Vox AC15 (TBX).. redesign?

[Sea Chief]

Hi chaps- hope you all keeping virus-free.

This Vox AC15 TBX model I have (marshall made, uk 90's) is good, & has all the basic ingredients it should I think, but sounds oddly lacking. Compared to the AC30 TBX, which is known as a very good reissue/ only decent one made, its kinda frustrating.

It does have two noticeable differences to an orig, an MV & a (crap) reverb added; but apart from that I believe its an AC15 top boost including the tube rectifier.

Does anyone know this amp/ circuit? I'm wondering whether its conceivable to 'up' it (mods) or even rebuild it to bring out the goodies, to a 'real' AC15 sound. I mean it must have the basic xfmrs & tube layout as an original.. plus it has a celestion blue: so is the potential there?

Thanks, SC

[pdf64]

TBX -
Stiff HT, EL84 cathodes not properly bypassed https://el34world.com/charts/Schemat...x_ac1596pa.pdf
Type 3 / crossline master vol, hodge podge preamp, neither one thing or another, nothing glaringly odd https://el34world.com/charts/Schemat...x_ac1596pr.pdf

But the absence of T&B tone controls on the original may be somewhat offputting - were 60s AC15 ever fitted with them?https://el34world.com/charts/Schemat...x_ac151960.pdf

Note that the TBX is missing the EF86 pentode channel.

The pcbs on your TBX may have many failing solder joints, are you sure it's working right?

The oversized chassis holes for the 'through valve' design are not ideal for a total conversion project.

[Sea Chief]

TBX -
Stiff HT, EL84 cathodes not properly bypassed https://el34world.com/charts/Schemat...x_ac1596pa.pdf
Type 3 / crossline master vol, hodge podge preamp, neither one thing or another, nothing glaringly odd https://el34world.com/charts/Schemat...x_ac1596pr.pdf

But the absence of T&B tone controls on the original may be somewhat offputting - were 60s AC15 ever fitted with them?https://el34world.com/charts/Schemat...x_ac151960.pdf

Note that the TBX is missing the EF86 pentode channel.

The pcbs on your TBX may have many failing solder joints, are you sure it's working right?

The oversized chassis holes for the 'through valve' design are not ideal for a total conversion project.

Hi pdf,

I can make only partial sense of the info there, but thanks I get a vague gist. Its working fine. No bad solder joints: I'm simply describing the inherrant sound all users describe (not describing faults with it, as it is, that need fixing up).

So if not a rebuild possible if the chassis isn't accomodating.. are there any mods I can do then to get more from it, dare I say get it more original sounding? it does sound pretty 'stiff' that's a good word.

[pdf64]

The vintage AC15 has a C - L - C filter between the rectifier and OT CT, whereas the TBX has a more modern arrangement where the OT CT feed is taken from direct from the reservoir cap C2. So my approach would be to replicate the vintage arrangement.
eg replace R6 with a suitable Vox type choke such as http://www.hammondmfg.com/pdf/EDB194E.pdf reduce C2 to 16 or 22uF, and move the OT CT connection to C6, same as the feed to the screen grids. Put a link across R4 to fully bypass the EL84 cathodes, then re-assess the amp's sound, to see if all that's had any benefit.
The standby is Marshall's typical hot switching arrangement, so it would be best not to use it.

[Sea Chief]

The vintage AC15 has a C - L - C filter between the rectifier and OT CT, whereas the TBX has a more modern arrangement where the OT CT feed is taken from direct from the reservoir cap C2. So my approach would be to replicate the vintage arrangement.
eg replace R6 with a suitable Vox type choke such as http://www.hammondmfg.com/pdf/EDB194E.pdf reduce C2 to 16 or 22uF, and move the OT CT connection to C6, same as the feed to the screen grids. Put a link across R4 to fully bypass the EL84 cathodes, then re-assess the amp's sound, to see if all that's had any benefit.
The standby is Marshall's typical hot switching arrangement, so it would be best not to use it.

Hi pdf, that's an interesting suggestion/ appreciated. Sounds feasable to me: I wonder if anyone else agrees this a good plan? I'll put the idea to my colleague at Rift (super busy not right to ask him directly), if I can simplify the idea.

Could you briefly say what the aim in your idea is for an idiot to understand? I added a choke to my '78 twin reverb -Rift's idea, similar?- & possibly changed the odd cap value etc, in that case to add a bit more authentic 'blackface' to the SF circuit I had to rebuild anyway due to conductive board: but as to the thinking behind it alas I didn't know / just blindly followed advice! & it sounds great of course.

[pdf64]

The idea is that at higher signal levels, the amp will respond to, and compress, note attack. Didn't you have a 5E3? If so, a little more like that.
As it is, the TBX's response will be as stiff and plinky as it's feasible for it to be; which in the grand scheme of things, isn't very
I'm sure none of this will be news to Mr Rift.

[Sea Chief]

The idea is that at higher signal levels, the amp will respond to, and compress, note attack. Didn't you have a 5E3? If so, a little more like that.
As it is, the TBX's response will be as stiff and plinky as it's feasible for it to be; which in the grand scheme of things, isn't very
I'm sure none of this will be news to Mr Rift.

I did have a 5E3, I built, but had no idea the vol-vol interaction thing existed at all at the time.. so it just sounded a bassy lump to me/ got shot of it pronto.

When you say 'at higher input signals'.. do you mean higher amp volume, or more oomph from a guitar's pickups, or more volume (somehow) from any guitar? input signal I thought was unable to be altered you see (if a gtr's pickups are what they are) hence my non-understanding.

I find this page (never known of before).. was this the general overview you got your basic (+choke etc) suggestions from? http://www.voxshowroom.com/uk/amp/15tbx_hood.html

cheers, SC

[pdf64]

I did have a 5E3, I built, but had no idea the vol-vol interaction thing existed at all at the time.. so it just sounded a bassy lump to me/ got shot of it pronto.

When you say 'at higher input signals'.. do you mean higher amp volume, or more oomph from a guitar's pickups, or more volume (somehow) from any guitar? ...

Ah yes, I seem to remember now that it was too loud for your situation.
By higher signal levels, I meant at the power amp, ie when it's pushing up to and past its max power output, into overdrive. There won't be compression effect at quiet levels, at least not one from HT sag.

...I find this page (never known of before).. was this the general overview you got your basic (+choke etc) suggestions from? http://www.voxshowroom.com/uk/amp/15tbx_hood.html...

I know that site but can't remember that page; it doesn't seem to mention the points I noted? Rather I've fixed up several of these TBX amps and got to know them pretty well, hence the heads up about the solder joints. Generally, I'm into Vox amps from the early - mid 60s.
Regarding the voxshowroom site, it's written by by someone with only a basic level of technical competence, so there can be errors, the info there isn't gospel.