question on biasing a Marshall JCM 800

[markmalin]

I have a JCM 800 that I just put 4 new 6550's in and want to adjust the bias.  I could solder in 4 1 Ohm resistors from the cathode to ground, but kind of hate to modify it so I wanted to use the transformer shunt method.  It's been years since I've done this, but I put the negative lead on one of the Pin 3's on the output tubes and the positive needs to go to the CT of the output transformer.  Assuming this is the same node as the HT fuse in the diagram below, I did that but just get a hum and readings like 5 ma.  (the amp sounds fine but I want to dial it in before I sell it).  What am I missing?


Mark

[shooter]

fwiw, I copied this, might be helpful, or not

[Ritchie200]

Mark

You are checking the right places.  Are you sure it wasn't 50mA and maybe your meter auto-ranged it to 5.0?  I'm just guessing here.   Be VERY careful with your meter probes.

Jim

[markmalin]

Thanks guys!


So I happened to have a pile of 1 Ohm resistors from Doug, so I decided to solder them in on all 4 sockets.  There's so much written about where to bias a class AB (fixed bias) amp, holy cow.  I wound up dialing in about 50 ma (if I understand correctly since this is a 4 tube amp each tube is seeing 25 ma).  Seems on the cold side, but to be honest it sounds very nice so I'm going to leave it there unless someone suggests different. 


(I had forgotten how nice a Marshall sounds.  May have to build one for myself)


Mark

[mresistor]

You could also use this bias caclulator.  http://www.tedweber.com/bias-calc   Most people I read are saying they like their JCM800s at 60% of max plate dissipation. What is your plate voltage? I would think the correct bias would be around 47 to 48 milliamps.  35w per tube at 60%  35x.6 =21w.  Say your plate V is 450V   then  21/450=.047. That is 47 milliamps.   I would say if you are reading  50 milliamps you are throttling  the 6550s at around 63-64% max dissipation.  You are running it 50 milliamps per tube.

[markmalin]

You could also use this bias caclulator.  http://www.tedweber.com/bias-calc   Most people I read are saying they like their JCM800s at 60% of max plate dissipation. What is your plate voltage? I would think the correct bias would be around 47 to 48 milliamps.  35w per tube at 60%  35x.6 =21w.  Say your plate V is 450V   then  21/450=.047. That is 47 milliamps.   I would say if you are reading  50 milliamps you are throttling  the 6550s at around 63-64% max dissipation.  You are running it 50 milliamps per tube.

Thanks!  Good info.

I'm confused about the value for a 4 tube push-pull setup.  Using the transformer shunt method, if I read 50 ma it would be 25/tube on that side.  That makes sense.  If I measure a 1 Ohm cathode to ground resistor on one tube and it's seeing 50 ma, it makes sense to me that that tube is seeing 50 ma, but the stuff I've read (or mis-read as the case may be) seems to indicated even this method you divide the reading in half (i.e. 25 ma per tube) on a 4 power tube amp.

[Willabe]

If I measure a 1 Ohm cathode to ground resistor on one tube and it's seeing 50 ma, it makes sense to me that that tube is seeing 50 ma, but the stuff I've read (or mis-read as the case may be) seems to indicated even this method you divide the reading in half (i.e. 25 ma per tube) on a 4 power tube amp.

If you measure the current draw across a single 1 ohm R on a single tube, then that is what that single tube is drawing for current.

[sluckey]

If I measure a 1 Ohm cathode to ground resistor on one tube and it's seeing 50 ma, it makes sense to me that that tube is seeing 50 ma

That's correct. You did say you put a 1Ω resistor on each of the 4 sockets, right? And I'm assuming that you don't have any wires connecting pin 8 of any socket to pin 8 of another socket?

So, if you measure 50mV across a 1Ω resistor, that means 50mA is flowing thru that resistor and that same 50mA is flowing into the cathode of that tube. You need to repeat this measurement for all four tubes.

[mresistor]

Don't be too surprised if the readings of the other tubes are not the same. It is rare that you get a perfectly matched quad set of tubes. What I do is try not to run any one tube past say, in your case with a JCM800, 68% max plate dissipation. In other words make sure the tube with the highest reading is not higher than 68-70% plate dissipation, knowing that the 6550 is 35 watts max. Once I know which tube has the highest current out of the 4 then I use it to set the bias.