Power toggle confusion

[Oddvar R]

I am a bit puzzled by the power button on the Express. Normally (in my perspective) the power cord goes to ground, the second to the fuse to the on/off switch and to the transformer. The third goes directly to the transformer. Is that possible to do here? But I guess there is a smart explanation. And what is the purpose of the Mov cap?

I also wonder about the OT CT, why to the standby switch?

Pardon for being in the woods her...

[pompeiisneaks]

I am a bit puzzled by the power button on the Express. Normally (in my perspective) the power cord goes to ground, the second to the fuse to the on/off switch and to the transformer. The third goes directly to the transformer. Is that possible to do here? But I guess there is a smart explanation. And what is the purpose of the Mov cap?

I also wonder about the OT CT, why to the standby switch?

Pardon for being in the woods her...

There are three connections. Green (earth) white (neutral) and black (hot).

Hot is connected to the fuse, then to one side of the DPDT switch, then to the PT so you have a fused/switched hot into the PT.
Neutral is connected to the other half of the DPDT switch so it is also switched but not fused and then off to the PT.
Next neutral goes straight to a chassis bolt.

The dual switching is just more safe, you get both hot and neutral switched off/on for safety. If you don't have a DPDT power switch you can just connect the neutral (white) directly to the PT yes.

the MOV is not a cap, it is an MOV. Metal Oxide Varistor. It isn't mandatory, but it's job is keeping the mains voltage spikes from hitting the amp. They're a common safety part of most modern electronics.

The OT CT to the first side of the standby just means the OT and the power tubes are going to always have power to them at their anodes. Not sure it's a huge deal. Maybe it helps let current move faster in the power section as soon as the standby is switched so they're ready to conduct. Someone may be able to more intelligently answer why Ken did that.

~Phil

[pdf64]

...Next neutral goes straight to a chassis bolt...

Oops, typo / brainfart alert; it's earth / green that gets bonded to the chassis
Avoid using a power transformer bolt for the eyelet, as it's bad practice and in breach of international code to use the most mechanically stressed fastener in the amp to also attach its only safety critical connection.
Instead, use a dedicated screw, star washer, eyelet then nut for the safety ground chassis connection.

[martin manning]

the MOV is not a cap, it is an MOV. Metal Oxide Varistor. It isn't mandatory, but it's job is keeping the mains voltage spikes from hitting the amp. They're a common safety part of most modern electronics.

And it has to be matched to the local line voltage ;^)

[Oddvar R]

Thanks guys, the reason I was puzzled was that I got the 100w lamp lightning up like Christmas. After a lot of measuring, I started to wonder if the on off switch was totally messed up, since I never understood how the works, (I do now, thanks.). In the end I had to swap PT, and everything was OK. So in the end, first time, the PT was gone, totally new. Very sad and and a lot of stress.

So now I am looking for a new PT.

[Oddvar R]

So what do you think of this for a PT, Hammond 372JX?:

[pompeiisneaks]

...Next neutral goes straight to a chassis bolt...

Oops, typo / brainfart alert; it's earth / green that gets bonded to the chassis
Avoid using a power transformer bolt for the eyelet, as it's bad practice and in breach of international code to use the most mechanically stressed fastener in the amp to also attach its only safety critical connection.
Instead, use a dedicated screw, star washer, eyelet then nut for the safety ground chassis connection.

OOPS good catch lol

yeah let's attribute it to both typo and brainfart

~Phil

[pompeiisneaks]

Thanks guys, the reason I was puzzled was that I got the 100w lamp lightning up like Christmas. After a lot of measuring, I started to wonder if the on off switch was totally messed up, since I never understood how the works, (I do now, thanks.). In the end I had to swap PT, and everything was OK. So in the end, first time, the PT was gone, totally new. Very sad and and a lot of stress.

So now I am looking for a new PT.

Wait, you killed a PT WITH a light bulb limiter? I thought that was the point of them, to protect PT's.

have you ensured you don't have the PT wired wrong? Have you checked that the windings are shorted and showing less resistance etc?

~Phil

[Oddvar R]

Thanks guys, the reason I was puzzled was that I got the 100w lamp lightning up like Christmas. After a lot of measuring, I started to wonder if the on off switch was totally messed up, since I never understood how the works, (I do now, thanks.). In the end I had to swap PT, and everything was OK. So in the end, first time, the PT was gone, totally new. Very sad and and a lot of stress.

So now I am looking for a new PT.

Wait, you killed a PT WITH a light bulb limiter? I thought that was the point of them, to protect PT's.

have you ensured you don't have the PT wired wrong? Have you checked that the windings are shorted and showing less resistance etc?

~Phil

No, not with the bulb, it must have been dead from the start. When I started the amp first time, the light was bright and shiny. So I tried to change everything, nothing happened. Then I connected a spare transformer I have, and then the bulb was very dimmed. I could even turn it on without the bulb connected.

[pompeiisneaks]

Thanks guys, the reason I was puzzled was that I got the 100w lamp lightning up like Christmas. After a lot of measuring, I started to wonder if the on off switch was totally messed up, since I never understood how the works, (I do now, thanks.). In the end I had to swap PT, and everything was OK. So in the end, first time, the PT was gone, totally new. Very sad and and a lot of stress.

So now I am looking for a new PT.

Wait, you killed a PT WITH a light bulb limiter? I thought that was the point of them, to protect PT's.

have you ensured you don't have the PT wired wrong? Have you checked that the windings are shorted and showing less resistance etc?

~Phil

No, not with the bulb, it must have been dead from the start. When I started the amp first time, the light was bright and shiny. So I tried to change everything, nothing happened. Then I connected a spare transformer I have, and then the bulb was very dimmed. I could even turn it on without the bulb connected.

oh gotcha it was doa

~Phil

[Oddvar R]

So, another problem popping up. I have about 550v at the rectifier, but only 1.8v at the other side of the diodes and therefore no voltage to the filter C's. Is there any obvious answer to why this happens. The 1.8v occurs many other places, except on the bias, about 250v-

[pompeiisneaks]

sounds to me like either you've got either bad diodes or you've installed them backwards? Or maybe you;re testing for AC volts at the other side? that's 550 VAC on the one side but there should be basically no VAC on the other side, only VDC. (in other words, make sure you've not left your volt meter on AC volts)

Edit: Now that I hit send, I realize if they were backwards you'd still get rectified DC on the other side, it would just be negative and destroy your filter caps, so ignore that option. Either you have dead diodes or the other thing I mentioned above.

~Phil

[pompeiisneaks]

I also missed the point about the bias voltage being so high, are you sure the dropping resistor is 220k? I would think after that and with only half of the rectified voltage through a single diode it definitely should be way lower. The more important value, though, is what you're reading on the other side of the 47k resistor?

~Phil

[Oddvar R]

Both sides of the 47k are about 224v each, but it seems to be -224, does that make any sense?

The diodes are as in the layout, and I have changed them all.
On the DC side of the diodes, what voltage should be there?

[pompeiisneaks]

The - voltage for bias is right but the voltage range makes no sense. You shouldn't have that much voltage. Also you're going to destroy those bias caps as they're rated at 150V and you've got over 200. The 220k dropper should be lowering 1/2 of the 550 vac side down quite a bit, and by the time it's gone across the other resistors as well, it should be down to near the bias range the amp would normally need.

Something's definitely wrong with your connections. Do you have pictures of the board in that area?

~Phil