ECL86 Rocket... any reason I shouldn't?

[bean]

A few years back I built an amp kit that had two 12ax7 preamp tubes and a pair of EL84's. Three gain stages and a cathodyne PI. I liked it, but didn't love it.

I'd like to reuse the chassis, OT and PT and don't want to try to add another preamp tube socket.

One idea I came up with was to build a rocket using a pair of ECL86's as the PI and power tubes.

The PT is a 220-0-220 @ 125mA and it has 4A for heater current.
The OT has an 8K Primary and is rated for 15W.

So could I build a rocket with SS rectification and no choke with 2 x 12AX7's and 2 x ECL86's? Or is it a crazy idea?

[matt h]

(deleted)

[bean]

Thanks Matt!

I have at least one pair of ECL86's that were pulled from a Tandberg reel to reel recorder. So having the tubes on hand influenced the decision to go that route.

I did notice the PT voltage is a bit lower than the stock. But figured with SS rectification and not using a choke would make up some of that difference.

The mosfet idea is definitely worth considering as well!

I'm not looking to build a Rocket clone... just something rocket-ish!

[matt h]

(deleted)

[gingertube]

..idea I came up with was to build a rocket using a pair of ECL86's as the PI and power tubes.

The PT is a 220-0-220 @ 125mA and it has 4A for heater current.
The OT has an 8K Primary and is rated for 15W.

So could I build a rocket with SS rectification and no choke with 2 x 12AX7's and 2 x ECL86's? Or is it a crazy idea?

This should work very well. ECL86 / 6GW8 are getting a little difficult to get hold of although bargains still happen - I got 6 NOS Mullards from my tube supplier for $5 each as recent as 6 weeks ago.

ECL86 are absolutely best with a 10K Raa OT but I've certainly used them with an 8K Hammond 1608.

One of the favourite AMPS OF THE local Guitar God is his Marshall with the ECL86 PP Power section. He managed to talk me out of a pair of those Mullards for his amp.

Cheers,
Ian

[bean]

Should be a fun project! I'm sure I'll have tons of questions as this will be the first time building an amp that isn't already well documented.

First question... the amp I'm converting this from had four B+ nodes... I see the Rocket has five. Can I run both 12AX7's from one B+ node? Or should I add another power supply capacitor?

[gingertube]

Bean,
Power Supply nodes are something which can be screwed up easily, such that you get oscillation problems when the amp is complete.

The "Rulz" are, however, very easy.

You can run 2 triode sections maximum from each node and then ONLY if the triode sections have the opposite signal phase. Thus a "normal" inverting triode stage and the following stage (whether it is inverting or not) can share the same power supply node.

The usual differential Phase Inverter has signals of both phases present and MUST therefore have its own dedicated supply node, which can not be shared.

Keep those simple "Rulz" in mind and you should never have a problem.

Cheers,
Ian

[10thTx]

IF you are looking for a low wattage Trainwreck version .................

I built this So Low Watt amp and used ECL84's. It was around 3-4 watts.

I thought it had an amazing bass response for a small tube.

https://tubeamparchive.com/viewtopic.ph ... w+watt+amp

http://www.el34world.com/Forum/index.php?topic=13438.0

Perhaps you could adapt a Trainwreck preamp into it?

So, in keeping with the idea of a Trainwreck Rocket preamp into the SoLow Watt LTPI and power amp, I drew this up. Note the value on the V1-3 cathode cap is smaller than the 22uf. I think this lower value may be needed & I don't think losing the 22uf will make much difference given the bass response of the ECL84 tubes.

I attached an editable ExpressSCH version also.

With respect, 10thtx

[10thTx]

OR ........... you could use the 6BM8 tube which I strongly prefer over the EL84. And the 6BM8 is still being produced.

This is simply a DRAFT idea that I haven't built. However, I have built Geezer's Little Wing (mini-bassman) and his HoSo56 amp using a 6BM8 & very much liked the sound of it. So the LTPI, power section and B+ rail are from a proven design. I simply grafted a Trainwreck preamp topology on.

I have built 6BM8 amps using the 6BM8 triodes for the LTPI. Sounded good.
However, I actually prefer using a 12A_7 with the 6BM8's because I like to be able to use a 12AT7 or 5751 for the LTPI. Just a personal preference and both sound good. I would not hesitate to use the 6BM8 triodes if you wanted to.

With the 6BM8, you will have to have some lower value caps to ensure that the amp will sound transparent and not get too much grit or mud.

The post LTPI coupling caps should be .01 to .02 range.

The preamp cathode caps should be 5uf to 2.2uf range, although you could try 10uf on the first triode cathode cap.

I attached a ExpressSCH version that is editable also.

With respect, 10thtx

[10thTx]

This is likely heresy, but it'll give you something closer to a rocket. Use a MOSFET for the CF.

This is how I do what Matt H. suggested in his post. I used this type of Mosfet approach successfully in numerous builds including the D'Mars ODS.

The entire schematic as a whole is a DRAFT unproven idea for consideration and shows how I would personally approach it.

I attached an ExpressSCH editable schematic for anyone interested.

With respect, 10thtx

[10thTx]

And if anyone is interested in experimenting ............

On EL34 World, several of us came up with DRAFT ideas for different low watt push/pull amps:

http://www.el34world.com/Forum/index.php?topic=12526.0

Some of the ones listed were actually built and are proven designs. Those should be indicated in the thread.

And attached is a data sheet for anyone to ponder an idea doing something like that.

With respect, 10thtx

[bean]

Definitely some good ideas! Thanks 10thTx!

The name of the game for me is to reuse what I have on hand. This includes transformers, chassis, board and whatever I can find in the drawer. Hopefully this will allow me to gain some knowledge instead of just putting together another kit! It will also keep costs low as it's hard to justify building more amps to the Mrs.!

The mosfet idea is really interesting, but I have the ECL86's on hand and might as well put them to good use!

The board I'm repurposing was setup using radial capacitors. I hope to reuse those. They are installed across the board near the circuit block they provide power too. They are as follows: 47uf-22uf-22uf-22uf. There will be no choke used. Some of these are slightly lower than the stock Rocket... but can I assume they will work?

Also, for the B+5 power node that I will need to add, can I use a 47uf cap that I found in the drawer? Any problems following a smaller capacitor with a larger one? I will see if I can use that as C2 and move that 22uf to the end of the chain.

[10thTx]

Well, this is interesting!

I found a schematic for a Trainwreck Express Baby using ECL86 tubes.

Maybe this will be useful to you?

I noticed this has some strong similarities to the Trainwreck 6BM8 schematic I posted a few posts up. Since that was done with an ExpressSCH editable schematic, you could use the SCH version to document your tweaks.

And I have done a smaller cap on a B+ node 1 and then followed that with a larger cap and it worked out just fine. In fact, I think Bogens did that very thing on some amps like the CHA-20?

With respect, 10thtx

http://www.bluetone.fi/fi/wp-content/up ... 08_pdf.pdf

[gingertube]

Equivalents for info:

ECL82 = 6BM8
ECL84 = 6DX8
ECL86 = 6GW8

The ECL86/6GW8 is the most interesting as its triode section is identical to 1/2 a 12AX7 and its pentode section is close to a slightly derated 6BQ5/EL84.

The ECL82 Triode is more like 1/2 a 12AT7 and its pentode section is slightly lower power again.

I have not used ECL84/6DX8 for a guitar amp but have certainly used them in a low power push pull HiFi Amp - It was gorgeous. The current production Russian 6DX8 are seriously good, as good as any of the NOS 6DX8 I've tried. I would have no hesitation in trying them for a new guitar amp design/build.

Another Tube which you could look at, pentode only NOT a triode pentode is the 6M5/EL80. Good for about 7 Watts in Class AB PP. The 6M5 was very common in Oz tube gear back when and was used in some Oz Guitar Amps, both in SE and in PP.

Cheers,
Ian

[zendragon63]

FWIW bean, I have built several pseudo wreck clones using the ECL82 (6BM8) and they actually sound pretty good. They work great and are plenty loud for small gigs and if you need a little bit more, just slap a mic in front of it. Don't weigh a ton either. The excess gain of a Wreck proper doesn't suit my playing or taste so I will put a 12AY7 in the front end. Attached is a recent build--feel free to cut and paste. Regards.

dennis