First time #183 Build

[jam-mill]

Thanks Martin!

First, set the bias trimmer to minimum (most negative) voltage, -64V.

B+ voltages look good.

The heater voltages on output tube pins 2 and 7 look strange. Are you measuring AC to ground? I'd expect ~3.4-3.5VAC, and matching values on those two pins.

Had meter on DC! I just measured p4 123Vac and p9 128Vac??? That's a bit odd! I have both green wires (which the MM PT spec sheet indicates 6.5v @5.5A) connected to p2&7 on V7. (see pic).

http://www.classictone.net/40-18004.pdf

Voltages on the preamp tubes look good if the pin designations (not the electrode names you listed) are correct. 1, 2, 3, and 6, 7, 8 are plate, grid, and cathode respectively.

You are correct. Pins are correct. I was referring to the wrong layout.
Searching "12AX7 pinout" doesn't necessarily bring up that pinout! Best to leave off "pinout" in search parameters!

How about the relay supply voltage? Can you get the relays to pull in with the panel and foot switches?

Relays do not pull in. I have @ 12v to each. See pics. I may have the foot pedal sw upside down? Wires are correct from relay to switches. see pics for comments.

Looks like the ground is missing on the footswitch connector.

[martin manning]

I just measured p4 123Vac and p9 128Vac??? That's a bit odd! I have both green wires (which the MM PT spec sheet indicates 6.5v @5.5A) connected to p2&7 on V7. (see pic).

I suspect your PT doesn't have a grounded center tap on the filament winding. If not, you need to add a ground reference to the filament circuit. Connect 100-150 ohm resistors from a power tube pin 2 and a pin 7 to ground (see T.Albany layout).

Pins are correct. I was referring to the wrong layout.
Searching "12AX7 pinout" doesn't necessarily bring up that pinout! Best to leave off "pinout" in search parameters!

Works fine for me (?).

Relays do not pull in. I have @ 12v to each. See pics. I may have the foot pedal sw upside down? Wires are correct from relay to switches. see pics for comments.

Yes, looks like a missing ground.

[MusicMaker]

I suspect your PT doesn't have a grounded center tap on the filament winding. If not, you need to add a ground reference to the filament circuit. Connect 100-150 ohm resistors from a power tube pin 2 and a pin 7 to ground (see T.Albany layout).

Just the two green (looks blue in pic) wires. Ok, I'll add the resistors. I seem to remember reading about that grounding trick at some point.

I'll add the ground.

Thanks jam-mill!

Edit:

Added ground. Relays and foot switch now function!


I have a question on the T.Albany layout. See pic. arrows.
What is the purpose for the 330R and diode? Limiting 1/2 the sine (rectify?) and current to protect the pilot light? I didn't use these. Must be a different power light than mine?

[jam-mill]

I suspect your PT doesn't have a grounded center tap on the filament winding. If not, you need to add a ground reference to the filament circuit. Connect 100-150 ohm resistors from a power tube pin 2 and a pin 7 to ground (see T.Albany layout).

Just the two green (looks blue in pic) wires. Ok, I'll add the resistors. I seem to remember reading about that grounding trick at some point.

I'll add the ground.

Thanks jam-mill!

Edit:

Added ground. Relays and foot switch now function!


I have a question on the T.Albany layout. See pic. arrows.
What is the purpose for the 330R and diode? Limiting 1/2 the sine (rectify?) and current to protect the pilot light? I didn't use these. Must be a different power light than mine?

330R is the current-limiting resistor for the power indicator LED and the diode is the rectifier.

[MusicMaker]

Connect 100-150 ohm resistors from a power tube pin 2 and a pin 7 to ground (see T.Albany layout).

Just dropped by my local electronic junkyard and got two 1W 150 ohm carbon resistors (probably older than I am!)

1W enough?[/quote]

[martin manning]

Sounds good, 1W is plenty.

[MusicMaker]

Success! 6.95Vac on the heaters! Center tap works like a charm!


I have to admit for a moment there I thought the smoke got out and a new PT was in my future! Just a little heart palpitation is all...


the glass should be here any day now.

Any other tests I need to perform to perfect this powerup before installing the tubes and making some noise?

OT?

[MusicMaker]

Been reading up on setting the bias.


Got a guitar plugged in, and a speaker on the output. Speaker has a hum.

[martin manning]

From this I assume you have installed the tubes? Did you set the output tube bias?

It's very easy:
- Connect a DVM to the black test jack and any one of the red test jacks.
- Set the meter to read millivolts.
- Power on, standby on. If the bias pot is still set for minimum voltage you will see perhaps a few mV on the meter at this point.
- Turm the bias pot up until you see 35 mV (this indicates 35 mA idle current, which is a reasonable place to start for EL34's with ~440V on the anodes).
- Check the voltage at other red jacks to see if the other tubes show similar idle current.
- Measure anode voltage and calculate the percent of maximum dissipation as (Va * Ik)/25W, where Ik is the average cathode current measured above, and 25W is the maximum anode dissipation for EL34. Actual anode dissipation is a bit less than this, since cathode current includes screen current.

[MusicMaker]

Yes installed the tubes. Attempted to set the bias.

However, I think there's a problem.

I got zapped from turning off the standby switch! Not by the switch, but my hand brushed the speaker output jack when turning from operate to standby. I put a meter on it and saw 400vdc on the output jack ground!!

Something tells me this is not correct! Scared the bejeezus out of me!!
The cable is good. I used it just before on a different amp.


There was also an unusual buzzing sound (400v into the speaker might do that!). It suddenly stopped being as loud. Speaker is ok, just tested it on another amp.

To isolate the source I pulled the trim & master ground connections (see pic), and never moved off of 8ohms impedance.

It's definitely the ground off of the OT! Unless the OT scheme drawing is incorrect (doubtful) There's nothing else attached but ground wires to plastic jacks. I tested for continuity to the chassis and no connection.


Not sure where else to look.

I did turn amp on to check tube glow with no speaker for a few minutes, but not off standby.

Did I blow the OT somehow?

[martin manning]

Looks like you missed the OT secondary ground. Oddly enough this also grounds the OD cathodes, according to the C-tone layout. Fix that, reconnect the loop jacks, and then see what happens.

[MusicMaker]

Ok, I installed the ground. That stopped the jack from being 400v hot.

Reconnected the loop wires.

Decided I would go back to the beginning measuring voltages. See attached.


With the 40W limiter and tubes in the bulb glows and no dimming. I'm thinking this is what it's supposed to do with tubes in? (will this harm the valves?)

I went this route because with full power, if I get bias to 2 mA it goes into a VERY LOUD buzz and I shut it down.

There must have been something that fried when I missed that ground wire?

What do I check to troubleshoot this noise problem?




I included the blank voltage sheet for anyone who want to use it.

[martin manning]

If you are getting a near full-brightness glow from the bulb that would indicate excessive current draw. The power tube voltages look good (these are without the bulb limiter, I assume), and you didn't report any smoke or smell, so it doesn't seem that there is anything seriously wrong. What happened when you turned on the main power? What happened when you switched from standby to play?

You don't show grid or cathode voltages for the preamp tubes. Were they zero or did you just not measure them?

The loud buzz could be due to reversed PI outputs or OT primaries, either of which results in positive feedback to the PI. A quick test for that would be to disconnect the negative feedback from the impedance selector switch. Do you have a diagram of the OT you can post?

[Structo]

Here is another voltage chart that will accommodate most amps.

[JazzGuitarGimp]

The presence of 400VDC on the speaker jack is not just a missing ground. The output transformer provides isolation from primary to secondary. Assuming the transformer does not have an internal primary to secondary short, there is a serious wiring error, and simply adding the missing speaker jack ground is going to cause a short from +400V to ground.

I do not mean to discourage the OP, but my advice is to take this build to someone with more experience to sort out the wiring errors - do it before you cause another injury to yourself (or someone else), and before serious damage is inflicted on the amp itself.