Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on January 05, 2021, 12:37:25 pm
-
A recent project got me thinking about standby switches, once again. Although I know that standby switches are not helpful in terms prolonging the life of my amps tubes, I do like the ability to mute the amp while changing guitars, pedals, and when my boss is calling.
I had been reading Merlin’s ideas regarding power and standby switches -- http://www.valvewizard.co.uk/standby.html -- and decided to install the simple large resistor across the SB switch in my one channel Hoffman DR, which has been under the knife due to cabinet changes. This amp was built using Doug’s one channel DR board and schematic (with one exception), but with 6V6 tubes, and transformers from a prior two channel DR. The exception is that the initial filter caps (parallel 16uF) are positioned before the standby switch (see the uploaded illustration). Otherwise the HT section is identical to a two channel DR with 6V6 tubes (eg, 10K 3 watt internode resistors and 16 uF filter caps, instead of the smaller resistors and 22uF as used in the 6L6 version).
The result is in, and here are the measurements and observations: My Wall Voltage is 124 VAC.
47K resistor installed, power on, standby closed:
All voltages are VAC measured with a crappy (orange) multimeter
Pre Node A: 452
Node A: 452
Node B: 451
Node C: 369
Node D: 324
When the standby switch is is opened – eg, the 47K resistor is now active:
Pre-Node A: 483
Node A: 202
Node B: 210
Node C: 165
Node D: 145
So, the circuit is passing about 6mA at standby, and the resistor is dissipating about 1.7W.
Behavior of the circuit:
Disconnecting a guitar: sometimes a pop occurs, but not always. If the guitar is played: soft, but unpleasantly distorted tones occur.
Discussion: This standby switch mod may make sense to an electrical engineer; however, it is not helpful to this guitarist and was summarily dispatched from the circuit.
-
Maybe a 100K resistor would work better.
-
If all you want is a mute, there’s a mute switch in the input jack- just pull the plug out 1/2”.
-
100K should be about right. Since the unit is on the operating table as we speak, worth a try.
pdf, you're assuming I have the coordination to pull the plug just 1/2" -- LOL, that ship sailed 20+ years ago. :icon_biggrin:
-
>> If all you want is a mute, there’s a mute switch in the input jack- just pull the plug out 1/2”..
> you're assuming I have the coordination to pull the plug just 1/2" -- LOL,...
-
Purple switch goes in tomorrow!
-
Just turn the knob!
-
Sorry for resurrecting an old thread, but I have a question related to this.
JTM45 build with GZ34 rectifier. The layout I am following (Metro) has the standby switch before the reservoir cap, and I would like to implement Merlin's suggestion of moving the SB after the cap, and using a 47K resistor (probably 100K) across the switch terminals as described above.
I've also read about placing a 0.047uF poly cap from one of the SB switch terminals to ground to prevent popping. If I do this, is best practice to to ground the cap at the power supply star ground, or can it go to the nearest accessible point on the chassis?
-
I would connect it to the same point the PT CT and reservoir caps are connected.
-
The more I look at it, this is looking exactly like the Dr Z "touch switch" found in the EMS amp as discussed a while back, where Jordan said it may just be there to prevent popping.
https://el34world.com/Forum/index.php?topic=27888.0 (https://el34world.com/Forum/index.php?topic=27888.0)
0.047uF cap from reservoir cap to ground.