More Fun With Magnavox: The 9300 Series

Hi folks,

I modded my Maggie last year and everything has been performing fine, except I went to bias the amp last week and now the channels aren't balanced. I bought a new matched quad of JJs, and the imbalance remains.

I'm wondering what could cause the imbalance? Just trying to find a place to start looking...


*edit* the only voltages with discrepancies between the channels is that pin #2 (grid) on the left channel is showing -462 whereas the right channel is reading around -115.

Thanks in advance!
 
Last edited:
not balanced in what way, sound-wise or current-wise?

Grid should be pretty much zero volts, not negative several hundred. Are you missing a decimal in there?
 
gadget73 said:
not balanced in what way, sound-wise or current-wise?

Grid should be pretty much zero volts, not negative several hundred. Are you missing a decimal in there?
Click to expand...

Not balanced current-wise.

I'll double check to make sure I'm reading the DVM correctly. I have a semi-dumb question: the left channel is the one with the lower bias—the cathode (#4) connection on the left channel wasn't strong due to the wafer sockets. Would this in any way effect the differences in current?
 
if it wasn't connected, yes. I'm guessing you have the one resistor per channel setup and not one per-tube ? Also, cathode is pin 3 on an EL84. With an open cathode connection you'd get basically 1/2 the amount of voltage reading across the resistor.

You can actually test the tubes individually in this setup. Just plug one in at a time and read across one of the resistors to see whats going on. If all of them read about the same, the tubes are fine and you've got something funny with the socket or whatever is wired to that socket.

Also worth checking to see if the cathode resistor has gone off value for whatever reason. That can be tested in circuit with the power off.
 
gadget73 said:
If you are rewiring the sockets, I might suggest replacing them. Those wafer sockets just aren't that robust, and shuffling wires around is somewhat likely to make them fail.
Click to expand...

Agree with that. After working so carefully, so many times, to try and save those sockets- I've basically said to heck with that. The cost of a socket is cheaper than the time cost to save one of them- especially when a pin will occasionally break off an old wafer socket, no matter what you do.

Regards,
Gordon.
 
Okay, I need to jump in with a total NOOB question before I burn down my own build.

The original Magnavox 9300 schematic specifies electrolytic coupling caps (.047 mmf), whilst Dave's redesigned 9300 appears to show non-polarized caps of the same value.
Questions:
1. Is my understanding of this correct?

2. Does the original Magnavox circuit require electrolytic coupling caps, or would non-polarized caps work satisfactorily?
 
The originals were not electrolytic coupling caps. They were paper, which can be replaced with film like Dave used. They are not and should not be polarized. The originals may have had the outside foil marked, which is not the negative side.
 
ArchieType said:
Okay, I need to jump in with a total NOOB question before I burn down my own build.

The original Magnavox 9300 schematic specifies electrolytic coupling caps (.047 mmf), whilst Dave's redesigned 9300 appears to show non-polarized caps of the same value.
Questions:
1. Is my understanding of this correct?

2. Does the original Magnavox circuit require electrolytic coupling caps, or would non-polarized caps work satisfactorily?
Click to expand...
You are reading the new schematic correct the coupling caps are non-polarized. Orange drops work well here, but any poly film type caps with the correct voltage rating should be fine. Like a Mallory or Vishay.
 
gadget73 said:
The originals were not electrolytic coupling caps. They were paper, which can be replaced with film like Dave used. They are not and should not be polarized. The originals may have had the outside foil marked, which is not the negative side.
Click to expand...
Oh boy, now I'm confused for a brand new reason. I always thought that those old paper caps with polarity markings were electrolytic. Aren't they?!

2. My second question remains unanswered: if one were to replace the (EDIT) .047mmf (END EDIT) coupling caps in the original Magnavox 9300 circuit, would you need to use polarized caps or not? (And if not, why not, since the schematic specifies them?)
Sorry for being dense here.
 
Last edited:
Exactly which cap are you referring to?

Broadly speaking, if its a decimal microfarad value, its a non-polar paper or film cap. Whole number microfarads are polarized electrolytic. Whole number with no units, or whole number mmf, mmfd, or pf are non-polar ceramic or silver-mica.

The 47pf (mmf is the old way of saying that) are high frequency stability caps. Usually a ceramic, and rarely need to be replaced but you can use ceramic or silver-mica for that spot.

A 47mf (uf now) would be a polarized electrolytic.
 
The schematic has a + on one side of the electrolytic caps to show polarity. No + , then it's not electrolytic.
 
gadget73 said:
Exactly which cap are you referring to?

Broadly speaking, if its a decimal microfarad value, its a non-polar paper or film cap. Whole number microfarads are polarized electrolytic. Whole number with no units, or whole number mmf, mmfd, or pf are non-polar ceramic or silver-mica.

The 47pf (mmf is the old way of saying that) are high frequency stability caps. Usually a ceramic, and rarely need to be replaced but you can use ceramic or silver-mica for that spot.

A 47mf (uf now) would be a polarized electrolytic.
Click to expand...

"Usually a ceramic, and rarely need to be replaced but you can use ceramic or silver-mica for that spot."

I"m replacing the ceramic .0015 cap with the following 1500pf film cap, is that ok?
https://www.mouser.com/ProductDetail/667-ECQ-E6152KF
 
gadget73 said:
Exactly which cap are you referring to?

Broadly speaking, if its a decimal microfarad value, its a non-polar paper or film cap. Whole number microfarads are polarized electrolytic. Whole number with no units, or whole number mmf, mmfd, or pf are non-polar ceramic or silver-mica.

The 47pf (mmf is the old way of saying that) are high frequency stability caps. Usually a ceramic, and rarely need to be replaced but you can use ceramic or silver-mica for that spot.

A 47mf (uf now) would be a polarized electrolytic.
Click to expand...
I'm referring to the .047mmf coupling cap in the original Magnavox 9300 schematic. It is labeled as .047 mmf, which I realize is the old convention, which I used because I'm referring to the original schematic.

That cap has a schematic symbol with one straight plate, and one curved, which I believe indicates a negative and positive side. Like this. I thought that meant electrolytic.Capacitor-symbols.jpg
 
gadget73 said:
Exactly which cap are you referring to?

Broadly speaking, if its a decimal microfarad value, its a non-polar paper or film cap. Whole number microfarads are polarized electrolytic. Whole number with no units, or whole number mmf, mmfd, or pf are non-polar ceramic or silver-mica.

The 47pf (mmf is the old way of saying that) are high frequency stability caps. Usually a ceramic, and rarely need to be replaced but you can use ceramic or silver-mica for that spot.

A 47mf (uf now) would be a polarized electrolytic.
Click to expand...
These big white ones, with a line on the (presumably) negative side.
20190426_104013.jpg
 
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
 
nj pheonix said:
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
Click to expand...

What about replacing the ceramic capacitors with film capacitors?

I'm going to replace the .0015 ceramic capacitor in the 9304 with a film capacitor 1500pf?
 
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