Marshall 50w Jmp 6550 bias consensus

[ckpop]

I am curious to see what other techs opinion is on biasing 6550's in a low voltage NMV Marshall. The voltage is 395v.
I seen others argue about the proper bias point for 6550's at that voltage.

[chief mushroom cloud]

~30W per toob on the hot side.....mebbe 25W per toob to be safe
it's very likely the PI is clipping first anyway...so the difference is prob subtle when over driven.....
and that's if the PT has the cajones (as in current capability) to keep up

try it...see if you like it...if you don't...dial it down until you like it...that's what I doo

[ckpop]

Hey chief, before I have do anything the measurement is about 15w a tube so I know they can come up some. The person before me didnt change the 220k grid leak resistors back to 150K so I will do that.
I can hear a difference and the way the amp attacks with EL34's between 150K and 220K for grid leak resistors but have to see what happens with 6550 going from 220k to 150K. Is it agreed upon to lower the 22Ok resistors for more of a leash on the 6550.

[Gaz]

Reducing the grid leaks is somewhat necessary for the 6550s, but it will change the RC time constant with the PI coupling caps, reducing the bass. If you'd like to get it back to where it was, just approximately double the value of the caps (e.g. .022uf to .047uf). I've never heard one of those low voltage Marshalls, but I 'heard' they are amazing. I bet they only put out about 30W though...

[ckpop]

Gaz, I guess some of these amp are rare with the lower voltage. I switched to 150K which at metro amps they go down to 82k but that seems like to much of a tone change to me. The amp will be used with the channel jumper so it's easy to dial the normal and bright channel for the right bass balance.
As far as the switching the negitive feedback to the correct tap for 6550's I will experiment because that is another way to change the low end a bit.

[drasp]

I hope nobody minds me bringing a Zombie thread back to life. . .

I'm biasing a (new to me) '77 JMP 2100 right now. Original GE 6550s, fresh cap job, otherwise all stock amp. I'm measuring 395VDC for plate volts (between pins 3 & 8 on the power tubes). Currently getting a reading of 39mV & 37mV using a Weber bias probe the basic one that plugs into your meter (mV on the meter is supposed to = mA bias, if I understand correctly)

Looking at this chart:

http://www.webervst.com/tubes1/calcbias.htm

At the bottom, it lists 44mA as "Cool"/50% for 400V. Should I bump it up? Is the chart a good reference? How to do I calculate watts from this?

THANKS FOR ANY HELP/INFO!!!

[Stevem]

I need to first ask this, what is it you want that the 6550s have when compared to a good set of 34s?

[Leo_Gnardo]

At the bottom, it lists 44mA as "Cool"/50% for 400V. Should I bump it up? Is the chart a good reference? How to do I calculate watts from this?

6550 or KT88 I set at 40 - 45 mA in 400V supply Marshalls. Yes there's a lot to like in those "lame wattage" Marshalls. Don't worry about only 30W output - there's plenty of volume for just about anybody and tone to die for.

I've seen as low as 82K grid leaks, but a 12AX7 has trouble driving that. If you go there try a 12AT7 as an inverter/driver. 120K-150K seems a good compromise.

[Gaz]

Assuming the 6550 is a 35W tube, what's the idea with setting the bias so cold? Is the concern taxing the PT?

[matt h]

(deleted)

[Leo_Gnardo]

Assuming the 6550 is a 35W tube, what's the idea with setting the bias so cold? Is the concern taxing the PT?

The way I was taught to bias, with a scope AND ears, is put a sine wave into the amp, dial down the bias voltage (thereby dialing up the bias current) until the crossover notch disappears. And you can hear that too. And that's all you need. More bias current doesn't make better anything, except sales for worn out tubes. For 6550's in those 400V-supply Marshalls, 40 to 45 mA usually does the trick. All that extra power not used in setting bias - you can turn into signal instead of heat. Being in one of those amps is a pretty cushy job for a 6550.

Then there are those who always underbias (in terms of current) for a permanently distorted tone. "Because that's what you rock guitarists want, distortion all the time", from a famous NYC amp tech. (Not a member here far as I know.) If that's what floats your boat, fine, but it's not for everybody. I've never had a customer complain about a properly biased amp, even the ones who play distorted tones all the time.

[Gaz]

Huh, you know I've never really used the crossover/scope method because of Aiken's articles. Using the 60-70% cathode current method would have those 6550s biased more around 60mA, which is a pretty significant difference. Either way, still pretty easy on the 6550s

[pdf64]

When considering static dissipation settings, it's beneficial to consider the capacity of the power supply, the power tube plate dissipation rating and the performance in regard of crossover distortion.
These amps especially so, as they seem to have been designed around power tubes with a 25W plate dissipation limit, and then tweaked for use with power tubes that have a 35W plate dissipation limit.
And then there may be little point running something at say 100% if it's meeting the required performance criteria by 50%.
Pete

[pdf64]

Doh!

[pdf64]

Sorry, after clicking 'submit' there was some sort of 'failed email' webpage.