5F6A verb & Trem revisited

[tubeswell]

Rather than add more and more to to the old thread (which is getting nigh impossible to navigate your way through), I thought I'd post this as a single item here to make it easier for people to help me answer my next question/revision which is the bits in red on V12d (see attachment).

I would like some feedback on the wisdom of adding this extra bit of decoupling of the power supply between the LFO circuit driver and the reverb driver. I was thinking of a 470R to 1k 1W resistor (so as not to drop the voltage too much, but to be enough to sufficiently decouple the power rail between the two sub-circuits).

This build is 75% complete and if I'm going to stick the extra decoupling RC in, I would prefer to do it sooner rather than later.

Cheers

[txstrat]

Hi TW
Though my knowledge leaves much to be desired, but isn't the reverb transformer decoupling enough?

[tubeswell]

Hi txstrat, Yeah I did initially wonder about that myself, but then I wondered if - when the reverb driver is idling, that maybe I don't want the LFO to come thumping into the other side of the RT primary via the power rail (if that was going to happen). So if it were to otherwise happen, then if I put a small resistor in between the LFO and the RT supply, with a decoupling cap to absorb any thumping, I might keep the signal going into the reverb pan clean when the reverb driver was idling. Anyway that's my take on it at the mo'. But if the RT primary is enough of a buffer, I'm happy to go with that. Since I can't make up my mind, I've decided that this is one of those situations where my overconfidence isn't going to carry the day, so I thought I'd better resort to some collective wisdom. Thanks for your feedback.

[txstrat]

I think I get what you mean. It's pertaining to those situations when you don't wanna shut the trem off via the footswitch, ain't it? Cause if you switch it off there'd be no thumping. Besides, I believe the bias vary is actually causing the thumping in the power tubes, not the LFO tube.

[tubeswell]

Yep the main voltage wiggle comes out of the CF cathode going into the output tube grid bias resistors, but might not there also be a chance of a ripple going back through the power rail from the B+ side of the plate resistor for the LFO? (I'm not sure whether there would be, or how much of a one there might be if there is, but seeing as how the reverb is the most sensitive circuit in the amp, I thought this might merit some further scrutiny)

[txstrat]

As far as I understand there'd be a current flow only when the tubes draw the current. No draw no current flow anywhere and therefore no ripple.
BTW the ripple in the B+ is already smoothed by the choke and the cap after it. Don't you think that's sufficient? IME the choke is one of the best parts to smooth the ripple.

OTOH it wouldn't hurt the amp if you put the extra decoupling in there. Or at least leave some space on the board to add it later.

[tubeswell]

Thanks txstrat - Having thought about this a bit more, I don't know whether I should be concerned about it or not.

The ripple/wiggle I am uncertain about is what I suspect would come from the LFO plate back through the 470k plate resistor and thence into the DC rail - lets call it back-wiggle because that is probably a more accurate description of what it is. I understand that this could be present at all times unless the trem is switched off. If somebody has an opinion on whether I am chasing butterflies in this regard, please sing out.

Even if the primary winding on the RT acts like an inductor/coil, I am not confident that it would work to inhibit any back-wiggle, as it would be such a low frequency that the inductor probably wouldn't retain enough surplus electrons to cancel out the wiggle (if you can understand my - possibly flawed - understanding of inductors). Therefore I don't know how much good extra decoupling would be either, come to think of it. So am I fretting about nothin' much at all?

[txstrat]

TW
I've found the same design in the 6G16 Vibroverb schematic. The spot (B) right after the choke is connected to the plate of one of the LFO tubes and to the reverb transformer as well.
http://www.schematicheaven.com/fende...6g16_schem.pdf
Should be no issue.
Concerning the hum I've found, that the higher sensivity to hum in a reverb circuit is more related to the recovery tube, anyway.

[tubeswell]

Hi txstrat - duh - I should've looked at that scheme again before posting. Thanks

(PS Sorry for wasting your time )