Francesca Schematic vs Photos

[JammyDodger]

Hey All,

I've noticed that in the photos I have there is are 2.1k resistors to ground off of pins 2 and 7 of one of the EL34's. This dosen't show up on any schematics that I have. Also, there is a ceramic disk capacitor on the power switch. I can't read the value.

Does anyone know what the purpose of these are?

Cheers, Mike

[gearhead]

That's not a disc cap, it's a MOV:

http://en.wikipedia.org/wiki/Metal_oxide_varistor

[Jack]

The resistors provides a virtual center tap.

[gearhead]

See here:

https://tubeamparchive.com/viewtopic.ph ... centre+tap

[joe6v6]

I the franseca pics I see what looks like the standard 100 ohm pair coming off of pin 2 & 7 of one of the power tubes, these form the artifical center tap for the heater circuit. . On the Trudy schematic it shows the 100 ohm pair AND 1 - 470 ohm resistor to ground from one of the power tubes. Aparently the addition of this resistor reduced the ammount of noise created by the heater circuit even further (on this particular amp)
- - pure speculation on my part - -

[novosibir]

On the Trudy schematic it shows the 100 ohm pair AND 1 - 470 ohm resistor to ground from one of the power tubes. Aparently the addition of this resistor reduced the ammount of noise created by the heater circuit even further (on this particular amp)

No, it rather would have worsen it.

But maybe Ken Fisher has used an elevated virtual center tap for the heater supply, to further reduce the hum & buzz?

Larry

[joe6v6]

But maybe Ken Fisher has used an elevated virtual center tap for the heater supply, to further reduce the hum & buzz?

If an elevated heater supply were utilized there would be no 100 ohm pair to ground or the lone 470 ohm resistor.

No, it rather would have worsen it.

The Trudy schematic is of an unaltered Express, what are the other possible explanations for that resistor?

[novosibir]

But maybe Ken Fisher has used an elevated virtual center tap for the heater supply, to further reduce the hum & buzz?

If an elevated heater supply were utilized there would be no 100 ohm pair to ground or the lone 470 ohm resistor.

Since your description was not very concrete - and to make my thoughts some clearer:

When you attach an elevated heater supply in an amp and the heater's secondary doesn't have a CT, then by any means you have to make a virtual center tap - usually with a pair of 100 ohms or a pair of 47 ohms resistors.

As for the mentioned 470 ohms resistor - and remembering, that people again and again have misread resistor values in amps, I was assuming, that you've maybe misread a 47K as a 470 ohms resistor. Note: Orange might have turned to brown due to excessive heat over the time - so you might read a 47K as a 470 ohms

With a voltage divider, where the upper resistor is i.e. a 220K and the lower resistor a 47K I may obtain on its junction the desired voltage for the virtual ground point of an elevated heater supply. Of course, you didn't mention still another resistor, going to this point. But I don't know, how clear the photo has been, where you've seen the possibly misread 47K - or it might have been below the power supply board, connected to V1's supply node, and from there a cable to the 'possibly misread' one...

Shure, it also might be, that sometimes I'm simply thinking too much, but OTOH I've already seen so many cunning ways, how people have tried to hide their 'special tweaks' from the eyes of others

Larry