12 Series Plexi Mod for Peavey Windsor 100W Head

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danman
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Re: 12 Series Plexi Mod for Peavey Windsor 100W Head

Post by danman » Thu May 24, 2018 9:03 pm

One very common issue in Marshall style builds is using a 470k resistor in the PI instead of the called for 470r. The mistake comes up quite often and produces exactly the same issue you described. It'll be the 470r located between the two 1meg grid resistors in the PI. If the color bands are hard to read, you may need to unsolder it from the board and check it with your meter.

Captain chunk
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Re: 12 Series Plexi Mod for Peavey Windsor 100W Head

Post by Captain chunk » Thu May 24, 2018 9:06 pm

I checked that as well. The peavey had a 470ohm resister in the phase inverter. I just reused that resister. I checked it again with my multi meter and it reads 469ish ohms.

Captain chunk
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Re: 12 Series Plexi Mod for Peavey Windsor 100W Head

Post by Captain chunk » Fri May 25, 2018 10:13 am

Thought this info might be useful. Here are the voltages on the phase inverter.
Pin1, 165v
Pin2. 27v
Pin3, 27v
Pin6, 167v
Pin7, 18v
Pin8, 27v

10k 1m 1m 470ohm junction 27v

danman
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Re: 12 Series Plexi Mod for Peavey Windsor 100W Head

Post by danman » Fri May 25, 2018 8:12 pm

Unfortunately since this thread is over five years old, most of the members that were involved in this Windsor mod are now gone. I'm not familiar enough with the Windsor to offer any suggestions. A full set of voltages throughout the entire amp plus some clear, up close pics of all wiring may be helpful. Do you own a scope? If not, you could still use your meter to check the AC signal voltage at each gain stage to determine whether or not the power amp is getting the full, expected voltages to drive it properly. You will need a way to inject a steady signal into the front end though. If you don't own a signal generator, I believe their are some apps available that will allow you to do this.

A couple of questions though...what are your power tubes biased at? Does the presence pot work as expected? Have you tried lifting the nfb wire from the circuit to see if the volume comes up? When testing the amp, the lo input should be much lower volume than the high input. Are you testing through the high input and does the lo input drop volume when plugging into it?

danman
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Re: 12 Series Plexi Mod for Peavey Windsor 100W Head

Post by danman » Fri May 25, 2018 8:33 pm

Judging from your voltages shown above, your plate voltage in the PI is much lower than the standard Marshall. You should be seeing around 220-250ish on pins 1 and 6. This may mean that your PT produces much lower voltages than a Marshall would or that you have a wrong value resistor in the B+ dropper string feeding the preamp. This wouldn't cause a huge volume drop but more of a browner, compressed sound. As I mentioned above, a full set of voltages would be helpful.

Here is a voltage chart for a Plexi. This should be very close to what you should see in your JCM throughout the amp even though the preamp is slightly different. Just scroll down to step 18 where the chart is shown...http://valvestorm.com/sites/default/fil ... MP_KIT.pdf

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