mkII vs plexi 50W v1 pin 1 value

awangotango · Post by **awangotango** » Fri May 11, 2007 8:08 pm

this site lists the average plexi (early) 50W as having a value of 180v, whereas the mk II's seem to have just over 100V on V1 pin 1. Anyone know what effect this lower value has and why it is different than an earlier 50W?
http://members.shaw.ca/house-of-jim/Html/Marshall.html

SDM · Post by **SDM** » Fri May 11, 2007 9:37 pm

That "MK2 Marshall 50 W. Volt. Chart" concerning V1 pin1 and 3 really makes no sense for a master volume amp or a 1987 style circuit, so not sure if that's someone's recording error there or what.
In accordance with the rest of the voltages, that chart should read approx V1 pin1: 150V, pin 3: 1V for a Mk2 1987 50 watter.

awangotango · Post by **awangotango** » Fri May 11, 2007 9:55 pm

the reason I asked is because I was concerned about a '72 50W V1 pin 1 value of 119. (B+ of 414.) So this might still be too low? When I got the amp, the high and low normal channel inputs were rotated in such a way so that the inner tabs touched one another and wondered if that shorted out something, maybe the input resistors. I can't measure those without taking them out, but all the other resistors around the preamp read normal. So, still wonder if I should be concerned about the 119 v value ? I should add, the value goes up to 275 when V1 tube is pulled out ..

SDM · Post by **SDM** » Fri May 11, 2007 10:25 pm

The 119V volts to pin 1 may be okay, but your pin 3 should be around .8-.9V, not 2.9 Volts as that voltage charts shows.

Does your V1 pin 3 cathode resistor measure around 820 ohms, and indeed the voltage there around .8-.9V? If so and your amp has the two 10K (or one 10K one 8.2K in series) for the first two B+ line dropping resistors (ie- no jumper in place of one of them), than your voltage would seem okay.

In my post above I figured the voltages in an amp that did use a jumper instead of two series resistors, so there's circuit variations to keep in mind.

In no case though, should V1 pin 1 be around 100V with 2.9 Volts on its cathode (pin 3) as that chart indicates. Hope that makes sense.

awangotango · Post by **awangotango** » Fri May 11, 2007 11:09 pm

I see. Yes, the relationship for my amp is 119V pin 1 (with v1 tube in) and .9 on pin 3. And the 820 ohm resistor measures 810. Not sure aboiut the B+ resistors but it's a bone stock '72, standard 2-10k's tied together at one end like this one:
http://www.amparchives.com/album/Marsha ... _left.html
thanks for the info

awangotango · Post by **awangotango** » Fri May 11, 2007 11:30 pm

another oddity about this amp. When the amp is take off stanby and goes to full power, the bias goes from upper 100's DOWN to it's normal bias of 39ma. This happens within the span of 2 or 3 seconds but this is not a good thing i can't imagine?

SDM · Post by **SDM** » Fri May 11, 2007 11:57 pm

The early 50 watters had a flaw in the way the power was sent to the bias section. The way they were wired originally, the bias circuit only gets voltage when out of standby mode. This means the B+ and bias circuit get voltage at the same time. The bias circuit needs a sec or two to build up and make the tubes idle correctly, and as you see this also causes a current surge that helps kill these earlier 50 watt rectifiers and PT's.

Anyway easy fix there. Just move the wire feeding the 220K (at the start of the bias circuit) to a different lug on the standby switch that also has a wire on it going straight into the PT (as opposed to where it is now on a switch lug that shares a wire going straight to the rectifier). Basically just move the wire one lug vertically on the standby switch. That way the bias circuit gets power the second the amp is turned on, and gets power even when the amp is in standby mode. Highly recommend doing this to help keep your amp from frying those important bits. Hope that made sense.

EDIT: Should also note it's possible the wire from your 220K bias resistor may not go to the standby switch, but run directly from the 220K to a rectifier leg instead. If that's the case, it's the same as above to fix, but you may need to extend the wire so you can move the end of the wire off the rectifier and over to the standby switch now instead. Then solder it to a switch lug with a wire running straight into the PT on it, and the bias/standby problem will be fixed.

Bill Durham · Post by **Bill Durham** » Sat May 19, 2007 12:34 am

Interesting thread... my 73 MKII 50 watt with 6550s tracks that voltage chart pretty closely. My B+ is about 20v low, but the voltage on pin 1 of V1 varies depending on the particular tube in the socket. My tubes are old pulls that draw different amounts of current but on average the voltage on pin 1 is in the 118-125v range. No where near the 160v that is on the tube voltage chart I have. Pin 6 of V1 has 152 volts on it.. which falls close to being 20v lower than my chart which would be consistent. I think that the reason for the lower voltage is the cathode bias of the tube with the 330uf cap and 820 ohm resistor causes the tube to draw more current, causing the voltage to drop across the 100k plate resistor.
I think my chart, which comes from the same time as the amp probably has numbers from the shared cathode models instead of the split cathode 1987.

Also.. I moved my bias supply source wire to the other side of the standby switch so the bias voltage is applied all the time. Seemed like the way to do it.