Marshall 6550 to EL34 conversion - what is the right way?

[11 Gauge]

It appears that the "tech" that modded my Marshall (when it was previously owned by someone else, did a half-vast job. With a good set of new tubes, it will not bias any hotter than 27 mA - way too cold.

I'm getting mixed information on doing the conversion the right way:

1. A "pro tech" source online indicates that only the bias range resistor (47K) needs to be cut to half that value (parallel it with another 47K), and move the negative feedback loop wire from the 4 to 8 ohm tap.

2. Marshall clearly states that the bias feed resistors need to be bumped up from 150K to 220K, the bias range resistor needs to go up to 56K, and the bias supply resistor needs to go from 120K up to 220K. They make no mention of doing anything with the feedback wire.

These 2 simply do not agree with each other.

The only "mod" that was done to mine was to knock the 47K bias range resistor to 38K. I can't imagine dropping it to approximately 23K!

So what is the tried and true method?

[Wally]

Going back to the schematic for EL-34's cannot be a bad thing...and Marshall does take the NFB wire off of the 8 ohm tap. This needs to be done no matter which method you use.
There are four resistors to change if you go back to the El-34 schematic...the two 220K's in place of the 150K's, one 56K in place of the
47K, and one 27K that needs to be changed to a 15K near the IN4007 diode in the bias circuit.
Obviously, paralleling the 47K is simpler. IIRC, some folks advocate changing the 220K's to 100K's on these Marshalls, so the 150K's might be good as is???
Maybe Tim Swartz will chime in here and share his experience with Marshalls. I would prefer to go back to the schematic, I suppose. The Plexi Palace Marshall forum would surely give you some good feedback.

[Tim Swartz]

If you're not intimately familiar with the inards of tube amps, take this to someone who is. This requires working over live circuits with 400+ volts. It ain't worth frying to save a couple $$.

OK, here goes: Original EL34 circuits have 220k bias splitters, not a bad thing to do IMO, but not a show stopper if you don't. Piggy-backing a bias resistor is widely accepted, and I'll do it if the customer is "budget concious". I explain it is like a bandaid fix. I'd much prefer to have the customer spend a few more dollars and have it done correctly (pull the board and replace the resistor). To determine the value what I typically do is set the bias pot in the middle of the range and connect a resistor box thingy (a switch to change about 100 values from 1ohm to 1M) with the switch set to say 100K (a 100k pot would also work)) to each side of the bias resistor (47k?). Then while monitoring the current I will switch to lower values until I'm at the point I want (typically 30mA - 40mA give or take with consideration to plate voltage). At this point I measure the resistance and that is the value I'll use. If piggy-backing just use the value on the switchbox or pot, if replacing use the combined value of the switch box/pot and the existing resistor.

As for the NFB connection, it is not critical, but will impact the intensity of the presence control. Experiment and put it on the tap you like best.

[Ben Harmless]

Tim nailed the way I did it.

...Actually he nailed the way I fixed it from the way it was done before I got it.

Some people actually pull the NFB wire completely to get a slightly looser tone. I think it sounds sloppy. They probably think it sounds "not so squelched."

[Tim Swartz]

I think no NFB in a Marshall would be a bad thing, plus your presence control would be rendered useless.

[11 Gauge]

Thanks, guys. Great news that I'll now be replacing (or piggybacking) one resistor instead of four (not that four resistors is a lot of work).

And thanks, Tim, for giving me the skinny on the incidentals of the 220K vs.150K feeders/NFB to which tap kinda stuff.

I will probably pull the board and do the full nine yards (I want to add some other goodies to the circuit, and move the location of the tone stack), but if I'm pressed for time, it's nice to know that I can get this thing biased right and out to the next gig.

Thanks again.