Self split cascode?

[Illinest]

I wonder if anyone has made a self split cascode output stage for a small guitar amp?

I found info about cascode configurations in one place, and a bunch of different self-split output stages (like this one) http://i740.photobucket.com/albums/xx46/jim-in-co/firefly.jpg
But I have never seen the ideas together.

 

[Illinest]

I guess that the impedance matching wouldn't work, would it? You would need almost 50k for a pair of 12au7?

 

[Printer2]

http://www.mif.pg.gda.pl/homepages/frank/sheets/168/7/7695.pdf

 

[Tubelab_com]

I made a little amp for the Hundred_Buck Amp Challenge. It used a self split output stage. It worked but its offspring used a mosfet splitter, which works better and makes more power.

The original design is in post # 1021 and post 1279.

The final version with two mosfets is in post #1622. It ROCKS, thanks in part to the dial a crank "saturator" control in the input stage.

 

[Illinest]

Ya know...
I'm not exactly prejudiced against sand myself, but I have no experience with it. I am barely making tubes work.

I am delighted by the practical design challenges but I resent math and charts.
But it's holding me back, not knowing that stuff better. Isn't it?

I will probably just end up doing what others have done. I found some schematics that use ecl84 tubes. I really like the idea of reducing the number of parts, or using the same parts, but in a novel fashion.
I was all worked up to do the ecl84 thing until I realized that it's another tube that isn't being produced anymore. For a moment I thought that maybe the self-split cascode idea was sufficiently strange that it hadn't been done yet, but if I understand printer's datasheet, it looks like those tubes are pretty much cascode equivalent within one bottle?

I need to clear my head and come up with a real plan.

 

[Printer2]

Ya know...
I'm not exactly prejudiced against sand myself, but I have no experience with it. I am barely making tubes work.

I am delighted by the practical design challenges but I resent math and charts.
But it's holding me back, not knowing that stuff better. Isn't it?

I will probably just end up doing what others have done. I found some schematics that use ecl84 tubes. I really like the idea of reducing the number of parts, or using the same parts, but in a novel fashion.
I was all worked up to do the ecl84 thing until I realized that it's another tube that isn't being produced anymore. For a moment I thought that maybe the self-split cascode idea was sufficiently strange that it hadn't been done yet, but if I understand printer's datasheet, it looks like those tubes are pretty much cascode equivalent within one bottle?

I need to clear my head and come up with a real plan.

No, they are two separate tubes. I have four of the 50V version. SE one tube runs into 1100 ohms. Not much use for the transformers, used one at 2200 ohms with a 15CW5.

 

[Tubelab_com]

I am delighted by the practical design challenges but I resent math and charts. But it's holding me back, not knowing that stuff better. Isn't it?

I worked in engineering at Motorola for 41 years. I started there with only a high school education, and worked my way up to design engineer. THEN they paid for me to attend college and obtain two engineering degrees, learning.....and promptly forgetting all that advanced math.

I was the lead design engineer on several products from cell phones to two way radios, and RF modem modules. I never used anything more powerful than the "scientific calculator" built into Windows. That is still the case today. There are computer programs and simulators to handle the serious math stuff. The guys that design those really need that serious math. I would rather be designing hardware. It is better to understand what works and why circuit wise than spend time with advanced math that you will rarely use. That knowledge only comes from experience.

Spreadsheets, charts and such, yes I used them, and still do.....mostly to keep track of all the parts, what they cost, and where to get them. Now I have spreadsheets full of tubes, pinouts, characteristics, prices, sources, and stock levels.........

No, they are two separate tubes......Not much use for the transformers, used one at 2200 ohms with a 15CW5.

I tried one of these with UL84's (45B5's) which are a 45 volt version of the 6CW5/15CW5 during the Hundred Buck Amp Challenge. The up side is that this circuit works well with repurposed power (mains) transformers as OPT's. The downside is low power output. I was getting about 4 watts from a pair of UL84's with this circuit. It didn't sound like a cranked pair of '84's either.

Note....there should be a dot (connection) on the schematic where the 80uF output cap intersects with the 8 uF and RC2.

The same pair of tubes in conventional push pull made 15 watts from the same 300 volt supply. The OPT was an Antek power toroid. This one went to 11.......I'm still looking for that amp. It was stuck in a box two moves and 1200 miles ago.

 

[Printer2]

I tried one of these with UL84's (45B5's) which are a 45 volt version of the 6CW5/15CW5 during the Hundred Buck Amp Challenge. The up side is that this circuit works well with repurposed power (mains) transformers as OPT's. The downside is low power output. I was getting about 4 watts from a pair of UL84's with this circuit. It didn't sound like a cranked pair of '84's either.

Think I am getting about 4W from a single 15CW5.

 

[Illinest]

Thanks for saying that Tubelab. I've been feeling a little bit overwhelmed.

I re-centered myself yesterday.
The reason that I asked about self-split cascode is because I (like everyone else, seemingly) want to build a low-watt push-pull guitar amp that doesn't sacrifice any tone.

But that's only about half of what I've been planning to build, and I think that I should compromise on the power amp.


My idea is to shove a Firefly and a Champ into a single box. I'm not looking to make a 2-channel amp. I want two completely different signal paths - driving different speakers. Bi-amping is an important part of the idea.

I want to plug my guitar into a shared 1/4" input jack, split the signal to separate pre-amps, then send each pre-amp through independent series f/x loops and on to independent power amps, independent OTs and independent speaker outs.

I want one side to be fairly clean and have a bit of headroom. (I envision that as the champ side). I have a preamp schematic that starts with a Pentode in V1, gain pot, then into a cathode follower, then a Fender-ish tone stack and volume pot, gain recovery, f/x loop, then a single 6V6. 3 tubes in all and something like 5 watts.

I want the other side to have cascading gain stages with two gain pots as well as a volume pot and tone stack. Then I want that to drive a push-pull output section that puts out very low watts. I don't know how much I need, but I want it to be about as loud (dimed) as the other side is when it's still clean. (I was figuring 5 watts to 1 watt might be about right)


If everything goes well, then I ought to be able to cross-connect the power amps (via the f/x loops) and send the "clean" preamp signal to the push-pull power amp, and vice-versa.

I'm excited about the idea but I'm not sure if there are some hidden barriers that could de-rail the project, and I'm not entirely satisfied with the idea of using push-pull triodes in the output. Pentodes definitely beat triodes for sound imo.

But I'm also trying (as a design challenge) to use only current production tubes. I was really really tempted by the ECL84/6FP4 as I said - but it's not being produced.
So I'm either going to have to use EL84s - which would be too loud - or ECL84s - which aren't being made anymore - or perhaps something that I overlooked - or I can just use the 12au7a.
12au7a is probably my best bet.

 

[jazbo8]

So I'm either going to have to use EL84s - which would be too loud - or ECL84s - which aren't being made anymore - or perhaps something that I overlooked - or I can just use the 12au7a.
12au7a is probably my best bet.

ECC99

 

[Illinest]

ECC99

I heard those are good too.

Also 12bh7, I think.
Which - come to think of it, maybe having three different tube options would be a good idea.

 

[Printer2]

Hate to say it, but I think simplicity is the best thing for a small low output amp. If anything build an amp and feed it from a VVR.

 

[Illinest]

I do have a VVR 18 watt derived circuit up and running. VVR is good. It definitely helps, but the amp doesn't sound as good if you reduce B+ too far.

 

[Printer2]

I do have a VVR 18 watt derived circuit up and running. VVR is good. It definitely helps, but the amp doesn't sound as good if you reduce B+ too far.

Thinking of a Champ.

 

[Struth]

Hi Guys

A 12AT7 in a self-split circuit can produce 1W easily, which is too loud if you play through a real cabinet. (Power Scaling easily tames this) Yes, such a circuit requires a high-z primary for the OT but this is easily attained using a Hammond 125-series.

Low tube-count circuits inherently restrict the potential for getting great tone unless you are after a simple clean sound. Still, that clean sound is most often improved with another gain stage added providing another veil of tube character. Note that with two dual-triodes available for preamp functions, you can have ANY tone you wish, then add the dual-triode for the self-split output and you have a great small amp for recording, practice, and for use as a preamp into larger amps.

Power reduction schemes like VVR are good inasmuch as they allow players to dial down the sound pressure a bit, although not enough, but... they are implemented incorrectly. Power Scaling is the original power control form and it works exactly as is claimed. It allows the player to choose how quiet "quiet" is and does not change tone.

Mark Stephenson builds a 1W amp pedal called the StageHog. This has Power Scaling because 1W into a real cabinet produces 100dB of sound! So, even 1W is too loud for home use.

A cascoded output stage goes in quite the wrong direction with respect to intrinsic output impedance. Tubes have no problem with voltage, so cascoding devices is of little use in a transformer-coupled output stage.

Regardless of the tube count you end up with, try to avoid the usual error of using the tightest box possible as this leads to difficulties wiring it, servicing it, and it is likely to be unstable. Space is your friend. use an airy layout and the circuit will be easier to build and to tweak.

Have fun

 

[Illinest]

Hi Guys

A 12AT7 in a self-split circuit can produce 1W easily, which is too loud if you play through a real cabinet. (Power Scaling easily tames this) Yes, such a circuit requires a high-z primary for the OT but this is easily attained using a Hammond 125-series.

Low tube-count circuits inherently restrict the potential for getting great tone unless you are after a simple clean sound. Still, that clean sound is most often improved with another gain stage added providing another veil of tube character. Note that with two dual-triodes available for preamp functions, you can have ANY tone you wish, then add the dual-triode for the self-split output and you have a great small amp for recording, practice, and for use as a preamp into larger amps.

Power reduction schemes like VVR are good inasmuch as they allow players to dial down the sound pressure a bit, although not enough, but... they are implemented incorrectly. Power Scaling is the original power control form and it works exactly as is claimed. It allows the player to choose how quiet "quiet" is and does not change tone.

Mark Stephenson builds a 1W amp pedal called the StageHog. This has Power Scaling because 1W into a real cabinet produces 100dB of sound! So, even 1W is too loud for home use.

A cascoded output stage goes in quite the wrong direction with respect to intrinsic output impedance. Tubes have no problem with voltage, so cascoding devices is of little use in a transformer-coupled output stage.

Regardless of the tube count you end up with, try to avoid the usual error of using the tightest box possible as this leads to difficulties wiring it, servicing it, and it is likely to be unstable. Space is your friend. use an airy layout and the circuit will be easier to build and to tweak.

Have fun

[IMGDEAD]https://i.imgur.com/wdQRzdQ.jpg[/IMGDEAD]

 

[Illinest]

Hi. I appreciate all of the advice from everyone.
I'm hoping to soldier on with this. I really like the idea but I want to make sure there aren't any major problems with the block diagram before I start to figure out bias points, tone stack specifics, supply rail - etc...