PowerScaling my Liverpool: Success!!!

[ODwan]

Hello,
for a long time I wanted to powerscale on of my Trainwreck clones but was afraid to do so with the Alpha pots that are available here in Germany. They do not have the voltage withstanding ratings that are mostly required. UR12 told me in another thread that he uses them with no problems. I checked the datasheets and now think that in circuits wth up to 400v they are ok. Anyway, I've tried it and it works just beautifully. I scaled just the plate and screen of the EL84s and converted my Cut control to a Limit control. The powerscale pot I put on the back of the amp. The sound is good, just a bit harsher than before, but I have to check preamp voltages.
Here is the (simple) circuit I used.
Greetings,
Timo

[Fischerman]

Are you also scaling the bias voltage? Power scaling will generally just drop a few volts. If it's not too late you might want to consider putting the Scale control on the front of the amp...as you play with it a bit I think you'll see that dialing in the Scale control involves a lot of back-n-forth tweaking of the Limit control (because whenever you adjust the Scale control you also have to adjust the Limit control just the right amount). After you've played it a while you'll find the sweet spots for each control so it may not be necessary...just a thought.

You also might want to mess with the NFB resistor...if your Limit control is any sort of post-PI MV then when you turn it down you're also reducing the NFB (your Presence control won't even work at low post-PI MV settings).

[UR12]

Odwan

Glad the Alpha pot thing worked out for you! Have you ever tried scaling the whole amp and getting rid of the drive control. This works great and to me it's the best way to go. It's really the only way I scale an amp anymore.

Just my 2 cents

[ODwan]

Hello,
after the initial euphoria I soon got sober with the scaled amp. On the "household" level output the sound was a bit buzzy and lost much of it's feel. Cleanup with the guitar volume was still great but most of the magic was gone. I checked preamp voltages and found them about 40 volts too high. Turning up the scale to full output power brought them down by about 20 volts, so it had to be PT load related. I cured this with some zeners as shown in the schematic. Now preamp voltages stay the same, but still something is missing.
UR12, I think I will try scaling the whole amp, maybe make it switchable, in the days to come. I'll report back.
Fischerman, I have turned the Cut control I had originally in the faceplate design to a Limit control. The Scale control I have put on the back of the amp, as I dont want to ruin the faceplate. After all, it's an experiment. And yes, the NFB is affected by only scaling the powertubes. The NFB ratio goes down and you hear it! I just turned the Presence control all the way down to keep some resemblance of NFB.
Greetings,
Timo

[Jack]

If you keep the preamp voltages the same while reducing the power amp voltage, the power amp has less and less headroom. In other words, the power amp is slammed harder and harder. It similar to raising the (relative) preamp's volume vs the power amp. That's what makes it buzzy at high attenuation level.

To cure this, you can either use a "drive compensation" control which is a MV that reduce the relative preamp signal vs the power amp to keep "tone" constant. or you can scale the whole amp.

The disadvantages with the former is a) using a marshall MV the lack of PI scaling. You scale the poweramp, reduce the preamp signal but miss the PI effect since it receives a lower input signal (unless you scale the pi + power amp and not the preamp). b) with a PPIMV, the presence/feedback loop is not constant.

With the later, the disadvantage is that at low volume, the input signal slams the first preamp stage and makes buzzy tone. To cure this, reduce the guitar volume or use a "low input" like 4 holer marshalls.

Fisherman: it's a cathode biased amp so no need to "scale" the bias. It does so automatically.

[ODwan]

Moin,
@Jack: I have a drive comp which I called Limit in my posts. It takes a bit of trial and error to know how to operate the Scale and the Limit control against/with each other, but I managed.
@UR12: have switched to scaling the whole amp just a hour ago, but it didn't realy satisfy me. Touch sensitivity was almost lost and on the lower scale settings the tone was very dark and didn't distort very much.
Timo

[drz400]

Moin,
@Jack: I have a drive comp which I called Limit in my posts. It takes a bit of trial and error to know how to operate the Scale and the Limit control against/with each other, but I managed.
@UR12: have switched to scaling the whole amp just a hour ago, but it didn't realy satisfy me. Touch sensitivity was almost lost and on the lower scale settings the tone was very dark and didn't distort very much.
Timo

The Badger from Suhr uses a pre phase inverter master as a limit control. It is cathode biased, no presence. The scaling is on the power tubes only.
The one I heard sounded killer at any volume. Suhr said he tried the post phase master and didnt care for it at all and also tried the phase inverter and the whole amp. Whatever he wound up with it works

[Fischerman]

UR12, When you say 'scale the whole amp' do you include the entire preamp or just the PI? It seems that scaling the preamp would affect the tone and yield the results that ODwan got; i.e. dark and not very distorted at low settings but I'm just guessing and have not tried it.

I've been meaning to change my power scaled Express to also scale the PI (it only scales the power tubes and bias right now) and then change the post-PI MV to a pre-PI MV. I've been interested to find out what Suhr did with the Badger but that's an EL84 design which usually rely more on power tube distortion than other tube types. I was actually about to yank the Express circuit (which is now actually a Komet circuit) and just go with a modded-2204 circuit but maybe this will change my mind. I'd really like to keep it 'wreck-ish' if I can get it sounding good...well...great...it sounds 'good' now. Good isn't enough!

[gtrcollectr]

I scaled the entire amp on mine and it sounds killer....I do not have any sort of drive control and it is not needed until you scale down to sub-bedroom level.

[UR12]

UR12, When you say 'scale the whole amp' do you include the entire preamp or just the PI? It seems that scaling the preamp would affect the tone and yield the results that ODwan got; i.e. dark and not very distorted at low settings but I'm just guessing and have not tried it.

I've been meaning to change my power scaled Express to also scale the PI (it only scales the power tubes and bias right now) and then change the post-PI MV to a pre-PI MV. I've been interested to find out what Suhr did with the Badger but that's an EL84 design which usually rely more on power tube distortion than other tube types. I was actually about to yank the Express circuit (which is now actually a Komet circuit) and just go with a modded-2204 circuit but maybe this will change my mind. I'd really like to keep it 'wreck-ish' if I can get it sounding good...well...great...it sounds 'good' now. Good isn't enough!

Yes that's what I meant. Scale the Preamp, PI and power amp. Look at it this way. If your using anything from a 820 ohm to a 2.7 ohm resistor on the cathode of the first preamp stage your putting about 1 - 2 volts of bias on the first preamp tube. A normal guitar pickup will put out about 1/2 to 1 volt on the grid of the first preamp tube. As you drop the voltage using the scaling control on the first pre, the bias will also drop. When the bias voltage on the 1st pre gets below the output of your guitar pickup you will be overdriving the front end. At that level you can probably hear the strings on your guitar just a loud as what your hearing coming out the speaker. I don't care what method you use, when your speakers aren't seeing enough power to move them efficently you will notice a change in the tone/sensitivity of the amp. That is going to happen no matter which method you use. If you limit how low the voltage can go (Say drop it down to 20v for instance) by adding a resistor to the bottom of the scaling pot, then you can set the range so that the amp can never go low enough to overdrive the 1st preamp stage.

You can still get power tube distortion at lower voltages and you can still get preamp tube distortion at lower voltages. I would guess that the range below moving the speakers efficently or overdriving the front end at very low bedroom levels is where you are hearing the tone change or get darker.

[jaysg]

Question: Did you implement a Standby switch? If so, where does it go - in front of PS, in the middle somehow, after it?

[UR12]

Yes, just put the scaling circuit right after the standby switch so you can still put the amp in standby,

[Fischerman]

I wasn't sure where to put the SB switch...because I wasn't that comfortable/familiar with the whole power scaling circuit I didn't want to put a standby switch in a potentially 'bad' spot. What I did was made the standby switch connect the plate node filter caps to the OT center tap and the screen node filter cap to the screens (using both poles of a DPDT switch). So my preamp isn't included in SB...basically the whole amp except the power tubes is up and running in SB. It works and there are no pops or anything (I put snubber caps across the SB switch).