Hi all,
I have just finished my latest amp. It is loosely based on the self splitting DiyParadise Simple EL84 amp. This has been a project in the planning and evolution for at least 5yrs now so its a great relief to have it finished and working.
It differs in a number of key areas though. I chose to use PL84's (16V EL86) because they are cheap and readily available and allow for lower impedance ratio Output Transformers. I also chose to use a 6AU6 as the driver. I am running both the PL84 and the 6AU6 in pentode mode, and this allows me to run the input and driven valve as a partial feedback combination. I have used LM317's in the tail of each of the outputs as CCS and have linked them for the self splitting using back to back 1000uf caps to allow fixed DC standing current and full differential output. The antitriode is run as a pentode. I am running 8V toroidal power transformers as outputs. Almost everything in the amp is salvaged so it has cost me almost nothing.
I was expecting it to sound good, but am a little surprised at how good it sounds. It has a slightly warm but very detailed presentation, and has masses of bass punch. I would have to agree with others impression of this configuration in describing it as having SE warmth with PP punch and extension.
If anyone is interested I will attempt to lash up a schematic.
Shoog
I have just finished my latest amp. It is loosely based on the self splitting DiyParadise Simple EL84 amp. This has been a project in the planning and evolution for at least 5yrs now so its a great relief to have it finished and working.
It differs in a number of key areas though. I chose to use PL84's (16V EL86) because they are cheap and readily available and allow for lower impedance ratio Output Transformers. I also chose to use a 6AU6 as the driver. I am running both the PL84 and the 6AU6 in pentode mode, and this allows me to run the input and driven valve as a partial feedback combination. I have used LM317's in the tail of each of the outputs as CCS and have linked them for the self splitting using back to back 1000uf caps to allow fixed DC standing current and full differential output. The antitriode is run as a pentode. I am running 8V toroidal power transformers as outputs. Almost everything in the amp is salvaged so it has cost me almost nothing.
I was expecting it to sound good, but am a little surprised at how good it sounds. It has a slightly warm but very detailed presentation, and has masses of bass punch. I would have to agree with others impression of this configuration in describing it as having SE warmth with PP punch and extension.
If anyone is interested I will attempt to lash up a schematic.
Shoog
I've read about the 6AU6 being an undiscovered gem several times. Or as they say in the states: sleeper
Once again it is confirmed. 🙂
Once again it is confirmed. 🙂
Me too (was suggested by one of my gurus before i read of it -- was suggested as an alternate to EF86)... i got some cryoed and have a box overflowing with them, at some point i'll get a chance to use some of them up.
dave
The thing with the 6AU6 is that it can handle respectable current (ie 5mA+) which is way more useful than most of the piddling little small signal pentodes which seem good for just about 1mA. this is especially important with partial feedback amps because the feedback current is liable to swamp the output signal otherwise. I have used it in my Tabor clone and now this one and both have sounded sweet without any hint of pentode harshness. There are also different voltage versions which makes it quite an economical tube.
The antitriode is the slave valve in a self splitting output stage which simply supplies the current to balance DC and AC signals in the output transformer. The stiffer it is the better it forfills its intended role - so configuring it as a pentode works best.
I will attempt a schematic tomorrow.
Shoog
The antitriode is the slave valve in a self splitting output stage which simply supplies the current to balance DC and AC signals in the output transformer. The stiffer it is the better it forfills its intended role - so configuring it as a pentode works best.
I will attempt a schematic tomorrow.
Shoog
6AU6 is a good tube, but I've found EL84 still the best driver tube. Try it. Using PL84 as drivers you have an option to power filaments from the same source.
It's an "asymmetric" push-pull amp using a triode on one side and an antitriode
on the other. An antitriode can be built from a pentode, MOSFET, BJT, or even
a triode. THe circuit works a lot like an LTP.
It's called "antitriode" because it follows the inverse transfer function of the
triode, i.e. it exactly mirrors the triode. When the mirrored signal is combined
in the OPT with the original signal, there is no cancellation of the 2nd harmonic
as there is in a conventional push-pull circuit. The result is a push-pull topology
with the harmonic signature of a SE amplifier.
I have built a few amplifiers using this idea in one form or another (series, parallel,
parafeed...) and they all have the same SE harmonic signature.
http://www.diyaudio.com/forums/showthread.php?t=118725&highlight=antitriode
http://www.diyaudio.com/forums/showthread.php?t=128950&highlight=antitriode
Neither Microphonics or hum have been issues with the 6AU6;s I have. Did you hit a bad batch.
Shoog
I have a bunch of 6AU6, several hundred, maybe 1000. So of course I have experimented with them. Yes many are microphonic, some ridiculously bad, you can talk to it and hear yourself in the speakers. There are some hummers too, and some that snap crackle, pop, or hiss at you. Granted all of my tubes are old military stock and many are used, and all at least 50 years old, but there are a significant percentage (10 to 20%) that work fine except for hum, noise or microphony. If I were the only one to observe this, I would say that my tubes are just junk, but others have observed the same thing.
Would better CCS's not make it sound better? Or put differently why did you choose the lowly LM317?
I have used them in a number of projects and they have worked fine. My philosphy is that they are so easy to use, they are robust, and in this application, even if they drop to 100R at high frequencies - that is still 100X the resistance of the capacitor which still leaves the cap dominant.
To sum it up they seem more than adequate for this particular application. I would not consider them for plate loads though. Broskie seems to agree with me on this one which is a big enough thumbs up for me.
Shoog
To sum it up they seem more than adequate for this particular application. I would not consider them for plate loads though. Broskie seems to agree with me on this one which is a big enough thumbs up for me.
Shoog
That's a cool idea, using partial feedback to turn the master tube into a triode
and the slave into an anti-triode. I think you have a winner here.
The LM317 is not by definition "lowly"; it has it's applications where it works well.
But in this circuit I don't think the CCS are in parallel with the caps, they are
in parallel with each other (AC) and with the cathode resistance of the output
tubes (1/gm). Still not a very high impedance needed but maybe more than
100 ohms.
Cheers,
Michael
PS have you experimented with different amounts of local feedback with this circuit?
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I haven't actually calculated the impedance of the LM317 and suggested the 100R as an absolutely worst case scenario.
The gain of this circuit is a little high at the moment. I think an implementation with a PL84 as the driver might solve this problem and also allow a slighly lower amount of partial feedback to boot - which would probably sweeten up the sound a little. I have the valves to try this, but don't much fancy changing the sockets so won't be trying it any time soon.
Shoog
The gain of this circuit is a little high at the moment. I think an implementation with a PL84 as the driver might solve this problem and also allow a slighly lower amount of partial feedback to boot - which would probably sweeten up the sound a little. I have the valves to try this, but don't much fancy changing the sockets so won't be trying it any time soon.
Shoog
I like it. Congratulations!
In guitar amps I used triode strapped EL84 (left) and pentode (right), they shared common cathode load on an incandescent bulb.
Yours is better!
Hey Shoog,
Even if pentode-drivers for some reason are in fashion, triodes might be the way to go. Especially if sensitivity, as you say, is on the high side. With a little more Schadeish approach, it might loook like this with a triode or triode-strapped pentode(PL84, 6E5P etc):
Even if pentode-drivers for some reason are in fashion, triodes might be the way to go. Especially if sensitivity, as you say, is on the high side. With a little more Schadeish approach, it might loook like this with a triode or triode-strapped pentode(PL84, 6E5P etc):
An externally hosted image should be here but it was not working when we last tested it.
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