DC on input jacks?

[Randall]

I wonder if the mojo fiberboard is conducting. I know had a recent AB763 build have to be ripped out due to moisture absorbing fiberboards from mojo that caused scratchy pots, as well as a few other problems. Having said that, I did two 5E3s 6 months ago on black fiber from them that are problem free. Wonder if they are into a bad batch of boards? It took a little time, but after refunding my money they admitting that fiber can and does absorb moisture and conduct, causing issues, and that they sell it to provide for authentic vintage reproduction amps. Not knocking mojotone, I like them. But, I know I am done with fiber after that.

On another note, one thing I don't like on this build is how close the 270 Ohm cement bias resistor is to the bias cap. Looks like it's touching it. That resistor gets hot, and could cook that cap. I'd move it 1/4" away.

[Steve A.]

Do you get DC voltage from all 4 jacks? The two channels go to different tube stages. Have you measured the DC voltage from the tip of a cable to chassis. and to the shield of the cable- are they the same?

You might also want to measure all of the DC voltages to chassis ground from the V1 tube pins 1,2,3,6,7 & 8 plus the AC voltages to ground on pins 2,4,5,7 & 9.

Steve Ahola

[ngteleblues]

voltages taken with an ElCheapo DVM Bright Normal
V2 V1 jack 2: .02v .02v
Pin 1- 146.2 119.3 jack 1: .18v .10v
2- .03 .01
3- 1.2 1.92
4- 2.9 AC 2.9 AC
5- " "
6-180.0 108.5
7- 2.83 .14
8- 44.3 1.9
9- 2.9 AC 2.8 AC

[ngteleblues]

Somehow this got screwed up when I posted. I tried to do it in a chart format and it got rearranged. I'll try again.
V1 pin 1:119.3, pin 2: .01, pin 3: 1.92, pin 4 and 5: 2.8 vac, pin 6:108.5, pin 7: .14, pin 8: 1.9, 2.8 vac
V2 146.2 .03 1.2 " " 180.0 2.83 44.3 " "
input jack bright #1: .18V #.02V input jack normal #1: .10V #2: .01V

[ngteleblues]

ok I'm giving up now......

[Tom Phillips]

ok I'm giving up now......

Just print out that section of the schematic, write the voltages on the schematic and post a picture. It won't get scrambled and the result is better than a table or narrative description. Like the attached ... but I couldn't figure what you were trying to post about the voltages on the jack pins themselves

[Steve A.]

ok I'm giving up now......

So what pin has the 44.3v? Was that vdc or vac? That was the only reading that jumped out at me.

Steve

[ngteleblues]

That was 44.3vdc at pin 8 on V2. I checked my wiring from the transformer and decided to add a standby switch (at the ground switch) I got that done today and the amp just sounds great but the problem still exsists and true bypass FX pedals pop like MF when the switch is pressed. Plug straight into the amp and it sounds wonderful except when you use the guitars volume control or a volume pedal.
Does anyone have the proper voltage readings for this amp?

[g1]

The only voltages of real concern with regard to the inputs are pin 2 and 7 of V1.
But post voltages for V1 pins 1,2,3, and 6,7,8

[Steve A.]

That was 44.3vdc at pin 8 on V2. I checked my wiring from the transformer and decided to add a standby switch (at the ground switch) I got that done today and the amp just sounds great but the problem still exsists and true bypass FX pedals pop like MF when the switch is pressed. Plug straight into the amp and it sounds wonderful except when you use the guitars volume control or a volume pedal.
Does anyone have the proper voltage readings for this amp?

The schematic shows pins 3 and 8 sharing the same cathode resistor and bypass cap so the voltages on pins 3 & 8 should be identical. I'd investigate why the voltages are different (perhaps a bad solder joint on the jumper between P3 and P8?)

I would keep a printout of the 5E3 layout drawing handy to make sure that I got the tubes and pots wired up correctly. (I have a photo from Bruce Collin's Mission Amps 5E3 kit instructions to make sure that I got the input jacks wired up correctly- a good reason to buy one of Bruce's kits!)



Steve Ahola

EDIT I took another look at the photos you posted and it looks like you did not add a jumper from V1P3 to V1P8 (assuming that I am reading the photo correctly)

[ngteleblues]

yes V1 Pin 3 is connected to Pin 8

[g1]

44VDC sounds fine for pin8 of V2.
How about voltages for V1.

[ngteleblues]

V1pin 1: 119.3, pin 2: .01, pin 3: 1.92, pin 6: 108.5, pin 7: .14, pin 8: 1.9

[Steve A.]

44VDC sounds fine for pin8 of V2.

So the 44vdc was for V2P8? I thought he was referring to V1P8...

Formatting is less of a problem if each pin is on its own line:
V1P1 119.3vdc
V1P2 .01v dc?
V1P3 1.92vdc
V1P6 108.5vdc
V1P7 .14v dc?
V1P8 1.9vdc

[g1]

So pin2 has very little DC, pin 7 has a little.
Do all the input jacks have the problem or just the ones that route to pin7 ?