Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Switching devices at tube amp voltage levels

Status
Not open for further replies.
Since 12AX7/ECC83 is the most widely available, I'm sticking with that. Just looking to clone a few preamps that share a common topology in one box, switching between the different R and C values they don't have in common. Last preamp I built was something I use for bass guitar that I'm pretty happy with, save for a slightly higher background noise/hiss content than I had hoped for. I don't believe I properly returned every ground sequentially to the right filter cap for that stage so that's probably the culprit. This time I am planning on doing a PCB instead of using turret board which is more likely to be properly thought out in advance on my end.
 
Thats possible, however a less than ideal ground is more likely to cause hum.
Hiss can be caused by impedance mismatches, or just cheap valves, if its more of a rattling sound thats shot noise and is as far as I can remember caused by old tubes where the anode face is battered by electrons with use.
 
Attempting to post a quick idea from an iPhone so please bear with me.

I think I attached an image of an example input stage. Assume there are two of these stages that will combine at the point where I have written "out" and they originate from the same 1/4" input jack on the far left.

Question 1: would a p channel get work as shown in the cathode of the triode? With a logic low, the channel should be open and current flowing therefore the 1k2 and .68uf cap would be in parallel with the 2k7 (save for the rdson of the jfet itself). With logic high, they should be a high impedance resulting in the 2k7 standing essentially alone. Correct?

Question 2: the P channel and N channel fets Q1 and Q2 I am thinking would work in the following fashion. With logic low into Q2 (N channel) gate, the gate of Q1 (p channel) will be pulled to roughly the Source voltage and therefore be off. This should form a large voltage divider for the signal off the plate equal Rd/(Rd+rdsoff). Assuming at least 3Mohm off impedance (is that reasonable?) that will cut the signal about 21dB. Meanwhile, the other triode will have its Q2 see the opposite logic, which would result in no voltage divider forming. The second triode's output will be at least 21dB louder than the parallel triode, which on an input stage I am assuming is loud enough to negate the others contribution.

I chose 330k for Rd and Rs based on keeping current consumption very low (less than 1mA) and also because the filter formed with the .68uf cap and parallel 330k resistors puts the -3dB point at 1.4Hz which is below audible and saves plenty of room for component tolerances. .68uf is chosen since it is the largest value film cap I typically see used in guitar amps that is rated for the high voltages found on triode plates. Also ignore the parallel 10Meg resistor in parallel with Q1 - brain fart that rdsoff will be dictated by the fet anyway.

Hope this made sense and isn't terribly wrong.
 

Attachments

  • image.jpg
    image.jpg
    1.7 MB · Views: 171
Tie the bottom end of the 1k2 and the bottom end of the 0.68 together and short them to ground with just 1 fet, you dont need the both of them, unless you want to control gain with the 1k2 and freq response with the 0.68.

your thesis for the second part sounds correct, however I dont think relying on 10mr of a fets open resistance is a good idea, I suspect all kinds of parasitics will come into effect and the circuit either wont work or will have a odd and possibly constantly changing freq response, either that or you'll pick up radio moscow.

Years ago this kind of thing would have been done using vari-mu valves, however I think if you want switchable gain why not form a voltage divider on the o/p and switch in/out extra resistors on the low end of the divider, you can follow this with an op amp if you want low o/p impedance.

The actual voltage on the fets in your drawing most of the time will be low, taking into account the mu of the valve assuming the input is a guitar will only produce a few volts at the cathode, including bias will still only be a few volts, so a standard fet would work once the circuit is running.
However as valves warm up esp if the ht comes on first voltages might not be as you expect so using high voltage fets probably isnt a bad idea.
 
Thanks - I actually planned to switch each independently along with a few other components in the cathode omitted for clarity sake.

Your caution about high cathode voltages as the preamp warms up is noted. I'll have to include a soft-start to keep the HT off until I have an adequate heater supply up to voltage and the valves sufficiently warmed. There are a million soft start variations on the internet so that shouldn't be an issue.

You mentioned radio Moscow - if I could pick that up, id claim success haha. All kidding aside, could you elaborate on why the off resistance of the fet shouldn't be relied upon? Would a parallel high-value resistor stabilize things? Or possibly a low-capacitance to ground in parallel with the next triodes grid resistor that forms a low pass filter out of the audio spectrum to filter out such possibilities?

As I spent all this time on everything I am reminded of the LED/LDR combinations I believe VACTROL or some name like that makes. Those would be seamless and probably perfect for signal-level switching. But they are expensive. But, then again, these high voltage fets are approaching those numbers all considered.

It was also mentioned having a set gain and using voltage dividers to take it down. The issue isn't gain as it is frequency response. Each combination of cathode R and C will produce different gains across the audible spectrum as bypass caps are switched in that don't completely bypass the cathode resistor for the entire audio spectrum. The cathode is also the lowest voltages in the amp, and therefore the cheapest and easiest to switch.
 
Makes sense.
Vactrol do make some interesting optical resistors, I cant remember the make now but I recall repairing an amp for someone that had a led/ldr in it, the manufacturer made their own from a led/norp12 and a pot knob as the body!
10mr off resistance and radio mosco are linked, if you have such high impedances in your circuit noise will be an issue, and you might even end up picking up radio stations, rectified off the incomming cables.
Its just a little outside my understanding but I wouldnt reccomend relying on the fets off resistance for the reason just mentioned and also I wouldnt have thought the off resistance would be that stable with temp or frequency. To switch gain your much better off shorting something like a resistor to ground rather than opening to a large resistance.

Your probably aware that cathode stripping is an issue on larger bottles like the el34 and kt66, which is probably why loads of soft starts are knocking about, in the days gone by it wasnt such an issue as rectifier tubes took longer to warm up than output tetrodes so you didnt get ht before you wanted anyway.
 
If the off resistance is too high, a parallel 2M2 resistor from Drain to Source would effectively provide an upper limit to that resistance. And as long as rdson is much lower than 2M2, which it is, the parallel combination there should be just about equal to rdson.

I may give the circuit a shot on a small scale to see how it works. Truthfully I think the Vactrols are a better choice.
 
If the circuits in a grounded metal box that might be ok.
Any hum on the supply might be coupled, but I spose thats a valve thing anyway.
 
a few years back i made a tube preamp "stomp box" using "subminiature" tubes, with plate voltages at 24V. it actually worked quite well. at low gains, the tube characteristic added just a barely audible even harmonic distortion, cranked up to full gain gave a nice smooth tube fuzz..... i don't remember the tube type off the top of my head, but i have a bunch of them at home.... basically a submini dual triode similar to a 12AT7, or 12AU7 (not as much gain as a 12AX7). since they were military tubes, they're extremely shock resistant.....

if you put your "component ladder" and switches (i also think relays and a mute circuit is the right way to go) on the cold side of a plate coupling cap, and keep DC out of the mix, you won't have to worry about thumps. also, another good place to do this is in an amp with an effects loop, where it's all line level (max 1.5Vrms) signals anyway, or parallel to the reverb circuit.
 
I built a tube amp when I was in my last semester of college using those subminiature tubes. I think I used a 6021 if I'm not mistaken. Dual triode and then a 5xxx type number pentode (can't remember the exact number) in class A single ended. I powered it off of a 6V lantern battery that I charge-pumped up to a higher voltage for the plate supply. Fun project to build.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top