Do any of the guys on the forum know the best way to do this? i know about the first pin on the preamp tube to ground and doing it that way but was curious if anyone has a better method?

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Luck won't do it and ignorance can't

George Van Eps.

Hi Gorgon
I saw this in the week but didn't have time to reply. Sorry.

Having a look at the schematic, it looks like pin 1 of V1 is a good method to drain the capacitors. You already have 110K R on that line making the drain capacitor friendly.

Looking over the schematic you could drain off the negative of C22. But put a 100K resistor in line with your earth lead.

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No no and no

Thanks Nick, i appreciate it. yeah i saw an amp tech do the method you're describing when he was draining the caps from an amp because it had some kind of obscure preamp valve in V1 where pin 1 was actually a heater pin and not the plate output so he rigged up the resistor you're talking of to do the job.

I won't be tackling it yet but i'll be doing it sometime ahead.

thanks mate.

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Luck won't do it and ignorance can't

George Van Eps.

I don't have any amp info for you Gorgon but I just want to compliment you on your choice of amp.The 4010 is the combo version of the 2204 probably the best 50W amp Marshall ever made,with that master volume you can get real lovely raunch at volumes almost suitable for a living room.

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'65 Strat,65 Mustang,65 Jaguar,4 more Strats,3 vintage Vox guitars,5 Vox amps,'69 Bassman with a '68 2-15 Bassman cab,36 guitars total-15asst'd amps total,2 vintage '60s Hammond organs & a myriad of effects-with a few rare vintage ones.

hi gorgon as guitslinger said great amps i have a 1981 jcm 800 but the 100 watter version , i wouldn't part with it for anything , it has everything i need from an amp . Also i'm not to far away from you , i'm in wishaw just outside motherwell as is fellow forumite snowy , we'll need to arrange a get together sometime mate , cheers
Alan

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Thanks guitslinger yeah these old JCM 800's are out of this world. They give me most everything i'd ever need. Maybe not quite as good a clean sound as a fender but man they are kickin' amps.


Hey Alan, man your JCM is a year older than mine! On opening up the chassis the other day to bias the tubes i saw the 1982 date written on the inside of the chassis.

You've got the 100 Watter, even better, more powerful. You know in all the years since i bought it i've looked at the JCM900's, the DSL's, the TSL's, and all the rest but when it comes right down to it i don't think there's anything to beat these older single channel Marshall's without all the channel switching stuff. Also as technology is getting more advanced, for some reason the older models start sounding even better! I don't know maybe they just put more build quality into it back then.

You're in Wishaw! this is a small world! I'm in Barrhead just outside of Glasgow, a few miles from Paisley, yeah we'll definitely have to arrange to get together and swap notes.

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Luck won't do it and ignorance can't

George Van Eps.

hi gorgon , i know borheed well mate , done a few jobs down that way , look forward to hooking up with you at some point mate and i'll ask you questions about how to bias a jcm 800 mines has never been done in it's puff , cheers
Alan

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Well join the club! neither had mine! this is the first time it's been done and it was pretty easy to do, and you do know this place well! using it's proper pronunciation! Barrhead just doesn't sound right! way too posh!

Gary

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Luck won't do it and ignorance can't

George Van Eps.

hi garry , as said i know it well any special equipment needed to do the biasing ?, and did you notice any difference in the sound from your combo ? i'd be interested to hear your thoughts mate , my 800 head still has the original valves in it , i think about changing them evey now and then but it sounds so good and i forget all about it till the next time endless cycle cheers
Alan

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Hey Alan all you need is a regular multi meter and a thing called a bias probe, basically you put the bias probe in where your power tubes go and then plug the tube back in on top of the bias probe. Anyway it all gets a bit technical but rest assured it sounds more tech than it is, it's actually a pretty easy procedure to do. I'll include a link to the Eurotubes site where you can see a tech re-biasing a Marshall like ours, his is a 50W JCM 800 head but the innards are basically the same as the combo's.

I wouldn't do it if you're happy with the way your amp is performing. If it's cool leave it as it is. I'm just a bit of a tinkerer and always want to know how to do things.

As for the sound i did notice a big difference. The sound was more browned out, more saturated the way it was before i re-biased it, after the bias the tubes were not running as hot. You measure the bias figure in ma milliamps, so i checked it before i did it and it was way up at over 49ma, i sent Marshall an email and they recommended a figure of 40ma for the setting. So i brought the bias down to just over 40 and it sounds good. The tubes will last longer when it's not running so hot, but i must admit i liked that fiery sound! just have to turn it up louder now!

Anyway your amp will have the same bias set up as mine, you have to remove the chassis from the amp to get at the bias pot. There's a little black switch inside the chassis that takes a very small screwdriver and you adjust the bias that way. A lot of newer amps have a bias control built into the amp so that you don't have to take the chassis out, which is a lot more convenient.

The problem with removing the chassis is you can expose yourself to lethal voltages if you're not careful. There are a lot of places inside the amp that store power even when the amp is off and unplugged. The filter capacitors can store power for ages so you don't want to touch anything you're not sure of.

Here's the link Alan, this gives you a good idea of the steps involved.
http://www.eurotubes.com/euro-Generic-Bias.htm

Cheers Alan, what type of music do you like?

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Luck won't do it and ignorance can't

George Van Eps.

Set my 100watt for 35ma per pair. It's the 4211 combo.
I set my KT88 KK head at 58ma per pair.
Did you get a plate voltage reading before biasing.

An example of how to work it out.

I think the average plate voltage of those 50 watters is 480v DC. Divide the max power of the el34 by the plate voltage (25watts/480v) gives 0.052. (Or for our purposes 52). The you want 70% of that, 0.0364. So bias to 36ma.

Please don't take that plate voltage guess as a given fact. Thats my guess based on vague memory.

(remember fellas, left hand in pocket, right hand in amp.)

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No no and no

Thanks Nick, no i didn't take a plate voltage reading. I know how to do it, touch off on the 3rd pin of the power tube to ground, but i didn't do that. The stuff i was told said that the bias on this amp could be set reliably in the 35/40ma setting. That wasn't from Marhall that was from another source. Marshall said 40ma. I wouldn't like to bring the bias any lower than what it is now as there is, as i said to Alan, a big difference in the sound. Bringing it down from 49+ma to around 40ma has changed the really browned out sound and cleaned it up to an extent.

Thanks Nick, BTW i echo your advice about safety, for anyone reading this don't under any circumstances poke around inside the chassis of a tube amp! there are lethal voltages in there and you can kill yourself if you don't know what you're looking for and start randomly poking around. Lots of people think if the amp is off and unplugged they're safe, this is not true, the nature of the capacitors inside the amp mean that they can soak up and retain massive voltages for long after the amp was last used. As you say Nick, and it's great advice, one hand only working in the chassis preferable the other one in your pocket out of the way.

This is the reason i wouldn't, at this point in time, do filter cap replacement or resistor replacement; i don't know enough and am not confident enough to do that. I hope to be able to do it in the future, but not now. I can replace tubes and re-bias fine but at the moment that's as far as i'll go. Through the reading up i've done and what i've learnt so far one lesson is well learnt and that is never to take any chances with electricity and to treat it with respect.

For anyone who is not willing to take the time to learn a bit about it, then they're better off taking their amp to a tech to get a re-bias done. But for someone who is interested and is willing to read up and learn a bit about it then it's a simple procedure.

I think your calculation figures are spot on Nick as that's the same advice i've had as well

Gary

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Luck won't do it and ignorance can't

George Van Eps.

An important thing to remember is that plate voltage can wander over time. Just like bias can.
Don't take it as a given fact that it is one set value. Especially on older amps.

That said, theres no point in having an amp that doesn't sound right. If it sounds right, it is right.
That's my rule of thumb.

It can lead to a very empty wallet and a good reputation with Hotrox though.

Did you get the bias probe from the Chinese chap I linked to?

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No no and no

Good advice Nick. No i didn't get the probe from him i bought it from the Eurotubes site the one where the tech guy is doing the bias setting. they did a pretty good deal on the probe and a fairly cheap multi-meter that did the job. Only thing is i had to wait on it coming from the states, only took about a week though.

Yeah that's the truth about the sound being the thing. from watching and reading what other guys are doing, some set up their bias with the ballpark figure but then fine tune it to what their ear wants to hear. Some guys like to run them hotter for more saturation some like to back it off. Having the bias high you'll burn through them a lot quicker and HotRox will be pleased! i get stuff from them too! great company, cool stuff!

How i wish i lived down there Nick where you are, i could pick your brains for more tech info! I'm always hunting for more knowledge.

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Luck won't do it and ignorance can't

George Van Eps.

hi gary , thanks for the link mate , i'll check that out later on , and i know what you and nick are on about with valve amps , i have no probs tearing guitars to bits but i leave my amp work to someone who knows thiere stuff , and the biasing doesn't sound to tricky to do if i ever get the urge as for music i like it's mostly 60's rock and blues stuff , i'm going to see zappa plays zappa on saturday ( i saw frank in 88 ) should be good , taking my 20 year old daughter to see dweezil do his dads stuff , cheers again mate
Alan

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