Are These Stock Jubilee (2204) Bias Readings Correct?

[Jube2550]

Do these seem ok for a stock 2204 50 watt chassis? Just got my biasrite. These readings are with the stock 20 year old tubes and how the amp came out of the box. Tube A readings is the EL34 closest to the preamp tubes or first to the left looking at the back of the amp. Stock plate voltage ideal is 460 for this amp. Anyone? Failing screen resistor or is this ok? Thanks for any suggestions. I have 3 sets of NOS rft and will try some other readings.

A = Vp 466 (seems high) Ik 35.8
B = Vp 447 (seems low) Ik 30.3

[Jube2550]

I'll answer my own question, according to Webers site it is correct 17 watts per tube or 60-70% cheers.

[John Phillips]

Be careful. The plate voltage readings are too far apart for that all to be correct - they should be within a volt of each other (ie no more than one digit out on a DMM).

There are a few possible reasons, the simplest is that the bias probe is not making proper contact with the tube socket (on the side with the low plate voltage and low current) and the extra resistance is affecting things. I have heard that some of these probes tend to do this, especially if the socket has been used with some power tubes with incorrect pin dimensions - which NOS RFTs shouldn't be though.

Another potential cause is that the bias probe may be making the amp unstable and self-oscillate outside the audio range (so you don't hear it) - this can give all sorts of very wrong readings, and is a hazard to the amp while it's doing it. If this happens you'll most likely see one or both tubes glowing oddly, and may hear 'crinkly' noises as they heat up too much.

The worst possibility is a damaged OT winding; but the voltage readings are so far apart that that is unlikely - the voltage drop across the primary (if working properly) should only be in the region of 1.5V, so even a totally shorted winding shouldn't change the plate voltage that much - although a broken winding that's just making a (poor) connection could have the same effect as the tube socket problem.

The side with the high current may be a tube pulling excess screen current too, although that wouldn't explain the plate voltage error.

This is why I don't like bias probes - too many variables, so it's difficult to isolate the causes of any odd readings, and they can cause other errors of their own. Better than nothing, but be careful about taking the results as exact.

The problem with all the ways of measuring the current accurately and independently of other factors are that they're dangerous if you're inexperienced, since they involve working on the tube sockets from the inside... even fitting 1-ohm resistors on the sockets doesn't put you very far away from the high voltages (unless there are proper test points as well).

[Jube2550]

Thanks for your help John, I really appreciate it. I dropped in some brand NOS RFT tubes and ran some new readings.

A = Vp 462 Ik 38.1
B = Vp 460 Ik 37.4

Not bad, no drift after 20 years. Either there as a misreading or the 20 yr old tubes were failing, imagine that LOL. I have replaced the screen resistors on an old Twin so that was my first suspect. Now thanks to your very imformative post I am wise to the ways of these testers. I hope this helps anyone biasing a 2204 or similar 50 watt Marshall. If you stick with matched NOS siemans (RFT) they are direct replacement drop ins.

The caps are swollen so those are getting done next. I am still amazed how well built these JCM800 era amps are. Again, thanks and cheers! Happy Holiday and God bless.

[mmorse]

I use a biasrite on my 2204 and my 2550. My A/B readings are never exactly matched either although they aren't that far off. One thing I found with the biasrite is that if you have the chassis standing on end rather than flat, (easier to get to the pot) the weight of the probe and tube can cause a shift within the socket due to gravity and cause some rather bizarre readings. I know this from personal experience. :rolleyes:

So now I always have the chassis flat and propped up equally on both ends so I can get to the bias pot underneath. BTW, the plate voltage on my 2204 is only in the 365 range while my 2550 is around 445. Not sure if 365 is too low for a 2204 but the amp sounds killer without making your ears bleed (still pretty loud when it needs to be) and it must be easy on my prized pair of XF2s so I'm leaving it alone.