Gretsch Playboy bias supply & weird preamp problem (pic inside)

[SirElwood]

Hi.

Does this bias supply work http://www.gretschguitars.com/resources/schematic_playboy.pdf ? I remember seeing similar circuit somewhere, but I can't remember where. However, to me that looks somehow wrong. Please enlighten me.

[sluckey]

Yes, works well. You can see that same circuit in the Marshall JCM-900 and several of the old Ampeg amps.

[SirElwood]

Thanks so much sluckey!

[PRR]

The signal at the bottom of the 0.047u (from the side of the FWB) is half-wave rectified unfiltered AC, several hundred volts.

The 0.047u and the 56K form an AC voltage divider. However this is loaded by D5 and all the rest of that stuff. The smaller AC bumps are rectified-off through D5 etc. You can also study it as a step generator or charge-pump: the 0.047u dumps pulses of charge onto the 10u cap. But the pump is leaky due to the 56K and 15K resistors.

The result is much less than hundreds of Volts of DC. Yes, it does work.

You don't compute these things by hand.

You don't "approximate" the values (say 0.033u in place of 0.047u).

You don't expect a proven design from one amp to work on another amp. Go ahead and plagiarize, but bench-test and measure the actual output before you stick tubes in. Trimming is probably best done with small changes to the "15K" part, not the 56K part.

When in doubt, take the smallest 120V:12V transformer you can find, feed your 6V to the "12V" side, take the resulting 60V from the "120V" side, and rectify and divide. The hardly-loaded under-volted transformer will last forever, the bias is solid.

[SirElwood]

Thanks PPR. I did build this amp, and bias supply indeed works fine. I was just a little cautious... However, W_a is now 12.5W. I need to see how I can get it little lower.

Schematic btw. has atleast one mistake. First 500pF cap should be in anode (pin1), not in grid as it's drawn. And here is my second problem: I have a werd thing going on whit that preamp. I can disconnect that same 500pF cap from anode, and I still get sound! I can also disconnect next grid (pin7), and yes, Still some sound comes thru. What the hell? I have done all "standard" things, changing tubes and so on, but it still "leaks thru". Layout problem? It's a thight fit, but nothing I haven't done before. Or is there some other mistakes or "mistakes" in that schematic?

[sluckey]

I can disconnect that same 500pF cap from anode, and I still get sound!

How much sound? Same level? Barely hear it?

3 possibilities come to mind...

• Bad tube, but you've already eliminated that.
• Bad filter cap at node C. Just bridge another 20µF across the node c cap to eliminate this.
• Layout issues, wiring error, or bad ground.

Aside from the obvious error with the 500pF cap, I don't see any thing on the schematic that would cause this. The trem cathode follower looks whacky to me, but I doubt that's the problem. You could always just disconnect the wire from V1B cathode that connects to the trem CF.

I can also disconnect next grid (pin7), and yes, Still some sound comes thru.

If pin 7 has nothing connected to it, all kinds of unpredictable things could happen. Try grounding pin 7 instead. Does that kill the sound?

[SirElwood]

How much sound? Same level? Barely hear it?

I can hear it very well, but it's not loud by any means.

3 possibilities come to mind...

• Bad tube, but you've already eliminated that.
• Bad filter cap at node C. Just bridge another 20µF across the node c cap to eliminate this.

I changed filtercap as well. No change.

• Layout issues, wiring error, or bad ground.

Bad ground was my 1st idea (and I somehow still think it is). But I didn't find any yet. There is also some hummmm, but I think that might be another issue (I have old power tubes whit too high bias and so on). Wiring error is of course always a possibility.

Aside from the obvious error with the 500pF cap, I don't see any thing on the schematic that would cause this. The trem cathode follower looks whacky to me, but I doubt that's the problem. You could always just disconnect the wire from V1B cathode that connects to the trem CF.

Tremolo works ok as far as I can tell for now, and preamp problem is there even if I remove tremolo tube. There is a little "thumb" noise, but nothing else. But what do you mean about "whacky"? I'm always open for modification ideas!

If pin 7 has nothing connected to it, all kinds of unpredictable things could happen. Try grounding pin 7 instead. Does that kill the sound?

I will try this. Thanks!

[sluckey]

But what do you mean about "whacky"?

I initially thought the intensity pot was wired wrong, but I now see that it's fine.

[SirElwood]

Okay... Some random tinkering. I replaced parallel 470k and 10pF combo that goes to V1b grid with random piece of wire. Guess what... No more "leaking volume pot"! I don't understand at all. I quess there is actually two "potentiometers" in series? One adjustable (of course) and 470k and 270k forms another "potentiometer"?

I also reduced the bias to about 8W and no more hummmm. Sweet things. Well, not quite. Now I have small amount of DC on tone potentiometers, but only when tremolo is on. And yes, some clicking when tremolo is on, but I don't think it's that bad. Alot less than in my vibro champ.

Other than that, amp sounds actually really nice. Somewhat on the bright side, but not icepick bright. Can't wait to test it whit les paul.

[SirElwood]

Slow progress today. I changed that 470k/10pF combo with 100k resistor, and now it seem to be better, but it's not silent when volume is turned down. Damn! That jumper wire works, but then there is waaaay too much gain. Oh well. And there is still some DC on tone potentiometers when tremolo is on.

Here is a hi-res picture: http://www.mia-amps.com/images/SDC10012.jpg

Any ideas anyone?

[VMS]

I don't know if this would help, but I would ground that volume pot to the same point where you have grounded the 270k and V1b cathode components 1k and 25uF. Mid pot should probably be grounded to the same point where the 1M resistor ground is or the 33k resistor.

BTW, I have had similar experience with volume not going to zero when I had resistors between vol pot and the next tube grid.

[SirElwood]

Thanks VMS. I will try to change grounding.

[VMS]

Don't thank me yet. 

The reason for the volume problem could also be that, because there is those resistors after the vol pot the grid doesn't go to ground when you turn volume down and therefore it can pickup the signal from somewhere. I think.     Maybe someone could confirm or debunk this theory of mine. 

If you have too much gain with jumper wire you could try 470k-1M (series) resistor before the vol pot.

[SirElwood]

Don't thank me yet. 

All ideas are welcome.

If you have too much gain with jumper wire you could try 470k-1M (series) resistor before the vol pot.

I need to try this too.

[SirElwood]

Short report... I changed 470k/270k divider before volumepot, and now volume pot works fine! There is still some DC in tone controls when tremolo is on, but I don't care... It's not that bad.

Thanks for all your ideas and help guys.

Here's a quick picture:


EDIT: All problems solved! Thanks!