Amps Amps in the shop currently

CapnDenny1

Student Of The Blues
I just got this in on Friday, a Marshall JCM800. Has some weird dropout issues. He showed it to me, but it was weird, so not sure what is wrong yet. Should be a rocker!

IMG_3775 by Dennis Kelley, on Flickr

I just finished a bass combo, an Ampeg BA115HP, the 220W version, 15" bass speaker, and a horn tweeter. A got it from craigslist, from a guy who converted the amp into sa speaker by removing the mp, so he was selling it as a 15" bass cab for $50 or something. I asked if he still had the broken amp and he said yeah, so I gave him another $20. It had a shorted op-amp, a $0.75 part. Of course I did something dumb and blew all the main transistors out! But that was only another $10. It sounds nice. But I put it on craigslist for $300, probably drro it to $250.

IMG_3761 by Dennis Kelley, on Flickr
 

CapnDenny1

Student Of The Blues
And lastly I have an ebay find, an Egnater Rebel 20. They said they think it has a blown transformer. I am hoping they are wrong, that's big $$. So far the transformer seems OK, but I haven't found it yet. The bolts on top are to fix the broken cabinet with a brass plate inside. I may fix that a little nice, and get rid of the Frankenstein look. These amps are really packed in there. They have a lot of stuff in a small space, and lots of wires. this eis the little brother to my Rebel 30, which I love. So it should sound good once I get it going.

s-l1600 by Dennis Kelley, on Flickr

I take these pictures from a lot of angles before I touch anything. Otherwise I would be lost to put all the connectors back in the right place. You have to take them all off and remove the boards to work on them, then put them all back in again to test them. Then repeat, with your next guess.

EgnaterRebel200011024 by Dennis Kelley, on Flickr
 

Shodai

Blues Junior
My dad tried to get me into electronics when I was a kid, but it never made sense to me. Still doesn't. I'm kinda of envious that you can do stuff like that. Very cool.
 

CapnDenny1

Student Of The Blues
So I started in onthe Rebel 20. I had to pull it all out and back in 5 times. It took me 4 hours to find the culprit. One of the rectifier diodes was failing at a voltage oer 20 volts in the direction that it should block 1000 volts. That was tricky because any measurements with an ohmmeter read that it was ok. So far it needed a new 1N4007 diode, costing about $0.25. Kind of an unusual failure mode.

I will have to find 7 or 8 tubes. It has 3 or 4 12ax7’s and a pair of 6v6 and of el84 tubes.

It doesn’t appear to be anything wrong with the transformer. Phew!
 

CapnDenny1

Student Of The Blues
I replaced the bad diode and after I re-assembled it I noticed the bias sense resistor was damaged. It readout as 4 ohms and should be 1 ohm. So took it all apart and replaced the resistor.

I went ahead and put tubes in and surprise, it works! I adjusted the bias on the tubes,and plugged in and played a bit.

So now, on to the Marshall!
 

CapnDenny1

Student Of The Blues
It looks like the JCM800 Model 4010 is a tube socket issue. I checked it out and the HI and LO inputs are not merely a 2:1 ratio like on a Fender, but on the Marshall JCM800, at least this model, the HI input is run thru an extra gain stage ahead of the LO input. In fact it routes the signal to the other half of the first 12ax7 tube, and then right back to the input jack. So it's basically a 2 channel amp, but without a relay or any switches. I was able to wiggle the tube and get it to drop out. The reason it is much worse on the gain channel is that it runs through that bad socket twice when plugged into the HI input. Kind of a clever way of having 2 channels.

Some models are more like a Fender input, and just have a slightly larger signal on the HI input. From what I read, this is the preferred method. It's a wildcat allright! So when buying one of these Marshalls I guess you need to be careful about the chassis model, not just the JCM800. There seem to be about a dozen different JCM800's?

I will change out the socket and see if that fixes it.

Oh yeah, I also found a potentially really bad issue. The speaker cable had a 1/4" phone plug that goes into the amp chassis. But it looks like it was smashed and kind of broken. As a result it would and would not make contact with the ground side of the plug. So that means it was probably opening up and having the amp have no load at all. That is a good way to ruin an amp. It causes the output transformer to generate extremely large voltage transients. They can be big enough to arc through the transformer's insulation from winding to winding. Once it does that it forms a path and it's easy for it to do it again, even with the speaker connected. A lot of amps have a diode across the output to protect the transformer if the operator tries to run it without a speaker load. This amp doesn't have that. I won't know if it's OK until I fix the socket issue. I hope for the customer's sake that the transformer is OK. An output transformer for this amp is probably between $100 and $200, and that's for an aftermarket one. If he wants a period correct Marshall one, it would be even more. So cross your fingers?
 

CapnDenny1

Student Of The Blues
Not so fast. I hanged the socket and installed his new caps. There is still a problem. I couldn’t get it to mess up by wiggling the 1st tue anymore, so the socket issue is fixed.

I decided to check the bias setting. I measured 1mA on both tubes? Yes, that was 1mA!? It is supposed to be around 35mA for this amp. I tried adjusting it and at the minimum bias voltage (least negative) of -40v I read a bias current of 4mA.

Perhaps I was doing something wrong when measuring the current, but I don’t think so. I have seen where people have had to adjust the bias voltage circuit to allow it to go to -35V in order to get it biased. I will try that and see if that helps. The tubes we have now are different from back in the day. I may try another set of el34 tubes just to see where the are.

What amazes me is the amp kind of works. It doesn’t sound good, but I am surprised it even works with 1mA of bias current?

Here’s an inside view of the mighty Marshall.

1B33AF11-B046-405E-A203-DA0A618327BC by Dennis Kelley, on Flickr
 

CapnDenny1

Student Of The Blues
I made some progress again. I determined that the reason the bias can't be adjusted for the EL34 tubes is that this amp was designed to have 6550 power tubes. They need a bias voltage of -50V instead of the -36V the EL34 meeds. That's approximate, but there wasn't enough range as it was. There were a couple of resistors that changed between setting the amp up for el34 and 6550, at least terms of the bias voltage. There is a 128K resistor that needs to be a 220K resistor. I didn't have that value in a size that could handle the B+ voltage, so I put a resistor in series with the existing 128K. That allowed me to get the bias current up about 33mA, which is about 60%. The amp sound much much much better now.

But it still has something that happens shortly after power up when using the gain input. It drops in volume and is barely audible. Turning the Pre Gain causes a lot of distortion, or just strumming the guitar hard makes it not work. It has to be in that first stage because it doesn't appear the LOW input has the issue. Here is the preamp section of the schematic.

preamp by Dennis Kelley, on Flickr

You can see the HI input goes thru the V1a part of the first tube, where if you connect to the LOW input it goes directly to V1b. Since this problem doesn't happen when plugged in the LOW input, it seems that the problem is the V1a section. I suspect one of the two caps connected to that tube are the issue. Problems where this only last for a short time are hard to actually catch in the act. So I may just replace those caps and see what happens. Next I will try the resistors. I think I've tried different tubes there, but I will try that also just to be sure.

So i will need to contact the customer to make sure he wants to use EL34 tubes in the power section, instead of 6550. If so I will put the right parts in the bias circuit for those tubes.

It's getting there. This is one of the first Marshall amps that I actually really like, other than the DSL40C that I fixed a while back.
 

CapnDenny1

Student Of The Blues
I am 99% sure I found it! It seems that it was the connection thru the LOW input after the 1st tube stage. When you plug in to the LOW input, you are connected directly from your plug to the amp’s jack, so it doesn’t go thru the jack. It was real sensitive to any touch with a wooden cleaning stick. So I de-ox’d it and scrubbed it a bit. Now it’s rock solid.

So once I get the 220k resistors I will get the bias circuit fixed, and then set the bias for real. I verified that the customer does not want 6550’s, so I will change the bias cktto what Marshall uses in an el34 amp.

The customer claims he biased it before he brought it over. I told him it could not be done with that circuit. He thinks perhaps he did something wrong. He said at firstit wasn’t working so he switched the meter to 2000 an then he got a reading. I don’t know what he rea, but I don’t think it was mA?
 

CapnDenny1

Student Of The Blues
Just to follow up.

I received the 220K 1/2W resistors from Mouser and installed them. There was a 47K that should change to a 56K, but I was able to set bias with only changing the 220K, so I left the 47K in there. I set the bias right at 35mA.

I removed all the pots and input jacks to get access to the bottom of the pcb to change the parts. So I did a thorough cleaning of the HI input jack along with the LO input.

Put it all back together and played it for a while. That is one LOUD amp! I had my Strat into the HI input with the Pre Gain at 50% and the Post Vol at 30% and it was about as loud as I could stand it sitting right in front of it. The amp sound pretty good, but I just don't care for that Celestion speaker with the cream colored magnet. A nice Eminence Texas heat would really sound good in this amp, or perhaps a Tonker which is more Vintage 30 like.

So customer is coming by today to pick it up. I told him total was $120. 4 hours labor, $20 in parts.

Here is what I did:
1. Replace all 6 electrolytic caps. 1hr
2. Troubleshoot noise isue, replace tube v1 socket. 1 hr
3. Debug low bias issue and fix by replacing 220K resistor. 1hr, $5 shipping.
4. Troubleshoot noise, clean input jacks as culprit. 1hr
5. Replace speaker plug. 1.2hr, $10 parts.

I probably actually spent about 6-8 hours on it. But some of that was me spinning my wheels. I don't think it'sfair to charge people for time sitting there with a dumb look on my face trying to figure out, or how to go about figuring the actual problem. A lot of that time is spent doing other things like watching TV or staring out the window wondering.
 

Jay Dee

Blues addicted
Capn you are awesome! I wish I was closer to you. I have a Crate Palomino V 32 that needs some help.
 

CapnDenny1

Student Of The Blues
Aw shucks, cut it out!

Those Palomino's are nice. The V series Crates are designed by Obeid Kahn, who plays in a local band called Wrath of Kahn. Great hard rodk, blues player. He wanted to play guitar for a living and his parents said, no you are going to college. So he got a degree in Electrical Engineering and got a job in St. Louis at St. Louis Music, who made Ampeg and Crate. On the side he was playing in his band, and modifying Marshall amps to improve the sound for people. His company found out he was playing in a band and how good he was, but saw that he was using a Marshall amp. They said, "Hey why don't you use one of our amps?" he told them, because our amps sound like crap!

So they told him to meet a certain cost target, and design a series of amps that don't sound like crap. And the Crate V Series amps were born. Awesome sounding well built amps. But they have Crate on them, which is known for cheap crap pretty much. So the V Series amps go for way less than their sound should make them worth.

I think the Palomino series, with the white Tolex, and gold faceplate and knobs are just awesome looking amps. And being part of the V-Series they are also awesome sounding amps.

Here is a video of Wrath of Kahn in the studio. That's Obeid on guitar. he is playing through a line of amps he did. He made a 30W tube amp portable! Those small amps are his design. he made a tube amp small and lightweight, but kept it as a tube amp. great sounding little amps. I think his company went out of business unfortunately.

https://www.youtube.com/watch?v=AW2j_hlYVGg
 

Jay Dee

Blues addicted
Aw shucks, cut it out!

Those Palomino's are nice. The V series Crates are designed by Obeid Kahn, who plays in a local band called Wrath of Kahn. Great hard rodk, blues player. He wanted to play guitar for a living and his parents said, no you are going to college. So he got a degree in Electrical Engineering and got a job in St. Louis at St. Louis Music, who made Ampeg and Crate. On the side he was playing in his band, and modifying Marshall amps to improve the sound for people. His company found out he was playing in a band and how good he was, but saw that he was using a Marshall amp. They said, "Hey why don't you use one of our amps?" he told them, because our amps sound like crap!

So they told him to meet a certain cost target, and design a series of amps that don't sound like crap. And the Crate V Series amps were born. Awesome sounding well built amps. But they have Crate on them, which is known for cheap crap pretty much. So the V Series amps go for way less than their sound should make them worth.

I think the Palomino series, with the white Tolex, and gold faceplate and knobs are just awesome looking amps. And being part of the V-Series they are also awesome sounding amps.

Here is a video of Wrath of Kahn in the studio. That's Obeid on guitar. he is playing through a line of amps he did. He made a 30W tube amp portable! Those small amps are his design. he made a tube amp small and lightweight, but kept it as a tube amp. great sounding little amps. I think his company went out of business unfortunately.

https://www.youtube.com/watch?v=AW2j_hlYVGg
Aw shucks, cut it out!

Those Palomino's are nice. The V series Crates are designed by Obeid Kahn, who plays in a local band called Wrath of Kahn. Great hard rodk, blues player. He wanted to play guitar for a living and his parents said, no you are going to college. So he got a degree in Electrical Engineering and got a job in St. Louis at St. Louis Music, who made Ampeg and Crate. On the side he was playing in his band, and modifying Marshall amps to improve the sound for people. His company found out he was playing in a band and how good he was, but saw that he was using a Marshall amp. They said, "Hey why don't you use one of our amps?" he told them, because our amps sound like crap!

So they told him to meet a certain cost target, and design a series of amps that don't sound like crap. And the Crate V Series amps were born. Awesome sounding well built amps. But they have Crate on them, which is known for cheap crap pretty much. So the V Series amps go for way less than their sound should make them worth.

I think the Palomino series, with the white Tolex, and gold faceplate and knobs are just awesome looking amps. And being part of the V-Series they are also awesome sounding amps.

Here is a video of Wrath of Kahn in the studio. That's Obeid on guitar. he is playing through a line of amps he did. He made a 30W tube amp portable! Those small amps are his design. he made a tube amp small and lightweight, but kept it as a tube amp. great sounding little amps. I think his company went out of business unfortunately.

https://www.youtube.com/watch?v=AW2j_hlYVGg


I have the white tolex with gold panel. It sounds awesome I got it at a pawn shop for 2 bills. I put a larger reverb tank and changed the tubes. The pots need to be replaced but I have never soldered on a pcb :cool:
 

CapnDenny1

Student Of The Blues
The JCM800 is back in the shop. I had a hard time remembering what I did with it the first time? I actually searched on here to find it.

I am going to start doing a better job with invoices. Then I can search them based on the amp or owner. That way I have a record of what was done.

The customer said he wasn't happy with the sound. He connected up his bias probe, and measured the current to be 48mA. He said that was way too high. So he adjusted it and could barely get it down low enough. He got it to 38mA, but that was at the edge of the pot adjustment range. He tried another set of JJ EL34L tubes and those were at 60mA and he couldn't lower them.

I am in favor of people doing their own work. I just wish they would keep doing it. Instead of a little knowledge being a dangerous thing, and then they screw it up, and bring it to me. Because I never know what they did, because often they don't know what they did either. OK, enough complaining. Hey, it's Monday!

So I removed the power tubes and checked the voltages and the bias voltage range, to make sure it was working. It seemed OK, so I put it on the AC with the tubes in. Turned on the power. Waited 20-30 seconds turned on the Standby switch, and here was a brief sound, like a split second of hum, then silence. It blew the 3.15A main input fuse?

I traced it to one of the tubes he had installed. I happened to have a pair of JJ EL34L tubes. I put those in and cautiously powered it up, and no worries. I set the bias to around 35mA, and it seemed stable and all. I played it for a bit, and it seemed fine.

I looked into his claim that I had it set too high. The plate voltage was 440Vdc, and with a max power in a EL34L of 25W, that means the idle power is 70% of 25W.

Max idle power = 0.70 x 25W = 17.5W

So with a plate voltage of 440V and a bias current of 35ma the power is at

P= 440V x 0.035A = 15.4W or 15.4W / 25W = 62%.

The actual max current with a plate voltage of 440V is

Bias Current @ 70% = 17.5W / 440V = .0398A = 40mA

Now this would agree with his number of 38mA that he claimed was ideal. However, most bias probes measure the current at the Cathode as it exists the tube, just before it reaches the Chassis Gnd. At that point there are two sources of current into the tube. Current enters through the Plate connection but also through the Grid connection. that is the connection through the 1K resistors.

pa by Dennis Kelley, on Flickr

By measuring the voltage drop across the 1K resistor, it's easy to know the current through the grid. Just divide the voltage across the 1K by 1,000 ohms and you have the current. In this case it was 4V, and therefore 4mA. Of course your meter is sitting at 400V when you do this, so extreme care is needed.

This is important because all this 70% stuff is talking about the Plate dissipation. That is the voltage on the Plate, multiplied by the current through it. The Plate current is the Cathode current minus the Grid current. So when I measured 35mA above, the actual Plate current was only 31mA. So the actual power dissipation at idle was P = 440V x 0.031A = 13.6W , not 15.4W as I calculated above.

Another thing happens when you increase the current through the power tubes at idle when setting the bias current. When you incease the current, the Plate voltage drops, due to the resistance in the output transformer, and the limits of the power transformer.

In the example above when I increased the bias current to close to 40mA, the Plate voltage dropped from 440V to 416V. So under those conditions the actual power dissipated by the Plate is

P = (40mA - 4mA) x 416V = 14.98W or 15W.

This is a percentage of the max of 25W of
Percentage = 100* 15W / 25W = 60%.

This is all well and good, if you want to wrestle with all this stuff. I actually enjoy the theory behind it all. Maybe I should have been an engineer? Oh yeah, that's right, I am. But the point is, there isn't just a number, of 40mA or 38mA, or 28mA. You have to take the voltage into account. Most tube amps don't have screen resistor. So you can easily measure the current. I usually just ignor it, and set it to around 60% to 70%, and rely on that current as a safety buffer. Quite often the tubes are out of balance a bit. One might be at 35mA and the other at 30mA. So I don't mind setting the hotter one to a higher current than ideal, in order to keep the lower one from being too low. I know there is some margin in the 60% number, and in the screen current that I haven't subtracted from my measurements.

So I am going to use the tube tester on his tubes, and see if one is indeed shorted. One of them did have a big black mark across it? Makes you wonder? Once I do that I will call the customer and have a discussion about tube bias, etc. Not to scold him or anything, just to try and give him the full story.

I've worked on several Marshall amps, and this amp is only the second that I really liked. The other was the newer DSL40CR. I've been told you have to crank the Marshall to get the right sound from them?
 

Silicon Valley Tom

It makes me happpy to play The Blues!
Well done Dennis! :)

Theory and practice. So many engineers can design electronics equipment but have absolutely no clue how to repair it or what caused a problem. I enjoy reading your accounts of equipment repair. It goes to show how much knowledge, effort and experience are required to repair something. :thumbup::Beer:

Tom
 

Silicon Valley Tom

It makes me happpy to play The Blues!
When I was 19, I had my first job as an electronics technician, and solved many problems that were very serious. One of our top engineers asked me: "Tom, how come you know so much"? I answered: "I watch the old timers"! If you want to earn, watch the old timers. :)

Tom
 
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