Best MV with Power Scaling

[jodie]

[quote="Fischerman"]If it's cathode-biased then don't worry about the bias issue I mentioned...that's for fixed-bias.



If it´s cathode biased, can´t you just use something like the vox presence control without a cap?

[DerStever]

Since the Liverpool uses NFB / Presence then I should Powerscale both the PI and the OTRan OTubes? So how does one do that?

RIght now the powerscale consists of two blocks which are between the B+ for the OTrans and its CT - the 2nd is between Screen Supply and screens. How would I get in between the supply for the PI, OT, and Screens with this? It seems like there are 3 break points for the supplies and only two PS inputs and outputs to do this.

[Fischerman]

Think of it as two separate B+ supplies...the scaled (i.e. variable) supply and the non-scaled (i.e. fixed) supply. You use a dropping string of resistors (or choke) and filter caps for each supply just like you normally would.

So if you did not want to scale the PI then you would connect the variable B+ to the filter node for the OT center tap...then that would connect to the 1K/25W power resistor (like normal). So the OT center tap node and the screen node would both be scaled. Then you would connect the 9.1K resistor (or whatever your usual dropping resistor from screens-to-PI is) from the fixed B+ supply to the PI filter node...and then run your preamp dropping resistors and filter caps like normal from there. So there's your fixed B+ supply. You might have to adjust that first dropping resistor to get your exact target fixed preamp voltages.

Now if you want to scale the PI along with the power tubes all you have to do is move that connection of the first 9.1k (or whatever) dropping resistor from the fixed B+ to the screen node (like it normally would be). Now the PI will scale along with the power tubes. But so will the rest of the preamp so you'll also have to then disconnect the dropping resistor between the V2 node and the PI node and connect it to the fixed B+ supply. So now V1 and V2 will remain fixed.

But in short...just think of it like two separate supplies...one variable and one fixed. Then run all subsequent filter nodes like you normally would. Hope that made sense.

[d95err]

Think of it as two separate B+ supplies...the scaled (i.e. variable) supply and the non-scaled (i.e. fixed) supply. You use a dropping string of resistors (or choke) and filter caps for each supply just like you normally would.

No, this was DC Power Scaling and does not work like that. This is a new technology by KOC that will be described in TUT6 (and is discussed briefly in TUT4). It uses one MOSFET regulator for every power node that is scaled. Thus, the original power supply string is left intact.

To scale the PI tube, you need an extra MOSFET regulator stage. I'm guessing that some of the DC PS kits include this and others don't.

[DerStever]

Think of it as two separate B+ supplies...the scaled (i.e. variable) supply and the non-scaled (i.e. fixed) supply. You use a dropping string of resistors (or choke) and filter caps for each supply just like you normally would.

No, this was DC Power Scaling and does not work like that. This is a new technology by KOC that will be described in TUT6 (and is discussed briefly in TUT4). It uses one MOSFET regulator for every power node that is scaled. Thus, the original power supply string is left intact.

To scale the PI tube, you need an extra MOSFET regulator stage. I'm guessing that some of the DC PS kits include this and others don't.

Which brings me back to my original questions - whats the best MV ie Drive Compensator to use and how is it wired or where can I see the best example of it? I did post to Powerscaling and no one is responding. BTW I'm using SB-2 which came with the drive comp pot which is a 1 meg pot.

[Fischerman]

Thanks d95er and sorry DerStever...my power scaling is the old style (although it's still scaling DC voltage...so I don't get the name but whatever). I don't think the power scaling type/technology will affect what MV to use. If you scale the PI then just use a regular ole 1M-A pot before the PI just like a JCM800. If you don't scale the PI then use a post-PI MV...I prefer the one with the 250k-A dual pot and 2M2 resistors in place of the bias splitters. Some people have problems having the dual-pot in their bias supply...but I guess I would ask them what they use to adjust their bias?

[d95err]

my power scaling is the old style (although it's still scaling DC voltage...so I don't get the name but whatever).

Yeah, DC Power scaling is a bit confusing. Regular power scaling is usually directly after the rectifier so there is a lot of ripple coming into the regulator. The main filtering comes after the PS regulator. This means the B+ line needs to be split in order to get non-scaled voltages for the preamp.

DC powerscaling works after filtering, so it adjusts smoother voltages (closer to pure DC, hence the name). The disadvantage is that is apparently generates a bit more waste heat, and that you need multiple MOSFETs - one for each scaled stage (power, screen and optionally the PI). The control system is also a bit more complex with a number of transistors needed.

There are several benefits. One is that the B+ line is not affected. Instead, the regulators are inserted between the B+ feed and the scaled tube. This probably makes it easier to install and easier to undo.

Another advantage is that the control voltage can be really low, which makes it possible to run the control voltage close to preamp voltages with no problem, and that you can use any cheap pot to for the power control. You can also change the power level instantly (even through an expression pedal) without having to worry about filter caps discharging slowly.

[DerStever]

Kevin says:Wire the Drive Comp as the improved bootstrapped-MV. Basically the pot replaces both 1Ms in the splitter. Each wiper goes to the tube grid on its side.

I can see where the wipers go - to my blue grid wires - but I'm not clear on where the connections go on the other side. In TUT3 8-5 he talks about EQ going to the MV in wiper to PA front end ie grid.

Using my picture below can someone tell me how this should be wired? What kind of pot? I wish I had a complete schematic or drawing that shows the whole circuit....

The photo below is pre powerscaling.

thanks very much for your help.

[Fischerman]

Just a guess DerStever but I think you still use a 1M-A dual pot. Each pot replaces each of the 1M grid load resistors except the grids are connected to each wiper. Look at a schemo of the PI and you'll be able to figure it out.

Did KOC say to use this if you are scaling the PI? Or if you are not scaling the PI? I'm guessing the former but wanted to be sure.

[DerStever]

Just a guess DerStever but I think you still use a 1M-A dual pot. Each pot replaces each of the 1M grid load resistors except the grids are connected to each wiper. Look at a schemo of the PI and you'll be able to figure it out.

Did KOC say to use this if you are scaling the PI? Or if you are not scaling the PI? I'm guessing the former but wanted to be sure.

I'm scaling only the output tubes and OT. On his instructions it says you can install drive comp pot which is a double 1meg pot, either before or after the splitter - see tut4 for further options. The attempt at drawing below is Pre PI so do I place this after the .1 caps on both sides of the splitter ie plate from V3 and the NFB Presence side? and remove the 1 meg resistors. How is this connected to the 39K & 1K in the PI?

[John_P_WI]

DerStever,

I think you are making this more difficult than it is. As the Liverpool is designed the input cap to the PI is the DC blocking cap from the plate of the previous stage. The pre-PI master that you sketched would be more difficult to impliment than Kevin's modified bootstrapped master with a dual ganged pot.

If you can't find the (2) 1 meg resistors on the schematic then I would suggest that you have someone assist you.

Just make sure that the gang halves are wired similarly, for example the caps are connected to lug X on their respective gang, wipers goes to their respective grids etc. Kevin described exactly what to do in the sentence that you posted.

Hope this helps,

John

[clarkydaz]

i tried to register at powerscaling.com and it refuses me because i am on AOL, any way round it?

[Structo]

Open a free hotmail account.

[davent]

i tried to register at powerscaling.com and it refuses me because i am on AOL, any way round it?

You'll need a non-freebee email account to register, hotmail, gmail, yahoo etc. won't work. I was able to register using the email account my ISP issued me after trying to register with my usual free accounts failed.

Take care
dave

[DerStever]

Got it to work as described - removed 1 meg resistors from the PI and inserted the 2 gang 1 meg pot - wiper to grids. Thanks for your help.