Gibson EH-150 square/round corner cab speaker questions

[murrayatuptown]

Search tool is not helping me (no finds) tonight, so I'll just create a thread.

I found an EH-150 with some mods at a fraction of usual pipe-dream asking prices. It's en route to me & the store that had it took the photos down after it sold, so I'll get pictures when it arrives.

I'm curious about how many vendors supplied field coil speakers in EH-150 amps. I have seen three brands described as 'standard' in articles online.

Utah, CTS and a third I forgot (Rola?) get named as being in these amps. If there were multiple vendors, I wonder how similar or different they sounded, or if it mattered to Gibson).

The one I got has a 'Fender Special Design' 12" PM speaker. I was expecting a 10" so I'm already wrong. The amp works, so I cringe to think of how far the sound may be from what it used to be.

There is also a Hammond (Canada, not B3) logo visible, so I suspect someone replaced a transformer (or added a choke).

I shall see. I spent a couple days reading about rewinding field coils and downloading data for late 30's Philco replacement field COILS for other brands than Philco, to get some context for field coil resistance and current. Rabbit hole, indeed.

Before buying the amp, I looked at various used field coil speakers, thinking it was part of rationalizing another amp adoption.

Gotta have a hobby, right?

[olejason]

If it's had a Fender Special Design installed I'd imagine a choke was added. I'm surprised that speaker fit, I'd be curious to see how it was mounted. The 10" is usually mounted right up against the edge of the cabinet on the rounded corner version. I guess there's a bit more room in the squared version.

Let us know how it turns out. I'm using a Vintage 47 VG-185 but there's always a part of me who wants the real deal

[murrayatuptown]

I haven't hit the bottom of the rabbit hole yet.

I had a seller cancel one FC speaker transaction because they ripped the cone packing it. I told them it's 80-90 years old & I anticipated restoration...relist it!

I found 5 or 6 different new old stock replacement speaker field coils of different resistances and a couple pole piece diameters.

Some have datasheets that identify series and parallel resistances by suffix letter in the catalog number. I didn't know (one use for) the tapped field coils were for flexibility in applying a replacement in different circuits.

There is a wealth of info scattered across the 'web. Having the datasheet presents opportunity for some context. That will also provide some enlightenment about a la carte DIY field power sources.

The field coils were about USD30 each shipped. I can't imagine paying less than that to have one rewound. Heyboer rewinds them, and are sort of neighbors to me, maybe 15 miles away. They gave me a tour of the factory about 30 years ago when I needed some components would for work.

I've contacted my local speaker repair place I had my BR-9 Jensen FC speaker reconed by, to make sure they can still collaborate.

If I had gotten three of the same 12" types, imagine a 3x12 field coil cab. Maybe that's available to those who qualify for the necessary mortgage.

I've embarked on some stupid projects in the past. Hopefully this is not another (I retired after almost 50 years in industrial electronics, so it's not "automatically" stupid).

How deep is a rabbit hole, anyway?

[wintermoon]

Why not wait until the amp actually shows up before you dig any deeper Bugs?

[Woody Sound]

If it's had a Fender Special Design installed I'd imagine a choke was added. I'm surprised that speaker fit, I'd be curious to see how it was mounted. The 10" is usually mounted right up against the edge of the cabinet on the rounded corner version. I guess there's a bit more room in the squared version.

I *think* from around '37- they had a 12".

[ruger9]

Let us know how it turns out. I'm using a Vintage 47 VG-185 but there's always a part of me who wants the real deal

I have a search saved on reverb, almost bought a couple over the last year (priced not -TOO- badly)...

For good measure, some good tones...

[murrayatuptown]

thank you

[murrayatuptown]

Style III:
6F5-6C5x3-6L6x2-5U4 No tone controls, has Normal/Bass Mode switch.
Gibson 12" Ultrasonic Transducer FC speaker has been replaced with Jensen/Italy C12N PM speaker & PT has been replaced with Hammond 274BX. 3-wire cord is installed.

Handle has no leather. One bottom metal corner is missing.

I haven't looked inside yet. It's tested/working so I should listen to it first then see what they did to the guts.

13k s/n, 1940 or later?

[olejason]

I have a search saved on reverb, almost bought a couple over the last year (priced not -TOO- badly)...

For good measure, some good tones...

Oh yeah that's the tone! I need to save a search too. I've seen a few pop up under $3k but not in the condition I was looking for. Seems like deals are still to be had on these.

[murrayatuptown]

I got really lucky on the price. (Style III if I didn't figure that out & say it already).

I figured out (I think) a usable leather handle that can be adapted to the already riveted mounts & contacted a company to ask about the cross-section of one style handle they make and the mounting method of another. It may not be possible without some crazy quantity purchase. As purchased, the amp is already significantly non-original, so I may as well do what I need to until I figure out whether there are any irreversible internal changes. I don't anticipate an easy return to full originality since the original speaker is gone and the power transformer is different.

I put all the tubes/valves back in and tried it out. I was surprised it didn't sound much like the vintage online videos, but the guitar I used was different and the speaker certainly is. When I look inside I may find a different output transformer, an added choke, and other added parts components to make up for the missing field coil. I think I need to check inside now that I've confirmed it works & sounds good (can't comment whether it sound right).

I tried the microphone input with guitar because I've read it's an option but I didn't like the amp behavior and took that as a warning sign. Not sure what I was hearing...if there is any such thing as low frequency microphonics I'd call it that, only when the volume was increased beyond a certain point. I later looked at a schematic and realized the mic input goes thru the 6F5 tube and the instrument inputs do not use it. So that could be source of the unique unpleasant sound. There were some overall microphonic tendencies with the 6C5 input also. I have never been up close to one of these amps before so I don't know how well behaved it is or isn't.

I ordered some NOS metal (non-G/GT) tubes excluding the rectifier for now. Currently has a 5U4G. I was so obsessed with metal tubes (which are heavily attracted to the huge ceramic magnet C12N speaker), I didn't even think to look up whether a metal 5U4 existed...it probably did but I didn't find any last night.

I think I should inspect the internal circuitry before continuing to use it in the interest of maintaining its health.

I don't know how many variations of field coil speaker existed or how different they might sound. If there were different manufacturers, they may have sounded different, as differences exist today with PM designs.

I am glad didn't get the earlier version with the 'strange' 6N6 tubes, which don't seem very common or easily substituted. Those are obviously not at all like modern 6H6_/6N6_ Russian tubes.

It definitely feels quite warm and smells warm. I suspect being Class A the output tubes do run hotter (continuously, regardless of music signal) than a modern AB1 amp. Between the 5U4G and the 6L6's I could feel significant heat when reaching for the rear panel controls.

It only has the 'Normal'/'Bass' switch and no tone controls, and I am surprised at how much low end it has. I did not have speakers larger than 10" (maybe 8") before this year so everything sounds different to me.

Once I know what's inside, I'll feel more comfortable using it and have a 'back-burner' project to start researching field coil speaker restoration (by others, not me). I don't know how important the Gibson "Ultrasonic Transducer" field coil was to the sound quality. It seems there were Utah, Rola and CTS over the various style EH-150 range. & I have to remember EH meant Electric Hawaiian.

[murrayatuptown]

Nice blur, Mur, on that front photo. I uploaded these from my phone before checking them. I know better than to upload blurry images

[wintermoon]

Most people I know use the mic input on old Gibson amps, I know I did during an approx 8 year stretch, especially live when Gibsons were all I used.

[murrayatuptown]

Thank you Wintermoon.

I may have been hearing microphonics, but I had never heard sounds like I was hearing. It has to be unique to using that input, so I'll try a different 6F5 when it arrives.

My first time in the same room with such an amp, so I was unsure what to expect.

[murrayatuptown]

I guess when you buy enough lottery tickets, you are guaranteed some losers, and you might get a winner.

I have had 4 restoration-grade field coil speakers (three 12" & one 10") and 5 NOS universal field coil replacements (Waldom & Carron, both out of Chicago) delivered recently.

One of the 12" purchased was canceled by the eBay seller as damaged or unavailable. I inquired and said if it was damaged I didn't care. They had torn the cone but I talked them into relisting it...it was described as coming from a Coronado 2M radio, but it's marked C12B and has Jensen patent numbers, as well as patents Jensen acknowledged. There are other numbers I don't recognize. I don't know when EIA manufacturer codes started, but this may be too old. It may be a good candidate some day after reconing to replace the aftermarket Fender/Jensen C12N that's in it. Hard to say...might be hard to convince me to change the amp if it's sounding good.

The 10" Utah is the most unfriendly for identification and online resources, having only one ink-stamped number. It has a (so far) unmarked transformer attached also. Seller thought it was from 1932.

M

[icr]

I tried to save the field coil speaker on my Gibson amp.

I did get the field coil speaker working for a few years, but the paper inside the field coil deteriorated and rubbed on the small, odd sized voice coil, destroying it. Eventually I gave up and fitted a ceramic speaker. I did keep the field coil in the circuit.

[icr]

[powerwagonjohn]

I will keep an eye on this thread. I have a 1950 Gibson GA-77 with a 15" Jensen F15N. I am too busy at this time to work on it but hopefully this winter when it's cold outside. I also have a 1952 GS-75 that has a permanent alnico P15N in it. I can't wait to check out the differences between the two drivers.
Thanks John

[murrayatuptown]

Thanks for including the photos.

The wrinkle texture paint on that amp (EH-125?) is like new.

I had only seen that L7-style pickup/guard/rest style once before and had no idea what I was looking at.

I took a tour of the interior of my amp last night. I can see all the footprints the last trespasser left behind.

I think I am going to try to find some photos of an unaltered EH-150 because only seeing a modified one, I don't know all that's been changed, just the obvious and suspicious items.

It works, and I don't want to disturb any more than necessary.

Had you had hand surgery? I had two trigger fingers released a month ago and am not too impressed with reduction in discomfort. I was told it would take about 3-4 months in my case.

[murrayatuptown]

Most of my steel tubes have 1941 date codes. If they stopped making the metal ones a really long time ago, then I can't conclude they are original. They could be 'new' replacements.

[murrayatuptown]

I'd ask 'Nyah...what's up Doc?' either way.

[murrayatuptown]

I watched the Uncle Doug dissection a day after studying the interior of mine. Some other trespasser made significant changes to mine probably around 2007 or later (all new parts with readable date codes were 2007).

UD confirmed there were more parts under the terminal board. I'll try a dental mirror or borescope. I'd call that Edgar Allen Poe construction style...at least that was my first attempt at describing it. Old Fenders are relatively serviceable. This looks rather unfriendly. Something one might only do when there's no choice.

I do think I need to know what's still under that tagboard. Maybe another day. I found the wax coupling caps had been replaced (topside) with 6PS 600 VDC Orange Drop types. Maybe I got lucky and the prior owner had the stomach to pull that hardwired tagboard out and replaced the cathode bypass capacitors already. They replaced the yellow box-potted filter caps with multiple axial electrolytics on perf-board. Maybe all the dirty work is done and I can work on planning my own dirty work Phase 2.

[wintermoon]

Some techs hate working on old Gibsons because they're a bird's nest of wires and caps inside unlike Fenders that are neatly arranged.
My tech won't even touch an old Gibson

[murrayatuptown]

I consider myself to be my tech, having been an industrial one & sometimes having little choice but to jump into an ugly situation. Especially where cost isn't an object, only success is. Sometimes someone else has given up, which actually takes some pressure off. Unless you don't like being called a magician.

Gotta plan, visualize, make as many sketches as needed (+1 you forgot & realized it would have been helpful).

[murrayatuptown]

I found someone's schematic for a Style III EH-150 that shared a lot I can see in mine. The Echo speaker had a switching jack that grounds one end of the output transformer secondary, with the other winding end going to the internal speaker voice coil. When an 'Echo' (remote) speaker is plugged in, the switching jack selects a tap drawn as a 'center tap' suggesting two equal impedances in parallel need half the secondary impedance. (to split hairs, transformers are tapped by turns ratio, not impedance ratio, which is the square of the turns ratio. So a 4 ohm tap wouldn't be 'centered' relative to 8 ohms but at 0.707 instead of 0.50 (or maybe 1-0.707 = 0.293).

I wonder what kind of speaker the 'Echo' speaker was...also an electrodynamic/field coil type? If so, there is no means of obtaining power for a second field coil, nor would you want it disturbing the internal one, as it would affect the voltages to all the amplifier stages.

So, my vote is that the Echo speaker was passive. Another reason a 35 feet displaced remote cab wouldn't run on the EH-150 power supply (because it makes no sense). Maybe someone has, or knows someone who has a Gibson accessory Echo speaker, if one was offered by Gibson.

I got the impression in a 1931 Jensen patent that there were 'permanent' magnet speakers at the time, and that electrodynamic speaker invention was an improvement over existing technology.

By modern PM speaker progress, field coils seem antiquated to us. There might be a lost chapter we modern 'historians' don't know, before permanent magnets got 'serious'.

I should say don't know completely.

[murrayatuptown]

I wonder what kind of speaker the 'Echo' speaker was...also an electrodynamic/field coil type? If so, there is no means of obtaining power for a second field coil, nor would you want it disturbing the internal one, as it would affect the voltages to all the amplifier stages.

So, my vote is that the Echo speaker was passive. Another reason a 35 feet displaced remote cab wouldn't run on the EH-150 power supply (because it makes no sense). Maybe someone has, or knows someone who has a Gibson accessory Echo speaker, if one was offered by Gibson.

The guy who does my field coil reconing has seen an Echo Speaker in person...confirms it was a permanent magnet speaker.

Me saying running B+ to a remote speaker field coil makes no sense...have to qualify that my saying '...makes no sense TO ME'.

Gibson did not do that, but I found an old scanned supplier catalog that showed Jensen PA systems with field coil speakers and a pair of 25' 4-conductor cables that sent B+ and output transformer secondary (voice coil power) and presumably ground/earth to remote speakers.