JFETs as switch in 2-channel tube amp

[Mooly]

Originally Posted by tubelab.com

I just found this thread. I see that there has been considerable discussion regarding a fairly complicated fet based channel switching scheme for a guitar amp.

I use simple LDR's.......

That could be a good alternative for a guitar amp I haven't tried LDR's as attenuators but they have quite a following in the lightspeed volume control.

[muadib25]

Originally Posted by Mooly

The supplies should hold up under all possible loading that can be presented.

The cap across the zener can't work as a reservoir cap because the zener clamps the peak voltage to 15v. As the AC cycle continues the voltage can only fall across the cap. If the cap were before the 150K then it would maintain a constant voltage (and hence constant current) across and through the zener. If you were to scope the 15 volt zeners I think you would see the supply isn't clean.

If you are simulating it then replace the zeners and their feed with a constant voltage supply (two 15 volt supplies) and get the circuit to work correctly from that

Unfortunately I don't have an oscilloscope, not yet anyway...
Anyway, I bought extra components for implementing what you suggested before, but first I tried replacing the resistors I was talking about to 2M2 and 33k, since it didnt require any drilling and rearrangement on the protoboard. I am attaching the pic with the voltages I measured.

However, the amp does not sound nice, there is still distortion present...

UPDATE:
I had a look at 2N5638s datasheet. In the 'Switching Characteristics' section it mentions Vgs=-12 V. Does it mean that I can/have to provide it with more negative Voltage for it to turn off?

[muadib25]

Originally Posted by Mooly

Make sure that the zener stabilised supplies are OK under all switch conditions. I would have thought it better to rectify and smooth the HT first and then apply to the zener via a resistor or at least split the 150K witth a cap. You have to have -15 volts across the zener.

I missed to mention that. The V- rail supply was not stable with the 470k-6k8 pairs, but got stable with the 2M2-33k values!

[muadib25]

Originally Posted by muadib25

However, the amp does not sound nice, there is still distortion present...

Skip that, too! I had the FX loop pot near zero... I'll play tomorrow, it's past 22:00 now and the neighbours won't be as happy if I start playing riffs in the middle of the night!

[Mooly]

Originally Posted by muadib25

UPDATE:
I had a look at 2N5638s datasheet. In the 'Switching Characteristics' section it mentions Vgs=-12 V. Does it mean that I can/have to provide it with more negative Voltage for it to turn off?

No, it means it will block peak signals up to the value of the Vgs voltage. Look at my link way back in post #11

If the sound is distorted then the first thing to do is to simply bias the gates on via a 6K8 and 470K divider (as in your circuit) and make sure the sound is clean. Feed the divider from a clean 15 volt supply. Or just use a 9 volt battery and recalculate either of the two resistors.

[Mooly]

Thread moved over to "Intruments and Amps" as it has been suggested as a more appropriate forum for this type of thread.

[tubelab.com]

Has anybody used HEF4066 to switch audio signals.

I have used several types of CMOS analog switches. They work great in a circuit where the total peak to peak voltage range of the analog signal applied to the switch is between Vdd and Vcc of the chip (usually 5 volts). Atempting to go outside this range activates the ESD clamp diodes on the input or output pins. This can cause distortion or chip death depending on the available drive current. They are useful in a solid state amp design, but don't work so well in between a pair of 12AX7's.

The cheap LDR isolator that I specified is rated for 60 volts. I have used it to well over 100 volts of signal in high impedance circuits without issues. It is also rated for 50 mW maximum dissipation. There are LDR isolators rated for 1/2 and 1 watt and a few hundred volts. These can be used for variable plate load resistors with a 12AX7.

[muadib25]

Originally Posted by tubelab.com

I just found this thread. I see that there has been considerable discussion regarding a fairly complicated fet based channel switching scheme for a guitar amp.

I use simple LDR's. It is a Cadmium Selenide photoresistor and an LED inside a sealed package. LED on... resistance is about 100 ohms. LED off resistance is about 10 meg ohm. They work good for channel switching. The resistance curve is not linear but they are good enough that you can make a channel transition with a pot based foot pedal.

Soldano used LDR's for channel switching in some amps (SLO100). They have one extra stage in the overdrive channel so that the channels are out of phase at the summing junction. This way any leakage cancels.

Soldano used Vactec VTL5C1 which are expensive. I use these:

Hello Tubelab!

Yes, LDRs sound like they can handle the gain in a tube amp. The original design had LDRs but I chose to experiment with JfETs primarily due to their low resistances and their sturdiness in time, although i may be wrong here... I will definitely look into them!

[muadib25]

Originally Posted by Mooly

No, it means it will block peak signals up to the value of the Vgs voltage. Look at my link way back in post #11

If the sound is distorted then the first thing to do is to simply bias the gates on via a 6K8 and 470K divider (as in your circuit) and make sure the sound is clean. Feed the divider from a clean 15 volt supply. Or just use a 9 volt battery and recalculate either of the two resistors.

Mooly,
I was able to make some measurements while playing tonight. The amp sounds clean only in low gain, meaning I had to have the Volume pot on my guitar to 1-4 to get clean sound. Higher than that, there is distortion.

I then tied my DMM to each channel's JFET gates with crocodile clips and the voltage was swinging from 0.2 to -4 at the ON FET's gate and from -8 to -2 at the OFF FET's gate when i was playing, especially when strumming chords and the low strings(high output).

Btw, on sunday I'll put the extra caps/resistors to the power supplies, as you suggested, and test/measure voltages again.

Is a Source-Gate resistor relevant here? Can it be used to offset the signal voltage to Vc?

[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.