EVH 5150 III 50W Help

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marshallnoise
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EVH 5150 III 50W Help

Post by marshallnoise » Wed Apr 03, 2019 1:33 am

I purchased a EVH 5150 III 50W about a month ago from Guitar Center for a good price. When I showed up, it didn't have a fuse holder so I couldn't test it. They found one for me to use for a test but I didn't pay any attention to the value of the fuse I borrowed. Either way, the thing sounded good, functioned well and I was pleased as punch.

$499 later, I contacted EVH and they sent me the fuse cap. I ordered a foot switch and a bunch of 250mA fuses and jammed one in the fuse holder. Amp would power on, then die immediately upon flipping the standby switch. Shit.

I test out some basic voltages and find one of the voltage regulators for the DC heaters to be burnt out. I then replace it and those tubes come back to life.

Same situation as before though. Still popping fuses.

I discover that I am not getting any negative bias for the power tubes and decide to build a light bulb current limiter and then go through Rob Robinette's transformer testing for dummies section to find out if there are any shorts. The answer is no. No shorts in the power transformer. That's good. My thought process here was that perhaps a power tube shorted and took out the bias supply tap in the transformer.

But...same situation and I am still popping fuses.

Still not getting any negative bias voltage, I figure out where all of the Pico fuses are on the board and test them. All are good except for F5. I order it and swap it in. I am now getting SOME negative bias voltage. But my readings are all over the place and not where they should be.

Same situation. Still popping fuses.

Here are some pics of the board for any one's reference. https://www.flickr.com/photos/150885180 ... 4088330612

Here are my test voltages. Now, I had to take all of the voltages with the current limiter on. Limter has a 60w bulb in it. I used my TP51 (negative, rectified raw bias voltage) as the mathmatics to scale up the readings. It runs with no limiter as long as the stand by is off. I got -73.1 VDC off limiter and -46.2 VDC with it on. So I used those ratios to scale the test voltages up. This kind of seemed to work. I will do some googling to figure if that is a reliable tactic when working with a current limiter.
Image5150 III 50W Test Voltages by Paul Abbott, on Flickr

Here is the schematic (click on the image and you can zoom in better).
ImageFender-EVH-50W 1 by Paul Abbott, on Flickr

ImageFender-EVH-50W 2 by Paul Abbott, on Flickr

ImageFender-EVH-50W 3 by Paul Abbott, on Flickr

ImageFender-EVH-50W 4 by Paul Abbott, on Flickr

So there are some clear problems with voltages on this amp. My observations are that I am not getting enough negative bias voltage/current for the power tubes. I should be getting 60mvDC and I am getting 1/10th of that. My plate voltages seem about 100 volts off. Just a few of the preamp voltages appear correct...sigh.

I guess I need to figure out if I should be replacing every transistor in this thing as there are many. Resistors and caps are easy to replace too. I still haven't figured out how to get good capacitor readings unless I pull them from the circuit and even then...capacitors have always confused me.

Any ideas?

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Wed Apr 03, 2019 1:47 am

Trying to answer my own questions a bit. According to PRR at EL34world.com...https://el34world.com/Forum/index.php?topic=17168.0

"You can't usefully measure voltages while on a dim bulb. It may be slitting your 120V as 100V to the amp 20V to the lamp, or 80V to amp 40V to lamp... you just don't know the split. Or if you did measure the split, and try to correct the internal voltages by that ratio, you'd be wasting your time. The internal voltages do not go down in proportion to the plug voltage because the cool heater/cathode makes the tubes lame."

Fack. :palm:

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Wed Apr 03, 2019 2:41 pm

I have read a lot about poor soldering jobs on these amplifiers and that most can be remedied by using leaded solder so that's on order. I am going to reflow the entire board and test certain items along the way. Pics to come.

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Tue Apr 09, 2019 4:54 pm

FYI, this is a giant pain in the ass. Its going to take me forever to remove the old solder and install new solder...But I am liking the results of the joints though. Nice and shiny. :D

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Wed Apr 10, 2019 1:52 am

Can anyone give me some insight here? I measured the big power supply section electrolytic capacitors and I am getting some weird readings. First, the circuit board that these are attached to is completely discharged and out of the chassis. Second, I did not unsolder them from the circuit.

Part Meas Should Be
C77 126uf 47uf
C127 126uf 47uf
C66 124uf 47uf
C128 124uf 47uf
C86 78uf 47uf
C85 77uf 47uf
C84 52uf 22uf
C80 30uf 22uf
C79 24uf 22uf

I am going to go do some research on measuring electrolytic capacitors in circuit and also price out some replacements anyhow. If my method was ok, then it looks like only one capacitor is remotely within spec. I measured some 4700uf capacitors, in circuit, and they tested out exactly right so...

Edit: Upon further research: No, I have to lift a leg of the circuit to test properly. That'll be for another night.

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Sat Apr 13, 2019 11:35 pm

I lifted the filter caps and they all checked out just fine. I screwed up one of the 22uf 450v caps so I replaced it with a 22uf 500v cap. Its slightly longer but it went in just fine.

So I got the board back together in the chassis after replacing about 35% of the solder joints with leaded solder. Pleased to report it fired up on the current limiter like it should. I switched to the 150W bulb and had the same success. Then I took it off the current limiter and turned the power switch on. Fired up and looked good. Over 1-2 seconds after I flipped the standby to play switch, the amp died out.

It blew the F3 fuse which is spec'd as 250ma 250v Fast Blow Fuse. I put in a 250v 6.3A fuse in its place, turned it on and it ran fine. I tested the plate voltages and received 505VDC back. So I shut it all down and tested the power from the wall and received 122.6VAC. So my wall voltage is a bit hot and I am thinking that is what is causing this F4 Fuse blow situation.

I turned it back into run mode and tested some voltages again. Everything was reading good except for the bias mv which is spec'd as 60mvdc. It was reading 42mvdc. So I turned the bias pot until I received 60mvdc.

My next move is to get some 500ma 250v Fast Blow Fuses and some 1A 250v Fast Blow Fuses and experiment with them.

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Thu Apr 18, 2019 5:03 pm

I quickly experimented with the amp today with different fuses. It popped 250ma fuses regularly, it popped 500ma fuses as well so I plopped in a 1A fuse and it is holding fine.

I have some noise I am going to chase down, but that's a start at least. I feel better about that fuse instead of the 6.3A fuse I had previously.

FYI, for 5x20mm fuses, Amazon has a set of 15 different rating fuses, 10 of each, for $9.99 that includes a cool little box.

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Fri Apr 19, 2019 3:22 pm

Awesomely, the amp plays great and sounds better since I biased the tubes.

The only problem now is I have managed to do something to introduce a 60hz hum! LOL

https://www.youtube.com/watch?v=rxizx5QltxQ

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Fri Apr 19, 2019 10:38 pm

So I adjusted the bias down to minimum and the hum dropped a bit, but it was still present so I put it back.

My thought process is that it can't be the preamp tubes in the preamp circuit because it is present in both the two preamp circuits. I also bypassed the preamp by engaging the return jack on the effects loop and the hum is still present.

I will keep digging.

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Sat Apr 20, 2019 1:37 am

Alright, I reran all of the voltage tests and the biggest problem I see is that TP44 I should have 263VDC and I am getting 32VDC.

This is the Phase Inverter V9 pin 6. I have swapped tubes and I had the same condition.

The only thing I can think of is that the Flame Proof 1W 100K resistor is shorted. When I measure it in circuit...I get OL. So that seems like its a bad resistor.

Once the amp cools down, I will de-solder it and test it out of circuit.

Getting there.

EDIT:
Yep, bad resistor. I will be installing a new one tomorrow morning and seeing if that fixes it!

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Sat Apr 20, 2019 5:19 pm

Ok, so I figured it out. The 100K resistor swap did the trick. Voltages back to normal.

I still had the hum as evidenced below.

https://www.youtube.com/watch?v=8Del8Jz9jbc

Then I did some tube swapping.

Still had the hum. But I was getting some mechanical noise from the power tubes. Not rattling but a vibration when I tapped the tubes. See below.

Boingggggg!
https://www.youtube.com/watch?v=Ta020uihZCc

I then grabbed another set of 6L6s and plugged them in. Noise gone!

I think plugging in a good set of 6L6s while the amp was still a mess killed them. Which sucks, but that's just money.

:champ:

Still requires a 1A fuse instead of the 250ma fuse. But that's not a huge deal. I do think a full recap might be a good idea. Its not that old but I don't know the history of these caps or the amp. Gonna rock this amp this weekend at church and see what happens.

Oh, and the mod I settled on for reducing the channel 1 volume discrepancy between green and blue was R79 increasing to 47K. I tried R88 increasing to 380K and it REALLY screwed up the blue channel's tone. I always felt the blue channel was too much anyway so increasing the output on the green channel to match the blue did the trick. Yes, it overdrives faster, but for me, running channel 1 low on preamp gain gives me plenty of juice on blue and that leaves red for the heavy stuff!

Thanks for sticking with me on this!

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Sat Apr 20, 2019 11:51 pm

I played this at church tonight (rehearsal for Easter) and it was flawless. VERY happy with this amp.

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Mon Apr 22, 2019 10:56 pm

Went through the entire Easter service without a hitch. Quieter than my buddy's Friedman 20w.

Annoying pop when switching channels but that's not a deal breaker. :evh:

marshallnoise
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Re: EVH 5150 III 50W Help

Post by marshallnoise » Tue Nov 19, 2019 5:34 pm

So the amp developed a hum problem when turned on and left idling for over an hour. Or when you cranked the sucker up it developed a hum. I took it to Top Gear in La Mesa, CA and they figured out what happened. The repair I made on the elevated DC heater supply on ONE side of the tubes had a bad trace that was broken. Apparently it was intermittent. Which is a bitch to figure out.

But they took care of it for me for a bench fee.

Still love this amp.

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