More Fun With Magnavox: The 9300 Series

I’ve been of the impression that only the first cap after the rectifier needs to be limited in value and as for the others, the bigger the better.
Bigger will filter more but could effect transient response, use psud2 to check the effect, be sure to use stepped load.
 
Trying to get my shopping list together.
I'm building the amp using the existing output transformers and the additional buffer circuit.
Does anyone have a parts list?
 
I have a couple of quick questions that I couldnt find answers to. Chiefly that I cant seem to figure out where the output of the lm337 connects to the other channel. Am I just supposed to connect to the same location on the other channel through a separate 10 ohm resistor? I may be over thinking this but I don't want to make a mistake. Also, where is the "input sensitivity reduction network" located in the circuit? Is it optional? I got a matched quad of el84s, a 5ar4 and components to shotgun the chassis. Those two things tapped out my electronics "skillz" though. I'd appreciate any help.
 
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I'm guessing this is very similar to the Dynaco st35 application.
If you look at the original Tronola article or you go to Dave's store and you look at the st35 PS efb board build manual , I'd bet you could ape the application
 
yes, output of the LM337 goes through a second 10 ohm to the second channel. This gives you a bias test point per-channel. The far side of that 10 ohm goes to the cathode on the other channel's pair of EL84 tubes

The reduction network is optional. if you've got a fairly hot output preamp it would be useful to use. If you do not plan to use a preamp or if your preamp is low output, don't use it. If you install it, it sits between the input jack and the grid of the 6EU7.
 
Thanks for this great thread. Thanks Dave.
I've learned a lot from here.
 

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Just yesterday I fired up the old Dave G Magnavox 9300 and I forgot how wonderful it sounds! I built a Dynaco ST70 clone recently (VTA version) and have been playing exclusively that for the past 2 months. Well, it was out in my shop where I was taking some measurements, so I connected the Maggie to my Klipsch Heresy IIIs for the first time and Wow! That's one sweep little amp. The bass is a bit looser for sure, but it is so in a very musical way. I am glad to have both.

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Koi -- May I be the first to welcome you to AK! You'll find many knowledgeable and helpful people here for all of your audio projects, so thanks for joining in!

Regarding your build, your work is very nice indeed, but if I may, I would point out two opportunities for you to consider:

1. You are missing the power supply dropping resistor and filter section for the screen grids of the output tubes. While the choke does a good job of filtering the B+ on these models, not having the screen grid filter section will make the amplifier more susceptible to 120 Hz power supply hum if the output tubes are not perfectly balanced. I would encourage you to add the missing filter section when you get a chance.

2. In the modified amplifier, there is a small 47 pF cap that connects between the Blue OPT primary lead, and the cathode of the 6EU7 input stage. You appear to have installed these caps, and one side is in fact connected to the cathode of the input stages. However, the other side of these caps appear to be connected to the RED lead of the OPTs (i.e., the B+ leads), not the BLUE leads as they should be. The effect of this mis-connection is to eliminate the NFB path from the upper output tube plate back to the input stage, with the result that HF response and stability will be significantly compromised. If this is as it appears to be, it needs to be corrected for the amplifier to operate properly.

I hope this helps!

Dave
 
Hi Dave, thank you very much for your personal guidance.

I saw the mis-connection on the 47pF . But I look at the schematic I don't understand where the power supply dropping resistor and filter section for the screen grids of the output tubes are.
Can someone help me display it on the schematic?
Does the two red circled place on the power schematic at attached files?

Koi
 

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The 375v tap goes to the red leads on the output transformer
the 370 tap goes to the screen. The 330 ohm resistor is the screen dropping resistor.
The 340 tap goes to the 6EU7 tubes

Basically you need one more capacitor in the power supply.


The two resistors that go to the LM337 are taking the place of the hum balance pot. It ties into the cathode circuit like the original hum pot did.
 
Hi Koi -- The two resistors circled on the schematic that are labeled " To Output of LM337" are not part of this discussion. Those resistors exist to raise (or "bias") the heater voltage above ground by a small DC value, and in a balanced fashion, so as to help minimize any noise that might be generated in the input stage tubes if the bias were not otherwise used.

The focus of my comment about the missing filter section centers on the missing 330Ω resistor (shown after the choke on the power supply page), and final filter cap that are not installed in your build. Note that on the schematic after the choke, power is applied to the OPT center tap connections (being the 375 volt supply point), and also the next filter section -- the missing 330Ω resistor -- that then supplies the 370 volt output tube screen grid supply point (pin 9 of the tubes), and also the next filter section after that (the 10K resistor) as well. In your build however, there is no 330Ω resistor. The point after the choke feeds both the OPT center tap leads AND the feed to the output tube screen grids, as well as the 10K resistor supplying the driver tube supply point. In other words, the screen grid supply point as shown on the schematic is completely missing in your build.

To correct this, you need to replace the 10K resistor with a 330Ω resistor, and move the screen grid supply lead from its current position after the choke, to the cap after the 330Ω resistor. From that point (after the 330Ω resistor), you also need to then reconnect the 10K resistor, and have it tie to a new isolated terminal, where a new (additional) single power supply cap is connected between there and ground (the 30 uF cap at the new 340 volt supply point shown on the schematic), and from which power is then supplied to the driver tubes.

I hope this helps!

Dave

Edit -- Looks like Gadget covered this for you as well.
 
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Thanks Gadget and Dave,

I fixed the 47pf mis-connection yesterday!
Is it correct on the attached files?
What is different if i use 33uf or 47uf 450V on last capacitor in the power supply?
The store only sell 33uf or 47uf. Can't find any 30uf or 40uf capacitor.
 

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Your understanding is perfect now. Either the 33 or 47 uF will be fine for the last filter cap. The 33 uF will be slightly smaller physically and so might be easier to install, but either will work fine electrically.

Dave
 
I got some problem after finished work.
It can't adjust BIAS over 0.32VDC when turn the 5k knob to the bottom. I measured the voltage at home only 104V. Is it the problem?
This 9302 didn't do transformer buck.
 

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You can turn the buck winding into a boost winding. Simply reverse the brown and brown/yellow connections. That should put voltages closer to correct.
 
104v?
Before doing changes, I'd put fresh batteries in your meter.
If possible check with another meter.
104 seems very low for line in (even in a brownout).
If true, I think I'd test your line in from utility, perhaps local power issue.
 
Consider his location. Probably 100v mains is normal for him. I don't pretend to know what Taiwan runs, but Japan at least does have 100v mains.
 
That being the case he could boost instead of buck and build a boosting transformer as well
Probably get him into 115vac territory
 
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