Maximising clean headroom

[alhe]

My first post here, so let me first say what a great forum this is.

If one wanted to maximize clean headroom, assuming a JTM/early Plexi 50 watt build, what are your options? What are the determining factors for clean headroom?

I get that the maximum headroom is limited in these amps, and that it won't sound like a Fender Twin. The goal then would be to delay breakup, and to get as much clean volume as practically possible. The amp could then be used a pedal platform.

[alhe]

I was looking at my amp under a scope yesterday. The preamp starts to distort at "2" with a 0,2mV sine wave (let's call that humbuckers) and at "4" with 0,1mV sine (single coils). I guess this is fairly normal for a Marshall tho'.

Surely you could you up the voltages gong to the pre/PI for more headroom? I noticed in the JTM45 schematic that there's only a single 8k2 B+ resistor, whereas later amps have 8k2 + 8k2. Maybe I should try a 12at7 in the PI as well for less gain and more current drive.

[neikeel]

For more clean headroom you could use the ECC81 in V1 and V3, use a 4k7 dropper after the choke (before the PI) and use a bigger choke with higher H rating and current rating.

Next you raise the value of the 820R cathode resistors on V1 and V2a to 2k7 or so (up to 5k6) and use SS rectifier.

Personally I would go for a 50w using 1986 Bass spec keeping a highish B+ for the preamp, keep the NFB at 27k/16 and maybe 1k5 or 2k7 on V1 cathode and ECC81 in preamp, probably use 6L6s or 6550s in output section.

[alhe]

Thank you for taking the time to comment, and some really good advice!

My build is already fairly close to a 1986 Bass/JTM-style circuit. Definitely some nice tones to be had there. By simply paralleling the two 8k2 dropping resistors I should now be fairly close to 4k7, which I also see is Bassman spec. That should certainly bring up the voltage and increase headroom a bit. I did the NFB and a 2.7k cathode resistor on V1. Only had one ECC81 at hand so that went in the PI. I will try to crank the amp a bit tomorrow to see where to go from there.

[herbvis]

I was gonna say to raise the cathode value of v2a but Neil beat me to it. Try a few different values there say 1.5k, 2.7k, 3.3k etc and see how increasing it one by one affects the tone until you find your value.

[alhe]

Reporting back. Had some trouble with my build. Some nasty breakup happening past a certain level.

A 4k dropping resistor and a 2.7k V1 cathode resistor seems to go a long way for clean headroom. Also seemed to make the amp more punchy. Made me realize how much of the classic Marshall sound is early preamp distortion. I tried a 2.7k for V2 as well. That dropped volume way too much (with a 5751 in V3), so went back to a 820.

[Carbia]

To get clean, clean up the phase inverter. High percentage of marshall gain comes from phase inverter.

[alhe]

As in a ECC81 tube swap or hardwired mods?

[sixpakldp]

An ECC81 in the PI position will def clean things up a good bit compared to an ECC83. A fairly non evasive way to get there. Use the low input, should be pretty clean and loud.

Also, if you are using the bright channel if you have a highly value bright cap like 5000 Pf, that will make the amp distort early. I have my bright cap on a push pull so I can switch it in and out.

[Haze13]

You can use a 12dw7 tube at v2 - it's one half 12ax7 and other half is 12au7. Use 12au7 section as a tone-stack driver. JJ makes tubes like these and you do not need to rewire the V2. Lower the 100k cathode resistor to ~ 60k.
If you want to use 12au7/ecc81 as PI, you need to check your NFB. Since you will lower the gain, you need to lower the NFB too.
Marshall is a bad amp for cleans. Make it more clean-frendly and you will loose it's dirty sound.

I use have JCM800 and when I want a clean sound, I just play at neck pick-up with a volume rolled-off a little and with chorus - ON. Add an Analog Delay (make it a reverb-like) and you'll have a very good clean A VERY GOOD CLEAN!

[alhe]

Thanks for the input, now that the site is back up.