bias for a 73 sunn mode t

[pombagira]

hi everyone, it's been a while, so I was wondering if someone could tell me what the bias setting should be on a sunn model t from around 73..
I have one where my amp tech set the bias at around 26 but that seems to be running the valves at a very cool level. Wondering if I could run more than one 4x12 cab off that with that low bias.
Any help will be gladly appreciated
Thanks Pete

[doc10039]

What is the voltage at pin three on the output tubes? I just re-tubed my Fender Bassman with 6550's and I am running those at 40 to 41 mA. What kind of tubes are they? What is the output power of the tubes? You can multiply the output wattage of the tubes by .70 to get 70% of maximum power. Then divide that total by VDC at pin 3 to get what your bias can be set. I used 27watts as my base only because I did not know the output. I am still learning also but I have picked up on a few things.

Doc

Metro Plexi 100 Watt kit(tubed with 6550's)
SUNN 200S

[jaywalker]

Short answer is yes you can run the cabs as long as the impedances (ohms) are correct.

What brand and type of power tubes do you have?
"26" whats? Watts?  

There seems to be some variation of the number for maximum plate dissipation amongst the 6550 family. Looks like your number "may" be just fine being between 65 and 70% depending, of course, upon the answers to my two questions.

btw we are talking about maximum plate dissipation or the ability to dissipate heat expressed in watts, not tube output power. We set the bias at idle to the lower percentage 'cause when we put the pedal to the metal more watts are happening and we want to stay below the max because..well..it's the maximum it can take. The same is true of speaker wattage ratings...if heat keeps building up beyond the abilty for the device to get rid of excess something is going to melt or warp like a voice coil...China syndrome!

[pombagira]

unfortunately the watts reference was surely wrong, however the bias did read around 26/27 but I was told that was low for a sunn model t. The valves are JJ 6550s. Sorry I can't be of more assistance because I'm no amp techician. I have watched people work on my model ts and noticed when I was told the bias was low. Explanation given was that this would lengthen the life of the valves, while reducing amp output. Not sure I'm making this any clearer..

[jaywalker]

The only spec sheet for the JJ 6550 I could find is for the "new and improved" KT88/6550 which has a max plate dissipation of 42w. Your tubes could be the pre "new and improved" and possibly had a lower m.p.d. so I think you're good right where your at...at worst case you could be a little cold at 62% of 42w (=26w). If those tube did have a lower m.p.d. number you would not want to push it up any. Does it sound good? If so then live long and prosper! You could always take it to a tech and carefully raise it up and watch for redplating. Either way running another cab doesn't make any difference regarding the bias. You will be moving more air though.

[EdBass]

unfortunately the watts reference was surely wrong, however the bias did read around 26/27

What unit of measurement are you referring to, and how was it measured?

[jaywalker]

unfortunately the watts reference was surely wrong, however the bias did read around 26/27

What unit of measurement are you referring to, and how was it measured?

He just has a number and from that I am really making some guesses that we are talking about plate current in mA, hopefull measured in some acceptable fashion. 26 seems too low a number to be bias voltage and called "cool"...maybe not? Everything that I said is based on my assumption and we all know what that means! That's why I'm refering him to a tech if he wants to have it tweaked some. Regardless, I don't think the bias issue is relevant to the use of a second cab.