My take on a Vox hybrid

[pompeiisneaks]

Here's a schematic I've put together, but not sure it will work, would love some feedback. here's the nitty gritty

1. EF86 main channel, with a tone stack
2. Top boost channel with cathode follower into tone stack
3. switchable between cahnnels with either, 1 or 2, or both.
4. Switchable OD like Dumble, I've heard these may not work so well with EF86's but we'll see.
5. Split cathodes on the power tubes like a liverpool to allow pulling two tubes easily.

I'm sure there are things I've done horribly wrong, please let me know what they are.

I've not put the relays on the schematic, but the footswitch should have 3 buttons, A, B and OD. I still need to work out that part

Imagine the 'switches' are all relays instead.

MainSchematic.PNG

~Phil

[JazzGuitarGimp]

From what I've read, pentodes have a higher output impedance than triodes, and are easily loaded down by tonestacks. You might consider adding a cathode follower after the ef86 into the tone stack. Also from what I have read, (Merlin) pentodes sound good as an OD stage when they are hit hard on the input - an alternate OD circnuit might be a single 12AX7 switched between the input jack and the ef86.

[ChopSauce]

Well, I'll watch this topic. I'm sure I will learn things. I am very puzzled by the OD entrance section, amongst other things.

One more question (if you don't mind...)

- any particular reason for the 25uF cathode bypass caps?
(I've been told that would be too much and I've been recommended to use 10 or 4.7uf rather - with less gain stages than your schem. reads - to keep the outcome from being muddy, and undefined )

[tictac]

Since the gain in this amp is higher that a AC30 type amp you might want to increase those 100 ohm screen resistors to 1k or higher. KOC recommends 2k.

Also 25W is a bit overkill on the 120 ohm cathode resistors. 10W is more than adequate.

TT

[pompeiisneaks]

From what I've read, pentodes have a higher output impedance than triodes, and are easily loaded down by tonestacks. You might consider adding a cathode follower after the ef86 into the tone stack. Also from what I have read, (Merlin) pentodes sound good as an OD stage when they are hit hard on the input - an alternate OD circnuit might be a single 12AX7 switched between the input jack and the ef86.

That's why I put another 12AX7 stage after the pentode to provide more handling for the tone stack, or are you implying I should convert it to cathode follower from the EF86 directly instead?

~Phil

[pompeiisneaks]

Well, I'll watch this topic. I'm sure I will learn things. I am very puzzled by the OD entrance section, amongst other things.

One more question (if you don't mind...)

- any particular reason for the 25uF cathode bypass caps?
(I've been told that would be too much and I've been recommended to use 10 or 4.7uf rather - with less gain stages than your schem. reads - to keep the outcome from being muddy, and undefined )

That's what all classic Vox's have used for some time, 22 or 25uF. They sound great to me, so I'm sticking with what works, and plan to tweak as needed ;P

What specifically about the od entrance? It is an 'on or off' switch just like the dumble OD section. EIther the signal is directly routed to the PI, or it takes a bypass into the OD and then back to the PI.


~Phil

[ChopSauce]

I look forward to reading about your experiments with this circuit. I could read that the bass had to be "held" (read cut) at the early stages, and then released but I could see OD setups which were reported working better doing the opposite, so...

... and about the OD entrance, I was rather curious at the resistor network (with the 500 pF in parallel to ground.)

[JazzGuitarGimp]

From what I've read, pentodes have a higher output impedance than triodes, and are easily loaded down by tonestacks. You might consider adding a cathode follower after the ef86 into the tone stack. Also from what I have read, (Merlin) pentodes sound good as an OD stage when they are hit hard on the input - an alternate OD circnuit might be a single 12AX7 switched between the input jack and the ef86.

That's why I put another 12AX7 stage after the pentode to provide more handling for the tone stack, or are you implying I should convert it to cathode follower from the EF86 directly instead?

~Phil

My mistake; that's what I get for not clicking the schematic for a high resolution image. I saw the volume pot and thought is was the tonestac! Carry on.....

[pompeiisneaks]

I look forward to reading about your experiments with this circuit. I could read that the bass had to be "held" (read cut) at the early stages, and then released but I could see OD setups which were reported working better doing the opposite, so...

... and about the OD entrance, I was rather curious at the resistor network (with the 500 pF in parallel to ground.)

Oh I grabbed a random OD from one of the amps, I think it was this one:

https://tubeamparchive.com/viewtopic.php?f=5&t=12871

But i'm not sure that's the 'best' OD to use, if not, I'd be happy to try one people think would be better suited for this schematic. I see others have another pot you can use, one for level one for drive, this only has drive. but I'm still learning about the Dumble style OD for sure.

~Phil

[ChopSauce]

Thanks for the link! I was only aware of the OD sections with the many controls. This one looks as a simpler alternative

I suppose the 500pF cap acts as some kind of tone control for the OD section (EDIT:) I mean, of course I'm naive but I feel this schematic has many tone tuning components spreaded here and there, which are somewhat unexpected on a prototypical/draft schematic - in a general matter.

[Guy77]

I love the sound of EL84 tubes with a cathode follower preamp . This is just like the Rocket amp topology, my favorite amp along with the JM Wonderland.
I ran my Rocket at 340 volts B+ and my JJ tubes started to red plate when I used the 100 ohm resistor. Switching to 120 ohm for each pair fixed the problem.

Very interested in how it turns out.

Cheers!

Guy

[pompeiisneaks]

Yeah It's still a ways out, gotta finish this dumble, and I may be doing a rack mount preamp between now and this as well, but it's almost completely designed, and we'll see how it turns out.

~Phil

[lord preset]

No, drop everything else and build this one. We want to see how it turns out.

[pompeiisneaks]

Heheh not so fast mister! I'll get to it, I promise

~Phil

[M Fowler]

Hold on a second, haven't you already built a Vox AC30 shown in one of your earliest videos?

Mark