AC30 build lives - a bit of a hummmmm, though

[markmalin]

FWIW, most, if not all of the schematics with the top boost channel show a 25µF cathode cap on the first stage and no cathode cap on the second stage. The only place I've seen that 22µF cap on the second stage was on the original 'top boost' mod that was a separate module to be added to an amp. Looks like by the time the top boost channel was incorporated into the AC30, that second stage cap had been eliminated.

Interesting.  Just so I'm clear, you're saying you typically see 25uF in place of the 0.1 uf for C1b in the picture, and no C5?  If so, that's about where I'm at. 
I've never built anything based on the AC30 circuit until now, so it's all very different from the Fender/D'clone style amps I've built in the past.

[sluckey]

Just so I'm clear, you're saying you typically see 25uF in place of the 0.1 uf for C1b in the picture, and no C5?

Yes. Take a look at the AC30 schematics in the schematics library at the bottom of this page.

[markmalin]

Just so I'm clear, you're saying you typically see 25uF in place of the 0.1 uf for C1b in the picture, and no C5?

Yes. Take a look at the AC30 schematics in the schematics library at the bottom of this page.

Ah, I see. 
So help point me in the right direction.  What's troubling me is, I've sort of put a band-aide on the hiss by reducing the gain, haven't I?  The amp sounds ok now, but I haven't played it long enough to know whether it's lost any of that sweet EL84 "ping".
If I understand correctly, taking the bypass caps out allows audio negative feedback and effectively "reduces" the gain.  Please correct me if I'm wrong.  If it's not an issue having these out, and if I feel the tone of the amp is good, I guess I'm done -- but if I want to add more of that bite back into the tone, wouldn't I want to have some value bypass cap in V1 and V2 (the cathode follower), but maybe change the cathode resistor to lower the gain instead?
I don't mean to confuse the issue.  At the end of the day, I want a nice "EL34" tone amp.  I need to grasp what these changes have done, whether it's OK to leave them out, and what I would do if I wanted to introduce a little more gain.

[sluckey]

What's troubling me is, I've sort of put a band-aide on the hiss by reducing the gain, haven't I?  The amp sounds ok now, but I haven't played it long enough to know whether it's lost any of that sweet EL34 "ping".

Gain is directly proportional to noise (hiss). Higher gain = higher noise. No way around that. This is certainly not what I'd call a 'high gain' amp and from just looking at the schematic, I'm surprised that the hiss is too high for you. But it is a bright preamp and hiss is heard better in a bright amp. I kinda suspect the reason Vox eliminated the second cathode cap was to intentionally lower the gain, maybe to reduce hiss. Just a guess.

I don't consider removing that cap as a 'band aide'. I'd like to think you are now copying the real Vox circuit. I'd also increase the value of that first cathode cap to .68, or 1, or 5, or even 22. The real Vox uses 25. Some say a 25 makes for a muddy sound but with a 500pF plate coupling cap the mud should be eliminated. Heck, I'd probably replace that 500pf with a .0022 or even a .022 and even remove that 100pF bright cap from the volume control. If you do all this you should have a quieter amp and I bet you will like the tone. At least give it a try so you'll know.

You're not really gonna get an EL34 tone from an EL84 amp.

If I understand correctly, taking the bypass caps out allows audio negative feedback and effectively "reduces" the gain.  Please correct me if I'm wrong.

That's correct.

[markmalin]

Thanks, Sluckey.  That was super helpful. 
I too was thinking of either removing or increasing the size of the bright cap (I put  120pf in there, but i may just take it out all together).  Was also wondering about increasing that 500pf.  I'll play around with these things.

Again, many many thanks

You're not really gonna get an EL34 tone from an EL84 amp.

D'oh!  fat fingers.  I meant EL84.  (corrected above)

[labb]

For  a pretty good read on Hiss reduction go over to the AX84 site and do a search for a thread titled "Experiments in Hiss Reduction". Makes for a pretty good read. Will be on about the second page when you do a search for "hiss"...Thread is about three years old.

[markmalin]

For  a pretty good read on Hiss reduction go over to the AX84 site and do a search for a thread titled "Experiments in Hiss Reduction". Makes for a pretty good read. Will be on about the second page when you do a search for "hiss"...Thread is about three years old.

Thanks labb.  I will.

[markmalin]

To close the loop here, in the end I used a 25uf cap on the first gain stage cathode (C1b), and a 5uf on the second stage (C5).  C5 was a tossup, but it seemed to sound better with the master volume in and the input volume set high (i.e. to get overdrive).  The gain is less and the hiss is minimal.  So:

C1b is 25uf
Plate resistor on V1 changed to 100k
C5 changed to 5uf
47pf treble cap value the same, but changed to silver mica and hung off the treble pot

Here is a picture of the build.  I added the MV, which works, but I need to modify that a tad.  Thanks for all the help guys, the amp is sounding great :)  FWIW - Here's a link to an iPhone video for the guy I'm building the amp for to update him on the progress.  (it's just an iPhone, so quality is not good, but at least you can hear the amp).  https://youtu.be/Iqss6zkv52E

[vicentebd]

see good bye happy with amp