Redplating jcm900

[dynaman]

I'm a little confused by the references to mv and ma and VDC. which is it? what are your voltages at all the pins ? pin 5 is the bias voltage.

Just a side note about tube quality. I recently had a pair of NOS Mullard XF2's the amp had a short in the bias circuit and the tubes were pulling over 100ma per side and NO redplating. this was at 400VDC on the plate. it sounded shitty of course. But , I was astounded at the ability of those tubes to take that kind of abuse. They do not make them like they used to and I frequently get crappy tubes right out of the box these days. I have had the best luck recently with Sovtek, Electroharmonix etc. everything else has been a crap shoot, especially JJ and chinese.

-55v bias sounds more like a 5881 where as an el34 would be more like -45v. That is bias volts not ma. that would not account for the redplate though, in fact the opposite.

Yeah, what are you measuring? Bias voltage or the tubes' current draw?

[madmartigan]

I set my multimeter to 200vdc and thats how i get a measurement with a bias probe. One of the ones that plugs into the tube socket and you plug a tube into it. VDC is the only way I can get any type of reading. Am I doing something wrong? There is a setting on the mm that says ADC but that does not give me any reading, and never has on any other amp. What do you guys set your meters to?

[madmartigan]

The only way I get a reading on pin 5 with everything turned on is if I set the mm to 2000m vdc. It starts at about 50 and then keeps climbing past 300 but by then the tubes are red hot and smoking so I shut it down.
This is with the tubes in.
Without tubes I get about -8 millivolts dc.

[dynaman]

Depends on which type of "probe" you have. Some contain a 1 ohm resistor in them that's put in series between the sockets' cathode pins (#eight). For this type, you'd set your meter to its lowest DC volts range. You'll be measuring millivolts. The "volts" reading will be the same as what your tubes are drawing in current (milliamps).

The other type has no resistor and simply breaks this connection and puts your meter in series to complete the circuit. Here, you're directly measuring the current and your meter should be set to mA.

Does your meter have multiple connections for the leads? Mine does. Make sure the red ("hot") lead is in the proper jack (amps vs volts).

[madmartigan]

yes multiple leads plugged in correctly. Cant tell if theres a resistor cause the guys who built it filled it full of epoxy. its called a Biastool by SRS.

I do have some 1ohm resistors on order but theyre shipping from Hong Kong so theres no telling what their quality will be like.

BTW I happen to have some 1ohm 1/2 watt resistors lying around. Should I just wait for the 2 watters from HK or will 1/2 watt suffice?

[dynaman]

1/2W is OK. If it blows, then it's acted as a fuse.

Ya gotta know which type of probe you have to know which meter jack to use. SRS makes both variations.

Set your meter to measure resistance and check the probe's leads (out of the amp). If it reads 1 ohm, then you use the volt setting on your meter when the probe is in the amp with a tube in it.

If your meter reads nothing or infinity, then you need to set your meter up to measure milliamps with the probe in the amp.

You should have both tubes in the amp when doing your measurements.

[madmartigan]

When I measured the leads I got infinity so plugged them into amps socket. It started at 12 and climbed to infinity. Something is giving me over 200ma bias current on the tubes
update: Tried the same with the old tubes and they did the same thing only slower. I stopped once they passed 80ma

[dynaman]

Is this your amp?

http://schematicheaven.com/marshallamps ... xx_50w.pdf

[es345]

The only way I get a reading on pin 5 with everything turned on is if I set the mm to 2000m vdc. It starts at about 50 and then keeps climbing past 300 but by then the tubes are red hot and smoking so I shut it down.
This is with the tubes in.
Without tubes I get about -8 millivolts dc.

On Pin 5 (grid 1) you should have something between -40V to -50V. If you have only -8 mV then definitely your bias circuit has a defect.
Pull out the power tube, switch on (both switches) and start measuring.
Reference is the JCM 900 50 W power amp diagram.

1. AC voltage at the connection C15,R7: should be something around 100V AC
2. Connection D1,C15,R29: should have around -90V
3. Connection R27, R26: around -42 to -48

[madmartigan]

I cannot get any of these measurements. they are all less than a volt.

[JD0x0]

I cannot get any of these measurements. they are all less than a volt.

thats why you need to put your meter on a decimal setting or lower the range.

[dynaman]

I cannot get any of these measurements. they are all less than a volt.

thats why you need to put your meter on a decimal setting or lower the range.

If he wants to read anything over 2 volts, he'll need to set his meter to a higher range...

The voltages in your very first post are wrong. They should all be about 100 volts higher. And I wouldn't expect to see such a large difference between pins 3 and 4 on the output tubes.

I'm kinda suspicious of the diodes in the power supply. I had to replace those in a JCM900 before.

[madmartigan]

I had the meter set to 200vac.

I checked the diodes earlier and they seemed fine, got a reading in one direction and infinity in the other.

[dynaman]

Only the first measurement he cited should have been checked with your meter on AC. The latter two need to be checked the meter set to DC volts.

[madmartigan]

Yes switched it over to dc when i didnt get a reading but still got nothing. And its definately not supposed to be millivolts? mVDC?