocks and they are quite reasonably priced. (3 or 4 dollars each) but I'm not sure what type these are or if they'd work. Electrolytics seem to be stupid expensive in these voltage ratings and sometimes not available as single purchase. (the one 500v rated cap min purchase quantity was like 300) mylar dipped caps seem available in these voltages easily as well.What recommendations do you all have for what should I be replacing this with?

author: floobydust
author_rank: Super Contributor
timestamp: November 30, 2017, 09:28:27 pm
content: I think you are looking in the wrong places for the capacitors.Antique radio parts, axial capacitors, electrolytics to 600V etc. here:http://www.justradios.com/Aluminium electrolytic capacitors are fine for those parts, if you change them to EIA standard values it is much easier to find. No harm in rounding up.But 450VDC is the max. electrolytic (common) rating so the 500V part is a bit of a challenge.I would use a polypropylene film cap, Digikey has several 8uF 700V from Epcos or Kemet.Replace all the electrolytics.C15 -> 4.7uF 450VC16 -> 22uF 450VC17 8uF 500V sameC12 16uF 50V (28139) -> 22uF 50VC10 25uF 25V (28141) -> 22 or 33uF 25VThese parts will be much smaller than the originals, so the leads are short and you must add teflon tubing and some wire.I use a dab of silicone caulk to glue the film cap upside down, and tack the electrolytic caps side to the chassis if they are too loose.I have a 503 or 700 amplifier in my junk box.Very hard to find 6SC7 that are not microphonic for the input tube.You can test the amp with a temporary power resistor (4.7k 25W) in place of the electrodynamic loudspeaker field coil, with ordinary speaker connected to the output.edit: looked at Model 24-A schematic

author: Seekonk
author_rank: Super Contributor
timestamp: November 30, 2017, 10:07:23 pm
content: There is another problem. Electronics of that age expects to see 110V AC. I've been working on SUN Distributor Machines of that vintage and the rectified voltage is way over the 500V of their caps. 220Uf 200-250V electrolytics are cheap and small for C17. Jut put three in series with some 330K-470K resistors across each for balancing.

author: Cyberdragon
author_rank: Super Contributor
timestamp: November 30, 2017, 10:56:28 pm
content: Quote from: Seekonk on November 30, 2017, 10:07:23 pmJust use 600V caps. You really shouldn't be running it at that high of a voltage anyway, it could damage the tubes. Use some sort of multi-tap or continuously variable transformer to step down the mains.

author: lilshawn
author_rank: Regular Contributor
timestamp: November 30, 2017, 11:06:16 pm
content: The nameplate specs 115v AC 150 watts 50/60 cycles. A little closer, but probably should still drop it down a bit. our power is usually about 120 to 125v ACI've been thinking about using a transformer to step down the voltage for the amp. 120 in 100 out. I think i have some from some arcade videogames kicking around.could you do up a quick mspaint schematic for how to wire this up with the 3 caps and resistors?thanks!

author: jmelson
author_rank: Super Contributor
timestamp: November 30, 2017, 11:52:36 pm
content: Quote from: lilshawn on November 30, 2017, 03:19:31 pmAughh! I got a blast of that waxed-paper smell when I saw that picture! I haven't smelled that smell in at least 30 years!All in the mind, of course!Jon

author: lilshawn
author_rank: Regular Contributor
timestamp: December 01, 2017, 12:28:02 am
content: yup... the unmistakeable smell of wax and dirty oil. let's not talk about the smell of the drive motor with the fried start winding. (must have got stalled one time)

author: Cyberdragon
author_rank: Super Contributor
timestamp: December 01, 2017, 01:25:58 am
content: Let's not forget the smell of a dying or overloaded tranny about to go thermonuclear.

author: bills
author_rank: Frequent Contributor
timestamp: December 01, 2017, 01:42:42 am
content: With old electronics post your question here.http://antiqueradios.comyou will find a bunch of old guy's that know their stuff.A lot of members monitor and take part on this forum.BTW your question is a very common one.You have received some good advice and some not so good advice.

author: lilshawn
author_rank: Regular Contributor
timestamp: December 01, 2017, 03:59:00 am
content: Thanks, I'll give them a try too. I'll update back here once I have an affirmative plan of action.

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2017, 12:33:05 am
content: I typically gut those multi-section electrolytics and stick some modern capacitors inside. Then you retain the original appearance but with reliable modern internals.115V gear should be fine on 120V but if you are worried you can use a 120V-6V transformer wired in buck configuration to drop the voltage down a hair. It doesn't take a very big transformer when used like that, the secondary rated current just has to be at least as high as the load current.