Dual Cathode bypass capacitor

[pjd3]

Hi there, I was hoping to get some feedback on the potential of a mistaken order I just made.

At Amplified Parts I just ordered some caps. 2 of those were the preamp tube bypass caps for a "BlackVibe" Fender amp that I'm building now, actually just collecting parts for .

What I didn't catch was that the two 25uF/25V F @ T preamp tube cathode bypass caps I ordered are actually double caps, which appears to be 2 caps with a common ground. I was curious to why the cost was so dis-proportianatly high for these. I thought they were singles.

My question is, since the 2 bypass caps are right beside each other on the board layout, would I be OK to use this double cap as opposed to two single caps? I actually sent an email to thier customer service to cancel the order but, if I'm OK with using this cap in place of the two, then I'll just call them to update the order.

This is my first build, at least for a tube amp, and Rob Robinettes layout would seem to accomodate a dual 25uF/25V cathode bypass cap due to the totally close proximity of the layout of them on the eyelet board. I'm quite sure I could accommodate the Large cap and the 2 accompanying 1.5K cathode resistors.

Sound like a go on that?

Thank you for coming by.

Phil

[sluckey]

Fender used thousands of those dual caps during the blackface era. Heck, the Fender layout may be the reason those caps were ever made. They were cheaper than two individual caps back in the day, but not today. The high price tag is not worth it IMO. Here's a pic of a 1964 Deluxe Reverb AB763...

[pjd3]

Very interesting. Thanks Sluckey.

Well, that pretty much convinces me that I'm in no trouble at all for having "mis-ordered" what I thought was 2 single 25/25's. They are pretty big, I think 2" x 1" was the toted dimensions. And it would just be a matter of settling the resistors in somewhat like they are in the 1964 deluxe photo that you sent.

Common sense (whatever that is) told me that it would probably be fine but, it wouldn't have been the first time that something sensical based on my experience may give way for some issue that I wasn't seeing, so, I"m very glad I asked, and very happy that you could get back to me in such short notice.

Thank you so much!

Phil

[Roe]

those caps may sound good. but 2u2 or 3u3 is sufficient at the first gainstage IMHO

[Stevem]

But smaller caps also produce less gain and a main function of any first gain stage is to provide get a good signal to noise floor ratio going by means of gain.

Your first gain stage or two also has the least amount of AC ripple voltage on it and to not take advantage of that makes no sence to me!

[Roe]

But smaller caps also produce less gain and a main function of any first gain stage is to provide get a good signal to noise floor ratio going by means of gain.

Your first gain stage or two also has the least amount of AC ripple voltage on it and to not take advantage of that makes no sence to me!

yes, but 3.3u only reduces low bass that is unnecessary in a guitar amp. actually the big cathode caps tend to produce bass flubb when you turn up the volume

[Tony Bones]

If we're fighting about it, I'll point out that the bypass cap also bypasses 50Hz or 60Hz hum coupled to the cathode from the heater. So a really big cap, especially at the first tube, can reduce audible hum.

[gingertube]

Even so 2u2 to 4u7 is plenty. Then you can use a film cap instead of an electrolytic.
Bypass cap value and frequency response graphs here:
http://www.valvewizard.co.uk/OtherStuff.html

Cheers,
Ian

[tubeswell]

Another vote for big Ck on V1a to improve S:N ratio

[sluckey]

Sunn liked to use a 250µF on the first triode.

[SoulFetish]

I think there is some data to suggest that a very large cathode bypass capacitance in the way of 250µF can decouple some heater noise which may couple in via the cathode. (I think I remember Malcolm Irving mentioning that in a thread on MEF?)
But, making the assumption of a 12AX7 stage with a 100k load and a 1M grid leak load on the following stage, a 3.3µF bypass cap put the halfboost frequency at around 37Hz(give or take).