1936 Gibson EH-150 and a story about parasitic oscillation and lead dress.

 

This one was fun. I came out of semi amp fixer retirement to tackle this beauty. It's a very early Gibson EH-150 built around 1936.

My client got it a while ago and it's been sitting. So he contacted me with some photos and details and it piqued my interest enough to go for it.

I have a thing for early amplification. This amp is so simple. Other than the two cathode resistors there are no other resistors, not even in the power supply. And other than the two cathode capacitors and the filter can there are no other capacitors, none in the signal chain at all. Instead of coupling capacitors you simply have a driver transformer which has this beautiful copper shield around it. 

It's a big piece of iron, around the same size as the output transformer. Rather than having any power supply resistors it has only chokes, one on the field coil speaker and the other inside of the chassis. 

The tube compliment is ultra simple: one huge 6A6 tube, which eventually would evolve into the 12AU7, and this drives a pair of 6V6 tubes and it has a 5Y3 rectifier. I'll call this amp version 1B. I know version 1 had an 80 rectifier and 6F6 tubes driven by that lovely 6A6. 

I love the tube shields.... GOAT. I mean, come on. It looks like a sarcophagus and it's stamped GOAT!

After inspecting and cleaning the speaker I determined it is okay, which is good since I don't wish to ship it off to be rebuilt. Field coil speakers have a lot of wires..... It had a nasty distortion that was cleaned up by removing the thick layer of 70 years o' dust on the bottom. Sometimes if you are lucky that is all it takes. 

Before my client dropped this off he mentioned he had not seen any of these with this knob before. I have strong reason to believe this amp left the factory without a volume pot. That is not all that unusual on such an early amp. I worked on a primitive Gibson like this, link here:

http://leightonamplifiers.blogspot.com/2018/11/ancient-gibson-amplifier-circa-1930s.html

I've also worked on some primitive Rickenbacher amps that were all metal and built with no volume knob. And yes, I spelled Rickenbacher correctly, that is how the name was spelled back then. 

So anyway this amp had a peculiar problem. When you turned the thing on it made a high pitched squeal like a bomb falling followed by a loud pop. The squeal would go away if the volume knob was turned all the way up or down, but the pop would come and go and with the pop went a good amount of volume. I tested all of the tubes and they thankfully tested very strong. 

So if looked at the next culprit and heated up every ancient solder joint. The two on the driver transformer did need help but that squealing and pop issue did not get resolved.

So issue number three: That volume knob. It's an ancient 500k pot that had a 500k resistor across the wiper and ground terminal. Something looked a little fishy to me, like a cover up..... Then it dawned on me. This is a classic parasitic oscillation issue. I took my plastic chopstick and the squealing increased or decreased by moving the signal wires around. So I replaced the wires with some boss looking Gibson shielded cable for integrity. Most amps I would use Mogami but this one I'd rather use some period correct looking stuff!

I did replace the badly tarnished input jacks as well, they couldn't be saved. When I replaced them I also sanded the chassis and used tooth washers to make sure the ground is tight. I wound up removing that 500k resistor as well, it's not needed now and only robs your signal a bit. I believe it was added to try to tame that oscillation. The kind of thing one does before gaining experience at this. I'm sure I would have tried the same thing at one point if I were pulling my hair out!

So there you have it. Even in an ancient amp lead dress is important. Very important! And while you don't always needed shielded cable sometimes that is what solves the problem.

Now bear with me here. I want to share a story about a modern boutique amp that came across my bench around 2005 or so that had an identical issue. It was a "Cowtipper" model, basically a Fender Twin Reverb in a head clone. I'll just go into it.......

My client, who is someone I count amongst my many friends, bought this thing and had it shipped. When it arrived there was no sound so he called me right away since he was a professional musician and needed this thing to be right. I opened it up and gasped. The no signal problem was easy as pie to track out.... most no signal issues tend to be easy, and in this case it was a resistor that got broken in shipping. It was the 68k input resistor. Why did it get broken? The wire used was that heavy 18 gauge green cloth wire used for heaters. It's solid core and not the least bit flexible. I pointed this out to my client who was reluctant to replace that piece of wire since he was led to believe the manufacturer was some kind of guru who would only use the best part for the job. This is the input wire and it's not shielded. Not a big deal, old Fenders didn't have shielded input wire until the early silver panel amps.....

But then I noticed two more issues. One was the same wire was used in the power supply. It was on the standby switch and UNSOLDERED..... Just a mistake. I've had a white Fender Bandmaster come across the bench with one junction that never had solder in the tremolo circuit and it worked for 40+ years. But that's the tremolo circuit. This was the main power line with 450+ volts sitting there!! It was making contact and well wrapped, it was just never soldered. So I pointed this out and soldered it.

The third thing was this very strange arrangement of the signal wires. One was the input wire which was hot glued to the wire coming off of the volume control (first gain stage). They were wrapped in some kind of foil and hot glued to the top of the tone capacitors. The foil bothered me since it wasn't grounded. It looked sloppy and I wondered why it was there. Was this an attempt to shield it?

So I pointed this out to my client and said "my man, I hate this arrangement. Please let me change it! It's trouble down the road!" 

He said no, it's gotta be there for a reason. This guy is like an amp god so don't mess with the tone! The amp was working and sounded like an amp so I left it at that.

He took it home and a few hours later he called me in a panic! "Jef! I have the amp on 10 and my neighbors aren't complaining! It's as loud as an acoustic guitar!" This was an 80 watt amp..... I knew what the problem was right away: PARASITIC OSCILLATION... due to that awful wire arrangement. 

My client came by first thing the next morning and I shared my thoughts but he didn't want me to touch it. He said the builder was going to call me that day to "talk me through the process." 

Ooh, I was looking forward to that call! But it never came. I looked at that amp collecting dust every day for a week, and never got a call. My client was anxiously calling me daily and was losing faith in his guru. So I got sick of looking at this thing and opened it up one morning and hit that weird wire foil arrangement with a chopstick. BOOM! Louder than I like it. Exactly what I thought. I took all that bullshit wire and tin foil out and installed some shielded cable and called my client. He was thrilled and asked "so he called you???" I said no, he never did but the problem was simple and exactly what I thought. 

It was a really good amp after that.....

Moral of the story is, all of this is a learning process. Fender made mistakes, I still make mistakes and whoever did this little Gibson amp made them too. If that builder had called me he might have learned something that day, and I bet this problem didn't show up in 95% of his amps. Lead dress, grounding, these are things that vary from amp to amp. None of it is set in stone. Only experience reveals the solution. Trial and error and being able to admit we don't always get things right.

Oh yeah, how does the EH-150 sound? I think these have a beautiful voice. Very rich and clear. Nothing at all like what one associates small tube amps with: it doesn't distort, rather it sings. I would love to build one but I mean, does anyone make a field coil speaker today? I've yet to meet one I didn't like. They are limited, this isn't an amp I'd use to rage out on, it is one I would use at an acoustic session for low volume situations. That warmth is unbeatable. 

A bit of demo:

J

Bronson Amplifier circa 1949, in the original box with the matching lap steel

 Today I got to see a stunning piece of history. This Bronson amp dated 1949, still with the original shipping box. It spent some of it's life in New Mexico at the Aloha Conservatory of Music. Apparently this school was affiliated with the Bronson brand. I would love to know more! American musical instrument manufacturing is a fascinating subject for me. Especially when it is tied to schools. 

Nothing to really write here, the amp isn't NOS, but it's as clean as one can be for a 72 year old amplifier. The customer wants to use it so sadly I needed to change every capacitor in the house. All of the wax caps were leaking badly (yes even with little use, they likely leaked right out of the Valco factory!). And I also replaced the electrolytic caps and the power cable at the customers request. Easy job. These are so well made. 

So the insides are all "before" pictures for your amusement. 

How does it sound? We happen to have a lovely old Kalamazoo model A with a real Charlie Christian pickup. Lovely combo. I'm a fan of a good field coil speaker which this has. Nice low end.

The filter cans are as follows: 40uf (big for the time!) and a dual 10uf can for the preamp and screens. 

Tubes are 5Y3, 6V6 pair, 6SC7 phase inverter and 6J7 pentode for the preamp. I'm a big fan of this arrangement having built a few amps with a 6SJ7 preamp into a 12AX7 or 6SL7 phase inverter. Big, warm sound with nice low end. 

Valco knew how to build a good amp. This thing runs pretty quiet. And it's a nice thing to look at. Easy on the eyes. An amp should be able to double as furniture so it doesn't get stuck in the closet when not being used. Potted plants should go near it alone with your sofa. These manufacturers had style back then. Not an ugly rock box or just some industrial looking thing that belongs in the garage. This is something you want to relax with and play some sweet music through. 

J 

1964 Fender Deluxe Reverb. Re-capped but still too much hum. What can I do?

 Here's the desert island amp for many of us and I think after all these years I'm one to be on that island myself. A really clean 1964 Fender Deluxe Reverb. 

About the amp: Transformers are original, speaker was replaced with a Celestion G12M greenback 30 watt. I'm a fan.... Amp was re-capped well enough. All of the electrolytic caps were top shelf expensive Sprague Atoms. Some of the work wasn't exactly tidy but done well enough.

But, I just didn't like the amount of hum I was still getting. I right away saw some things to improve. First is the bias supply cap:

Whoever re-capped this amp went with the original value. I mean, why wouldn't you. That's what Fender used in 1964 so.... 

Nah. When I worked for Jeff Bober in 1994 he just told me to upgrade it to 100@100v rather than 25@50v. I've been doing it ever since. Better regulation and less hum. I do not detect any change to the sound of the amp and I have well trained ears. And for what it's worth Fender upped it later to 70uf then 100uf cause they could. Caps got a little better. So just do it. If you think that's blasphemy try it anyway and put the old value back in if it freaks you out.

Yup, being dramatic here but I'll share a story. I once had a client who paid me to convert an AB165 Bassman to an AA864. That's no small task. Big job. I made one mistake and one executive decision. He took the amp home and inspected the work under a microscope. The next day I had an irate customer who brought his amp back and he (understandably) expressed his trust had been burnt. So I gladly changed the one pot value that I missed to the correct one. But he was really upset about my little executive choice: using a 100@100v for the bias supply. I explained to him why I did this: with a 25@50v cap it will hum. He didn't believe me one bit and I could tell I was losing his business.

So we gave it a listen, I then replaced it with a 25uf cap.... HUM! And not a teeny tiny difference. It was significant enough that he heard it and looked a bit embarrassed. He asked me if it would make a tonal difference and I said no but if he thinks it does bring it back in a week. I put my little 100uf cap back in, fired it up and he was happy with the quiet. I never saw him again. All is well that ends well!

So here ya go. Use a pretty blue Philips Vishay cap if you will! I like them cause they look nice and European:

But wait! We're not done yet. Amp was much improved but I still wasn't 100% satisfied. This next part involves the signal caps.

If you have been inside as many old Fender and other high quality amps you'll notice even the signal caps have a direction. No they're technically not polarized like an electrolytic cap is, but there is a direction you want them to go in. The have an inside foil and an outside foil. Some old caps like my favorite Ajax blue capacitors you find in Fender amps even tell you which side is the outside foil.

Rule of thumb: always have the outside foil closer to ground. In other words, if one side of the cap is on the 100k plate load resistor and the other is connected to the 1Meg volume pot, outside foil goes on the 100k resistor. Fender almost always got that right! I've only seen a few exceptions. 

Another example is the tone stack. The .1 bass cap goes to a 250k pot and the .047 mid cap goes to a 6.8k resistor. On the other side is the 100k slope resistor then the 100k plate load resistor for a total of 200k. So the bass cap should have the outside foil attached to that slope resistor and the mid cap should be going the opposite direction with the outside foil connected to the 6.8k resistor on the bass cap. Like this:

The phase inverter has three .1 caps. Two are connected to the plates of the 12AT7 while the third is connected to the grid. On the plate load side you have a 100k and an 82k resistor going to each plate. On the other side you have two 220k resistors going to the bias supply. So the outside foil goes to the plates, inside to those 220k resistors. The third cap however is connected to a 1meg resistor on one side and a 47 ohm resistor going right to ground on the other side. So that cap goes in the opposite direction. In this case the previous owner used my least favorite cap, the Sprague 715P. No indication for inside or outside foil so you just read it from left to right. It will look like this:

Notice how the print is going in the opposite direction.........

Here's the dry channel tone stack. I had a spare old polyester orange drop cap that has a stripe on it for outside foil. The mid cap you'll notice reads left to right with the left going to the 6.8k resistor. If you are experienced you'll notice the .047 600v blue Ajax cap for the final stage. Sadly entire channel was orange dropped. But I have a bag of .047 Fender "Death Caps" from the AC balance. I never throw these out when I ground an amp. I save them for classy amps like this one!

One cap backwards will not make all that much of a difference but in this case I counted 5. It adds up. In the hundreds of old Gibson and Fender guitars I've seen over the years, the tone caps are almost always done in this arrangement: outside foil to ground. If the good people at those companies were sticklers for such a detail on the treble bleed cap, there's something to it.

So there you have it. The amp behaves like my own '69 model now. Quiet! Of course it's not possible to eliminate all hum and noise but it is possible to reduce it using these little steps. And the less noise the more music dig?!

Thanks for reading, hope this was helpful and feel free to leave a comment of ask a question below! J

1956 Voice of Music8 watt amplifier

This was a fun one. 1956 (or so) Voice of Music 8 watt amplifier. I like that it's just called "8 Watt Amplifier". No fancy school names, nothing clever to shift more units of these babies!



Complaint was it's dead! Lights on but nobody home. That's a good thing. Always easier to fix a dead amp than one with annoying intermittent problems. In this case all those beautiful Mica Mold capacitors were just plain dead. I also replaced the filter caps and added a cap to the 300 ohm cathode resistor on the 6v6 output tubes. This helps low end response while also helping tame some hum away. So it ain't on the schematic. Who cares? Let's improve it!

Super cool slide out chassis:




10" Jensen Alnico 5 speaker! Worth the price of admission.




Some shots of the not 'mil spec' bowl o' spaghetti design:



Mica Mold Capacitors! I'd love to hear a fresh set, never have. They're almost always leaky. One read a whopping 114 vdc on the potentiometer side!



But either way, they sure are pretty. I replaced them with a set of Mojo Dijon for the preamp, and Sprague 225P polyester Orange Drop caps for the phase inverter. An amp like this can benefit from that setup, very clear and warm, but not mushy.



Shot of the filter cap. I upgraded the 470ohm 3 watt resistor with a 500 ohm 5 watt. Also used 2 watters for the 10k power resistors. I like to never see an amp again once it leaves so why not?



Cap dated 47th week of 1956!




Schematic included! I love that. No guesswork if something is burnt.





I love the speaker connectors.




How does it sound? Perfect for what I like. It's not your Neil Young rage box, it's subtle and sweet. Very open and clear sounding. This was never intended for guitar, it's a PA amp. It's the kind of thing I'd use in my living room while someone is playing a big Gibson J45 or  Martin D-18. It gets only slightly louder than one of those at full strum. Perfect accompaniment amp.

Single 5Y3, pair of 6V6 tubes driven by a 12AX7 and a 12AY7. Same tube compliment as a Tweed Deluxe but completely different sound due to a completely different circuit. So if you like something fresh and unusual, seek one out! This is a lovely amp to look at as well as play. And I like nice looking things in my living room. 

-J

Little Silvertone 1471. 5 watt killer!

Another gem from Indigo Ranch that's been sitting on a shelf since the 90's! This belongs to Sonic Circus as well.




It was totally dead, ripped mouse eaten speaker and all. Found a cool Utah to throw in, re-capped it and it's making music again.

Circuit is similar to your classic tweed Princeton (Champ with a tone control) and it's got a great early Eric Clapton type of sound. Smooth breakup, warm but good bite.. I particularly liked the low end. Smooth and creamy not unlike Billy Gibbons. I just want to play some ZZ Top on this amp.

Simple 6V6 driven by a 12AX7 with a compact 6X4 rectifier.

Here's a funny video of it's first few notes after 30 years of being mothballed.

https://www.youtube.com/watch?v=sumDxmeaeP0



And some photos!











Planet Capacitor, guaranteed for 1 year!!!





So there you have it. These are cheap as chips. Yes, built rather crappy but who cares. Just don't let it get wet as they are made of pressed paper! Fantastic tone. Who says you need a pine box to cut great tracks anyway?

j

1950 (or so) Gibson BR6F

The first good small tube amp I bought was a Gibson BR6F. I found it at Guitar Exchange in Catonsville, Maryland back in 1992.

I....love....these....amps!

6SJ7 driving a 6SN7 phase inverter feeding 2 hungry little 6V6 tubes. 5Y3 rectifier. 10 watts into a 10" field coil speaker.

A word about the field coil speakers. They often sound lovely. Don't do what I did to mine: run a bunch of pedals into it and blast off for hours. I killed mine and once that speaker is dead, that's it. It'll never be the same. You want to treat the elderly nice and gentle, with respect. Leave those dumb ass pedals in the bag and learn to play better!

I probably could have had that speaker rebuilt, but I blew mine up in 1995. No internet in those days and tough to find someone to rebuild it properly

If you do blow it up, check these cats out:

http://www.thespeakershop.com/home/speakerrepair.html

his one was at Southside Guitars. It's long sold. I re-capped it and replaced the power cable. I hope the new owner just plugs in and plays it!

And a video of my friend Eric playing it.....

https://www.youtube.com/watch?v=_PUzJnwjELg

The amp:

40's - 50's Rickenbacher "Electro" amplifier

Here's an extremely rare amp. An old Rickenbacher metal amp from the 1940's.

No idea what model it is, there is one that looks like this called the M10 but this has a 12" Jensen PM speaker rather than a 10".

It looks like a space heater!



With this speaker grill and metal box we were expecting a not good sounding amp! Though the circuit is oddly similar to my main amp that I built: 2 6V6 tubes fed by a 6N7 phase inverter with a lovely 6SJ7 preamp. 5V4 rectifier tube.

What does it sound like? This was the closest we could get to the AC/DC sound. Raw, raunchy and open. I'm usually not a fan of the old Jensen PM speakers, too mellow for me, but not this one! It was perfect!

Re-capped it, changed the power cable and took out the death cap. This was another Indigo Ranch Studio amp that had been sitting on a shelf in Vermont for the last 2 decades. Now it's back in service.

I love that it lived in Cumberland Maryland and belonged to "Jaguars International"! I'm a Maryland native. I just wonder what music was made on this peculiar amp?





















Matching grey power transformer!







I'm a fan of the 6SJ7 preamp tube sound. They're cheap as chips for a good NOS one. Lots of harmonics and a big open sound. This is an amp I'd like to find another of for myself. But then again I can always visit my friends at Sonic Circus and cut tracks on this one anytime.

J