To choke or not to choke? (Express)

[Matthews Guitars]

That's a question I've wondered about, as I prepare to start building my Express clone since I think I have everything I need now.

Choke or no choke?

Pros and cons of each?

If I do, what choke value is recommended?

[Roe]

it kills tone and resonse and tube life unless you keep the 1k resistance

[Mark]

That's a question I've wondered about, as I prepare to start building my Express clone since I think I have everything I need now.

Choke or no choke?

Pros and cons of each?

If I do, what choke value is recommended?

A choke was used in the Komet amps, thought they also used a valve rectifier.

Rightly or wrongly, I thought the 1K choke resistor sets the sag in the amp. Again rightly or wrongly I seem to recall that the 1K choke resistor cause the power supply after the resistor to sag by 40VDC when the amp is on 11.

I haven’t heard the amp with a choke.

[Roe]

That's a question I've wondered about, as I prepare to start building my Express clone since I think I have everything I need now.

Choke or no choke?

Pros and cons of each?

If I do, what choke value is recommended?

A choke was used in the Komet amps, thought they also used a valve rectifier.

Rightly or wrongly, I thought the 1K choke resistor sets the sag in the amp. Again rightly or wrongly I seem to recall that the 1K choke resistor cause the power supply after the resistor to sag by 40VDC when the amp is on 11.

I haven’t heard the amp with a choke.

True, although the Komet 60 and 50s got a lower Z that puts less stress on the screens (5k8 and 4k8 respectively).

Often the screens in the express will sag more than 40v, but this is greatly dependent on tube types. the RFTs draw the most screen current, whereas the Chinese ones (and the Russian Tung-sol el34b) draw much less screen current, something that undermines the screen sag that is important to the Express sound and response.

[Bergheim]

A choke in place of the 1k resistor will allow much more current through the screens when overdriven. With a 6k6 OT primary impedance, you shouldn't be surprised to see the EL34 screens glow up with the amp dimed. You'd probably also lose the clean-to-mean character of the express, since the screen node voltage won't sag very much.
A choke in series with the 1k will improve AC filtering in the screen node, without much sonic impact otherwise (I think).

IMO, there's only two reasons to use a choke instead of a resistor; (1) to reduce compression/sag as much as possible, and (2) to maximise power output by keeping the screen node voltage as high as possible. The design of the Express is quite the opposite of those, as far as I can see.

[jelle]

I tried a choke in an Express years ago. With the Choke, an Express feels more punchy, aggressive and clear. Its louder too. It takes it closer to a Marshall Plexi in feel.

IMO its too hard and upfront for this circuit, but I am going off what I know an Express sounds like.

With the 1K resistor, there is a compression that I feel this circuit needs. Especially since compared to the Plexi, the Express does not have the compression of the Plexi's Cathode Follower.

If you are looking to build a very direct, uncompressed amplifier, the choke in an Express will get you there. Maybe its jut the ticket for you?

[Mark]

Good to see you here again Jelle. Thanks for your comments.

[Richie]

I've also tried the choke on the express, and went back to the 1k resistor.

[martin manning]

I suspect the 1k keeps the screen from over-dissipation.

[jbrrrrr]

Only if consensual and discussed beforehand.

[Mark]

I suspect the 1k keeps the screen from over-dissipation.

In my experimentation with this circuit I found it dials some sag into the preamp of the amp too. I found the power supply to be pretty stiff otherwise.

[Mark]

Only if consensual and discussed beforehand.

That can only get you into trouble regardless.