AX84 SEL with VVR Power amp only schematic check

[caps8419]

Hi Everyone,

I've been lurking and learning for a while with this being my first post. I recently completed an Ax84 SEL build with the entire amp being scaled which has been working fine. I'd like to try scaling just the power amp. The original SEL schematics have the screens and part of the preamp sharing the same B+ node so I adjusted the schematic by adding another node for just the screen and added the VVR itself in circuit. This is my first build so I'd like to check if my changes look correct. My main concern is if i need another dropping resistor before B+3 or is that what R6 is doing (I think that's the case..) Any way, I'd like to confirm before I start soldering. Thanks!

-Michael Cappiello

[Colossal]

Yes, you'll need to add another dropping resistor and maybe even another cap for the preamp rail.

Like this...

[caps8419]

Thanks Colossal. That was an incredibly fast reply!

For my own knowledge, where did you come up with the 18-22kOhm resistor value for R3a? Thanks!

-Michael

[Colossal]

For my own knowledge, where did you come up with the 18-22kOhm resistor value for R3a? Thanks!

-Michael

The voltages in the SEL are close to an 18W Marshall. Lots of guys have built VVR'd 18W Lite IIb amps where just the phase inverter and power tubes are regulated and the preamp split off from the main rail in the same way as what you are doing. I just ballparked the 18-22k value based on the voltages. YMMV in reality.

It is good you have a reservoir cap in front of the VVR.

[martin manning]

Here's my put. Dave, you need a resistor between the reservoir and the added filter cap.

[caps8419]

Thanks guys. Looks like a 20k should do the trick.

[martin manning]

Do you have it wired up? If so where did you put the VVR?

[caps8419]

not yet. it will be before R2 after C1 as in my original schem.

thinking outloud... maybe i'm off by a decimel...

going off of voltages in the original SEL schematic i'd find the current across R3:

(356-346)/1000 = .01A

if i leave the VVR before R2 the V is 366 which is then sent to the preamp. So if i need to get to 346 from 366 i need to drop 20V. 20/.01 = 2k Ohm resistor ( R3a) before B+ 3 ( in Colossal's updated schem). This would bring 366 down to 346.

Is this correct or am I off and should be looking at R4 like i originally calculated?

[caps8419]

Martin,

Do you have the VVR installed after R2 for a particular reason? it looks like our schematics are pretty similar (adding the dropping resistor into mine) except for that point. Thanks!

[martin manning]

After R2 and C2. There is an extra stage of filtering before the plate in this design, so why not let the VVR work with that lower ripple, slightly lower DC voltage? BTW, the preamp only draws about 5mA, and there will be another ~1.5mA for the extra bleeder. Given that, Rx in my schematic should be about 1k5, and R3 should go up to 2k. These changes are edited in above.

[caps8419]

Thanks Martin, That makes sense. I'll implement your design when my Mouser order arrives and see how it goes!

-Michael