Early JCM800 2203 2.2k 10w screen grid resistors

Iv recently bought an early 800 2203. Looks a very clean example with no obvious mods as far as I can see. Whilst peeking inside I noticed 4 x 2.2k 10w resistors where the 1k 5watts should be. Any idea why someone would do this and more to the point is it going to cause any issues. Valves have biased ok at 60%. Should I be looking to getting them replaced with the correct values 

It will restrict the maximum power output very slightly but otherwise there’s no downside and it should be a bit less hard on the valves.

It may have been modded for KT88s at some point - apparently the Kerry King model which has those also has 2.2K screen resistors, but I’ve never seen one. If so check the bias circuit has the correct values for EL34s as well - although I’m guessing it does if it’s at 60% (which is correct) and the trimmer isn’t right at the hot end.

I love reading these threads where I know all the words are english but I have no idea what they mean :-D  I really should set a new years resolution to learn some technical stuff.

ICBM said:

It will restrict the maximum power output very slightly but otherwise there’s no downside and it should be a bit less hard on the valves.

It may have been modded for KT88s at some point - apparently the Kerry King model which has those also has 2.2K screen resistors, but I’ve never seen one. If so check the bias circuit has the correct values for EL34s as well - although I’m guessing it does if it’s at 60% (which is correct) and the trimmer isn’t right at the hot end.

Plenty left on the trim pot. I think I’ll get it in for a service at some point before I start gigging it. Probably ask them to put correct values in whilst it’s in. It’s still got the original xf4’s and caps in it so probably could do with a once over 

HHwarner said:

Plenty left on the trim pot. I think I’ll get it in for a service at some point before I start gigging it. Probably ask them to put correct values in whilst it’s in. It’s still got the original xf4’s and caps in it so probably could do with a once over 

Make absolutely certain they don’t throw out the valves. Even if you don’t want to gig it with them in to preserve them, those - and possibly RFT preamp valves, if they’re original too - are worth a good amount, even old and well-used... not to mention if you want to record with it or something.

The caps are more likely to need doing, but even then I would assess first - the originals are very good quality and can often survive this long. That said, if any are questionable then replace the lot, including the bias caps.

The El34's in the 2203 draw ~ 8mA screen current when idle, but when driving the power section the screens draw ~50mA

So what that means is with the stock 1K screen grid resistors your screen voltage will drop 50v + the voltage drop from the power supply when cranking the power amp.

With the 2.2k resistors the screen voltage will drop 110v + the voltage drop from the power supply when cranking the power amp.

The screen voltage tells us two things. It directly effects the power output of the amp and the change in screen voltage effects the compression of the power amp.

So from a playing perspective the effect of this is as you start to drive the power section, the amp with the 2.2k resistors will compress more and put out less power compared to the amp with 1K resistors.  This is how the trainwreck express and liverpool go from clean to mean without massive volume changes.  The effect with your amp will be less though as the power supply arrangement is different.
You may not actually like the extra compression depending on the type of music you are playing.

Two things to remember:
If you never play the amp properly loud then this is much less of an issue.

It is likely a tech fitted the 2.2k resistors to stop the screens melting causing failures in the amp. You can actually watch the screens glow when playing on some amps. if that is the case then keeping them at 2.2k would be a good idea.

It does sound pretty compressed. Suppose it covers up my sloppy playing. Bias trim pot resistor is correct for EL34 (47k) but looks like it’s been replaced at some point. Bias caps look to have been replaced also. 

HHwarner said:

Bias trim pot resistor is correct for EL34 (47k) but looks like it’s been replaced at some point. Bias caps look to have been replaced also. 

Yes, I’d guess that’s been converted to and from either KT88s or 6550s and back, but they left the larger screen resistors - or possibly the bias caps started to fail and the resistor was changed to try to compensate (a common bodge when someone doesn’t realise the fault is leaking caps) and the larger screen resistors were part of that in an attempt to make the valves run cooler, then someone else fixed the bias but left the big screen resistors.

I’ll probably change them for 1k 5w and just keep an eye on it.