Biasing a Pro Mod with KT-88's

[gitrman65]

I bought my Pro Mod used on ebay and I didn't get the power tube boxes with the ratings on them. Anyone know if the JJ KT-88 are rated the same for all the amps. If not, has anyone biased their amps and know where the bias adjustment pots are and what plate current is recommended for the amp. Any help would be greatly appreciated. My amp is pretty new and the tubes sound great, but I like to be prepared just in case!

[firebird]

Good question. I am in the same boat and that would be good info to know.

[parntz145]

I e mailed Scott about this a few weeks ago. He told me that the Pro Mod with KT88 should be biased at 50ma. I know that is low (on the cold side) but that is where he said it should be. I think the bias adjustment is the brass screw on the inside of the chassis.

[angusdeadking]

Can I revert my ProMod into a Quickrod if I install el-34's?

[LooseCannon]

angusdeadking said:

Can I revert my ProMod into a Quickrod if I install el-34's?

Yeah but you'll need to change one of the resistors or something.

[gitrman65]

I got my new tubes in and biased. For some reason, when I checked the bias the current was only 29mA, so I adjusted the bias pot all the way out and could only get 41mA with the bias pot all the way out, so Scott told me to replace the piggy backed 22K resistor in spot "R30" with a 47K to get the within the bias range, it worked great and the amp sounds much better. I bought this amp used and someone might have already "tinkered" with it. It would probably be good if you have a used Splawn to check this situation out. The values of the piggy-backed resistors when I started were both 22K Which is the color code " red red orange " On Scott's advice I clipped the top resistor off and changed it to a 47K resistor whose color code is " yellow violet orange". But for those wanting to go the other way you could probably go the other way with a lower ohm resistor or remove the piggy-backed resistor completely to lower the bias adjustment within range for the EL-34's

[Ayton(e)]

How did the sound change after your hotter rebias?

[erock503]

hmm, very interesting thread. I've been contemplating trying some KT88 in my Nitro, but I'm wary of messing with such a great amp if it's only for curiosity. If it's that simple though, that doesn't sound bad.

What would be the best kind of solder to use on a PCB amp circuit, and what other specs should the resistors be besides actual resistance. ie type(metal film, carbon comp, etc.), wattage, tolerance, etc...

[gitrman65]

The re-bias made the amp even louder and eliminated some wierd noises that I thought were coming from the electrical outlet. It seems to have made the amp quieter also, or maybe because it's louder I just don't hear as well when I'm not playing! I used a 1/2 watt 5% tolerence resister and silver bearing, rosin core, .022 dia. solder, both from Radio Shack. The resistors match some that came with the amp. If you check with some other tube websites you will find that the 50mA bias setting is more toward the low side, but this is what Scott recommended. I haven't tried increasing it because 1) it sounds good 2) the tubes will last longer at lower bias settings and 3) I am a poor musician who can't chance burning something up. I feel that Scott has probably checked many different settings and found this to be the best compromise of sound and tube life.

[erock503]

thanks for that info man, much appreciated.

[xjohnx]

I was having the same problem. I couldnt get my pro mod to bias past 30ma. I tried your fix and got different results. With the 47k ohm resistor I cant get the bias lower than 120ma!! Radio Shack has since closed but tomorrow I am going to try a 33k. Does that sound like a logical idea?