More Fun With Magnavox: The 9300 Series

Rule of thumb you can always put a film in where a polarized cap was but not visa versa (assuming value the same and voltage rating adequate)
Generally when a cap is over value (in mf) it's size will prevent it from physically fitting if it were a film. That is the electrolytic will be smaller.
I'm not sure where you got schematic but I suspect it's a misprint. The cap you're referring to is not polarized
This Magnavox original schematic shows it as polarized.
Magnavox9300schematic_zps31ad6fd4.jpg
 
Hard to see on my phone the indication that it's polarized (if in fact its there). I believe you are referring to C106, 206 , which are not polarized in the drawing
 
Hard to see on my phone the indication that it's polarized (if in fact its there). I believe you are referring to C106, 206 , which are not polarized in the drawing
C106 is shown diagramatically with one flat plate and one curved plate. C106 is shown with a pair of flat plates.
As Gadget and others have explained, that indicates the foil side of the capacitor.
I mistakenly believed that always indicated a polarized capacitor.
I hope this hasn't confused the issue further.
Capacitor nomenclature has always been a mess, and the revisions to the system have muddled it further.
 
Curved side symbol was usually outside foil. Symbol for elyticaps may show thicker straight line for positive electrode, positive side may also show cross symbol. The eliticap may have an line along one side for negative pole electrode or the end/ ends marked for polarity.
 
Ok then. I have motored through about a third of this thread. I am about to wade into my own rebuild. I have a few questions. My program is as follows:

I am leaving the cabinet, the tuner/pre, the MPX, the turntable and the speakers intact. I am going to skip the bucking scheme so as to leave the Molex connection intact to feed the tuner/pre and MPX As I understand it the purpose of the transformer rewiring is to take down the voltage to the heaters/filaments which is high when fed with today's house current. What are some other simple and inexpensive ways to better regulate the heater voltages without resorting to rewiring the transformer?
 
You could make an external bucking transformer. This would be a good solution if you plan to keep it inside the original console.
XFMR_Buck_Boost.gif
 
See what the pilot lamp voltage measures with all of the stock stuff plugged in. You may find that its reasonable with the extra load from the tuner. I suspect at least part of the high voltage stand-alone is just reduced load on the power transformer.

You'll have to fool with the B+ stuff to keep the right voltage output to the tuner as well. The voltages inside the amplifier need to match with Dave's schematic, so you may need to add an extra node with a drop resistor to get the tuner power voltages correct.
 
See what the pilot lamp voltage measures with all of the stock stuff plugged in. You may find that its reasonable with the extra load from the tuner. I suspect at least part of the high voltage stand-alone is just reduced load on the power transformer.

You'll have to fool with the B+ stuff to keep the right voltage output to the tuner as well. The voltages inside the amplifier need to match with Dave's schematic, so you may need to add an extra node with a drop resistor to get the tuner power voltages correct.

Thanks so much. This gives me a way to get started. That is, a way to get started after I replace all the PS electrolytics in the entire rig.
 
IMG_20190817_163837480.jpg
My 9600 is a work in progress which is going well, thanks to this terrific post! I removed the hum pot from my circuit, which still allows the amp to function... But I was just noticing by doing that I no longer have anything feeding the pin 3 of all 4 6BQ5 tubes... they are simply linked to each other's pin 3 via yellow wire. 12 volts yes? Shouldn't I tap in somewhere to correct this? Thanks!
 
Morrism -- You know the old saying -- the only bad question is the one that wasn't asked!

In each channel, pin 3 between the two output tubes is connected by a jumper, therefore, two jumpers. Then, from each channel's jumped cathode terminals (pin 3), there would be a 10 ohm resistor connecting jumpered pair to the output of the EFB regulator. Therefore, at the output of the regulator, that connection includes:

1. One 10 ohm resistor to one channel's jumped cathode terminals,

2. One 10 ohm resistor to the other channel's jumped cathode terminals,

3. The 20 uF bypass capacitor, and

4. The two 220 ohm resistors from the heater winding.

Therefore the four cathode terminals are not all connected together, but connected together in two pairs, two for each channel.

Kevin -- I was really trying to stay within the boundaries of the topology that the original design represented. There are soooo many 6BQ5 designs out there that use the AF Amplifier/Cathodyne formula, that I thought it would be nice to stay a little truer to the original format. But in doing that, the original paraphase inverter needed so much help, that the logical move was just to modify it to the floating paraphase type. It represents a significant improvement over the original design.

Thanks for following along!

Dave

Dave I'm not implementing the EFB at this time but I DO need to manage applying proper voltage to all the pin 3 of all 4 6BQ5. I HAVE removed the hum pot circuit. What should I do?
 
Tonyg88....... you've obviously never played with the Dynaco ST-35 or the SCA-35.
 
22C60BD7-09DD-49F5-88A4-57691B9DC916.jpeg I just picked up a 9304-20 and don’t know where to start. I guess, I need to figure out which wires on the molex plug to tie for power. There is no balance pot so I am expecting to add some resistance at the input.
The good news is that I bought it from the original owner and before I could object, he powered it up. It works but I need to get it going as a stand alone amp and see what it can do.
 
Jumper from pin 1 to pin 10. I’d still use a variac and current limiter to power up in stock form. As far as the rebuild, I’d recommend starting at the beginning. Good luck!
 
View attachment 1610314 I just picked up a 9304-20 and don’t know where to start. I guess, I need to figure out which wires on the molex plug to tie for power. It works but I need to get it going as a stand alone amp and see what it can do.

Well this thread will point you in the right direction start at the beginning on the first page, it's a long read so be patient it's well worth it.
 
Jumper from pin 1 to pin 10. I’d still use a variac and current limiter to power up in stock form. As far as the rebuild, I’d recommend starting at the beginning. Good luck!

You may have to hold my hand here. Since the pins aren’t numbered, is 1 top left? Please confirm which colors get connected.
Yes, I’ll bring it up slowly on a variac.

I’ve owned a ST-70 and a bunch of EL84 integrateds over the years, but it’s been a few years since I messed with them. This thing is also a different animal.
Thanks.
 

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If you’re in a hurry to just get her up and running, or just while you’re figuring out a plan to build it the DCG way, what I’d recommend is replace the power supply capacitors (the can) and the two (white) .047 coupling capacitors. You’ll be good for quite a while. Ask away anything you need to know, there are a ton of folks here very willing to help.
 
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