Power Scaling the Liverpool

[Fischerman]

Not because of any voltage on the power scaling pot...that pot is mounted to the chassis just like any other pot. The PS pot that Kevin sells has a 2KV 'case' rating. There might be oscillation issues but I really doubt it.

In other words...I would think you could mount them next to each other no problem. Mine are right next to each other but I didn't scale the PI so it's a post-PI MV.

You might investigate the Suhr Badger amp...I think it's an EL84 amp with power scaling but I don't even know if they scale the PI.

Oh, and your revised MV schematic is correct.

[withnail_jet]

Not because of any voltage on the power scaling pot...that pot is mounted to the chassis just like any other pot. The PS pot that Kevin sells has a 2KV 'case' rating. There might be oscillation issues but I really doubt it.

Thanks, I'll try them next to each other for ease of control then, and relocate as necessary.

Oh, and your revised MV schematic is correct.

Cool - thanks, gtrcollectr, for posting that up before I had a chance!

[withnail_jet]

OK...so bear with me here. I understand that if you scale the entire amp, the power scaling regulator simply goes between the rectifier and the filter section. But, if you're only scaling the power section, for instance, how do you separate the filtering for the fixed B+ supplies from those that are scaled?

[img:654:263]http://glasselevator.net/images/lpool_filter.png[/img]

[Fischerman]

Start at the rectifier diodes, it's 'normal' up to that point. Then you make two 'branches' from the rectified voltage..one is for the Variable B+ including all the filters for the power tube plates/screens (if that's all you're scaling). The second 'branch' is for a new Fixed B+ supply with it's own filters (these are added filters to the amp)...this new Fixed B+ supply with it's filters and dropping resistors is supposed to simulate the effect of the plate/screen filters (which are now a different 'branch'). Connect the 18K resistor to this new Fixed B+ supply and that's pretty much it...well...aside from the actual power scaling circuitry.

Notice in my PSX: See how there are two branches coming off the end of the rectifier string? The black filter caps on the left are for the new Fixed B+...there is a wire connecting the Fixed B+ to the preamp filter cap board near the back of the amp. That extra blue colored filter cap and resistor (on the preamp filter cap board near the back) is for an elevated DC for the heater center tap (probably wasn't necessary).

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

[jaysg]

The PS pot that Kevin sells has a 2KV 'case' rating.

Slight tangent --- would you please tell us exactly what pot this is that Kevin ships? I would expect a JAN or RV4 marked Clarostat/PEC/A-B. If there are other sources, it would be helpful.

[withnail_jet]

The PS pot that Kevin sells has a 2KV 'case' rating.

Slight tangent --- would you please tell us exactly what pot this is that Kevin ships? I would expect a JAN or RV4 marked Clarostat/PEC/A-B. If there are other sources, it would be helpful.

It's an RV4 PEC, AFAIK.

[withnail_jet]

Start at the rectifier diodes, it's 'normal' up to that point. Then you make two 'branches' from the rectified voltage..one is for the Variable B+ including all the filters for the power tube plates/screens (if that's all you're scaling). The second 'branch' is for a new Fixed B+ supply with it's own filters (these are added filters to the amp)...this new Fixed B+ supply with it's filters and dropping resistors is supposed to simulate the effect of the plate/screen filters (which are now a different 'branch'). Connect the 18K resistor to this new Fixed B+ supply and that's pretty much it...well...aside from the actual power scaling circuitry.

So, like this?

[img:720:468]http://glasselevator.net/images/lpool_filter_scaled.jpg[/img]

Since the 1k resistor on the new filter section preceding the fixed B+ supplies no longer feeds the output section, does it still need to be 25W as indicated in the original schematic, or can I downrate it to 2 or 3 watts?

[withnail_jet]

Any feedback on the proposed schematic above? I have soldering iron in hand and am ready to do some damage...

[Fischerman]

That schematic is pretty much right. In the PSK you don't actually build the filter section for the power tubes twice like you did. The filters and dropping resistors for the fixed B+ are different values but what you have there would work just fine I think. Then there's all the scaling circuitry.

I don't think you'd need 25W for the Variable B+ but you'd still probably need something pretty stout and the next common size down is 10W...so....your call. The dropping resistors for the preamp can probably be 2W or 3W. I never seem to be cramped for space so I tend to oversize things...in my build I just used the stuff that was supplied in the kit.

[withnail_jet]

That schematic is pretty much right. In the PSK you don't actually build the filter section for the power tubes twice like you did. The filters and dropping resistors for the fixed B+ are different values but what you have there would work just fine I think. Then there's all the scaling circuitry.

So, if I understand correctly, the idea is just to have some level of filtering preceding the fixed section to compensate for what was converted to scaled B+ and moved to the new "branch". And, having exactly the same filtering values is not strictly necessary, as long as they are close, right?

Oh, and on the resistor, I'll just leave the 25 watters in there. Because of my new layout, I will have all kinds of real estate in there and I can lay these guys off to there own spot in the chassis to cool.

Thanks for all the help! I am getting anxious, but still a little uncertain about the whole PS thing, so I don't want to fudge anything.

[gtrcollectr]

Since you are not using the B1 and B2 voltages from the fixed side it seems like you could use a single 120uF cap and place the 1k7 and 18k in series after the 120 cap and achieve the same effect with one less cap. You would still have the same filtering and voltage dropping but a little cleaner layout wise. Theoretically you could use an 19.7k 25 watt and loose one resistor but I don't know if that exists or how massive that part would physically be.

I may be way off base but I think that is right......Somebody please explain why this circuit would still require both filter caps. Is the method I described above valid?

Thanks.

[RossH]

The PS pot that Kevin sells has a 2KV 'case' rating.

Slight tangent --- would you please tell us exactly what pot this is that Kevin ships? I would expect a JAN or RV4 marked Clarostat/PEC/A-B. If there are other sources, it would be helpful.

JaySG,

rv4naysd105A PEC (Power Scale)

rv4naysd253A PEC (Sustain/Sag)

HTH,
Ross

[jaysg]

k, those are readily available from Digi-key ($7.51 ea) and the NTE brand from mouser are probably PEC also. They're rated to work up to 500V if within the 2W dissipation.

[Fischerman]

Those pots aren't the exact same pots that Kevin uses...but he said they were suitable. I know that because I broke the shaft on mine, got the info from Kevin to get a replacement, then tried like hell to find a replacement. I could never find the pot that Kevin uses (I did find those pots pictured) and I liked the plastic casing (as opposed to a metal casing like most pots) and 2KV rating (also a 2W rating)...so I contacted Kevin and he just sent me one...on the house. What a nice gesture!

But I had to modify the pot so the knob would sit flush with the other knobs...I just had to shorten the threaded outer shaft, it protrudes too far through the chassis...nothing a Dremel and a cutting wheel couldn't handle in about 30 seconds.

[jaysg]

Those pots aren't the exact same pots that Kevin uses...but he said they were suitable. (clip) I liked the plastic casing (as opposed to a metal casing like most pots) and 2KV rating (also a 2W rating)...so I contacted Kevin and he just sent me one...on the house.

That's why I asked. RV4's aren't rated to 2KV. What are the one's Kevin provides?