AC30 power supply: rectifier, choke, and filter caps

[Roe]

I've experimented with different power supplies in my ac30 build. there are three different ways of doing this.

1) traditional ac30 with gz34. most choose to increase filtering from the original 16+16 to at least 22+22 or 32+32. Personally I preferred 64uf on mains and 33uf on screens. (If you want to experiment easily, just use the vox ac30s with switching between 22+22 and 44+44.)

2) 1970-80s rose-morris ac30 with ss rectifier. Sounded a little hard and stiff, using a sagr resistor helps (although the 70s 22R is rather small)

3) CLC or Pi-filter ala Dave Peterson/Brian May (and the earliest ac15s). Sounds quieter than the others, have better bass. can sag alot if you use gz34 and a choke with high dc resistance.

I ended up combining 2 & 3: first uf4007 rectifier, then 32uf cap into hammond 300mA 5H 52R choke to 32uf cap to OT CT. This setup gives a little less sag than 1) the traditional approach but is less stiff than 2). the bass response is great. amp sounds punchy and articulated with sweetness and minimal hum

[Roe]

I experimented a little with plate and screen voltage. I actually preferred keeping the screen voltage a little lower than the plate, using a common 470R screen resistor (where the choke usually is - keep in mind that an ac30 choke typically has a DC resistance around 400ohms) and 100R individual resistors.

Plate voltage is 325-330v, screen voltage almost 10v lower. This is with 50R cathode resistor and a 560VAC 250mA PT.

[Reeltarded]

This is very good! Roe, could you describe (3) or point to more about that?

Thanks!

[Roe]

This is very good! Roe, could you describe (3) or point to more about that?

Thanks!

there are schematics available for the brian may ac30 as well as the 1959 ac15. For the latter, see http://www.voxamps.com/downloads/circuits/ac151959.jpg.

the idea is to have a big choke immediately after the rectifier and before the OT, not after OT and prior to screens as is usual

The Brian May/Dave Peterson choke is speced at 7h 260ma, 78 ohms. I used a Hammond which worked fine.

You'll find more info online if you search CLC filter or Pi filter.

[Reeltarded]

Ok I see it. Thanks, Roe!

[rooster]

Yes, there is something to be said about putting a choke in front of the OT. Nathan Daniels used this on his '59 Silver Medalist and if you switch things around, attaching the OT directly to the rectifier before the choke, the amp doesn't sound as good. ..And then there's the 5E5-A Fender Pro, same thing, along with other tweed Fender amps of the day. I wouldn't know but since Western Electric was so influencial on so many of the amp builders of the day, it all reads like a follow-the-leader type of thing.

However it's also true, when the volume wars hit, nobody was running the choke first. And a lot of 'feel' went away, IMO. Eh, so here we are.

In reference to the Pro, I know that Ken liked the 5E5-A tone and feel because he stated as much in a Vintage Guitar column of his back in the day. Frankly, I think his choke (1K ohms) is an attempt to get some of this type of feel into his Express.

Anyway, Roe, I like that you did this to an AC30 circuit. I bet it sounds and feels pretty good.

[Mark]

https://tubeamparchive.com/viewtopic.php?t=20221

[Roe]

thanks for adding link with schem and info

[randalp3000]

here's the super schematic also
http://www.webphix.com/schematic%20heav ... _schem.pdf

I really need to try this on an AC build

thanks

[Ondrej]

The full DP conversion is descibed here:
http://www.mikeryde.com/%5CDP_Amp%5CBMA ... P_Spec.pdf

I was wondering whether my MM 20h, 100mA choke would work. I currently have 64uF mains filtering and I believe 40uF at screens, no standby switch. I've added two diodes before the GZ, as Ceriatone does (thanks Roe). I guess I am still hard on the GZ, but with lower filtration, I was getting some noticeable ghost notes. Now the sound is kind of stiff. What would be your choice (brand/specs) if you had to get a new choke for this?

Roe, are you still with your low coupling caps? I guess you said you had 15nF PI caps and 10nF coupling preamp->PI? I followed your tweaks. Could the stiffness of my amp be due to these "trebble" values?

[Roe]

same 15nf caps here. works fine for me.

for choke you want something rated for 250-300mA, 3-7H, 50-110ohms

[Ondrej]

Roe, the Hammond choke you talk about, is it the four-hole "193L"? Would it fit the chassis? Found it here:
http://www.hammondmfg.com/193.htm

BTW, I guess I used 1n4007's in front of the GZ. Do you expect difference from your uf4007's?

[Roe]

Roe, the Hammond choke you talk about, is it the four-hole "193L"? Would it fit the chassis? Found it here:
http://www.hammondmfg.com/193.htm

BTW, I guess I used 1n4007's in front of the GZ. Do you expect difference from your uf4007's?

yes, I'm using the 193L. I had to drill new holes in the chassis. there is a very slight difference btw n4007s and uf4007s