Gibson EH150 restoration

Discussion in 'Amp Tech Center' started by TenaciousP, Feb 22, 2019.

  1. TenaciousP

    TenaciousP Tele-Meister

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    Hey y’all,

    I’ve had this non functional Gibson EH150 sitting around for a couple of years now. A friend of a neighbor was looking to turn it and a Gibson EH125 lap steel into some quick cash. So I ended up buying them both for a reasonable price. Here are a couple of pics from the day I went to take a look at them. Clearly the original speaker is long gone. Replaced by this lovely 70’s RadioShack Realistic ceramic mag speaker. :eek:

    28D15090-03A1-4161-964F-4948966F5CDF.jpeg
    0A9281F9-0C7B-4D41-989C-A1BA8D6B92C6.jpeg
    E789D43B-40F0-426E-84F1-187567DE9E75.jpeg

    So anyhow, the amp has been in my shop for a couple years now and I’m thinking about getting it up and running. So I removed the chassis from the cabinet and pulled the tubes for safe keeping. I’m not sure if they are still good, but i’ll deal with them later.

    132A558D-C683-4AD8-8D91-41DF06ED0070.jpeg

    Here is a shot of the guts. Looks like some of the caps were replaced decades ago. I believe the big paper tube capacitor with the wires coming out of one end is one of the original multi caps. It also looks like a couple of large resisters were installed in parallel across where the original speaker field coil would have connected as a power supply choke.

    2EC5E3F5-DC30-436B-B775-F14697172875.jpeg

    So I am planning to recap it and I have a vintage Utah field coil speaker that I want to us in it. I’ll discuss the speaker later on as well.

    C4087ABA-1E69-43DC-A547-3693E882CFDC.jpeg

    I’m still in the planning stage of this project at the moment and I have a question for those of you that are more knowledgeable about tube amp design. So looking at the schematic, I see that the first filter cap is (well, all of them are actually) a 10mfd. But on close inspection of the original INCCO multi cap, I can see that it was a 20-10mfd.

    8027FB03-D145-4C75-A8D8-BAA2AFECA855.jpeg

    0724C3AA-B9CF-4D1D-A773-62538821AA8F.jpeg

    So looking in a copy of the 1950 RCA tube manual, it says the max filter input capacitor for a 5U4G (not 5U4GB) is 10mfd. So how was it acceptable for Gibson to use a 20mfd instead of a 10? Also, maybe I am just making the assumption that the 20 would be the first cap. I don’t really have a way to tell If the blue wire or the red wire (mostly gone) was for the 20 half or the 10. I know where each wire was connected in the circuit, but no way to know the values since the cap is way beyond no good. Any words of advice? Should I just replace with all 10 mfd’s per the schematic. I’ll post updates as I progress through the project. It should be fun and interesting.:)
     
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  2. peteb

    peteb Friend of Leo's

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    Nice amp, nice project, nice porch.



    10 or 20?


    Let others who have tried both tell you for sure, but either should work and 20 should be tighter.
     
  3. TenaciousP

    TenaciousP Tele-Meister

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    Thanks! The porch was not mine. Ha ha! It was at a neighbors house when he called me over to look at the amp to see if it was something I was interested in.
     
  4. TenaciousP

    TenaciousP Tele-Meister

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    Oops double post!
     
  5. TenaciousP

    TenaciousP Tele-Meister

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    So on the capacitor value, I’m not so much concerned with the sonic difference between the two values. I would like to have as good of filtering as possible but at the same time, I don’t want to cause excessive current in the rectifier tube and damage it. I suppose the simplest thing would be to put all 10’s in the power supply per the schematic. It just makes me wonder about the 20 being in there originally which is not per the schematic. I’m fairly certain that is one of the two original multi caps because I have seen pics online of these amps with the exact same 20-10 capacitor. Hopefully someone will educate me on sizing filter caps because if the 20 is original, it seems like they would be exceeding the max rating of that rectifier tube. but then again, perhaps it makes a difference depending on whether it is the first cap or the second. I normally think of a higher value on the first cap, but I don’t know if that is a definitive rule.
     
  6. peteb

    peteb Friend of Leo's

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    Are you limiting yourself to the 5U4G?


    Fender runs two 70 mF in series in the first cap position after the 5U4GB for a total capacitance of 35 mFd.


    This is typical Fender, over running the technical specifications.







    742B840F-C20F-4F8F-8DCC-E7B25E25AB7B.gif
     
  7. David Barnett

    David Barnett Doctor of Teleocity

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    [​IMG]

    P-13 baby!
     
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  8. TenaciousP

    TenaciousP Tele-Meister

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    Well, yeah kinda. Mostly because the 5U4G is what is in the amp. So assuming it’s still good, I’d like to use it since I have it and I really kinda like the look of the ST (coke bottle) style glass. But I do recall that a 5U4GB allows for up to a 40 mfd cap according to the RCA book. I suppose swapping to that tube could be a possibility for me. For now, I’m thinking of just using 10’s for all four caps per the schematic and see how it sounds. I reckon I can always change the first cap to a larger value later if I think it really needs it. As far as over running technical specs goes, I’m sure it’s possible, I’m just not knowledgeable enough to know when it’s acceptable and how much you can over run. I’m still very much a newbie to amp repair and building. I guess I feel better playing it safe for the time being.
     
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  9. David Barnett

    David Barnett Doctor of Teleocity

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    How was the amp rigged to work with a PM speaker?
     
  10. TenaciousP

    TenaciousP Tele-Meister

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    Yeah, it’s pretty cool. Sadly the pickup was dead when I got the thing. I noticed that the cover was already loose from the baseplate, so I suspect it had already been molested by someone in the past. I opened it up but couldn’t find where the break was. It read an open circuit on my multimeter. :( So I spent like a couple hours carefully removing the paper tape that was over the coil hoping I could perhaps find that one of the leads was pulled loose or something fixable like that. But The adhesive on the tape was dried out and bonded to itself and the coil... so well that when I got down close to the coil, pulling the tape up tore the wire. :cry: Perhaps there is another method of old tape removal that could have been employed that I am not aware of, but what is done is done. It pained me to cut out the old coil wire. I’m sure that’s where all the mojo is right? Anyhow, I rewound it with new (mojoless) 42 PE wire and reassembled it all. It plays and sounds good now but I still wish the P13 was all original. But oh well, it is what it is. :)
     
  11. TenaciousP

    TenaciousP Tele-Meister

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    Two 1500 ohm 10 watt resistors in parallel were attached across where the field coil would connect at the speaker plug. That should give the specified 750 ohms of the field coil. But obviously without the filtering capability. The two FC wires were removed at the plug.

    CB8FCD21-0784-4CB6-ADF9-7384FF9B2671.jpeg

    The speaker I picked up on eBay reads about 780 ohms across the field coil, so I figure that’s plenty close to work. The frame is actually like what I usually see in the slightly earlier (version 3 I think) EH150. But I figure if I want a field coil speaker for the thing, that’s probably about as close to original as I’m gonna get. In fact I scoured the Bay for like 6 months before I found that speaker. Now, the voice coil impedance is a problem. I suspect it’s only a 3.2 ohm since it only reads about 2.5 ohm DCR. The cone has a couple of small holes in it anyway so the plan is to recone it with an 8 ohm VC.
     
  12. flyswatter

    flyswatter Friend of Leo's

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    Cool project. For the filtering, you might split the difference and go with the 16uF caps made by F&T, either the individual blacks commonly used to recap Deluxes, or the 2X blue can used in Marshalls. At any rate, I doubt that an increase to 20uF is going to hurt. If the tubes are indeed fine, consider that they were probably run for decades on that higher capacitance -- and we aren't talking stupid high anyway. The notion of 10uF being max for a 5U4 does not sound right anyway. I've seen 5U4s in all types of amps (Princetons, etc,) yet can't think of a single amp that had a mere 10uF as the first reservoir cap value.

    If you haven't already, be sure to check all the caps in the amp -- including those black Aerovox. We usually think of coupling caps as surviving much longer than electrolytics, which they do, but even they can stray and acquire unacceptable ESR over time. I recently serviced a late-30s Hammond organ that had Aerovox caps that were toast, due simply to age and heat.

    In your last photo, that 2W carbon comp resistor at the bottom of the photo beside the blew is cracked... in case you didn't notice.

    Best of luckl
     
  13. TenaciousP

    TenaciousP Tele-Meister

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    Thanks for the advice on the filter caps. I’ll think on that for a bit. I’m certain the 1950 RCA manual shows the 5U4G with 10mfd for “typical” capacitor input to filter. I suppose typical doesn’t necessarily mean maximum.

    38C2B5B4-5CC9-4285-A954-17E483EC9465.jpeg

    I read the recent tread about Filter cap increase.
    http://www.tdpri.com/threads/how-much-filter-cap-value-increase-is-too-much.920175/
    I see lots of references to certain amps exceeding data sheet values for certain rectifier tubes. I just wish I knew the science behind doing that. Is there a calculated way of determining how much is too much? Not to say 20 mfd in this amp is too much, because obviously it was done. I just wish I knew more about why. I did see where most rectifier tubes in the RCA manuals will have an * saying something about the input cap value can be increased but the effective plate-supply impedance may have to be increased to accommodate it. I don’t know what that means.... ha ha! Something to do with the circuit design I assume? I may change my mind back to the 20 mfd. Anyhow, as far as the other caps go, I don’t have an ESR meter (probably should get one) so I was gonna make the assumption that all of the other caps in this thing were bad and replace them all. I feel like that would be the safe thing to do. And capacitors aren’t that expensive so why not. The Aerovox caps are replacements already anyway.

    I did notice the crack in the top of that resistor you pointed out. I debated whether or not it was just a surface defect or a deep crack. I probably should replace it. I believe the color bands indicate that it is a 12k 10% and it reads about 13.4k on my meter so it’s still pretty close. But I should probably replace it to be safe.
     
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  14. flyswatter

    flyswatter Friend of Leo's

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    Tenacious, interesting: The manual does indeed recommend 10uF. Here's a detail from the Princeton AA1164 schematic, with 5U4 tube and all 20uF/450VDC filter caps.

    AA1164 detail.PNG


    I know that some people change to a GZ34/5AR4 in the Princeton, but my understanding it that it is due to the lower current draw of the GZ than a problem with 5U4 vs. 20uF caps. I'm thinking that if so many Princetons have survived and thrived so long, there's nothing about 20uF that's going to blow up a 5U4. I recently serviced the amp from a 1930s Hammond organ (a really early one) that also had a 5U4 (a really old one!) and used 20uF X2 for the filtering. The caps were toast but the rectifier was fine.


    About your more technical queries, I don't know either.

    For resistors that look cracked, I usually give them a couple of mean pokes with a chopstick. If they are truly gone they will usually fall apart, then I replace them. Cheers!
     
  15. TenaciousP

    TenaciousP Tele-Meister

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    Well, unfortunately I haven’t done much of anything on the amp other than look at it and think about how I want to proceed. So I was look at it for a few minutes to night and noticed something that looked kinda funky.

    So I believe these would be the preamp tubes cathode bypass caps.

    31AED0AA-A6ED-4EE1-B962-8648A46D15A8.jpeg

    So on electrolytics, I have always believed that the negative lead is on the end where you see the metal end of the casing and the case is electrically connected to the negative lead. On the positive end, there is some sort of insulating end cap that separates the positive lead from the case. On these, the labels clearly mark the positive end. On close inspection, I see the one on the right in the first pic has a reddish rubber insulating end cap on the positive end (yes it appears to be ruptured I think:eek:).

    FDF3DCC8-77FF-49F2-BBFF-C12FA5AAEE59.jpeg

    But the one on the left has the metal end of the case at the end labeled as positive.

    1137990E-45FF-4F31-963A-6E6BD0541AAD.jpeg

    Was this label installed backwards at the capacitor factory? I thought a polarized capacitor had to be installed in a certain orientation in order to work right. Right? Negative to ground? What do y’all make of this.
     
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  16. peteb

    peteb Friend of Leo's

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    Those are cool shots of the wires




    I’m interested
     
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  17. flyswatter

    flyswatter Friend of Leo's

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    Not sure about the reversal business, Tenacious, but, yes, they are the bypass caps for the preamp cathodes. I'm thinking the polarity is correct or else the cap would likely have exploded. I would just replace them... at least one of them clearly leaks.

    Nichicon make nice little 22uF/50V caps that work great in those locations. Digikey sells them.
     
  18. TenaciousP

    TenaciousP Tele-Meister

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    Yea, I actually think both of them have leaked and I am planning to replace them both.

    But looking at the capacitor construction, disregarding what the label says, I really think the one on the left was actually installed backwards. I don’t know enough to say if a reversed polarity on a cathode bypass cap would cause it to explode or not. I don’t think there is much voltage applied to it. But i’m pretty sure reverse polarity will cause the cap to not do what it’s designed to do in the circuit. But like I said, I’m planning to replace them both anyway so it doesn’t really matter at this point. I just thought something was odd about it. Especially since those are the original factory installed caps.
     
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  19. peteb

    peteb Friend of Leo's

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    at least the ends marked positive are the ends going to the tube cathodes.


    that's right.




    are you saying they are marked wrong?
     
  20. TenaciousP

    TenaciousP Tele-Meister

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    Yes. That is what I am saying. The cap on the left (V2 cathode) in the below pic is labeled incorrectly.

    In this view you can sort of see that both caps are marked positive on the ends going to the tube cathodes.

    6A023F9F-AAC8-4984-BDF6-B42320788519.jpeg

    Here is a pic showing the V1 cathode cap (on right in above pic) where you can see the reddish brown rubber disk that closes out the positive end of the capacitor body.

    051885B0-D8DD-478B-AB68-9C420708CADF.jpeg

    Now here is a pic of the V2 cathode cap (on left in top pic) showing the all metal (negative) end of the capacitor going to the tube cathode even though it is clearly marked as positive on the paper label. On this cap the rubber disk (actual positive end) is on the side going to the grounding location.

    6162225C-49FA-4250-BE0B-66F141D8164E.jpeg

    Anyhow, they both have a ruptured spot on them so they are both ruined and need replacement. The polarity/mislabeling thing is not really a problem at this point. I just thought it was really weird and it was something talk about until I get some real actual work done on the amp.
     
  21. David Barnett

    David Barnett Doctor of Teleocity

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    Looks like a factory f-up. Or two factory f-ups, since the cap factory messed up and Gibson didn't notice.

    Just to be clear, there's a red plug/vent at the grounded end of the "backwards" one?
     
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