Dark half-power Hoffman AC30

[MORE_Guitar_Solos]

Hi folks,


I've got a 2x el84 Hoffman AC30 that performs really well, but is kinda dark. Compared to the blackface Bassman I've got here (which is very bright)  it sounds like it has a blanket over it. I've built a couple of these before, as well as the full size version, and it seems like the 2x el84 amp is always a bit darker, perhaps due to having less headroom, but this one is just darker than I'd like. The full power AC30 I have is as bright as anyone could ever want.


It's basically standard according to the Hoffman version of the schematic other than some larger filter caps in the preamp section of the power rail, and voltages are in line with previous builds I've done (~300v on el84 plates; ~140v on plates of first 12AX7), and I've juggled coupling caps and bypass caps in the preamp with no satisfactory change. I'm wondering if making some changes in the tone stack would get the "air" and "sparkle" this amp seems to be missing.


Attached is the tone stack in question. I've played with a tone stack calculator a bit, and it seems like raising R16 (to maybe 27k?) and/or lowering the value of the treble pot R17 (maybe 250k?) as well as raising the value of the 47pf tone cap to around 100pf could essentially raise the eq shelf from 10k up a few dB. Does anyone have experience with this and do these seem like sensible or effective changes to make? I'm hoping not to make this into a different amp, just need more of that Vox shine at the top.


Also, I'm wondering what changing the value of the cathode follower resistor (V2-B) would do as this is not included in the tone stack calculator I'm using. Vox (and Hoffman) use a 56k but I notice that both 5F6A and Marshall 1987 use a 100k here. What would changing up to 100k do to the top end in a Vox?


Thanks in advance everyone
Dave in Japan

[stratomaster]

If the half power version is significantly darker than your full power one then I'd look elsewhere for a problem, not at the tonestack for a tonal tweak. 

Once you've ruled out a problem like an incorrect value (say a gridstopper accidentally 10x larger than intended) or a signal short (especially something like the carbon layer of a shielded cable contacting the signal line) then start looking at tweaks.  The one I'd try is not either resistor but the 100k. Bump that up to 120-150k for less lows and more highs.

[MORE_Guitar_Solos]

Hi Stratomaster,


Thanks for your reply. Never wrong to check basics, but I don't think there's any major circuit errors in this one as I built it nearly ten years ago and I've been through it a few times.


Are you referring to R15, the 100k slope resistor? If so I'll give that a shot and report back, cheers!


Dave