EL34 or 6CA7... and bias question

[catalin gramada]

You are misunderstanding me I think. I am only talking about Fixed Bias operation.

K, let me to understand , please.
You said: "G3 in an EL34 is better tied to the negative bias supply rather than ground." but in a fixed bias amp the bias supply is applied to G1 control grid through the grid leak resistor, whatever is. The G3 will not follow the lowest impedance path to prevent excedentary emission to bounce I to the screen, so I wonder why you doing so? Can you send me to any power circuit which use it to try to understand why is doing so ? Thanks

 

[BassmanPaul]

Re read post 15.

 

[catalin gramada]

Re read post 15.

Allright, I did. That meant also tied to ground as time is tied to the big decoupling cap. But still don't get it why? It is something you discovered yourself it works better than normal aplication? I asked cause in years as electronic hobbyist I didn't found this configuration. Does it have any benefit for tube operation or is something it sounds better for some of you particular circuit? Pretty curious about any commercial amps which used before.

 

[catalin gramada]

Re read post 15.

Short research and found one circuit made by Traynor (yba-3) using negative voltage to g3 connection . Can you tell me please screen grid current will be affected? How much, relative to g3 to cathode ? Do you know if g3 will start to draw cathode current? Did you remarked any change in voltage into bias circuit in those two situations, please? Still don't get what is ideea behind. Thanks.

 

[BassmanPaul]

Sadly I can’t help you more. While tubes were still current technology when I went to school, three strokes have erected a barrier which only goes down when it feels like!

 

[Wasnex]

Short research and found one circuit made by Traynor (yba-3) using negative voltage to g3 connection . Can you tell me please screen grid current will be affected? How much, relative to g3 to cathode ? Do you know if g3 will start to draw cathode current? Did you remarked any change in voltage into bias circuit in those two situations, please? Still don't get what is ideea behind. Thanks.

If G3 is negative and the cathode is tied to ground (0V), Electrons cannot not flow from cathode to G3. When electrons hit the plate they tend to bounce, the suppressor basically reflects them back to the plate so they can be soaked up. In many amps G3 is hooked up to the cathode and both are typically at ground potential. By making G3 more negative, the force pushing the derelict electrons back to the plate is strengthened. So this should reduce screen current. Suppressor grid - Wikipedia

Keep in mind different tubes have different internal connections and different operating characteristics, and some modern tubes do not comply with historical tube specs. So research and some degree of caution is advised when you are swapping tubes to make sure the circuit will behave as expected.

 

[Wasnex]

Short research and found one circuit made by Traynor (yba-3) using negative voltage to g3 connection . Can you tell me please screen grid current will be affected? How much, relative to g3 to cathode ? Do you know if g3 will start to draw cathode current? Did you remarked any change in voltage into bias circuit in those two situations, please? Still don't get what is ideea behind. Thanks.

I can't tell you how strong the relationship is between G3 voltage and tube current, but I believe such a relationship exists.

It would be advisable to check bias after the mod is done. A tech could bring the amp up on a variac and monitor tube current.

For someone without a variac, I would suggest setting the bias for minimum tube current before the mod; then resetting bias after the mod is complete.

 

[agedhorse]

I would certainly monitor for bias stability under a variety of conditions, especially if the impedance of the bias supply is not low.

 

[Wasnex]

Regarding checking the bias yourself. I too recommend you avoid the plate methods as they exposes you to high voltage and more risk of electrocution.

Here is a page that provides lot's of info: The Last Word On Biasing

The easiest and least destructive way is to buy a bias probe. These work by inserting an adapter between the tube and tube socket. All of the pins are typically wired pin to pin between the top and bottom of the adapter, except the cathode.

The cathode circuit is broken between the pins. There are two options here. Some probes insert a milliammeter in the cathode circuit, and some insert a one or ten ohm precision resistor. With the milliammeter you measure the current directly, and with the resistor you measure you the voltage drop across the resistor and then use ohms low to derive the current.

Ohms Law E=IR where E is voltage, I is current, and R is resistance. Solving for I gives you I=E/R

If R = 1 ohm, I = E...so 18mV measured across the bias sense resistor indicates 18mA of cathode current. (0.018/1=0.018)

If R = 10 ohms, I = E/10 or E*0.1...so 180mV measure across the bias sense resistor indicates 18mA of cathode current. (0.18/10=0.018) or (0.18x0.1=0.018)

I have a fairly large collection of amps. Some have built in bias sense resistors and others have the cathodes wired directly to ground like your amp. I bought set of bias probes rather than installing resistors in various amps.

Since you only are concerned with the one amp, it might make sense to have 1 Ohm bias sense resistors installed. However it won't really do you much good unless a variable bias mod is installed. Without such a mod, you will still need to send the amp to a tech to get the bias adjusted.

 

[catalin gramada]

If G3 is negative and the cathode is tied to ground (0V), Electrons cannot not flow from cathode to G3. When electrons hit the plate they tend to bounce, the suppressor basically reflects them back to the plate so they can be soaked up. In many amps G3 is hooked up to the cathode and both are typically at ground potential. By making G3 more negative, the force pushing the derelict electrons back to the plate is strengthened. So this should reduce screen current. Suppressor grid - Wikipedia

Keep in mind different tubes have different internal connections and different operating characteristics, and some modern tubes do not comply with historical tube specs. So research and some degree of caution is advised when you are swapping tubes to make sure the circuit will behave as expected.

That I assume aso

If G3 is negative and the cathode is tied to ground (0V), Electrons cannot not flow from cathode to G3. When electrons hit the plate they tend to bounce, the suppressor basically reflects them back to the plate so they can be soaked up. In many amps G3 is hooked up to the cathode and both are typically at ground potential. By making G3 more negative, the force pushing the derelict electrons back to the plate is strengthened. So this should reduce screen current. Suppressor grid - Wikipedia

Keep in mind different tubes have different internal connections and different operating characteristics, and some modern tubes do not comply with historical tube specs. So research and some degree of caution is advised when you are swapping tubes to make sure the circuit will behave as expected.

Yes, but this "bounce" will have directly two consequences, in my understanding as: push more current to screen grids as time the anode current will be limited (not good), strong limiters screen necessary to keep operation to a safe mode determine suppressor grid currents (also not good). Both possible and probable as time the tube current is determined by bias and screen grid voltage. This are just my thoughts only. So, in this conditions what is the benefit which this connection to negative supply coming from?

 

[catalin gramada]

Regarding checking the bias yourself. I too recommend you avoid the plate methods as they exposes you to high voltage and more risk of electrocution.

Here is a page that provides lot's of info: The Last Word On Biasing

The easiest and least destructive way is to buy a bias probe. These work by inserting an adapter between the tube and tube socket. All of the pins are typically wired pin to pin between the top and bottom of the adapter, except the cathode.

The cathode circuit is broken between the pins. There are two options here. Some probes insert a milliammeter in the cathode circuit, and some insert a one or ten ohm precision resistor. With the milliammeter you measure the current directly, and with the resistor you measure you the voltage drop across the resistor and then use ohms low to derive the current.

Ohms Law E=IR where E is voltage, I is current, and R is resistance. Solving for I gives you I=E/R

If R = 1 ohm, I = E...so 18mV measured across the bias sense resistor indicates 18mA of cathode current. (0.018/1=0.018)

If R = 10 ohms, I = E/10 or E*0.1...so 180mV measure across the bias sense resistor indicates 18mA of cathode current. (0.18/10=0.018) or (0.18x0.1=0.018)

I have a fairly large collection of amps. Some have built in bias sense resistors and others have the cathodes wired directly to ground like your amp. I bought set of bias probes rather than installing resistors in various amps.

Since you only are concerned with the one amp, it might make sense to have 1 Ohm bias sense resistors installed. However it won't really do you much good unless a variable bias mod is installed. Without such a mod, you will still need to send the amp to a tech to get the bias adjusted.

Mounting an low watage, say 1/2w as bias resistor in cathode circuit get some other advantage as it can really act as a protection for "catastrophic failure" without to affect nothing in functionality of circuit, as is a general trend.

 

[Wasnex]

That I assume aso

Yes, but this "bounce" will have directly two consequences, in my understanding as: push more current to screen grids as time the anode current will be limited (not good), strong limiters screen necessary to keep operation to a safe mode determine suppressor grid currents (also not good). Both possible and probable as time the tube current is determined by bias and screen grid voltage. This are just my thoughts only. So, in this conditions what is the benefit which this connection to negative supply coming from?

Suppressor is between the screen and plate. Without the suppressor, electrons that bounce off the plate will be absorbed by the screen. The role of the suppressor is to mitigate this problem. I have seen comments that suggest making the suppressor more negative will increase screen current, but I think this is wrong.

I believe if you can understand this page you will come to the same conclusion: Electrons in Screened Grids and Pentodes

 

[catalin gramada]

Suppressor is between the screen and plate. Without the suppressor, electrons that bounce off the plate will be absorbed by the screen. The role of the suppressor is to mitigate this problem. I have seen comments that suggest making the suppressor more negative will increase screen current, but I think this is wrong.

I believe if you can understand this page you will come to the same conclusion: Electrons in Screened Grids and Pentodes

The problem is a lot of electrons will be repelled but the tube current (cathode currents t) will be almost the same because it is determined by the bias and screen grid voltage. What is happen with this excess ? Part of bounced electrons will be aglomerrate to screen grids determining more traffic current. This seems to be very plausible.
Of course this a possible scenario, nothing sure....how simple is to do a quick test...but never meet it a pentode tied as so before...BassmanPaul was the first guy heard talking about...

 

[Wasnex]

The problem is a lot of electrons will be repelled but the tube current (cathode currents t) will be almost the same because it is determined by the bias and screen grid voltage. What is happen with this excess ? Part of bounced electrons will be aglomerrate to screen grids determining more traffic current. This seems to be very plausible.

I believe you will find that the voltage on the all of the grids has some control over cathode current, however the suppressor voltage has the least control.

I don't know how to say this more clearly. The suppressors job is to reflect the secondary emissions from the plate away from the screen and back to the plate. If the suppressor was missing or disconnected, the secondary emissions would pass on to the screen grid. Making the suppressor more negative should make it more effective.

Keep in mind that when electrons are flowing from cathode to plate they are shaped into a pattern. The grid of the suppressor has little impact on the electrons as they make their initial journey. Secondary emissions are a different matter. They are spread out and the suppressor is designed to prevent them from going where they should not go.

Go through this document Invalid Link Removed , search for the term "suppressor", and read.

 

[catalin gramada]

I believe you will find that the voltage on the all of the grids has some control over cathode current, however the suppressor voltage has the least control.

I don't know how to say this more clearly. The suppressors job is to reflect the secondary emissions from the plate away from the screen and back to the plate. If the suppressor was missing or disconnected, the secondary emissions would pass on to the screen grid. Making the suppressor more negative should make it more effective.

Keep in mind that when electrons are flowing from cathode to plate they are shaped into a pattern. The grid of the suppressor has little impact on the electrons as they make their initial journey. Secondary emissions are a different matter. They are spread out and the suppressor is designed to prevent them from going where they should not go.

Go through this document Invalid Link Removed , search for the term "suppressor", and read.

Thanks