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Advice needed on use "general purpose" film capacitor as coupling cap in a tube amplifier.

gadget73 said:
guessing Metalized Polyester, aka mylar, but could also be Metalized Paper.
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Didn't expect that metalised polyester/ paper existed that early.
My knowledge is mostly based on Sansui equipment were they start using it, that I know of, around 1980.
 
GrisPato said:
So, because i just looked at the schematic to put some capacitors to an order for some other amplifiers I did check lead length.
Decided to open up the AU-70 to check if they would be long enough.
Not for all of them!
So checked out the mentioned Jantzen caps, I get them here in the Netherlands, and for a very reasonable price.
Just for some of the smaller values probably have to go for the 716P for they have longer leads.
Sansui also used some bipolar electrolytic 4 uF for coupling and they are big with long leads. I think I will use some Jantzen for them to.
These 100 nF caps were labeled mp in the service manual, 2 of them look like this the other 2 are Suzuki paper oil caps. The 2 in the picture look like Mylar to me.

View attachment 3844873

I am happy I asked some advice and looked inside !
Many thanks!
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"Sansui also used some bipolar electrolytic 4 uF for coupling and they are big with long leads."

I am surprised Sansui uses a bipolar electrolytic for coupling. Evidently the engineers believed they could get away with it in that particular part of the circuit. Reason being they have never got an to a point of building an electrolytic without some leakage of DC which will upset the bias of the grid of next tube. At least this is what I have been taught, heard.
 
where are they using a cap that large for coupling in a tube circuit? The leakage alone would be a problem, and the size is easily 10x as large as would typically be required

unless its a screen bypass cap on a small signal pentode? That could be a sane value. Seems too small for a cathode bypass cap.
 
gadget73 said:
where are they using a cap that large for coupling in a tube circuit? The leakage alone would be a problem, and the size is easily 10x as large as would typically be required

unless its a screen bypass cap on a small signal pentode? That could be a sane value. Seems too small for a cathode bypass cap.
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Screenshot_20260928-090053.png

Here they are, marked with blue.
It looks to have to do with the bypass switching of the tone control stage.
 
They are not directly tied to the grid of the next tube so the engineers could get away with using NP electrolytics in that part of the circuit.
 
Also its downstream of a standard film or whatever capacitor, thats why it works.

looks like its 20 volts. I might sub that with a 4.7uF film. They exist at that value for not-stupid money.
 
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