Marshall 900 prob

[496dart]

I just re-tubed a jcm 900 SLX head . this a 100w with a half power switch. 4-EL34's . Im getting strange bias readings. In EITHER mode its reading 32 to 35ma . This is with the pot as high as it goes. I would think in 100w mode, i should see around 70ma on my meter . The bias doesnt change when i switch it from low to high.
It does have the 150k bias resistors , and measures -38v at the split of those.

The amp sounds fine...

[stokes]

How are you measuring the current draw.If you are reading just one tube,then it wont change when you switch from low to high.I am not 100% sure without looking at the schem,but if it is like most low/high switches it just lifts the cathode on one tube from each side of the OT.If you are using a bias right type of current tester then you are reading only one tube.If you use the transformer shunt method then you will see the change in current draw as you are now reading the entire side of the OT.I dont quite remember why at the moment but there are certain Marshalls that the transformer shunt method doesnt work right.

[496dart]

Hi stokes --yes im using the transformer shunt method . Thats why im confused at the low reading

[stokes]

I dont recall what the exact problem with certain Marshalls and the transformer shunt method was about,or which models it was related to,I'll take a look in the books I have and see if I cant pin it down,but I know it doesnt work in some Marshalls.I think it had something to do with where the screen was connected.Anyway,can you check the current with a bias right type set up,so you are only checking one tube at a time?Maybe the 1ohm cathode resistor?You say the amp sounds fine,so I assume that means the half power switch is working correctly,and I suspect the transformer shunt method is not right for that amp.I know when I used the shunt method on a 45watt Marshall I was getting a reading that was just about half the actual current draw.That was a long time ago,I verified the current by measuring the actual plate current with a home made bias right adapter.I later learned about the issue with the shunt method.

[496dart]

i guess im going to have to put a 1ohm in . But.... maybe this 35 ma IS actually half and im probably in the ballpark . plate voltage is 494.

[stokes]

I'm thinking you're okay,but suggest checking with the 1 ohm on the cathode just so you can be sure what each tube is actually drawing.

[496dart]

Luckily i had a 1 ohm here. Threw that in and it biased at about 70mv at each pair . Everything looks fine .
I did notice that the cathodes go to a couple fuses before ground . I dont think thats the shunt method issue . Im curious as to why you cant bias these on the OT .

[stokes]

Been looking for where I read about the Marshall and the shunt method issue,I'm thinking it had to do with the screens being tied into the plate and when you shunt the tranny the tube becomes a triode.I just checked the schem for the JTM 45 which is the amp I had the issue with and the screen and the plates are at the same point in the B+ rail.I do know the shunt method is not reliable in certain amps,I think some Tweed amps have the same issue.If I find the full explanation,I'll pass it on to you.Just be aware for future reference if you see questionable results with the shunt method.I like the shunt method,but I have a rig I use that will measure the plate or 1 ohm cathode current at the flip of a switch,the difference you see between the 2 is your screen current,which will alert you if there is too much screen current.

[Mark Black]

A double-check I like to use is to measure the DC resistance of each side of the OT primary and then check the voltage drop across that resistance, doing the math to arrive at current.

On a similar note: What about a bias-measuring device that reads out directly in watts? Seems like it would be do-able if the measurement was made on the plate side across a small resistance, as both current and applied plate voltage would be available to sample and a microcontroller could do the math. Not that I'm lazy or anything...

[stokes]

Mark,by measuring the resistance of the OT..etc you are doing the long version of the shunt method.Just use a current meter and forget the math and extra steps.

[Mark Black]

Just came across an explanation of the problems using the shunt method with Marshalls:

http://www.duncanamps.com/technical/lvbias.html

[stokes]

Thats the one I was looking for,thanks Mark.I knew it was different than the Fender Tweed issue,which was in one of Webers books.

[drewl]

This is why I'd rather just do the math!



BE WARNED...for all practical purposes, a meter set to measure CURRENT is equivalent to a STRAIGHT WIRE. This means that as soon as you touch either probe to the high voltage circuitry, THE OTHER PROBE NOW CARRIES THE SAME VOLTAGE. If you drop the probe and it lands on your arm or leg, you could be electrocuted. If it lands on the chassis (or anything else that is at earth or circuit ground potential) a huge spark will be generated, along with a noise like a small firecracker. (Please don't ask how I know this. ;-) The probe tip will be partially melted, and at the very least, the meter's internal fuses will blow. At worst, the meter will be history. Shorting the HV to ground isn't especially good for the amp either, and may blow the amp's fuse or damage the circuitry. You can easily kill a rectifier tube this way. BE ESPECIALLY CAREFUL NOT TO LET A PROBE SLIP OFF A TERMINAL AND HIT THE CHASSIS WHILE YOU ARE TAKING A READING! BE *EXTRA* CAREFUL TO MAKE SURE YOUR FINGERS DON'T SLIDE DOWN THE PROBE AND COME INTO CONTACT WITH THE METAL TIP!! And make DOUBLE DAMN SURE you know which two points in the circuit you are supposed to touch the probes to, because if you accidentally touch the bias supply and the plate supply at the same time, you won't *believe* what happens. IF YOU'RE NOT *SURE* WHAT TO PROBE, *DON'T* PROBE IT!!

[Amp Kat]

The two safe ways with out a doubt are a Bias Rite and the Cathode Current Sense method. The problem is I don't have a Bias Rite meter yet and most PCB amps are too difficult to get between Cathode and ground so it's either shunt or scope and scope isn't very accurate for cross over distortion soooooooo. Yeh things can happen with those leads but after several times and several screwups like blowing open power resistors and getting the crap scared out of you when the cap pops like an M-80 when the lead hits ground. Don't do those things ! Use leads and make sure they are on good without slipping off and isolate your meter leads away from each other. Then all you have to do is sit back and watch of course with your ONE hand on the standby switch in-case something does happen. I had a meter (Fluke) that was reading OL on the CT to winding when checking OT ohms and it would read across the transformer fine. I thought the OT 's were bad when they really weren't so now I disconnect the secondarys and inject a small voltage and check the voltage ratios on the primary from CT to each side and across it. This test works very well and is extremely accurate but I guess everyone has their own ideas so all I can say is just be careful, Merry Xmas & a Happy New Year !

[TD_Madden]

I made my own "bias-rite" using an old tube base, socket, resistor, and some wire with banana-plugs.....pretty easy to do.