Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Cirrus on August 22, 2022, 06:20:00 pm

Title: Tighten up some flub on a half-power AC30?
Post by: Cirrus on August 22, 2022, 06:20:00 pm
As per a post from a couple of months back, I converted an AC30 1x12 into an AC15 by just re-wiring the bias scheme to work with 2 x el84s, and replacing the GZ34 with a 5v4 to combat the rise in voltages that the reduced B+ current draw produced. That meant I could put a single Celestion Blue in the cab.

And honestly, I'm really happy with the result. It's been perfect at band practices and lets me run it right in the edge-of-breakup, clean but compressed place that Voxes excel at.

The only thing I'd like to change is in tightening up a little flub in the bass end as it starts overdriving - comparatively it feels like the bass is more loose than when it was an AC30, or in comparison to my AC30 2x12 with two blues.

My two thoughts are

- Smaller output transformer, since at the moment it's 15 watts going into a 30 watt transformer. On paper I quite like the idea of the slightly saturating output transformer, and if that reduces the sub energy and maybe focuses things into the mids a little at high volume it'd do it for me.

- Smaller coupling caps. Obviously way cheaper, but price isn't an issue to get the sound right. At the moment it's 0.047 before the PI and 0.1 after, I was thinking of reducing the ones after the PI to 0.022 and seeing how that works.

My gut feeling is a smaller OT might be the ticket, because if that would cut sub bass a few DB then the low end would stay deeper and full when it's not being pushed, which I like, and only tighten up/ focus in on the mids more when pushed, which is where the flub starts to come in.

Anyway, thoughts welcome!
Title: Re: Tighten up some flub on a half-power AC30?
Post by: sluckey on August 22, 2022, 07:43:19 pm
The original used .01 after the PI.
Title: Re: Tighten up some flub on a half-power AC30?
Post by: Cirrus on August 23, 2022, 10:15:48 am
Quote from: sluckey on August 22, 2022, 07:43:19 pm
The original used .01 after the PI.

it's interesting how much the spec varied between the ac30/4 and the revisions going into the 60s. And now it seems vox has pretty much settled on 0.1 before and after the PI for modern ac15s and 30s.

I've got some 0.022 and 0.01 caps inbound, will play around.