6080 Triode cascode guitar amp
I'm planning on trying some 6080 triodes in a cascode output- transformer coupled guitar amp.
Here's my design so far, the voltages I have on the cascode are just estimates.
It seems like the 6080's would be a good candidate for a vacuum tube cascode output due to the low plate voltage and dual triode.
Any comments or suggestions would be welcome.
Mark
I'm planning on trying some 6080 triodes in a cascode output- transformer coupled guitar amp.
Here's my design so far, the voltages I have on the cascode are just estimates.
It seems like the 6080's would be a good candidate for a vacuum tube cascode output due to the low plate voltage and dual triode.
Any comments or suggestions would be welcome.
Mark
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This should be in the Instruments and Amps forum area.
Cascode output will give boomy bass, but this may be what you want?
Cascode output will give boomy bass, but this may be what you want?
Couldn't agree more.
The only cascode output stage I have ever played through was the Musicman's, I think they were driving the cathodes in a common grid configuration, extra power and increased bandwidth I believe, seems kinda pointless in a guitar amp. I remember them to be really loud but bias towards class B, didn't have the nice warm sound I like.
Lately I built an amp with a cascode as the input gain stage. I use a very efficient speaker cab so I only need 10-15 watts to hit my sweet spot, so I wired some 6P3P in triode mode, sounds really nice clean, and it's a great blues/classic rock amp. Here's a schematic.
The voltage divider after the cascode is because I do not know how to do potentiometers in LTSpice.
Edit: R18 is a mistake and isn't in the amp.
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famousmockingbird,
You are correct on the Musicman. I did a repair on one a while ago and they run very close to class B.
They are a different beast, hybrid, with BJT in the lower cascode driving an EL34 run at >700V on the plates and half that on the screens.
It seems that using a pentode in a cascode is unusual but I believe the BJT serves mostly as a current gain in that circuit so the pentode may be called for.
That is a sweat design, cascode instead of cascaded gain stages and no NFB. Did you find 300V sufficient for the 12AX7 cascode. I got to learn LTSpice.
Mark
You are correct on the Musicman. I did a repair on one a while ago and they run very close to class B.
They are a different beast, hybrid, with BJT in the lower cascode driving an EL34 run at >700V on the plates and half that on the screens.
It seems that using a pentode in a cascode is unusual but I believe the BJT serves mostly as a current gain in that circuit so the pentode may be called for.
That is a sweat design, cascode instead of cascaded gain stages and no NFB. Did you find 300V sufficient for the 12AX7 cascode. I got to learn LTSpice.
Mark
Not sure.
I haven't read a good explanation on how to calculate a loadline for cascode. I know the 6080 has a very low ra but in a cascode I'm thinking it would be closer to a pentode?
My plan at this point is to use a typical 4k P-P OT and then sub different values of RL to find the optimum loadline and go from there.
I might try one of these toroids http://www.antekinc.com/50-va/ since they have dual pri I could wire it P-P
Mark
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Some of the issues that you face were covered in this thread. Also the 6080 does not seem to be a good candidate for the cascode if that's really the topology that you want to use for the output stage.
Thanks jazbo,
That's what I was looking for, I must have missed that thread.
I'm rethinking the whole cascode output idea, perhaps I'll try the 6080's in parallel P-P, DC coupled.
That's much more doable, although you still need to take care of the large drive swing required since its mu is so low.
Right. here's what I was thinking since I already have this inter-stage tranny.
I will start a new thread for this since the cascode idea is all but dead.
Thanks
Mark
Hi Guys
The use of a cascode output stage does not make the amp "boomy" or impart any other specific tone.
The Musicman amps referred to above used tubes only in the output stage - the preamp is opamps with gain values and EQ to mimic a fender. The cascode output stage allowed the tubes to be operated cold but without the detriment that would have in a grid-driven situation. The MMs are extremely reliable because of the cascode and cool operation. The BJTs have a local feedback loop around them to set the idle current for each tube, all of which is enclosed within an opamp feedback loop that controls the power amp.
With respect to the 6080 cascode amp, I would build a standard output stage first. This will give you experience with the 6080s which require a tonne of drive voltage remember, their mu is only 2. They are a prime candidate to use BJTs for the lower position but you lose some of the magic of the power triodes.
Be careful of the heater-cathode voltage. Even at 300V, it is easily exceeded in a push-pull cascode.
Have fun
The use of a cascode output stage does not make the amp "boomy" or impart any other specific tone.
The Musicman amps referred to above used tubes only in the output stage - the preamp is opamps with gain values and EQ to mimic a fender. The cascode output stage allowed the tubes to be operated cold but without the detriment that would have in a grid-driven situation. The MMs are extremely reliable because of the cascode and cool operation. The BJTs have a local feedback loop around them to set the idle current for each tube, all of which is enclosed within an opamp feedback loop that controls the power amp.
With respect to the 6080 cascode amp, I would build a standard output stage first. This will give you experience with the 6080s which require a tonne of drive voltage remember, their mu is only 2. They are a prime candidate to use BJTs for the lower position but you lose some of the magic of the power triodes.
Be careful of the heater-cathode voltage. Even at 300V, it is easily exceeded in a push-pull cascode.
Have fun
Short answer: you gain nothing and lose a lot in such configuration, plus triodes are the worst choice for Guitar amps.
Somebody suggested useing EL84.
They , or any other pentode, will be much better than any triode.
Now if you were making a class A, no NFB Hi Fi amp. 1930's style, then I might consider it.
But for a guitar ....
Hi Guys
For hifi or MI you can try anything and use any circuit you wish - there are no absolutes about what is good or bad. Anyone who tells you "you can't do that for guitar" or "you can't do that for hifi" should learn a bit more about what is possible; maybe figure out why they limit themselves so much.
Trying out different ideas is how we learn and how fashion evolves. Triodes are great for guitar. if they weren't, we would all be using EF86s and 6AU6s in our preamps. To say triodes are bad as output tubes for MI is stinkin' thinkin'. Triodes make excellent power boosters if you want a relatively conventional circuit as a Super Scaler that retains the tone of your small amp driving it.
Suppose you build a triode power amp and it has a neutral sound? That becomes part of the system character and you add character in other places.
Have fun
For hifi or MI you can try anything and use any circuit you wish - there are no absolutes about what is good or bad. Anyone who tells you "you can't do that for guitar" or "you can't do that for hifi" should learn a bit more about what is possible; maybe figure out why they limit themselves so much.
Trying out different ideas is how we learn and how fashion evolves. Triodes are great for guitar. if they weren't, we would all be using EF86s and 6AU6s in our preamps. To say triodes are bad as output tubes for MI is stinkin' thinkin'. Triodes make excellent power boosters if you want a relatively conventional circuit as a Super Scaler that retains the tone of your small amp driving it.
Suppose you build a triode power amp and it has a neutral sound? That becomes part of the system character and you add character in other places.
Have fun
Why do you have to mention preamps when we are talking about power amp output tubes?
Maybe you don't have a succesful power amp example using triodes?
Not a single one? 🙄
Replace your insulting stinkin' thinkin' label by "what everybody has done during the last 60 years" (including Fender , Marshall , VOX , Ampeg , Laney , Peavey , Soldano , MB , Gibson , Randall , Engl , Dynacord , ..... insert 10000 MI amp brands here ...)
Maybe all of them were stinkin' thinkin' too?
Please let me suggest that insulting people who think different won't carry you far.
Maybe you know something nobody else does? 😛
Oh, I see.
I had to google it , go figure, and found it's your new product 🙄
You feel your pet idea threatened. 😉
So you want to sell a power triode based power amp, so anybody stating triodes don't cut it are nuts or morons.
Well, maybe you are running against 60 years of successful pentode MI designs.
Thanks for confirming that power triodes have bland sound.
You might as well go straight SS and simplify your life.
Oh !!!!!! looks that you did indeed go for SS 😱
Cut and paste from: Super Scaling Q&A
Funny how you start vehemently pushing triodes and then switch to ...... MOS FETs 😱 (SS stuff of course).
Looks like Bait-and-switch - Wikipedia, the free encyclopedia (talk expensive tubes and then offer cheaper SS)
Even more funny is that MosFets, if anything, behave like a pentode, not a triode ... talk about self contradicting 🙄
OK, let's go away from your Marketing and back to what LPMark wants to do, it's his thread after all.
Why did I criticize that particular output stage? (I wanted to keep it short but your derailing the thread forces me to provide a longer and more comprehensive answer):
cascode connecting power tubes :
1) provides no advantage.
Why?
Because the "bottleneck" in tubes is usually available current.
You can reasonably say that they are "current limited" and won't be far from the truth.
So the typical way to increase power in tube power amps is to put more tubes in parallel to increase current capability and so increase power in a straight way.
So if 2 tubes can provide 50W, then 4 tubes can provide 100W, and so on.
Simple, huh?
2) in fact, lowers efficiency, in a gross way.
a) it does not provide more current, which as we just saw is the big bottleneck.
2) it wastes / provides much less voltage.
Which directly causes a gross power loss.
You see, if those tubes were connected the classic way , meaning cathode to ground, plate to +V through an OT winding, you will available from (in your case) 385V (idle) to some 60 to 80V (saturated) , some 300V swing.
Good.
Enter the cascode.
a) to begin with, and without further analysis, we are adding at least an extra 60 to 80V drop, across the extra tube, without adding one iota of current.
To simplify analysis I'm not considering cathode resistor drop, because it's not the main problem and in fact can easily be eliminated by using fixed bias.
b) in fact the loss is higher, you are estimating a plate voltage at the lower triode between 100/180 V DC ... which must be substracted from available 385VDC .... added to the 60/80V dropped across the upper tube.
So we are going from original 300V swing available down to 160/200V ... a huge voltage (and power) loss in exchange of nothing.
3) the cascode configuration makes sense in a Musicman amplifier, because the driver is an NPN Silicon transistor, which:
a) can feed as much current as the upper tube demands and then some.
We are talking 3 to 10A, depending on actual transistor used, while the upper tube can handle around 400mA tops .
Even driving two tubes per side in the 130W models, tube current demand would be around 800mA , piece of cake for the transistors used there.
b) saturation voltage for a transistor can be as low as 0.5V .
Even if it were supplying maximum current possible (which it's not, by a wide margin), datasheets usually give 4V saturation voltage at maximum current.
A far cry from 60/80V lost in a cascode tube amp (at least, and probably much higher in practice) .
Hi Guys
J, I don't see how defending lethargy is going to be helpful.
Guitar amps are about tone creation and are therefore entirely subjective in how well they achieve the goal. There is no 'right' or 'wrong' provided components are not being damaged.
With respect to the original post: The 6080's low mu will require a paraphase stage between the splitter and the outputs. This is just a dual-triode with a shared Rk+Ck, separate plate loads and grid-leaks. A 12AT7 here will provide a tonne of gain at medium impedance drive. I built a 6080 amp like this about thirty years ago, and many triode amps for guitar since then.
If you have the 6080s then definitely build a guitar amp with them. Remember that ALL power amps in guitar amps are hifi designs so there is nothing wrong at all in using something that others here think is "too obviously hifi". Have confidence in your desire to be creative and to explore the edges of design possibility. After all, this is your own amp.
Triodes are not bland. I find them warm and full-bodied inasmuch as they sound like a 6L6GC to me. That is 'neutra'l as far as tube tones go for me.
Most people's impressions of how triodes sound or how UL sounds is tainted by the decisions made mostly by Fender. Fender used hifi circuits for their guitar amps. The voicing of the amps is obviously hifi as there is no frequency emphasis at all coming from the gain stages or the power amp. The EQ provides a little compensation for what the speakers do to the sound and that, combined with the character of successive tube stages, yields the classic tone we expect.
As music and the MI biz evolved, Fender made the amps higher powered and more hifi, peaking in the silver-face era with designs that cost fender some sales. Their use of UL was not a mistake but their overall implementation was. Anyone who reads my books knows that the problem with Fender tone is in the feedback loop values. Changing these makes the amp more dynamic even at the same gain and this should have been done when the UL mode was chosen. This would have given the higher power desired but with tone more or less intact.
The way to warm up the UL amps like the Twin reverb is to rewire one pair of outputs as triodes. Misinformed people would expect the amp to be cleaner, and in one respect it is, but the effect is to remove harshness and thus make the tone smoother and warmer. (Again from TUT). In a UL amp with only one tube pair, skewing the drive slightly will warm it up as will changing the loop values.
The character of pentode/tetrode mode is predicated on the distortions of that mode. Triode mode is less distorted, so sounds "dull" if pentode/tetrode is the reference. UL is in between. So, amps with half-power switches that are really triode-pentode switches have caused confusion to players with respect to tube operating modes versus tone. It is in our nature that brighter tones impress us more, so the more distorted pentode tone is brighter and taken as "better" by many players if they flip the switch back and forth without letting their ears rest. Again, there are usually other circuit details the player or ill-informed tech has overlooked which contribute to these impressions.
Be true to the tone in your heart and ignore negativity.
Have fun
J, I don't see how defending lethargy is going to be helpful.
Guitar amps are about tone creation and are therefore entirely subjective in how well they achieve the goal. There is no 'right' or 'wrong' provided components are not being damaged.
With respect to the original post: The 6080's low mu will require a paraphase stage between the splitter and the outputs. This is just a dual-triode with a shared Rk+Ck, separate plate loads and grid-leaks. A 12AT7 here will provide a tonne of gain at medium impedance drive. I built a 6080 amp like this about thirty years ago, and many triode amps for guitar since then.
If you have the 6080s then definitely build a guitar amp with them. Remember that ALL power amps in guitar amps are hifi designs so there is nothing wrong at all in using something that others here think is "too obviously hifi". Have confidence in your desire to be creative and to explore the edges of design possibility. After all, this is your own amp.
Triodes are not bland. I find them warm and full-bodied inasmuch as they sound like a 6L6GC to me. That is 'neutra'l as far as tube tones go for me.
Most people's impressions of how triodes sound or how UL sounds is tainted by the decisions made mostly by Fender. Fender used hifi circuits for their guitar amps. The voicing of the amps is obviously hifi as there is no frequency emphasis at all coming from the gain stages or the power amp. The EQ provides a little compensation for what the speakers do to the sound and that, combined with the character of successive tube stages, yields the classic tone we expect.
As music and the MI biz evolved, Fender made the amps higher powered and more hifi, peaking in the silver-face era with designs that cost fender some sales. Their use of UL was not a mistake but their overall implementation was. Anyone who reads my books knows that the problem with Fender tone is in the feedback loop values. Changing these makes the amp more dynamic even at the same gain and this should have been done when the UL mode was chosen. This would have given the higher power desired but with tone more or less intact.
The way to warm up the UL amps like the Twin reverb is to rewire one pair of outputs as triodes. Misinformed people would expect the amp to be cleaner, and in one respect it is, but the effect is to remove harshness and thus make the tone smoother and warmer. (Again from TUT). In a UL amp with only one tube pair, skewing the drive slightly will warm it up as will changing the loop values.
The character of pentode/tetrode mode is predicated on the distortions of that mode. Triode mode is less distorted, so sounds "dull" if pentode/tetrode is the reference. UL is in between. So, amps with half-power switches that are really triode-pentode switches have caused confusion to players with respect to tube operating modes versus tone. It is in our nature that brighter tones impress us more, so the more distorted pentode tone is brighter and taken as "better" by many players if they flip the switch back and forth without letting their ears rest. Again, there are usually other circuit details the player or ill-informed tech has overlooked which contribute to these impressions.
Be true to the tone in your heart and ignore negativity.
Have fun
Wow where do I start.
JMFahey, very close minded thinking I have to say
If everyone jumped off a bridge would you? I didn't think so. The majority of people are tone deaf and are happy to listen to music on their laptop speakers
Does it make it ideal or correct? Sure, but everyone is different. Besides valve type we need to discuss output impedance as well, after all a triode has "internal" feedback, most of your beloved pentode amps use global feedback, regardless it's all feedback, meant to reduce distortion, lower output Z, and increase bandwidth. I personally don't like having the output transformer in the loop for issues like stability, so to lower Z and distortion I went with triodes or (pseudo-triodes) 🙂
I just played a gig using triodes in the finals, I got several comments from other musicians about how awesome my guitar sounded, people never compliment about amps (I was using a cheap Epiphone dot deluxe w/ Seymour Duncan PAF 59's). If I told them that I was using triodes instead of pentodes would they run for the hills? No, they wouldn't even know what the hell I was talking about. My 6.5 watt amp sounded a lot better (my opinion, and remember we all are entitled to one) then the 100 watt EL34 pentode beast that was on the other side of the stage. My cleans win hands down, for metal tones his dirty/distorted sound wins. Not taking anything away from when my amp clips, because it has a very lush overdrive sound, just apples to oranges. I do not sell my amp so don't look for my website and self promotion, I promise it's not there, because I don't have a website😉
For the record I do not want to sound like the majority of guitar players, hence why I have been tinkering with guitar amps for some time now.
Here is a schematic to my inferior amplifier (R10 and R12 are just modeling a potentiometer). Let the bashing begin, I am not an engineer and don't claim to be, but to my ears this amp sounds good. How long will it last? Not to sure, I have a bad case of let me rip my amp apart and try this. I want to go class AB2 with mosfets as drivers but I really don't want any more power so I think I will leave it as is. I think my next project will be a real triode, single ended, class A2, DC coupled etc.......
JMFahey, very close minded thinking I have to say
If everyone jumped off a bridge would you? I didn't think so. The majority of people are tone deaf and are happy to listen to music on their laptop speakers Does it make it ideal or correct? Sure, but everyone is different. Besides valve type we need to discuss output impedance as well, after all a triode has "internal" feedback, most of your beloved pentode amps use global feedback, regardless it's all feedback, meant to reduce distortion, lower output Z, and increase bandwidth. I personally don't like having the output transformer in the loop for issues like stability, so to lower Z and distortion I went with triodes or (pseudo-triodes) 🙂I just played a gig using triodes in the finals, I got several comments from other musicians about how awesome my guitar sounded, people never compliment about amps (I was using a cheap Epiphone dot deluxe w/ Seymour Duncan PAF 59's). If I told them that I was using triodes instead of pentodes would they run for the hills? No, they wouldn't even know what the hell I was talking about. My 6.5 watt amp sounded a lot better (my opinion, and remember we all are entitled to one) then the 100 watt EL34 pentode beast that was on the other side of the stage. My cleans win hands down, for metal tones his dirty/distorted sound wins. Not taking anything away from when my amp clips, because it has a very lush overdrive sound, just apples to oranges. I do not sell my amp so don't look for my website and self promotion, I promise it's not there, because I don't have a website😉
For the record I do not want to sound like the majority of guitar players, hence why I have been tinkering with guitar amps for some time now.
Here is a schematic to my inferior amplifier (R10 and R12 are just modeling a potentiometer). Let the bashing begin, I am not an engineer and don't claim to be, but to my ears this amp sounds good. How long will it last? Not to sure, I have a bad case of let me rip my amp apart and try this. I want to go class AB2 with mosfets as drivers but I really don't want any more power so I think I will leave it as is. I think my next project will be a real triode, single ended, class A2, DC coupled etc.......
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Wow, the kind of logic my MOM used to apply 😉Wow where do I start.
JMFahey, very close minded thinking I have to say
Loved her but not exactly a guru on Tech stuff.
Back to the "bridge" example, I say that a million people walked that bridge and happily got to the other side, I saw nobody jump.
You say that somebody should?
Out of curiosity or boredom maybe?
Sorry buy I'll follow the standard million instead of the exciting jumper.
Wow !!! You certainly are not shy to use words 😀
Don't worry, no problem.
Statistically, not "my" beloved pentodes but 99.99% people loved.
Still no problem, let's go on.
Me neither; personally I build with no or very little NFB (barely above 6 dB, the minimum to have some waveform correction and symmetry), but the point is moot because as soon as you clip even a little you lose NFB.
As in: the loop may try to do what it can (not much), usually just slamming the tube or transistor against a rail (or ground, which is also a rail), but achieving nothing.
Well, they use the word "clipping" for some reason, don't they?
Ok, if it works for you, who am I to disagree?
https://www.youtube.com/watch?v=vBSWyWTpTpw
Most probably 😉
If it works for you, be my guest .
Not inferior if you like its sound.
And no need or justification for bashing, it has no glaring errors visible.
My main comment (and I hope it was useful) regarding the OP cascode power triode amp was not based on tone (although I called it bland in a later post, nobody should commit suicide because of that) but because of the much more important huge voltage swing loss , in exchange of ... nothing.
That is a big loss and not based on taste but simple logic.
As in: if I spend double, have double trouble building and get half power as before ... is is worth it?
On the contrary, cascading power transistors IS useful, and a very different thing, because then an important supply voltage doubling is achieved.
In 1974 (yes, 40 years ago 😱 ) revolutionary Designer James Bongiorno published Ampzilla, the first commercial SS power amplifier, capable of 200/8 and 300/4 per channel, unthinkable before him.
And cascaded/series power transistors lose just a couple volts, no big deal.
Excellent my friend🙂
I actually agree with you, and so does the OP because he is no longer going to try it.
I guess what I am trying to say is that with large scale production the engineering behind said product is usually quite good for the masses. Sometimes it's just fun to do something different that kinda doesn't make any sense. I guess I was using my amp as an example because it's kind of a joke, most players who have asked what it is and what tubes it uses (I have only been using it for a month or so) look at me funny when I say it's output power is under 10 watts, especially with two 6L6's, most amps that people are familiar with that use two 6L6's are around 30-50 watts. But the amp is still loud because of the efficient driver choice. Anyway I am rambling about music instrument gear as being part of the overall art associated with the artist🙂
I actually agree with you, and so does the OP because he is no longer going to try it.
I guess what I am trying to say is that with large scale production the engineering behind said product is usually quite good for the masses. Sometimes it's just fun to do something different that kinda doesn't make any sense. I guess I was using my amp as an example because it's kind of a joke, most players who have asked what it is and what tubes it uses (I have only been using it for a month or so) look at me funny when I say it's output power is under 10 watts, especially with two 6L6's, most amps that people are familiar with that use two 6L6's are around 30-50 watts. But the amp is still loud because of the efficient driver choice. Anyway I am rambling about music instrument gear as being part of the overall art associated with the artist🙂
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