Silvertone 1474 high gain mod

[jamesedmunda]

I've got a Silvertone 1474, the 100 watter, 4 6l6 that been converted to a head. I have a single Fender 100 watt output transformer installed and all new e caps, it sounds great but the reverb is mia. No chance of finding an original piezo tank, so I'd like to add a gain stage using the reverb intensity pot. My 1474 only has an intensity reverb control no balance or threshold. Can some one point me in the right direction.
 Thanks James

[tubenit]

I had a Dano Centurian 275 with the piezo reverb.  It sounded awful.  I switched it out for a 12AX7 "one tube reverb" that I thought sounded great.


Have you considered the "one tube reverb"?   
         
Adding 1-tube reverb (el34world.com)




I find the 1474 schematic somewhat challenging to make sense of, but if you compare it to the 1484 .......  it may be useful in figuring out the signal flow.  Silvertone1484.pdf (el34world.com)


With respect, Tubenit

[maksim]

James, other folks:
Long time lurker, this is, I think, my first post. I just did some extensive work on one of these for a friend, and thought I may be able to contribute just a hair. First I've ever thought that...........
I'm gonna try to attach the schematic for version 2 of this essentially one-year-only amp. The second incarnation is vastly different (reverb implementation mostly) than the first, and because of the single reverb knob it's a reasonable guess that yours is version 2. The one I just did was version 1.
Lotta pros here to correct me if I mis-speak, and I welcome that, but as I waded through the one I had here I finally realized that it's actually a cathodyne PI off the 12AX7 (V3), which sends the out-of-phase signals not to the power tubes but to a third gain stage (V4) - the 6CG7 (or, in your case, I'd guess the "second" 6CG7). I had just assumed, from the outset, that V4 was the PI, but seems it's not. I've never seen a design like this before, been doing this stuff for fun over 10 years since retiring.
At any rate, either version already has 3 stages of gain. Ironically, besides being (to me) out of place, the third stage is a tube with a 20 amplification factor. I don't get that a bit, but I'm no EE or even a pro tech. I also looked at using the reverb circuit as a gain stage, but didn't - I just removed the circuit and its tube from the signal path and reconfigured Channel 2 to pass to the second stage, not through the reverb.
I just finished a D clone of #124, which yes has 4 gain stages. But two of them are switched in and out, together, via relay for the OD. Point is, I didn't see what purpose adding a fourth stage to this amp would serve, other than take it away from what it is after a load of rewiring. I thought it would probably be better to address the topology and use a 12AX7 for the third stage, probably even relocate it in front of the PI, but decided against all that.
This amp has a lot of (weird?) things going on, maybe why it was one year only. PT so close to a preamp gain stage? Two preamp tubes running DC heaters, helping to bias the four output tubes? Funny thing is, running right, it sounds awesome and the owner - before my work and after - loves it.
Just my half a cent............ymmv.
PhilW

[PRR]

Yes, a twisted drawing of a low-level cathodyne. The low-Mu push-pull driver has many fathers but this one is probably in-tribute/stolen-from Williamson, a VERY popular hi-fi amp of the 1950s. Williamson was driving power triodes (albeit strapped pentodes) so needed BIG strong grid swing. A quad of 6L6 might need big swing, but we know it has been done with less than all this, but a simple LTP has less voltage gain.

[jamesedmunda]

Very interesting, I've added a gain stage in  front of a bunch of amps using the Supro Thunderbolt mod.