Dumpster Recovered Gibson EH-150 Help/Guidance

[Reli-Teli]

"Long history of how I acquired it"

Whats up, I'm new here and to amp servicing in general. I'll start off by saying my family is infamous for being at the right place at the right time and getting only deals you hear about and end up paying an arm for. None more than the day my dad started bringing antiques home from some fellas dumpster in a neighboring town. He said it was a fella who's last parent had died and we guessed he thought it was all junk, or memories. Being the nice guy my dad was he gave him some cash each time he stopped by. My dad was very open about what he found he didn't mind showing my Victrola parts, Cobbler tools, 50's Life Mags etc...etc...But when I happened to walk by his open garage door and saw some small open back tube amp and an extension cab It was like a tractor beam. No soon that I took a couple steps in that direction he slammed the door down and said don't worry about that. About a year later I happened to see it in another building (Dads a Hoarder). I asked him about it the first chance I got and bugged him for about 4 years until I got an Epi Lucille in a trade. I proposed a trade idea to him but was very coy about what I had. It took him about .5 sec to say yes. While loading it up in my trunk he said "Oh it's a Gibson, you knew what this was didn't you?" I only ever referred to it as that old guitar speaker or that old speaker at that point. As is with anything of my dads I was after over the years it's more about the rescue instead of my own gain. The building the amp was stored in was deteriorating so it was a matter of time before it got the amp too.


When I brought her Home

Upon bringing it home I gave it a visual inspection and cleaned it up a little then it was all systems go. I didn't know about the Mic/Gain Channel at that point so I plugged into the instrument jack and let me tell you I was definitely surprised. Not only did it work but had next to no hum and sounded really smooth. It wasn't as loud as I would have expected it, there was a sliver of break up as I increased the volume. After turning it on to play it for the 4th time the Rectifier tube put on a dazzling light show then kaputen. I was advised to check the filter caps. I'm not much of an electronics guru but I have been modding pedals and building guitar wiring harnesses for awhile. I wouldn't have any fear about jumping into this Gibson amp. I'm a working family man and only have so much time to scour the web for resources.

Questions
Is there a really good amp servicing guide for dummies around? I'm not fluent in schemanese but I can see a lil of whats going on when I compare the EH Schem to a picture of her circuits. Or even better is there an image somewhere that has the parts labeled? And which Schem would mine be? There are two versions on Gibsons Sight. Thanks for any help you may have, I know I should have cleaned it up and serviced it before firing it up but I just had to.

[David Root]

There are four versions between '36 and '41 IIRC. I have built the last version, the '41, with three 6SQ7s, and a few others on TAG have built that or earlier versions.

This one one looks like an earlier version, maybe the 2nd or 3rd versions. Somewhere I have all four schems but can't quickly locate them. The two big yellow caps are obviously not original. V1 in your amp, the one on the right with the plate cap, should be a 6F5, the next two are probably 6C5s, the next one a 6N7. The power tubes look like 6L6GCs which would work but are not vintage correct of course.

Pity about the black paint!

[rp]

You need for Colossal and MFowler to chip in they are the EH150 guys here. Primitive amp but an advanced undertaking for someone who doesn't know his way around a schematic. The speaker's a field coil so hopefully it isn't rotten. It's great looking, if it is shot I'd have Weber recone it and keep it as field coil, assuming they can recone field coils. You need to do the caps, that big cardboard box should also be caps, but you should check that the PT is ok first, make sure the cloth wires are sound, poke and look through it all to make sure they aren't crumbling, but should be ok seeing how clean the amp looks. IMO you should just replace the 2 coupling caps off the bat, little chance those held up, I would get some old sound molded caps off ebay for this one. I would test the resistors seeing how few there are, the power resistor at the end of the board looks toasty. If the wiring isn't rotten a tech could get this running perfect with little effort, maybe $150 in parts and time if you're not in someplace like NYC, might be worth it, just make sure to take it to someone good at proper restorations who loves these things as they are. Find a tech here on TAG to service it right.

There are a bunch of posts on the net about getting paint off tweed and tolex. Might come off easy, might not. I wouldn't recover it either way. It's got its vibe.

Wish I could find an EH150. Been ages since I got to mess with something original. If I wasn't in Bumfucco I'd do this for free for you.

[rp]

Who paints over TWEED!?!?

same people who paint over doors made of American chestnut, or oak windowsills. At some point that wood looked too victorian and 1920s sad ass so in the jetstream age they painted it all white. In the 60s or 70s your amp looked too Shanana to someone and they painted it black. Can't hold it against them, you gotta rock.

[Phil_S]

If transformer lead insulation is brittle, it should be dealt with by someone experienced. I understand that Mercury Magnetics will re-lead a transformer for a reasonable price. Because this is an antique and you want the mojo of the original iron, this is only a DIY job if you know exactly what to do before you do it. You don't want to find out that you've ruined the stuff after experimenting with it.

Since the original tweed(?) covering was painted, you may want to consider stripping it and recovering it. Google is our friend here...you can see what it used to look like and should be able to get new tweed that looks period correct. It may take some patience to find what you need.
https://www.google.com/search?q=Gibson+ ... 44&bih=731 Taylor (who has a sticky in cabinets and speakers section here) is probably the goto guy on this and if not he's a good place to start.

Very nice dumpster dive. I am envious.

[Colossal]

That looks like a Type II or III depending on your serial number. 10" speaker? The three small transformers on the inside are (from R to L), field coil, power supply choke, and interstage (doing phase inverter duty). The tube compliment from R to L (looking from the back) is 6F5, 6C5, 6C5, 6C5, 6L6, 6L6, 5U4G. As Mr. Root said, if you are going all vintage, you'll want a pair of 6L6G ST (coke bottle type) power tubes and they can still be had at reasonable prices with a little diligence. Sorry to hear your rectifier tube blew, but without a doubt that orange wax box Aerovox 10uF capacitor is done for. Those yellow capacitors are very much the wrong value both for vintage spec and performance. There are some capacitors under the board that have probably remained unmolested. The amp looks pretty good otherwise.

[Leo_Gnardo]

Excellent! I've got a '150 that's been lurking in my junque room for decades, just the chassis - no speaker nor box. I'll be watching this thread for pointers on what to do.

[rp]

Hey Colossal, any idea on the values of that PI choke? A link to a modern sub? Though with specs I could get one wound easy enough. Any idea on the resistance of that field coil too? This is about the only Jurassic thing I still would love to build.

[Colossal]

Hey Colossal, any idea on the values of that PI choke? A link to a modern sub? Though with specs I could get one wound easy enough. Any idea on the resistance of that field coil too? This is about the only Jurassic thing I still would love to build.

Instead of a tube and LTPI or paraphase, the PI in those old amps is a Thordarson T-8790, an interstage transformer. Mercury winds a direct replacement/clone of it. I have measured the DCR in field coil in several 150s and I think (don't have my notes in front of me) it was somewhere around 820R. Don't quote me on that. I'll try to look tonight and see what I recorded. The PT is a Thordarson T-40069 and it is 408-0-408. Yes, that is correct! The plate voltage is a mere 275-300VDC. There is a 20k 25W power resistor to ground in the power supply (left of the two wirewound resistors in the photo). The cathode bias resistor is 200R, no bypass cap. 19-20VDC at the cathodes.

[rp]

The PT is a Thordarson T-40069 and it is 405-0-405. Yes, that is correct! The plate voltage is a mere 275-300VDC. There is a 20k 25W power resistor to ground in the power supply (left of the two wirewound resistors in the photo). The cathode bias resistor is 200R, no bypass cap. 19-20VDC at the cathodes.

WTF, I guess with Mr. Roosevelt's TVA and Hoover Dam now providing cheap plentiful electricity... You think Gibson just had too many 810V PTs and needed to use them up??? That'd be just whack. Did you build your clone(s) this way w/ the big resistor to ground? No Energy Star rating for you! Do you think it makes a difference to the feel or sound - or enough of a difference to warrant the thermal excess? I'll likely just go with a normal PT to get 300V on the plates. Knowing I can get the interstage unit now makes this more of a possibility. I have little love for MM's business style but they sure are comprehensive to the guitar community. Anyone in Italy want to give me a job so I can blow my paycheck on amp parts? Maybe you own an Italian amps parts store, you can pay me in chits redeemable only at the company store like in the Grapes of Wrath.

Colossal, all the notes you want to share I'd be happy hear.

My parts box is near empty but I do have a Watts chassis. If I punch it for an EH150 I'll be forced to build it

[M Fowler]

Dave (colossal) great info as usual buddy.

https://tubeamparchive.com/viewtopic.ph ... bson+eh150

https://tubeamparchive.com/viewtopic.ph ... bson+eh150

https://tubeamparchive.com/viewtopic.ph ... bson+eh150

[rp]

I went through those posts real quick, including David Root's build that got me thinking EH150. One thing that turned me off then was the claim of 500V on the plates of the orignals. Seemed odd to me too, as someone mentioned, for the 1940s with 6L6 metals or 6L6Gs so I figured incomplete info or just crazy amp to avoid.

Do we now know that the real ones whether Type I/II/III and whatever secondary voltages all ended up with ~300V on the plates? I hope so, I'm not qualified to build a crazy amp.

[Colossal]

I went through those posts real quick, including David Root's build that got me thinking EH150. One thing that turned me off then was the claim of 500V on the plates of the orignals. Seemed odd to me too, as someone mentioned, for the 1940s with 6L6 metals or 6L6Gs so I figured incomplete info or just crazy amp to avoid.

Do we now know that the real ones whether Type I/II/III and whatever secondary voltages all ended up with ~300V on the plates? I hope so, I'm not qualified to build a crazy amp.

I built a clone for a jazz player that is based on a Type IV, but it was running Class AB with 6L6GCs, not Class A push-pull, like the original. That amp did have 500V on the plates and adjustable cathode bias. The bias resistance of course was higher than the 200R in the originals. I can confirm that the originals run 275-300VDC on the plates, Class A push-pull. I built another clone based on the original specs and have reproduced the voltages found therein. RP, you are correct that 6L6Gs are not up to handling 500VDC. The high voltage clone I built sounded remarkably similar to the originals which ran at lower voltage. Bigger, bolder low end and maybe a bit less creamy, but this is what the owner preferred. That amp was mated with a Vintage 30 of all speakers, and it just killed. I have repaired and played originals and they are not particularly loud at 12-15W or so, have a rather dark "telephone" midrange quality to their voice, and idled remarkably quietly (no buzz or hum). They are great for swing jazz of course and very transparent with Selmer type guitars often used for that playing style. The Mic channel is very gainy and gives a really great singing sustain, raucous blues tone!

As to the PT, my guess is as you said, that Gibson could get the Thordarson PTs cheap (being right there in Olde Chicago) and simply adapted their design. So the rectified voltage is high, but the field coil is dumping some voltage, the 20k 25W bleeder resistor would certainly help smooth the rail, and then there is another choke as well. Touch sensitive but stiff(ish). All of the old ones I have been in have been 275-300VDC on the plates with modern mains AC input voltages.

[M Fowler]

I wonder if a guy just used 560vct PT and 6k6 ot with 6L6GC if would get that EH150 tone.

Keep the plate voltage way down. Here is where the Stancor iron or even TW iron might be very good for a DIY build.

[David Root]

I originally built mine to 400V plates (not with an 800VCT PT with a big-ass resistor to ground) and it was on the clean side unless really cranked. I did not use a coil speaker, I used an early '60s P12N.

Then I changed out the power supply to about 280V plate by the simple expedient of changing it from cap fed to choke fed, and using the existing big-ass choke. I think I detailed that in my post on my EH-150. All I had to do was rewire the power supply, and now it's very close to original specs on voltage and sounds pretty good too.

Yes I know it's not the original-style PS but that was not exactly purpose selected iron either. Choke feed is definitely old school though!!