Installed VVR3 in Express, bias too high?

[LamontGrady]

I just got done installing one of Dana's VVR3 boards into my Ceriatone Express copy.

Before I installed it, I had the (external) bias knob set to 38 ma (measured at external bias points or with Weber BiasRite).

The vvr bias pot is turned all the way down. After I installed it, the bias was about 68 ma. When I turned the amp's bias knob all the way down it gets to about 40 ma and everything seems to work as it should!! Very interesting to turn the vvr knob down and see the bias and the voltage both drop! Both bias pots increase the bias as they should when turned up so I don't have the vvr's bias pot turned the wrong way or anything like that.

It sounds great, everything seems fine on the power side. I already changed R31 on this amp from 47k to 56k to be able to get a pair of JJ 6V6 cool enough. Should I just bump it up a bit more? Or is this symptomatic of me doing something wrong? I'm not sure what else to test, but the bias voltage is around -35.4vdc before the vvr board (between R30 and R31) and -32.7v after (between R16 and R17)?

I should mention that I removed a ppimv and as far as I can tell, everything was back to stock which pretty much exactly matches the 12/4/08 Express schematic.

I bypassed just the bias side of the vvr, and the bias dropped from 40 to 16 ma.

[Richie]

i'm sure their are many ways to do this, Dana may chime in later,as he plays music on sundays.
I usually set theVVR board bias pot to be max bias voltage. Then i use the bias pot on the amp, to set the bias. I think you can do this either way, as long as one is maxed,and use the other for adjusting the bias.

I just do it my way, then its like you'd be adjusting the bias the same way,or useing the amps bias pot, like you would if the VVR was not there.

Alot of people change the bias feed resistor in the amps bias circuit, from 220k to 180k or 150k, this will give you more bias voltage range to play with.

I think some pics are on here of some express amps Dana built using the VVR, might try to find those.

[UR12]

Like Richie said. You need to use one bias pot or the other. Set the one on the VVR to max Bias and then use the original pot to adjust your bias or set the original pot to max negative voltage and use the one on the VVR to adjust your bias.

[LamontGrady]

I must be misusing some terminology.

With both pots turned all the way in the "cooler" direction, the very lowest I can get is 40 ma on EL34. If I put 6V6 in there they'll fry. If I turn either pot the current goes UP from there. If I turn the vvr's pot it goes way up. If I bypass the vvr's bias functionality the bias drops back down to the range I would expect with the amp's bias pot.

You guys are saying if I decrease R29 the bias will get "cooler?"

[UR12]

I must be misusing some terminology.

With both pots turned all the way in the "cooler" direction, the very lowest I can get is 40 ma on EL34. If I put 6V6 in there they'll fry. If I turn either pot the current goes UP from there. If I turn the vvr's pot it goes way up. If I bypass the vvr's bias functionality the bias drops back down to the range I would expect with the amp's bias pot.

You guys are saying if I decrease R29 the bias will get "cooler?"

Forget about current measurments for a minute. Take the power tubes out and turn the bias pot on the VVR board so that it points towards the rear of the board (Away from where the dual pot hooks up). set your volt meter up to measure DC volts. Turn the dual pot (VVR power control to full Clockwise (max power) Use your volt meter and check the Bias OUT voltage on the board. It should be a neg voltage. Then turn the VVR Dual pot fully CCW and take another reading. It should measure about 10% of the first voltage you measured. If that is working then the VVR is functioning properly. What voltages did you measure?

[LamontGrady]

Thanks for the help guys!!

It seems ok, bias out of vvr board goes from -33.1 vdc to -3.06 vdc. At the bias into the vvr board it's -36.

Turning the bais pot on the vvr clockwise (towards dual pot) takes the measurement gradually from -33 to -0.6

Turning the amp's bias pot ranges -33 to -24.5.

Ironically, my wife is at work so I'm all jacked up to jam somewhat more quietly than usual

[LamontGrady]

Also wanted to mention I did have those pictures right in front of me when I was figuring out how to install, definitely helped.

[UR12]

Thanks for the help guys!!

It seems ok, bias out of vvr board goes from -33.1 vdc to -3.06 vdc. At the bias into the vvr board it's -36.

Turning the bais pot on the vvr clockwise (towards dual pot) takes the measurement gradually from -33 to -0.6

Turning the amp's bias pot ranges -33 to -24.5.

Ironically, my wife is at work so I'm all jacked up to jam somewhat more quietly than usual

Personally, I would like to see a little more range on the original Bias pot and I think this was what Richie was aluding to as far as changing a resistor value on the power supply board. The Express usually has a 220k resistor coming off of one side of the high voltage winding going to the Bias diode. You can change the value of the 220k to a 180k or 150k and get a little lower (higher negative number) bias voltage. This might get you a range of -40 to - 28. You would just have to play with the values to get what you want. So just leave the bias trimmer on the VVR maxed out (pointing towards the back of the VVR board) and set the power control (Dual pot) fully clockwise (max B+) and adjust the bias using the original bias pot on the Express power supply board.

As a side note, the reason there is a bias trimmer on the VVR is because I want to make sure it can be used in just about any amp. I also use them in my own amps and use the trim as my only bias pot. Some amps have a fixed bias that is not adjustable. With the trim pot on the VVR board, you can make those amps become an adjustable fixed bias amp. So it is just a feature on the VVR board that you may need or not need. That's why I say just leave the VVR bias trim at max and use the original .

[LamontGrady]

Ok cool. So it seems like everything is working the way it should. There's still something about all the Ohms law stuff vaguely remembered from high school electronics class combined with tubes that makes my head hurt and I know I'm not quite getting it.

I'll change R31 from my current 56k back to 47k so it's more "normal/stock" again and experiment with the 220k. I changed it to get the three pairs of tubes I've tried so far to be able to get in range and it worked swimmingly. But this amp is already pushing the high side.

Was I wrong to assume that putting the vvr board in would essentially leave the existing bias pot's range intact? This isn't the last one of these I have to install

I'm all for the onboard bias pot!! Yes complexity always invites confusion. I've put bias pots in two amps so far so I'm pretty much in your target market of the diy crowd. Someone who is willing to take a soldering iron to their amp because they enjoy learning how it all works, and wants to be responsible for it.

I don't know how much time it would take for an experienced tech to figure out how to put a bias pot in a given amp that they may have never seen before or that had unknown mods or whatever, but for a diy person like myself I'd probably spend several hours perusing forums and whatever else I could find to reassure myself I had a basic understanding of what was going on and was getting good advice from people who knew more than me about that particular amp.

Making multiple changes that all needed to be tested and thought about and listened to, in my inexperience, would take days.

So being able to throw in one magic black box that also adds the bias pot is a wonderful feature.

Ok, if I squint really hard, I can understand why decreasing R29 will lower the range of current, but increasing R31 has the same effect.

[UR12]

Was I wrong to assume that putting the vvr board in would essentially leave the existing bias pot's range intact? This isn't the last one of these I have to install

The VVR board will drop a couple of volts from the bias between input and output. Normally, if your range is good enough to begin with, it isn't really an issue.