Design Error 2 extra

Hi to my tormenters,

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the tube amp question left most of you a tad agog.

Yet it was simply a case of noting that opening the s/b switch altered the topology of the amplifier in a not good way.

Since transient over-voltage on the plates leading to insulation failure is a common thing with tube amps generally, many makers apply a simple fix using high voltage diodes.

Two or three 1N4007s in series or a 4kV multi-chip diode is strung across the plate and ground of each tube in a pair to stop the plate voltage ever going below zero.

With them in place, the max positive plate voltage cannot exceed twice the B+ supply due to transformer action in a push pull output stage. If one side ever goes to zero, the other will be at double B+.

But this simple fix could not be safely applied to the Fender amp in my question.

Why ?

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..... Phil

Reply to
Phil Allison
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You characterize the issue as the amplifier operating with a different topology, with the output transformer and tubes presumably operating from the 20uF cap through inductor TR2.

But at the instant of switch-off, TR2's operating current should first make a negative-going flyback voltage, whose magnitude may be partly attenuated by winding capacitances, but ultimately will be limited by insulation breakdown. A reverse diode across TR2 would eliminate any flyback.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

TR2 is a choke, no opposite winding to connect a diode network to.

The diodes from the endpoints of the TR3 primary would only protect against TR3, but not against a flyback voltage from TR2, which would cause a reverse breakdown of one (or both) of these diodes.

What could have offered better protection is:

- Either a 600V rated MOV between TR3 primary midpoint and GND.

- Or a a pair of at least 600V rated diodes as follows: - one in reverse direction across the standby switch and - the other in place of the MOV above, with anode to GND.

- Alternatively, a 600V rated TVS diode across the standby switch (non-bidirectional, with the cathode towards the supply side)

In the first case, the MOV should be chosen to have enough thermal mass to absorb at least 20 uF, better some 50 uF worth of stored energy.

In the last case, the TVS diode does not need to dissipate as much as the MOV, because the flyback voltage is clamped non-dissipatively, but it still needs to dissipate significant energy during undershoots.

Dimitrij

Reply to
Dimitrij Klingbeil

I've been working on amps using that circuit for oh what 40 years, never had any issues. I hit the standby when at full power all the time, no surges, no nothing. Unlike that older circuit where they put the SB switch in series with the power transformer CT, that one blows rectifier tubes. But the discussed circuit has always worked fine in my experience. Some makers put a RC snubber across the switch but never bothered, whenever I have to replace the switch it's from mechanical damage (wore out or broken) rather than electrical stress. Despite being ran way above its rating.

Intuitively seems like there should be significant kickback from the choke when the SB switch is opened, but it's only passing a few milliamps (screen and preamp supply) and the output transformer absorbs it. With a proper load the output transformer CT is effectively resistive (if not the frequency response would suck!) and perfectly capable of absorbing fast transients. But it's probably not a good idea to operate the standby switch (or for that matter the amp itself) with no load.. that's why the output jack has a shorting contact.

Terry

Reply to
Terry Newton

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** Eee - yep.

** Eee - nope.

Not with a 20uf cap at one end and a 100mA of load on the other.

** Fanciful with loads at both ends.

** There is about 50 volts p-p of 100/120 Hz ripple appearing across the choke at full power. 35uF at 250mA load, I = C.dv/dt

But only about 10 volts of DC drop.

See the issue?

FYI to all:

Win walked into that one like a revolving door ...

How did the man ever get to be a circuit designer ?

This stuff is soooo basic.

.... Phil

Reply to
Phil Allison

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** Spot on. We have ourselves a design genius.

That exact event happened on my bench when I fitted HV, fast diodes to the 6L6 tube sockets.

First shot, under drive and load.

** That could work.

** Not good practice, bridging a switch with a big safety aspect to it with a diode. Safety for some poor repair tech that is.

** Hmmm - about 5 Joules. Doable.
** YOU are not just a pretty face...

..... Phil

Reply to
Phil Allison

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** That is not possible.

** From the inrush current surge at switch on.

Some recent all tube amps have the same flaw - like the new Vox AC30s made in China.

** They would not do that if there were NO issue.

In fact, if you *slowly* operate the s/b switch under drive and load - you can hear an arc form across the contacts. Open one and you can see all the cabonisation after years of such abuse.

** Ones I've seen all have badly burnt contacts.

** Not kickback, the 6L6s are driving the choke like IT is the load.
** At full power, screen current is not small any more. When clipping, the plate voltage drops to about 70V while the screens stay at full supply - so pass heaps of electrons. This is the major reason for so many EL34 burn outs in Marshalls - the thin screen wires glow and melt.
** Shame that jack is rendered useless if a lead is plugged in and not terminated.

.... Phil

Reply to
Phil Allison

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