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:
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.
A bit of demo:
J
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