Gibson EH 150

[aabbs20]

Great stuff guys!

Dave:

The LC feed goes: PT > 5AR4 Rectifier > Choke > Reservoir Cap?

Colossal:

I think I understand what you're saying, but I'm not positive. Do you have a photo or layout handy? I've looked for pictures of your finished build but haven't had any luck. I might be searching the wrong way again.

M Fowler:

You've got a much higher higher B+ than the original, does it have a lot more clean headroom or does it still break up early? There is no judgement here, just curiosity...does it sound like the original? Are you happy with the tone?

[RWood]

Search function takes a bit of learning.
Note that Mark used EH+150 in his search.
Use of the + sign is important.

How about someone putting up a sticky on this site's search engine.
I find it returns far too many results to be useful to anyone who doesn't know it's parameters. I made this request long ago without any responses.

[David Root]

Aaron, yes, I forgot the rectifier!

[Colossal]

Colossal:

I think I understand what you're saying, but I'm not positive. Do you have a photo or layout handy?

The field coil is about 750R, not 850R-1k as I said earlier. The choke between the plates and screens is about 5H. I've used a 7H JTM45 choke and that worked quite well.

[M Fowler]

I should recheck my voltages.

The expected voltages using the transformers specified by Tino Z are shown below. My transformers specs are what Tino suggested.

[aabbs20]

M Fowler:

Your voltages look right in line with Tino's. I have that book as well and I'm trying to decide if I want to build it to his specs or go with the original B+. Does yours still have the classic CC tone? What speaker did you use?

Colossal:

Thanks for that. I hadn't seen that version of the schematic yet, it seems like there are a bunch of different variations on the EH 150. If I was to leave out the field coil speaker in that design, would I use a second choke in its place?? Maybe a big dropping resistor? If I was to search for a field coil, is there a good source or do I just have to comb Ebay to find the right one?

Thanks for all your help guys, I'm getting really exciting about building this!

[David Root]

I would forget about a field coil speaker. They cannot handle being overdriven and you would very likely blow it up.

Just use a reasonably sized PT and a rectifier tube and choke that will get you to the plate voltage you want, and a decent alnico speaker. A 250-0-250 PT with 150mA should get you close to 280V or so with a 5U4 or 5R4 rectifier.

You might have trouble finding 150mA at that voltage, might have to go to a 275-0-275 eg a Hammond 270EX or FX and pick a choke or power resistor that will drop some of that overvoltage. Musical Power Supplies makes a 285-0-285 @ 150mA for a reasonable price. I used one in my Maggie 113 and it works fine.

[M Fowler]

M Fowler:

Your voltages look right in line with Tino's. I have that book as well and I'm trying to decide if I want to build it to his specs or go with the original B+. Does yours still have the classic CC tone? What speaker did you use?

I used a variety of speaker cabs but prefer Celestion Vin 30 or WGS for this dark of an amp.

This amp is loud, nice mellow octal preamp tone and also has high gain channel so a variety of sounds are possible. I don't play C.C. style.

[Colossal]

I used a variety of speaker cabs but prefer Celestion Vin 30 or WGS for this dark of an amp.

This amp is loud, nice mellow octal preamp tone and also has high gain channel so a variety of sounds are possible. I don't play C.C. style.

I too found a run-of-the-mill Vintage 30 to work REALLY well with this amp. Very full, warm, punchy, and balanced. Not bright or shrill at all. With an archtop, you hear the guitar very clearly. Smoky and jazzy, but not in a muffled way as if the tone were rolled all the way off. Comfortably loud.

Colossal:

Thanks for that. I hadn't seen that version of the schematic yet, it seems like there are a bunch of different variations on the EH 150. If I was to leave out the field coil speaker in that design, would I use a second choke in its place?? Maybe a big dropping resistor? If I was to search for a field coil, is there a good source or do I just have to comb Ebay to find the right one?

Thanks for all your help guys, I'm getting really exciting about building this!

Yes, use a choke in place of the field coil and add some series resistance to reach about 750R-1k total. You need to be dropping about 100VDC across that first Pi filter if you use a 390VAC transformer. The target plate voltage is 275-300VDC. You can go Class AB with a higher plate voltage however, like Tino's version, and it sounds quite similar to the real deal.

[aabbs20]

I too found a run-of-the-mill Vintage 30 to work REALLY well with this amp. Very full, warm, punchy, and balanced. Not bright or shrill at all. With an archtop, you hear the guitar very clearly. Smoky and jazzy, but not in a muffled way as if the tone were rolled all the way off. Comfortably loud.

This is exactly what I want to hear! Thanks.

Next step is ordering parts and building. I'll keep you guys posted.

[aabbs20]

Hey Guys.

I've been working hard on my build and finished up the wiring this morning. This is my first chassis punch and I changed my mind a few times...forgive me if its a bit messy. I followed Tino Z's schematic and layout with a few small changes. I plugged it in this morning and the pilot light came on with no smoke, which was a good start. I then plugged in the 5U4 rectifier and the filaments lit up, but when I flipped the standby switch I got a short laser light show in the tube and blew the fuse. I tried another tube and another fuse, same thing. I changed out the tube socket and fuse and tube and tried again...same thing! I have checked and rechecked my wiring and it looks fine. The pins on the rectifier socket read ok I think, 380AC on pins 4 and 6 and ~2AC on pins 8 and 2. What did I do wrong here????

[Murrayatuptown]

Late to the party, but this post is very helpful to me as I bought a Style III EH-150 which had been modded/rebuilt, from a Music-Go-Round store online. Relatively low price, reduced from I don't know what.

s/n 13195 6F5-6C5x3-6L6x2-5U4G Fender/Jensen(Italy) C12N. Pretty close to one of the 6C5's which wanted to visit the magnet as I reinstalled the metal tubes. Store had the intelligence and thoughtfulness to remove all the tubes & bubble-wrap them. I think they didn't know what to do with the 6F5 as that was bubble-wrapped and secured in the cabinet with the shielded grid lead still attached :@).

PT is Hammond 274BX

I haven't looked under the hood yet because it works. Smells warm. I don't know what other magnetics are underneath. I should probably look instead of blindly using it, if I want it to remain healthy.

Sounds better than I expected...archtop w/humbucker. I tried Instrument input, 'Normal' & 'Bass' mode tone switch. I briefly tried Microphone input & it's a little weird in ways I can't describe yet, but don't want to use that. It seemed microphonic but only at low frequencies. Maybe with a lower output pickup.

I hunted down a couple 12" (Philco) & one 10" (Utah) FC speakers for future plans. While awaiting delivery of the amp I found 30's replacement Philco field coil catalog data which gives some perspective for the range of types that were in use.

Last thing I saw on my way down the rabbit hole was NOS Waldom & Carron replacement field coils. The datasheets in the box show the range of f.c. resistances by letter suffix for each p/n, and their use in series or parallel (the purpose of a center tap on a f.c. wasn't obvious to me). They have a variety of pole piece diameters and length I hope offers flexibility with a local recone service.
I have not seen (yet) much discussion online about current and power in the field. I wonder if the size and field coil power had some ratio vs. the amplifier power output.

Finding all that field coil data might be a piece of history worth putting into one place. I think a good bit of that isn't widely known by the living.

Decomposed leather over wire bail handle is basically skeletal now, allowing study of how it was assembled and what challenges that presents to repair.

Last price I saw on out-of-stock California leatherworker's replacement was up to $125, and a vintage one sold at auction for 258...(I forget whether that was Euros or US$ but that's getting crazy).

I had vague plans (as most of mine tend to be) to convert a Heathkit Williamson amp to an EH-150-inspired guitar amp, definitely PM speaker...but a little enthusiasm about restoring f.c. drivers might change my direction.

I was a little discouraged to see a phase-splitter/inverter xfmr in the EH-150 schematic but realized they are only a headache if you have to find, or pay for, the right one.

The tubes are easier to obtain than those in some versions.

Murray

[Murrayatuptown]

Another perspective on the 20K resistor to ground after the series field coil as filter choke. It has a more important function than just a bleeder resistor or dropping excessive plate voltage as is sometimes suggested.

But wait, the older version with 6N6 power tubes has a 6K resistor to ground between the field coil and '2nd' filter choke. The 6N6 schematic doesn't show a 'normal' cathode resistor, so I won't guess at the 6N6 cathode current.

The original 6L6 build being Class A it already has a substantial load on the B+ all the time (unless the 6L6's are unplugged, roughly 20 V across the 200 ohm = 100 mA cathode current. Th B+ drains quickly when the amp is turned off. If the 6L6's are unplugged, then the only load on the B+ is the preamp tubes. For the Style III version with 6F5 + 3 6C5's there is only a few mA being drawn. So, yes, the 20k resistor draws more than the preamp tubes, and it is bleeding there, but that's not all.

Just to pick a number for the sake of discussion (or monologue), say there is 290 VDC across the 20 K 'bleeder'. 290/20000 = 14.5 mA, 4.2 W dissipated in the 20k resistor, and the 'extra' 14.5 mA from the 'addition' of the 20 k resistor only drops 10.8 V across the 750 ohm field coil resistance. Now, go back and look at an older version/Style of EH-150, the 6N6 output version. Assume the same 290 V across the 6k resistor to group after the field coil. That's 48.3 mA just thru the resistor, 13.8 W dissipated in that resistor. That's pretty wasteful for a bleeder. Call it 48 mA. That drops 36 V more across a 750 ohm field coil than if the 6k were not present. They're not obsessed with the plate voltage as much as setting up the desired field coil power to get the electrodynamic magnet flux to the level they want.

Back to the more-discussed 20 K 'bleeder' and 750 ohm field coil scenario.

I have not personally examined an original 'Ultrasonic Transducer' speaker for markings. I don't know if it's a Jensen, but if you look up old Jensen speaker catalogs (mid 30's & mid-late 40's), for a 12" field coil A-12 Concert speaker, field coil power range was recommended to be nominally 14W, minimum 10W and maximum 18W. Hammond service manuals for some models (I don't know what year) ran the speakers a little over 10W, 13W and 14W. Maybe the M-3. AO21055-2 is an F12NS Jensen speaker. AO15888-5 and A015888-6 (on my living room floor) have way different f.c. resistances (5300 and 246). The 2nd is series (filter choke) and the first is shunt powered. (For the 5300 ohm example, 51 mA at 272 V, per a spreadsheet calculating the current that produces 14W of field coil excitation also 10 & 18 W). One can't power an amplifier with a series 5300 ohm field coil, so it has to be shunt powered.

P R fc I fc V fc
18 750 0.155 116.19
17 750 0.151 112.92
16 750 0.146 109.54
15 750 0.141 106.07
14 750 0.137 102.47
13 750 0.132 98.74
12 750 0.126 94.87
11 750 0.121 90.83
10 750 0.115 86.60

The 3rd column is the current thru a 750 ohm field coil to reach the 1st column fc power, and the last column is the voltage drop across the field coil for the desired fc power.

Look at the 10 W line. 115 mA of field coil current. Increasing the current thru the field coil by 14.5 mA, in the assumed example of 290 V across the 20k resistor to ground, brings the field coil power nearly to 13 W, assuming it had been at 10 W. But without the 20 k resistor, the plate voltage would have been a little higher and the cathode current a little higher. Chicken or egg. But the field coil power and voice coil gap energy was more important than how quickly the power-down discharge of the B+ was. A secondary purpose of a bleeder resistor is to draw more current to 'regulate' B+ by minimizing load fluctuation. It's a Class A amp. That serves no purpose there.

In a 'modernized' build with a PM speaker, and a lower voltage power transformer, the plate voltage is reduced far more by the alternate transformer choice than the small additional (vs. not present) current in the 20k resistor. For someone building a 150 on steroids, with higher B+, running Class AB1, the 20k is only functionally part bleeder resistor and part superstition. My assumption of constant plate voltage above for the analysis of the field coil voltage drop and coil power wasn't accurate in terms of all other things remaining equal. A PM build changes more things than the liberty I took in my example.