A strange bypass cap in my old 5D3

[Tyrannocaster]

I'm working on my build of a 5D3 preamp with a 5E3 power amp and I was looking at my old '55 5D3. The cathode bypass cap on V1 is not an electrolytic - somewhere along the line it got replaced with what is obviously a coupling cap - and it's a .01! The cathode bypass resistor is the stock 820 ohm. I'm guessing that this cap is essentially the same as having no bypass cap in the circuit - am I right? I ask because this particular old 5D3 is one of the best (if not the best) sounding tweed Deluxe I have ever encountered (at least for how I play, anyway) and I had planned to blueprint it. But this is so weird! If a mistake results in something better I will replicate it, but I don't want to just copy without understanding. For now I think I will proceed without a bypass cap but I would like to have that theory confirmed by somebody who knows a lot more than I do.

What do you think?

[thelonious]

The cathode bypass cap on V1 is not an electrolytic - somewhere along the line it got replaced with what is obviously a coupling cap - and it's a .01! The cathode bypass resistor is the stock 820 ohm. I'm guessing that this cap is essentially the same as having no bypass cap in the circuit - am I right?

I, too, would like to hear from the experts on this, but it seems to me that you could call that an "oscillation boost cap".

Make sure you test its value. If it has drifted significantly upward in value, perhaps it is causing some kind of audible effect.

Have you tried A/B-ing the amp with and without it?

[Tyrannocaster]

Have you tried A/B-ing the amp with and without it?

The amp sounds so good that I have become quite paranoid about altering it in any way. Superstition or not, I just don't mess with this amp unless it's absolutely necessary. I really don't like to de-solder components on that 57 year old fiber board. By now most (if not all) of the caps in the amp have been replaced and some of the resistors, too, but it still sounds like a million bucks. I used to get calls for studio work in Hollywood just for the amp, which wasn't really all that flattering, when you think about it.   

I can't try to A-B it in the new amp yet because it's nowhere near ready to fire up.

[phsyconoodler]

yes it acts like a bypass cap.The .01uf value passes more highs.It may be too bright but let your ears decide.

[Cups]

Those amps tend to have big (dark sounding) coupling caps so maybe it was modded with the super small (bright sounding) bypass cap to compensate?

[John]

yes it acts like a bypass cap.The .01uf value passes more highs.It may be too bright but let your ears decide.

Yup, in fact I'm experimenting with that now. It made a big difference on V2 for me dropping to 2.2uf, and I'm lowering the cap on V1 as well. I'm trying to make mine brighter.

[thelonious]

The amp sounds so good that I have become quite paranoid about altering it in any way. Superstition or not, I just don't mess with this amp unless it's absolutely necessary.

I don't blame you one bit! "If it ain't broke..."

So I have a question: The cathode resistor in the 5D3 appears to be shared by both triodes of V1. If I understand correctly, because twice the current is flowing through the resistor, it drops twice the voltage, biasing each individual triode more like a 1.5k resistor would. Is that true?

If that is the case, would the corner frequency of that bypass cap be calculated based on a 820R resistor, or a 1.5k resistor?

Hopefully this is not a thread hijack! I think it's related because I'm trying to find an accurate corner frequency for the bypass cap, but I will gladly open a new thread if you prefer, Tyrannocaster.

[Tyrannocaster]

I don't care where it goes as long as I get my question answered too! 

[Tyrannocaster]

I believe HotBluePlates already answered my question. I thought there was something on this in an old post but I get these messages mixed up even when I don't actually forget them, and after searching through all of my posts on this board I found it. This was in his response to the schematic I posted of the proposed amp, including the .01 cap:

Which reminds me: a 0.01µF cap against 820Ω will have a -3dB point high enough that you might as well not have a bypass cap. I calculated a number almost as high as 9kHz, but that does include a simplifying error that will guess the roll-off a bit low.

So I think I will just go ahead and build it with no bypass cap. I can always add values easily later.

[HotBluePlates]

Can you post a picture of the interior of this amp?

I think in that old thread I also suggested simply unsoldering one side of that cap and seeing if you hear any change. Did you ever do that?

[Tyrannocaster]

Can you post a picture of the interior of this amp?

I think in that old thread I also suggested simply unsoldering one side of that cap and seeing if you hear any change. Did you ever do that?

I could post a photo but it's a recent vintage cap like others I have obtained from Mouser and other places for use in coupling stages - it's not some ancient "magic" relic. One of those tiny light yellow things - can't see a brand on it. All the actual coupling caps in the old 5D3 are Orange Drops (not original) and the power supply caps are Atoms (replaced those, too), but I don't know what this is and since it's such a tiny value I'm pretty sure it isn't doing much. However, as already mentioned, the amp sounds so good that I won't do anything to it until it is absolutely necessary, and experimenting on it would not be a necessity.

I certainly don't remember putting this non-electrolytic .01 in there but I can't remember anybody else working on the amp in recent years - I'm thinking it's simply an error. If so, luckily it didn't impair anything and it may even help, who knows. Later, when my current build is working (listen to him - so sanguine!) I will try putting one of these or something like it across that resistor and see if I hear a difference; that way I don't have to mess with L'il Fender (my name for the 5D3 a long time).