Sluckey Dual Lite with 5879 and ECL84 Quad

[boodle]

Summited for your approval...(channel Rob Serling's voice)  Hoping you tube gurus can help catch any glaring mistakes and make suggestions for my schematic attached!

The schematic is a redesign of Sluckey's Dual Lite using a 5879 and the four triodes and pentodes from  four ECL84's.  I borrowed heavily from Tubenit's SoLowWatt 12A_7 and CPI designs.

I have never built with toroidal transformers before, so I am giving them a shot.  Antek has very good pricing on the PT, OT, and Choke that I needed.  Likely due to the non-standard tube compliment, finding a transformer that had ~200Vac and 6.3Vac @3+A was a challenge.  Spent quite some time looking for EI trannys and stumbled upon the Antek PT that fit the bill.  So, toroids it is!

If my research is correct, the PI from the SoLowWatt grew out of the DMars/TOS design process and is sourced from Dumble ideas.  Sounds great in the SoLowWatt CPI Source Follower amp I built, but don't know if that is the "best" topology for a Vox&18W amp. 

Also, the 5879 topology was lifted verbatum from the SoLowWatt.  Again, not sure it that will yield the "best" Voxy tones. 

Thus, submitted for your approval...

[Reposted pdf of schematic.  Egregious error!  Had the pentodes pins for EL84 not for ECL84's!  Took the tube quad part of the schematic from an AC30 .sch file.  Should be good now.]
 

[AmberB]

Question for you, since that Antek power transformer has 2 high voltage coils at 200 volts each, are you going to wire them in parallel for this amp?

[tubenit]

A couple of thoughts for you to consider ...............................

I think the amp design will work for you.  Having said that, a pair of ECL84's are around 3-4 watts I'd guess?  So a quad would be 6-8 watts I'd guess.
A pair of 6BM8 tubes is around 7-9 watts & the 6BM8 tube is still being manufactured and they sound fantastic to my ears.

If you have a large stash of ECL84's ............. then perhaps stick with your design.  If not, consider the 6BM8 which is one of my favorite tubes tone wise.

I did built an amp with a 5879 channel and a 12AX7 channel. Yes, there was a difference between them  ..............  but not nearly the difference that I was expecting to be there.  The 5879 is somewhat chimey-er and fatter than the 12AX7 but the gain (in the context that I used them) wasn't that far off from one another.  I ended up eliminating the 12AX7 channel and left the 5879 as V1.

Lastly,  you could simplify the "tone stack" for the 5879 with a SPDT center off design switch where you parallel two other caps with a coupling cap from the 5879.  I'd probably try a .001, .0047 and .01            Just another thought to ponder.


OR you could add a mosfet CF to the 5879 and use something more traditional in the way of a tone stack.


With respect, Tubenit

[boodle]

Question for you, since that Antek power transformer has 2 high voltage coils at 200 volts each, are you going to wire them in parallel for this amp?

AmberB - yes, I will parallel both the 200V secondary and the 6.3V secondary.  In chatting with Antek, they commented they put 2 of both to allow the PT to be as flexible as possible.  I actually would have bought the AS-05T200 (no "x2" in the model name) but they were out of stock.  The difference between them is the x2 has parallel 200V and a 5V for a tube rectifier.  The non-x2 has a single 200v (rated "the same" output as the paralleled 200V's) and a 180V tap, but no 5V.  The non-x2 is nominally cheaper.  If I choose to repurpose the PT in the future, I guess the 5V tap might come in handy.  I am hoping this Dual Lite version will be a main amp and no repurposing will be needed.

  So a quad would be 6-8 watts I'd guess...6BM8 tube is still being manufactured.

Tubenit, I did consider the 6BM8.  It sounds like a great tube.  One of the considerations for this particular project was minimizing the operating costs over time.  Both the 5879 and the ECL84 fit that bill.  While watching upfront costs, as long as they are reasonable, I don't mind them as long as the end product is of quality and useful.  So, it is kind of an economic and electronic experiment that I am running, I guess.

I do have a small stash of both tubes, so I am semi-future proofed.
The real deal sealer for both tubes is how much I like the combo in the SoLoWatt CPI Source Follower.  Great stuff you (and others?) put together there in your design.

I did built an amp with a 5879 channel and a 12AX7 channel...

Glad to hear of your experience with both tubes.  I will keep running with the 5879 in the Vox channel then as chimey and fat are both terms I enjoy!

Lastly,  you could simplify the "tone stack"...

I will consider that in the modding phase.  I want to hear what Sluckey's 2P6T switch brings to the table as I have never done that type of frequency control before.

Just to make sure I understand your premises: 
Cathode caps: the 2P6T switch would be completely gone and tone shaping would be accomplished just by switching cathode bypass cap values?  If I remember correctly, a cathode bypass cap increases gain and adds a high-shelf boost and follows f=~1/(2piRC) to calculate the shelf frequency.  If so, I imagine that gain wouldn't change much for the different caps but the shelf frequency would.  I would also assume that tube impedance behavior, plate load resistor, and grid resistor all play a role in frequency, but if standardized values for plate and grid resistance are used the f equation gives you a general starting point use cathode resistor and cathode bypass cap values?
MOSFET source follower:  I will keep that in my back pocket.  I did add what I think was your MOSFET SF design in the SoLowWatt CPI to decrease signal loss of the tone stack to good effect.

I appreciate all of the help!  I will begin with the board layout.  I have a long piece of fiberglass board material and turrets.  It is not the correct height for a Hoffman style board, but it has been great for 3 or 4 other builds.  I didn't see an existing Hoffman board to buy for Sluckey's Dual Lite, so I will use his layout as a guide.

More input certainly welcomed by all.

[sluckey]

If I choose to repurpose the PT in the future, I guess the 5V tap might come in handy.

If this means you may use a 5V rectifier tube, then you would need to connect the two 200V windings in series for 200-0-200 operation. This would give the same voltage as the parallel windings with a FWB, but only half the current capacity. Or, you could leave the windings connected in parallel and use a hybrid FWB, one 5Y3 and two silicon diodes.

Just to make sure I understand your premises: 
Cathode caps: the 2P6T switch would be completely gone and tone shaping would be accomplished just by switching cathode bypass cap values?

I think tubenit means to replace the 6 pole switch with a wire. Then use the SPDT center off switch to connect one of two caps in parallel with the .01µF plate coupler cap that's connected to V1 pin 8. Expect the switch to pop when changing positions. The 6 pole switch is nice but I soon found my preference position and just leave it there.

[boodle]

Sluckey,
Got it.  I will try using single caps in the 6T switch location first to sample what you have done and then try the .001 in parallel in 56k 5879 plate resistor and then tubenit's values in parallel. 

If those experiments allow simplification, I will experiment with removing the Cut control altogether and replace the 6T switch with a standard tone control.

See any issues with any of that I should expect?

[sluckey]

I like the cut control. I'd hate to sacrifice it.

[boodle]

I like the cut control. I'd hate to sacrifice it.

Noted!  Thanks

[boodle]

Also, according to the Miniwatt tube data sheet, each pair of ECL84 pentodes in P-P is capable of 5.4W at a plate voltage of 250V and a common cathode resistor of 117 ohms.  So a quad should deliver an upper limit of ~10W.

[boodle]

Schematic pdf modified and updated.  Posted in original post.

Added tubenit's Vox channel tone control as an option to Sluckey's design.  Notes explain option.
Corrected several small errors.

[sluckey]

You're missing a volume control and this is what tubenit's suggestion would look like...

[boodle]

Sluckey,
Ah...missed the "coupler" part.  Thanks for catching the volume control and the schematic snip.

Schematic pdf has been updated.  In original post.

[boodle]

Sluckey,
Do you know what cap is selected in your favorite setting on the 6T switch?  That would give me a good place to start with experimenting.

[sluckey]

It's been a while. My favorite is either position 1, 2, or 3. The smaller caps sounded too thin to me.

[boodle]

Thanks, I'll start there.

[DummyLoad]

Seems to me that plan will have a propensity to be highly unstable: The layout of the first gain stage in such close proximity to output stage, to me reads like a futile attempt. Don't mean to rain on your parade, but those were my first thoughts when reading your schema.


Respectfully,


--Pete

[boodle]

The layout of the first gain stage in such close proximity to output stage...

Pete,
By that do you mean because a triode and pentode reside in the same glass envelope in the ECL84?

My experience thus far, with ECL84's is that having both reside in the same tube is not an issue in the SoLowWattCPI_Source_Follower.  The SoLowWatt uses the ECL84 triodes for the PI and has a similar layout to the Vox side of Sluckey's amp design I am tinkering with.

It could see that triode/pentode proximity being an issue especially is the preamp design has cascading gain stages for more pre-amp distortion. 

It might also be an issue with the 18W side my DualLite application with ECL84's.  It remains to be seen.

Thanks for the heads up,
Tim

[sluckey]

The triode in those triode/pentode power tubes is usually used in the power amp, for example, the PI. Sometimes the triode may be used for tremolo. But I've never seen that triode used in a preamp circuit. Not saying it will be, but it could be a can of worms.

[boodle]

could be a can of worms.

It might be.     I'll live the "nothing vertured, nothing gained" headspace.  Will be interesting and informative to find out if the tri/pent is an issue when tri's are used in a preamp feeding the pent. 

However, since Dummyload's post, I have been thinking through a backup plan in case it is chaos:
Ditch the 18W side and repurpose those 2 triodes. 
Add Brown Princeton type tremolo using one triode. 
Add a standard Vox tonestack with a cathode follower with the other triode.  I would be banking on the low gain of ~1 of the CF making it less interactive with the PA side of the amp.

Hoping none of that is necessary.

If the CF is problematic, add a mosfet source follower and orphan one of the triodes. 

Regardless of which specific outcome, should end up with a tasty little amp.

Progress to date: debugged schematic, modeled the layout with DIYLC, built the turret board, drilled the chassis for hardware mounting, calculated high pass frequencies for the brilliance cap experiments. 

Will likely end up with part of Sluckey's and part of tubenit's Brilliance control ideas.  Have a fixed cap before the selectable caps to keep the DC voltage jump out of the signal when switching caps, but use simplified on-on-on/on-off-on for cap selection. Thinking I will end up with 3 brilliance settings.  Series caps like Sluckey, simpler options per Tubnit.

I believe Sluckey's design has a selectable high pass knees at 17, 35, 71, 148, 200, and 640 Hz.  I believe the settings that he mentioned above are the 17, 35, and 71Hz knees.  I am guessing that the 71Hz setting might be very useful when playing with a band.  Thumps and chucks will be less meaty, but the tone would stay in its lane better.  I can see up to 148Hz possibly being useful in a dense band mix as FOH might lop off everything below 200 Hz in the guitars anyway.  Sometimes as high as 250Hz for some.  I'm predicting at this point I will end of with the 3 settings being ~35, 71, and 148. Playing alone @ 35, small band @71, big band @148.  I will see what the cap experiments say.

Hope to make some more progress this weekend.