Deluxe Reverb Build Problem!

[Penguin]

Hey, friends. My name is Michael and I'm a guitar player from Toronto, Canada. I enjoy working on guitars and building amps. This is the second amp I've built. The first one was a modified 5E3 that sounded absolutely incredible! With that being said, on to the problem...

This amp started out as a Fender Deluxe Reverb Reissue. I decided to gut it and rebuild it using a Mojotone Deluxe Reverb Parts Kit as I'm a fan of the simplicity and workability of hand wired amps. I finished the build and the amp sounds superb; arguably better than any Deluxe Reverb I've heard! However, there is a loud buzz I can't seem to eliminate!

Things I've noticed:

Pulling V4 stops the hum but I can still play the Normal channel.

Grounding pin 7 on V4 stops the hum but I can still play the Normal channel.

Grounding the right lug on the Intensity pot stops the hum but I can still play the normal channel. Is this strange?

Tapping on almost every capacitor and wire associated with V4 can be heard loudly through the speaker. This is prominent on the wire from V4 pin 7 and where the red wire from the Reverb transformer meets the green wire from the filter cap on the board.

Both Volume pots affect the buzz.

The amp buzzes with everything on zero regardless of if a guitar is plugged in or not.

Moving wires around doesn't change anything.

I've checked all grounds and everything appears to be fine.






Here is an audio clip of the noise. It is louder than it sounds.
https://soundcloud.com/mike-mastandrea-1/amp

There is also another problem. I have the following modification installed. It literally has NO affect on the amp. I even tried switching the orientation of the wires but to no avail.
https://robrobinette.com/AB763_Modif...ative_Feedback

I'm running out of ideas here, haha. Any and ALL help would be greatly appreciated! I'm not too familiar with technical terms so please be as descriptive as possible in your replies.

Cheers!

[pdf64]

Images aren’t viewable. Photobucket is useless unless you upgrade.
Try imgur etc instead.

Does the reverb control affect the buzz?

If the NFB mod switch doesn’t do anything maybe the NFB isn’t working, eg something isn’t wired up right. Remove the mod and wire up the amp stock in the first instance.

[Penguin]

Thanks for the heads up! I changed the picture host.

The Reverb control affects the buzz but only very slightly. The Volume controls affect it way more than anything else. Grounding the points I mentioned kill it completely.

I'll return to the NFB to stock and see what happens with that.

[Bob M.]

Penguin,

When you get this right you'll be in tone heaven. Well, the first thing I noticed is that you're not using shielded wires on the grids (pins 2 and 7) of the first two tubes which has become a pretty standard practice. I see a ground buss wire running in back of the pots. I myself haven't had good luck with this type of grounding scheme in a Fender amp but I know others have.

But this may not be the source of your problem. As you've seemed to have localized the hum the in and around V4, I would re-check your wiring in that area very carefully, double check, then after that, triple check it. You could replace the V4 tube as a precaution. Measure the voltages on both plates and cathodes and make sure that they are the expected voltages for those points. The same is true with measuring the ohmage of these points to ground (including the grids) but that data isn't as readily available. If you've missed grounding something however, it'll show up then. You could temporarily un-wire the reverb circuit as to eliminate it as the source of problems and hum and once the hum is eliminated, re-wire the reverb circuit back to as it should be. I have on occasion used shielded wires from the reverb pot on the front panel to their various respective points on the circuitboard. Sometimes, this helped and other times it made no difference. I would also check in and around V2. Also, I see a 220K resistor on the inside back, going between jacks. Now, if that is a grid load resistor, I'd ground that to where the cathode parts from that same stage are grounded.

The Hoffman site has a good drawing of a good grounding scheme for their builds. Also, the Glass Audio article, 'The Remaking of a Champ' has an excellent grounding discussion. I'd measure in ohms every ground point in the amp to the transformer bolt and make sure all grounds read the same with your ohmmeter. In fact, if it were me, I'd only ground to ONE transformer bolt, not several. That may or may not make a difference, although it is alot of work to re-do.

This is most probably a wiring error/mistake but it could also be something inherently flawed with your ground scheme. At any rate, I'd add those shielded grid wires to V1 and V2 as a first step. When you solve your hum problem, please report back and tell us what it was and how you fixed it as it will help others who will come after you. It is not uncommon to have some problems with a new build, just stick with it and be methodical.

If all fails, as you're in Toronto, there will be a very sharp tech there who loves a challenge. You'll have to do a little investigating to find him.

Good luck,

Bob M.

[xtian]

How are you getting NFB? I can’t see any NFB wire connected to the output jack.

[Penguin]

How are you getting NFB? I can’t see any NFB wire connected to the output jack.

You're right! I can't believe I missed that wire. Thanks, Xtian!

[Penguin]

UPDATE!

The NFB switch is now working. On the setting with no NFB, the buzz is still prominent. On the setting with the stock 820 ohm resistor, the buzz is slightly reduced.

I decided to take some preamp voltage readings.

V1
Pin 1: 172
Pin 6: 179

V2
Pin 1: 177
Pin 6: 181

V3
Pin 1: 405
Pin 6: 405

V4
Pin 1: 177
Pin 6: 181

V5
Pin 1: 401
Pin 6: 354

V6
Pin 1: 192
Pin 6: 188

Is it me or are some of those too high?

[pdf64]

Regarding the voltages, what is the HT voltage and power tube plate or cathode current at idle?
And the mains and heater V ac?

Regarding the buzz, that symptom is often related to the heater circuit. What is the resistance between any point on the heater circuit and any circuit 0v point, eg chassis?

[Mick Bailey]

Are both hum balance resistors making good connection to ground? they appear to be going into a crimp eyelet.

[Penguin]

Regarding the voltages, what is the HT voltage and power tube plate or cathode current at idle?
And the mains and heater V ac?

Regarding the buzz, that symptom is often related to the heater circuit. What is the resistance between any point on the heater circuit and any circuit 0v point, eg chassis?

I hope I calculated this correctly, haha. Please bear with me as I am still fairly new to this.

Wall Mains: 122 VAC

OT Voltage: 414V

Both Power Tube Plates: 405V

Heater Voltage: 6.3V

Voltage Drop: 9V

OT Resistance to Plates: 21 ohms

9V/21 ohms = 42.8mA

Only one heater reads "0" in relation to ground. Is this normal?

Keep in mind that grounding V4 Pin 4 as well as the right terminal on the Intensity pot kills the buzz.

Both hum balance resistors are securely grounded.

[Mick Bailey]

Have you got one side of your heaters grounded?

Edit - check at the panel light to make sure you don't have a short.

[Penguin]

Have you got one side of your heaters grounded?

Edit - check at the panel light to make sure you don't have a short.

I certainly hope I don't have one of the heaters grounded, haha. When I touched one heater it read "0" to ground. When I touched the other the reading stayed at "1."

By "panel light" do you mean the bulb assembly? I have the green wires from the OT and heater wires connected to it. I also have the hum balance resistors connected from the bulb assembly to ground.

[Mick Bailey]

Hoping isn't the same as checking. Did you check that there isn't a connection to ground at the back of the bulb assembly that would short out one of your hum balance resistors? Your heater chain should 'float' with either side of the heater winding connected via each 100 Ohm resistor to ground. Because the heater winding is low resistance, then you should read both 100 ohm resistors in parallel (50 Ohms) when measuring resistance from either side of the heater chain. Your readings suggest to me you have a short and it could be one of your resistors is touching the grounded mounting bracket on the bulb holder or you have a solder bridge.

To put it another way, your heaters should each read 3.15v AC from either side to ground with the amp powered.

[pdf64]

Assuming that the measurements are correct, the 6V6 (?) are idling crazy hot, >17W.
Half that current is appropriate.
1 ohm cathode resistors are helpful in making accurate power tube current measurements.

[Mick Bailey]

That OT resistance of 21 Ohms is way off what I'd expect.