With solid state it's pretty easy, and for the sake of simplicity I'd start off with guitar amplifiers. Take your favourite overdrive pedal, connect it to the power amplifier chip and you have your custom guitar amplifier done.
How doable is it with tubes?
Say, I want to build a Orange Tiny Terror clone but make a low power design, and use 12AU7 in PP as a power amp section. Is it just a matter of taking the preamp section of a Tiny Terror and a power amp section of a Marshall DSL1 and just connecting them up?
How doable is it with tubes?
Say, I want to build a Orange Tiny Terror clone but make a low power design, and use 12AU7 in PP as a power amp section. Is it just a matter of taking the preamp section of a Tiny Terror and a power amp section of a Marshall DSL1 and just connecting them up?
Using 12AU7 in PP for output is a PITA because it's hard to find a transformer with a high enough Z...
In my hi-fi designs, it's as simple as building the stages and connecting them together, but decoupling the stages is important for stability.
In my hi-fi designs, it's as simple as building the stages and connecting them together, but decoupling the stages is important for stability.
High enough for what? Maximum power output? Minimum distortion?
For home playing, 20% THD at 60mW (instead of 10% @ 600mW) may still be too loud.
Last I checked, a 17AU7 wanted like 27k Ra-a...
Perhaps using a toroidal PT for the OPT? if you can find a dual 120V primary and a 3V secondary, you'll be pretty close.
Perhaps using a toroidal PT for the OPT? if you can find a dual 120V primary and a 3V secondary, you'll be pretty close.
So at 6k load maybe 0.2 Watts. I used to annoy my father with 0.1 Watts. Worth a try.
Why use a PP design for such low power output? A SE circuit is much simpler and easier to implement.
A while back I modded a little Magnavox SE 6BQ5 console amp to use 12BH7 output tubes with the two triode sections run in parallel. The ideal load impedance was determined to be ~10.75k. I believe the stock output transformers are ~5.8k into a 4 ohm secondary so if you use an 8 ohm speaker that's ~11.6k which is close enough.
Lots of SE console stereo amps or mono portable record players have OTs with similar specs.
Push pull is cheaper to implement. That's why the only SE amp I've ever built used a MOSFET source follower output stage (IXTH6N50D2 loaded with a 300W 120V light bulb).
- no transformer 🙂
- no transformer 🙂
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At this power level a salvaged SE OPT from a console stereo or record player is your best bet. There is only so much you can do to raise the impedance - running a 16 ohm speaker on a 4 ohm tap will get it up in the 8k range on one of those trafos. Video output pentodes make cheap higher gain alternatives to 6V6 - especially at the high z load and lower B+ you’d want in order to make a watt or less. Some of those pentode types are REALLY sharp cutoff and will work well at even 100V or less B+ which makes it easier to dial down the power.
The problem with using a power toroid trafo as an OPT is you’re limited to push pull (typically). At this low a power level you might get away with wasting DC current in half the primary so you can run SE. Using half of it just for DC balance and contributing no power. A solid state CCS that servo’s to the cathode current of the active stage. Don’t know how well that would work for guitar as it would null out some of the distortion that you really want. But it is an avenue to explore.
The problem with using a power toroid trafo as an OPT is you’re limited to push pull (typically). At this low a power level you might get away with wasting DC current in half the primary so you can run SE. Using half of it just for DC balance and contributing no power. A solid state CCS that servo’s to the cathode current of the active stage. Don’t know how well that would work for guitar as it would null out some of the distortion that you really want. But it is an avenue to explore.
Agreed that a salvaged SE OPT from an old radio or console would be perfect. But these (believe it or not) are now getting valuable. It might cost less to simply buy a single, cheap OPT. Edcor makes some small 10W-rated SE OPTs for $26 plus shipping (and add the wait time).
It would be pretty easy to take a typical guitar preamp section (several examples can be found in MerlinB's "Building Tube Guitar Preamps" book) and have that drive a single-ended triode-wired 6K6GT or something cheap like that. Get like a watt of output, maybe.
Or, take a 6N6P, ECC99 or 5687, parallel the two triodes, and you have an output triode with roughly 6W total dissipation (the 6N6P data sheet says 8W). Power out should be roughly a watt or two, depending on OPT. The rp of the paralleled 6N6P, ECC99 or 5687 would be about 1.5k ohms, or about the same as a triode-wired 6L6. A 3k ohm primary should work. The gain of that output triode would be about 18x, similar to that of an EL84 in triode. Extremely easy to drive (grid bias of from -4V to -10V, depending on your plate voltage).
I'd use plain old self-bias using a cathode resistor with capacitor in parallel. No need for hi-fi tricks in a guitar amp.
If you want a pentode output tube, perhaps something like a 12C5 or 12CA5? Max plate dissipation is listed as 5.5W. These also come in versions for 17V and 25V heaters, and are cheap and plentiful. Perhaps use the 17V version heated with 17VAC, rectify that and regulate down to 12VDC for the preamp tubes...
If you're in the USA, these tubes are very cheap. If you're in Europe different types would be chosen, but there are lots of possibilities once you start looking at old car radio tubes.
If you need lower SPLs, try using a less sensitive loudspeaker. Guitar speakers tend to be quite sensitive (94+dB SPL 1W, 1m). Maybe use a cheap, small 'hi-fi' woofer with a paper cone, with something like 88dB SPL 1W, 1m. Or one of those paper cone full-range drivers some folks use for hi-fi, but use a closed box instead of an open baffle or transmission line, to reduce the sensitivity?
It would be pretty easy to take a typical guitar preamp section (several examples can be found in MerlinB's "Building Tube Guitar Preamps" book) and have that drive a single-ended triode-wired 6K6GT or something cheap like that. Get like a watt of output, maybe.
Or, take a 6N6P, ECC99 or 5687, parallel the two triodes, and you have an output triode with roughly 6W total dissipation (the 6N6P data sheet says 8W). Power out should be roughly a watt or two, depending on OPT. The rp of the paralleled 6N6P, ECC99 or 5687 would be about 1.5k ohms, or about the same as a triode-wired 6L6. A 3k ohm primary should work. The gain of that output triode would be about 18x, similar to that of an EL84 in triode. Extremely easy to drive (grid bias of from -4V to -10V, depending on your plate voltage).
I'd use plain old self-bias using a cathode resistor with capacitor in parallel. No need for hi-fi tricks in a guitar amp.
If you want a pentode output tube, perhaps something like a 12C5 or 12CA5? Max plate dissipation is listed as 5.5W. These also come in versions for 17V and 25V heaters, and are cheap and plentiful. Perhaps use the 17V version heated with 17VAC, rectify that and regulate down to 12VDC for the preamp tubes...
If you're in the USA, these tubes are very cheap. If you're in Europe different types would be chosen, but there are lots of possibilities once you start looking at old car radio tubes.
If you need lower SPLs, try using a less sensitive loudspeaker. Guitar speakers tend to be quite sensitive (94+dB SPL 1W, 1m). Maybe use a cheap, small 'hi-fi' woofer with a paper cone, with something like 88dB SPL 1W, 1m. Or one of those paper cone full-range drivers some folks use for hi-fi, but use a closed box instead of an open baffle or transmission line, to reduce the sensitivity?
26 bucks is good for a trafo - that is even compatible with the HBAC. But free is better if available. 12C5s or anything in that whole family are “good” but don’t discount the video output pentodes which are better as flea-power (and up to 7 or 8 watt in push pull) amps. More gain and often with a free triode in the bottle.
Hi-fi speaker drivers tend to have the wrong characteristics - but can be made to give a guitar speaker sound. Put 10 or even 20 ohms in series and raise the Qts. This also raises your plate load and lowers the SPL. Use a driver with a very high Qms (10 is good) to KEEP the impedance peak at resonance, and use as small of a sealed box volume as you need to push this resonance up where you want it (80 to 100 Hz). Too low a Qms and the (electrical) resonance is damped - as the series resistor damps it even further. Want a huge breakup peak? Parallel LC across the series resistor. That will get it “screaming” in the upper midrange. DSP would be “easier”, but you use what you have not what you wish you had.
Hi-fi speaker drivers tend to have the wrong characteristics - but can be made to give a guitar speaker sound. Put 10 or even 20 ohms in series and raise the Qts. This also raises your plate load and lowers the SPL. Use a driver with a very high Qms (10 is good) to KEEP the impedance peak at resonance, and use as small of a sealed box volume as you need to push this resonance up where you want it (80 to 100 Hz). Too low a Qms and the (electrical) resonance is damped - as the series resistor damps it even further. Want a huge breakup peak? Parallel LC across the series resistor. That will get it “screaming” in the upper midrange. DSP would be “easier”, but you use what you have not what you wish you had.
Yes you can do that. Obviously the power section is going to sound rather different from the original Tiny Terror, but the amp will work. You can treat the preamp and power amp as independent blocks, more or less
For guitar it feels like PP design is better to my ear.
I am located in europe, soviet tubes are plentyful here.
Probably would try a 6N1P in PP in output from a schematic I found. So far I've no idea how to find a transformer with a suitable Z for a preamp tube (god, using letter 'Z' and word 'soviet' in same reply makes me throw up a little, but I'll manage). But Hammond 125 series would probably suffice I'm thinking. I could get it for 30 bucks from uraltone shop.
I am located in europe, soviet tubes are plentyful here.
Probably would try a 6N1P in PP in output from a schematic I found. So far I've no idea how to find a transformer with a suitable Z for a preamp tube (god, using letter 'Z' and word 'soviet' in same reply makes me throw up a little, but I'll manage). But Hammond 125 series would probably suffice I'm thinking. I could get it for 30 bucks from uraltone shop.
For that sort of tube you want a realtively high impedance transformer, like 10k. Looks like most of uraltone's transformers are out of stock. An alternative is to use a 100V 'line matching' transformer which might be easier to obtain. The impedance will be Z=100^2 / P, where P is the tapping you choose to use.
For example,
COM, 2W, and 0.5W tappings will make a 20k transformer (2W is the centre tap)
COM, 4W, and 1W tappings will make a 10k transformer (4W is the centre tap). I'm using this one in a prototype:
(If you get really stuck you can even use an ordinary 6V toroidal transformer with dual primaries.)
For example,
COM, 2W, and 0.5W tappings will make a 20k transformer (2W is the centre tap)
COM, 4W, and 1W tappings will make a 10k transformer (4W is the centre tap). I'm using this one in a prototype:
(If you get really stuck you can even use an ordinary 6V toroidal transformer with dual primaries.)
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Thanks for ideas. So now if I decided to build a schematic like that I posted. To make it for guitar, should one rid or add some parts from that design?
Also, coming back to the main question, adapting a tube preamp design to it, is it just a matter of connecting it up, or are there some additional stages necessary?
I assume if I wanted to do the Marshall style presence control I should modify the feedback chain to be changeable with a pot?
How do you figure it out when its not written in the datasheet?
Also, coming back to the main question, adapting a tube preamp design to it, is it just a matter of connecting it up, or are there some additional stages necessary?
I assume if I wanted to do the Marshall style presence control I should modify the feedback chain to be changeable with a pot?
How do you figure it out when its not written in the datasheet?
Start with an impedance that is twice the rp of the triode, or a little higher. For guitar it is not very critical; 10k (ish) seems to work ok with a lot of preamp tubes.
Just connect your favourite preamp up to the poweramp section. Post the circuit here and someone will check it for you. I'm not sure exactly what you want to acheive, but it might look like this for example:
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I just noticed I showed two volume pots in a row, my mistake. Hopefully you get the idea though.
Yes I do. In that sense it really looks quite simple. This schematic is out of an old russian radio magazine. I know they teach there how to wind your own transformer for that particular purpose, they describe what laminates to use and what thickness wire with how many windings, but don't tell you what measurements it should have.