software

I've got a Marshall JTM30 if that's relevant.

It's kind of one of those things where quot;if you have to ask...quot;

Don't want to see you electrocute yourself because the voltages inside an amp where you'll be working are lethal!

I'd recommend one of those Weber Bias Probes or the Bias Rite product to get you started. You remove your power tubes, plug in the Bias Probes, plug your tubes into them, and then you use a multimeter to read the current.

Much safer if you're a total novice.

Thanks! Lew

Cheers Lew. Now I'm gonna sound like a right noob. There's supposed to be some sort of dial or screw or something to tweak the bias right? Where is it?

be very careful man...you can fry yourself real easy if you mess with the wrong thing in an amp. if your set on doing it yourself take some time and search the internet and find everything and anything you can about doing it.

-Mike

Well I found this about my amp. It looks like it doesn't need rebiasing. Although if anybody knows otherwise please tell me. Also would it be possible to put in EL34's? It currently has 6L6's in. from : localhost/66.102.9.104/search?q=cache:X...ient=firefox-a

Once you locate the mini pot, which is probably somewhere near the power tube sockets, you'll want to know that it's adjusted so the amp sounds it's boldest, tightest, and most rich.

If you're smart enough to deal with a chassis that's turned on and exposed, you can listen to the amp plugged in, while you use extreme caution in turning the bias pot gently.

Of course, it's best to take Lew's advise and buy a $20 Bias Tool and Amp meter, so you can make sure the 6L6's are around 32mA and EL-34's are around 38mA. As long as the multimeter is giving you a reading that's close to those numbers, you won't burn up tubes doing it without the guidance of a meter...........doing it with the mA bias tool will allow you to stay near those numbers while hearing the amp to get it sounding perfect, even if it's a couple mA more or less than the numbers I mentioned.

MY DISCLAIMER: Don't wear loose hanging metal bracelets, while standing in a bucket of water while you bias your amp.

Cheers GJ, brilliant disclaimer. I read somewhere that I might have a fixed resistor instead of a pot on the bias and that it would be good to get a pot in there, what do you reckon?


Originally Posted by APB_4Cheers Lew, brilliant disclaimer. I read somewhere that I might have a fixed resistor instead of a pot on the bias and that it would be good to get a pot in there, what do you reckon?

Take a look and see. Boogies are notorious for having a resistor instead of a pot but my Marshalls have all been adjustable. Does your amp have a standby switch? If so, turn the amp off but leave the standby switch in the on position and play a few chords on your guitar until you hear no sound coming out of the speakers...at that point your filter caps are probably discharged and it will be safer to take a look inside. Unplug the amp before opening it up.

Follow Gearjoneser's instructions for locating the bias adjustment pot. Most likely it'll be somewhere near the output tubes....maybe within a few inches of them but inside the amp.

Never looked inside your model of Marshall though.

Lew

Thanks Lew I'll give it a try.

PM Simon. He has the same amp, and has probably seen if it has a bias pot or not. Unless a qualified tech does it, don't swap the 6L6's for EL-34's. It may be out of the range of the bias pot, or it could possibly require a different transformer.

If it were mine, I'd put some JJ or Svetlana 6L6's and use a highgain tube like a GT12AX7C or M. Sovtek 12AX7LPS is a great tube for Marshalls too. Tungsol 12AX7's are what's in my Jubilee.

Cheers GJ I PM'd him a while ago about this and he said quot;I think it probably can be, but I've never done any biasing myself.quot; I've found some schematics of it if anybody wouldn't mind taking a look I'll email them to you. Or you can download them from here from : localhost/www.blueguitar.org/new/schem/marshall/jtm30.zip

Well thanks for everybodies help. I opened her up and apart from the controls there doesn't seem to be a pot in sight. Here's a pic photobucket shrank it though and it was blurry in the first place

Do you think it would be worth getting a pot put in? I have changed the power valves and they look and sound as though they are running fine.

This thread may be vault worthy. I need to bias my Rivera and get some tubes from Eurotubes, and I've found the info here to be a great launching point.


Originally Posted by GearjoneserPM Simon. He has the same amp, and has probably seen if it has a bias pot or not.

I'm flattered by GJ's confidence, but I've never attempted to bias an amp in my life. That's probably how I reached the ripe old age of 44!

Actually, mine is/was a JTM60, but it's now sold to another forum member. Better not take it to bits.

OK, so what do I need? Lots of dangly metal jewellery and a large bucket of water? Probably best to drink a few beers first, right? Dutch courage and all that.

i heard that KT66 tubes in 6L6 fender BF amps were completely out of range of the pot, and you'd have to run the pot at almost full to get the 'right' breakup of the tube. Is this true that KT66's can handle alot of current?

Bump. Anyone?

It's very possible that your model of amp had the bias set at the factory with a resistor rather than mini-pot. If your new tubes made the amp sound good to your ears, it's probably fine. Right around the late 90's, tubes became a bigger deal to guitar players. The cold war ended, and Russia, China, Germany, and Yugoslavia bombarded our market with cheap tubes. I noticed that a lot of amp companies like Marshall and Fender took the cue and began putting the bias pot on the back of the amp, rather than inside.

That was one of the smartest things amp companies have ever done! On the Marshall DSL's and Fender Pro Tube Series amps, all you need is a multimeter touching the bias points on the back of the amp, along with a mini-screwdriver. I wish all amps had that convenient feature!

the 65 reissue twin also has it on the underside of the chassis, very handy. As well as the balance pot...