50kΩ is correct for AF, but applies when the anode power dissipation is 36W. The R value can be scaled higher in proportion to a reduced power burn in the anode.
A DC-coupled follower can improve 300B grid-bias stages, and can greatly reduce the risk of blocking distortion, compared to RC coupled endstages.
A DC-coupled follower can improve 300B grid-bias stages, and can greatly reduce the risk of blocking distortion, compared to RC coupled endstages.
I would actually drop the anode current by 5-10mA. As is, it's on the high side for a 5.2K load and Class A1 operation. One just gets less power. With 420V anode voltage and 5.2K it could get easily 9-10W at 3% THD.
IME, the only reason for running it at 80-85mA with 5.2K is that the actual tube is less linear than the boogey.
IME, the only reason for running it at 80-85mA with 5.2K is that the actual tube is less linear than the boogey.
The schematic in #38 and other parameters in posts are differs from each other.
The amp schematic shows 3k5/5k OPT, but in the pics only 3k5 presents.
The amp schematic shows +420V, the PSU schematic +400V, the measured voltage is 396V.
If you use +396V, the suspected anode voltage about 386V.
Ua= 396-DCR of OPT*Ia (for example DCR:125R, Ia:80mA)
The fixed bias voltage is -87V on schematic, Ia 80mA ... these are not fulfilled at B+=396V.
If you use 300B at 386V, 80mA operating point (bias about -78V), the expected power -up to A1- with 3k5 OPT about 7W at 5% distortion.
http://trioda.com/tools/triode.html
The amp schematic shows 3k5/5k OPT, but in the pics only 3k5 presents.
The amp schematic shows +420V, the PSU schematic +400V, the measured voltage is 396V.
If you use +396V, the suspected anode voltage about 386V.
Ua= 396-DCR of OPT*Ia (for example DCR:125R, Ia:80mA)
The fixed bias voltage is -87V on schematic, Ia 80mA ... these are not fulfilled at B+=396V.
If you use 300B at 386V, 80mA operating point (bias about -78V), the expected power -up to A1- with 3k5 OPT about 7W at 5% distortion.
http://trioda.com/tools/triode.html
Grid current.if you drive the 300b grid above 0 volts,into positive bias,that grid current has to flow through the bias resistor which creates a vd across the bias resistor altering the operating point of the 300b.thats why driving a dht with a low source ac impedance and dc resistance is benifical such as directly connected cathode follower or I.T transformer. There is also the charge generated on that coupling cap which also creates bias instability.
You need to compare on the same basis. I don't know that particular amplifier but most Jap 300B amps use 2.5-3.5K OPT at lower plate dissipation than yours.
If you have barely 400V anode voltage and run it at 80-85mA with 5.2K load with stiff fixed bias, there less room left on the positive grid side and your driver will likely distort more. Grid current flows before it gets positive and you have mainly two issues: 1) lower input impedance of the 300B (roughly "delta" Vgk over "delta" Ig, so can be some 30K-50K already for small grid current of the order of 0.1 mA before the grid voltage actually gets positive!), 2) blocking distortion kicks off because of RC coupling.
I am not even sure what your servo-bias does in the presence of grid current. Grid current is a reason for limiting grid resistors. With cathode bias the tube can "manage" itself. Stiff fixed bias + RC coupling is the worst combination, IME.
If you want to keep that stiff fixed bias, try setting the anode current of the 300B lower than 80 mA if use 5.2K. Find the best spot for your amplifier. Can also try to reduce the grid resistor to 100K.
Don't be stuck on generic numbers, other amplifiers and forget simulators (they are not reliable to assess distortion just because your actual tube might have curves that don't quite match with the "typical" ones). Generally speaking, 70mA is more than enough to get 12W into that load at 5% THD. Your real limit is set by linearity and anode voltage, IMO.
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Your assumption is wrong because up to 8.2w there can be no grid current, I have carefully monitored the driver distortion and it is clearly visible but even up to 8.5w the driver resists instead the end tube passes a THD >5% so I believe that up to 7.8w the end tube works in class A and only above this value enters A1. The amplifier has all voltages stabilised and the anode voltage is 420v, the driver allows 20-30% more attack than required for 8w power and the servo system works normally. I think under certain circumstances fixed and auto negative do the same thing and are just options, my opinion...
Yes, it's grid current - but leakage current from the grid (stemming from gas corruption of the vacuum, or unwanted emission from the grid).
If there is some gas in your 300Bs, the cathode emission can collide with gas molecules and displace electrons from them. THe gas molecule, now a positive ion, is drawn to the grid, and a (small) current is established.
This current passes through the Grid-Leak resistor, and creates a voltage. This voltage biases the valve hotter - more anode current, and in turn, more collisions, more grid volatge.... and so on until - if the gas was bad enough to begin with - the anode current runs out of control and damages the valve through overheating.
With autobias, this is much less of a problem, since the cathode resistor acts against the increased bias. And if you are not running the valve at max current or power, there is more safety margin in the first place; both of these considerations allow the use of a proportionately higher grid leak resistor.
If there is some gas in your 300Bs, the cathode emission can collide with gas molecules and displace electrons from them. THe gas molecule, now a positive ion, is drawn to the grid, and a (small) current is established.
This current passes through the Grid-Leak resistor, and creates a voltage. This voltage biases the valve hotter - more anode current, and in turn, more collisions, more grid volatge.... and so on until - if the gas was bad enough to begin with - the anode current runs out of control and damages the valve through overheating.
With autobias, this is much less of a problem, since the cathode resistor acts against the increased bias. And if you are not running the valve at max current or power, there is more safety margin in the first place; both of these considerations allow the use of a proportionately higher grid leak resistor.
You mean, that B+: 420V, 5k2 OPT the commercial, capacitor coupled 300B at 8.2W in A1 mode?
I would be very surprised.
This is not about being wrong or right. You asked why the grid resistor should be limited.
Your driver is a mu-follower and it can work fine into a lower load if the grid current is not too much however the grid current will also charge the coupling capacitor. You have to measure the grid current to know precisely.
Once you are certain you have no grid current then the problem is your 300B. It can be better than that. Maybe you simply don't have the best samples of 300B. It's not the brand, it is what you have in hand. And it doesn't take much to adjust the bias for 70 mA. Aren't you curious?
OPT is not commercial but is designed and made by me and the occurrence of grid current may be due to the voltage excursion in the driver where for positive alternation this voltage is Ug=0 or higher for the Ua-Ug characteristic of the 300b tube. In fact the driver supports a small grid current without significant distortion i.e. it accepts light input in class A1 however the amplifier is only built for class A.You mean, that B+: 420V, 5k2 OPT the commercial, capacitor coupled 300B at 8.2W in A1 mode?
I would be very surprised.
