Marshall Standby switch deletion and bias tap design

[ViperDoc]

I'm looking at a Marshall-style build without a Standby switch with SS rectification. The normal bias circuit design seems to take the bias voltage from the HT secondary before the standby switch, thus allowing the bias circuit to heat up in standby mode. Without a standby switch, is there a problem heating the entire circuit simultaneously? I would think not.

[pdf64]

There’s no problem powering all circuits up simultaneously.
However it would be beneficial to protect the DCCF in V3 (?) from excessive grid to cathode voltage at power up, add a resistor and diode, see http://www.valvewizard.co.uk/dccf.html

[ViperDoc]

Much appreciated, pdf64! Thanks for the link.

Here's the base SCH I'm basing the build off of:

TL 2550X schemR11.JPG

Due to the placement of the FX loop, does the DCCF protection circuit apply?

[xtian]

There is no cathode follower in the schem you just posted.

[pdf64]

My understanding is that a diode across the relay coil (in addition to the one across the switch) is good practice, to kill back emf spikes.

[ViperDoc]

I'll be using Hoffman relay boards which have the coil diodes.

Xtian, no CF, that's for sure. Thanks!

I figured I would build this chassis with the stock FX loop since I've never tried it, but I might like the loop in between the treble pot and master volume, not sure.

[Reeltarded]

That is where to put the loop. If it's earlier it sounds terrible and if its later you constantly have to adjust literally everything for gain changes.

[ViperDoc]

That is where to put the loop. If it's earlier it sounds terrible and if its later you constantly have to adjust literally everything for gain changes.

Which "that" do you mean, my good man? Original Jube location pre-tone stack, or Treble:Master?

[Reeltarded]

loop between the treble and master!

[Roe]

The preamp and phase inverter filter caps in marshalls are only rated for 350v to 500v (depending on the amp). In many cases you risk blowing these caps unless you use the standby switch. For example, a 100JH has 600VDC across the standby switch, while the phase inverter cap is only rated for 450v.

[pdf64]

Bear in mind a cap’s surge voltage rating, eg for an ecap, might typically be 1.1 x working voltage rating for 1minute not more than say 4 times per hour.

[Roe]

Bear in mind a cap’s surge voltage rating, eg for an ecap, might typically be 1.1 x working voltage rating for 1minute not more than say 4 times per hour.

Still, I recommend using the standby switch on the high voltage marshalls/plexis (typically 100w amps and jtm45/100s, not to mention Majors)

[Badside]

The preamp and phase inverter filter caps in marshalls are only rated for 350v to 500v (depending on the amp). In many cases you risk blowing these caps unless you use the standby switch. For example, a 100JH has 600VDC across the standby switch, while the phase inverter cap is only rated for 450v.

And that standby switch is only rated for 250VDC

[ViperDoc]

Here's the datasheet for my F&T 50 uF + 50 uF can caps.

https://s3.amazonaws.com/tubedepot-com- ... 1400177538

Says U"p" or Peak Voltage = 550 V 5 x 1 min per hour.

Good enough? Or should I start shopping for thermistors?

[pdf64]

It depends on the likely max initial or unloaded voltage your PT sends out their way.
You’re probably fine, though of course it’s always best to verify.
But then thermistors are good things to have anyway.