Bias settings for Sunn 350b / 2000s ?

[spellcaster]

Looking for bias settings for the 350B amp.
My amp has the original tung-sol 6550's and is currently showing 25v.
Anyone have the correct bias value for the 350B or 2000s?

[JoeArthur]

The only correct bias setting is measured in volts and is shown on the schematic.

(awful and terrible snide comment about "tube tone guru's" and their biasing methods that was originally here intentionally deleted)

[spellcaster]

Ok, I've looked at the 2000s schematic and where I would expect the see the bias voltage I see an "E".

[mckinnon audio]

  Hi there,look at the end of the power supply and you'll see "e" and right alongside it you'll see -55 vdc.This is your bias volts.It's the 2000S schematic on this site.Good luck,Mel.

[Soundmasterg]

Personally I don't like biasing by a negative voltage alone as you don't know what the current draw is on the tubes and if that is exceeding the max dissipation of the tube in question or not. But I don't want to start any arguments. You can search other amp related forums and sites, and there is plenty of info about suggested bias methods. If you bias using that -55v, the amp will probably work just fine.

Greg

[spellcaster]

Thank you all very much for your replys.

Yeah, I would like to have the amp gone over by a good tech but for now I'm limited to the meter that plugs into the tube socket and wanted to make sure it's not too far off.

 On the 2000s/350B I believe the tubes are wired in pairs and if the bias is originaly set for 55v should I be ideally getting a read out for 27.5 per pair?

[mckinnon audio]

  Hi there,no you will get -55 v at each cathode of each tube,no halving or sharing of the bias volts,each tube gets the same.Good luck,Mel.

[spellcaster]

 Still sounds good but looks like the original tubes might be getting old because I can not get the bias up to 55v. 

[mckinnon audio]

  Hi there,just a thought,check the bias cap. and diode just to make sure they're alright.They'll be at the end of the red/blue wire from the trans.Good luck,Mel.

[Soundmasterg]

Which meter are you plugging in to the tube sockets? Bias King? Bias Rite? These read the cathode current indirectly as a voltage across a 1 ohm cathode resistor. The cathode is usually grounded in a fixed bias amp like the Sunn, but inserting that meter allows you to read the cathode current. This will not be the same as the -55v that is listed on the schematic, which is what the negative voltage should be at the grid on the power tubes. I've seen that particular negative grid voltage be low and not get up high enough when the filter caps are bad in the amp. My 200S would not go even half as high as it was supposed to go until I replaced the filter caps in it because the caps were partially shorted.

The cathode current is a combination of the screen current and the plate current, but the screen current can usually be ignored as it is much smaller. If you want to set the bias with this method of measuring, then you also need to be able to measure the plate voltage and multiply that by the reading on your meter. This will give you the plate dissipation, which you then compare to the specs of the tube to make sure you are not exceeding the max dissipation rating of the tube in question. A KT88 is a 42 watt max dissipation if I am remebering correctly. So since this is a fixed bias amp and you need to allow for some leeway, if you shoot for somewhere between 60% and 70% of that 42 watt number, then it should work fine. Measuring the plate voltage requires you to open the chassis and requires a DMM or other applicible meter, and it exposes you to possibly lethal voltages. So if you don't know how to be safe doing this, then it is best to take the amp to a tech to have them set the bias for you.

An example of the math involved. Say your plate voltage is 550v and you have 25 showing on your cathode current sensing device. What that 25 really means is milliamps, so 25 becomes .025. Multiply 550 x .025 and you get 13.75 watts. If the max dissipation of the KT88 is 42 watts, then 70% of that is 29.4, so your bias would be set very much on the cold side at the moment. As you adjust the bias pot to where you get more cathode current, this runs the tubes hotter, but also makes your plate votlage change, so everytime you make an adjustment, you have to remeasure the plate voltage and multiply by what is on your meter to get an accurate idea of what the wattage is that is being dissipated.

If you dispense with your meter and just set things so the grid shows -55v like the schematic says, then according to Sunn back in the day, you bias would be set correctly. Of course this doesn't take into account that different tubes will draw different levels of current either. I think since most techs back then biased using the oscilliscope method, then this is why most manufacturers put the negative voltage number on the schematics. Asking what current you should be setting your bias to is an incomplete question as you also need to know the plate voltage and multiply the numbers out to find out where your bias is adjusted if you use the cathode current method.

Hope that is helpful.
Greg

[JoeArthur]

  Hi there,no you will get -55 v at each cathode of each tube,no halving or sharing of the bias volts,each tube gets the same.Good luck,Mel.

I believe you mean grid and not cathode.

Still sounds good but looks like the original tubes might be getting old because I can not get the bias up to 55v. 

Tubes and/or their condition does not dictate what bias voltage can be placed on their grids. That is a function of the bias power supply. (As suggested by mckinnonaudio with his suggestion to check the bias cap and diode).

[mckinnon audio]

  Thanks for correcting me Joe,to many late nights this week I guess,Mel.