# Converting my YBA-2a back to 12AX7 preamp



## nbs2005 (Mar 21, 2018)

Hi all, 

The '67 YBA I have is modded and running an EF86 tube in V1. I bought this thing to mod and learn a little bit so I wanted to talk about how I would return this to running a 12AX7 in V1. I'm at the very beginning of this journey in learning how to do this.

First off, I need to get a hold of the schematic for the stock amp. I'm pretty sure I can dig that up. Next, I'd need to figure out what was done. @Jim Jones, can you shed some light on what you followed to make the change? Also I think this amp only uses part of the 12AX7 stock and it's fairly common to wire it up to take advantage of all the gain in the tube. 

Thanks for the help.

Jeff


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## loudtubeamps (Feb 2, 2012)

great little amp.... you should be able to return it to stock quite easily.70's build of the yba's use both sides of the ax7....have fun.
Traynor-Bassmate-YBA2B-Manual-Schematic.pdf


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## dtsaudio (Apr 15, 2009)

There are several versions of the YBA2. Traynor was renown for making changes and not documenting them. If you think yours only used one half of the first 12AX7 it may be this one.
Note the first tube is listed as a 12AU7. I doubt that is correct. I've seen this same schematic with a 12AX7 in V1, but can't seem to find it.
The EF 86 is a pentode, not a dual triode, so wiring is going to be very different. It also depends on whether it was wired in full pentode or in triode mode. I've attached an example of pentode wiring. This is just an example and things are going to be different, but it will give you an idea of what to look for.


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## traynor_garnet (Feb 22, 2006)

I've owned a couple of early YBA-2 heads that have 12au7 written on the tube chart. I think they were trying to cut down on breakup as it was technically a low powered bass amp. You can pop a 12ax7 right in there and get a ton more break up.



dtsaudio said:


> There are several versions of the YBA2. Traynor was renown for making changes and not documenting them. If you think yours only used one half of the first 12AX7 it may be this one.
> Note the first tube is listed as a 12AU7. I doubt that is correct. I've seen this same schematic with a 12AX7 in V1, but can't seem to find it.
> The EF 86 is a pentode, not a dual triode, so wiring is going to be very different. It also depends on whether it was wired in full pentode or in triode mode. I've attached an example of pentode wiring. This is just an example and things are going to be different, but it will give you an idea of what to look for.


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## dtsaudio (Apr 15, 2009)

That's interesting, as the gain would be way lower than with a 12AX7. The resistor values denote a 12AX7 as well. An AU7 wouldn't be run hard enough.
Learn something new every day.

As for the OP's request, there would be a massive difference between the gain of an EF86 and 12AU7. Would be closer to the X7 for gain.


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## Greg Ellis (Oct 1, 2007)

How does it sound with the EF86?

Do you want to change back to 12ax7 for tone reasons, or just to restore the amp to original as a fun electronics project?


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## nbs2005 (Mar 21, 2018)

It sounds pretty bight with all but one of my guitars and it's hard to tame that down. And I've wanted to dip a toe into amp mods for a bit; this seems like a logical choice for a start.


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## Jim Jones (Sep 18, 2006)

Hi Jeff,

I changed the cathodyne phase inverter to a long tail pair style and replaced the first gain stage with an EF86. In its original incarnation only one half of the first 12AX7 was used. Before I swapped the 12AX7 out altogether I paralleled both sections for awhile. That’s a cool sound, too.

Sorry to hear it’s not working with your guitars as-is. Most of my guitars are darker tonally and have pretty hot P90’s so it was a great fit with those!

I’m pretty sure I have some pics of the board pre-mods. If I can find them I’ll send them your way.

Jim




nbs2005 said:


> Hi all,
> 
> The '67 YBA I have is modded and running an EF86 tube in V1. I bought this thing to mod and learn a little bit so I wanted to talk about how I would return this to running a 12AX7 in V1. I'm at the very beginning of this journey in learning how to do this.
> 
> ...


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## Greg Ellis (Oct 1, 2007)

I think I'd be inclined to chase the wires as they lie now, and sketch out a schematic of the current setup, at least the preamp section.

See if there's a bypass cap on the volume pot creating this over-bright character. Or it might be the choice of cathode bypass caps, or the interstage coupling caps. 

Messing around with a few minor things like that would be a lot simpler than ripping it all out and rewiring for a completely different preamp tube.


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## dtsaudio (Apr 15, 2009)

Jim Jones said:


> I’m pretty sure I have some pics of the board pre-mods. If I can find them I’ll send them your way.


Do you have the schematic of the original?


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## jb welder (Sep 14, 2010)

dtsaudio said:


> Do you have the schematic of the original?


pg.4 of this set? Also includes the 6BQ5 variant.


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## dtsaudio (Apr 15, 2009)

Thanks JB. I just wanted to varify the 12AU7 version i posted was the same.
BTW, Jim, that isn't a concertina phase splitter. Just nit picking, but if it was you could do the first stage and phase splitter with one tube.


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## Jim Jones (Sep 18, 2006)

Ooh, fair enough, it’s been a looooong time since I changed the PI. IIRC, the original phase inverter was the version in the second schematic in the three posted above.



dtsaudio said:


> Thanks JB. I just wanted to varify the 12AU7 version i posted was the same.
> BTW, Jim, that isn't a concertina phase splitter. Just nit picking, but if it was you could do the first stage and phase splitter with one tube.


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## jb welder (Sep 14, 2010)

dtsaudio said:


> Thanks JB. I just wanted to varify the 12AU7 version i posted was the same.


I have one schem. showing a 6AV6 first stage, but I'm not sure it's legit, looks like tube name has been edited.


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## dtsaudio (Apr 15, 2009)

That may be typical traynor. But it would be strange.


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## nbs2005 (Mar 21, 2018)

Appreciate all this insight. For fun I swapped the 6v6 tubes from this (a Phillips and a Sylvania) into my Harmony and the Tube Store matched set (Russian, don't remember brand) from that into the YBA. Both amps sounded better (same speaker and cab for both). Go figure.


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## jb welder (Sep 14, 2010)

jb welder said:


> I have one schem. showing a 6AV6 first stage, but I'm not sure it's legit, looks like tube name has been edited.





dtsaudio said:


> That may be typical traynor. But it would be strange.


So, did a bit of research, and as is often the case with Canadian amps, ended up back at this forum. 
Here's a thread with a link to some documentation at vintageguitar, and a schematic.
https://guitarscanada.com/index.php?resources/early-yba-2-schematic.50/download&version=50
YBA-2 clone (6V6 version) conception


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## dtsaudio (Apr 15, 2009)

Looks like the very first version. Must be a rare amp. 
I can understand the 6AV6 in v1 as it is a high mu triode if you ignore the diode part of it.
Novel approach.


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## Greg Ellis (Oct 1, 2007)

Why would Pete choose a 6AV6 in that V1 spot, if he doesn't need the diodes in the circuit?

Was he just sitting on a big box of 6AV6 tubes and inventing a way to use them up?


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## traynor_garnet (Feb 22, 2006)

I owned a few of the very early ones that run a 6AV6 in V1 (with a 5Y3 rectifier too). Great amps (that is actually my old amp in the Vintage Guitar write up). They are basically Tweed Harvards with beefier transformers.


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## dtsaudio (Apr 15, 2009)

Greg Ellis said:


> Why would Pete choose a 6AV6 in that V1 spot


 Cheap and easy circuit or maybe just trying to do something different.


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## Jim Jones (Sep 18, 2006)

The triode section of a 6AV6 is half a 12AX7, why not?


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## nbs2005 (Mar 21, 2018)

Because I'm a fan of closing loops when possible, I thought I'd share my recent thoughts on this amp. I've been playing lately through the second input, which I believe is lower gain, and using it for clean tones. That, along with some guitar tone tweaks, is getting a really nice tone (and different ) out of all my guitars now. The real surprise was my Esquire; that guitar was tough to use with this amp. But again, if the goal is clean, this amp can easily get really nice tones even with that guitar. 

So for the time being, changes (and GAS) have been avoided.


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## Jim Jones (Sep 18, 2006)

Good to hear! If you wanted just a bit thicker tone out of the amp you could experiment with upping the coupling caps to the power tubes. I’m pretty sure I used .022’s there. .047 or .1 @ 400V will add a bit more beef without changing the overall character of the amp.


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