Trying to figure out how much I want to bite off - and if I'll be able to chew it....
Im wanting to build a guitar head in the vein of an Egnater Rebel 20 or a Mesa Blue Angel - something that uses both El84 and 6v6 power tubes.
I have a Rebel 20 and LOVE it. However I've been wanting to do a real amp build and if I could manage something like this I would be stoked!
My first hope was to find a schematic for the Rebel 20 and modify it - however I cant seem to find a schematic that has component values.
I did find a good one for the blue angel though...
Both designs have a few things I would like to remove, both because I don't use them, and because I want the design and build to be simpler (as simple as possible!).
I want to leave out the ability to switch between power tubes or to "blend" them as the Egnater does.
I just want a static 50/50 mix of the el84s and the 6v6s. Don't need a built in attenuator like the Egnater. Don't need a reverb circuit. Simple tone controls are fine. I'd like an effects loop.
So I don't have specific questions yet - but I'm curious to get some ideas / feedback / advice on how to approach this.
There don't seem to be a lot of amps that use multiple types of power tubes at the same time - anyone have experience with this? Done any builds that do this?
Im wanting to build a guitar head in the vein of an Egnater Rebel 20 or a Mesa Blue Angel - something that uses both El84 and 6v6 power tubes.
I have a Rebel 20 and LOVE it. However I've been wanting to do a real amp build and if I could manage something like this I would be stoked!
My first hope was to find a schematic for the Rebel 20 and modify it - however I cant seem to find a schematic that has component values.
I did find a good one for the blue angel though...
Both designs have a few things I would like to remove, both because I don't use them, and because I want the design and build to be simpler (as simple as possible!).
I want to leave out the ability to switch between power tubes or to "blend" them as the Egnater does.
I just want a static 50/50 mix of the el84s and the 6v6s. Don't need a built in attenuator like the Egnater. Don't need a reverb circuit. Simple tone controls are fine. I'd like an effects loop.
So I don't have specific questions yet - but I'm curious to get some ideas / feedback / advice on how to approach this.
There don't seem to be a lot of amps that use multiple types of power tubes at the same time - anyone have experience with this? Done any builds that do this?
Using four valves of two different types in the output, essentially doubles the complexity of the output stage. It is getting well away from "simple". And I think it's a really good idea to keep a first build simple.
If you'll forgive me for playing devils advocate for a moment - why go to the trouble and expense of building something that does exactly what your Rebel 20 can already do? Wouldn't it give your more versatility to build something very different from the Rebel 20?
The only Rebel 20 schematic I turned up has no component values on it. If you're determined enough, you could inspect the PC boards in your amp carefully, and match the real physical components in your Rebel 20 one by one to their schematic locations, and so find all the part values. Once again, this is not very simple, and probably not a great idea for a first amp build.
-Gnobuddy
The 2 most obvious configurations to consider would be (1) a PP amp with an EL84 on 1 side and a 6V6 on the other side, and (2) a PP amp with EL84s driving 1 OPT and 6V6s driving 1 OPT, and the OPT secondaries paralleled.
Considered different build types - although I am really taken by the concept of a mix of power tubes. As to building something different than what I have already - I'm not interested in building an amp to get a different sound (I could buy something that sounds different) - I'm interested in building an amp for the experience and the sense of accomplishment.
Probably going to be a nightmare.... But I think I'm gonna actually attempt this! I would be stoked to compile the information and update the schematic I have with the component values for other people who need it as well! Kinda amazed it isn't out there..... (Also probably take some good detailed photos of the inside in case anyone wants to help)
As to the suggestion of using multiple OPTs - what would be the value in this? Both the Mesa and the Egnater use a single OPT for multiple tube types.
It may be a little insane, but I think I'm gonna push on with this project - starting just by making a parts list for the Rebel 20. Maybe slowly starting to redraw a simplified schematic for my own build.....
Tubelab (George) was talking about amps like this that he used to design and build. He mentioned that the only user complaint was that these amps did not have "traditional" guitar sounds in them, because the output stage was always out-of-balance.
Personally, I think this idea is very interesting, and worth exploring. But it's not very much like the Rebel 20 the OP was asking about (schematic is on Electrotanya).
This would definitely fit in the "not simple" category, no? 😀
The Rebel 20 schematic shows an approach using only one OPT. Each end of the OPT is connected to one EL84 anode, and one 6V6 anode (i.e. the two valves are in parallel). The two EL84s share one (fixed) bias setup, the two 6V6's share a second bias setup. Each valve has its own independent screen grid resistor. You can vary the amount of grid (G1) drive to either the pair of EL84s, or 6V6s, so you can blend the two sounds.
I've never played through a Rebel 20, so I don't know how it all works out in practice.
-Gnobuddy
Good luck with the project! It would be a nice thing for other diy amp builders to have, although in these crazy times the Egnater legal department might have a different opinion.
There is a gentleman on these forums who is a retired mental health specialist. He says his colleagues (in the same profession) seemed a bit concerned about his own mental health, considering that he was a DIY valve guitar amp builder.
Yup, DIY valve amp building may very well be inherently a little insane, at least according to society's current priorities!
-Gnobuddy
Those amps evolved from a WTF moment when I stuck a 6L6GC and an EL34 into a Fender Bandmaster about 25 years ago. It made some unique tones at about half throttle.
I then built a grand experimental breadboard with 4 output tubes feeding a single OPT. Each output tube had its own bias adjustment and drive control. I could stuff anything from a 6V6GT to a KT88 into any of the 4 sockets. It was a great tool for learning, but in the hands of someone who didn't understand each pot, it was a sure fire way to blow up tubes.
I distilled this all down to an amp with two output tubes, and made about a dozen amps. Like my Turbo Champs, no two were alike, but most had purposefully mismatched output tubes. The most popular combinations were a 6L6GC and an EL34 or a 6V6GT and a 6L6GC. A few amps had a pair of 6L6GCs and got their mismatch from drive and bias offsets. Each amp had a 10 (I think) position switch with resistors and trim pots to upset the balance. You could select anything from a push pull pair to an SE amp with the other tube idling to keep the OPT happy.
With a pair of mismatched tubes the only clean tones were in SE mode which wasn't all that clean because the OPT was too low impedance (half of a 6600 ohm OPT is 1650 ohms, to low for class A SE).
The original breadboard was probably pretty close to what the Rebel 20 does since you could dial up one set of tubes, the other, or any blend of the two. I used octal tubes in most of my guitar amps, but for an amp capable of blending two different sets of tubes, the EL84 / 6V6GT combo would be good, because as I have said for years, the 6V6 was born to sing the blues, but the EL84 (or 6BQ5 as I knew them) was born to ROCK!
Well, it is funny at least to me that you would poke fun at my solution as not so simple, and then describe something which to me is much more complex than what I suggested, and the OP has said is not what he wants.
Anyways, I am out of here, I don't have the patience to put up with your verbal diarrhea. Too bad you don't get paid by the word.
Dude, I have no idea what you're on about. Are we reading the same thread? 😕
-Gnobuddy
How about one more switch position, which puts the two output valves into parallel class A mode? At least the OPT primary impedance would be correct. Lotsa DC current through it, though.
-Gnobuddy
Why not use cathode bias on the outputs; pretty simple...
The two EL84s would use one cathode resistor and the 6V6s would use another, different value. That way, both sets of tubes would be happy and the driver would be simple without the extra caps and bias adjustment pots
The two EL84s would use one cathode resistor and the 6V6s would use another, different value. That way, both sets of tubes would be happy and the driver would be simple without the extra caps and bias adjustment pots
I had the same thought, that's probably the simplest way. But not everyone likes the sound of an overdriven cathode-biased output stage.
-Gnobuddy
danFrank´s approach is interesting and apparently simplest of all-
Mesa´s SimulClass goes the full way, but besides being patented , it requires a custom built OT: different taps for each tube type, all 4 working at the same time, from same +B , but each pair getting its preferred impedance.
That said, aiming at an intermediate compromise impedance and not being anal about *exact* 50% power sharing, which in any case I suspect is irrelevant, then supplying proper bias and drive to each pair is, by comparison, trivial.
Mesa´s SimulClass goes the full way, but besides being patented , it requires a custom built OT: different taps for each tube type, all 4 working at the same time, from same +B , but each pair getting its preferred impedance.
That said, aiming at an intermediate compromise impedance and not being anal about *exact* 50% power sharing, which in any case I suspect is irrelevant, then supplying proper bias and drive to each pair is, by comparison, trivial.
The switches in the amps that I built were the most difficult thing in the amp to wire. I had acquired a box full of large NOS Mallory rotary switches with 6 or 7 wafers on each one. Wiring them up without making at least one mistake was not trivial. When I get around to doing this again I will use a microcontroller, probably an Arduino compatible called a Teensy driving mosfets and relays
Meanwhile I have started work on the grand project that I have talked about for at least 5 years, but so far I have been all talk. At least for now I have started on my take on a vacuum tube kilowatt. It will use a Teensy because as the name suggests, it's small, powerful, has plenty of I/O, capable A/Ds, but the DAC's for things like tube bias will have to be an off board chip or two hanging on the SPI bus. There will be plenty of learning, probably some fried parts, and several setbacks, along the way. This will be an exercise in how to make tubes, silicon, SiC, and a microcontroller all play nice with each other.
If the push-pull output stage is slightly off balance, and crossover distortion is not an issue, the distortion spectrum shape is more musical, more like a single ended stage. The 2nd harmonic dominates. If the balance goes further off, and the tubes are biased class AB (not pure A), the distortion goes up to the point where you can easily hear the I.M. distortion that is generated by the same "mechanism" that brings up the 2nd harmonic (asymetric non-linearity).
Combining dissimilar tube types is tricky because each tube has a different gain, needs a different bias voltage, and has a different plate impedance that will react differently to the output tranny and speaker load. If I tried to do that I would want to have adjustments for gain and bias, separate for each tube, so I could figure out what actually sounds good. Ideally, one pair of output tubes would start to saturate just before the other tube type pair, thereby giving you a more gradual overdrive waveshape. You definitely don't need two output trannies, and the added weight is not desirable. Output trannies can cause severe voltage transients when not loaded properly, which can blow everything up, so you're going into risky waters unless you know what you're doing. I'd recommend a regular output stage where you can vary the gain on one of the P-P output tubes, for distortion effect. That's relatively safe, and will give you some nice and somewhat flexible distortion.
Combining dissimilar tube types is tricky because each tube has a different gain, needs a different bias voltage, and has a different plate impedance that will react differently to the output tranny and speaker load. If I tried to do that I would want to have adjustments for gain and bias, separate for each tube, so I could figure out what actually sounds good. Ideally, one pair of output tubes would start to saturate just before the other tube type pair, thereby giving you a more gradual overdrive waveshape. You definitely don't need two output trannies, and the added weight is not desirable. Output trannies can cause severe voltage transients when not loaded properly, which can blow everything up, so you're going into risky waters unless you know what you're doing. I'd recommend a regular output stage where you can vary the gain on one of the P-P output tubes, for distortion effect. That's relatively safe, and will give you some nice and somewhat flexible distortion.
You said simple but complex... Two types of tubes... Have an idea... Just search for Angela Super Single Ended amp on the internet, and you will get a PSE design with two 6V6's and you will be pretty much able to look at it, modify it and make something more MESA lookin'... Parallel single ended gives you the ability to have your harmonics non cut off like every push and pull, and you will be able to use two tubes... and still get the harmonic richness that P-P cuts down...
There are a few ways around the "push-pull cancels even harmonics" problem.
One way is what Bob Richards outlined - deliberately unbalance the phase splitter stage a little bit. It only takes about 1 dB of imbalance (12%) to regain much of the SE tone.
I found another way, too. Push-pull only cancels harmonics that are generated by the push-pull stage. If you generate the harmonics you want before the signal ever gets to the push-pull output, those harmonics go right through to the speaker.
That leaves you with the choice of preamp, power amp input stage, or power amp driver stage to generate the single-ended pentode harmonics you want to hear. I find that putting a small signal pentode somewhere in that chain works very well.
Of course parallel single-ended can work too, but the price to be paid is a huge, heavy, expensive output transformer that can tolerate a lot of DC current flowing through it.
This last point is a matter of opinion - we may or may not all agree on this. Personally, I like the clean tones from a single ended amp, but single-ended overdrive often sounds ratty and harsh to me (Eric Clapton and his Champ on Layla being a classic example). On the other hand, I don't like the clean tones from a push-pull amp, but prefer their overdriven sounds, which can be cleaner and less harsh.
So my preference is for an amp that produces single-ended (mostly 2nd harmonic) distortion at low power, and transitions to mostly 3rd harmonic as the power level goes up and overdrive starts to happen. The tricky part is making this transition happen!
-Gnobuddy
The Egnater rebel 20 is not that difficult to clone.
It has separate post Phase Inverter Masters for EL84 pair drive and 6V6 pair drive and you can blend anyway you want. Also if you run both these Masters down a bit you can get Phase Inverter overdrive distortion.
Both EL84 and 6V6 need 8K Raa output transformer for a pair so something around 4K would suit when you have one of each on each push pull side.
B+ not greater than say +350V max.
Something like a Vox AC30 output tranny would do.
The 35W rated Hammond 1750J would suit (but only has 4 Ohms secondary) but I would probably go to 1750M which has 50W rating and 2,4 and 8 Ohm secondaries.
Cheers,
Ian
It has separate post Phase Inverter Masters for EL84 pair drive and 6V6 pair drive and you can blend anyway you want. Also if you run both these Masters down a bit you can get Phase Inverter overdrive distortion.
Both EL84 and 6V6 need 8K Raa output transformer for a pair so something around 4K would suit when you have one of each on each push pull side.
B+ not greater than say +350V max.
Something like a Vox AC30 output tranny would do.
The 35W rated Hammond 1750J would suit (but only has 4 Ohms secondary) but I would probably go to 1750M which has 50W rating and 2,4 and 8 Ohm secondaries.
Cheers,
Ian
If you really want to learn the deep levels of guitar amp design, I'm sure I don't know it all, but I've researched designed and built about 8 guitar amps, and several of them are explained in a lot of detail on my hobby website. You might enjoy seeing the pictures and reading about the tradeoffs and how it all comes together there.
My latest guitar amp, The Deuce Jr., is something I'm very proud of. Lot's of hard earned lessons went into it's design. There's no other amp I know of that I'd rather have. I explain in more detail than you wanted, why it's like it is, and if I didn't show all the math, my "Music Box" tube Hi-Fi amp shows most of that, in more detail than you wanted. It's at: Bob's Website
My latest guitar amp, The Deuce Jr., is something I'm very proud of. Lot's of hard earned lessons went into it's design. There's no other amp I know of that I'd rather have. I explain in more detail than you wanted, why it's like it is, and if I didn't show all the math, my "Music Box" tube Hi-Fi amp shows most of that, in more detail than you wanted. It's at: Bob's Website
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