My first build, What should I do before thuning it on?

[Mont]

The pilot light is 125v from Watts tube audio, He has some cool stuff. The light bulb I am using is 25 watts is that too small? could that be why it only glows bright when the tubes are in?

[paulster]

Aha! Try it with a 100W bulb and see how that goes.

[Structo]

A 25 watt bulb limits the amp to just under 250ma.

I use that the first time I start up an amp.

Then I will move up through higher wattage bulbs, 60, 75.

I would definitely find the fault before going ahead.

[Mont]

My limiter is wired correct. I moved up to 75 watt and same thing I replaced all the thin wire with 20 awg. fired up the amp(no tubes) I get 6 voltts on the filaments and 112 at the on\off switch. all the caps read about 370 is this correct?

[geetarpicker]

I'd suggest doing more voltage measurements and posting them here. Specifically at each tube socket, since you say it goes short (limiter stays bright) once tubes are installed. I'm assuing the bulb limiter goes bright the instant you power it up, even before the tubes warm up?

Something like this:

V1
Pin 1- ?
Pin 2- ?
Pin 3- ?
Pin 4- ?
Pin 5- ?
Pin 6- ?
Pin 7- ?
Pin 8- ?
Pin 9- ?

V2
Pin 1- ?

etc...

[marcoloco961]

My limiter is wired correct. I moved up to 75 watt and same thing I replaced all the thin wire with 20 awg. fired up the amp(no tubes) I get 6 voltts on the filaments and 112 at the on\off switch. all the caps read about 370 is this correct?

Yeah, everything there seems to be within tolerance.

If indicator light is a 125v and it lights with no tubes, but does not light when the tubes are in, then it sounds like a short between one of the high voltage pins and a ground on or coming from a tube socket. Makes sense. Once tubes are in, they become the vehicle which takes the hot to ground. No current, no power to caps, no voltage to light the indicator light, because it is all shunting to ground somewhere.

Turn the amp off and dis-charge the caps. Locate all positive voltage pins on the sockets. Get your continuity meter out. Clip your negative to a good ground lug like where the center taps connect to the chassis. Start with no tubes and no power plugged in at all. Check plate pins 1 and 6 on all three 12ax7's in V1, V2, and V3 positions with the positive probe of your meter. Then check grid pins 2 and 7 on all three tubes. You should have no continuity btween ground and any of these pins. If your meter has a beeper and it makes a noise this is where the problem is. There should be no continuity on any of these pins. Heater pins 4,5 & 9 will have continuity.

Now repeat the process on pins 4, 5, and 6 of the power tubes, V4 and V5. These also should have no continuity between them. Heater pins 2 and 7 will show continuity.

Put the tubes in with no power connected and repeat. Everything should be the same, but doing it both ways might narrow it down somehow.

If this all seems to check out. Double check the wiring on the power tubes. The layout, if not looked at carefully can be hard to follow around the power tubes.

Let me make a few clarifications for you to verify.

Looking at the bottom of the tube socket, find the notch on the inside of the hole. Pin 1 is the first pin clock wise from the notch. there should be three diodes connected to it and to pin 3, with the lines on the diodes facing 3. pins 2 and 7 are heaters. the screen voltage coming from the bottom of the 25watt resistor and the positive of the B+2 supply cap should tie to pin 6 on both power tubes, there should also be one end of the 1K 5-7watt resistor tied to pin 6 and the other end ties to pin 4. Your feeds from the PI should come in thru resistors to pin 5. Make sure both of the leads on these grid resistors are cut as short as possible and do not touch the chassis or a ground point. Depending on whether you have bias test points installed pins 1 and 8 might be tied together and to ground.

[Mont]

Let me ask this first, does it take awhile for the limiter bulb to go back down to a small glow when the tubes are in. Again all filter read 420 v, 6vac on heaters, over 500 on the 2yellow leads that connect to diodes. my meter only goes to 500.bias pot at center -25.

V1 V2 V3 V4 V5

Pin 1-390 P1-400 P1-400
Pin 2-0 P2-0 P2-0 P2-0 P2-0
Pin 3-0 P3-0 P3-0 P3-420 P3-420
Pin 4 P4-420 P4-430
Pin 5 P5-(-17) P5-(-17)
Pin 6-390 P6-400 P6-430 P6-420
Pin 7-0 P7-0 P7-0 P7-0 P7-0

[marcoloco961]

Let me ask this first, does it take awhile for the limiter bulb to go back down to a small glow when the tubes are in. Again all filter read 420 v, 6vac on heaters, over 500 on the 2yellow leads that connect to diodes. my meter only goes to 500.bias pot at center -25.

V1 V2 V3 V4 V5

Pin 1-390 P1-400 P1-400
Pin 2-0 P2-0 P2-0 P2-0 P2-0
Pin 3-0 P3-0 P3-0 P3-420 P3-420
Pin 4 P4-420 P4-430
Pin 5 P5-(-17) P5-(-17)
Pin 6-390 P6-400 P6-430 P6-420
Pin 7-0 P7-0 P7-0 P7-0 P7-0

Your voltages don't look bad. I believe your power supply is correct. AGAIN, YOU NEED TO TURN THE BIAS POT TO THE MOST NEGATIVE POSSIBLE VALUE. Around (-35) volts or lower before putting in tubes or you will fry them puppies. You could have fried one already by ignoring this after several people have told you to do it.

And yes, with tubes in the light comes on bright and gradually fades to a lower glow.

Pins 2 and 7 of V4 and V5 should have 6.3 V A/C on them if they are wired properly.

Turn your bias down and do some continuity checks next, then let us know what happens.

[Mont]

ok, all heater pins show continuity, i get a beep. all pins showed no continuity on both preamp and power tube sockets. the same with tubes installed. I did get the beep on pin 2 V1 with the volume turned down and no beep with the volume turned up . I hope this helps. one other thing I was told I could leave out the bright switch so i did.

[Mont]

sorry about the bias pot thing. I did check all the filaments and they were all the same. am I just paranoid? about how long should it take for the limiter bulb to go down?

[M Fowler]

I would hook up a speaker, a guitar, install tubes and plug into the wall and give it a try. I am not sure that limiter is right, besides with all this testing those caps have to be formed by now.

Mark

[marcoloco961]

sorry about the bias pot thing. I did check all the filaments and they were all the same. am I just paranoid? about how long should it take for the limiter bulb to go down?

It takes 10 to 15 seconds sometimes before it get to it's dimmest point. If it is not dimming, I would not take it off of the limiter yet by any means. The fact that the indicator light doesn't light with tubes in still bothers me. Make sure the bulb is screwed in tight, mine was a screw in type and it was intermittant to start with until I tightened the bulb in the socket. Follow the build guide step by step for what to do after tubes are installed with the current limiter in line. You need to make sure and plug in a speaker cabinet and set to proper ohms before powering the amp up with tubes in it. THIS IS A MUST or you can fry you output tranny. Try to turn on both power and standy switches with tubes in, bulb limiter installed and speaker plugged in. If the light stays bright, turn it off. But if it just glows try to finish your voltage checks for us with the tubes in once. watch to make sure the tube plates do not start to glow red.

[marcoloco961]

sorry about the bias pot thing. I did check all the filaments and they were all the same. am I just paranoid? about how long should it take for the limiter bulb to go down?

On the bias pot settings. Mine were all between -30 and -32 to achieve a "hot" bias of approx 45 mA. At only -17 you are most likely far over the tubes rating, probably throwing hundreds of mA at the tube in an idle setting. Add signal and it's bye bye tubes. That is why it is so important to start at -40 or better if possible and slowly bring it up. If you cannot get it to go that low you will have to change a resistor in the bias circuit to lower the voltage some more. Let me know if this is the case and I will direct you to the proper resistor.

[Mont]

My pilot light is fixed so it is not a problem. I set the bias pot at -43 v and tried it with tubes. Again no pilot light, the bulb limiter stayed bright for 15 seconds and the filaments were showing no signs of lighting up so i turned the amp off and again no charge.

[marcoloco961]

My pilot light is fixed so it is not a problem. I set the bias pot at -43 v and tried it with tubes. Again no pilot light, the bulb limiter stayed bright for 15 seconds and the filaments were showing no signs of lighting up so i turned the amp off and again no charge.

When the bulb limiter lights bright it is a sign of a short. The fact that it only happens after the tubes are installed points to the to the sockets or the components directly connected. Check the values of all resistors like the 1M resistors that go between signal path and ground. You might have to lift one end to get an accurate reading. Double check the values of the cathode resistors and the 1M on the input jack.