'70 PA20 - do I need to re-bias

[surfguy13]

I have a 1970 PA20 and I'm not sure if it's Class A or A/B or neither!! It only has the one on/off switch which made me think it might be A or A/B. Photos here:

Marshall PA20 1970 pictures by surfguy13 - Photobucket

The amp was serviced by Marshall but not sure how good the replacememnt valves are so wanted to re-tube and wasn't too sure if I need to re-bias too.
Any help would be much appreciated!

 

[demonufo]

You're cathode biased, fixed by a resistor. As long as you're using EL84's, you're fine to fit and forget.

 

[demonufo]

I should add that class of operation has nothing to do with it whatsoever.

 

[surfguy13]

Hi Demonufo
Brilliant, thank you. To be honest I'm not technical at all, sure you guessed that so the reference to 'class' was simply down to my assumtion that 'class A' or 'A/B' meant that you could change tubes without re-biasing. Like in my old 50s princeton.

So I can just whack EL34s in there and go? Do they need to be a matched pair? Thanks again, really appreciate your help.

Cheers

Guy

 

[hbach]

Hi Guy,
I built a clone of a 2061 which is similar. I think actually exactly the same power section, and they are class A/B.
Because EL84s use a different socket than EL34s you can only use them.
I recommend you should have the bias of the amp checked (If you feel confident working with the lethal high voltage in a tube amp you can do it yourself, but if you have the slightest doubt take it to a tech). To change the bias the tech will have to change the cathode resistor. Generally these amps run at a VERY hot bias by design due to their very high plate voltage (around 15 Watts dissipation at idle). I changed the cathode resistor in mine so I have around 12 Watts at idle (recommended maximum overall) in mine. It sounds very good to my ear and goes a long way to improve the life of the output tubes. Some more delicate brands might even not work with such abuse and red plate. The beauty of the cathode bias amps is that the bias will adjust "automatically" with slightly different tubes. But these ones are a bit to rough on tubes by design in my opinion.
You should definitely use a matched pair, as the two tubes share the cathode resistor.

 

[surfguy13]

Hi Hbach

First of all many thanks for underlining that I need EL84s and not EL34s!! I now see that Demonufo said they needed to be EL84s but I stupidly assumed they were EL34s.

Right, that makes good sense to me and as I'm taking my old Vox head to the guy that works on my amps next week I'll get him to check the bias on the PA 20 and ideally change it to 12 watts at idle.

The tubes that Marshall put in there are Marshall branded EL84s so no idea what they use as default when servicing. Any advice re a good all-round output valve? I will definitely make sure I get a matched pair too.

Cheers

Guy

 

[plexifier]

...Any advice re a good all-round output valve?

JJ EL84's sound good in my 18w. There may be better NOS available. ?

 

[hbach]

Hey Guy,

Talk to the tech and try it out. There are lots of folks that are adamant the good sound on these cathode biased EL84 amps, VOXes, Dr. Z, 20W Marshalls etc. comes from such grinding conditions and you will have to live with (meaning pay for ) the strain they put on tubes. I guess it is not black and white, but what compromise you are willing to make. I was going to run the amp now for a bit and then up the ante and run it with a bit hotter bias see how long the tubes live in a comparison. Usually you will even find that the recommendation for cathode bias is 85-90% of maximum dissipation (about 10,5W) at idle. My Mesa TA-15 which also has an EL84 cathode biased power section is set to that bias by design. I have reviewed the power dissipation with signal in theory for my amp and I can go higher as my OT has a 9,2K primary impedance (usually it should be around 8K).
Sorry if any of the jargon confuses you. Just tell me if I'm just speaking chinese to you and something is not clear, although I'm not a tech, just a hobbyist, I know the 2061 which is very similar quite well now.

For tubes: I like JJ EL84 as they sound good to me and are a fair price. Electroharmonix are quite similar but I somehow prefer the JJ. I prefer both to Sovteks. I have a pair of Mullard reissues I've been meaning to try, but I end up fiddling so much with my amp that I want to run it for a bit before I install anything new.

 

[surfguy13]

Thanks guys, REALLY helpful! You both suggest JJs and strangely enough I've just bought a set of tubes for my '62 AC30 head from the US (I'm in the UK). They soumnd pretty damn good to my ears and believe it or not the tubes I took out are the original Mullards! I shall go for JJs for the PA20 and see how they go.

You mentioned the Mullard reissues Hbach - I've been trying to find someone who can give me an opinion on them. Seems like the logical choice for an old AC30 and what I was going to do was buy a set of the Mullard reissues from the same source for my '64 AC30 combo and then A/B them with the JJs.

I will discuss the PA 20 with the guitar tech as he's a friend and easy to talk things over with. And no, you're not speaking Chinese, I can sort-of understand what you're saying and will print out these posts and take them along for Andy to read. It's incredibly helpful. Tubes are starting to get expensive now and it is a consideration whereas 20 years ago I didn't give it a second thought. I'd prefer a compromise where the amp wasn't running so hot it is caning the tubes. I understand what you mean about this being an integral part of the sound though - the AC30s run REALLY hot and these old ones have rubbish vents so that also has to be considered. I guess it is part of the sound though, as you say, and if you reign it in too much you run the risk of changing that. For me there is nothing like an old AC30 but if you run them hard for a couple of hours you can fry eggs on the top of the cab!
Thanks again guys, much appreciated.....