Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jack2911guitarblues on December 08, 2014, 09:32:45 pm
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Hi there!
Just newly joined and have a question about the Hoffman Hotswitch. I built a Tube Depot jtm-45 plus, after ironing out a few problems, pretty happy with the end result. Sorry about the questions, but is there a way to have less gain so that there is less problem with volume jump and squealing etc when engaging the Hotswitch? I have a DPDT switch installed, and when it's on, signal goes from middle of Volume control to pin 7 of V1B, thereby cascading the gain from 1st stage.
Is it possible to put a resistor of some sort between the 2 stages (wiper of Volume 1 and pin 7 V1b) to "calm down" the gain a little before being cascaded? Would this work Or am I asking the impossible?
Love tube amps, and thoroughly enjoyed building mine, but don't quite understand how everything works just yet, but enjoying the experience as I go along.
Would be very grateful for any assistance here, I know that there is an enormous wealth of knowledge out there!
Much appreciated!
Jack (from down under)
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Put a 470K resistor between C4 and the top side of VR1.
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Thanks for your quick response!
I probably don't quite understand it right, but will that cut the gain in just the "cascaded" setting? Or will that affect the normal "plexi" mode as well, when I switch the DPDT? I only really want to affect the cascaded gain and lower it somewhat.
Thank you, Jack.
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What I proposed will affect gain for either position of the hot switch. Try this instead...
Put a 470K resistor between C3 and the top side of VR2.
When you did the hotswitch mod did you change the resistors and remove cathode bypass cap as indicated? These changes help reduce the 'over-the-top' gain.
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Hi,
Thanks again for your input. Problem with my tubedepot build, is that I no longer have a VR2. Have used it as a Master Vol. control, so I no longer have a Bright & Normal input and am only using the V1a side as the Plexi side.
the V1b side is only switched on for the cascaded gain. Otherwise it's totally back to a normal input. I've included the Schematic & Layout for you to have a look at, if you would be so kind.
By the way, is this the best way to do cascading gain? I have seen circuits (one wire mod I think) whereby the Bright & normal inputs are retained, and used for the Plexi side, and yet you can still get cascaded gain if you need it.
Again apologies for all the questions, just trying to get my head around everything, and get this cascade gain option going.
Also to answer your question, I did remove the 470k res. and the 500p which is the cathode bypass cap. And I also changed the resistors from 2.7k to 2.2k as was required. My Tubedepot JTM-45plus schematic is now the same as the Hoffman Plexi50 with Hot switch Gain Mod schematic.
Bye, jack