Thanks for that.
I haven't checked but I guess the ECL82 has the same pinout as the ECL86 - so I could throw this idea together with a cheap 82 and still trade up to the 86 if it shows signs of promise.
I do have a second idea based on the same circuit. Instead of a small amp triode/pentode/OP transformer/speaker - I was considering triode/pentode/OP transformer/load as an overdrive pedal. And possibly still using the risky positive feedback idea... Watch this space
Pete
I haven't checked but I guess the ECL82 has the same pinout as the ECL86 - so I could throw this idea together with a cheap 82 and still trade up to the 86 if it shows signs of promise.
I do have a second idea based on the same circuit. Instead of a small amp triode/pentode/OP transformer/speaker - I was considering triode/pentode/OP transformer/load as an overdrive pedal. And possibly still using the risky positive feedback idea... Watch this space
Pete
They don't have the same pinouts... I've just built a SE PCL82 "hifi" amp - partly due to the transformer I had to hand. There seem to be plenty available and so far I like the sound. I had some PCBs made up which I guess could be used for guitar easily enough. Merlin also had some on his web store, but I think they're sold out...
Hi,
a single ECL for guitar is a bit quiet the 82 more than the 86. For positive feedback you can try a link tying both (unbypassed) cathode with resistor/cap play with them to stay in stable area. I once tried PF from the cathode follower to the preceeding cathode stage turning the pot until it SQWEEEEEEE.....Finally i delete it because the thing was not switchable without loud pops.
Have you some germanium transistors left? you could built a treble booster like the range-master or better (to me) this one: npnboostv2 which can be adapted (bias & polarity) for pnp germanium as well.
a single ECL for guitar is a bit quiet the 82 more than the 86. For positive feedback you can try a link tying both (unbypassed) cathode with resistor/cap play with them to stay in stable area. I once tried PF from the cathode follower to the preceeding cathode stage turning the pot until it SQWEEEEEEE.....Finally i delete it because the thing was not switchable without loud pops.
Have you some germanium transistors left? you could built a treble booster like the range-master or better (to me) this one: npnboostv2 which can be adapted (bias & polarity) for pnp germanium as well.
I have a hunch you may not have enough voltage gain, unless you have high-output humbucker pickups and plan to play only rhythm guitar with 100% clean tone.
One possibility to consider: add a MOSFET input stage ahead of the ECL82. Something like the little LND150 ( LND150 - Depletion Mode MOSFETs ) might do the trick, wired as a common source gain stage, maybe with an unbypassed source resistor to set the voltage gain to where you need it.
I have seen a variant of this idea that uses a JFET / MOSFET totem-pole (cascode). The JFET is on the "bottom" and provides low input noise and a little quadratic nonlinearity for "valvey" warmth. The MOSFET is the upper device, converts the JFET output current to an output voltage, and isolates the JFET from the several-hundred-volt power supply.
I encountered the "positive feedback inside negative feedback" thing many years ago, in an op-amp circuit that synthesizes a negative output resistance. I built and measured it, and it was well behaved and quite stable. For years I thought I had invented both the circuit and the concept, but no, it turns out somebody got there long before me and spoiled my fun.
Thanks for the reminder! We were snowed in for about four days, so I didn't get my Valentine's Day shopping done. I may have to stop at a few shops on my way back home tonight!
-Gnobuddy
Thanks for those ideas.. I am kinda expecting this one valve amp won't have enough gain. Otherwise the world would be full of them. So maybe positive feedback will just add enough to save the day? I'm planning that the volume control adds the PFB as you turn it to the loud end. I'm also thinking of back to back diodes on the PFB loop to create some distortion if needed.
Cascade FETs are a neat idea, particularly if you operate them from conventional valve HT. I admit I have looked at cathode bias DC voltage in the past and wondered if I could use that as a form of LT for a transistor stage, such as effect loop or reverb driver. I'm much more at home with silicon - and I once designed a 200w bass amp. But for these amps I have taken a "no transistors" vow
Cascade FETs are a neat idea, particularly if you operate them from conventional valve HT. I admit I have looked at cathode bias DC voltage in the past and wondered if I could use that as a form of LT for a transistor stage, such as effect loop or reverb driver. I'm much more at home with silicon - and I once designed a 200w bass amp. But for these amps I have taken a "no transistors" vow
You may find some ideas for positive feedback in this book /page65 http://www.tubebooks.org/Books/Atwood/Crowhurst Cooper 1956 High Fidelity Circuit Design.pdf
I prefer watching Peter Weir's Picknick at Hanging Rock todayMental note, buy card and chocolates - because Valentines day tomorrow. I might need to grovel a bit before all these parts for "yet another amplifier" arrrive
!Best regards!
Fair enough!...for these amps I have taken a "no transistors" vow
My suggestion is, leave room on the chassis for one more valve socket. I think you're going to need it!
The most recent valve guitar amp I built puts out a whopping 200 milliwatts. Just like you, I started out with a single triode-pentode valve, in my case a 6JW8 for which I paid the princely sum of $1 (USD). The little pentode in the 6JW8 is quite sensitive, biased to Vgk ~ -4 volts in my circuit, so it takes about 8 volts peak to peak at the control grid to start overdriving.
Maximum peak-to-peak output voltage at the speaker is about 3.6 volts at 200 mW RMS output, so voltage gain of the output stage is roughly 0.5 or -6 dB (this includes the voltage step-down in the output transformer.)
The triode gain stage driving the pentode has a voltage gain of roughly 30x (~ 30 dB.) With no tone control and wired with no volume control, I found I had just enough gain - with a humbucker-equipped guitar - to hear low-volume clean tones from the speaker. Not a trace of overdrive, though.
So I added another incredibly expensive one-dollar valve, this time a 6AQ6. I don't remember exactly what voltage gain I got out of it (around x50 or 34 dB, I think), but the combined voltage gain was enough to let me insert a gain control between the two triodes, and a modified Fender passive tone control in between the 6JW8 triode and pentode. There is enough gain to go from clean tones to early classic rock levels of overdrive, which covers most of the music I'm interested in.
Bottom line, I needed 60 dB of voltage gain in the amp to deliver a tiddly 200 mW to my speaker, allowing for insertion loss in a tone control, and a little overdrive.
In my experience, the output stage invariably ends up with a voltage gain of order (of magnitude) unity, from control grid to speaker terminals. Say somewhere between (-6 dB and +6 dB). I don't think you'll manage more than 30 - 35 dB voltage gain from the triode. I think this is going to leave your amp with a gain deficiency of at least 20 dB, particularly if you plan to use a tone control with significant insertion loss.
Your amp is probably also going to put out abut ten times as much output power, so you'll need three times the end-to-end voltage gain, i.e another 10 dB, all else being the same.
It's possible I'm wrong, of course. But my bet is that you are going to want 30 dB more gain than you'll get from the single valve.
-Gnobuddy
Fair enough!
My suggestion is, leave room on the chassis for one more valve socket. I think you're going to need it!
.......
It's possible I'm wrong, of course. But my bet is that you are going to want 30 dB more gain than you'll get from the single valve.
-Gnobuddy
Yes, I fear you are right there. There won't be a tone stack in this amp for reasons of signal loss, and I do reserve the right to use a reverb pedal up front - which might give me +6dB (there goes my "no transistors" rule..).
It will be interesting to see if PFB can pull back some of the lack of gain. It isn't supposed to be a serious amp really, more of an experiment.
But you never know. We all hope to stumble across some new amp topology that will make us famous, eh?
Pete
For years, I've wanted to try an "ultra-nonlinear" output stage. If you get "ultra linear" with a little NFB to the screen grids, what happens if you cross the two screen grid connections for PFB, and dial down the amount of feedback for stability? Ultra-nonlinear, and maybe interesting guitar tone?
Personally, I've never wanted fame. But I wouldn't mind inventing an amp that made me rich enough to retire, leave the city, and move to some beautiful place in the countryside.
Yeah, right. Pigs will fly first, I expect. They probably have by now, in some large transport aircraft or the other!
-Gnobuddy
I built my little ECL82 amp as a bit of fun and as a basis for experimentation. I assure you that it does amplify my P90 equipped guitar, but not as well as my son's Victory Kraken! If you're looking for sufficient gain, as gnobuddy says, to go from clean tones to classic rock levels of overdrive then you've probably got your work cut out employing a single ECL82. However, there's no reason why you shouldn't build the basic circuit then knock yourself out experimenting with ways of boosting its performance. I share with you how interesting and satisfying it would be to achieve even half decent results from a single valve!
In the USA, there was a brief period in the 1960s when valve manufacturers stuffed up to three different valves into one glass bottle with a 12-pin base. The result was known as a Compactron, and the hope was that these would stave off the semiconductor assault a little longer.
I guess the strategy didn't work very well, or for very long, because it seems a lot of NOS Compactrons ended up on dusty warehouse shelves and in people's attics.
One of these Compactrons - the 6AF11 - contains two triodes as well as a power pentode. Decades after their heyday, it seems several DIY valve enthusiasts all had the same thought: "Can I make a one-valve guitar amp with a 6AF11 Compactron?"
From what I've seen on the 'Web, most attempts failed because of incurable feedback problems that resulted from having all that gain stuffed into a single glass envelope less than 2" (5 cm) in diameter. The unavoidable inter-electrode capacitances were enough to make these amps oscillate and turn into RF transmitters.
But at least one person succeeded. Here is his one-valve Squirrel Monkey amplifier: The Squirrel Monkey One Tube Guitar Amp
-Gnobuddy
LOL!However, I'm tempted to say that the Squirrel amplifier is nuts!
Here's a (fluffy) tail of even more squirelly nuttiness, which I think you're going to like: YouTube
I actually bought a couple of 6AF11s, and a pair of Compactron sockets, right after I first encountered the Squirrel Monkey. To my shame, I've never done anything with them - all those stories of uncontrollable oscillation put me off.
I like the sounds the designer / builder / guitarist got out of the Super Squirrel Monkey. How on earth did he put even more gain into that little bottle with his additional external gain stage, and get away without endless oscillation?
Sadly, not everyone who tried managed to tame that ferocious little monkey: Super Squirrel Monkey - First from-scratch tube amp build, first guitar amp build
-Gnobuddy
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