Peavey 6505 bias mod

[disschord]

Hi i have been considering moding my peavey 6505 so i can set the bias.
I did a search and found some people saying that the 6505/5150 is set cold for a reason.
I only ever play the dirty channel on my amp. Will i gain or loose anything by setting my bias hotter.

Can anyone give me any info on how setting the bias hotter/cooler will effect the lead channel.

[Enzo]

It won't affect the preamp channels in the slightest.

If you want to make the bias adjustable go ahead. The reason they are biased cold from the factory is that that way, you can plug most any tube in there and it will work fine. No one can damage their amp mis-adjusting the bias. Any amp maker knows that if he makes his amps adjustable and some kid burns his up messing with it, the kid wil be on Harmony Central in an instant complaining about what crappy amplifiers the brand makes. Peavey choses to avoid that.

ANother reason is that the existing power amp arrangement is loud and clean. The tone in that amp comes from the preamp

So there is no compelling electronic reason it is not adjustable, you can make it so.

[disschord]

Thank you for your reply. if i were to do the mod and change the stock tubes to say JJ' 6L6. i have an AMP-HEAd bias tester and a multimeter are there any set bounderys.
I think i read 35/40mA.
Is that about right or is there more to it.

any links with info about setting bias would be great.

[Enzo]

DO some searching for setting bias.

You need to know the B+ voltage. I usually expect to see about 465 in a 5150, but it might be 480 or something. B+ volts times plate current gives plate dissipation, which is what to set to. Me, I like them a hair cooler, so with that B+ I am more like 38-40ma, but others would tell you 40-45ma. A lot of guys use the 70% rule. Set the idle dissipation at 70% of the tube max. A 30 watt tube like the 6L6 would then have 21 watts as a target.

[disschord]

so am i right in saying divide the tube power by plate voltage multiply by say 70%

Eg assuming plate voltage is 470V 30/470=.063x70% = .044

or should i be assuming the tube wont be running at 30WATTS and callculate at 21WATTS

eg 22/470=.046x70% = .32

[Enzo]

Huh?

It's like setting the idle speed on your car engine. It doesn't matter how fast you plan to drive it, it still gets set for 750rpm or whatever at ilde.

Even if you never turn the volume past 1, the thing still gets biased. The tube is a 30 watt tube, regardless of what you do with it. 70% of 30 watts is 21 watts. DOn't discount the tube 30% then discount it again for the bias.

The 70% thing is a guideline, no one can say you MUST use it. if it sounds good at 30ma to your ear, that is all that matters.

[disschord]

ok got it cheers