Greetings!
Last year I acquired a Vox AC15H1TV, the Heritage reissue from 2007.
This is the model with an EF86 normal channel and a Top Boost channel, with pentode/triode switching in the power amp.
I love the EF86 channel, it sounds really good.
However, there seems to be a bit of bleed between the channels, and more specifically - when you turn up the bass control of the Top Boost channel, it introduces hum.
I never use the Top Boost channel, and while troubleshooting I discovered that pulling the preamp tubes from the Top Boost channel eliminates the noise completely and that the normal channel now is great sounding and lacks the low end hum.
However, pulling the preamp tubes for the Top Boost channel raises the B+ on the unused filter caps to around 350VDC - the same as the ratings for the filter caps themselves. The B+ for the normal channel is still the same 343VDC or so.
I am thinking that that the raised B+ for the Top Boost will cause an issue and kill the filter caps, and that the normal channel B+ is a little too high for comfort, especially when taking into account possible wall voltage variations and potential voltage spikes.
I was thinking about recapping it anyways as the amp is 15 years, and I am thinking that it would be sensible to use caps with a higher voltage rating, and that pulling the Top Boost preamp tubes would be safe, since the B+ for the normal channel seems uninterrupted.
Based on the schematic and cap values it seems that the amp is very conservatively filtered compared to the newer reissues (10/22uf vs 47uf) and that this maybe could be the culprit for the hum?
A quick google search shows that this is something that other owners have experienced, and that it's a part of the design.
https://music-electronics-forum.com/forum/music-electronics/50517-vox-ac15-heritage-tb-channel-bleeds-into-the-ef86-channel Anybody here have any experiences with this model and these issues?
Topic: Vox AC15H1TV - Hum and channel bleed (Read 2172 times)