G7 question for Jakob

[scott_humphrey]

Hi Jakob,
If the feedback circuit is not being used in the G7, is there any reason to use the 220 ohm resistor that is connected to the tube's cathode resistor and capacitor? It seems to me that (if feedback is not being used) it would be better to leave the 220 ohm resistor out, and instead connect the cathode resistor and capacitor directly to ground. What are your thoughts on this?
Thanks in advance.
-Scott

 

[alk509]

It's still part of the cathode biasing arrangement, feedback or no feedback. I guess you could put a 1.78 Ohm resistor in place of the 1.6K one, and put a jumper in place of the 220 Ohm... but why bother?

Peace,
Al.

 

[Gus]

With the 220 ohm and not using the feeback from the transformer you still have a little negative feedback.

With a jumper for the 220 ohm the circuit will be as close to no feedback as possable and have a little more gain.

 

[Jazzy_Pidjay]

if i remove the 220
and replace the 1,6k with a 1,78k

i obtain more output gain ??

i have correctly understand :?:

 

[scott_humphrey]

[quote author="Jazzy_Pidjay"]if i remove the 220
and replace the 1,6k with a 1,78k

i obtain more output gain ??

i have correctly understand :?:[/quote]

In general, as you decrease the value of the cathode resistor you'll increase the gain. The tube will be self-biasing with a cathode-resistor-biasing configuration. The tube should work with a range of cathode resistor values, but the gain and frequency response will be affected.

What I'm not sure about is the effect of connecting the cathode resistor's bypass capacitor to the 220 ohm resistor vs. connecting it directly to ground. Maybe someone can explain the effect that this would have.

 

[Jazzy_Pidjay]

anybody ?

 

[Guest]

If you essentially change the 1k6+220=1k82 with a 1k78 you will get a very slight increase in gain due to the little bit of extra current you can get through the cathode, but (without doing the math) it will be negligable at these operating conditions.

 

[Gus]

One resistor 1.6K to 1.8K bypassed will be fine jumper the 220 spot.

The 220 ohm is there only for the transformer feedback I believe.

When the cathode bias resistor is bypassed with a big enought cap value, the resistor value is just the cathode bias setting resistor. -1 to -1.8 volts should be fine for bias of the tube.

This is a cathode bias circuit. Read about fixed and cathode bias. Look for thing like stablity with the tube aging etc. Noise and Gain for hints.

 

[Jazzy_Pidjay]

ok it's almost clear now !!

thanks :thumb:

 

[hitchhiker]

I think I like my G7 better with the 220r bypassed.

Is this a harder on the tube?

Lance

 

[scott_humphrey]

[quote author="hitchhiker"]I think I like my G7 better with the 220r bypassed.

Is this a harder on the tube?

Lance[/quote]

It shouldn't matter.

 

[Gus]

hitchhiker

Look at my post above.

Like Scott posted it should not matter to the tube. yes it will run with maybe a little more current but not much.

Look at the neumann schematics like the u67 you can find on the web and calculate the current in the plate section. The voltage drop across the plate R / by the plate R or even the voltage across the cathode r and / by the cathode r(basicly no grill current so this is close)

Most tube microphones seem to be under 1ma.

 

[hitchhiker]

thanks Scott and Gus.
I'll leave the 220r out. I like the way the sibilant
frequency sounds to go up a bit. With the 220 in, the M7 capsule sibilance was a bit spitty ,but not as bad as the MT711 fet circuit though. With the sibilance at a higher frequency it's clear and crisp.

cheers,
Lance

 

[gyraf]

I'll leave the 220r out.

You'll need to replace it with a jumper..

Jakob E.