running Marshalls cathode biased

[EETStudent]

Does anybody have any experience with this? Would it be recommended?

[Steve Conner]

Well why not? It's not like it'll make the amp explode or emigrate to New Zealand. The worst that'll happen is that it won't sound like a Marshall any more.

The 18 watter would probably not change its character too much, since EL84s work well and give plenty of power with cathode bias.

The larger EL34 models would probably lose quite a lot of power, however you might like the tone more. Matchless used cathode-biased EL34s in some of their amps, and people seem to like them fine.

[d95err]

The 18 watt is already cathode biased...

The only problem with running cathode bias in a 50 or 100W amp is that the cathode resistor will generate a lot of heat. This may require extra ventilation inside the chassis, or perhaps mounting the cathode resistor on the outside of the chassis.

[jrfrond]

Another problem with this would be the power transformer VA rating. You will get increased current draw vs. grid biasing during amplification. If the transformer cannot handle it, it might cook. After the conversion is performed, you should monitor the power transformer temperature after hard playing. If it is too hot to leave your hand on there for more than 10 seconds, you have an issue.

[daz]

If you copied one of the matchless schematics done like this, what then would be the risk? getting the wrong PT, and if so in what way? I'd like to know because i've been going back and fourth over whether to do a 4-EL84 amp or a cathode biased EL34 to get the EL34 tone i love but at a less than nuclear level.

[Leka]

Why not make it switchable?

I guess it's really hard to say what you would like more.
I've found that I'm using the fixed bias a lot more in my amp...
IMO the clean sounds are a bit warmer with cathode bias.

[EETStudent]

Thanks guys for your comments. Ive been working on a project where Ive installed a plexi-style preamp into cathode biased EL34s. I know that most Marshalls, except for the 18watters, are fixed bias and Im thinking that a change to fixed bias might be in order if I want to achieve that tight, marshall tone. I was just unsure of the tonal differences between the two.

Speaking on the subject, my EL34s are drawing a screen current of 123mA and a plate current of 114mA. I think Im cooking these tubes, correct? And is it affecting my tone at all?

[Bruce / Mission Amps]

...Speaking on the subject, my EL34s are drawing a screen current of 123mA and a plate current of 114mA. I think Im cooking these tubes, correct? And is it affecting my tone at all?

!!??!!
123ma of screen current at idle? Yikes, and I mean YIKES!
What is your plate voltage for 123ma of plate current?
And are those numbers for both tube added together?

[d95err]

Speaking on the subject, my EL34s are drawing a screen current of 123mA and a plate current of 114mA.

Like Bruce said, that doesn't sound correct. Perhaps you'd better take that measurement again. If you get the same result, tell us how you measure it and we can check if you're doing it right.

[Enzo]

I was wondering how that measurement was taken.

Any chance that was a voltage across the 1k screen resistor?

[EETStudent]

http://www.diyguitarist.com/GuitarAmps/PT-Biasing.htm

cathode current= cathode voltage/cathode resistor
So, 37/300=.123(123 mA correct?)

screen current= 9(voltage across resistor)/1000(resistor) = .009

plate current= .123 - .009 = 114mA plate current

Still sound wrong?

[d95err]

http://www.diyguitarist.com/GuitarAmps/PT-Biasing.htm

cathode current= cathode voltage/cathode resistor
So, 37/300=.123(123 mA correct?)

screen current= 9(voltage across resistor)/1000(resistor) = .009

plate current= .123 - .009 = 114mA plate current

Still sound wrong?

That sounds more reasonable (although I have no idea if it's safe for EL34s or not). It was the 123mA screen current that was way off (typo I guess, I'm sure you meant cathode current in the first place).

[imaradiostar]

When I was still living in Nashville I was told by some session guys that the hot ticket for a big marshall sound in the studio (at least a few years ago) was a cathode biased Matchless EL34 amp set mostly clean (breaking up just a touch to add EL34 flavor) with one of the many "marshall pedals" in front of it- Catalinbread DLS, King of the Britons, etc. This leads me to believe a correctly set up cathode biased marshall will sound great. Dr. Z makes an amp with a marshall preamp and cathode biased 6v6's and it enjoys a good tonal rep as well.

jamie

[jrfrond]

There's nothing wrong with setting up a 50W or even 100W Marshall for cathode-bias operation, and I LOVE the way they sound, but you need to be aware that the extra idle current MIGHT stress the PT (depending on how you choose to bias them; I like 'em a bit h. The best way to accomplish this evaluation (unless you monitor current draw AND have the PT VA specs) is to temporarily add the cathode-bias resistor and cap to the power tube grounds and ground the grid leak resistors where the fixed bias was, then cook the amp into a proper load with a sine wave at half-power for an hour or so. If the transformer gets too hot to touch and leave your hand on for about 10sec., then you probably have an issue. I know this isn't a rocket-science approach, nd having the transformer specs and current draw figures IS best, but this will get you in the ballpark.

The only realistic resistors to use for high-power amps are 25W resistors with brass heat sinks that bolt to the chassis. I use the Arcol brand carried by Mouser, and use a dab of heat sink grease to allow more efficient heat transfer to the chassis. In a 100W amp, use one resistor per pair, and mount the cathode bias cap(s) AWAY from the resistor(s) to minimize heat damage, and make sure the caps are rated 105°C.

[Gingertube]

Some time ago I came into possession of an old Australian AWV PA Amp which ran EL34 Outputs.
As a quick exersize one Sunday I ripped the 70V line output tranny out and fitted a spare 50W Marshall Output Tranny in its place.
Cathode biased EL34 Outputs.
Modified the preamp to a 3 stage preamp - Trainwreck Express Circuit.

I lent it to the local Guitar "God" while I did some repair work on his Fender 75. He rang back the next day to say that the amp had given him a "chubby" and he did'nt want to give it back.

So it (the amp) is a cathode biased (around 35W) Marshall Power Amp with Trainwreck Express (A0) circuit preamp from here.
http://amparchives.com/Amp%20Archive...s%20A0-A9k.pdf
And it is stunning - another vote for cathode biased EL34 Outputs.

Cheers,
Ian