Biasing for Variac?

[motrock]

Hey guys, I am going to be having my 50watt Plexi rebuilt within the next couple of weeks and its getting a new OT. Currently it has Sylvania 6CA7s. I was told that it was biased hot to 50ma per tube at 120Volts on the Variac, then the Variac was dropped to 90Volts. I want to make sure that is right so I don't blow anything! Can the amp be rebiased with the Variac at 90Volts? What should the 6CA7s be at when the Variac at 90Volts? Also, would you guys run the Variac at 95Volts? Or does it NEED to be at 90Volts?

Just want to make sure I have the straight so I don't damage the tubes or the transformers.

[vanhalen5150]

Welcome to the worm cannery factory Motrock. Cause everyone is going to tell you something different.

Personally I would set your amp up to lower the voltage via variac and see what the heaters do. Some amps can drop to below 80v, and the heaters only change slightly. This is where the PT impact is. That determines the heaters. Dropping your heaters too much is tube death.

[motrock]

My buddy doesn't think we will have to bias it. As long as we don't swap out the PT. I am going to have him check it where its at right now, then we will keep it at that bias when we rebuild it. Dave Friedman originally didn't mine. He said he did 50ma at 120v and then dropped it to 90V. And that keeps a good balance apparently. I just want to make sure I have it straight. the amp has run since 2009 at 90V with the same Sylvania 6CA7s. No problems!

[vanhalen5150]

I understand what your saying there. The biggest thing that happens with the variac is what the heaters do. That is what the Variplex completely gets around. Everyone keeps trying different components and PTs, but never measures what the heaters drop down to.
This should be a good thread.

[rgorke]

I have used Sylvania 6CA7s for the past several years with the voltage 80-90 volts...more toward 80 these days. My heaters are below 3. I haven't seen any problems. Great for helping control volume and adding that sag we all like.

[vh junkie]

Assuming that the variplex does not have some sort of bias compensation circuit built in already, and you are using el34/6ca7 power tubes, and the amp has the little 1 ohm bias resistor(s) connected to pin (1/8) of the power tube sockets:

Need to record the plate voltage on the power tubes(pin 3) at what ever variac voltage you are thinking of using.
Then apply the following formula:
17.5 / plateV = mV
then adjust the bias to get the resulting mV across the little bias resistors.
Ex:
17.5 / 440v = .039mv
17.5 / 400v = .042mv

[motrock]

It should be noted that the VariPlex is no longer the VariPlex. Its a 50watt 68 Plexi clone. So everything that was in the original VariPlex is no longer there

[vh junkie]

Ok, then if you have the 1ohm bias resistors then the way I described is the way to go... you could bias with the variac at 120v and then turn the variac down without damage... the bias would go a little cold as you turned the variac down, but a lotta folks like that!

[vanhalen5150]

The Variplexs are biased around -37mv. I realize yours is no longer running with the Blankenship design but i guess I'm pointing out that it goes down to about 1 watt output yet sounds exactly the same. I've found that once heaters go below 2.5 bolts the sound really starts to suffer.

[rgorke]

I e found that once heaters go below 2.5 bolts the sound really starts to suffer.

Maybe that is why my amp sounds bad? All this time I thought it was my playing.

[motrock]

The Variplexs are biased around -37mv. I realize yours is no longer running with the Blankenship design but i guess I'm pointing out that it goes down to about 1 watt output yet sounds exactly the same. I e found that once heaters go below 2.5 bolts the sound really starts to suffer.

Yeah, the variac is no longer internal. The extra transformer was removed, and we now have a regular power cord. Standard Variac is being used to drop the whole amp. I keep it right at 90volts. Sounds good that way to my ears, its just some of the internal stuff is wrong. Like the 330uF on v1b, the .1uF cap is nowhere to be found(instead its a .022uF), the Negative Feedback is cutting out half the gain, there is a weird 220k resistor on the PI(Which Roy was confused about), and there is no bright cap. So we are redoing the whole thing to real Plexi Spec with the NFB at 100k 4ohm tap; like Eddie's amp. And a new OT by M E R R E N.

I have all new SoZo Next Gens coming.

BTW, the new SoZo Next Gens sound WAAYYYY better then the other SoZo's. My buddy has all of them, including the Next Gen in my amp. He is considering switching his out for new Next Gens because they sound WAAAYY more vintage and nearly identical to Mustards. We were both surprised BTW! SoZo went above and beyond with these.

Anyway, back on track, I asked about the bias so I don't damage the tubes. We will check where the bias is right now and match it when the amp is rebuilt!

[rgorke]

Glad the new Sozo's are gonna work out. Boy I was way off base earlier. The question is...when building an amp and you want to make it a true vintage clone, does one use the originals or do we use these that may not "look" exactly right?

I suppose it comes down to what comes out of the speaker but some of us are obsessive with how the amp looks with its skirt up.

[jnew]

IMO, vh junkie had the best answer. There is no set bias for a variac set at 80, 85, 90 or whatever VAC you choose to use. You have to choose one, then measure the VDC on pins 3 of the output tubes. Then apply the formula as he outlined. That's the right way to do it and then see how your amp responds. For example, my amp at 85VAC on the variac puts my plate voltages at 245VDC. (very low as the PT is a JTM-45 type) So at 70% plate dissipation, this put the mA's at about 71. Sounds nuts but that's proper. Also bear in mind that when you change those power tubes, this has to be repeated because different tubes will draw different currents resulting in a change in plate voltages. The other effect, as mentioned is the heaters. I won't go into this as the guys have already talked about and you have some pertinent info already. So you can try different voltages with the variac but keep the bias true for more best and most accurate results.