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Messages - Steve Conner

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Amplifier Discussion / Re: SSGuitar Contest: Most Interesting Enclosure
« on: February 24, 2011, 05:44:11 AM »
This is a very old amp, and a hybrid too. (OK, it is all tubes except for a solid-state EQ and a MOSFET/diode clipper.)

But the enclosure involved a lot of work. I started with a Euro rack chassis, and made the top, bottom, back and front plates out of 1/8" alumin(i)um. The front and back are made from a hard alloy that was quite a pain to work with.

I used only hand tools except for the front, where I got some help from a friend who had a milling machine in his garage. (I did drive the milling machine myself, though :) )


Amplifier Discussion / Re: KMG SS Poweramps
« on: February 15, 2011, 05:11:03 AM »
Yes, I agree. No reason to be scared of SMD nowadays. Annoyed maybe, scared no. :)

For these circuits, wouldn't a mains transformer be close enough to use as an OPT? The type with two 120V primaries and a low voltage secondary.

Toroidal ones have pretty low leakage inductance, but they're awfully sensitive to DC imbalance, and saturate more suddenly than EI transformers, which gives harsher distortion. The EI type with chambered bobbins probably have too high leakage inductance.

Amplifier Discussion / Re: Fet version of Marshall Superlead Plexi
« on: February 08, 2011, 07:25:12 AM »
Well, that's the issue. I guess some people have this holy grail of a "tube replacement" block that will drop straight into a classic circuit, with the same voltages and component values outside of it. But that is hard, and it's not very forward looking either. KMG's approach is the best I've seen.

I'm used to designing my own tube circuits from scratch (and I don't really care much if other people like the sound of them :) - see for details) so the approach suits me fine too. I would just take KMG's building block as a strange kind of tube with its own characteristics. I'm looking forward to trying them with my transformer driven power amp.

KMG: You used an emitter follower to buffer the bias voltage on the FET source. Is the temperature coefficient of Vbe a problem there? Or does it actually improve stability of the circuit? Likewise for the tempco of the germanium diode.

I understand what the Ge diode is for now! :) I thought it was part of the "scalable diode" circuit that emulates grid current. But you said it was to make the cutoff softer, and that makes perfect sense.

I'm sold on the topology, but I'm determined to use only current production parts in my designs, after all, that is the main reason why I'm uneasy about using tubes in the 21st century. So do you think a Schottky would do instead of a germanium diode?

Tubes and Hybrids / Re: LND150 tube emulator attempt.
« on: February 04, 2011, 11:00:48 AM »
No. It does it on purpose, because I stuck some resistors from drain to gate.

See this article for why I put the resistors there:

They also help the LND150 to bias up hotter than it otherwise would, especially since it needs resistance in its source to bring its gm down in line with a 12AX7.

Tubes and Hybrids / LND150 tube emulator attempt.
« on: February 04, 2011, 10:34:33 AM »
OK, so following the recent discusion on FET-based tube emulators, I did some more work on my own ideas. Friday afternoon at work, and all that.  ::)

First I took the dual gate MOSFET cascode one and compared it against a Mullard ECC83. After some playing with the component values, they gave similar traces, but it was extremely temperature-sensitive. Someone had borrowed the heat gun, but there was no need for it: even breathing on the MOSFET caused about 10V shift in drain voltage. That made me think that the variation between different MOSFETs would be even worse.

So, I tried the LND150 in a circuit somewhere between the Trioderizer and the scalable diode. It worked surprisingly well. The clipped waveforms were as good, if not better than the dual gate circuit, and temperature stability seemed much better. Also, the component count is lower.

The major drawback is that it leaks DC out of its "grid". In conjunction with the usual 1M grid-leak resistor found in a tube circuit (well, 910k, as I ran out of 1Ms) this creates the positive voltage needed for the LND150 to have the right bias point. But tube circuits often use a volume control, or a different value to 1M, as the grid leak.

So, I'm working on that one. For now, this circuit would only be usable in a design that took the DC into account: it might not work if you just plugged it into a tube socket.

The scope shots show the LND150 circuit working alongside the Mullard ECC83, also set up with a 100k plate and 1.5k cathode resistor. Both are running off the same 250V supply. Top trace is the input signal, and the bottom two are the outputs from the ECC83 and LND150. I will leave you all to figure out which is which. :)

To paraphrase the Monty Python sketch, "Bring out your FETs!"

The LND150 is a great device for this kind of application. I've seen people substitute it directly for a tube. I managed to get some samples recently, but I've not tried them yet.

Hi JM,

You could be right about the spread between units. Somehow I thought the pot in the drain was the right thing to fix that, but I might have it backwards.

I think I figured out the "scalable diode" thing. The thread on it was in Russian, but the schematics and scope shots were pretty clear. If the amplifying device were a BJT, the scalable diode would be a Baker clamp: its purpose is to make sure that "grid" current begins before the amplifying transistor saturates. This hides the fact that the transistor's saturation behaviour is different to a tube's.

I think my curve correction approach does something similar. It gives the same rounded bottoms to the clipped wave, because it's a physical model of a triode. It has the same saturation behaviour, so there's no need to hide it.

I was inspired by the "Trioderizer" concept that feeds the curve correction signal into the gate of a regular FET along with the signal to be amplified. The Trioderizer has great characteristic curves, but because the curve correction signal is a kind of shunt feedback, its input impedance is nothing like a real tube's.

I messed around with arrangements of multiple FETs, but the dual gate MOSFET made the most sense. They are still widely available, sadly in tiny SMT packages.

Yes, I fully agree. The grid current charges the interstage coupling capacitor, and that changes the duty cycle of the clipped waveform.

It adds interest to the tone by changing the harmonic content, in exactly the same way as the PWM effect on an analog synth.

I experimented with DC servo loops to make JFET stages stable with temperature, but as an unwanted side effect, it adjusted the clipping duty cycle to 50%. That made them sound really dull and "dead" as you say.

Well, it was seeing the Fetzer Valve that got me started messing around with this stuff.

The pot in the drain kind of makes sense. FETs that bias up hotter will have more gm, so will need a smaller drain load resistor to get the amount of gain the designer wanted.

I just feel that sticking more voltage into the circuit will somehow make it sound more like a tube circuit.

JM: No, I figured out what those ones do. I meant the transistors used as emitter followers hanging off the drains.

My best "triode" so far is a cascode of a dual-gate RF MOSFET and a MPSA42, running off 250V. I previously experimented with a JFET/MPSA42 cascode, but it had curves more like a pentode, and needed a negative bias arrangement similar to KMG's. It didn't sound too exciting when overdriven, either.

By dividing down the "plate" voltage and feeding it back to the MOSFET's second gate, the curves are changed into triode-like ones, and it seems to run OK with the source just grounded, no negative bias voltage is needed. The divider ratio sets the "mu" of the tube.

A zener on gate 1 protects it from ESD in the reverse direction (which the RF MOSFET needs badly!) and simulates "grid" current in the forward direction. Peavey seem to have a patent on that.

I haven't tested it for thermal stability yet, though. (From the datasheet, the 2SK216 has a really nice tempco at "tube amp" drain currents, which will be hard to beat.) Nor have I run it alongside a 12AX7 as KMG did, or even listened to it. It's still sitting in a drawer at work, so the above should be treated as just random musings. :)

Very cool! :) I like your other projects too. Thanks for taking the time to make them available in English.

How is the temperature stability of your FET building block?

Can you explain the purpose of T1, D1 etc? I'm guessing the diodes are to emulate grid current in a tube, but why the transistor?

Tubes and Hybrids / Re: AnyWatt Load box
« on: February 01, 2011, 05:06:39 AM »

I once made something similar, but using an old RS telephone isolating transformer that was lying around.

I can't find the transformers for sale any more. xP

FWIW, I found that I got better results using an ordinary DI box like yours, feeding an Alesis EQ that I set up to emulate the frequency response of a speaker.

Tubes and Hybrids / Re: Airline GHM945B suitcase record player
« on: February 01, 2011, 05:03:05 AM »
Hmm, yes, be careful with that one... A rule of thumb is that if the tube numbers begin with anything other than "6" - or "E" for European tubes - put it back in the dumpster, Salvation Army heap or wherever.

Then again, as a high school student, I managed to shock myself pretty bad, trying to convert a reel-to-reel tape deck that did have 6.3V tubes and a transformer. I still don't know what I did wrong.

Tubes and Hybrids / Re: My new hybrid- demo and schematic
« on: January 31, 2011, 10:37:08 AM »
Hi joecool!

The MEF is - what started out years ago as "Ampage".

There are a lot of great engineers, techs and amp builders over there, but they mostly can't see past tubes. I love tubes, but like you, I can't see a long-term future for them. So I thought it was a great idea to create a forum that focuses on solid-state. In the long term I would love to see SS boutique amps that hold their own against high-end tube ones. But only in blind tests, where the cork sniffers can't see the absence of tubes. :)

Re JM's Hewlett-Packard comment, I actually do design scientific instruments in my day job. I've never worked for anyone as big as HP/Agilent, though. I found a niche doing custom instruments for really bizarre applications. Right now I'm working on a DSP for sniffing out faults in high-voltage cables. :)

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