Reverb driving methods

[tubeswell]

Thanks pdf64 and Merlin.

Ver 12 f attached. I'm pondering whether the dwell control could be 250k or 1M? (I suppose 1M is better). If need be, I can put a grid stopper on the 2nd 1/2 grid pin later.

In driving the first 1/2 of the 12AT7 from the buffer stage (as I have shown now), is a grid stopper really necessary? (I have shown one in any event). I guess the voltage swing off the CF cathode is large enough to put surplus charge the 0.1uF coupling cap, and if there is a 1M grid leak at the 12AT7 first 1/2, then you might get problems, but does the low impedance output of the CF stage mitigate that somewhat (or is it irrelevant?). Besides that, there is a 100k cathode resistor right there which may help bleed excess charge to ground (methinks?) - in addition to allowing a signal to be taken off the cathode.

I'll leave the 2nd 1/2 of the 12AT7 bypass to later experiment.

I've also shown 100k on the recovery stage grid (but obviously 47k will provide sufficient impedance bridging as well for minimal noise). I note that the 6G15 has a whopping 250uF bypass cap, but I wonder why? I thought it might be fun to see if increasing it to that ballpark makes any difference.

I'll leave the recovery stage tone stack issues until I get the reverb intensity sorted. (at the moment there's good treble response but not good bass response - and I'm hoping this will balance out more if I can get more drive into the pan)

Lastly, I guess its still okay, with this revised topology, to power the 1st 1/2 of the 12AT7 from the same supply node as the V1 plates?

I appreciate your support. Cheers :-)

[unparalleled]

Hammond used to drive "dangly spring" reverb with a 12BH7 & PP transfomer.
(you could use a $10 Jaycar 100V line xformer)
12BH7 is pin compatible with ECC..... and can do 3.5 watts per plate!

[Gregg]

How about skipping the reverb driver transformer the way Trace Elliot did it (pg. 2):

http://www.mif.pg.gda.pl/homepages/t...bonneville.pdf

[tubeswell]

A happy conclusion to this tale.

I went and had a yak with Simcha Delft yesterday, and while I had been planning to cascade the 12AT7 in any event, I actually got some sagely tips about the biasing, gain structure and impedances.

So the new (and improved) driver setup is a cascaded 12AT7 with 47k on the first anode (which is powered from the same decoupling node as V1), and a 1k5 cathode resistor bypassed with 22uF, and a 22nF coupling cap and 1M grid load resistor going to the second stage, which has 435V on the anode driving the RT primary, and a 4k7 cathode resistor bypassed by 22uF. The dwell control (and I'm glad I built one into the amp because it really is necessary here) is in front of the first gain stage. In the end I left the recovery stage in the BF format, and the 'standalone' bypass/buffer CF stage is still there with everything mixed thru 250KN. I took out the coupling cap going to the DC pair.

With this setup there is bucket-loads of reverb (everything from Dick Dale downwards ), so I am over the moon about it. Thanks once again to everybody here for your helpful suggestions and participation.

The only thing still to sort out is the trem - but I'll finish that episode on another thread.