Converting a 50 watt Marshall Plexi From 6550s to EL34s The Right Way

A vintage Marshall "Plexi" arrived here that was in bad need of some refreshment. These are pretty much the ultimate rock and roll amps-loud, brash, edgy and in your face. So they are well worth the effort to bring them back to life.

In no particular order it needed a fresh suite of electrolytics and some general freshening up before retubing and troubleshooting.

It was an ebay find I think.

As part of my standard procedure I noted that someone had "converted" it in a prior time to use EL34s but it didn't have any bias voltage, which makes me think that's why whoever had it gave up on it a long time ago. They didn't "convert" it either-just some half assed hack job.

Amps that were destined for the USA were equipped with 6550 power tubes by the distributor in the US which was, I think, Unicord. They changed the sonic signature of the amp, and it didn't sound very British any more.

There are four steps that must be taken to effect the conversion.

First, the bias splitter resistors should be changed from 150k to 220k metal film resistors. For some unknown reason the board was marked 82k.

This is relatively easy to desolder as all these things are by lifting up the board and putting a small light inside, like one of those LED lamps you strap on your forehead. Then you can see what you're doing.

 The 47k resistor should be changed to 22k. Don't bother with mickey mouse scabbing resistors on top of one another as some folks suggest.

It looks crappy. Do it right.

The two 10uf or 8uf 160v electrolytic capacitors need to be replaced. They do not last forever.

Now here comes the tricky stuff.

Install a 220k bias level resistor as shown on the schematic coming from the high voltage tap on one side of the power transformer to the  bias supply diode. Set the bias pot to the lowest setting. You can set up your bias checker with a set of power tubes, make everything ready and briefly throw the standby switch on, take a quick reading and then put it back on standby because you're probably drawing too much current.

Now, take two test leads and parallel the 220k resistor to reduce the bias level down to about half. Then you can see whether you're close enough to do a fine adjustment with the bias pot. If you are, measure the paralleled resistance and find something relatively close.  I ended up with a 100 k resistor. Or you could start with a 100k resistor, or you could fine tune it with a pot and then measure the total paralleled resistance.

The last step is to relocate the negative feedback wire from the 16 ohm speaker out[ut tap to the 8 ohm speaker output tap-which hadn't been done in this amp.

Once you've done all this check and check your voltages again before you button it up, and do a sound check.

You might find, as I did, that this amp has an oscillation problem which is likely how it got shelved to begin with.

If you're ready to button it up, clean off all the flux and schmutz with some anhydrous isopropyl alcohol and an acid brush cut off to about 3/8 inch. It makes a dandy cleaner.

Rock on.