Two Rock CustomReverb

[pula58]

I was looking at a schematic of a Two Rock custom reverb (on this forum, I forgot exactly which thread I found it in). The first preamp has 100k plate Resistor, 1.5K Cathode resistor (so far, just like Fender)...but...4.7uF cathode cap. How come the cathode cap is so much smaller than the typical Fender value of 25uF?

Thanks!

P

[exocet]

In my view the bypass cap on the first stage is 4.7 uF because that amp is derived from the Dumble circuit and not straight Fender. It has the skyliner tone stack as well if we are both referring to the hand drawn schematic that is listed on this forum somewhere?

The Fender value is probably in turn derived from the old RCA application notes for small signal valve stages based on 12AX7. These were based on "Hi-Fi" requirements rather than guitar. Larger bypass caps give extended bass response (potentially useful for Hi-Fi, less useful for guitar).

In short I'll stick my neck out and say that you don't need to go larger that 10 uF on the 1st valve stage when using with a guitar. It's useful to put smaller values on subsequent stages to trim the bass response.

[ER]

#124 w/fender reverb and two-rock loop.

[pula58]

Here is the schematic I am referring to:

https://tubeamparchive.com/files/two_ro ... em_667.pdf

By the way, can anyone comment on the reverb circuit. Where is the reverb driver tube??

[wjdunham]

I've built an amp with that configuration - the reverb driver and receiver is not shown explicitly, it's just a box labeled "Rev." in that schematic. I built a standard Fender Twin type reverb circuit, a 12AT7 driver using both halves, and 1/2 of a 12AX7 as the receiver. The other half of the 12AX7 is used as a post FX loop recovery stage with a trimer for the gain level. Sort of 1/2 of a Dumblator, without the cathode follower driving the loop.

Bill

[butwhatif]

I have 22uf on mine, and 15uf on v1b, it provides bass in dry rooms. 5uf isn't even close to enough in some situations, or w/some guitars. you can always turn the bass down, but not up if it isn't there.

[marcos]

Here´s my 2 cents:
I played this amp ( the actual amp shown in the schematic), and liked it a lot.It is not an exact copy of #124, different coupling caps,
470 pF snubbers (higher than I would recommend, but sounded great in this specimen), built in loop etc.
It had a lot of bass, nice Carlton-ish tones, not too versatile.
However, just played it in somebody´s living room, so I don´t know
how good it works in real-world situations like gigging and recording.

Hope this helps a bit Marcos

[pula58]

so, in my Fender AB763 -style build, is there anything to be gained by changing the 25uF cathode caps to a lower value?

I can see that I might lose some bass response, but, will I gain anything?

And, interestingly, last night I tried to find some axial-lead low voltage (like 25V) electrolytics that had smaller values (like 5, 10 or 15uF) but could only find radial lead electrolytics with those values and voltage rating.

[marcos]

so, in my Fender AB763 -style build, is there anything to be gained by changing the 25uF cathode caps to a lower value?

I can see that I might lose some bass response, but, will I gain anything?

And, interestingly, last night I tried to find some axial-lead low voltage (like 25V) electrolytics that had smaller values (like 5, 10 or 15uF) but could only find radial lead electrolytics with those values and voltage rating.

IMHO you won´t gain anything in a clean amp like the AB 763.
HAD also used 22 µF in clean amps like the Steel String Singer, sometimes even in Overdrive Specials.The 4,7 µF helps to keep the bass more defined when in OD mode, 4,7 µF on all four preamp stages tends to give the amp more of a low-mid emphasis in my experience.Hope this helps a bit

Marcos

[topbrent]

so, in my Fender AB763 -style build, is there anything to be gained by changing the 25uF cathode caps to a lower value?

I can see that I might lose some bass response, but, will I gain anything?

And, interestingly, last night I tried to find some axial-lead low voltage (like 25V) electrolytics that had smaller values (like 5, 10 or 15uF) but could only find radial lead electrolytics with those values and voltage rating.

Nichicon makes great axial 50v caps in any uf value you would like to try. Spend 10 bucks and buy a handful of all of the usual values and find out for yourself what the different values sound like. It is a cheap experiment.

Mouser: http://www.mouser.com/catalog/catalogUSD/643/813.pdf

[pinkphiloyd]

I was thinking about implementing this somewhere, maybe, someday, in permanent "rock mode." Anybody care to look at this and tell me if I've got it right? I'm not really familiar with Dumble topology to begin with and coupled with trying to extract stuff from this hand drawn schematic...well, I'd like a little confirmation. Also, I know this isn't the ideal pic or posting method, but I'm at work and it's the best I can do right now.

[heisthl]

so, in my Fender AB763 -style build, is there anything to be gained by changing the 25uF cathode caps to a lower value?

I can see that I might lose some bass response, but, will I gain anything?

And, interestingly, last night I tried to find some axial-lead low voltage (like 25V) electrolytics that had smaller values (like 5, 10 or 15uF) but could only find radial lead electrolytics with those values and voltage rating.

Get the preamp plates down to 200 and you will want to make the bypass caps smaller. Your amp probably has 240v on the preamp plates, if you lower it the tone gets rounder and the 25uf will be too much.

[metalguy]

I was wondering if that 470k resistor (before Send) is not a mistake. If the reverb is Fender type I suspect too much of reverb getting into the sound (compared to the 3M3/220k in Fenders)or at least that pot should not be linear but log. Anyone tried that in any real build?