More Fun With Magnavox: The 9300 Series

Thank you all for your concern.
Normal voltage in my location is 114V, 107V after the evening.
After the boost winding the dc voltage from:
375V 370V 340V <-Dave standard
305V 304V 288V <-no boost winding
312V 310V 300V <-boost winding
The same result when replacing JJ 5ar4 rectifier.

It still can't adjust over 0.32Vdc,Is there any other way?I am not an electronics engineer,Can't fix it by myself.
Slightly asked, if I run 0.32Vdc BIAS, in addition to the power reduction, does it affect my vacuum tube?
 
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Its probably just the slightly low voltage. If the output tubes or the rectifier are tired, that would also do it.

Just to confirm, you're using the 5AR4 / GZ34 rectifier right? The stock 5U4GB will drop too much voltage.

If the tubes are known to be in good shape, try changing the 15K resistor from the bias adjustment pot to ground to a 22K and see if that will make it go.
 
If I may, it's the other way around. To move the range of the bias control in an upward current direction, the 15K resistor should be reduced to about 10K. Doing so will cause there to be less cathode voltage, and therefore cause greater current flow through the tubes. If the resistor is raised to 22K, it will allow more cathode voltage -- which is more bias voltage -- causing the current flow to drop even further.

It is easy to get confused, thinking that the circuit operates like conventional cathode bias. It is cathode bias in that the bias voltage appears at the cathode, but after that, all similarities end. With a cathode bias resistor, a greater cathode bias voltage equates to greater current flow -- all else being equal. But this circuit is not a conventional cathode bias circuit. It is truly a fixed bias circuit, so that more bias voltage equates to less current flow -- exactly as occurs in any conventional fixed bias design. To elevate the quiescent current then, bias voltage needs to be reduced, meaning that the 15K resistor should be reduced to 10K.

I hope this helps!

Dave
 
Use the 5AR4 / GZ34 rectifier or 5U4GB got the same 310V and 0.32v BIAS.
I will change 15K to 10K after work.

Hi Dave,
Another question I read about EFB regulator from here:
http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf
If I want to provide individual bias control for each channel in the 9302,
Is this circuit correct? If the two channels are unbalanced and there are no matching tubes.
 

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Yes -- That is the correct circuit.

Dave
I'm intrigued by this, but not sure I understand. In order to have individual bias control for the two channels, would you need to install a pair of LM337s?
If not, how would you adjust them separately?
And if I were to follow Koi's circuit diagram, do I still connect the two channels as indicated?
Also, I seem to remember that you specified a 2amp fast blow fuse for the amp is that correct? I'm in the guitar world usually, where slo-blo is the rule.
 
I think it should be just one LM337 and two 2k VR.

Situation
LFT_CH RT_CH
0.44vdc 0.48vdc Turn right 2k VR to make the right bias value to 0.44vdc
0.51vdc 0.44vdc Turn left 2k VR to make left bias value to 0.44vdc
0.31vdc 0.40vcd Turn 2k VR on any side to make same bias values on both sides, then adjust 5k bias for 0.44vdc
 

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right, one regulator but it has a "feedback" of sorts for the adjustment pots. It tickles some positive cathode voltage back into the grid in order to allow adjustment.
 
I think it should be just one LM337 and two 2k VR.

Situation
LFT_CH RT_CH
0.44vdc 0.48vdc Turn right 2k VR to make the right bias value to 0.44vdc
0.51vdc 0.44vdc Turn left 2k VR to make left bias value to 0.44vdc
0.31vdc 0.40vcd Turn 2k VR on any side to make same bias values on both sides, then adjust 5k bias for 0.44vdc
Thank you, that makes sense.
One more question: in this circuit diagram the LM337s output goes TO EFB output, while in Dave's previous schematic the LM337s output goes to Other Channel. Is that the same thing?
Also; slo-blo or fast-blo fuse for this amp?
 
EFB output would go through the 10 ohm resistors to each channel's output tube cathodes.

I use slow-blow types on tube amps, though you can get away with fast blow if you have a thermistor to limit inrush current.
 
Attached file was modified the Dave circuit diagram for you.
 

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If I may, the two 2K pots are shown wired incorrectly: At each of these controls, the bottom terminal should be grounded as shown, but the top terminal is what should connect to the new purple line and also what is now a 13K resistor from the bottom of the original bias pot, while the 47K resistor in each channel should now connect to the wiper of each 2K control.

Thanks to whomever stitched the schematic together!!

Dave
 
Can someone tell me how this DC Voltage Meter connect to 10K in the 9302 to show the Bias 0.44VDC.

sku_436670_2.jpg
 
Koi -- I'm not sure what your picture is showing. But if you want to measure the point in the circuit where .44 vdc should appear in each channel, then refer to the corrected schematic attached to post #781.

Using that schematic, and with your DC voltmeter set on a 2 volt range, the black probe should touch the red line on the schematic, which is the output of the EFB regulator. The red probe would then touch pin #3 of either output tube in the pair of tubes you are wishing to check the setting for.

I hope this helps!

Dave
 
Going back to Edcor OPTs, would the CXPP 25-8K or the CXPP 25-8K 23% model be preferred for a 9304? I know the difference has something to do with ultra-linear circuits or something, but that surpasses my meager understanding.
 
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