Marshall JMP MKII non master volume

[pnut5150]

386v

[pnut5150]

Ordered one today, been trying to get stuff a little at a time, I have a BK 30mhz dual trace scope, capacitance meter, a Snap On DMM, a digital soldering station, ordered the bias probe today, now I need a good tube tester.

[J M Fahey]

Not against them bias probe ( ok, I confess, I am against them) but you are way beyond them, just solder a 1 ohm resistor from cathode to ground (instead of the wire which is there now) and measure voltage across it.
Please post results.

Bias probes don't do any better and add unreliable connections , who needs them?

[pnut5150]

Lol I have ready put the ole girl back together..I'll try that on my next one though.

[Chuck H]

-38 volts of bias is fine for a 380Vp Marshall with el34's. I don't have enough experience with 6550's to say if that tube performs well for guitar amp purposes at that bias level. The -V is usually higher on those tubes. Not sure why you put it back together since there are still details like this to work out. You should also monitor the amp for a time to be certain it's stable. Checking voltages periodically, giving it a whack while monitoring, etc. Maybe you just like taking the chassis in and out of the cabinet? You'll be able to check current when your bias probes arrive. As long as the tubes are within a safe operating range you can let your ears decide what bias current is "correct".

[pnut5150]

Yea I stick the chassis back in the cab..I played it for awhile last night, it has great tone sounded killer

[Chuck H]

Yea I stick the chassis back in the cab..I played it for awhile last night, it has great tone sounded killer

Not surprising at all. They're great rock amps. If you're chasing early metal tones, hard rock tones from all eras or even most Indy tones you can hardly do better. What they don't do is jazz, American blues and country. And you can still get close enough to country if you don't need it real loud.

Just take it easy on the amp until you get the bias current checked and adjusted if necessary.

[g1]

Here is a voltage chart with plate voltages similar to yours. It shows -44V for bias for the 6550's.
Final setting should be made for idle current.

[52 Bill]

Lol I have ready put the ole girl back together..I'll try that on my next one though.

Do you own this amp or does it belong to a customer?

[pnut5150]

Friends

[g1]

There is another non-invasive bias method if you don't have a bias probe or can't install cathode resistors.
Measure the resistance of each half of the OT primary, write them down.
Now with amp warmed up, idling with no signal, measure DC volts across each half of the OT primary.
Voltage divided by the resistance you wrote down equals idle current for each tube.
Measure plate voltage. Idle current times plate voltage equals idle dissipation in watts.
Idle diss. watts divided by max wattage spec'd for tube equals % bias.

[Chuck H]

There is another non-invasive bias method if you don't have a bias probe or can't install cathode resistors.
Measure the resistance of each half of the OT primary, write them down.
Now with amp warmed up, idling with no signal, measure DC volts across each half of the OT primary.
Voltage divided by the resistance you wrote down equals idle current for each tube.
Measure plate voltage. Idle current times plate voltage equals idle dissipation in watts.
Idle diss. watts divided by max wattage spec'd for tube equals % bias.

Just to add... Do take a "null" reading of your meter probes before each reading. That is, touch the probes together and note the figure. Then take your reading. Then subtract the null amount from that figure. What's left will be a more accurate number.

[pnut5150]

Great info! Thanks

[The Dude]

Not against them bias probe ( ok, I confess, I am against them) but you are way beyond them, just solder a 1 ohm resistor from cathode to ground (instead of the wire which is there now) and measure voltage across it.
Please post results.

Bias probes don't do any better and add unreliable connections , who needs them?

Point taken, although I have to respectfully disagree. Firstly, you can buy a decent enough bias probe for 30ish bucks (small investment). Secondly, it's cheaper for the customer than me soldering resistors in every amp that comes in the shop, especially those that are built on printed circuit boards. I haven't found "unreliable connections" to be an issue with my bias probe in all my years of doing this. But,......to each his own.

[g1]

Another plus for the bias probes is if the bias pot has external access, you don't even have to remove the chassis to set the bias.