Vox AC15, EF86 channel only, head

[fabiomayo]

Hi everyone.

Hopefully, this will be my next amp. As people requested, here is the
untested and incomplete layout and schem. Just a Vox AC15 power
section with a selectable dual-ganged master volume and EF86
preamp. All very tight in a small head chassis - just like I thought I
would never build an amp again.

Any critique and suggestion to both the schem or the layout are
very much welcolme.

Would I be missing too much letting og of the Top Boost channel?

Thanks.

[Luthierwnc]

A couple years ago I made the attached head using the Matchless DC30 schematic. This one is 14 watts. I'd built another at 30 watts and sold it so I knew the schematic. I like both tones.

The Bassman/Marshall CF circuit on the other side uses 2 more tubes but not a lot of real estate on the board. It also feeds into the bottom of the phase inverter very cleanly. You have what looks like an on-on-on toggle for the coupling cap selector. That ought to be fine. I have the 6 cap version on mine but the middle ones are most useful. I wouldn't bother with the Master. You won't have any trouble getting dirty fast. The cut is really important, though.

These 2 EL84 amps are louder than you'd expect. Put this over a British 12 (or two) and stand back. "But honey, this is the lowest power amp I've got. Are you saying I need to make a new one?" Skip

[fabiomayo]

Thanks, Skip.

I chose the three-position cap selector based on reports like yours
that only a few positions are really useful. What do you think about
the cap values I've chosen as a starter?

I'm curious on the Bassman/Marshall side. How does i sound and
is it useful?

This project started for me with a second channel, a Top Boost and
a tremolo using the spoare triode left by the TB. Since it would make
the amp much more complex and bigger I decided to strip it down -
which is good cause I already have an aluminum blank, folded and
soldered that'll fit this circuit to a tee.

Any suggestions?

[echuta13]

I'm doing an EF86 SE build, and I was planning on just using an ON-ON switch for two different coupling caps. My question is which ones do you find yourself gravitating to on the dial? I currently have .022 & .01 on my schematic, but I'm wondering if they are too close together to make it worthwhile.

[CaseyJones]

These things sound fantastic! They're almost worth the hassles of sorting through a couple dozen EF86s to find The One that's least microphonic. What you get is a really thick warm clean tone. To me a distorted EF86 sounds nasty and I don't mean nasty in a good way.

A few tips: Use an isolation mount for the EF86 socket. Don't improvise, buy one.

These are a good idea as a head and a PITA as a combo. The buzzing EF86 will drive you nuts in a combo. For those of you considering a combo... don't say I didn't warn you!

[fabiomayo]

Thanks for the tip, CaseyJones.

What is this isolation mount? I've seen a tubing to roll around the EF86
to tame it's microphonic and rattle down a little.

I was planning on a head already. Tubes pointing up.

[CaseyJones]

What is this isolation mount? I've seen a tubing to roll around the EF86 to tame it's microphonic and rattle down a little.

https://taweber.powweb.com/store/chassis/chord2.html

Scroll down a bit, the part you want is listed as "9 pin shock mount".

Look at the pic, you can build your own if it's going to be problematic to get it shipped to Brazil. The socket itself is mounted to its own little sub-chassis with rubber grommets then the sub-chassis (really just an oversized washer) is mounted to the main chassis with springs on the mounting bolts. There's a lot of wiggle room in the assembly, that's what keeps the EF86 quiet.

Another way I've done it (although I don't have a pic) is to mount the socket to a small square of sheet metal then suspend the square of sheet metal under the main chassis with four little springs.

[fabiomayo]

Thanks, Casey Jones.

Ordering from Weber wouldn't be a problem except I was planning on
using other vendor (I already have most of the stuff and Mojo has
Heyboer trannies in stock). I'll have to try the lifted spring-mounted
square thing. Hope it's enough.

Also, just a mildly related question. I was thinking of using Mojo's Vox
AC15 replacements if the PT offer two 6.3v winding (for tubes and rectifier
heaters). But maybe I could/should use the Marshall 18watt replacement.
Any thoughts on that? (I also just checked and it's cheaper too ).


Regards.

[CaseyJones]

Also, just a mildly related question. I was thinking of using Mojo's Vox AC15 replacements if the PT offer two 6.3v winding (for tubes and rectifier heaters). But maybe I could/should use the Marshall 18watt replacement. Any thoughts on that? (I also just checked and it's cheaper too ).

I've worked on a lot of amps that have just the single 6.3 volt winding, the filaments simply continue from the rectifier to the rest of the tubes. It might not be the best way to do it but it works. I don't see any reason why an 18 watt replacement transformer intended for use with a 6CA4 rectifier shouldn't work perfectly for your application.

[dartanion]

Also, just a mildly related question. I was thinking of using Mojo's Vox AC15 replacements if the PT offer two 6.3v winding (for tubes and rectifier heaters). But maybe I could/should use the Marshall 18watt replacement. Any thoughts on that? (I also just checked and it's cheaper too ).

I've worked on a lot of amps that have just the single 6.3 volt winding, the filaments simply continue from the rectifier to the rest of the tubes. It might not be the best way to do it but it works. I don't see any reason why an 18 watt replacement transformer intended for use with a 6CA4 rectifier shouldn't work perfectly for your application.

Using the same filament winding for the rectifier tube and other tube filaments is a non-issue with an indirectly heated rectifier tube. the EZ81/6CA4 pulls the rectified HV off the cathode whereas the 5Y3/5V4/5U4/GZ34 etc. are all directly heated (besides a 5V filament), which means you are using the filaments as heaters and pulling HV off it as well.

[skyboltone]

The pentode section of radio tubes such as the 6EA8 seem similar to the EF-86. Given that the 6EA8 in NOS RCA and Tung Sol brands are plentiful I just wonder if it might be worth giving a try? I got more projects on my hands currently than I can keep up with, but if no one else trys it I'll give it a go one day and report back.

[dartanion]

There are quite a few pentode - triode tubes that were commonly used in TVs that are viable guitar amp tubes. You just need to experiment if you have the time and resources. One great thing with a pentode - triode tube is that you can use the triode as a CF to overcome the high output impedance of pentodes. Using the CF, you can drive a full TMB tonestack without much insertion loss. The uber gain found in pentodes is counteracted by the high output impedance when mated to a lossy tonestack. If you need a simple EF86/E80F tone control, use the one from a 5F2A or 1974 trem channel. Both are low loss and work well with pentodes.

[fabiomayo]

That's a good idea regarding the pentode/triode tubes. Perhaps it'd be
good to have two sockets wired in parallel so you can change back
and forth between them?

Good to know about the 6.3v secondary. What about the OT? Would I
benefit from using the 18 watt? Or would I loose something by not
using a Vox replacement OT?


Also, I had forgotten to update the schem (even though the layout is up
to date). I wish I could build this amp without a choke (place a 500R
wirewound resistor in it's place). What do you think about that? My reason
for that is space and noise (such a tiny chassis) and based on my
perception that it wont change tone or feel all that much.

[CaseyJones]

That's a good idea regarding the pentode/triode tubes. Perhaps it'd be
good to have two sockets wired in parallel so you can change back
and forth between them?

I try to keep things absolutely simple and minimalist. I see this again and again, a builder starts with a simple amp then goes crazy and makes everything switchable, tweakable and adjustable.

Parts left out cost nothing and never break.

Less parts mean the shortest possible signal path which usually means lowest noise and best response. If you're going to tweak make the signal path as short as possible and strip away everything you can do without.

It's a simple amp, build another one if you want to try a different tube lineup. Then you can play them side by side.

Good to know about the 6.3v secondary. What about the OT? Would I
benefit from using the 18 watt? Or would I lose something by not
using a Vox replacement OT?

I usually use whatever transformers I have on the bench at that moment. Purists will tell you that if you don't use the "correct" reproduction iron the amp "won't sound right"... which is total BS.

My take on EF86 amps is that they're great amps for clean tones. Clean tones equal hi-fi iron as far as I'm concerned so I'll grab an old output transformer from a hi-fi console.

The only drawback to using vintage iron is that if you find some magic tone then you're going to have to figure out exactly what you used if you want to do it again.

Also, I had forgotten to update the schem (even though the layout is up
to date). I wish I could build this amp without a choke (place a 500R
wirewound resistor in it's place). What do you think about that? My reason
for that is space and noise (such a tiny chassis) and based on my
perception that it wont change tone or feel all that much.

I'm on the fence with this one. If I have room for a choke I generally like what a choke does for my tone but... like just about every other amp builder I'm cheap. If I can save $15.00 by leaving the choke out it's tempting. If you have a choke available try it then see if you can do without it.

[fabiomayo]

Thanks for the tips, Casey.

I'm surely trying to make things simple with this amp.

About the OT, my thoughts were that the Marshall 18 watt OT
would give me a similar Vox vibe with added power (which
would perhaps mean a little cleaner). My question is if by
using this OT I'll be missing too much of the Vox sound. Guess
not. I'm not really down to this clean amp thing you speak of.
The PI should start to clip soon enough. Am I wrong?

Also, if you want clean, shouldn't I add a little bit of negative feedback?

Regarding the choke, the main issue was space and noise
(things will be tight as you can see on the drawing). Ditching
the choke and using a resistor seemed like a good way to make
thing simpler (cheaper is always a very welcome plus ).
Also, a big 10 watt wirewound resistor should have enough
inductance to make tone differences very small. What do you think?