Thanks for the WE data. The lowest 3rd harmonic is seen in the higher primary values, as expected. The only exceptions are at lower HT voltages.
But to be serious, I remember (and can point to) numerous discussions on the issue of blocking distortion with high values of grid resistance.
Only because the driver could not handle the load...
And if you love your 300b's don't let them run away...
What makes an output tube "run away"
And if you love your 300b's don't let them run away...
What makes an output tube "run away"
Last edited:
Why? :-o
This 300B settings is foolproof, the driver is separated by capacitor.
There is no chance to runaway if 300B is healthful.
This 300B settings is foolproof, the driver is separated by capacitor.
There is no chance to runaway if 300B is healthful.
Grid Current is one of the most poorly understood topic and so at the risk of teaching grandma to suck eggs I've written a little essay.
Grid Current and all that – remember that, due to historic limited knowledge, current flow is defined as from positive to negative whereas in actual fact electrons flow from negative to positive.
There are 3 types of grid current
Current flowing into the grid is known as POSITIVE grid current
• When the cathode is heated a cloud of electrons forms around the cathode known as a “space charge”
• Some of these electrons gather at the grid. These electrons then flow out of the grid which is the same as saying that current flows into the grid.
• This positive grid current generates a voltage across the grid leak resistor (Rg1)
• This voltage makes the grid more negative which ADDS to the bias
• If the grid leak resistor is large enough then this positive grid current can generate the entire required bias – This is known as “Grid Leak Bias”
POSITIVE grid current is a low level phenomenon and can easily be overshadowed by NEGATIVE grid current
I copied this from this thread here :
Amplifiers that exceed grid leak resistance
Grid Current and all that – remember that, due to historic limited knowledge, current flow is defined as from positive to negative whereas in actual fact electrons flow from negative to positive.
There are 3 types of grid current
Current flowing into the grid is known as POSITIVE grid current
• When the cathode is heated a cloud of electrons forms around the cathode known as a “space charge”
• Some of these electrons gather at the grid. These electrons then flow out of the grid which is the same as saying that current flows into the grid.
• This positive grid current generates a voltage across the grid leak resistor (Rg1)
• This voltage makes the grid more negative which ADDS to the bias
• If the grid leak resistor is large enough then this positive grid current can generate the entire required bias – This is known as “Grid Leak Bias”
POSITIVE grid current is a low level phenomenon and can easily be overshadowed by NEGATIVE grid current
I copied this from this thread here :
Amplifiers that exceed grid leak resistance
Last edited:
So on some further investigation 100k seems a more acceptable value and should have little impact sonically. Do you agree?
"I copied this from this thread here :"
... where:
- fixed bias
- guitar amps,
- ignore the recommended maximum Rg1 values;
etc.
Apples and oranges.
I used 300B for decades, haven't experienced runaway in cathode biased amplifier stage ... if driver stage and PSU is working properly.
... where:
- fixed bias
- guitar amps,
- ignore the recommended maximum Rg1 values;
etc.
Apples and oranges.
I used 300B for decades, haven't experienced runaway in cathode biased amplifier stage ... if driver stage and PSU is working properly.
@ Lampie519
You did not "write your essay", but simply copied it from the thread you mentioned (which you confirmed by editing your post).
When you read that thread to the end, the phenomenon of blocking distortion pops up.
It can happen when the 300B is driven into class A2, causing grid current.
In that case a high value grid resistor can cause blocking distortion.
You mentioned only one type of grid current (positive grid current).
The two other types are negative grid current and grid rectification current.
It's the last type I refer to wrt blocking distortion.
You did not "write your essay", but simply copied it from the thread you mentioned (which you confirmed by editing your post).
When you read that thread to the end, the phenomenon of blocking distortion pops up.
It can happen when the 300B is driven into class A2, causing grid current.
In that case a high value grid resistor can cause blocking distortion.
You mentioned only one type of grid current (positive grid current).
The two other types are negative grid current and grid rectification current.
It's the last type I refer to wrt blocking distortion.
Last edited:
You can look it up in the data. WE states maximum grid leak is 250K in self bias. You should be OK with 220K in self bias. Helps the driver a little.
https://frank.pocnet.net/sheets/084/3/300B.pdf
Yes.
But C3m driver in this schematic remains the most limiting factor:
- high impedance;
- limited driving capacity,
- too much gain, so sensitive to disturbances;
- too high distortion (compared to good triodes/trioded pentodes);
- too much power requiring for biasing.
I would rather use trioded C3g, which have enough low output impedance, good driving capacity, enough low distortion even at high swing.
Attachments
Agree with the C3g vs C3m issue, but cannot you lower the gain with the appropriate anode resistor?
Actually I guess that driving capacity of C3m should be good enough (current).
Actually I guess that driving capacity of C3m should be good enough (current).
Why not use a plate choke instead of an anode resistor if your driver is in triode? As you increase the current going through the stage, the HT voltage required for a decent value of resistor climbs to unrealistic levels. With a plate choke you can use a current of 15mA or more.
In addition, with a higher current you stand some chance of using an unbypassed cathode resistor of 100-200R if your plate choke has plenty of inductance. A humble Hammond 126C has 15mA gap and 106H inductance. Even an even more humble 157G - used in the Hagerman headphone amp - has 30H inductance and 40mA gap.
In addition, with a higher current you stand some chance of using an unbypassed cathode resistor of 100-200R if your plate choke has plenty of inductance. A humble Hammond 126C has 15mA gap and 106H inductance. Even an even more humble 157G - used in the Hagerman headphone amp - has 30H inductance and 40mA gap.
Last edited:
Internal resistance in tetrode mode 250k. :-(
If you decrease load, the distortion will be increasing (even at 220k->100k -300B- grid leak change).
If you decrease load, the distortion will be increasing (even at 220k->100k -300B- grid leak change).