Power Scaling a 36W

Stu-Daddy · Post by **Stu-Daddy** » Wed 01/17/07 12:49 pm

Anyone try power scaling their 36W yet ?
I just Beta tested an older 36W build that I had laying around with a power scaling board I built while back.
This Power Scaler allows me to dial down the plate voltage to the power tubes like a variac only I've only isolated the Power Tube plate voltages from the other power supply voltages so the heaters and everything else remains unchanged.
Here's a few jpgs of the unit and the mA current readings before and after scaling,remember you can dial the amp to anywhere you want from 36W to 1W and everything in bettween.
I haven't hooked up a drive compensator yet and I was wondering if anyone had yet?
The drive comp allows you to scale the driver too, so that you can adjust how hard the eq hits the power section.
This is nice if you've dialed down the power section and you're getting to much distortion and you wanna clean it up a bit.
I imagine that with the power scaler and the drive compensator and the master volume I'll be able to tweak all the parameters possible for tonal shaping.
I'm waiting on some enclosers to come so I can put the power scaler in a box and put it inside the amp cabinet.
If I'm at a gig and the amps too loud..........

Mauri · Post by **Mauri** » Sun 01/21/07 3:21 pm

I was planning to do the same but I am not at the building stage quite yet. Did you use the KOC's schematics for power scaling which are somewhere around on this forum? Did you have to tweak any of the values or ratings of the resistors? What mosfet did you use? I was also planning to have a PPI drive compensator, but my amp will have a presence control too, so i thought i might be better to have this before the phase inverted instead. Any thoughts?
Maurizio

ur12 · Post by **ur12** » Sun 01/21/07 3:27 pm

You should try power scaling the whole amp (Power and preamp stages) as you won't need the DCC. Plexi has scaled a few 36watters now using my boards and the amps work great using this approach.

Mauri · Post by **Mauri** » Mon 01/22/07 4:31 am

what is the advantage of scaling down the whole amp rather than just the power section?
What if my amp had 2 channels, a liteIIb type channel and a hi-octane type channel and 36W output section?would you still think that I should scale the whole amp? Where do I find the schematics and layout of your board, ur12?
Thanks for your help. Much appreciated.
Maurizio

cplus · Post by **cplus** » Wed 02/14/07 1:46 pm

ur12 wrote:You should try power scaling the whole amp (Power and preamp stages) as you won't need the DCC. Plexi has scaled a few 36watters now using my boards and the amps work great using this approach.

I'd also be interested in learning more about the advantages/disadvantages of scaling the whole amp vs. just the power section. Does it sound the same as it's dialed down? I may give it a shot this week.

My experience with scaling just the power section of a DC30 is that the sound gets a bit fizzy as it's dialed down to low power, even when dialing down the PPIMV (aka drive compensation).

Post by **jaysg** » Wed 02/14/07 1:57 pm

ur12 wrote:You should try power scaling the whole amp (Power and preamp stages) as you won't need the DCC. Plexi has scaled a few 36watters now using my boards and the amps work great using this approach.

er...what's DCC -- KOC's Drive Control or what we'd call a post-PI master volume?

In trying to follow some of the Badger discussion, I think Suhr said the whole amp is scaled, though I think he has a master on it too.

cplus · Post by **cplus** » Wed 02/14/07 2:12 pm

jaysg wrote:er...what's DCC -- KOC's Drive Control or what we'd call a post-PI master volume?

Yes, the drive control is a PPIMV, used to give some control over the amount of output tube distortion. (They distort earlier as the B+ voltage is scaled down.)

Post by **markh** » Thu 02/15/07 2:32 pm

Yes, the distortion tends to get 'splatty' when the power tubes are scaled down to 50% and the PI is still slamming at full tilt.

On the other hand, the overdrive almost goes away when you get too low -- there's likely an ideal ratio to scale the pre- and power sections at different rates for optimum tone as the scaling gets dialed in.

--mark

Mooreamps · Post by **Mooreamps** » Thu 03/29/07 3:01 pm

markh wrote:Yes, the distortion tends to get 'splatty' when the power tubes are scaled down to 50% and the PI is still slamming at full tilt.
--mark

Splatty ? Is this "splatty"?
http://www.youtube.com/watch?v=8hHG3EfDqK0

Bear in mind, this is the old model-5 which is going away, being replaced by the new model-10.

-g

ur12 · Post by **ur12** » Thu 03/29/07 4:38 pm

cplus wrote:
ur12 wrote:You should try power scaling the whole amp (Power and preamp stages) as you won't need the DCC. Plexi has scaled a few 36watters now using my boards and the amps work great using this approach.
I'd also be interested in learning more about the advantages/disadvantages of scaling the whole amp vs. just the power section. Does it sound the same as it's dialed down? I may give it a shot this week.

My experience with scaling just the power section of a DC30 is that the sound gets a bit fizzy as it's dialed down to low power, even when dialing down the PPIMV (aka drive compensation).

By power scaling the whole amp (Preamp and power amp) you don't need the "Drive Compensation Control "(DCC). The bias on V1 in most amps is about 1 - 1.5 volts. If you use 500mv as a arbitrary value for the signal level of the guitar for a reference and you start cranking the B+ level down on the preamp and power amp tubes, the amp will work just fine at lower levels but when the voltage on the pres drops enough that the cathode bias on the first tube drops below the level of the signal from the guitar you will start to overdrive / distort the first preamp stage. You can use your guitar volume control and turn down the level going to the first pre to bring it back in line just as you would with a DCC. So basically you can scale the power amp only and as you turn down the voltage you will overdrive the power tubes and have to use the DCC to turn down the level hitting the Power tubes or you can scale the whole amp and use your guitar volume to turn down the signal going into the first preamp tube so you don't slam it to hard. Richie and I have scaled the entire amp on the few 18/36watters we have done, and personally, I like this method best. YMMV. Try it both ways and see what you think

Post by **jaysg** » Thu 03/29/07 7:10 pm

I wonder how it would work if you used a diode for the bias...it won't drop below the diode drop....or you could run them all fixed bias off of a non-ps'd supply.

Pressed_Rat · Post by **Pressed_Rat** » Fri 03/30/07 3:53 pm

Splatty ? Is this "splatty"?
http://www.youtube.com/watch?v=8hHG3EfDqK0

Sounds pretty darn good to me. But, does manual adjusting the "brake" and the preamp gain act as a drive compensator?

I've never heard tone that fat from a Tele w/o Humbuckers. Is that a characteristic of the 1/4 pounder PU or the amp?

Thanks.

Mooreamps · Post by **Mooreamps** » Sat 03/31/07 1:19 am

Pressed_Rat wrote:
Splatty ? Is this "splatty"?
http://www.youtube.com/watch?v=8hHG3EfDqK0

Sounds pretty darn good to me. But, does manual adjusting the "brake" and the preamp gain act as a drive compensator?

I've never heard tone that fat from a Tele w/o Humbuckers. Is that a characteristic of the 1/4 pounder PU or the amp?

Thanks.

Act as a drive compensator ? Not heard that specific term before. The power brake controls the plate current conduction of the power tubes. Pre-amp gain dial controls are pre-amp gain dial controls.

It's the characteristic of the amp, both in the pre-amp and inside the twin PA power amp. It's not a standard layout.

-g

Pressed_Rat · Post by **Pressed_Rat** » Tue 04/03/07 6:13 pm

Act as a drive compensator ? Not heard that specific term before.

From Stu-Daddy's initial post...

I haven't hooked up a drive compensator yet and I was wondering if anyone had yet?
The drive comp allows you to scale the driver too, so that you can adjust how hard the eq hits the power section.
This is nice if you've dialed down the power section and you're getting to much distortion and you wanna clean it up a bit.

We're probably talking two different approaches here. It appeared in the video the power amp was dialed down and the preamp gain increased. Yet the sound reminds me more of power tube distortion than pre.

I've just started thinking about power scaling myself. I was shooting for a low power push-pull amp, but I like to really crank it when the wife's out (that's why I don't "listen" to her - I can't hear her

), and the low power amps just don't seem to have the same character as say a 1987 w/KT66s. There's some real good ones out there, but it's just not the same. That's what got me thinking about this power scaling concept.

Is your modified Epiphone a paralleled SE type - e.g. twin power amp? It sounds really good.

Mooreamps · Post by **Mooreamps** » Thu 04/05/07 1:59 am

Well, it's no longer an Epiphone. I pulled the factory card out to put in the 4 gain stage pre-amp and twin PA power amp. The power brake is for the power tubes only. In this layout, I was hoping for some pre-amp distortion, but it just didn't work out. But this layout is a high gain pre-amp that over-drives the power tubes very nicely when they are scaled down to about 5 mills of plate current per tube.

Now, how would all this be relivent to this forum. First came the 5 watt prototype, which was in the video clip. The 5 watt amp went away to become the 10 watt amp. Second came the 18 watt amp. Same layout as the 10 watt amp, except the PA tubes are an EL34 and a 6L6 running single ended. Next wil come a 36 watt push-pull PA. I'm thinking with proper PA tube plate loading, I'll run these two tubes in push-pull. I also re-built my power brake to handle the higher plate current for these two Power tubes.

Attached is the 18 watt SE parts installed on my bench chassis.

-g

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Power Scaling a 36W

Power Scaling a 36W