Grid leak resistors with fixed bias

Carbsandcaps

Carbsandcaps

Super Member
I’m working on a stereo organ 7591 pp amplifier. I’ve converted it to el34 because I’m not paying the ridiculous prices for 7591’s and I had el34’s laying around. It’s fixed bias and I’m trying to get the grid leak resistors nailed down. It originally had 150k resistors straight from the negative voltage supply going to the 1k ohm grid stoppers with no
Adjustments. I put 50k pots in place of the fixed resistors. I got the bias down but am now trying to figure out the grid leak resistors. Do they go between the stopper and the pot or do the go from the stopper to ground? If they go through the pot I’m guessing the pot becomes part of the circuit so it will have to be factored in. Also I can’t for the life of me find the grid leak values on any data sheet I look at. Anyone know the el34 fixed bias grid resistor value?
 
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700k, but I'd probably be more comfortable running it conservative at 500k. Or realistically, 470k since thats a common value.

keep in mind you'll need about double the drive signal for an EL34 vs a 7591. A lot of 7591 circuits don't have that much extra available for voltage swing. Bias voltage also will need to be about double.
 
Ok thanks. The bias is just about maxed out. Should the resistor go from grid stopper to ground or in between grid stopper and the potentiometer?
 
Grid stoppers are typically around 1k and sit between the coupling cap and the grid terminal of the tube socket, physically as close to the tube as possible. The grid leak resistors run from that junction to the bias supply.

example output stage

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The circled 330k resistors come from the bias supply. The 1K resistors are your grid stoppers.
 
gadget73 said:
Grid stoppers are typically around 1k and sit between the coupling cap and the grid terminal of the tube socket, physically as close to the tube as possible. The grid leak resistors run from that junction to the bias supply.

example output stage

View attachment 3314819

The circled 330k resistors come from the bias supply. The 1K resistors are your grid stoppers.
Click to expand...

And then they get the ground reference through the bias adjustment pots correct?
 
Carbsandcaps said:
Ok thanks. The bias is just about maxed out. Should the resistor go from grid stopper to ground or in between grid stopper and the potentiometer?
Click to expand...
To minimize the lead length, manufacturers often put the grid stopper and screen resistors across an unused pin to the signal grid or screen. For 6L6GC tubes pins 1 and 6 are unused --- so they tied the grid stopper from pin 1 to pin 5 (grid). And the screen resistor from pin 6 to pin 4 (screen).

Grid%20stopper%20and%20screen%20resistors%20-%20AB763%20mod_zpsuq5yqdau.jpg
 
Maybe you guys can help me with the phase splitter. I’ve tried to draw it the best I can which is still pretty bad. What I don’t understand is why the cathode in the second half feeds partially back into the grid. Is this some kind of feedback? Also what does the 300k capacitor do that also feeds into the grid on triode 2? IMG_6012.jpeg
 
It doesn't feed back, it biases off the cathode.

Thw 300pf in series with the resistor is a low pass filter for stability.
 
What you're drawing is the lower right. The grid resistor taps out of the cathode resistor chain to set the grid to cathode voltage difference. With a 12ax7 its probably about 1 volt. Thats the bias. Its also the path to ground, through the cathode resistor.


If the grid resistor went to ground no current would flow, or at least not enough to be useful.
 
Oh ok. I just looked at it again, I missed the ground on my amazing drawing. I couldn’t figure out where the cathode got it’s ground. IMG_0769.jpeg
 
yep, that'll do it. I didn't notice specifically but I knew what you were going for there. Its just drawn kinda upside-down.

The other way to do this is direct coupled from the first stage, but that involves getting the plate voltage of the first tube to the right spot where it biases the inverter triode correctly. Eliminates a phase shift point but requires a little more care with the design.

With tubes that have a fair bit less gain I suspect you may need to re-jigger that first stage to get more gain out of it. Or maybe it needs to be a pentode/triode tube to get the job done. Basically Dyna ST-70 type front end.
 
gadget73 said:
yep, that'll do it. I didn't notice specifically but I knew what you were going for there. Its just drawn kinda upside-down.

The other way to do this is direct coupled from the first stage, but that involves getting the plate voltage of the first tube to the right spot where it biases the inverter triode correctly. Eliminates a phase shift point but requires a little more care with the design.

With tubes that have a fair bit less gain I suspect you may need to re-jigger that first stage to get more gain out of it. Or maybe it needs to be a pentode/triode tube to get the job done. Basically Dyna ST-70 type front end.
Click to expand...
I suppose I could copy the st-70 7199 driver schematic. I’m using lower voltages though so might take some work
 
Well now you got my brain going. Since the 12ax7 probably won’t drive the el34’s correctly and I’ve got some 6gh8a’s laying around I think I’m going to go the st-70 direction.
 
Is there a reason you didn't want to use 6L6GC here? Plate loading and filament current requirements are the same as the 7591, whereas the EL34 is very different on both counts.
 
^^ extremely true. Probably should have mentioned that.

both have similar bias and drive requirements too.
 
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