Fender Long tail pair phase inverter

[guitman321]

Does anyone know why the capacitor in the fender phase inverter is specified as .1-200? Why specifically 200 volts? What was the original rationale?

[pdf64]

Perhaps because that's a voltage rating that was available in that cap range and ensures it won't be exceeded under foreseeable conditions?

[Tom Phillips]

The 0.1uF / 200V cap was in use from the tweed era amps through the CBS era amps. My speculation is
1) 200V is sufficient for the cap in that part of the circuit
2) It was common practice to use a "just good enough" part back in the old days. Especially since the price difference between a 200V part and a 400V part was a greater percentage of the part cost back then than it is today.
3) Today's manufacturers are more likely to minimize the variety of parts they stock and take advantage of the cost reduction (cost for an individual part) associated with larger quantity purchases.
4) Obviously, the lower voltage part spec was carried through as the Fender amp circuits evolved.

It's interesting to see that, in the Twin Reverb Re-Issue which came out in the 1990's, Fender was still specifying 400V for most of the caps in the circuit and they were then using a 250V part for the subject cap in the PI. I'm not saying that the change from 200V to 250V was required but it sure indicates that someone was still purposely deciding to use a different value there rather than just using the same 0.1uF / 400V part that was used in the tone stack of the same amp.

One more thing...I certainly don't think the lower voltage rating was selected for tone reasons.

[Enzo]

ANother possibility is that 200v became a less standard value than 250v. These days for example 63v is a common cap voltage rating, but 50 years ago it was not. The famous big blue Peavey 5000uf 55v caps make an example from the 1980s. Try finding a 55v cap today, it will be 63v or 100v or something.

In general, one has to consider what happens when say the amp is fired up without tubes. B+ voltage rises through the circuit, and coupling caps can have a lot higher than normal voltage across them. In the case of the PI cap, it is wired to a grid and cathode circuit that does not directly follow to B+. So 200v seems sufficient.

[pdf64]

The BF standard 400V rating used for the phase splitter plates to power tube control grids coupling doesn't seem to give much, if any, leeway at power up or if the phase splitter tube stops conducting etc.
The TRRI uses 630V there, which seems more resiliant.

[guitman321]

I find it odd that throughout the years, even after the CBS buyout, and in all models, that cap stands out as unusual. Fender probably bought thousands of .1-400v and still specified that odd cap in the phase inverter for no other reason than to save a penny? That is mind blowing.

[Tom Phillips]

...Fender probably bought thousands of .1-400v and still specified that odd cap in the phase inverter for no other reason than to save a penny? ...

I checked the 1962 Allied catalog to find the relative price difference between a 0.1uF/200V and a 0.1uF/400V tubular cap. To my surprise I found that the price was the same for two types and I found one example where the 400V cap was less expensive than the 200V part from the same cap line. That seems to blow away my previous speculation that they used the 200V part to cut cost. The 200V parts were a little smaller though. Go figure.

[guitman321]

Thanks Tom. I wish I had saved some of the old catalogs. Anyhow, that upsets the applecart as far as money saving reasons. Too bad the original designer can't chime in.