JFETs as switch in 2-channel tube amp

[Mooly]

Try and go back on basics and confirm the conditions are correct.

1. The drain and source points in the circuit that all the FET's connect to should have zero volts DC present. If those conditions are met then there should be no need for any "offset" which would be very difficult.

2. Measure the gate voltage at Q4 and Q5 collectors because measuring on the gates via the 1Meg resistors will not be a true reading due to the small current the meter draws.

I still think there is a problem with the supplies

Also without a scope and seeing what the peak signal levels are, then it becomes more difficult to see where what is going on and where things are going wrong.

[muadib25]

Originally Posted by Minion

If you can't get the fet switching working I have a pretty simple relay switching desgn that I use is a couple amps that works pretty good. It uses two DPDT relays to switch between 2 channels and it has LED indicators to tell you what chanel you are on.

Hey Minion!

I am starting to think that Vpp levels are too high for the FETs and their gate supply levels, although I have to measure more and try one more topology... If all else fails I will definitely fall back to relays (although I'd like to try out LDRs too). Thank you very much, I appreciate it!

[muadib25]

Originally Posted by Mooly

Try and go back on basics and confirm the conditions are correct.

1. The drain and source points in the circuit that all the FET's connect to should have zero volts DC present. If those conditions are met then there should be no need for any "offset" which would be very difficult.

2. Measure the gate voltage at Q4 and Q5 collectors because measuring on the gates via the 1Meg resistors will not be a true reading due to the small current the meter draws.

I still think there is a problem with the supplies

Also without a scope and seeing what the peak signal levels are, then it becomes more difficult to see where what is going on and where things are going wrong.

I nailed it!
I was curious this weekend to try out Kevin O'Connor's method which has the following differences:

1. Reversed biased 1N4148s on all FET gates.

2. +/-10 VDC for the Supplies, contrary to the -10/+0.2Vdc I was trying until now.

These 2 changes made the trick. And the experience gained is huge. i haven't really got the whole concept really well but it's a start!

I'll post the schematic when I get home.

Thank you Mooly!

[Struth]

Hi Guys

To use jfets as series switches requires that the control voltage for the gate exceed the signal amplitude being controlled. Since the signal is referenced to ground and is therefore bipolar, so must be the control circuit voltages.

For example, if the signal is 10V-peak, then the jfet control signal must be +/-12V or a higher. Using a unipolar gate voltage will cause distortion of half the signal.

The control circuit shown earlier in this thread is only good for shunt-connected jfets - not series ones. It is a simple matter to modify the circuit to provide this control range by adding NPNs tied to the -13V rail with their collector loads tied to +13V. The current here is negligible and can be drawn directly from any plate node that is well filtered.

For jfets used as series audio switches, the control lines to the gates should be ramped using a series 1M and 100pF to ground at the jefet gate-diode end. This will control thumps and provide a good transition from one channel to the other.

Generally, shunt switching is preferred whenever it can be used, as it is inherently quiet. The only downside is that the shunt element must work against a series resistance in the signal path. In a tube guitar pre, there are already series resistances, so this is not really a concern.

TUT has a 90-pg Switching Methods chapter. TUT7 (out next year) has another huge bit of info in the Automation chapter.

Have fun
Kevin O'Connor
londonpower.com

[Mooly]

Originally Posted by muadib25

I nailed it!

Well done you

Yes it would be interesting to see the final arrangement that worked here.

[muadib25]

Originally Posted by Struth

Hi Guys

To use jfets as series switches requires that the control voltage for the gate exceed the signal amplitude being controlled. Since the signal is referenced to ground and is therefore bipolar, so must be the control circuit voltages.

For example, if the signal is 10V-peak, then the jfet control signal must be +/-12V or a higher. Using a unipolar gate voltage will cause distortion of half the signal.

The control circuit shown earlier in this thread is only good for shunt-connected jfets - not series ones. It is a simple matter to modify the circuit to provide this control range by adding NPNs tied to the -13V rail with their collector loads tied to +13V. The current here is negligible and can be drawn directly from any plate node that is well filtered.

For jfets used as series audio switches, the control lines to the gates should be ramped using a series 1M and 100pF to ground at the jefet gate-diode end. This will control thumps and provide a good transition from one channel to the other.

Hello again Mr. O' Connor!

Glad to 'meet' you here! Your proposal works great. As you can see from the topic, I tried some other topologies that, despite the fact that they didn't work in my application, offered me lots of experience.

As for your books, I have 4 of them (TUT,TUT4, RSG and SPKR) and I totally recommend them!

Originally Posted by Struth

Generally, shunt switching is preferred whenever it can be used, as it is inherently quiet. The only downside is that the shunt element must work against a series resistance in the signal path. In a tube guitar pre, there are already series resistances, so this is not really a concern.

Shunt switching is what I wanted to use in the beginning, but I was unsure of the exact topology (resistance values etc.) that it required. I will be testing it, however, in a future project

Originally Posted by Mooly

Well done you


Yes it would be interesting to see the final arrangement that worked here.

Sorries for not having the time today, I will be posting it tomorrow with the voltages. Check my #36 post for Kevin's circuit, it's the one on the left.

Cheerz!

[muadib25]

Ok guys, here it is.

Now a question emerges... what if I wanted to raise the gate voltage a bit more than +/-7.15? The thought behind this is to have more headroom...

Would it be wise to change the 1MΩ resistors with something smaller, say 680kΩ? Or is there another way to do this?



Thanks!

[Mooly]

Easyone first (hopefully ). The FET gate draws zero current at DC (if the biasing is correct and within limits) so the 1meg value shouldn't matter. 100k, 1meg or 100meg, the gate voltage should be the same although not measurable accurately without a FET input voltmeter (or old type valved VOM).

Hmmm... so the diodes seem to get the desired result. If it works it works although I see a "floating" gate now...

[muadib25]

Originally Posted by Mooly

Hmmm... so the diodes seem to get the desired result. If it works it works although I see a "floating" gate now...

Yes, yet as I measured them while playing, the gate voltage changes according to the signal present in the channel! Those 1N4148 are really fast on this!

Btw, I really have to get a Fluke... I don't trust my $20 DMM anymore...

[Mooly]

Originally Posted by muadib25

Yes, yet as I measured them while playing, the gate voltage changes according to the signal present in the channel! Those 1N4148 are really fast on this!

Btw, I really have to get a Fluke... I don't trust my $20 DMM anymore...

I can see how you say the gate changes according to the signal. The gate has virtually infinite impedance at DC so the voltage on the other pins can "transfer" via leakage and at AC via the junction capacitance of the FET.

Its one of those things where you need the working circuit in front of you to probe and test and see exactly what is going on.