Benson Monarch

[shane]

Either an in line resistor or swap the hot and wiper connections on the pot.

[jazzyjoepass]

Just so happen to have the non-reverb Monarch.

The driver circuitry to the power tubes is totally different.
Something that I've never seen before.

Mine's got a blown dropping resistor in the power supply.
Anyone knows what the expected B+ voltages are ???

[Synchu]

@jazzyjoepass - you mean yours is different from the posted schematic?
Else what Aaron posted (at least "architecture" sort of) looks inspired by Fender 5B3.
Niki

[Colossal]

The driver circuitry to the power tubes is totally different.
Something that I've never seen before.

It's a paraphase. Check out the wide panel Fender Deluxe 5B3.

[dorrisant]

Just so happen to have the non-reverb Monarch.

The driver circuitry to the power tubes is totally different.
Something that I've never seen before.

Mine's got a blown dropping resistor in the power supply.
Anyone knows what the expected B+ voltages are ???

Can you post what the differences are?

Also, can you tell the value of the blown resistor?

[dorrisant]

Aaron... Am I to assume that the second filter node, after the 1k droppng resistor, is to go to the screens? And the second cap after the 10k is to go to the preamp tubes? Pretty sure of it, but you know about assuming.

Also, does the B+ node for the phase inverter get connected with the screen node? Where does the reverb tube B+ get connected?

Do you have any naked pictures... of the amp?

Thanks again for posting this. Gonna build it this weekend.

[Aaron]

Hi Dorrisant,

The node after the 1k does go to the screen. The three other tubes, 12AX7-preamp, 12DW7-reverb and 12AX7-PI all run off the node after the 10k. I forgot to take pictures before it was picked up. But it was mostly point to point from tag strips. I wasn’t even gonna attempt a layout
The balance pot in the PI is a 10kL, and I drew the reverb pot wrong, the wiper connects to the 250pf cap, pin 1 is the usual ground and pin 3 connects to the wiper on the volume pot.
I’ve measured the voltages so will update the schematic.

Thanks,
Aaron

[dorrisant]

Thanks again Aaron!

[dorrisant]

Aaron, should the reverb feed come after the .022µF to block the DC into the reverb tube grid?

[Colossal]

Aaron, should the reverb feed come after the .022µF to block the DC into the reverb tube grid?

The reverb driver is being run as a cathode follower off V1a.

[dorrisant]

Ok... I get it.

Here is a quick and dirty layout... Lemme know if you see anything wrong. I know it is not laid out perfectly but should be enough to wire one up.

Assume errors until this is reposted as VERIFIED!

**Edited to show a correction (resistor left out (R5)) and to reflect voltages at the tube pins.**

[pullshocks]

I have a type 4FBxxx reverb tank. Input impedence 1475 ohms, output impedence 2250 ohms. Will that work with the cathode follower capacitor reverb drive in this amp? I have used it in a 12DW7 plate driven cap drive reverb circuit.

Hope so, I'd like to try this circuit in my 2 x 6V6 platform.

[pullshocks]

Ampgaragers, could you give me a reality check on this:

So if I understand this right, the input impedance of the tank needs to match the output impedance of the driver stage, in this case a12AU7 cathode follower stage.

In Merlin Blencowes Designing Tube Preamps for Guitar and Bass Second Edition, on page 115, he states the output impedance of a cathode follower is 1 divided by the transconductance, and for a 12AX7 (transconductance 1.5 mA/V) this works out to 667 ohms.

For a 12AU7 the transconductance is 2.2 mA/V, so the output impedance would be lower, more like 500 ohms.

So my 4FBxxx tank is not well matched. If I've got this right, a xDFxxx EDIT 4EFxxx tank would be the closest match, at 600 ohms input impedance and 2250 ohms output impedance.

[shane]

Aaron:
did you get chance to post those voltages yet.

Dorrisant:
Have you had a go at one yet

[brewdude]

I am a little confused about the tone cap/pot.
Is the tone cap really the same value as the coupling cap?