Do you have 4 bias trim pots? One for each tube? With the KT88's not installed, what range of DC voltages do you get at pin 5 at each end of the bias pot rotation? We need the min and max voltages at pin 5.
Yes, you need to modify the bias circuit slightly. Conceptually, you need to move the pot toward ground a little more to lower the voltage out of the wiper.
On another note, one end of each 10R cathode R's shown in your schema are connected together........is this the case?
Here is the math to determine the bias circuit changes:
I don't know the R value of the bias pot, but I know the voltage drop across the 22K R to ground at the min bias pot wiper setting is 45V.
Using ohm's law, I get I=E/R=45V/22K=2.04ma, which is reasonable.
I also know that the voltage drop across the pot is 60V-45V=15V, so using ohm's law again I get R=E/I=15V/2.04ma=7.35K....I'm assuming that your pot is 7.5K.
The total R from 60V to ground is now 7.5K+22K=29.5K. Checking the math, calculate the current again across the whole R string I=E/R=60V/29.5K=2.033ma close enough (the difference is due to the 7.5K to 7.35K assumption).
So we want to keep the total R from the 60V to ground around 30K or so, but move the pot lower in the chain. So let's split the 22K and put some of that R above the pot and leave some below.
Let's try 60V-10K-7.5K pot-15K
Calculate the current thru the R's. Total R is 10K+7.5K+15K=32.5K I=E/R=60V/32.5K=1.84ma.
Calculate voltage drop across the upper 10K R E=IR=1.84ma x 10K=18.4V
Calculate voltage drop across the pot 7.5K E=IR=1.84ma x 7.5K=13.8V
Calculate voltage drop across the 15K R E=IR 1.84ma x 15K=27.6V
Sanity check: add the voltage drops to equal 60V 18.4V+13.8V+27.6V=59.8V, close enough.
So the new pot range is 60-18.4V=41.6V to 27.6V, a good range for EL34's.
On another note, one end of each 10R cathode R's shown in your schema are connected together........is this the case?
Here is the math to determine the bias circuit changes:
I don't know the R value of the bias pot, but I know the voltage drop across the 22K R to ground at the min bias pot wiper setting is 45V.
Using ohm's law, I get I=E/R=45V/22K=2.04ma, which is reasonable.
I also know that the voltage drop across the pot is 60V-45V=15V, so using ohm's law again I get R=E/I=15V/2.04ma=7.35K....I'm assuming that your pot is 7.5K.
The total R from 60V to ground is now 7.5K+22K=29.5K. Checking the math, calculate the current again across the whole R string I=E/R=60V/29.5K=2.033ma close enough (the difference is due to the 7.5K to 7.35K assumption).
So we want to keep the total R from the 60V to ground around 30K or so, but move the pot lower in the chain. So let's split the 22K and put some of that R above the pot and leave some below.
Let's try 60V-10K-7.5K pot-15K
Calculate the current thru the R's. Total R is 10K+7.5K+15K=32.5K I=E/R=60V/32.5K=1.84ma.
Calculate voltage drop across the upper 10K R E=IR=1.84ma x 10K=18.4V
Calculate voltage drop across the pot 7.5K E=IR=1.84ma x 7.5K=13.8V
Calculate voltage drop across the 15K R E=IR 1.84ma x 15K=27.6V
Sanity check: add the voltage drops to equal 60V 18.4V+13.8V+27.6V=59.8V, close enough.
So the new pot range is 60-18.4V=41.6V to 27.6V, a good range for EL34's.
Hi ,
i got -60vdc min bias and -30 vdc max bias voltage by changing 23k resistor with 15 k resistor .
now i can run both el34 and kt88 in my amps .
Thanks a lot for your help .
i got -60vdc min bias and -30 vdc max bias voltage by changing 23k resistor with 15 k resistor .
now i can run both el34 and kt88 in my amps .
Thanks a lot for your help .
Just noticed this thread.
Thought it worthwhile to point out that you may wish to modify the circuit somewhat. Failure mode of potentiometers is often for the wiper to lose contact. If this happens you will have 0V bias, which the tube will not like!
If you place a resistor between the wiper and negative supply of pot, if the wiper lifts, you will get a high negative voltage at the grid, saving your tubes.
look at the bias circuit of this guitar amp for example... http://www.ax84.com/static/corepoweramps/20W_PP/AX84_20W_PP_Poweramp_Schematic.pdf
Thought it worthwhile to point out that you may wish to modify the circuit somewhat. Failure mode of potentiometers is often for the wiper to lose contact. If this happens you will have 0V bias, which the tube will not like!
If you place a resistor between the wiper and negative supply of pot, if the wiper lifts, you will get a high negative voltage at the grid, saving your tubes.
look at the bias circuit of this guitar amp for example... http://www.ax84.com/static/corepoweramps/20W_PP/AX84_20W_PP_Poweramp_Schematic.pdf
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