Saturday, April 9, 2022

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:

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:


Monday, February 28, 2022

1970's Foxx Fuzz and Wah...... Acid trip waiting to happen!

 This was a fun one. It arrived DOA. It needed the battery clip to be re-soldered which was easy enough but then the fuzz part was barely audible. In this case it just needed a re-cap. These are reaching the half century mark so just do it already!

This is one of the non inductor wah models. I'm not terribly keen on this one myself but it does have it's own special trashiness that would be perfect in the right setting.....

Truthfully, I've yet to meet a Fuzz Wah I love but this one is pretty nasty and much better than many other makes. The octave section on the fuzz really goes there! Ear splitting but somehow has that psychedelic beauty only these old creations capture so eloquently.

Little bit of demo.....

Monday, February 21, 2022

My favorite flea power amp has been returned to me after theft. Thank you!

 This is a good story with a happy ending.

In June 2021 my favorite amp was stolen from my van in Brooklyn. I played Russian roulette with my gear many times, it was buried under blankets but on this night I'm pretty sure I was being watched by someone in McCarren Park as I locked my van up for the night. 

I discovered the theft on Saturday as I was picking up a friend. I noticed the passenger side door lock had been tampered with. The funny thing was I parked in a no parking zone for two whole days! So naturally I was annoyed but I pretty much accepted the loss. I did file a police report and unlike me I didn't have the serial number. All I had was a good description.... I put the oxblood bakelite knobs on myself and the smooth sky blue pilot light was my addition too. 

I was annoyed but what are you going to do? In a city of 10 million folks I'm a very low priority for the NYPD. So, that's the way it goes. I did what I could including posting photos on the social media that I have and I wrote to all of the NYC guitar stores I could think of.

So fast forward to a Friday in December and a good friend in Baltimore texts me with a Reverb listing and he asks me "Is this your amp?" 

Yup! I could see my signature on the filter can in the back and it even had the pack of DR Pure Blues strings still in it. The seller didn't seem to know too much about it but he had a years long history and knew that it was a rare amp. I have a feeling he may have seen it on my blog here since I was singing it praises. 

His asking price: $2000! Local pickup only.

So all of the emotions came popping up right away. The stuff I didn't really allow myself to feel back in June. Disgust, violation, rage..... I thought about setting up a meet and showing up and just taking it. But I figured at $2,000 it's not going to go anywhere fast!

So I contacted the Greenpoint precinct. I figured that was a long shot and it was. A petty larceny in NYC? The cops literally have more important things to do. They did get back to me on that Monday but by then I had already gotten it back.

How did I do it?

My gut told me the seller was not the thief. So I simply wrote to him and said "Wow, you found my favorite amp! That's amazing! I would like it back now as it was stolen in June."

I did my best to appeal to his higher self and it worked. I presented photos and social media posts in my first message. He pushed back for one half of a sentence then apologized profusely. He bought it at a table sale in east Williamsburg for $40. He showed me the text exchange he had with a friend about his lucky score.

Within an hour of contacting him he dropped it off at Southside Guitars where I work once a month. I paid him that forty bucks as a finder fee and while I know I didn't need to do that since he was selling a stolen good, I was happy to pay him. Why would I do such a thing?

If I had found a cool amp on at a sidewalk sale I would have done exactly the same thing as him. In fact that is part of how I make my living. It's survival. Sometimes you win, sometimes you lose. There is little to no accountability in our eBay Reverb world for hot goods. 

When I worked at a shop in Portland two street drug dealers brought in an old saxophone.... I could see it from across the room and knew it was something I could get $2k for. Rare and desirable. The store turned it down so I asked permission if I could buy it. Me and the dealers stepped outside and they told me it plays nice to which I replied "bullshit. I know you don't play so don't try to play me. How much fellas?" They said $40. I took out two Jacksons and sent them on their way.

The horn had been neglected for years. No case, badly tarnished but in good physical shape. I ran it through the Portland Police Department second hand goods three times (I think that was 6-12weeks?) before I had it fixed up to sell. No one seemed to be missing it. I won that time. I could have just as easily lost.......

And I remember the pawn detectives. I think when I started working at that shop the Portland PD had 3 pawn detectives. It was reduced to one soon thereafter.... one moved to Arson and the other was moved elsewhere. That's too big a job for 3 people let alone one.

So for the fella who was trying to sell my amp: Thanks for doing the right thing! It's a good way to live.


DOD Stereo Chorus Vibrato 565-B pedal. 80's gem!

 Ok.... another pedal. This one was fun. It belongs to a good friend and it's been with him since the 90's. It came sounding rather weak and with a lot of crackly chunder going on. So I put on "The Gentle Side of John Coltrane and in 4 sides (just over 1 1/2 hours) I re-capped it and cleaned it thoroughly. I also changed the LED as the original was going dim. We went with blue cause... why not??

The re-cap was pretty easy except for the nasty double sided circuit board. But the nice thing is I could remove the whole thing just by disconnecting it!

It's funny. I've never worked on a DOD. Financially speaking I would say no unless you love it. These are still cheap as chips. That being said, once I plugged this in to my two baby Magnatones for that stereo glory I found I love this box! Instant 80's inspiration! 

Will I buy one for myself? Doubtful but if I wanted a chorus box this would likely be the one for me! 

Good times baby!!


Monday, February 7, 2022

1973 Mu-Tron Octave Divider, fun!

 Got lucky with this one. I've been taking on some pedals at Southside Guitars. I kind of enjoy them.... it's nice when you can find a schematic and even nicer when you can get them to work!

This one is a 1972 Mu-Tron Octave Divider with a cool 70's Miles Davis era ring modulator function. When it arrived it only half functioned (bass only side worked, the rest did nothing!)

I got lucky. I did the routine: changed EVERY electrolytic and tantalum capacitor and cleaned everything thoroughly, touched up the solder joints. 

And yes! it came back to life. No IC changes necessary. But then two out of the three LEDs died!!! So I ordered a kit o' LEDs and replaced all of them. It's ready to go cut tracks in your Miles inspired band or EDM or whatever you wanna do! Go on and get your creativity on! If you buy this one it's a good one. Y'all know me, I'm a strong proponent of changing out those electrolytic caps (exceptions are old fuzz boxes that sound like the customer want's them to sound.... gotta wait till those die otherwise you'll get that "but it had this thing before!!)

Funny demo. I don't know what to do with this one!!

1969 Park 75! Rare as hen's teeth yo!

 This was a fun one. A very nice Park 75 from 1969.

In my 32 years of doing this I've had the pleasure of working on exactly 3 Park amps from the old Marshall 60's plexi era. I did work on a 70's one at some point but that one wasn't as memorable.

The first was a Park 45 a guy brought to me in Portland about 20 years ago. He had just bought it at a pawn shop in Vancouver Washington for $300. A wild man apparently had dropped about a dozen ancient Marshalls there so I dutifully drove there right away and bought several. They were all in various states of dis-repair but little things like, in the Park 45 the slope resistor had been removed.

That amp with it's KT66 tubes left an impression! I wish I was the lucky man who popped in that day. 

The second was a Park 150 I bought from a shop in downtown Portland for about $500 or so around the same time. That one didn't thrill me as much but it was pretty fascinating. I recall it being somewhat like a Marshall Major but far more interesting. It had an early "overdrive" channel and was just too much amp for me. Think Ritchie Blackmore. I did like that channel though, more primitive than the few Laney Klipp amps I've seen. That Park didn't stay with me long, I re-capped the power supply and flipped it like a burger. 

I do know that Park was built by Marshall, but with subtle circuit changes as to not totally compete with Marshall. The coolest thing about that 150 is there isn't a Marshall like it. He seemed to be in a groovy experimental place building that one.

Then last week this beauty came across my bench. 

As far as I could tell, and I didn't bother with too much analysis as it was a "get it ready to sell" job and it was already in well maintained great health, it's just a 50 watt Marshall with KT88 output tubes and different cosmetics. Beautifully built inside and out like a good hand wired Marshall is. 

The cabinet is loaded with Celestion "Greenback" speakers and it's a cool bass cabinet. It sounds exactly as one with expect with a good Les Paul: voice of God or Satan depending on what style you dig. Or the voice of good LSD if that's your bag! I'm a big fan of these early Marshall amps. Nothing sounds like an amp pushed to the brink! Master Volume is great as this is too loud for me today but that being said, nothing beats the real thing! 

I thought the JJ KT88 tubes that were loaded in this one sounded pretty damned good for modern tubes. Of course I would like to hear it with the OG Genelex glass but.... that's a lotta dough.... Glad these were pleasing. Thumbs up.

This amp already had all of the filter caps replaced in the 90's. No hum, great bottom end, no need to change anything. I like it when they show up like this. In and out in no time at all. 

Monday, January 24, 2022

Electro Harmonix Double Tracker


Second box of the day. This one arrive DOA. Someone had boogered it up inside with some shoddy tech work and ya know, why make an EH pedal worse than it already is? These charmingly janky boxes that do a good job of falling apart on their own.

I got this one working by re-wiring the battery cables. The switch was intermittent so I replaced that and it worked brilliantly for about 3 minutes then POOF!

Power supply issue. I needed to replace the 2n4302 FET in the power supply. They are rare and spendy. I first tried a recommended substitute but that failed after 10 minutes. I replaced all of the electrolytic caps with the same values and replaced the tantalum cap with another tantalum. I also replaced the 13v zener diode (this pedal is 18v, takes two 9v batteries.)

I don't find much joy in working on janky pedals but it is nice when there is success and I've learned something.

How is it? It's a two trick pony.... slap back and faster slap back plus blend. That's it. I can do better with just a delay box. However, it sounds wickedly trashy! If I had a studio I would own one of these. I'm not sure there is anything else that quite sounds like this box.....