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Post by oldgoat on Apr 22, 2017 12:26:01 GMT
My 532 amp is generally fine; although there is a bit of 'wonkiness' with the volume. (Specifically tied to the 12AX7 tube socket I believe). For awhile now, I've thought about getting an old 532 and rebuilding it. I figure it would be a good education for me and keep me outta trouble for a few weeks. My current amp was rebuilt by Bill Bickers. As part of that work, he zapped a trace so that I could use a ceramic cartridge (I have since added a preamp and magnetic cartridge). In addition, he zapped the circuit for the auto level. I remember noting that in a thread on Phonoland and was told by Ron? that he kept the auto level in place. While most records play at a similar volume, some are substantially louder or softer. Therefore, if I decide to get and rebuild an amp, I thought I would try to convert to ceramic but leave the auto level in place. Long-winded preamble, but here is my question. What do I need to do to convert the amp for a ceramic cartridge while keeping everything else in place?
A second question is what are the steps I should take to rebuild it and does the sequence matter? After general cleaning (any particular process or product recommended?) I figured I'd start by getting one of the 'kits' to recap it. I'll check the traces to see if I need to reflow anything and, of course, replace all the tubes initially. Is there anything else I should assume I need to replace?
Thanks
Brooks
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Post by Ron Rich on Apr 22, 2017 14:12:29 GMT
Hi Brooks,
Glad you found us---
As for What someone else does to "modify" equipment--that's anyone's guess. Either will need to axk that person, or, get the schematic out, and figure it out for yourself. I don't work on tube-type audio, but insist it must have an AVC circuit, in a jukebox, if at all possible.
For "cleaning", on the older type chassis, you have to be very careful, as the silk screen they used is subject to removal. I usually try a "non-ammonia glass cleaner", sprayed on a rag--and wipe quickly/carefully-- Ron Rich
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Post by oldgoat on Apr 22, 2017 15:06:10 GMT
Thanks Ron...hmm wonder who it was that was giving me a hard time about the AVC capability...Rob, Matt? Anyone else know the magic answer (as far as which trace(s) to eliminate so that I can run a ceramic cartridge)? I can try to bug Bill as well.
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Post by robnyc on Apr 23, 2017 4:24:29 GMT
Well, good to "see" you Brooks. When we last convened to assist on this topic the question was about reconstituting the AGC. I may have pointed you toward this site: www.verntisdale.com/Wurlitzerschem.htm & this schematic: www.verntisdale.com/schem/532a.jpgSlightly left of the center (on schematic) are two tubes; a 12AX7 and 1/2 of a 12AU7. Those are the AGC voltage amplifier and the section of a tube that actually controls the circuit gain, respectively. If the traces are still there reconstructing the circuit is straight-forward --with one exception; removing the AGC will allow too much gain to the following stages resulting in gross distortion. Whomever modded the amp likely added some attenuation in place of the AGC (reactance) tube which is the 1/2 12AU7. So Goat, look at the amp printed board and see what is there in that section which may include a search for "wurlitzer 532". Then, a missive to your amp rebuilder is in-order. Find out what (and why) was done in eliminating the AGC. I can scan the schematics and parts list from my very dogeared shop manual if Tisdale's aren't adequate As for downgrading to a ceramic: WTF-? Most of the change(s) needed has probably been done in accommodating the preamp...but I can't see why you would do this. None of the currently available ceramics are comparable to a decent, changer-rated magnetic. RobNYC |
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Post by oldgoat on Apr 23, 2017 11:40:58 GMT
Thanks Rob. I think I may have thrown you off base with my original post. My current amp works fine. I want to buy another amp and refurbish it. I will continue to use my magnetic cartridge, so I need to disable whatever circuitry is necessary to support that. However, I do not want to disable the Auto level volume. When I had my current amp worked on, Bill did that as part of the conversion to support a magnetic/ceramic cartridge, so I don't want to just look at my amp and recreate what he did. I also know myself well enough to know that there is at least a 50/50 chance I will mess something up; hence I do not want to try to restore my current amp's AVC circuit back. Therefore, I was looking at the mods needed to support the cartridge only. Hopefully, that make more sense?
Right now, my amp only needs one 12AX7. I was thinking this was one of the clues to the disabling of the auto volume. Out of curiosity, does the auto volume circuit have any impact to the sound? Second, is their any need for the two 12AX7's to be matched?
Thanks
Brooks
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Post by robnyc on Apr 23, 2017 12:27:21 GMT
Brooks, yes, one 12AX7 is the voltage amp-rectifier for the AGC. None of the small tubes need matching. Matched outputs are optional and of little use unless you run the machine at high volume. In those instances a matched pair of 6L6's might give slightly less distortion.
The AGC does have a slight affect on the highs when it is driven hard by a 'loud" record. The reactance tube adds capacitance and attenuates the highs a little bit more. Oddly, the Wurlitzer circuit is much less prone to this than the Seeburg one.
If you buy an old 532 be sure to look at pics of both the topside and underpan --especially around the transformers--. Look for any soot or boilout around the holes where the leads emerge. This would indicate a bad tran. Cracked circuit boards are another problem for an amp that has 'knocked around" outside of a machine for a while.
Again, make sure to look at the pic's carefully.
Rob
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Post by oldgoat on Apr 23, 2017 14:42:25 GMT
If you buy an old 532 be sure to look at pics of both the topside and underpan --especially around the transformers--. Look for any soot or boilout around the holes where the leads emerge. This would indicate a bad tran. Cracked circuit boards are another problem for an amp that has 'knocked around" outside of a machine for a while. Thanks for the advice. I knew to look at the circuit board but would never have thought about looking at the holes where the leads emerge. I really hope to avoid ending up buying an expensive boat anchor. (At least with the 532, the damn thing is heavy enough to serve as a reasonable anchor.) |
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Post by oldgoat on Apr 23, 2017 15:02:31 GMT
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Post by robnyc on Apr 23, 2017 19:00:02 GMT
-Yep, avoid that wreck. From the looks of it , it appears that some genius tried to power that relic, or the whole machine as they found it. So much damage happens due to a lack of knowledge and common sense.
Rob
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Post by robnyc on Apr 25, 2017 1:08:52 GMT
Brooks, I did an E-bay search for that or another compatible amp. Given what they are asking for what is essentially junk, I suggest getting in touch with your rebuilder about having the AGC ckt restored along with whatever other minor fixes/mods you might want. After you factor the cost of buying, shipping, replacement parts along with the risk that something major such as a tran may prove to be faulty, paying your guy to touch-up your amp will likely be cheaper and less risky.
FWIW: Thirty years ago when I was buying this sort of stuff, i would not have paid more than $10 for any of those amps currently being offered. They would mainly have been good for parts. Often, op's would just toss stuff like this into boxes of parts along with machines they were getting rid-of by selling them to me :-).
These 1950s'60s amps were in a period where Wurlitzer didn't paint or anodize their internal components. By the 1980s nearly all had some surface rust an all were badly discolored at the open area in back from hand acids and general dirt.
Rob
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Post by oldgoat on Apr 27, 2017 0:56:25 GMT
Yeah, I know that is the smart thing to do, but I figured out long ago I could either be smart or consistent...went with consistent. It is interesting that the 532's seem to be treated like gold. It seems like you can find other Wurlitzer amps, both older and newer, in much better shape for less money. For me, the education will be worth it. Not worth $200 for a piece of junk mind you, but I'd be OK with a $100 for a decent restoration candidate. Maybe none will ever surface.
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