1984 Mesa Mark II C+ Power Tube Diodes

[plexi50]

Would anyone happen to know what diodes i should use to replace the power tube diodes that work with the 60/100 watt switch on a Mesa Mark II C+. The script on them is Vargo VG4. I can't find any diodes that are listed as VG4.

[sluckey]

Looks to be connected between pins 3 and 8. If so, any 3KV 1A diode will work. Many people will use three 1n4007s in series.

[plexi50]

Looks to be connected between pins 3 and 8. If so, any 3KV 1A diode will work. Many people will use three 1n4007s in series.

Great info. Thanks much! It has been blowing the screen resistor at the far right power tube. The diodes are open and don't have any readings in either direction..

[sluckey]

Your pic doesn't seem to go with those schematics.

[plexi50]

All the resistors and screen values are the same as the schematic. Edited: I just talked to Mesa and i went over the OT which is good. I put (2) tubes in the center amp sockets and it is working fine without blowing a screen resistor. I pulled the diodes as he said not needed as the voltage in this amp is not that high. 483vdc. Bias -52vdc. Still have to figure this one out. Amp sounds loud and fine right this minute using two center power tubes. Let me find the right schematic if possible

[SILVERGUN]

Here's a different version:
Schematic Capture - C:\Users\danoc\Desktop\Amp stuff\Proteus Files\Mesa Mk2C+\Mesa MK2C+.pdsprj (googlegroups.com)


Maybe those diodes aren't factory?


EDIT- And, maybe they are
The Mesa/Boogie Mark IIC+ | Vintage Guitar® magazine

[PRR]

> same as the schematic.

Your  0.jpg and  00.jpg have obvious serious errors. The OT primary is shorted. The LTP driver is misdrawn more like a cathodyne. Diodes as shown make little sense.

The scheme Silvergun linked looks much more right.

[plexi50]

Mesa told me sometimes they used those diodes and sometimes not. Yes SILVERGUNN schematic makes more sense. Here are actual chassis pics and diode placement.

[SILVERGUN]

Well, your output stage doesn't match that schematic I linked to.
Does your amp have mixed power tube types?
1 set of EL34 and 1 set 6L6GC?
I guess there's more than 1 model to that model #

[plexi50]

Mesa says they made several different types. So it's a mixed bag. Ive been thinking this whole time that it must have used EL34 (OUTSIDE) 6l6gc (INSIDE) sockets. Owner say's no. Bought it new in 1984 and came that way with all 6L6GC.  I just lifted the leads of the CT and plates including speakers jacks. Getting no solid readings from the OT from CT to each plate lead. It is just fluxuating all over the place. Bad OT?

[plexi50]

It is working and producing full volume with (2) 6L6GC in the center sockets. Could it be that with all 4 power tubes the load is too much because the OT is damaged and on the way to exploding in smoke? I mean with no Ohms readings the OT shouldn't work at all!

[SILVERGUN]

You're right that you should have solid resistance readings across those windings.
It's an ominous sign that those 2 protection diodes were failed open.

[plexi50]

You're right that you should have solid resistance readings across those windings.
It's an ominous sign that those 2 protection diodes were failed open.

They said they don't always use those diodes and that they are not mandatory to have in place. A new OT is $180.00 plus shipping. They insisted that i send the amp to them because of it's value today. I think i can swap out the OT within a few 35 minutes without any problems. The power section of the amp isn't rocket science. I doubt the power transformer voltage is surging? I will check the choke as well while im at it.

[plexi50]

Chassis.

[plexi50]

You're right that you should have solid resistance readings across those windings.
It's an ominous sign that those 2 protection diodes were failed open.

The schematic link you provided is not the same as this Mark II C+ amp. Also those PS cap's and bias cap's must go.

[shooter]

should have looked at schematic 1st 
so the windings for the outside 2 read bad but the inside two measure solid?

Getting no solid readings from the OT from CT to each plate lead.

It is working and producing full volume with (2) 6L6GC in the center sockets.

guessing since it's working with2 tubes, your meter might be suspect reading ohms 

[plexi50]

should have looked at schematic 1st 
so the windings for the outside 2 read bad but the inside two measure solid?

Getting no solid readings from the OT from CT to each plate lead.

It is working and producing full volume with (2) 6L6GC in the center sockets.

guessing since it's working with2 tubes, your meter might be suspect reading ohms 

CT removed from standoff terminal. Plate leads removed from tube's (2) wires. Meter ground to CT. Test probe to each plate lead one at a time. Meter reading flying up and down all over the place. I made a very short video but i cant compress it enough to upload. I have 2 other meters. They show the same flying around readings. Never settles down to any number.

[pdf64]

Inductance can screw up autoranging. Try using manual range selection.

[PRR]

> Inductance can screw up autoranging.

+1. A fat unloaded winding can kick for more than a second. Jumper-clip short another winding, such as speaker winding when checking primary inductance. That makes self-inductance ~~1000X less, so less confusing for auto-range and quicker for manual tests.

[plexi50]

Ok. I grounded each plate lead 1 at a time and then probed the CT itself. It settled at 34.4 & 34.5 ohms. I was told 2 days ago that a fat thick wire / winding would sometimes not settle down and show a solid meter reading. Afterwards i put in a single 6L6GC tube of mine in the far right socket that was burning the screen resistor. Ran it for 10 minutes last night in the 100 & 60 watt modes and had no issues with that screen resistor burning up again. This morning i have ran it again for 20 minutes and tested voltages. All is fine so far. I have told the customer that he needs a new quad of power tubes. I marked the screen shorting JJ 6L6GC so it don't find it's way back into any other amp. All this Hooplah over bad tubes! Ah! But the tubes all test good in my tester!??? I looked down inside the bad 6l6 and you can see how close the elements are to one another. One of them is almost touching the one next to it. 

[pdf64]

Valve testers are BS, yet people treat them like they’re voice of god.
At best they prove bad, they can’t prove good.
My view is that a guitar amp with light bulb limiter is the most effective means of weeding out bad valves whilst mitigating damage risk.

[plexi50]

Valve testers are BS, yet people treat them like they’re voice of god.
At best they prove bad, they can’t prove good.
My view is that a guitar amp with light bulb limiter is the most effective means of weeding out bad valves whilst mitigating damage risk.

That's the best explanation i have heard and put plain and simple as well. I trust my tester and go through it every few years just to make sure it's calibrated. But your so right about reliability when it comes to (IS IT REALLY GOOD)? I flick a tube with my finger nail sometimes when i think i can get a bad tube to show itself (shorted) in a test position it shouldn't. Or just leave the power tube in the tester for 5 minutes and magically the short light appears. And sometimes they do. So your dead right on about testers. The amp is working just fine now and new tubes are on the way. Will they be good? It really suckz when you can not be sure of anything,anymore. Your work is a reflection of your knowledge, but that goes out the window with just (1) bad tube.

[plexi50]

Recapped the Power Supply. Found a crumbling carbon comp 2 watt resistor which going by the schematic (which is not fully accurate to this amp) that resistor is a 680 ohm. The schematic follows the RP11A power supply 1K rail. There are a lot of differences but for the most part gets me in the ball park. Waiting for a 680r to arrive. I have almost every part on the planet except for a lousy 680r. Let you know what happens after power up with load. Look at the heat that the solder took on the diode & invisible 680r & 470uf / 16 volt cap. Second row down in PDF. RP11 Designation.

[plexi50]

V4 no 680r as shown on schematic from 470uf negative side. Diode in place instead. A lot of differences and evidently C+ model changes.

[PRR]

Multiple errors in that PDF; ironic because it likely does correct many errors seen in the wild.

[plexi50]

Multiple errors in that PDF; ironic because it likely does correct many errors seen in the wild.

R88 680 r is all i have to replace now. It is like a jigsaw puzzle. Glad i don't have to go into the preamp LDR area. Like it say's in the into of the PDF / Resistor & Capacitor values are not official, Etc