Best MV with Power Scaling

[DerStever]

I am using Kevin's new DC Power scale on a Liverpool clone - when you back it down it gets fizzy sounding - so he recommends a MV as a drive compensator - I wired one in from a Kendrick book - the one with the Trainwreck pages - I used the 1m pot across the two outputs to the output section and it really isn't working correctly.

It will shut off the preamp to poweramp feed but it doesn't gradually increase gain like it should I'm using tabs 2&3 on the pot. So is there a better way to do this? WOuld a pre PI MV be better and how do I do that in on LP??

Help and thanks.

[bmcmusic]

You may want to try a PPIMV. Use a dual pot. 500k or 1M This is Ken Fisher's design and I have used it on a number of amps.

Bob

[Bob-I]

You may want to try a PPIMV. Use a dual pot. 500k or 1M This is Ken Fisher's design and I have used it on a number of amps.

Bob

Interesting. So Ken used the C- voltage as the ground reference instead of the actual ground?

[mat]

You may want to try a PPIMV. Use a dual pot. 500k or 1M This is Ken Fisher's design and I have used it on a number of amps.

Bob

I hope the original poster won't mind me asking but would this MV be valid for non power scaled build also ?

[tsl602000]

I hope the original poster won't mind me asking but would this MV be valid for non power scaled build also ?

I use this PPIMV all the time and it works great. Yes, you can use it without a non power scales build.

[mat]

I hope the original poster won't mind me asking but would this MV be valid for non power scaled build also ?

I use this PPIMV all the time and it works great. Yes, you can use it without a non power scales build.

tsl602000,

Thanks for the answer ! I'll put one 500k dual in order right now

[Fischerman]

Bob-I,
Yes, that is using the bias supply as 'ground' so you don't need any extra caps.

Mat,
Yes, you can use this in a non-power scaled amp. That was the original purpose...many people put them in Marshall type circuits (I have many times).

The circuit shown above does change the tone a little when fully cranked...you then have 220k in parallel with 500k for your bias splitters which does affect the fully-cranked tone. You also have changed the load on the PI output which affects tone. There is a very similar PPIMV that uses different values that IMO works better. Use a 250k-A dual pot with 2M2 resistors from wiper-to-'ground' (which is actually the bias supply). You also remove the 220k splitters from the board...the pots/resistors become the bias splitters. Now the load on the PI is very close to the stock 220k and the bias splitters are right at 220k when fully cranked. This is something I tried when I had run out of 500k dual pots and liked it better...then I learned that Larry (of Larry's Amps...a member here) had been using this and turned the Metro amp guys onto it.

My disclaimer: ALWAYS check your bias voltage right at the socket throughout the sweep of the MV pot before turning the amp from standby to on when doing these mods. You're messing with the bias supply and want to make darn sure you didn't make a mistake! The bias voltage should stay constant throughout the sweep of the MV.

EDIT: Derstever; I built my Express with power scaling and I decided to use caps to isolate the MV from the bias supply. The reason I did it is because when you scale the amp way down you're dealing with just mV of bias voltage and I was concerned that the MV might mess with that so I used a slightly different MV. I don't really know if it's a problem but I wanted to be safe. FWIW, my power-scaled Express MV is a 1M-A dual pot connected to the coupling caps with .47uF caps on the wipers...other end of .47uF caps to grid stoppers/220K splitters junction. Ground tab goes to 'true' ground...not the bias supply. This one is a compromise but I couldn't think of a better way to do it. Here's a schematic with what I used...the preamp circuit is something I was experimenting with at the time (power scaling not shown in schemo).

[John_P_WI]

DerStever,

Why don't you ask Kevin what he recommends? He is a good guy.

You could also ask over at powerscaling.com, Kevin and others participate there.

His TUT4 details an improved "bootstraped" MV that supposedly works very well, I have not tried it yet, but others say so.

I have played with most of the other MV types and don't like the tonal change. Certain PPIMV layouts do have bias dangers, so note the caution.

Good luck,

John

[DerStever]

Kevin basically likes to point you to TUT4 for answers, and I specifically am only going to do a couple of these so really don't want to buy the book - I have TUT3.

I will make a post up there but I was hoping someone here who's had the experience with Trainwrecks would enlighten me too as the TW is a different beast then most other amps.

Also my amp is cathode biased not fixed like the Express so is there still the bias issue you guys are describing above and Fischerman do you have a layout drawing or photo of what you did?? It would help me immensely.

Thanks guys.

[Fischerman]

If it's cathode-biased then don't worry about the bias issue I mentioned...that's for fixed-bias.

The only photo I have is when I originally built the amp and had this MV in it...I'm not sure how much it will help but here ya go. The 'scale' control is far left (plastic pot body with the Marshall knob on it), the MV is next to the right and I had a dual-pot treble bleed in there just to the right of the MV (where the Presence control usually is). You should see the .022uF couplers on the board connected to the MV (white and black wires; twisted pair)...then those blue caps are the .47uF from the pot wipers to the 220k bias splitters (the grid wires also connect here). The blue and white wires connected to the other end of the 220k bias splitters go to the bias supply (I have separate bias adjustment pots for each power tube and bias test points on rear of amp). Haha...but you don't need any of that crap...just try the MV above or the 250k-A dual pot with 2M2 resistors.

[img:800:600]http://i108.photobucket.com/albums/n4/m ... tshot1.jpg[/img]

[Bob-I]

Bob-I,
Yes, that is using the bias supply as 'ground' so you don't need any extra caps.

I see what you're doing, but I'm not sure I understand why this avoids the need for the additional caps. I also seems to add the complication of being in parallel with the bias grid resistors.

I guess I'm just not quite understanding this.

EDIT:

Ok, I looked over the schematic and now it makes since. By connecting the C- the pots are not seeing any of the - voltage, therefore they don't need to blocking caps. Duh

[txbluesboy]

The dual 500K pot mentioned in the earlier posts, Is that linear?

[Fischerman]

Audio pots.

The C- is on the pots...that's how it works because the grids still see/get DC bias voltage but the bias supply is AC ground. No appreciable amount of current flows in the bias supply though so there is virtually no voltage drop across the resistance (just like with the regular 220k bias splitters).

[Krinkle]

DerStever

Use the boot strapped Master Volume on Page 8-5 of TUT3. I used this before I changed to a Dual MV setup on my LP Standard and it was by far the best MV that I have heard. I haven't found anything that could come close since.

Darrin

[d95err]

These are my Powerscaling and MV experiences and thoughts:

* For an amp without negative feedback - scale powertubes only and use a PPIMV.

* For an amp with negative feedback - scale powertubes and PI tube, and use a conventional MV before the PI.

Negative feedback will work against a PPIMV; as the PPIMV is turned down, the feedback will attempt to compensate. This results in more PI distorsion and a change in tone. With a scaled PI tube and conventional MV, the amount of feedback in relation to the input to the PI remains constant, so the poweramp tone stays more constant.

I've tried a few different PPIMV versions. I prefer using a dual 250k pot that replaces the bias feed resistors (or ground reference resistors for a cathode biased amp). Add 2M2 safety resistors (as suggested by KOC) and you get about 220k with the MV on full. I.e. it is identical to the original circuit when on full.

I tried Kevin's dual 1M pot version, but it sounded dull when turned down low. I also tried the Matchless style cross-line single 1M pot. However it has a very bad workable range (all or nothing), and changes tone too much in the useful region.

I haven't tried KOC's improved bootstrapped MV, but intuitively it seems like it would be the same problem as a PPIMV. It reduces negative feedback as the volume is turned down, so you get the same problem as with the PPIMV. My guess is that the improved boostrapped MV works best in a non-scaled amp, but that's just speculation from me. Sounds like a good question to ask over at Powerscaling.com.