Replacing choke with resistor

[sbirkenstock]

Hi everybody,

I tried to replace my choke in a BF Vibroverb project.
Just to check it out and I do own a ValveTech Amp that has not choke and sounds awesome.

The Vibroverb´s choke (all transformes by TubeAmpDoctor) has a DC of about 100 ohms.
I checked on some other amps that come without a choke, mainly tweed Fenders and their resistors in that place are fairly large, 5K to 18K.
Can´t think of other examples that don´t have a choke.
If you can name one with schematics available on the Web, that would be great.
A friend of mine uses 2.2K. He recommends not to go below 1K.
So I was not sure and dropped in a 1k 5watt one.
It works, but the voltage drop is now around 57 volts.
The choke dropped the voltage about 9 volts.

So is it ok to put in a resistor with 100 ohms or so?
Since we have (almost) DC here, the resistor should have the same ohms as the choke measured for DC resistance?
I´m getting mostly insecure here since all the other examples I found are at or above 5K.

Thanks a lot,

Stephan

[Phil_S]

Voltage drop equivalence will probably be had with a resistor of about 250 ohms. You'll have to experiment some because it is hard to know exactly without having the amp present. But this misses the point.

A choke and a resistor are two very different things. A choke has inductance and a resistor does not. Inductance causes the choke to behave with some degree of variability. In other words, as flow across the choke rises, it will do more filtering and regulate B+. A resistor is a fixed value item and does not substitute for a choke. The result is that the amp is likely to sound different. Whether you like that or not is up to you.

You've been told not to go below 1K. The other thing going on is the decoupling of the power supply stages. 250 ohms may not be enough resistance to adequately decouple the plates and the screens. It may make plate voltage of the plates and screens (and the rest of the preamp tube plates downstream) a little bit interactive with the power tube plates, which is probably not desirable and for that reason I'd avoid it. Assuming 1K is as low as you can go (I don't know the answer), you can just live with whatever voltage drop it causes.

[pdf64]

Marshall have used a 100 ohm resistor between plate and screen grid nodes, eg http://schems.com/manu/marshall/jcm900_ ... x_100w.pdf

Not that everything Marshall can be used as an example of good design.

[roberto]

The amp will react differently, so I'd don't take care of the voltage drop by itself, but the dynamic interaction with the rest of the amp.

You could like something as high as 3k (gives to the amp a complete different feel), or as low as 150R. Just explore.