DR BF/SF PT and rectifier questions

[hokenpoke]

A friend has given me a cosmetically nice but only semi-operational SFDR to work on.  I expect a number of problems will clear up with contact cleaning and appropriate retubing and replacement of electrolytics.  This is, however a stock SF and I told him I would look at what 'blackfacing' involves and we could see if he wants to give it try.

I am pretty clear on most of the changes that are involved, but the rectifier tube question hasn't yet been revealed with the clarity I would like.  Obviously there was a change from GZ34 to 5U4 between BF and SF, and it would seem that one mod would be to drop in a 5AR4 with resulting increase in B+ and decrease in sag.  But a couple of things I have read seem to indicate that the SF PT has a hotter HT secondary and that this would jack B+ too high for the power tubes.

Does anyone have a more definitive answer?  If a new PT is needed to truly blackface this amp, that obviously changes the amount of work and the economics.

[thermion]

the deluxe reverbs (regardless of bf or sf) were running 6v6gt plates at 420vdc...already way over spec. you'll add maybe 30-40vdc to the b+ with the gz34 compared to 5u4. rca 6v6gt datasheet says 350vdc max on the plate, 5u4 will have you at ~460. if you use jj 6v6 you probably will be fine with bias adjustment.
also, there are ways of using a small auxillary transformer to "buck" voltage off of pt winding to get your b+ down some if you want to use vintage 6v6 examples. vvr comes to mind as a more elegant solution.
or get a bf pt... lotsa ways to skin this cat.
happy hunting.

[hokenpoke]

Cool - so the SF must indeed have a hotter HT.  So the options are change tube, change tranny, or both.  Or option B like a VVR.

If the answer seems to be just the rectifier and the power tubes are OK with it, would there be any noticeable effect in the upstream stages?  Or could I just up the value of the power resistor on the bus to compensate?

[jojokeo]

To me, worrying about a little bit of B+ isn't going to make any noticeable or disceranable difference as far as tone is concerned. Why bother and why throw money & needless work into something like this just to it because you can say you did? Worry about other changes that are much more worth it like different/better tubes (if necessary) & re-bias, speaker, improve preamp biasing & bypassing, adding a switchable presensce or at least adj/switchable NFB, - then hot-rod up the rarely used 1st channel into something that's actaully desitreable to play.

[DummyLoad]

read the schematics! 5U4GB and GZ34 versions use the same PT - p/n 125P23B which is 660VCT - use whatever recto you like and don't worry.

--DL

[Fresh_Start]

+1 on jojokeo's suggestion.  Does his amp have the "volume boost"?  There are so many other things you could do with that switch - alternative cathode bypass cap(s) for one.  As far as I'm concerned, the "Normal" channel is a blank piece of paper except for 2 triodes, volume, treble and bass.  Does it have "Bright" switches?  I built a Super Reverb where I turned the Bright switch into a guitar/harmonica switch.  Worked out pretty well too.

Cheers,

Chip

[jojokeo]

Chip, on the normal channel I've "simply" cascaded the two stages with the stack placed after and it made a teriffic gain difference. I seperated the shared cathodes and tweaked the Rk's, bypass, & coupling caps, and lastly a load resistor too. These changes made all the difference in the world to that weak channel.
I liked your amp I saw and looked really well done. Not sure I saw how you changed the bright switch, just that you did it and may have labeled it that way?

[worth]

Don't trust the voltages shown on old schematics. The ONLY way to be sure about your PT voltages is to put a meter on it. The AB763 DR shows a plate V of 415V using a GZ34. The AB868 DR shows a plate V of 415V using a 5U4. That can't be correct if the same PT was used on both. Either the voltages shown on the schem are wrong ,( which is probably the case, schematic states +/- 20% ) or the Blackface DR has a different HV secondary voltage than the Silverface.

[Fresh_Start]

JJ - here's the preamp schematic.  I used both poles of one of Doug's slide switches in each channel:



I like the idea of doing something really different on the Normal channel though.  Did you put the Volume between the cascaded triodes?  Mine was just a tweaked version of a brown Vibroverb.

Cheers,

Chip

[jojokeo]

I like the idea of doing something really different on the Normal channel though.  Did you put the Volume between the cascaded triodes?  Mine was just a tweaked version of a brown Vibroverb.

No, you don't want anything between gain stages like that except coupling cap, grid, & grid bias resistor. The treb, bass, & vol are all after the two stages cascading keeping you signal gain intact is the way to go. Here's what I did to a bandmate's '65bfdr, everyone loves the tone of this thing it just needed a bit of refinement is all. Here's what I ended up doing.
Jojo