Power Transformer Center Tap Standby

[Mr. SD-1]

I've encountered a few amp designs that employ a spst standby switch wired to disconnect the center tap of the power transformer high voltage winding from ground. This is not considered the best standby solution; nonetheless, I am curious about the theory. Why does the switch turn off the HV? Wouldn't it rewire the HV secondary to produce a single large AC signal instead of two, smaller AC signals of opposite polarity?

Also, I've noticed that Marshall Super Leads have the HV center tap connected between two of the filter caps, with the (-) output of the bridge rectifier grounded. What is the theory behind this configuration?

Lastly, some amps have no HV center tap, but use a spst standby switch to disconnect the (-) output of the bridge rectifier from ground. How does this arrangement work?

Many thanks,
Mr. SD-1

[schmidlin]

Yes, I have had the same questions, but it does makes sense when you think about it. HV MUST have a reference to ground, or it gets no 'traction'. Imagine a car that you lift off the ground. Then lower it with the standby switch. Same with CT and bridge rectifiers. But I feel it is a good standby solution. Works great.

With regard to the CT going to the 2 filter caps, I remember seeing something like that, but can't find the schemo, so others may chime in.

[SatelliteAmps]

Electricity has to have a path, meaning a beginning and an end. If you lift either one, then there is no path and current will not flow. Whether it is lifting a center tap (which is a ground reference essentially.), or lifting one side of the rectifier, it does the same thing.

[Mr. SD-1]

Thank you both for the explanation. I'm looking at a 5F6-A Bassman schematic. The 5VAC winding for the rectifier does not have a center tap ground reference. How does current flow from beginning to end in the 5V circuit?

http://www.schematicheaven.com/fenderamps/bassman_5f6a_schem.pdf

[WaltC]

It's AC power, not DC. Since the heater on the Rectifier isn't part of the signal path and the B+ taken off of it is heavily filtered after it is rectified, you don't need a ground reference to reduce filament hum.

[Mr. SD-1]

I see... The 5VAC is riding on the high voltage DC output of the rectifier, so ground reference of the 5V filament circuit would short the B+.

Back to the Marshall Super Lead circuit... Why is the center tap of the high voltage secondary connected between the first two filter caps?

http://www.drtube.com/schematics/marshall/1959u.gif

[VacuumVoodoo]

Back to the Marshall Super Lead circuit... Why is the center tap of the high voltage secondary connected between the first two filter caps?

http://www.drtube.com/schematics/marshall/1959u.gif

This rectifier circuit is a voltage doubler. Imagine the rectifier and ripple filter caps disconnected from the rest of the circuit. Now imagine the CT and mid point between filter caps connected to ground. You'll be seeing -230V DC on the cap that was formerly at ground and +230V DC on the cap that was supplying B+, both measured relative to ground. If you swap the CT ground and -230V point you get all of 460V between new ground and B+. It's about defining ground reference point. Confusing enough ;-) ?

[donnyjaguar]

It is a voltage-doubler of sorts, but not the classic style. Whether the centre tap is to ground or to the middle of your filter capacitors has no bearing on the resultant DC voltage. What it does do is stiffen up the supply in that it provides two paths for current into the filter section during each cycle of the AC. It also allows for more efficient balancing of the energy in each half of the first filter. Finally, it has the benefit of being able to use lower-voltage capacitors which are also generally lower cost.

From a personal opinion standpoint, I don't like the centre-tap lift style of stdby/run switches. I just like to switch everything on the positive side whenever possible.

[hasserl]

PT center tap standby works fine for cathode biased amps, but it's not the best scenario f0or fixed bias amps, as there will be no bias voltage until you flip the amp off Standby, allowing the tubes to operate momentarily with no bias voltage until the bias circuit charges up.

[Mr. SD-1]

PT center tap standby works fine for cathode biased amps, but it's not the best scenario f0or fixed bias amps, as there will be no bias voltage until you flip the amp off Standby, allowing the tubes to operate momentarily with no bias voltage until the bias circuit charges up.

Yes, there is a similar problem associated with many 50w Marshalls from the early 70s. The fixed bias feed was wired after the standby switch, so the tubes would cook with each flick of the standby.

[schmidlin]

Agreed. Unless you have a bias tap.

[SatelliteAmps]

Disagreed. If the bias supply is off the same tap as the B+ then they will activate in the same time frame. The bias supply is no faster or slower than the B+. They should achieve the same thing at the same time.

There was no major problem with Marshalls from any time period having tubes cook with each standby switch flick due to the bias supply not being fast enough.

[Mr. SD-1]

Disagreed. If the bias supply is off the same tap as the B+ then they will activate in the same time frame. The bias supply is no faster or slower than the B+. They should achieve the same thing at the same time.

There was no major problem with Marshalls from any time period having tubes cook with each standby switch flick due to the bias supply not being fast enough.

Wouldn't the bias caps discharge faster than the B+ caps when powering down?

[SatelliteAmps]

The amount of time for a difference doesn't really let the tubes run full on for enough to actually damage them.

[Mr. SD-1]

The amount of time for a difference doesn't really let the tubes run full on for enough to actually damage them.

The issue does not necessarily damage the tubes, but the associated current surge is very hard on the power transformer and rectifier diodes.

[SatelliteAmps]

Really? The diodes used in those amps are capable of handling surges well over what the power transformer is capable of delivering. And this has NO effect on the power transformer itself. It is going to be working at a nominal state at this point, and would only go up to a standard operating state. It would do nothing to add any stress to the power supply at all. Nothing extra, no extra current surge over or above what it is capable of.