How to bias this amp

[dude]

Can someone explain how to bias this Sunn Bass amp copy by Weber? Forgive my ignorance but I never encountered this bias set up. The jack by the KT88's, is this for a stereo jack and now does one go about biasing this amp.  I see two bias pots for each tube, does the jack input play a roll in biasing this amp?

[sluckey]

The stereo jack was meant to make it easy to monitor the mV on the cathodes. Ideally you would make up a special cable with a stereo plug on one end and two meter leads (both black leads connect to sleeve of the plug) to be connected to two meters. You could monitor the cathode current of both tubes simultaneously. Adjust the bias pots to set the cathode mAs. I wonder if anyone ever built that special cable. Probably not.

So, just forget the jack is even there and connect your meter to pin 8 of a power tube. Adjust the bias pot for that tube to set the desired cathode current mA. Repeat for the other tube, hopefully matching the current set for the first tube.

***CAUTION!!!***

Look carefully at the bias pots in the schematic. Notice that the bottom lug is connected to ground. THIS MEANS YOU CAN ADJUST THE BIAS VOLTAGE ALL THE WAY DOWN TO ZERO VOLTS ON THE OUTPUT TUBE GRIDS. DOING SO WILL DESTROY YOUR TUBES!

Look at page 2 of this pdf to see how to safely connect the bias pots to prevent the zero volt crisis. Shame on Weber for releasing such a stupid dual bias adjustment circuit!

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

[shooter]

WHAT HE SAID, n beat me oh keyboard warrior 

R42/43 should be 1 ohm, monitor them with 2 meters through the stereo jack while adjusting R25/41.  That's my guess 

[PRR]

> I wonder if anyone ever built that special cable.

You can use a mono cable and pull it to half-way to read V4. Potentially real sweet if you have a HeathKit VTVM with 1/4" input: use a guitar cord. (No, shorting V4 while reading V5 will do no harm.) However the resolution of a basic VTVM is <100mV, and with 1r check resistors we need <10mV.....

You wonder if brain was engaged.

[dude]

WHAT HE SAID, n beat me oh keyboard warrior 

R42/43 should be 1 ohm, monitor them with 2 meters through the stereo jack while adjusting R25/41.  That's my guess   

Yes, there is  a 1 on the schematic (R42/43) but not clearly written, thanks. If l make the cable, l’ll bias using the jack, thanks.

[dude]

The stereo jack was meant to make it easy to monitor the mV on the cathodes. Ideally you would make up a special cable with a stereo plug on one end and two meter leads (both black leads connect to sleeve of the plug) to be connected to two meters. You could monitor the cathode current of both tubes simultaneously. Adjust the bias pots to set the cathode mAs. I wonder if anyone ever built that special cable. Probably not.

So, just forget the jack is even there and connect your meter to pin 8 of a power tube. Adjust the bias pot for that tube to set the desired cathode current mA. Repeat for the other tube, hopefully matching the current set for the first tube.

***CAUTION!!!***

Look carefully at the bias pots in the schematic. Notice that the bottom lug is connected to ground. THIS MEANS YOU CAN ADJUST THE BIAS VOLTAGE ALL THE WAY DOWN TO ZERO VOLTS ON THE OUTPUT TUBE GRIDS. DOING SO WILL DESTROY YOUR TUBES!

Look at page 2 of this pdf to see how to safely connect the bias pots to prevent the zero volt crisis. Shame on Weber for releasing such a stupid dual bias adjustment circuit!

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

Thanks, yes l see no resistor to ground after the bias pots, your schematic shows a 10k bias pot. I bought a few small 50k PEC bias pots off eBay a while back. If l use the 50k bias pots, what size resistor with l use, you have 18k’s on that Sunn amp? Maybe like 10k’s..?

[sluckey]

I thought you had that amp and were trying to set the bias! Exactly what's the whole story here?

[sluckey]

Yes, there is  a 1 on the schematic (R42/43) but not clearly written, thanks. If l make the cable, l’ll bias using the jack, thanks.

Why bother?

It's possible that jack was intended to be used directly with Weber's fancy bias checker that they no longer offer.

[dude]

I thought you had that amp and were trying to set the bias! Exactly what's the whole story here?

The amp was bought from Weber a few years ago and a guy put it together but never finished. I was asked if I could finish it.  As you can see it's a mess, leads way too long, looks like a rats nest.  I haven't touch it yet, just looking at the schematic and getting a plan together. I noticed one of the small bias pots was missing and I have several small vintage 50K pots so I planned on using them.  The schematic calls for 50K but like you mentioned, no resistor between pot and ground, big trouble for the nice vintage Genalex Gold Monarch, brand new in the box tubes, he also handed me. Your Sunn amp in your link has 10K bias pots and 18K to ground, since the pots I'm using are 50K I'm thinking a lower R to ground, like 8 or 10K. I'm asking your opinion. 
You're probably right about that Weber stereo jack in there, Weber most likely sold a cable, now discontinued. The back of the face plate has a hole labeled for the biasing stereo jack. (Be nice to be able to bias the head without taking the chassis out so I probably will use it.
That's it...

[sluckey]

You can't just copy my dual bias circuit into your messy amp because my amp uses a dedicated bias tap on the PT and your amp uses the HT lead for bias source. Therefore, resistor values will be different.

A note on your schematic suggest there will be about -70v available from that bias circuit. I suggest replacing the 50K pots with 25K pots and 27K resistors connected between the pots and ground. This will give you a bias voltage range of approx -35v to -70v. Use this as a starting point. -35V is probably still low enough to redplate those KT88s so be prepared to increase the resistors to 33K or higher to prevent damage.

[dude]

You can't just copy my dual bias circuit into your messy amp because my amp uses a dedicated bias tap on the PT and your amp uses the HT lead for bias source.

Ok, thanks appreciate your help, but what is the difference if the bias gets its own voltage from dedicated tap on PT or off the leg of PT with correct resistor used to get the correct voltage? These small 50k bias pots l have will fit the holes in back of chassis, l guess l don’t understand exactly what size R to ground from 50k pot l should start with.
Is this true: the higher the R off the bias pot to ground the less negative voltage, upping that R the more negative bias voltage l get.
I planned on getting rid of the messy amp wiring and clean it up, put tubes in except power tubes and check negative voltage for around -70v. Just wanted to know what size R l should start with off the bias pots. Sorry for the confusion.
I know using a 50k pot will result in quick change of bias current with slight turn.
I see Weber s using his SS copper cap and a 39k R to feed the buss. No voltages are shown

[stratomaster]

The difference between having a dedicated tap vs pulling from the HT is essentially the starting voltage.  You need a big resistor to drop the voltage down to bias levels if you're pulling from HT.  You also need to be aware of where you're pulling from and where that is in relation to circuit interrupts like standby switches.  Finally you'll need to have some idea of the effective load of the bias circuit to be able to size your dropping resistor.

As for your resistor to ground, it's just the bottom leg of a resistive voltage divider.  To get half voltage at max resistance then you need a value equal to the pot to ground.  A 47k to 56k resistor should get you near enough. 

NOTE: Given that both legs to the grid leak resistors are in parallel, I would expect for there to be some level of interaction as you adjust the pots when biasing.  You'll have to go back and forth a few times before you get the bias voltage you're after.

[mresistor]

The difference between having a dedicated tap vs pulling from the HT is essentially the starting voltage.  You need a big resistor to drop the voltage down to bias levels if you're pulling from HT.  You also need to be aware of where you're pulling from and where that is in relation to circuit interrupts like standby switches.  Finally you'll need to have some idea of the effective load of the bias circuit to be able to size your dropping resistor.

As for your resistor to ground, it's just the bottom leg of a resistive voltage divider.  To get half voltage at max resistance then you need a value equal to the pot to ground.  A 47k to 56k resistor should get you near enough. 

NOTE: Given that both legs to the grid leak resistors are in parallel, I would expect for there to be some level of interaction as you adjust the pots when biasing.  You'll have to go back and forth a few times before you get the bias voltage you're after.

When you say "big" we assume you mean larger resistance and wattage shouldn't have to be 5 watts.   There isn't much of a load on the bias ckt, it is negligeable.

[dude]

  As for your resistor to ground, it's just the bottom leg of a resistive voltage divider.  To get half voltage at max resistance then you need a value equal to the pot to ground.  A 47k to 56k resistor should get you near enough. 

Thanks, the schematic in my first post above has a 39K, 5watt from Weber's copper cap, (SS diodes) supplying the bias with HT. He uses two 10K mini bias pots, they were missing when I got the amp, also Weber's schematic has no voltage divider R to ground, hence as Sluckey mentioned, turning the pot to zero would blow the power tubes. I'll use the 50K pot, and a 47K to ground. If I can't get the correct starting negative bias on the grids, I assume I can increase the R off the pot to ground or raise the 39K off the HT..?

[stratomaster]

You're not coming off the SS diodes in the wz.  You're coming off the AC side, so you need to think about this that way.  The 39k resistor is pre rectifier diode, so it affects the AC into the diode.  The diode rectifies the negative AC swing and the cap smooths it to usable negative DC.  If you need more negative DC voltage you need to increase the overall AC voltage into the bias supply rectifier diode. To do this you would DECREASE the 39k resistor. 

It's easier to mess with the resistor off the pot though.  You can get away with a 1/2 resistor or less.