Gretsch (Valco) Playboy 6156 OT question.

[ServiceBob]

Hi Tube guys!
A friend has asked if I could build him a Gretsch (Valco) 6156 "Playboy" amp. His first amp was a Playboy and he misses it. This would be my fifth build and my second PP amp. I found the schematic and it all seems doable and within my skill set. The only question I have is about the output transformer. It appears that there is a filter choke (I think) built into the OT. Is that what I'm seeing? I'm assuming that finding a transformer like this would be nearly impossible. Is there a way to duplicate the effect with a standard PP OT or how can I make this circuit work with parts I can actually find, without very much changing the tone quality? Thanks in advance for your help.

gretch5156.JPG

[sluckey]

That coil is a field coil, ie, electromagnet. That speaker does not have a real permanent magnet like modern speakers. You can replace it with a 1000Ω high wattage resistor.

[didit]

Hello -

There were a few different circuits associated with that amp model #, and other Valco "OEM" designs. Each is different sounding. Perhaps you know that and have researched it, but before major effort of a full build might be wise to confirm.

Best .. Ian

[thetragichero]

to further expand on what sluckey said, that is between the first b+ node (which goes to the ot center tap) and the second (which goes to the power tube screens). it's a f0nky way of drawing it, probably relating more to it's physical positioning (on the speaker, with possibly the output transformer is also attached to the speaker - that would be my guess based on the rectangle with ot and field coil connections) rather than ease of reading the schematic

[sluckey]

...that is between the first b+ node (which goes to the ot center tap) and the second (which goes to the power tube screens).

The OT CT and screens are both connected to the second node. Only the coil is connected to the first node. So the plate and screen current both travel through the field coil. That's why I suggested a high wattage resistor.

[didit]

...that is between the first b+ node (which goes to the ot center tap) and the second (which goes to the power tube screens).

The OT CT and screens are both connected to the second node. Only the coil is connected to the first node. So the plate and screen current both travel through the field coil. That's why I suggested a high wattage resistor.

Correct. It's a PI filter ahead of the power section entirely. Personally I would put a 10H 150mA choke, likely in series to hit 1000 Ohms, instead of a simple resistor. Also, the DC distribution string as drawn would yield major disappointment. However, seems fairly straightforward to infer what was intended.

Back to my previous comment: anticipate your friend's nostalgia is for a '60s era design around 6973 power tubes. Quite a different creature.

Best .. Ian

[sluckey]

There are two errors in the schematic posted by ServiceBob that will prevent the amp from working. Here's the corrected schematic.

[ServiceBob]

Thanks, folks!
Upon staring at the schematic more I see that I really don't understand quite a bit of this.
Now I see how a resistor or choke coil would solve my original concern about the OT.
But, as Didit has suggested, I also now see that I have no idea at all what's going on with the plate voltage on the 6SQ7 and on the second plate of the 6SC7, which I assume is the phase inverter. Where is the plate voltage coming from and by what process? I suppose I could just build it and maybe it would work - but I'd rather understand it, too. And it might be easy to infer their intent - but I lack the insight (and knowledge/experience) to make that inference.
And the point about there being more than one version of the Playboy is also a good one. I checked with my buddy and he is pretty sure his Playboy had 12ax7s. So I need to look further. But I'd still like to understand this problem I've been staring at all afternoon.
And where might I find another Playboy schematic?
Thanks for all of your help and education. It's priceless!
Bob

[ServiceBob]

I was writing whilst Sluckey was fixing!
That answered exactly what I was asking.

THANK YOU.

I was beginning to think I was stupider than I am!

[didit]

Hello -

Quick web search unhelpful finding another schematic but did yield it might be similar/identical to other Valco builds such as some Supro. My hardbound Jack Darr from the '70s includes exactly that older one posted above. Sorry.

Suggest you and your friend do a bit of research. The tele forum has a few threads on different variants - eg., https://www.tdpri.com/threads/1965-gret ... mp.744697/.

Best .. Ian

[ServiceBob]

Thanks, Didit, for the pointer! I found a lead to what appears to be the proper schematic.
https://el34world.com/charts/Schematics ... 510_54.pdf