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JFETs as switch in 2-channel tube amp
JFETs as switch in 2-channel tube amp
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Old 28th November 2012, 02:11 PM   #111
JMFahey is offline JMFahey  Argentina
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Location: Buenos Aires - Argentina
Hi guys, still struggling with this?
*If* the OP wants to, I may suggest a simple and working configuration, but previously need him to re-post here the schematic of the preamp where he wants to add switching, in fact showing conventional switches drawn in place, to see what the *actual* task the Fet will have to fulfill.
There is no "one solution fits all", so suggestions will differ depending on circuit point to be switched.
Many good ones have been suggested above, and in some cases even CD40xx chips might work, but not in others.
The only truly "universal" switch is a metal-to-metal one (hand/foot actuated switches or relays), followed by LDR if high "ON" resistance is acceptable; all others *can* work very well but with distinct limitations which must be considered.
Please post the schematic already converted to black-on-white so it can be easily edited; those colorful black background .sch type are good within the PCB design world, but not so much to exchange ideas in Forums.
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Old 29th November 2012, 01:32 AM   #112
Struth is offline Struth  Canada
Join Date: Aug 2012
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Hi Guys

The ideal switching element for a given circuit varies both with the circuit and the aesthetics of the design. Devices that break DC paths, such as mechanical switches, often require the addition of DC paths which might alter the circuit gain or frequency response. So, the decisions must be made with care.

Table 9-1 of the Switching Methods chapter of TUT shows the characteristics of the various switch elements commonly used. The OP's circuit is just a recto clone, which itself is a copy of an SLO with some of the extraneous bits removed. LDRs have been replaced with jfets, if you read the thread here and the secondary one at PowerScaling.com forum. The double switching of the EQ - both input and output _ is not needed except when using LDRs. The output switching using a series-shunt jfet configuration works much better, providing better channel isolation.

CMOS switches would definitely _not_ work in this app.

Have fun
Kevin O'Connor
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Old 1st December 2012, 01:46 PM   #113
muadib25 is offline muadib25  Sweden
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Location: Stockholm, Sweden
JFETs as switch in 2-channel tube amp
Originally Posted by Struth
Measure the voltage at the BJT collectors to make sure they are going from rail to rail when switched. If they are not, then the series base resistance is too high and needs to be reduced.
Yes, the collectors are going from rail to rail. Checked!
Originally Posted by Struth
If the BJTs are switching rail-to-rail as they should and the gate end of the 1Ms reads much lower, there is a leakage path somewhere that needs to be addressed. The 10M input of the meter will cause a misreading itself, but by no more than 10%.
I'll check. The only thing that stands between the 1MΩ resistors and the diodes are the 10nF caps to GND. Just to be clear, I am posting once more the Switching circuit in B&W using red only at the voltage reading points.

Generally, the switching circuit Struth proposed performs correctly, even at high gain situations!
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File Type: png SwitchingcircuitVoltages.png (353.5 KB, 33 views)
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