JTM 45/50 Build: Issues with high voltages

[bt2513]

Hi Everyone,

I'm relatively new to amp building and am new to this forum but have read/followed for a long time. I have successfully built a Princeton Reverb kit in the past so I thought this build would be much more straight forward. This was posted over at TDPRI as well but I have yet to resolve the issue. I want to thank everyone in advance for any help you can provide.

This is the British 45 kit from Mojotone - I made no part substitutions but would like to change to KT66s at some point after I get it working.

Link to Wiring Diagram
Link to Schematic

When I power the amp with tubes installed, there is no sound or white noise at all. The tubes glow a nice orange (no red plating). I have checked and rechecked all connections with a wooden stick and a conductivity probe. It is almost like the amp never comes out of standby.

Voltage readings on pins 4 & 6 of V6 are 350v which is appropriate. However, when I read voltages from the yellow 5Vac taps I am getting 35-40v+ with the recitifer in place. I powered off the amp and pulled the tubes. When I went to drain the caps, I had a very visible arc and loud pop at the filter cap that is connected to the standby switch. The cap appears to be grounded and I have done continuity tests on all ground wires to double check (against the chassis).

Here are pictures - please pardon my n00b-grade wiring. I'm getting better... also, my real camera had no battery power left last night so these were taken with my iPhone. I can take higher quality pics if that would help tonight.

[IMG:268:200]http://imagizer.imageshack.us/v2/280x200q90/850/hu5m.jpg[/img]
[IMG:268:200]http://imagizer.imageshack.us/v2/280x200q90/823/dux4y.jpg[/img]
[IMG:268:200]http://imagizer.imageshack.us/v2/280x200q90/842/cbx6a.jpg[/img]
[IMG:268:200]http://imagizer.imageshack.us/v2/280x200q90/840/yamc.jpg[/img]

Thanks again!

[xtian]

First, pull ALL the tubes out, and do some voltage testing. You should have ~400vDC coming out of the rectifier (the brown wire) regardless of the state of the standby switch.

How are you discharging the caps? If you're just shorting them directly to ground, of course you will get a loud SNAP and spark.

[bt2513]

That arc actually came from my meter probe before I drained the caps. Using alligator clips connected to a resistor. The reading was well over 500VDC (rapidly began to fall so didn't see the highest number).

[xtian]

OK, so your voltages may well be ok. I don't know what steps you've already taken to make sure the amp is running safely.

Have you gone thru these steps:

http://www.paulrubyamps.com/info.html#FirstPowerUp

Assuming all that stuff passes, we can move on to "why no sound." Make sure you have a load connected to the speaker out (known good speaker?).

Checked bias? How much current are the power tubes dissipating?

Do you have a signal generator to help troubleshoot?

[bt2513]

Thanks for your help - I had all the tubes out when I did this earlier. I will power it up when I get home and post voltage readings at the pins while the tubes are out.

[statorvane]

If you are getting 35-40 volts from your 5 volt taps, that is a problem. Almost seems like your bias supply is hooked up to your rectifier?

[bt2513]

The voltage threw me off too but I can't figure out where that would be coming from.

[Stevem]

since he has high dc voltage in the amp, the tube recto is working.
a far safer way to drain the power supply is to just turn off the and ac power switch and leave the standby on, if this does not drain the power supply, than the wiring is wrong in the power supply somewhere!

Do you have a negative 32 voltags on pin 5 of the output tubes with the standby in the off position?

[bt2513]

Pin voltages (in standby)

V5 (EL34)
3: 3.5vDC
5: -36.5vDC

V4 (EL34)
3: 3.5vDC
5: -36.6vDC

V3
1: 3.5vDC
6: 3.5vDC

V2
1: 3.4vDC
6: 3.5vDC
7: 3.4vDC

V1
1: 3.4vDC
6: 3.4vDC

[bt2513]

with the rectifier and EL34s in place, i'm getting 1000vAC plus coming out of the rectifier. this can't be right.

[martin manning]

Measure your AC line voltage and see if you get something reasonable. Got fresh batteries in your meter?

[bt2513]

AC line voltage is 119vAC - checked this out first.

edit: I should have been testing that in DC. When I do this, I get 472vDC out of the rectifier and into the filter cap with the recto and PTs installed.

When taken out of standby, rectifier voltage drops to 421vDC

[martin manning]

That all sounds good. You should be able to measure AC voltage at the rectifier output and get the ripple voltage.

Get voltages on all the tube pins with the standby in play mode. Don't forget to connect a load to the speaker output.

[bt2513]

Pin voltages (in play) (all tubes installed)

V5 (EL34)
3: 420
4: 413
5: -36

V4 (EL34)
3: 426
4: 407
5: -36

V3
1: 227
2: 25.7
3: 42.5
6: 216
7: 27.8
8: 42.6

V2
1: 165
2: .4mV
3: 1.1
6: 295
7: 164.2
8: 165.5

V1
1: 192.2
2: 0
3: 1.5
6: 196.1
7: 0
8: 1.6

[martin manning]

Got some way to measure idle current so that you can set the bias? 25Wx0.7/425V = 41mA would be about right. You can put the preamp tubes in, and get voltages on them too.