LDRs as pots?

[Enzo]

The public is seeing through the deception. The return to so-called vintage amps is a return to the fundamental principal that tubes are superior to transistors when it comes to guitar amplifiers.

Really? There are those of us who prefer tube tone, but go to a Guitar Center, look in Musicians Friend, look at Sears, Kmart, Target, Best Buy, etc and compare the number of vintage tubes amps to solid state amps. PVs most popular amp is their Bandit, not the 5150 or Classic 30. For every Mesa Boogie out there, there is a sea of Crates.

[tbryanh]

Really? There are those of us who prefer tube tone, but go to a Guitar Center, look in Musicians Friend, look at Sears, Kmart, Target, Best Buy, etc and compare the number of vintage tubes amps to solid state amps. PVs most popular amp is their Bandit, not the 5150 or Classic 30. For every Mesa Boogie out there, there is a sea of Crates.

I understand that crap is king. People are driven by price. But the serious musician can see throught the dark cloud.

[Enzo]

I am uncomfortable when we get snooty about our preferences. Tell BB King that SS amps suck, for example.

[tbryanh]

I am uncomfortable when we get snooty about our preferences. Tell BB King that SS amps suck, for example.

There are exceptions to every rule. He is one of the rare exceptions.

I'm not trying to be snooty. I suppose I could sugar coat my language, but I prefer to speak what I feel is the plain truth in a way that is clear.

If you love your hybrid or your pure SS amp. OK. I hate it. It sucks. You love it. I accept that you love it. Just accept that I hate it. Its garbage. Its trash. It makes me sick. I won't touch it.

[tbryanh]

I understand Neil Young uses a 10K watt PA system to project his music to the audience, but he still uses vacuum tube amps to make his sound.

If you have an enourmous amount of headroom, then transistors can work to reproduce tone, but not to generate tone.

BB King plays clean, so his amp is only reproducing the signal from from his guitar pickup. His amp does not shape the tone in any way other than maybe filtering and compression, but there is no distortion-type tone shaping through his amp.

Every serious musician who want distortion-type tone shaping uses a vacuum tube amp. a tr5ansistor amp simply does not perform well for this.

[tbryanh]

I understand Neil Young uses a 10K watt PA system

A 10K watt transistor PA system that is.

[Enzo]

This is my last comment on the social aspects, I remain interested in the electronics.

tbr, if I think this project is a bad idea, I can just go away. Or... How would you feel if instead I hung around and explained in great detail that I thought it was stupid, a waste of time, I hated it, and so on? It would serve to dampen free discussion, it would make some folks uncomfortable, it would urge the discussion towards "sides" and defending turf. "I like tubes and do not like solid state," is fine. "I like tubes and do not like solid state, and anyone who doesn't agree is stupid," is not fine. In my opinion. That was the distinction I was going for.

[Steve Conner]

Well, I think the social aspects are inseparable from the electronics. Electronic design is my day job, and when I'm asked to design something, the requirements are usually given in social terms, like "Make it cheap", "Make it very small and cute", or "It has to be ready by next week if you want your bonus". My bosses couldn't care less what circuitry I used to achieve the ends.

I think the design of a digitally programmable guitar amp should be approached in the same way. Use whatever technology gets you your desired goal with a minimum of cost and hassle, rather than shooting yourself in the foot with arbitrary restrictions like "I will not use semiconductors in the signal path".

I believe that tbr's distinction between reproducing tone and generating tone works even inside a hybrid guitar amp. You can use tubes in their non-linear regions to generate tone, and semiconductors in their linear regions, with plenty of clean headroom, to do other things. The transistorized graphic EQ in the early Mesa Boogie combo amps is an example of this. EQ is a form of tone shaping that's linear and doesn't depend on distortion, so tubes aren't needed.

I don't see how this is any worse than using a solid-state treble booster pedal ahead of your amp, or micing it up through a solid-state PA system.

Anyway, back to the subject in hand...

Using JFETs in the ladder circuits under discussion would be difficult, because they usually have a maximum voltage rating less than the signal swings seen in tube amps. They also require a drive to their gate that is referenced to the source, but the source is swinging with the signal. About the only thing I can think of that would do this with low enough stray capacitances and noise coupling is a photovoltaic isolator: a LED and tiny solar cell in a DIP package.

You can get tiny high-voltage "solid-state relays" made of two back-to-back 400V MOSFETs with their gates driven by a PVI. These would be ideal for ladder switches in our application, except they're expensive, and have high stray capacitance between terminals when turned off. I evaluated them years ago for audio switching, and the stray capacitance put me off.

[Tiago]

It might be that FETS could be used with a resistor string, but it does not appear FETs can be used with a resistor ladder.

I am not sure what you mean by the term "resistor array."

I guess I want to mean resistor string...sorry for my bad technical English...
I attached a picture so you can see what I meant.

But as far as I can see about the resistor ladder, a good Maxim ic multiplexer could be used for that, doesn't it?

[Enzo]

I guess my mental picture of JFETs was in a DC environment. Wasn't thinking about the signal adding to that. Also was thinking more of switching resistors to ground. Optocouplers could work.

[Gregg]

For those who will go the FET/relays way maybe you should consider Meder's SIL series relays with internal diode. They are small /20x5mm - 0.8"x0.2"/ and less expensive than an LDR:

http://www.mouser.com/Search/Product...RVylULtA%3d%3d

Datasheet:

http://www.meder.com/upload/produkte...Series%20e.pdf

[TucsonSean]

Possible Vactrol alternative

One possible alternative to the Vactrol is the H11FxM series of photo FET optoisolators from Fairchild. They can handle 60Vp-p of signal through them. Check here:

http://www.fairchildsemi.com/ds/H1%2FH11F1M.pdf

I'm still waiting for them to be released but I have some of the older ones from QT Optoelectronics (Fairchild bought them). When I get my lab set back up, I should be able to take some measurments. The only problem is it may be another month before that happens as I just finished moving. I really don't like moving.

Regards,
Sean

[Steve Conner]

I've used H11Fxs in my amps before (and mentioned it in this thread before too!) I got the idea from an industrial project I was working on that used them. They are fine for what they are, but the 60V breakdown voltage isn't really enough for every position in a tube circuit, and their saturation current is only a few mA with the LED driven as hard as you dare.

When used as a variable resistor rather than a switch, their linear range is very small indeed, but at least the clipping is quite soft and musical when you exceed it. RTFDS for more details.

I used two for a dramatic footswitchable gain boost: one to short out a large cathode resistor, and the other to release the bottom leg of a gain pot from ground.

I'm not too sure what TucsonSean means about waiting for them to be released, we were buying them by the dozens 2 years ago.

[TucsonSean]

H11Fx are difficult to find

Steve,

The H11Fx series has been discontinued by Fairchild (QT Opto, the original mfgs of these devices) and replaced with the H1FxM series that is RoHs compliant (I assume the previous were not RoHS compliant). I was thinking of using these replacements to the JFETs or relays in a resistor string for the tone controls. Depending on the range of the linearity, one may be able to implement them in a similar fashion to the LDR setup that Mesa uses. Though the voltage capability is lower, I believe the distortion performance is improved?

By the way, I saw that you had used them before (visiting your website) but I do not recall seeing them mentioned in this thread with respect to replacing either the Vactrols or the JFETs or relays in the tone stack. I'll review this thread again to make sure that I am not behind.



Regards,
Sean

[Don Moose]

RoHS compliance is only an issue if you're manufacturing NEW equipment for sale.

Adding RoHS-compliant components to a non-compliant board won't really change anything ... though you may need a bit more flux and a slightly hotter iron to get good wetting on the lead.

Hope this helps!