Location of Standby switch - before B+1 or after????

[JammyDodger]

Hey All,

I am again digging into my ODS-101 HRM to make a few tweaks. In researching the schematics I see some in which the standby switch is directly after the bias board (before B+1) and others in which it is between B+1 and B+2. In this case, since the OP center tap is on B+1 it will be getting power as soon as the power switch is turned on.

Is there an advantage to either?


Cheers, Mike

[ontariomaximus]

My preference is to KEEP DC OFF THE STANDBY SWITCH. You can do that by using a double pole switch and connect the 2 HV legs from the PT secondary to the diodes. I hate hate hate big time DC on switches.

[tubedogsmith]

Hey All,

I am again digging into my ODS-101 HRM to make a few tweaks. In researching the schematics I see some in which the standby switch is directly after the bias board (before B+1) and others in which it is between B+1 and B+2. In this case, since the OP center tap is on B+1 it will be getting power as soon as the power switch is turned on.

Is there an advantage to either?


Cheers, Mike

Connect the OT center tap to the cold side of the switch along with one side of the choke. The other side of the choke will connect to the screens.

[nickt]

My preference is to KEEP DC OFF THE STANDBY SWITCH. You can do that by using a double pole switch and connect the 2 HV legs from the PT secondary to the diodes. I hate hate hate big time DC on switches.

I'm guessing you're worried about arcing? But I don't think that's a problem given the load on the other side of the switch is relatively high.

OTOH connecting rectifier to switch to BIG ASSED caps means the switch contacts always get big assed currents as the caps charge up.

Is this a real problem? No idea...

[JammyDodger]

Nicht,

That's what I was looking for ... a reason to do it or not. Arcing does not concern me, current does.

Cheers, Mike

[butwhatif]

When u put the switch before the primary caps, they get a huge spike current flow. I've seen switches welded by this more than once. The caps should be connected to the recs, then the switch. I've never had a grounded switch, although they are not rated for hi voltage,

[ontariomaximus]

When u put the switch before the primary caps, they get a huge spike current flow. I've seen switches welded by this more than once. The caps should be connected to the recs, then the switch. I've never had a grounded switch, although they are not rated for hi voltage,

When you say "before the primary caps" do you mean at the secondary winding or after the rectification. Above I promoted the use of switching the AC only. The secondary resistance of the transformer should limit current (so I am thinking) and no DC to arc the switch contacts. Is the JTM45/Bassman type perhaps better, with a very small poly cap right after the rectification, then the standby switch, then the main B+ cap(s)?

[Tonegeek]

I lean toward butwhatif on this one. Put the SB switch AFTER the big primary caps. Put the input to the choke and B+1 (goes to the OT center tap) on the other side of the switch. Make sure you have bleeders to take voltage off ALL caps in the power supply when power is off (for safety reasons).

Pro: Stresses your standby switch and rectifier less

Cons:
when power is on and standy on you will still have your largest caps charged to the full potential of the rectifier, so you have be a little more careful. Also the caps will have a larger voltage on them doing it this way because there is no load on them until the standby switch is closed. There is some concern that this stresses the cap more. This may also stress the output tubes, and OT, as it puts an instantaneous high voltage (higher than normal) on the plates whereas when the caps are on the other side of standby, the caps ramp up the voltage gradually. Personally, I don' t think it is an issue.

[Structo]

Yeah, I've heard it both ways as well.

I like Tonegeek's reasoning.
It makes sense that if you have the SB switch after the big caps that there would be no load on them.
The instantaneous high voltage on the power tubes doesn't sound like a good recipe for long tube life.

Also, I believe it will thump the speakers hard when you switch after the caps.

[paulster]

I prefer switching AC, before the rectifiers with a DPST switch.

There'll be inrush current, sure, but by switching AC you'll have no popping issues and no arcing on the switch contacts. If you switch after the first cap bank then the first caps still have the high inrush current at power-on so it's not like you're really doing the caps a favour anyway.

Definitely one of those personal preference things though.

[butwhatif]

fwiw in my 40 years of working on, building, modifying, I've never seen a power tube, or cap blow because of this method of routing B+. But the quality is not like it used to be, and tubes aren't across the board the same. If you buy good stuff though, and don't buy cheap, u'll have no probs. Stressing the rectifiers by slamming them with a dead short big cap inst load is a recipe for disaster, even if it doesn't happen right away.

[Bob Simpson]

I learn better from pictures/diagrams.

I'm kind of confused, based on layouts I've seen, how the tubes get any voltage at power on/ standby off condition.

I don't know how to diagram in this editor, but I thought it went something like this...

DPDT on/off/on Standby switch: if 1 and 4 are the top 2 contacts, 2 and 5 are the middle contacts, & 3 and 6 are the bottom contacts....

1 4 :--> diodes to first filter cap bank + and 2
2 5 :--> 1 and 3 jumpered & OT center tap to 3, choke in to 1, choke out to 4
3 6 :--> 10W resistor in to 3, 10W resistor out to 6, 5 to pin 3 on octal sockets & 2nd filter cap bank +

Power on / standby centered ( off ) charges first filter cap bank, no power to OT
Power on / standby up ( on ), power to OT CT, resistor out to tubes and 2nd filter
Power on / standby down (on), power to OT CT, choke out to tubes and 2nd filter


Sorry for the cryptic explanation... how far off am I?

edit... this looked much better in the editor pane than it does here...

[Tonegeek]

Maybe this will help (uses a DPDT with center off):

[JammyDodger]

To All,

I'll weigh in with my own drawing. So my real question is.... Standby switch before B+1 or between B+1 and B+2. Advantages or disadvantages to each.

I've drawn the choke only version


Cheers, Mike

[Tonegeek]

To All,

I'll weigh in with my own drawing. So my real question is.... Standby switch before B+1 or between B+1 and B+2. Advantages or disadvantages to each.

I've drawn the choke only version


Cheers, Mike

The bottom drawing is incorrect, I think. The caps are ok on the hot side, but the B+ should come off the cold side of the switch. I listed some pros /cons in a previous post. BTW the hand drawn schematics of #124 is like your top drawing where all caps are on the cold side.