Question on Channel switching and relay voltage input

[ramvet]

In 1997  a build your own tube amp course was offered through Parks and recreation and was taught by a local luthier.  The participants selected the type of amp they wanted to build and parts were purchased.  The luthier flaked and folks were left on their own to put these amps together.  Having had some electronic foundation from junior high school and college, I studied tube amp books and put together an amp.  It has two preamps: one fender and one marshall with output to a Vox AC-30. 

The preamps are switched with a dpdt switch: one side switching the guitar line input to the separate preamps and the other side switching the corresponding preamp outputs to the phase inverter.  After all these years I don't remember why I did the switching this way. 

Now I am considering re-configuring, possibly with a footswitch arrangement. 

Question 1: is there a better method for arranging the switching configuration as stated above. 

Question 2: regarding the Hoffman relay channel switching circuits that utilize the 6.3 volt filament secondary from the OT, could the 5.5 volt rectifier secondary (which I am not using) be used as the input AC voltage for the Relay switching circuit instead?

Thanks for your help, Bob Mansfield

[sluckey]

could the 5.5 volt rectifier secondary (which I am not using) be used as the input AC voltage for the Relay switching circuit instead?

Absolutely. In fact, a totally separate winding is preferred.

[ramvet]

Awesome.
What do you think of the switching scheme: guitar line in and preamp outputs to the PI?

[sluckey]

Awesome.
What do you think of the switching scheme: guitar line in and preamp outputs to the PI?

That certainly works and if you are happy with using a DPDT switch, then using a DPDT relay will work just as well.

I prefer to ground the inputs of the unselected channel. That requires a two pole switch/relay. So, what to do with the outputs? Either use a couple 220K resistors to mix the two channels together or use a 3 pole switch/relay. Probably have to use two relays.

[ramvet]

Update on channel switching using Hoffman power supply board and relays.

The unused rectifier secondary was used to supply AC power to the Hoffman power supply board.  This measured 5.63 volts AC.  The output from the board is 4.53 volts DC.  A DPDT relay from Hoffman was used to switch the channels: one side switching the input to one of the 2 preamp channels and the second side switching the outputs of the preamp channels to the PI. Shielded cable (grounded on one side) was used.  Despite output from the power supply of less than 5 volts DC, the relay worked fine.
 
Unfortunately a significant 60 Hz hum is now present in both channels.  The hum is present whether the relay is activated or not.  The hum disappears when the power supply wires are disconnected from the relay!
I am not sure how the hum/noise is still apparent when the relay is not activated.  I assume the power supply is noisy. 

Questions:
1.  Would including the optional radial capacitor on the Hoffman power supply board help remove the noise? 
2.  Would a 0.1 mfd capacitor on the negative terminal of the rectifier connected to the AC ground help remove the noise?
3.  Would increasing the size of the 4700 uf capacitor help reduce the noise; 10,000 or 15,000 uf?
4.  Would installing a separate 6.3 volt filament transformer help prevent the noise?

Comments from the board are appreciated, Thanks, Bob Mansfield

[sluckey]

That 7805 regulator requires 7 volts to operate properly. I suspect it is causing some noise. Remove it and put a jumper on the board between the input and output pins to see if that's the issue. Those relays can operate just fine with unregulated voltage.

[ramvet]

Update on hum/noise from relay power supply

Removed the 7805 voltage regulator and jumped input to output.
Relay works fine.  Hum/noise is reduced but not significantly. 

Questions:
1.  Would including the optional radial capacitor on the Hoffman power supply board help remove the noise?
2.  Would a 0.1 mfd capacitor on the negative terminal of the rectifier connected to the AC ground help remove the noise?
3.  Would increasing the size of the 4700 uf capacitor help reduce the noise; 10,000 or 15,000 uf?
4.  Would installing a separate 6.3 volt filament transformer help prevent the noise?

I am thinking a separate filament transformer may be the best option.

Thanks, Bob

[pdf64]

A schematic showing the circuits you’re asking about would be helpful.

[sluckey]

You still have not confirmed that the noise is related to the relay power supply. To confirm, just disconnect the AC voltage input to the relay board. Do you still have the noise issues? If so, the noise is not coming from the relay power supply.

[shooter]

+1 and +1
is your relay board completely isolated, doesn't share ground with ANY of the signal path?
another member as having problems with the cheap C-made relays bleeding past open contacts fwiw

[ramvet]

Update on hum/noise from relay power supply

Attached file shows Hoffman diagram for Hoffman relay with power supply.
I am using the 5.5 volt output from the output transformer instead of the 6.3 volt filament output.

The relay board is completely isolated and does not share ground with anything.

There is no noise when the output from the power supply is disconnected from the relay board.

Thanks,
Bob

[sluckey]

Here's a better pic...



1. OK, since you are using a separate 5VAC winding as the ac source it's OK to connect the ground of the supply to chassis. Does that reduce the noise?

2. 5VAC is not enough input for that 7805 chip, so just remove the chip and put a jumper between the input and output solder pads for the chip. That's the two outside pads.

3. Disconnect all six of the pink signal wires from the relay board. Still got hum?