Metropoulos Forum

Well it's been almost 2 years now, and it's time: I need new power tubes for my 2204 Mod5. I know Mark used exclusively JJ El34's, but that's only step 1. Step 2 is figuring out what I gotta bias the damn thing to to let it sing properly. Now I got this second hand from an EBay-er, so I'm completely in the dark here about a lot of mod5 stuff. Everything I've learned about running and maintaining the Mod5 has been from you guys and research on Mark's old posts. So, any Mod5 owners, please chime in!!

There are no special bias settings for the mod5. It's like any ole' amp in that department.

25W divided by plate voltage, then multiplied by 70% = bias current

You should probably take it to a tech to have it biased if you are not that familiar with the innards of a tube amp. If you don't know what you are doing you can really hurt yourself (high voltage caps).

echoplexi1974 wrote:There are no special bias settings for the mod5. It's like any ole' amp in that department.

25W divided by plate voltage, then multiplied by 70% = bias current

You should probably take it to a tech to have it biased if you are not that familiar with the innards of a tube amp. If you don't know what you are doing you can really hurt yourself (high voltage caps).

Yea there's a great tech 5 minutes from my house that I trust with this, but I wanted to check if there was a specific way Mark biased the heads before I gave it to my tech to bias it. But as long as the tone and gain are still there, I'm happy!

http://www.rig-talk.com/forum/viewtopic ... 7#p1398312

"bias range is calculated by the b+ of the given amp.

1. for an amp with a B+ of 480dcv, 70%MPD would be 36mA
2. for an amp with B+ of 400dcv, 70%MPD would be 43mA

in example #1 45mA would be way hot.
in example #2 35mA would be way cold.

so its not as simple as someone saying "bias them at 35mA" unless they knew that that particular amp had a B+ of around 500dcv.

You need to find the B+ as part of the equation when biasing and then do the math.

Mark"

You could just re-bias the amp with the same tubes. Especially if it's been a couple years. Trust me, you might be surprised.

Simplified:

70% of a 25 watt tube is 17.5 watts

17.5 divided by your plate voltage measured on pin 3 of your power tubes = bias setting in mA

So if you measured 460v on pin 3

17.5 divided by 460v = .038 mA
So .038mA would be the max you could be without going
Over 70% max dissipation

You would need a bias probe or bias tool that plugs in the socket, or if your amp has 1 ohm shunt resistors on the the power tubes, you could measure the voltage drop a across it in mV. Since it is 1 ohm it would be times 1 or 38mV

This was how Mark learned this.