Tweaking a Marshall 4150

[WORMDIRT]

I have a Marshall club and country bass compressor combo, and I would like to do a few things to it to make it better suit my needs.

First of all, I'd like to take the compressor out of the circuit. It isn't very musical and, in terms of reliability and noise reduction, I think it would be worthwhile to cut it out. I'm assuming I just need to clip out FET 1.

Also, the schematic indicates something about 6550s, but the rest is cut off. I would like to use 6550s seeing that the general consensus is that Marshall amps benefit from using them and I love the feel of 6550s. I assume that I will need to alter the bias circuit slightly, if at all, can you guys confirm this.


Last would be filter caps and an impedance selector switch. I just need to source these/get recommendations for brands. I was thinking JJ caps based solely on the price point.

Any help is with any of these / any ideas of what to do to this thing will be appreciated

[Muziekschuur]

Did you went ahead and modify the amp? How did it turn out?

[WORMDIRT]

Did you went ahead and modify the amp? How did it turn out?

I replaced the power tubes with JJ el34s, and put new filter caps in. Still not very loud. I'm like the circuit alot, I just need to figure out how to unlock its potential for volume I guess

[Stevem]

What do have for plate voltage at idle on pin 3 of the output tubes and pin 5 which will be a negative voltage?
What does the plate voltage drop down to when the amp is run flat out?
When the amp is running flat out how much AC voltage output do you read across the speaker or whatever you are using for a load, and what is the resistance of that load and or impedance of the speaker ?
Please post back these findings!

[martin manning]

The clipped-off note on 6550 just says "6550 (USA)," and the bias change is there too at "RB" with the USA value noted. If it's not loud then something is wrong for sure!

[Stevem]

Good catch Martin!
Yes that resistor after the F3 fuse needs to be changed to the lower value from its current 27k to drop that - bais voltage probably by some 15 volts once in place!
With your current high bais voltage you might as well just have only one output tube in the amp!

[tictac]

If you're looking for more volume you could bypass R5 with a cap with a larger uF value than the stock .47uF. Try 22uF to 100uF.

Also bypass R9 will increase gain.

You could further increase gain by changing R9 to a resistor cap network like 2.2k/22uF.....

TT

[tictac]

Oh, just noticed R5 is 15k and not 1k5. That's going to significantly lower your 1st stage gain. If you're getting rid of the FET anyway You should change R5 something like 1k5 and bypass it with 22uF or more....

TT

[WORMDIRT]

What do have for plate voltage at idle on pin 3 of the output tubes and pin 5 which will be a negative voltage?
What does the plate voltage drop down to when the amp is run flat out?
When the amp is running flat out how much AC voltage output do you read across the speaker or whatever you are using for a load, and what is the resistance of that load and or impedance of the speaker ?
Please post back these findings!

Well so far I have -500v across 3-5, or -33 when on standby
Plate voltage is 500v, 3 to ground
The highest I can get the bias set to is 20ma (reading with a bias probe)
As far as running flat out, i started this project kinda late, so It'll have to wait till tomorrow.

[Stevem]

20 ma is far to little for idle current so you need to lower that -33 volts, would like to see you pulling 35ma.
Also that low of a bais voltage would be way too low to run 6550 tubes as they would be pulling a lot more idle current than they should .