Deluxe issue

Paul B · April 5th, 2006, 08:05 AM

Hello

I have built from scratch a 1965 Deluxe reverb.

But I have an issue. All the voltages on the preamps tubes are double the value than what the values are on the schematic chart.

I have checked the voltages on the rectifier tubes and all the power tubes and they are correct. The voltages on the electrolytic capacitors are also correct.

The sound is very distorted and the volume knob needs to be turned up all the way to get any sound heard through the speakers.

What is causing the voltages in all the preamp tubes to double ?

Regards

Paul

FMA · April 5th, 2006, 10:40 AM

I can't help you, but over at the Fender Discussion Page, they have a forum on amp mods and projects, frequented by a lot of tube heads and amp techs. They may have an answer.
Here's a link: http://www.fenderforum.com/forum.html

Wally · April 5th, 2006, 01:34 PM

Have you double/triple checked the values for the resistance along the power supply for the preamp section?

mark norwine · April 5th, 2006, 01:42 PM

voltage goes up when current goes down. If you're not conducting current through the tubes then the plate voltage will rise up to the B+ ladder voltage associated with that node.

I had a guy bring in a home mead amp with similar issues....instead of 1K5 cathode resistors, he had used 1M5. That shut off the tube, killed the volume and raised the plate voltage.

Could you have made a similar mistake?

JohnnyCrash · April 5th, 2006, 03:13 PM

Check pins 1 and 6 (your preamp tubes' Plates) on your first two preamp tubes... they go to 100k resistors...

Make sure your 100Kohm resistors here are NOT just 100ohm (uh, without the k, hehe).

I made that same mistake on a different amp, voltages checked out everywhere except the preamp's plates. Drove me nuts for about a week... even shows up "100" on a multimeter, but you gotta check for the "K"!

Paul B · April 5th, 2006, 05:38 PM

Thanks for the replies.

The issue is that a wire from the electrolytic capacitors that join to two 100k resistors that then split the voltage to pin 1 and 6 of first 12AT7 tube does not reduce the voltage down from 392 volts down to 170 volts.

Why has this voltage not been split down from 392 to 170 ?

All the preamps tubes have on pin 1 and 6 double the voltage that they should have. The pins have now 372 volts when they should have 180 volts !

Therefore what haven't I missed that has caused these voltages not to reduce ?

Regards

Paul

alotas123 · April 5th, 2006, 05:47 PM

You have a voltage divider that isn't working? If the resistors have the right values (check them) then it seems like only two possibilities. The first resistor is shorted or the connection to ground through the second resistor is broken.

Paul B · April 5th, 2006, 06:39 PM

I will replace the resistors and check that all wires to ground are soldered in place.

Then get back to you.

I have checked and the resistors do measure 110 k.

Regards

Paul

Paul B · April 6th, 2006, 06:50 AM

Have tried all this and no solutions.

Any one has any other ideas.

Wally · April 6th, 2006, 12:45 PM

Quote:

Originally Posted by mark norwine

voltage goes up when current goes down. If you're not conducting current through the tubes then the plate voltage will rise up to the B+ ladder voltage associated with that node.

I had a guy bring in a home mead amp with similar issues....instead of 1K5 cathode resistors, he had used 1M5. That shut off the tube, killed the volume and raised the plate voltage.

Could you have made a similar mistake?

Paul, did you check your cathode resistance values and grounding? Mark may be on the track with his observation of the entire tube circuit.

Paul B · April 6th, 2006, 07:11 PM

No I have not.

Which pins of the preamp tubes should I check to confirm that the earthing has occurred ?

In other words which pin is the cathods resistor ?

Wally · April 6th, 2006, 07:50 PM

Pins 3 and 8 are your cathodes. As Mark notes, the bias in these tubes is set by the cathodes. IF the resistance is too great, whether due to an incorrect cathode resistance value or a ground not made, the voltage would go up as yours has and function suffers.

Rick S · April 6th, 2006, 08:47 PM

Just to back up a bit and make sure we don't have a wild goose chase going on, Paul, would you please confirm that you have tubes installed in the sockets while taking the voltage readings?

Paul B · April 7th, 2006, 04:58 AM

Yes I took all voltages with the tubes in the socket and the power on.

Paul B · April 7th, 2006, 10:35 AM

Apart from using a multimeter to test for conductivity on all those points any other tests that I can do to check the earthing?

Rick S · April 7th, 2006, 05:56 PM

Quote:

Originally Posted by Paul B

Apart from using a multimeter to test for conductivity on all those points any other tests that I can do to check the earthing?

Let's review to see if we have this straight. You have correct voltage from the power supply rail to all of the 100K plate resistors. You have confirmed the 100K value on these resistors and have known good tubes installed and there is no voltage drop across these plate resistors. As Mark stated, this would indicate the tubes are not conducting. So, lets move to the cathode side of the tubes.

Wally told us that Pins 3 and 8 are the cathodes for each triode. If you follow the wire on these pins to your circuit board they will lead to the cathode resistor. These are the resistors you have paired in shunt (parallel) with all of the 25uf/25v bypass caps. The junction across the cathode resistors from the wire to the cathode pin on the tube socket should have a solid ground connection.

To check out the cathode circuits, hook the black lead of your meter to one of the power transformer mounting screw studs. Probe each of the cathode pins while checking your schematic for the value of the cathode resistor for each pin. For instance, the cathode on your first gain stage has a 1.5K resistor. We are looking for a reading within about 10% of 1.5K on this pin.

Also, while you are in there, confirm that you understand the polarity marking for your 25uf/25v caps and have the correct orientation, with negative side to ground.

Paul B · April 7th, 2006, 10:14 PM

I have checked the polarity of teh capacitors and they are all correct. The negative side are all going to earth.

The cathode on your first gain stage measures 1.5K for the first two 12AX7 tubes ( these tubes are linked to the volume pot. )

Still the same result. Can this be purely caused by bad preamp tubes ? How do you check tubes except to place them into another amp ?

My question is to check the earth with a multimeter can you place a probe on pin 3 and 8 of the preamp tubes and confirm conductivity by the beep noise that should come from the multimeter ?

Regards

Paul

April 5th, 2006, 08:05 AM	#1 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	Deluxe issue Hello I have built from scratch a 1965 Deluxe reverb. But I have an issue. All the voltages on the preamps tubes are double the value than what the values are on the schematic chart. I have checked the voltages on the rectifier tubes and all the power tubes and they are correct. The voltages on the electrolytic capacitors are also correct. The sound is very distorted and the volume knob needs to be turned up all the way to get any sound heard through the speakers. What is causing the voltages in all the preamp tubes to double ? Regards Paul __________________ Hipbluesman

April 5th, 2006, 10:40 AM	#2 (permalink)
FMA Tele-Afflicted Join Date: Apr 2003 Posts: 1,116	Paul I can't help you, but over at the Fender Discussion Page, they have a forum on amp mods and projects, frequented by a lot of tube heads and amp techs. They may have an answer. Here's a link: http://www.fenderforum.com/forum.html

April 5th, 2006, 01:42 PM	#4 (permalink)
mark norwine Tele-Afflicted Join Date: Mar 2003 Location: New Jersey Age: 50 Posts: 1,381	voltage goes up when current goes down. If you're not conducting current through the tubes then the plate voltage will rise up to the B+ ladder voltage associated with that node. I had a guy bring in a home mead amp with similar issues....instead of 1K5 cathode resistors, he had used 1M5. That shut off the tube, killed the volume and raised the plate voltage. Could you have made a similar mistake? __________________ "Progress is not possible without deviation from the norm." Frank Zappa

April 5th, 2006, 03:13 PM	#5 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	Check pins 1 & 6 resistors for... Check pins 1 and 6 (your preamp tubes' Plates) on your first two preamp tubes... they go to 100k resistors... Make sure your 100Kohm resistors here are NOT just 100ohm (uh, without the k, hehe). I made that same mistake on a different amp, voltages checked out everywhere except the preamp's plates. Drove me nuts for about a week... even shows up "100" on a multimeter, but you gotta check for the "K"!

April 5th, 2006, 05:38 PM	#6 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	Thanks for the replies. The issue is that a wire from the electrolytic capacitors that join to two 100k resistors that then split the voltage to pin 1 and 6 of first 12AT7 tube does not reduce the voltage down from 392 volts down to 170 volts. Why has this voltage not been split down from 392 to 170 ? All the preamps tubes have on pin 1 and 6 double the voltage that they should have. The pins have now 372 volts when they should have 180 volts ! Therefore what haven't I missed that has caused these voltages not to reduce ? Regards Paul __________________ Hipbluesman

April 5th, 2006, 01:34 PM	#3 (permalink)
Wally Friend of Leo's Join Date: Mar 2003 Location: Lubbock, TX Posts: 3,988	Have you double/triple checked the values for the resistance along the power supply for the preamp section?

April 5th, 2006, 05:47 PM	#7 (permalink)
alotas123 Tele-Meister Join Date: Aug 2004 Location: Hill Country Texas Posts: 470	You have a voltage divider that isn't working? If the resistors have the right values (check them) then it seems like only two possibilities. The first resistor is shorted or the connection to ground through the second resistor is broken. __________________ Play that thing, make it sing!

April 5th, 2006, 06:39 PM	#8 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	I will replace the resistors and check that all wires to ground are soldered in place. Then get back to you. I have checked and the resistors do measure 110 k. Regards Paul __________________ Hipbluesman

April 6th, 2006, 06:50 AM	#9 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	Have tried all this and no solutions. Any one has any other ideas. __________________ Hipbluesman

April 6th, 2006, 07:11 PM	#11 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	No I have not. Which pins of the preamp tubes should I check to confirm that the earthing has occurred ? In other words which pin is the cathods resistor ? __________________ Hipbluesman

April 6th, 2006, 07:50 PM	#12 (permalink)
Wally Friend of Leo's Join Date: Mar 2003 Location: Lubbock, TX Posts: 3,988	Pins 3 and 8 are your cathodes. As Mark notes, the bias in these tubes is set by the cathodes. IF the resistance is too great, whether due to an incorrect cathode resistance value or a ground not made, the voltage would go up as yours has and function suffers.

April 6th, 2006, 08:47 PM	#13 (permalink)
Rick S Tele-Meister Join Date: Nov 2003 Location: Tulsa, OK Posts: 493	Just to back up a bit and make sure we don't have a wild goose chase going on, Paul, would you please confirm that you have tubes installed in the sockets while taking the voltage readings?

April 7th, 2006, 04:58 AM	#14 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	Yes I took all voltages with the tubes in the socket and the power on. __________________ Hipbluesman

April 7th, 2006, 10:35 AM	#15 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	Apart from using a multimeter to test for conductivity on all those points any other tests that I can do to check the earthing? __________________ Hipbluesman

April 7th, 2006, 10:14 PM	#17 (permalink)
Paul B Tele-Meister Join Date: Mar 2003 Location: Australia Posts: 274	I have checked the polarity of teh capacitors and they are all correct. The negative side are all going to earth. The cathode on your first gain stage measures 1.5K for the first two 12AX7 tubes ( these tubes are linked to the volume pot. ) Still the same result. Can this be purely caused by bad preamp tubes ? How do you check tubes except to place them into another amp ? My question is to check the earth with a multimeter can you place a probe on pin 3 and 8 of the preamp tubes and confirm conductivity by the beep noise that should come from the multimeter ? Regards Paul __________________ Hipbluesman