ac30 build: tweaking the circuit

[Roe]

I've build a weber 6v30 kit with some upgrades and changes:
- magnetic components ac30 output transformer and 20h choke
- ef86 channel instead of tremolo, with 6 way eq swich ala dc30
- sozos and mustards because I have plenty already and they are perfect for this project
- carbon comps in selected positions

Amp is mainly based on the vintage ac30s - I'm using the vox ac30 guide.
also, it is somewhat inspired by the z wreck, the rocket and the dc30. I'm tweaking values to find what suits me. earlier testing have lead to the following results:
- the 50ohms cathode resistor is a must. I prefer 320v and the 50ohm over higher valued resistors as well as higher voltages
- the original 16+16uf filtering is too low. the mains needs to be increased, probably also the screens. Currently I'm running 48 at the mains and 33 at the screens. I'm gonna try 64 at the the mains later on.
- low preamp filtering works great. I prefer the bc caps in british amps. I'm using 10ufs in the preamp and 33uf for the screens
- originals used both 100k and 220k plate resiostors - I prefer the 220k
- I prefer the 3k cathode resistor on the first gain stages but 2k2 also works well (both on a single triode). I didn't care for 1k5 or 4k4 (all possible value from the original amps, although many used shared cathode so that a 1k5 shared becomes 3k and 2k2 shared becomes 4k4)
- I prefer the vox 22k and 10k voltage drop resistors in the power supply over some variants

so far it seems that the holy numbers are written down in the old schematics.

pics:

http://www.facebook.com/photo.php?pid=1 ... =860225267
http://www.facebook.com/photo.php?pid=1 ... =860225267

[Roe]

tried the strange 330k resistor (just in front of vol pot) and 220pf bright cap (bypassing 330k and connected to vol pot viper) used on the 30/6 treble models. this adds brightness but gives less gain and volume, esp when turned up. more brightness may be fine but the latter is a killer. switching bright caps works better for me.

[M Fowler]

Yeah that 220pf bright cap would be shrill city I would think.

You might have to go with a 2 meg volume pot to up the volume some

[Roe]

the 220pf sounds bright yes but could actually be useful for some applications (like playing loud and distorted with a dark sounding guitar)

[M Fowler]

Could use a two bright switch then.

[Roe]

update: tried different mustard coupling caps in the phase inverter. as you may know, originals used everything from .01uf (1960 ac30/4) to .15uf (later ac30/6 bass). I tried .015uf and added a .039uf in parallel for a total of .054uf. With different guitars, I always preferred the .015 because of the tighter bass and quicker response.

[loctal]

Thanks for the info, keep us posted. I am following you.

[Roe]

originals used .01uf and .047uf input caps on each side of the phase inverter. I'm using .01uf since this is more than sufficient for a full bass response (as randall aiken has shown)

[Roe]

one more detail, I comparred 270k and 220k mixer resistors. the difference gain was slight but the 220k seem to sound harmonically richer

I`ve upped the mains filtering to 32+32uf now. in order not to stress the gz34 (max 60uf), I've connected one 32uf before standby and the other after standby.

[M Fowler]

You upped to 32uf?

I have always used 40-47uf on 36w AC30 type circuits and 33uf on 18w.

Mark

[Roe]

upped to 64uf (dual 32) on mains.
screens is 33uf.
sounds good with the new magnetic component ot

[Roe]

comparred 56k and 100k cathode follower resistors - liked both, so I made it switchable. the 56k sounds compressed whereas the 100k gives bolder, punchier tones

[M Fowler]

Which MC transformer was that? Vox?

Did you go with the 220K/220K voltage divider like the Zwreck?

[Roe]

yes, its the magnetic components vox ac30 output transformer. I'm quite happy with it. certainly sounds like a vox.
and yes, I'm using the 220k mixer resistors, not the 270ks. the 220ks work as voltage divider like the z wreck when you turn down the normal channel volume.

[Roe]

got a question about grounding. on this build, my grounding scheme is a follows:
1 - wall ground only -
2 - mains caps and 50r cathode resistor with 250uf cap
3 - screens
4 - cathode follower
5 - everything else (all preamp tubes using a 22k voltage droper), including speaker jack and input jacks

1 is located to the far right near the pt but bolted through both chassises
2 is near the PT, but do not use the pt bolts unless you want noise
3 grounded to the hole near the mains filter cap (between pt and ot)
4 is grounded in a bolt running through both chasses, at the middle
5 is grounded in a bolt running through both chasses, nearer the output transformer
this grounding scheme work very well for me. its a slightly simplified version of the multi star ground scheme recommended by larry g and randall aiken