Hi everyone,
I built an AC15-style amp last year. It was my first attempt at making something with valves in it and it sounds pretty good. I have an idea for a second build, but my knowledge of some of the theory behind this stuff is still patchy at best
So with that in mind, I throw this idea out there to the forum in the hope that someone might very kindly sense-check it before I start buying parts etc..
Is it possible to build a low-wattage guitar amp using only 3x 6F12P (6Ф12П) valves?
The 6F12P is a split triode/pentode valve in a single envelope, with the pentode section having a max dissipation of 5 watts @ 330ma heater current.
I'm thinking you could use 2 of the pentode sections as a push-pull output stage, with the 2 triode sections joined together to form the phase inverter (LTP). If that works, you get a PI and output stage for only 660ma heater current. I'm guessing here... but I'd hope you could get something like 7 watts output from it (cathode biased)?
The third 6F12P would be the input stage. You could use the pentode section as the first gain stage, a bit like the old EF86-based Vox circuits. The remaining triode could drive a tone-stack or maybe act as a gain boost.
Worth trying? Or doomed to failure?
I built an AC15-style amp last year. It was my first attempt at making something with valves in it and it sounds pretty good. I have an idea for a second build, but my knowledge of some of the theory behind this stuff is still patchy at best
So with that in mind, I throw this idea out there to the forum in the hope that someone might very kindly sense-check it before I start buying parts etc..Is it possible to build a low-wattage guitar amp using only 3x 6F12P (6Ф12П) valves?
The 6F12P is a split triode/pentode valve in a single envelope, with the pentode section having a max dissipation of 5 watts @ 330ma heater current.
I'm thinking you could use 2 of the pentode sections as a push-pull output stage, with the 2 triode sections joined together to form the phase inverter (LTP). If that works, you get a PI and output stage for only 660ma heater current. I'm guessing here... but I'd hope you could get something like 7 watts output from it (cathode biased)?
The third 6F12P would be the input stage. You could use the pentode section as the first gain stage, a bit like the old EF86-based Vox circuits. The remaining triode could drive a tone-stack or maybe act as a gain boost.
Worth trying? Or doomed to failure?
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Start by calculating possible voltage and current swing , then peak available power, then optimum load impedance, then check whether it´s realistic.
I think you´ll need a 32K OT. not exactly common, and maximum theoretical power would be some 3.5W in crude Class B, so some 2-2.5W RMS ... tops.
Paralleling 3 pentodes in a single ended stage might give you some 6W peak, 3W RMS ; 2W would be more realistic; is that enough?
Load would be between 1800 to 2400 ohm, perhaps easier to find.
Main limitation is low pentode current, some 40mA peak and I would not go higher than 20mA at idle.
I think you´ll need a 32K OT. not exactly common, and maximum theoretical power would be some 3.5W in crude Class B, so some 2-2.5W RMS ... tops.
Paralleling 3 pentodes in a single ended stage might give you some 6W peak, 3W RMS ; 2W would be more realistic; is that enough?
Load would be between 1800 to 2400 ohm, perhaps easier to find.
Main limitation is low pentode current, some 40mA peak and I would not go higher than 20mA at idle.
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Hi, thanks for taking the time to respond and crunch some numbers.
It's taken me a few hours of reading to understand why such a high OT primary impedance is necessary. I found another post from you (How to calculate primary impedance of output transistor for class AB push pull stage? - Music Electronics Forum) that helped me get my head around what's actually happening.
I see (low power) push-pull OT's available up to about 20k primary impedance, but trying to find a 30k+ one looks difficult. And on top of that, 2 - 3 watts output is a bit lower than I wanted.
I can think of two things that might help...
- use a quad in push pull (double the power, half the impedance?)
- plug a 16 ohm speaker into the OT's 8 ohm secondary (doubling the load on the OT primary?)
Would that configuration get the OT requirement down to 8k (half and half again)?
> using only 3x 6F12P (6Ф12П) valves?
Why is this a goal?
These are voltage amplifiers. For rock & roll you want a Power Amplifier at the end.
Just because you have them? Tubes are rarely a dominating expense. As we are already seeing, even getting most of the small power possible leads to an odd transformer.
But don't diss "small amplifiers". A 1/3rd Watt in the house will annoy everybody, maybe neighbors also.
Why is this a goal?
These are voltage amplifiers. For rock & roll you want a Power Amplifier at the end.
Just because you have them? Tubes are rarely a dominating expense. As we are already seeing, even getting most of the small power possible leads to an odd transformer.
But don't diss "small amplifiers". A 1/3rd Watt in the house will annoy everybody, maybe neighbors also.
Use a 16 ohm speaker on a 4 ohm secondary on an 8k trafo.
Interleave a pair of Antek AN0207 power transformers. That will give somewhere around 36k with all 4 secondaries in parallel and a standard 8 ohm speaker. And cheap enough where if it doesn’t work out you can use them to power heaters on other projects.
6F12P is like ECF80 / 6BL8.
See Grant Wills designs
Lamington Junior – design | Valve Heaven
The Lamington Lite – design | Valve Heaven
But for ‘bedroom volume’ a single ended amp may be better.
6BL8 has also been used for a stand-alone reverb and phase splitter.
See Grant Wills designs
Lamington Junior – design | Valve Heaven
The Lamington Lite – design | Valve Heaven
But for ‘bedroom volume’ a single ended amp may be better.
6BL8 has also been used for a stand-alone reverb and phase splitter.
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Hi and thanks for your thoughts on this amp idea
To answer your Q: to learn, experiment, and to find out if it sounds any good. A 100% successful outcome would be a nice-sounding, distinctive, home/recording amp - but there's value learning what not to do.
I haven't bought any of the parts yet, it's all still theoretical at this point.
A fair point
. I was maybe a bit too hung up on hitting a certain output wattage, but now I think about it, the speaker sensitivity would probably have a more notable impact on volume than whether it's 2, 3 or 5 watts on paper.Thanks! All useful info that adds to the picture
I'll need to read up on line level transformers...
Single ended makes it easier, both on number of tubes (which can even be an odd number, you mentioned 3 of them) and impedance requirements.
Why 32k? (using just 2 of them in PP): simple Ohm´s law: once you set voltage,the lower the current the higher the impedance, as straight as that.
The pentodes are low current: 40mA peak so idle 20mA for Class A or, say, 5mA or 10mA for Class AB so that leads to higher than normal impedances.
Now IF you have many of them, you can use 2 or 3 pentodes per side.
people tend to stick to very few tube types (12A*7 , EF86 , EL84-34 , 6L6 variants, KT**) and not many more, because they are the few ones still in production, but there´s a zillion NOS tubes out there which are just as good, most of them so called "TV tubes".
You can do GOOD things with them, it only takes actual "designing" straight from datasheets, you can´t simply plug them into a "conventional" design.
Which isn´t really difficult, just practice around datasheet curves.
Only problem is that, for commercial designs, you should first make sure you have an adequate stash or they are *always* available; I bought 1000 ECC189 "TV tuner" tubes for peanuts (50 cents each) and am making all kinds of flavourful Guitar peamps with them, LOTS of Second Harmonic and compression when driven hard, they add strong Tube flavour to "cold sterile" SS amps, go figure.
Now for experimenting, just a few are more than enough.
Why 32k? (using just 2 of them in PP): simple Ohm´s law: once you set voltage,the lower the current the higher the impedance, as straight as that.
The pentodes are low current: 40mA peak so idle 20mA for Class A or, say, 5mA or 10mA for Class AB so that leads to higher than normal impedances.
Now IF you have many of them, you can use 2 or 3 pentodes per side.
people tend to stick to very few tube types (12A*7 , EF86 , EL84-34 , 6L6 variants, KT**) and not many more, because they are the few ones still in production, but there´s a zillion NOS tubes out there which are just as good, most of them so called "TV tubes".
You can do GOOD things with them, it only takes actual "designing" straight from datasheets, you can´t simply plug them into a "conventional" design.
Which isn´t really difficult, just practice around datasheet curves.
Only problem is that, for commercial designs, you should first make sure you have an adequate stash or they are *always* available; I bought 1000 ECC189 "TV tuner" tubes for peanuts (50 cents each) and am making all kinds of flavourful Guitar peamps with them, LOTS of Second Harmonic and compression when driven hard, they add strong Tube flavour to "cold sterile" SS amps, go figure.
Now for experimenting, just a few are more than enough.
Hey, those are really interesting. What I had in mind is pretty much half-way between these two designs, the output section very much like the Junior.
One of the reasons I want to try the 6F12P in a circuit like this is because, according to the datasheet, it should (?) be able to handle more power than an ECF80 / 6BL8.
(All these are pentode values)
Max plate dissipation for ECF80 = 1.7w, 6F12P = 5w
Max anode voltage ECF80 = 250v, 6F12P = 300v
Max cathode current ECF80 = 14ma, 6F12P = 22ma
Transconductance ECF80 = 5.5mA/v, 6F12P = 19mA/v
I can see how you'd get 2w from ECF80's push-pull, but the 6F12P sounds like it should be capable of a bit more?
Currently, I have 0, but I see people selling them on ebay in lots of 50 or 100, so I think there's plenty of NOS out there in Ukraine, Russia, Bulgaria etc.. I mentioned 3 at the start because I thought that would be the minimum number you'd need for a practical design. Not a problem to go to 4, 5 or 6
Sounds cool. Are there any sound samples I could listen to?
An unfinished homework
Can´t play sh*t by myself, need a good Guitar Player or two to record a "usable" one, otherwise I´ll push sales *down*
Since I am working isolated (literally, nobody gets through my front door, not even my Family) since March 2020, guess I´ll ask some Customer to do it for me, *or* send me a couple DI tracks to test at home.
Stay tuned
Can´t play sh*t by myself, need a good Guitar Player or two to record a "usable" one, otherwise I´ll push sales *down*
Since I am working isolated (literally, nobody gets through my front door, not even my Family) since March 2020, guess I´ll ask some Customer to do it for me, *or* send me a couple DI tracks to test at home.
Stay tuned
Thanks to everyone for their thoughts and comments on this idea.
I've found two push-pull output transformers from VVT that might work, but I think the rest of the circuit might change quite a lot depending on which one I choose...
1. VTP12298 - 15k impedance - 4, 8 and 16 ohm secondaries, rated 4W rms
2. VTP12566 - 8k impedance - 4, 8 and 16 ohm secondaries, rated 15W rms
Transformer #1 would seem to be the obvious choice. Connect 8 ohm speaker to 4 ohm secondary (or 16 ohm speaker to 8 ohm secondary) and you've got a 30k primary. My only concern is the 4 watt rms rating.
As it's not a common tube choice, the 6F12P isn't included in any of the online calculator tools designed to help builders estimate output, bias values etc... so I've looked at other triode-pentode valves that might be sort of similar to see what kind of power they can cope with.
If I assume the following...
Anode / Plate = 270v
Screen = 250v
Quiescent Current = 20mA
Primary impedance = 30k
...am I not going to end somewhere between 5 and 6 watts for the PP pair?
I'm not questioning the figures very kindly provided by people who know a lot more about this stuff than I do, just want to make sure I'm not going to blow up the OT by overloading it
Transformer #2 costs a bit more but can obviously take a lot more power. The downside of that however is that, all else being equal, I'd be limiting myself to 16 ohm speakers only - not great.
Any thoughts much appreciated
I've found two push-pull output transformers from VVT that might work, but I think the rest of the circuit might change quite a lot depending on which one I choose...
1. VTP12298 - 15k impedance - 4, 8 and 16 ohm secondaries, rated 4W rms
2. VTP12566 - 8k impedance - 4, 8 and 16 ohm secondaries, rated 15W rms
Transformer #1 would seem to be the obvious choice. Connect 8 ohm speaker to 4 ohm secondary (or 16 ohm speaker to 8 ohm secondary) and you've got a 30k primary. My only concern is the 4 watt rms rating.
As it's not a common tube choice, the 6F12P isn't included in any of the online calculator tools designed to help builders estimate output, bias values etc... so I've looked at other triode-pentode valves that might be sort of similar to see what kind of power they can cope with.
If I assume the following...
Anode / Plate = 270v
Screen = 250v
Quiescent Current = 20mA
Primary impedance = 30k
...am I not going to end somewhere between 5 and 6 watts for the PP pair?
I'm not questioning the figures very kindly provided by people who know a lot more about this stuff than I do, just want to make sure I'm not going to blow up the OT by overloading it
Transformer #2 costs a bit more but can obviously take a lot more power. The downside of that however is that, all else being equal, I'd be limiting myself to 16 ohm speakers only - not great.
Any thoughts much appreciated
Very cool
Never seen anything that uses LEDs instead of cathode bias resistors before.
Any audio clips of it in action?
Sorry - no clips. Didn't have the technology at the time. Have moved country and three houses since this first built. Currently wired as pentode, triode, triode, pentode, and built into a head cabinet. Next version will be triode, pentode, triode, pentode- still with led biasing on each stage. I did try pp pentode output (same opt) but was far too loud for my purposes. All versions under $100.