PI capacitor question

[Randall]

Working on a 5F8A tweed twin clone with bias problems, scratchy pots, etc that are all caused by leaky caps. I had a mixture of vintage Astron and Mallory caps in it. 5 of the 6 Astrons were leaking, so I changed them all to Mallory 150s. The last one I pulled was the .1uF in the PI, but it was actually a .02uF. I thought "aha builder, you slipped". But after I changed it to match the schematic I noticed the amp sounded a wee bit darker, and it seemed to break up sooner, more like a blackface. Before I changed it, the amp had a more chimey 3 dimensional sound.

So my question is, does this follow logically? If so, how does this work? I will never know if it was a mistake or was by design since the builder has passed. Customer was hesitant to change the caps at first or do anything that might change the sound because he loves the amp so. I am probably going to put the "wrong value" cap back in since he liked it when it was that way.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Leo_Gnardo]

I am probably going to put the "wrong value" cap back in since he liked it when it was that way.

Likely that's the best solution. The cap you're referring to, is that the cap that AC-grounds the second grid in the PI? Hm I've never experimented with altering its value from the typical 0.1 uF. This might turn out to be a good spot to do a little low frequency band-limiting to concentrate the amp's power response on the midrange and keep it from amplifying low frequency "fluff."

Some advice to ALL MEFsters: it's a good idea to name the amp. Not all of us translate Fender's model numbers instantly. Don't make us have to look up what's a 5F8A. Please ! ! !

[g1]

Is by any chance the other one (coupler from treble wiper) a .1 instead of .02 ? (in case the builder mixed the two up)

[Randall]

I actually did post the schematic, not sure why it didn't take. I have fixed it. Tweed Twin.

"Is by any chance the other one (coupler from treble wiper) a .1 instead of .02 ? (in case the builder mixed the two up)"

The other cap is the right one. I don't see any mods other than this one, and don't know if was intentional or not.

[Zozobra]

I'd say anything bigger than 10n would adequately decouple the 2nd grid:

Decoupling to 3Hz, C = 1 / (2 * pi * 3 * 2e6) = 26nF

Either it's blocking distortion or some sort of interaction with the NFB? Blocking seems kinda unlikely as pretty much every common garden LTPI uses 100nF so if that were a problem it'd be hella noticeable in modern amps with lots of lows added in the NFB loop.

[J M Fahey]

Iīd say, in principle leave as is.
Owner only complained about scratchy pots so correct that, so same value just good dielectric (anything but PIO).
If he says sound is perfect and he doe not want you to Mod anything, by all means please do.
"Factory mistake"? .... "earlier Tech Mod"? .... who cares?
The point is that *as is* it sounds killer, at least for owner.

Unless you see a gross safety problem, such as a 2 prong cord, no (or questionable) grounding, a chassis which tickles you, leave as Customer wants.

As of "Factory errors sounding good", I know of one famous case.

Just quoting his words, I didnīt even check the schematic, Forum Member John Frondelli runs *the* NY service shop, with sprinkled in fairies dust Customers (the list is *incredible*)

This includes U2, who before any extended Tour drop a truckload (literally) of equipment at his doorstep for him to check up and leave in perfect condition.

Once he got curious about a famous amp, the one they use to record Guitar since forever.
Itīs a small simple one, a Princeton.
They use many, but *one* is specially marked as *THE* one.
He checked it end to end and found nothing unusual, except that "a certain coupling cap (I guess itīs the PI coupling cap, but *might* be another) is .01 instead of expected .02 "
He triple checked, but yes, itīs Factory installed, soldering is untouched.
Clearly "that day they had run out of .02 and installed what they had on hand"

Itīs reasonable to suppose that a slight Bass cut made that one overdrive somewhat cleaner , I bet a regular user would find it a "normal" one, but a very focused user (maybe in a Studio with a Recording Engineer standing at his side or at the "fish tank") plugging into 5 or 10 of them trying to find the best sounding one, detected a difference,even if slight.

I compare it to horse raiding or races: when riding a horse alone, whether it makes 20 Miles an hour or 20.05 MPH is irrelevant and undetectable; but at a race, horses win "by a nose".


PS: for those interested about seeing the list of customers: http://www.dbmproaudio.com/customers.html

[Randall]

Well, the customer agonized over which value to put in there, but finally went with the schematic. As it turned out, he wasn't displeased with the subtle loss of high end since he plays a strat. So there you go.