help! my HRM overdrive sounds harsh

[novosibir]

whats your thought on using a MOV here instead?

The MOV should be across the wall voltage inlet. There's no benefit, to have a MOV across the standby switch.

Larry

[Pete]

OK, I think I'm not getting this. Where would there be more potential for a spike on the latter nodes, with standby switch before all caps or with the switch after the 1st big caps (keeping them charged)??

Mine is wired between the diodes and first cap but this way seems like a better idea. Is this just to keep from having such a large inrush when flipping the standby? I would think it is a bit easier on the diodes, fuse, etc.

That plus it keeps any high voltage spikes away from the later nodes that probably have lower voltage caps. The addition of a .02 1KV disc cap across the Standby switch is another good thing to add.

[novosibir]

Spikes will be produced from the rectifier diodes, that happens every time, when the AC cycle is going through the zero-line, because diodes usually aren't fast enough, to open, and especially 'to close' fast enough.

That's indeed producing RF artifacts, whose intermodulations products with the ripple frequency then is audible as the well known buzzzzzz.

You may minimize this by bypassing each diode with a .002/1000V cap - or with a .02/1000V cap from close after the rectifier to ground.

You may avoid this by using FRED diodes - i.e.:

Instead of 1N4007 use UF4007

or

instead of 1N5408 use UF5408

Don't forget a 5-th FRED diode for the bias topology!

Larry

[heisthl]

The addition of a .02 1KV disc cap across the Standby switch is another good thing to add.

When you have the first filter caps on the hot side of the standby switch, then I'd recommend the .02/1kV cap from the cold side of the switch to ground.

Larry

You might be right -
I haven't thought about that because why would you not have the first filter charged? Don't you think not charging it is flat out wrong? When you do it the right way the cap goes across the switch (but you know that ).

whats your thought on using a MOV here instead?

The MOV goes across the primary power transformer leads (i.e. the AC after it's switched) not the standby switch. I put a MOV in every amp I build, it's cheap insurance for running on surging generators or plugging into the occasional weird club oultlet wired for 220.

Edit: it would help if I read the previous posts before I start typing.

[heisthl]

OK, I think I'm not getting this. Where would there be more potential for a spike on the latter nodes, with standby switch before all caps or with the switch after the 1st big caps (keeping them charged)??

Mine is wired between the diodes and first cap but this way seems like a better idea. Is this just to keep from having such a large inrush when flipping the standby? I would think it is a bit easier on the diodes, fuse, etc.

That plus it keeps any high voltage spikes away from the later nodes that probably have lower voltage caps. The addition of a .02 1KV disc cap across the Standby switch is another good thing to add.

What I was talking about there was you have the diodes with no load (no filter to set the ouput voltage) so It has the possibility of being higher than 500 volts in the initial surge. If your charging the first filter stage when power is applied this can't happen because the voltage from the PT comes up slowly (relatively) and gets to a stable point at whatever the first filter sets it to (believe it or not a filter cap sets the voltage when there's no load - try changing the capacitance on an unloaded stage, you can change the voltage drastically with varied capacitance).

[Tonegeek]

I learn something new most every visit to this board! Time for a group hug

[Pete]

So how are original (real) ODS amps wired in regards to this part of the PS?
I thought I knew the ansewer to this. I have seen pics and the schems which show the standby switch following the diodes. This must have been a conscious decision to change from the "original Fender design". Just wondering about the evolution of the design, and if/why anyone might think it was better.

OK, I think I'm not getting this. Where would there be more potential for a spike on the latter nodes, with standby switch before all caps or with the switch after the 1st big caps (keeping them charged)??

That plus it keeps any high voltage spikes away from the later nodes that probably have lower voltage caps. The addition of a .02 1KV disc cap across the Standby switch is another good thing to add.

What I was talking about there was you have the diodes with no load (no filter to set the ouput voltage) so It has the possibility of being higher than 500 volts in the initial surge. If your charging the first filter stage when power is applied this can't happen because the voltage from the PT comes up slowly (relatively) and gets to a stable point at whatever the first filter sets it to (believe it or not a filter cap sets the voltage when there's no load - try changing the capacitance on an unloaded stage, you can change the voltage drastically with varied capacitance).

[heisthl]

I don't know about the early ones but the precision power supply ones definitly do it the correct way. The filter PCB has 2 holes connected to the first filter - 1 for the diodes and 1 to the switch. Then there is a cluster of 3 holes for switch return,choke feed and OT feed.

[skyboltone]

Ok, now, not by way of argument, but for information. How about lifting the center tap of the HV winding for standby?

[heisthl]

Ok, now, not by way of argument, but for information. How about lifting the center tap of the HV winding for standby?

Among other reasons not to do it that way, this would not work if you had a bias tap. You want the tubes at stable bias when you go to "operate" position.

[skyboltone]

Ok, now, not by way of argument, but for information. How about lifting the center tap of the HV winding for standby?

Among other reasons not to do it that way, this would not work if you had a bias tap. You want the tubes at stable bias when you go to "operate" position.

OK! Thanks.

[novosibir]

I haven't thought about that because why would you not have the first filter charged? Don't you think not charging it is flat out wrong? When you do it the right way the cap goes across the switch (but you know that ).

Not to be misunderstood! Nothing against the first filter caps on the hot side of the standby switch, as long as their voltage ratings keeps it on the save side. It's even better for the switch, because the peak loading current of the first filters doesn't run over it, so its lifetime will be enhanced.

I've only agreed, that an additional foil cap anywhere in parallel to the first filter caps will be helpful to kill some spikes (at RF electrolytics are too slow), and as soon as the amp is switched from standby to operate, it doesn't matter, whether this cap is on the hot side of the standby, or on the cold side.

I've only experienced, that when the filters are on the hot side of the standby switch and the additional foil cap on the cold side, then this additional cap also helps to suppress the possible popping of the switch.

Larry

[heisthl]

[quote="novosibir]
Not to be misunderstood! Nothing against the first filter caps on the hot side of the standby switch, as long as their voltage ratings keeps it on the save side. It's even better for the switch, because the peak loading current of the first filters doesn't run over it, so its lifetime will be enhanced.

I've only agreed, that an additional foil cap anywhere in parallel to the first filter caps will be helpful to kill some spikes (at RF electrolytics are too slow), and as soon as the amp is switched from standby to operate, it doesn't matter, whether this cap is on the hot side of the standby, or on the cold side.

I've only experienced, that when the filters are on the hot side of the standby switch and the additional foil cap on the cold side, then this additional cap also helps to suppress the possible popping of the switch.

Larry[/quote]
I've never done that but It makes sense - wouldn't you still want a cap across the switch to prevent arcing?

[novosibir]

... wouldn't you still want a cap across the switch to prevent arcing?

If the 1-st filter caps are after the standby (cold side), then I'd say that's certainly not a bad idea, since the peak current by switching and therefore the amount of arcing is fairly high.

With the 1-st filters on the hot side of the standby, I'd say not absolutely necessary, but certainly doesn't hurt

Larry

[JimiB]

Also you keep saying "foil cap" I thought we were talking about a 1KV .02 ceramic?