Loud hum and distortion on 5e3 kit

[Harvette01]

I built a 5e3 kit from China. I have an incredibly loud hum that increases with volume control. Amp has a low hum when V1 is pulled, unaffected by volume control.
Guitar comes through, but loud and very distorted. Checked and rechecked all wiring, reflowed all solder joints, replaced tubes and checked voltages.
All voltages are correct, except I have 0v on pins 2 and 7 on V1, and 2.4 volts on pin 7 of V2. Pin 7 of V2 shows 17v on all the schematics I have looked at.
I had mistakenly attached my C voltage to pin 7 of V1, now fixed.
Is it possible the .022uf cap off pin 1 of V2 is fried? Maybe other damage caused by the high voltage in the wrong place?
I don't own an oscilloscope or signal generator.

 

[Jon Snell]

What we need is the schematic you are working from, to give us an idea of what the build is.

 

[Harvette01]

What we need is the schematic you are working from, to give us an idea of what the build is.

Weber schematic (added standby switch) and StewMac '57 Tweed Deluxe.

 

[Harvette01]

Sorry, '59 Deluxe.

 

[Lowerleftcoast]

All voltages are correct, except I have 0v on pins 2 and 7 on V1, and 2.4 volts on pin 7 of V2. Pin 7 of V2 shows 17v on all the schematics I have looked at.

FYI
The voltage on pins 2 and 7 shown on schematics are VAC. Test points in an oval on a schematic denote AC voltage. V2 pin7 shows the *signal* voltage when a signal is introduced at the input. So when the tech supplies 100mV at test point TP12 then the gain stages should have AC signal voltage of 3.79 VAC at TP13, 300mVac at TP14, 17.4 VAC at TP15, etc.

For the louder hum... have a good look at the input jack area. There is a switch on the jack. Does it open and close properly? Are the 1M resistors connected to the proper terminals? Have a careful look in this area. Post some good pictures of the input jacks and the rest of the guts. The Shock Brothers and Sisters have some sharp eyes and may be see what might have been overlooked.

 

[Burning Fingers]

Have you tried swapping over the output transformer primary leads that connect to pin 3 on the output tubes ?.

 

[Jon Snell]

Have you tried swapping over the output transformer primary leads that connect to pin 3 on the output tubes ?.

Won't make any difference as there is no NFB loop on the schematic for the 5E3.

 

[2L man]

Input jacks have switches and they always are suspect when amp hums loud. Yours does look different kind problem but you could do this quick test next time you power up.

I am not 100% sure does this test work but if there is one jack switch which does not close, then inserting guitar to all inputs one at a time, the jack which switch does not function should make hum quieter when guitar impedance is inserted to the input.

 

[Harvette01]

Here's some pictures. Hope this helps! To date: Pretty loud hum with both volume controls turned all the way down. Ground reading at center tap of both volume controls. Horrendous loud buzz increases with either volume control. All tips on the input jacks read ground. Pin 2 of V2 reads ground with both volume controls turned all the way down, and resistance varies appropriately as they are turned up. Just a mild hum with V1 pulled.

 

[Harvette01]

Better pic of the power transformer codes. Previous post just showing off the cabinet I built.

 

[YellowBoots]

Have a look at this chart. It might help us zero in on where your problem lies. V2 pin 2 should read 500k to ground no matter the setting of volumes.

 

[Lowerleftcoast]

Do you happen to have a picture of the under-board wiring?

On the image below, it appears the center tap for the high voltage terminates here along with a blue wire that terminates with the 6V6 cathode resistor/cap on the board. Does this terminal, of this terminal strip, connect to the chassis at this location?

 

[mountainhick]

Looks like a blown dropping resistor in the power supply cap filter section. Begs the question why?

 

[Harvette01]

The terminal strip is grounded to the chassis. Here are some voltage readings: A is 386v, B is 342v, and C is 252v. Each is slightly higher than the Telenut62 levels. I looked at a picture of a Chinese prebuilt chassis, and they used the power transformer blue wires at 270v instead of the green wires at 330v. Does that make sense? Also, I rechecked the resistance at pin 2 of V2 and it does vary from 0 ohms and up with the volume controls. Pin 2 of V1 reads 500 k. This evening I will go through the resistance chart from andrewTneuman.

 

[Harvette01]

That's andrewRneuman's chart.

 

[Burning Fingers]

I noticed that your grounding system is one that has caused problems for others in the past.
I recommend using Rob's system as pictured below.
Also check that 22k dropping resistor that looks burnt... is it cactus ? and if so what has drawn too much current through it ...is the power supply filter capacitor on the right ok ?

 

[Harvette01]

I noticed the brown on the dropping resistor, so I checked it and it reads 22k. I believe it got heated from accidently running its output to the wrong connection, the one on the 68k's to pin 7 of V1, instead of the 100k's to pins 1 and 6 of V1. The resistors read correctly, but I still have a strong feeling that something got cooked. The power supply capacitor seems to read the same as the other 2. When I found the misplaced B+3 voltage wire, I double checked the other jumpers on the back of the board. I didn't take a picture of the back, unfortunately.

 

[Lowerleftcoast]

The terminal strip is grounded to the chassis.

In that case, I think what we have here is a ground loop.

If I am seeing correctly, the amp ground bus is designed to not use the chassis for return current. Only one place of the chassis can be used as a reference to ground. The ground bus is already attached to the chassis at the input jack. Connecting the high voltage CT to the chassis at the terminal strip will allow current to flow in the chassis and the ground bus (ground loop).

Lift the connection to the chassis at the terminal strip. Better yet, maybe you can run the high voltage CT directly over to the negative side of the first filter cap on the board and get rid of the terminal strip.

 

[D'tar]

Hmmm... Are we sure this PT is wired up correctly?

Which wires are 6.3vac heater?

Which wire is 6.3vac heater center tap?

Do we also have an artificial CT?

Which are 5v heater?

Which is HT center tap?

 

[Harvette01]

Ground loop wins it!! I cut the power cap ground wire to the ground terminal and, like magic, amp works as designed. Thanks a million for all the help. Up next is my Deluxe Reverb Rivera with minimal reverb.