Safe alternative to KT66

[tpatton]

My Quad II's are up and running nicely now, with the Russian 6L6-type tubes, R12 gone from 180R to 220R, and Obbligato Gold Premiums in for C1, not because I was sure it sounded better than the F104K from a Bottlehead kit, more just because I had the Obbligatos around with no other use in sight.

A final question for Henry's Cat and/or Ray P, along with much gratitude for their extensive help with this nonBottlehead project.  Thanks to all others who contributed too, and Paul Joppa in particular.

Question: with R12 now at 220R, what would happen if I put KT66 tubes back in?  Just lower output?

It turns out that one of the rectifier tubes in the amps as they came from the seller was a GZ34.  I'm loaded to the gills now with rectifier tubes, but if I ever need more, I could go with GZ32's, correct?

57 years of nostalgia have now been served, with the Quad II's sounding remarkably good.  A nice circuit, surely, though I'm hardly qualified to pronounce on that.

[tpatton]

Minor correction to my last post: ". . . . I could go with GZ34's, right?" 

That is, could I use GZ34's instead of GZ32's or some usual GZ32 equivalent?

[Henry's Cat]

Increasing R12 to 220 ohms for the 6L6 was recommended in the extract below from QUAD PAGES.  The higher value would create a higher grid bias voltage needed for the 6L6 so it would alter the conditions for a KT66. Not sure what it would do off-hand without looking at tube curves for a KT66 but it would probably lower output power.  I've used modern National GZ34's in my  Quad II's and they were fine but I currently have the original Mullard GZ32's.  Glad I could be of help.

Chris Beeching of PM Components has some very helpful advice on revalving:

"Wearing my PM hat, the Golden Dragon KT66 Retro has a filament current draw of 1.3amps, the same as the original GEC KT66. Confusion has arisen because of a (wrong) article which stated that the original GEC only drew 0.9amps. THIS IS NOT TRUE. The majority of the 6L6 family draw 0.9 amps, but the KT66 and KT88 (and particularly the EL34) are exceptions. A very good alternative to the GD KT66-R (if you're on a budget) is the excellent 350B which is shaped similalrly to a Mullard GZ32, but which is an output pentode, and will drop in without alteration. If you're running 6L6s you should raise the cathode resistor value to about 220ohms."

[tpatton]

Thanks, Henry's Cat.  I'll keep the KT66's out of the Quad II's unless I go back to 180R ohms for the resistor you mention, now 220R ohms.  But I have a bunch of tubes that 220R ohms works for, and will no doubt stick to them.  In my continued ignorance of the role of C1, near the input, and since I had a pair of Obbligatio Gold Premiums on hand and no other use for them in sight, I put them in as the C1's.  One way or another, the amps sound very nice indeed.  The circuit must be good, however antique.