Fired it up finally!!

[Tdale]

The bias pots measure a max of -37V, so I need to change the resistor. Do I need a larger or smaller value to get about 65V?

Also, between ground and the last rectifier diode, I measure 300 and some V, but between ground and the + side of the filter caps, I measure no voltage.. Is that normal

I had no tubes in it...

Tommy

[Tdale]

.....and it was on standby.... put it on operate, and I got about 480V on the filters...

Need to change the bias resistors, and I should be good to go!!!

Tommy

[Zippy]

I'd say that you just performed a satisfactory operational check of your standby switch!

[Bob-I]

The bias pots measure a max of -37V, so I need to change the resistor. Do I need a larger or smaller value to get about 65V?

Larger

[Tdale]

I have two 24K in there, and still get about -43V

The bias tap on the transformer is 50V. Can I get -60V from that......??

Tommy

[Normster]

Tommy, if the bias tap is -50V, you won't get -60V from it, but you also don't need that much. Your bias voltage will most likely run in the -30V to -35V range under load. There are two places that you can adjust the bias range. First, on the input side before the diode. Most Dumble schematics show 3k3 but Fender uses 470R. Decreasing resistance here will increase bias voltage. The second is the tail to ground from the bias pot. The higher the value, the higher the bias voltage. FWIW, I normally run a 470R, 10k bias pot, and 10k tail to ground.

Also make sure your bias cap is wired positive to ground. If it's the other way around, your bias voltage will be VERY low (and you'll most likely have to replace the cap). Don't ask me how I know this.

[Tdale]

I got a good value... and put on full power, with tubes in it.


A loud roar in the speakers, all volumes all the way down..... I shut it off in less than a second, but when I put it in standby again, the heaters don't work, and the pilot light doesn't work, even if it's not blown.......

Can the PT be blown...?

Tommy

[Normster]

Fuse?

Definately use a "bulb limiter" when you're testing a new build. I use a 40watt light bulb in series and it has saved me from a few disasters.

[Tdale]

Fuse is ok, pilot lamp is ok.

I removed all tubes, and put power on it again, and everything seems fine. pilot light lights, the bias voltage is ok, and when I apply full power, nothing happenes, other than that I get about 480V readings from the filter caps..

Looks like it's when the tubes are inserted that the problem occurs.. Must be some bad wiring?

Is it safe to just put in one tube at a time, and apply power, to see if it works ok with that specific tube in place?

Tommy

[Icetech]

I would double check every single wire before i put another tube in...

[Tdale]

I guess I have to do that.

I already did a check for the most vital wires.. I counted all grounds in the schem, and counted the grounds in the amp, and it added up.

Common on OT is grounded

I checked tube sockets, and found a couple of solders that was close to touching one another, and fixed that.

Everything seems to be in order, but I guess a more thorough check is in order.

I would have guessed that the sound it makes could indicate something. There is no guitar connected, and all volumes are turned all the way down, but it still screams....

Tommy

[tonelab2]

Hi Tommy,
Had a similar problem once with the amp howling on first bootup (can't recall if volumes were turned up) turned out to be the plate wires from the OT. Just swapped the blue/brown wires around and the amp was fine.

[Tdale]

hmmm. I'll try that, even though I can't understand why it would work...

By the way, I went through the wiring again...can't find any mistakes. Checked the wires actually are leading and aren't broken in some way. Followed the schematic part by part, and can't find anything..

Wondered though.. I have a combined standby/operate switch. It's a double pole switch, but the two poles are not connected, so that in mid postion lug 1 and 2 are connected and in top position lug 1 and 2 are still connected, but lug 3 and 4 are also connected.

The stand by is connected like it is in the hybrid-a... the switch is between the rectifier and the first filter. The voltage from the rectifier is higher than the 250V max, marked on the switch...but could this cause a problem like this?

Tommy

Tommy

[Normster]

Hi Tommy,
Had a similar problem once with the amp howling on first bootup (can't recall if volumes were turned up) turned out to be the plate wires from the OT. Just swapped the blue/brown wires around and the amp was fine.

+1
When you get the shorting issue sorted out, this is the exactly what you need to do. Often, the brown and blue wires are not coded correctly when the tranny is manufactured. You end up with positive feedback into the PI instead of negative feedback. As tonelab said, just switch the two and you're good to go.

[Tdale]

ahh...that makes sense... I think. I'll switch them now, and report back

But what do you mean by "the shorting issue"?.... the wire switch...or am I missing something?

Tommy