So I have been given free reign to gut a Blues Jr and do whatever I want with it

[mort]

I do have some 12au7's. I'm trying to keep the reverb to only two triodes if possible. Also trying to order the least $$ amount of parts possible. But I'd rather add a tube than have to buy a transformer and different verb tank.

[mort]

After some thought, I think I just better add a tube. That will give me the 3rd triode for the transformerless reverb and a triode to use as a cathode follower after the ef86 to drive the tone stack (it's been suggested this will be better than just the ef86 trying to drive the stack...)

So all day that's 4 preamp tubes. The ef86 front end, a PI, 3 triodes for reverb, and a triode to drive the tone stack. Sound like a good plan?

[mort]

Not sure of the 70s, but the Reverberocket 2, GU12, Rocket 2, all used 6U10 triple triodes...

Justin

I'm actually also building a GU-12 right now. But I don't have to think about design considerations because it's all getting pulled straight from the stock circuit.

[Leo_Gnardo]

Not sure of the 70s, but the Reverberocket 2, GU12, Rocket 2, all used 6U10 triple triodes...

I sure wouldn't build anything new with triple triode tubes. Tough enough to find 'em for the older amps, the ones we need for repairs & maintenance now practically nonobtainium.

Yes Mort it looks like you'll have to add another tube socket if you're going to use an EF86 preamp. To spare one triode in my project, I'm planning on a concertina type output drive stage, as in a Princeton.

[mort]

Just because I'm in a sharing mood, here's teh GU-12 I'm working on. And yes the cap has been moved to the side of the 10w sand resistor

This one is actually for another amp builder. He's busy building his own stuff, had been thinking about wanting this circuit so he can get his Exile On Main Street on, and asked me to build it for him.

[The Dude]

...... To spare one triode in my project, I'm planning on a concertina type output drive stage, as in a Princeton.

Funny,......that was my first thought when I read the question. I even pulled up a Princeton schematic to refresh my memory.

[mort]

What would be the audible difference between a concertina split load and a LTP?

[Leo_Gnardo]

What would be the audible difference between a concertina split load and a LTP?

My ears tell me, not much until you start pushing past the clean zone. Amp pushed into distortion, they'll act and sound differently. The old Orange amps OR120 & 80 also used concertina drive. In lower power amps it's a cool sound, think George Thorogood playing thru his dimed Princeton. Allen amps has essentially a double power Princeton (woops I forget which one), also a Cape Cod amp builder Gries, and I have one of his sitting right here, it sounds terrific to me and its owner. That's more or less what I'm going to try & stuff into a Blues Jr. box. In high power amps, the Oranges, to me the concertina sounds brittle when overdriven, and as we see in another thread here, it can stress the heck out of one side of the output push-pull pair, again when pushed very hard. I'm hoping that won't be too much of a problem as long as we stay out of the 500V + power supply zone.

[The Dude]

That's odd to me, Leo, I've always thought the concertina sound was less brittle or rather "warmer" because of the asymmetrical nature of the circuit. But then, I don't like a ton of swimming reverb either, which I think can add to the brittle sound. Saying that, I guess a consideration here would be, are we going for a "surf guitar box" or something with what I would call a "reasonable" amount of reverb?

Edit: Sorry to confuse the concertina circuit with reverb discussion, but IMO, there is a tonal interaction.

[mort]

In my case it just needs to have some reverb, so the err would be toward "reasonable".

[The Dude]

Here's one of several demos on YouTube that might help.

[Leo_Gnardo]

That's odd to me, Leo, I've always thought the concertina sound was less brittle or rather "warmer" because of the asymmetrical nature of the circuit.

Just my opinion, brittleness when pushing those old Oranges well into clip. Could have something to do with EL34 breakup too, also asymmetric by that point, being thrashed by 500V and more. These Blues Jr conversions won't be going there, I expect we'll be more in the Thorogood Princeton zone and that's a good place by me. IOW the brittleness of Oranges may have more to do with extra-hi voltage and crushed pentodes.

Thanx for the video, yes different tones but different speakers, EL84 vs 6V6, and who knows what all else, even where's the mic pointed. Princeton wins the warmer tone prize in this vid.

[The Dude]

I agree with that, Leo. Too many other variables to say it's the concertina circuit alone that makes the tonal difference. And, more likely it's not. I guess the point is that it's certainly possible to make a good sounding "warm" amp with that circuit and free up a triode.

[tubeswell]

If you want a spitfire-like circuit, but with a pentode-like pre-amp tube in V1, you could use something like a 12DW7 as a cascode, with the AU7 side as the input triode and the AX7 side as the output triode. This ought to produce good gain with pentode-like characteristics, but with more reliability than an actual EF86 pentode.

Or otherwise do a more conventional setup with a dual triode wired either as a parallel V1 stage with single and parallel - 'normal' and 'hot' - switching at the input jacks - see attachment. This also gives versatility for parallel sets of input effects.

Or a DC-coupled pair with a CF driving the tone stack

Or a BF style pre-amp with the 2nd triode as a tone stack recovery stage driving the LTP.

(The 47R resistor under the tail of the LTP in BF fender amps is part of the NFB loop, with something like a 2k7 resistor in the upper leg of the NFB voltage divider)

[mort]

OK so I've ordered and recieved some parts for this build, and I went ahead and got a reverb transformer and tank so all my option would be open. Here's where I'm at on the drawing, just added in the cathode follower between the EF86 and tone stack. Does that look about right? If so, I just need to settle on what type of reverb circuit to use and where to tie it into the signal path.