EVH 5150 III: 50 vs 100 watt

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hammered
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Re: EVH 5150 III: 50 vs 100 watt

Post by hammered » Sun Sep 27, 2015 1:12 pm

ledvedder wrote:Does anyone have pictures of the mods?
I`ve eamiled Howard Kaplan about modding the 50 watt head to accept EL34s and it was a big laundry list that went over my head as all I can do is change out tubes. I`m sure it`d be a breeze for a qualified tech but I`m gonna leave well enough alone. The amp IMO sounds like a beast as is

marl3y
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Re: EVH 5150 III: 50 vs 100 watt

Post by marl3y » Sun Nov 20, 2016 4:10 pm

Resurrecting this old post as I couldn't find a newer appropriate one. I'm planning on making some mods to my 50 watt 5150iii head, but the trouble-shooting started early on this one... I can't get the main PCB out. I know it sounds ridiculous, but I had to just take a break before a damaged something. Anyway all the necessary spade connectors have been disconnected, retaining nuts off the pots and 23 (labeled ST1-ST23, so I thinks that's all of them) PCB mounting screws have been removed. I also took out the loop/headphone PCB for easy access and three top mounted heatsink screws. When I lift up on the back of the PCB it lifts easily, but seems to be stuck in the center of the board, like around V5 and V6. Anyone experience this? Am I missing fastener or is there some kind of glue holding this? Any assistance is appreciated. Thanks.

marshallnoise
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Re: EVH 5150 III: 50 vs 100 watt

Post by marshallnoise » Sat Mar 16, 2019 11:17 pm

I know this is an old ass thread, but I wanted to chime in with a bunch of pictures that I think people would appreciate.

And also, you literally have to remove everything except the power socket to get the board out. There is just no way otherwise.

Pics!

ImageIMG_20190316_172416714 by Paul Abbott, on Flickr

ImageIMG_20190316_153304391 by Paul Abbott, on Flickr

ImageIMG_20190316_153257153 by Paul Abbott, on Flickr

ImageIMG_20190316_153317266 by Paul Abbott, on Flickr

ImageIMG_20190316_172639364 by Paul Abbott, on Flickr

ImageIMG_20190316_172631033 by Paul Abbott, on Flickr

I am fighting a problem with my amp where it is popping the fuse (yep, only one fuse) when I flick the standby switch. I built a current limiter and it didn't pop fuses with no tubes in it. Then I added power tubes and the phase inverter and it powered on and did not pop the fuse. Then I added one preamp tube in V6 along with the tubes mentioned above and then it popped the fuse anyway. All of that running through the light bulb current limiter.

There is nothing obvious on the circuit board. I tested every diode and they all are fine. I checked the entire power circuit board and it seems fine.

The only thing is that if I put in a higher amperage fuse, like 3 amp, the thing will not pop fuses. It seems like there is a component failure coming from somewhere.

I am really kind of stumped.

marshallnoise
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Re: EVH 5150 III: 50 vs 100 watt

Post by marshallnoise » Tue Mar 19, 2019 4:39 pm

So it turns out that these amps do have more than one fuse. They just happen to have Pico fuses which look like little resistors. Annoying. So I will be testing these out for a short when I get home.

marshallnoise
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Re: EVH 5150 III: 50 vs 100 watt

Post by marshallnoise » Tue Mar 19, 2019 10:25 pm

All of the Pico fuses are good and not shorted...

I measured the power transformer's resistance for shorts and they are all good. The primary is 23 ohms. The secondary taps are 2.5 ohms on each leg. The heater winding is 3 ohms.

:palm:

Waiting on a whole boat load of fuses to arrive so I can try this damned thing out again. I have until 04/03/19 before I can send it back to Guitar Center as a dud.

marshallnoise
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Re: EVH 5150 III: 50 vs 100 watt

Post by marshallnoise » Fri Mar 22, 2019 2:05 am

Ok, I think I found one of my problems. I went through and verified the heater voltages across the entire amp with no tubes in. The power section which consists of V7, V8 and V9 have 3.14 VAC everywhere it should.

The preamp section is jacked badly. The area before the heaters should have +15.6 VDC on one side and the other side is supposed to have -15.6 VDC per test points 31 & 36. V2, V4 & V6 have -15.83 VDC on pin 5. But they have 0 VDC on pin 4. V1, V3 and V5 have +19.85 VDC on pin 5 and 0 VDC on pin 4. The heaters should have +12.6VDC on V1, V3, and V5 pin 4 and -12.6VDC on V2, V3 and V4 pin 5.

That tells me that the U2 7815 voltage regulator on the + side is not working properly. U1 regulator is working fine. Diodes D12 & D13 test out fine. So I need to get one of those (maybe some spares?).

I know that pin 9 is each 12AX7's center tap for the heaters so I am thinking that the voltage might drop in half and be split evenly among pin 4 and pin 5 down to +6.3 VDC and -6.3 VDC when the tube is connected. I will check this tomorrow.

I wonder why R94, R74, R103, R62, R47 and R71 are not knocking down the VDC to the required 12.6VDC. I just had the meter on them and the Flame Proof 1W 20ohm resistors measure fine.

Oh well, I will get the voltage regulator first and see what gives.

marshallnoise
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Re: EVH 5150 III: 50 vs 100 watt

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

Alright, so I replaced U2, the 7815 voltage regulator. The voltages came down to what they should be prior to all of the pin 4s and pin 5s. I am not getting the correct voltage at the heaters. Half are correct, the other half is not correct.

Apparently channel 3 is the channel that is not getting the correct voltages because when I switch to it, there is next to no output.

Now, the caveat to all of the above is that the amp only fires up when connected to the light bulb current limiter. It behaves normally.

As soon as I remove the light bulb limiter and I flip on the power switch, it kills the replaceable fuse F3.

More voltage tests coming I suppose...

marshallnoise
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Re: EVH 5150 III: 50 vs 100 watt

Post by marshallnoise » Tue Mar 26, 2019 1:31 am

Alright, I think I have this figured out. After going through Rob Robinette's trouble shooting section for blown fuses, I concluded that the amp behaved normally until I plugged in the power tubes. Then the light would stay bright if either of the power tubes were inserted. After checking every component on the power tube board, I started looking for voltages based on the test points enumerated on the schematic.

When it came to bias, I got a 0 VDC reading. That's not good! So I tested the diodes in there and all checked out fine. Then I retested F5, which comes off the 52.5 VAC secondary that is for the bias supply. It is connected to a ribbon cable from the main board to the power board. Well, my VAC was fine, but it turns out that fuse F5 shows zero continuity. I had thought I tested all of them, but apparently missed this one.

So, I will be looking for one of those suckers on Amazon or something and soldering it in.

My assumption, for those who want to postulate with me, is that a power tube shorted, killed the negative bias supply fuse but still ran when you put in a higher rated mains fuse. I am not sure why this would happen, but it is what happened.

:palm:

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