Gretsch6151 preamp fun project - Goes silent when tremolo tube is put in - diyAudio
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Old 8th July 2013, 09:05 PM   #1
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Default Gretsch6151 preamp fun project - Goes silent when tremolo tube is put in

Hi everybody.

I had some scrap parts at home, so i found myself a little project to keep me busy. It is a Gretsch 6151 style preamp with tremolo.

Here is the schematic of my build:

Click the image to open in full size.

Only parts, that i have changed are:
C7,C8 - should be 4.7n, but i could not found any at home
C12 - should be 10n, but again i was out of stock (but this one should not really matter)
R4 - this is a potentiometer, originaly 500K
R11/R13 - should be an 470K resistor, i changed it to 220K resistor and a 250K potentiometer for depth of the tremolo.
Added a switch for disconecting the R2 from ground for Tremolo on/off.

The problem is, that with the Tremolo tube put in, the preamp does not amplify any signal (hum from heaters is audible). Turning the tremolo off does not help.
Without the tube the amp amplifies very vell.

Can you please give me any suggestions?


Thanks for your help.

Regards,
Tibor
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Old 8th July 2013, 10:57 PM   #2
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
Yes. One half of your trem tube is a cathode follower, U2. When it is out of the circuit, its cathode resistor is shared with that of triode U3, so U3 works normally.

When you install the trem tube, that cathode follower conducts current, and so places the cathode at a substantial positive voltage, which is also on the cathode of U3. It appears to me that that might be enough voltage there to bias U3 into cutoff, so no signal passes.

When the trem tube is installed, what DC voltage is present at the shared cathode, point TR on the drawing?
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Old 9th July 2013, 06:31 AM   #3
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Hi.

Thanks Enzo for your time and suggestion, but it seems it wont be needed.

I checked and rechecked everything possible and finally found a bad soldering on the grounding of R6/C6. Resoldering this point removed the problem.

The only thing now is the depth pot - 250K doesnt do much, so ill have to change it.

Again thanks for your time.

Regards,
Tibor.
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Old 9th July 2013, 08:58 AM   #4
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Quote:
Originally Posted by vitahamin View Post
The only thing now is the depth pot - 250K doesnt do much, so ill have to change it.
Presumably it's drawn wrong?, as it makes little sense as it is - so check how it should be wired.
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Old 10th July 2013, 06:31 AM   #5
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Quote:
Originally Posted by Nigel Goodwin View Post
Presumably it's drawn wrong?, as it makes little sense as it is - so check how it should be wired.
In the end i just took out the 220K resistor. With just the pot i get just enough Depth.

Can you please explain why does the drawing makes so little sense?

Regads,
Tibor.
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Old 10th July 2013, 08:21 AM   #6
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Quote:
Originally Posted by vitahamin View Post
In the end i just took out the 220K resistor. With just the pot i get just enough Depth.

Can you please explain why does the drawing makes so little sense?

Regads,
Tibor.
You mean apart from R13 isn't even shown as a pot?

I would imagine R13 slider should connect to R5, thus feeding a variable amount of tremolo to the grid of the valve. Connecting R5 as it's shown stops it having very much effect.

I've just downloaded the original Gretsch circuit - I didn't realise it was YOU adding a pot, and that the Gretsch circuit was just switched. So connect it as I suggested, and see how that works.
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Old 10th July 2013, 12:13 PM   #7
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Quote:
Originally Posted by Nigel Goodwin View Post
You mean apart from R13 isn't even shown as a pot?

I would imagine R13 slider should connect to R5, thus feeding a variable amount of tremolo to the grid of the valve. Connecting R5 as it's shown stops it having very much effect.

I've just downloaded the original Gretsch circuit - I didn't realise it was YOU adding a pot, and that the Gretsch circuit was just switched. So connect it as I suggested, and see how that works.
Yeah... sorry for R13 not being a pot - i dont have them in my LTC library.

If i understand you correctly, you want me to connect one side of the pot to the ground, slider to the grid stopper R5 and leave the other side of the pot unconnected. If so, that is exactly how it was connected from the beginning.

As i mentioned earlier, i bypassed the 220K resistor, which was connected between the R5 and the slider of the Depth pot and that removed my problem. Seems, that 470K on that position was just too large. With 250K i can go from just a mellow vibrato to a deep, choppy tremolo.

There are few things, that i have to resolve - hum from heaters and stability becouse of the tremolo, but those issues should not be mysteries for me (2x100ohm resistors and positioning of the oscilator circuit).
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Old 10th July 2013, 06:34 PM   #8
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Quote:
Originally Posted by vitahamin View Post
Yeah... sorry for R13 not being a pot - i dont have them in my LTC library.

If i understand you correctly, you want me to connect one side of the pot to the ground, slider to the grid stopper R5 and leave the other side of the pot unconnected. If so, that is exactly how it was connected from the beginning.
No, the other side of the pot to R11.
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Old 10th July 2013, 08:21 PM   #9
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Ok. Ill try that.


But another thing popped up, that riddles me. When i installed the 100 ohm resistors to the fillament supply i get no sound. The tubes heat up and i can hear a slight buzz, but the guitar doesnt go through.

Checking the B+ Gives me 320V, so that is okay.

The supply is a EZ81 with two 1N4007 (The transformer has no center tap). Fillament supply is 6.3V without center tap and is connected to all 3 tubes (EZ81 and 2x12AX7).
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Old 15th July 2013, 01:06 PM   #10
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Well now im getting pretty upset. One problem away other pop up.

So when i use any type of grounding the filament supply (grounding one side, 2x100 ohm, 420n capacitors) the amp powers on, starts to silently hum, but no signal comes through.

So i leaved the filament supply ungrounded.

this way the preamp worked but had a serious case of hum. Using it with my home made amplifier (Hi-Fi) it was tolerable (but anoying), but it is unusable with my Fender FM212R.

It sounded like a ground loop, so i completely reworked the grounding and elevated it (10ohm resistor parallel with 100n cap between signal ground and ground).
But the problem did not go away.

Working with the pre-amp i noticed, that when connected to power amp, which is turned on, the hum appears even when the power cord is pluged in (presumably picking up noise from surrounding equipment. The hum is audible even with no tubes pluged in.

I chcecked the 1N4007, they seem to be ok, and they are positioned correctly. I even tried to use a diode bridge instead, but with no succes.

Next i checked the power supply filter caps, but they are okay.

Right now im suspecting the power transformer. I already tried to shield it off with a steel box which i mounted to the chassis, but with no effect.

Can it be, that the transformer is faulty? Or is there an error in my grounding?
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