Author Topic: Gretsch Electromatic Amp, power amp question; Schematic  (Read 2401 times)

adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« on: July 22, 2006, 12:28:55 AM »
Hi

I am fixing up an old Gretsch Electromatic amp and came accross something I have not seen before and can not find on any other amps. On the power amp, PP 6V6, the cathodes are tied together and then have the normal resistor to ground shared between the two of them. The odd thing is tied directly to the cathodes is a lead from the power transformer. It is a green lead which is very often the sheild, but if it is that, or something else, why would it be tied to the cathode?

In general this is a kinda weird amp. When I have some time to scan I will post the schematic and some pictures for those of you who would be interested.

thanks,
adam
so there


Gus

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #1 on: July 22, 2006, 06:33:44 PM »
I think it is standard old school design

I would guess the wire is the center tap for the fil supply.

  I use this old trick sometime with cathode biased outputs with cap bypass.  it floats the fils above ground helping reduce fil noise and hum

adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #2 on: July 22, 2006, 07:01:47 PM »
Ahh, That is simple enough. This is the first pre1960s amp I have worked on, so the first time I have come accross this, could not find it in RDH4 either. Thats a good thing to know.

Here is the schematic as the amp is now, and a few pictures of it.

www.sdiy.org/oid/gretsch6169.jpg
www.sdiy.org/oid/gretsch1.jpg
www.sdiy.org/oid/gretsch2.jpg
www.sdiy.org/oid/gretsch3.jpg

This amp was full of band aide fixes and just weird modifications. Power switch was bypassed and the fuse used as a power switch, instead of replacing the filter caps they just just stuck another 40uF on the last filter stage, disconnected the tremolo and maybe did some screwy stuff with that as well. You will notice the 270k grid resistor on the 6V6 goes to the phase inverters grid, I am assume the tremolo connected at these resistors originaly and when they disconnected the tremolo they put the resistor on the wrong pin. It is deffinetly new solder on that spot, and the dust is cleared from that spot as well. At least I can not find any precedent of this being done yet, but I could not find one of the heater center tap being tied to the power tube cathodes either. At least who ever worked on it in the past just added to the circuit and did not make any real changes, nice and easy to get his stuff out.

thanks
adam
so there

Gus

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #3 on: July 22, 2006, 07:12:01 PM »
That is a cool looking amp.

  I would remove the tubes and measure the green wire to all the other power transformer wires.

  I guess my guess was wrong.

I wonder what the wire is for.

solder_city

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #4 on: July 22, 2006, 07:22:54 PM »
the trem osc will not work the way its drawn. that .05 cap should go to the osc grid instead of ground.

adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #5 on: July 22, 2006, 07:35:41 PM »
Whoops, erased the wrong line useing the digital white out. fixed. uploading now.

adam
so there

adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #6 on: July 22, 2006, 07:41:46 PM »
Quote from: "Gus"
That is a cool looking amp.

  I would remove the tubes and measure the green wire to all the other power transformer wires.

  I guess my guess was wrong.

I wonder what the wire is for.


Yeah that was the plan once I finished getting out all the mods, could not even see most of the original circuit under all the added parts. Just going to recap, throw in a new fuse holder and power chord, first. The amp had the awsome duct tape spliced 50 foot power chord on it.

What makes you think your guess was wrong? I guess I should mention I did not put that leed on the schematic yet since I had no clue where it went on the transformer yet.

adam
so there

adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #7 on: July 22, 2006, 07:51:57 PM »
Nope you were right, center tap for the 6.3V winding.

Anyone have any comments on what my logic tells me about the 6V6 grid resistor and the tremolo oscilator output?

Back to work

thanks,
adam
so there

dabo

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #8 on: July 23, 2006, 04:19:43 PM »
The 270k is likely in the correct place. This phase inverter is not the standard long tail pair approach to driving the output tubes differentially.

dabo

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #9 on: July 23, 2006, 04:27:29 PM »
Or perhaps it's supposed to be, but the earlier "repairs" mucked it up horribly. Hmm...


adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #10 on: July 24, 2006, 12:49:20 AM »
I have not found any phase inverters like this one yet, I have found a few instences close to this though. They all split the power tube's grid resistor and feedback to the other side of the phase inverter from the split, never through the full grid resistence. I figure I will just see what kind of voltage the trem oscilator is putting out, that will tell me what tube it was supposed to modulate.

adam
so there

PRR

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #11 on: July 24, 2006, 10:35:07 PM »
> You will notice the 270k grid resistor on the 6V6 goes to the phase inverters grid

This is a perfectly common phase inverter.

> could not find it in RDH4

Page 524, Figs 12.30A and 12.30B, but it is not quite like either one. It is one of a huge family of Paraphase drivers. The Patents and the RDH are bullplop; real designers who had to keep it cheap and keep their job did it as in your schematic.

>> the wire is the center tap for the fil supply.

> could not find it in RDH4

Page 540, section 12.10.G.2, suggests heater CT to a small bias voltage, but does not suggest the output tube cathode. (I suggest that this would be "obvious" to anybody who had spend too much time poking a meter in a tube radio.)

> I am assume the tremolo connected at these resistors originaly

Betcha it goes to the first stage cathode.

http://www.schematicheaven.com/bargainbin.htm
Scroll down to Gretch in the left column. They don't have your model, but the others have the same mix-n-match pieces.

adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #12 on: July 25, 2006, 02:39:41 AM »
Quote
Page 524, Figs 12.30A and 12.30B, but it is not quite like either one. It is one of a huge family of Paraphase drivers. The Patents and the RDH are bullplop; real designers who had to keep it cheap and keep their job did it as in your schematic.


I suppose they saved a resistor, and I am sure Valco wanted to get as much money as they could from gretsch. I have been going through the Methodes of Exciting Push-Pull amps section of RDH4 this past couple days, I never really spent much time bothering with these since I prefer single ended amps, but its time I learn.

Quote
Page 540, section 12.10.G.2, suggests heater CT to a small bias voltage, but does not suggest the output tube cathode. (I suggest that this would be "obvious" to anybody who had spend too much time poking a meter in a tube radio.)


Well thats my problem, I have never poked about a tube radio, just amps, and none this old. No, one tube radio, a big old Hellicrafters I fixed up for a friend, it did not have a center tap on the filament supply.

Quote
Betcha it goes to the first stage cathode.


Betcha you are right, last place for it to go.
Quote

http://www.schematicheaven.com/bargainbin.htm
Scroll down to Gretch in the left column. They don't have your model, but the others have the same mix-n-match pieces.


Missed the Gretsch schematics when I went there somehow, but I did go through the Valco and found a couple that were close.

This is pretty close to ready to try out, just need to run a wire from the tremolo and wire the power chord and output transformer. Although I suspect it is going to need coupling caps replaced. Who ever worked on it in the past just stuck a second cap of the same value in series with the original ones. I can only assume they were dealing with the leaky caps in their own way with that, cant imagine it sounded good with its cutoff shifted up an octave.

thanks,
adam
so there

adamasd

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Gretsch Electromatic Amp, power amp question; Schematic
« Reply #13 on: August 08, 2006, 03:03:29 AM »
Finally got a chance to finish it up and got it up and running, it is sounding great. Noticed this nice bit of wiring on the 5Y3s socket,

Notice the fillament wires (yellow), no solder! and by the looks of the oxidation, it came that way from the factory and has worked every day since without a problem. The wires were not even that tight on the tabs, easily moved. The worst part is by looking at the parts that were changed and added to the amp, it looks like two people worked on it in the past, and neither noticed. Thanks for all your help in getting this one sorted out.

adam
so there


 

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