LNFL and PAB

[ElectronAvalanche]

Hi gang,

I tweaked my 100W again (sic) and installed a switch to choose between 22M and 44M or no Local NFB on CL2.

To my surprise I finally found out, why non of my later amps had that rich Boost with the PAB on like my very first amp. Back then, I did not have 22M resistors and simply left out the LNFB.

With no LNFB, the boost is very rich and across the whole freq spectrum. With the LNFB one gets a Boost with the low mids and upper bass subdued. More NFB leads to even less low mids.

Which actually makes sense, since the 47nF NFB cap does cut the low content.

I played around with the cathode caps a bit but did not find too much of a difference. Actually not enough difference to decide for a value. Since I have the 220k/150k anode and 3k3/2k2 resistors, I wonder if the 100k/1k5 combo gives a stronger effect with the higher cathode caps. Anyone checked this?

Concerning the Presence pot:

Some schematics show the pot being wired as a variable resistor. In Gil´s pics you can see, that only the left most lug and the wiper are wired. I changed to this setup and found the presence much more pleasing, oddly enough.

Cheers,

Dominik

[Bob-I]

To my surprise I finally found out, why non of my later amps had that rich Boost with the PAB on like my very first amp.

With no LNFB, the boost is very rich and across the whole freq spectrum. With the LNFB one gets a Boost with the low mids and upper bass subdued. More NFB leads to even less low mids.

I experimented with this a bit last night. I didn't find the same thing at all. The PAB is not as rich as with my HRM amps but the LNFB didn't have the effect like you found. I'd like to find a way to fatten up the PAB on this amp, maybe switching on the mid boost at the same time would help.

I played around with the cathode caps a bit but did not find too much of a difference. Actually not enough difference to decide for a value.

Really? I find that V1 bypass have a big effect. V2 not so much especally on V2b. Even small changes on V1 make a difference. I don't have any 5uF caps so I've been using 4.7uF and I believe there's a difference with that small change.

Concerning the Presence pot:

Some schematics show the pot being wired as a variable resistor. In Gil´s pics you can see, that only the left most lug and the wiper are wired. I changed to this setup and found the presence much more pleasing, oddly enough.

Cheers,

Dominik

I'm not thrilled with my presence, what cap are you using? I'm using a 1uF electrolytic, same as the bypass on V2b. I find the presence boost a bit harsh.

I've tried using 2 lugs and 3, and I agree the 2 lug method seems best, plus the control is more even. Using 3 lugs I found that most of the change was between 2-5:00 where with the 2 lug it starts making changes at about 10:00.

[ElectronAvalanche]

Hi Bob!

Yes, the PAB is "fattest" with the Mid-Boost on and no LNFB. Maybe I should have worded things a bit clearer.

I will try to come up with a recording soon.

Maybe my ears are not so fine, but the effect of upping the cathode cap on V1b/V1a was not tremendous. Of course, there is some change in the high mids, but compared to say switching out the LNFB this was only a very subtle effect.

I have 5uF on the cathode caps now.

Cheers,

Dominik

[Darkbluemurder]

Concerning the Presence pot:

Some schematics show the pot being wired as a variable resistor. In Gil´s pics you can see, that only the left most lug and the wiper are wired. I changed to this setup and found the presence much more pleasing, oddly enough.

Dominik,

I can see a red wire going to the left lug and a purple wire going to the right lug but where do they connect? Do you have a schematic?

Thanks
Stephan

[ElectronAvalanche]

Hi Stephan,

the purple wire on the wiper goes to the + side of the 1uF cap, the red wire goes to the 390Ohm resistor.

cheers,

Dominik

[Darkbluemurder]

Thanks, that's what I thought. I tried it last evening but had not had much time to test. On first glance it worked the same as before but that's OK for me.