Valve amp HV switches

[Gareth Magennis]

I have a fairly vintage valve amplifier with a dead HT (standby) switch.

It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1500alaabb/switch-antibacterial-spst-black/dp/2068581


Now, this switches the rectified HT winding from the transformer, which is
around the usual 500v with no valves in place.


I've been looking at replacements, but have not found any such switch that
has a rating of 500v DC.
Here's a typical search result:
http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000006219&eq=N%3D204319%2B110173704%2B2031%26amp%3BNtk%3Dnarrow_desc%26amp%3BNtt%3Drocker%2Bswitch%26amp%3BNtx%3Drel%2Bgensearch_store_specific%252cstatic%2528P_STORE_MARKETING_RANK_FARNELL_UK%252cascending%2529%2Bmode%2Bmatchall%26amp%3BNty%3D1%26amp%3BNtpc%3D1%26amp%3BNtpr%3D1%26amp%3BD%3Drocker%2Bswitch*%26amp%3BDn%3D2031%2B2031%2B2031%26amp%3BDx%3Dmode%2Bmatchall&st=rocker%20switch&pageSize=25&showResults=true&aa=true&pf=110125431,110173704&vw=


Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst-toggle-switch-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything else I
have managed to find, but definitely not 500v rated.




I'm particularly concerned that the switch originally fitted, and similar
generic types available from Farnell only seem to be rated to 24v DC.
No wonder it is faulty.

Unless I am missing something here.




Gareth.

 

[pfjw@aol.com]

On Wednesday, August 10, 2016 at 3:07:10 PM UTC-4, Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.

It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1500alaabb/switch-antibacterial-spst-black/dp/2068581


Now, this switches the rectified HT winding from the transformer, which is
around the usual 500v with no valves in place.


I've been looking at replacements, but have not found any such switch that
has a rating of 500v DC.
Here's a typical search result:
http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000006219&eq=N%3D204319%2B110173704%2B2031%26amp%3BNtk%3Dnarrow_desc%26amp%3BNtt%3Drocker%2Bswitch%26amp%3BNtx%3Drel%2Bgensearch_store_specific%252cstatic%2528P_STORE_MARKETING_RANK_FARNELL_UK%252cascending%2529%2Bmode%2Bmatchall%26amp%3BNty%3D1%26amp%3BNtpc%3D1%26amp%3BNtpr%3D1%26amp%3BD%3Drocker%2Bswitch*%26amp%3BDn%3D2031%2B2031%2B2031%26amp%3BDx%3Dmode%2Bmatchall&st=rocker%20switch&pageSize=25&showResults=true&aa=true&pf=110125431,110173704&vw

Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst-toggle-switch-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything else I
have managed to find, but definitely not 500v rated.

This gets you to 400VDC: http://76.12.210.115/pdf/s-toggle-sw-rocker-400v-dc.pdf

and then, if you use a relay, you do not risk arc-welding the switch:

http://www.vsholding.com/datasheets/7107%20D3F500D6.PDF

Hope it helps. Keep in mind that at higher voltages, arcing is the concern. But you knew that already or you would not be asking the question.

Peter Wieck
Melrose Park, PA

 

[whit3rd]

On Wednesday, August 10, 2016 at 12:07:10 PM UTC-7, Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.
Now, this switches the rectified HT winding from the transformer, which is
around the usual 500v with no valves in place.

I'm particularly concerned that the switch originally fitted, and similar
generic types available from Farnell only seem to be rated to 24v DC.
No wonder it is faulty.

I'd be tempted to rewire the unit to activate a high-side PMOS power FET
switch. The plastic switch could bias it from any positive or negative
low voltage with a NPN level translator (emitter driven from low voltage
DC on the replacement switch). Depending on the current you're switching,
even an optoisolator might suffice.

 

[N_Cook]

On 10/08/2016 20:07, Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.

It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1500alaabb/switch-antibacterial-spst-black/dp/2068581



Now, this switches the rectified HT winding from the transformer, which
is around the usual 500v with no valves in place.


I've been looking at replacements, but have not found any such switch
that has a rating of 500v DC.
Here's a typical search result:
http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000006219&eq=N%3D204319%2B110173704%2B2031%26amp%3BNtk%3Dnarrow_desc%26amp%3BNtt%3Drocker%2Bswitch%26amp%3BNtx%3Drel%2Bgensearch_store_specific%252cstatic%2528P_STORE_MARKETING_RANK_FARNELL_UK%252cascending%2529%2Bmode%2Bmatchall%26amp%3BNty%3D1%26amp%3BNtpc%3D1%26amp%3BNtpr%3D1%26amp%3BD%3Drocker%2Bswitch*%26amp%3BDn%3D2031%2B2031%2B2031%26amp%3BDx%3Dmode%2Bmatchall&st=rocker%20switch&pageSize=25&showResults=true&aa=true&pf=110125431,110173704&vw=



Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst-toggle-switch-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything
else I have managed to find, but definitely not 500v rated.




I'm particularly concerned that the switch originally fitted, and
similar generic types available from Farnell only seem to be rated to
24v DC.
No wonder it is faulty.

Unless I am missing something here.




Gareth.

Is it a problem with the make or break? any HV cap over the contacts?
Parallel up a DPST switch, perhaps NOS would be better

 

[Gareth Magennis]

"N_Cook" wrote in message news:nofufa$il$1@dont-email.me...

On 10/08/2016 20:07, Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.

It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1500alaabb/switch-antibacterial-spst-black/dp/2068581



Now, this switches the rectified HT winding from the transformer, which
is around the usual 500v with no valves in place.


I've been looking at replacements, but have not found any such switch
that has a rating of 500v DC.
Here's a typical search result:
http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000006219&eq=N%3D204319%2B110173704%2B2031%26amp%3BNtk%3Dnarrow_desc%26amp%3BNtt%3Drocker%2Bswitch%26amp%3BNtx%3Drel%2Bgensearch_store_specific%252cstatic%2528P_STORE_MARKETING_RANK_FARNELL_UK%252cascending%2529%2Bmode%2Bmatchall%26amp%3BNty%3D1%26amp%3BNtpc%3D1%26amp%3BNtpr%3D1%26amp%3BD%3Drocker%2Bswitch*%26amp%3BDn%3D2031%2B2031%2B2031%26amp%3BDx%3Dmode%2Bmatchall&st=rocker%20switch&pageSize=25&showResults=true&aa=true&pf=110125431,110173704&vw=



Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst-toggle-switch-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything
else I have managed to find, but definitely not 500v rated.




I'm particularly concerned that the switch originally fitted, and
similar generic types available from Farnell only seem to be rated to
24v DC.
No wonder it is faulty.

Unless I am missing something here.




Gareth.

Is it a problem with the make or break? any HV cap over the contacts?
Parallel up a DPST switch, perhaps NOS would be better







This is a vintage(ish) amp that has snap-in rectangular Mains and standby
switches. The Mains DPDT double width, the Standby SPST, single width.

I don't really want to be drilling holes for another switch that isn't even
rated to 500v.


Gareth.

 

[Gareth Magennis]

wrote in message
news:7bb703cc-6d67-468e-a261-9353a8788713@googlegroups.com...

On Wednesday, August 10, 2016 at 3:07:10 PM UTC-4, Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.

It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1500alaabb/switch-antibacterial-spst-black/dp/2068581


Now, this switches the rectified HT winding from the transformer, which is
around the usual 500v with no valves in place.


I've been looking at replacements, but have not found any such switch that
has a rating of 500v DC.
Here's a typical search result:
http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000006219&eq=N%3D204319%2B110173704%2B2031%26amp%3BNtk%3Dnarrow_desc%26amp%3BNtt%3Drocker%2Bswitch%26amp%3BNtx%3Drel%2Bgensearch_store_specific%252cstatic%2528P_STORE_MARKETING_RANK_FARNELL_UK%252cascending%2529%2Bmode%2Bmatchall%26amp%3BNty%3D1%26amp%3BNtpc%3D1%26amp%3BNtpr%3D1%26amp%3BD%3Drocker%2Bswitch*%26amp%3BDn%3D2031%2B2031%2B2031%26amp%3BDx%3Dmode%2Bmatchall&st=rocker%20switch&pageSize=25&showResults=true&aa=true&pf=110125431,110173704&vw=


Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst-toggle-switch-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything else
I
have managed to find, but definitely not 500v rated.

This gets you to 400VDC:
http://76.12.210.115/pdf/s-toggle-sw-rocker-400v-dc.pdf

and then, if you use a relay, you do not risk arc-welding the switch:

http://www.vsholding.com/datasheets/7107%20D3F500D6.PDF

Hope it helps. Keep in mind that at higher voltages, arcing is the concern.
But you knew that already or you would not be asking the question.

Peter Wieck
Melrose Park, PA






Interesting that the switch you linked to uses a magnet to steer the arc, as
used on some Speaker Relays.

I'm guessing that valve amp manufacturers have used underated HT switches
for decades now, in much the same way that they have often put way higher
voltages on pre-amp valves than they are designed to cope with. (Fender,
Mesa reverb drive circuits for instance).

So I guess I shouldn't be so shocked at this revelation.



Gareth.

 

[Phil Allison]

Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.

** Is the failure mechanical or are the contacts burned up ?

It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1500alaabb/switch-antibacterial-spst-black/dp/2068581

Now, this switches the rectified HT winding from the transformer,

** Where is the switch connected - in the AC feed ?

which is
around the usual 500v with no valves in place.

I've been looking at replacements, but have not found any such switch that
has a rating of 500v DC.

** And you won't.

Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst-toggle-switch-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything else I
have managed to find, but definitely not 500v rated.


I'm particularly concerned that the switch originally fitted, and similar
generic types available from Farnell only seem to be rated to 24v DC.
No wonder it is faulty.

Unless I am missing something here.

** You have missed the fact that the breaking capacity of a switch depends on the current flowing at the time. That rocker switch will break 24V DC and 250V AC all day long as long as the current does not exceed 16 amps.

In the Standby position of a valve amp, it maybe has to break 0.4 amps at most - so the voltage rating goes way up and it can be expected to do the job fine.

Plus any 250VAC mains switch is insulated to way more than the nominal voltage to allow for spikes etc - so that is OK too.

The best idea is to have the switch in the transformer wiring, breaking the AC voltage rather than the DC after the rectifier. Marshall did this in most models, using a double pole rocker.

But Fender got it *horribly* wrong in a great many models - like the Bassman 100 - connecting the switch in the one spot it should never be.

http://www.thevintagesound.com/ffg/schem/bassman_100_schem.gif

Imagine the switch is operated when the amp is on and running at high volume - maybe because of a sudden acoustic feedback.

Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now in SERIES with the filter choke as the main electros are disconnected. Audio frequency current flows in the choke and generates a very large AC voltage pulse - making the switch contacts arc and the same voltage also appears on the output valve plates.

I have measured this voltage at more than 3kV rms.

A similar thing happens when the HT fuse in a70s or 80s Marshall blows, since it is wired in the same place.

Explains a lot of arc burnt valves and octal sockets.


..... Phil

 

[Phil Allison]

Gareth Magennis wrote:

** You have missed the fact that the breaking capacity of a switch depends
on the current flowing at the time. That rocker switch will break 24V DC and
250V AC all day long as long as the current does not exceed 16 amps.

In the Standby position of a valve amp, it maybe has to break 0.4 amps at
most - so the voltage rating goes way up and it can be expected to do the
job fine.

Plus any 250VAC mains switch is insulated to way more than the nominal
voltage to allow for spikes etc - so that is OK too.

The best idea is to have the switch in the transformer wiring, breaking the
AC voltage rather than the DC after the rectifier. Marshall did this in most
models, using a double pole rocker.

But Fender got it *horribly* wrong in a great many models - like the Bassman
100 - connecting the switch in the one spot it should never be.

http://www.thevintagesound.com/ffg/schem/bassman_100_schem.gif

Imagine the switch is operated when the amp is on and running at high
volume - maybe because of a sudden acoustic feedback.

Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now
in SERIES with the filter choke as the main electros are disconnected. Audio
frequency current flows in the choke and generates a very large AC voltage
pulse - making the switch contacts arc and the same voltage also appears on
the output valve plates.

I have measured this voltage at more than 3kV rms.

A similar thing happens when the HT fuse in a70s or 80s Marshall blows,
since it is wired in the same place.

Explains a lot of arc burnt valves and octal sockets.


Thanks, Phil, that makes a little more sense now.
This switch is connected to the main filter caps, as in the Bassman
schematic, though I don't think it has a choke, it's a pretty sparse 50W
EL34 design (Roost, made in UK).
One Mullard EL34 is toast, the other is OK.

** The bad EL34 may be responsible for the burnt switch.

The owner may have tried to use the standby switch when the valve was passing a large DC current - like a couple of amps. At that level, a continuous arc would form and quickly destroy the contacts.

I see there is no DC choke in the schem but the fusing would allow that sort of current to flow.

http://www.andy-duke.co.uk/roost/Non_reverb_schematic_2006_05_14.png

Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off.


..... Phil

 

[Gareth Magennis]

"Phil Allison" wrote in message
news:66d4080c-49ef-41a4-b28f-1d59c03fe004@googlegroups.com...

Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.

** Is the failure mechanical or are the contacts burned up ?

It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1500alaabb/switch-antibacterial-spst-black/dp/2068581

Now, this switches the rectified HT winding from the transformer,

** Where is the switch connected - in the AC feed ?

which is
around the usual 500v with no valves in place.

I've been looking at replacements, but have not found any such switch that
has a rating of 500v DC.

** And you won't.

Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst-toggle-switch-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything else
I
have managed to find, but definitely not 500v rated.


I'm particularly concerned that the switch originally fitted, and similar
generic types available from Farnell only seem to be rated to 24v DC.
No wonder it is faulty.

Unless I am missing something here.

** You have missed the fact that the breaking capacity of a switch depends
on the current flowing at the time. That rocker switch will break 24V DC and
250V AC all day long as long as the current does not exceed 16 amps.

In the Standby position of a valve amp, it maybe has to break 0.4 amps at
most - so the voltage rating goes way up and it can be expected to do the
job fine.

Plus any 250VAC mains switch is insulated to way more than the nominal
voltage to allow for spikes etc - so that is OK too.

The best idea is to have the switch in the transformer wiring, breaking the
AC voltage rather than the DC after the rectifier. Marshall did this in most
models, using a double pole rocker.

But Fender got it *horribly* wrong in a great many models - like the Bassman
100 - connecting the switch in the one spot it should never be.

http://www.thevintagesound.com/ffg/schem/bassman_100_schem.gif

Imagine the switch is operated when the amp is on and running at high
volume - maybe because of a sudden acoustic feedback.

Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now
in SERIES with the filter choke as the main electros are disconnected. Audio
frequency current flows in the choke and generates a very large AC voltage
pulse - making the switch contacts arc and the same voltage also appears on
the output valve plates.

I have measured this voltage at more than 3kV rms.

A similar thing happens when the HT fuse in a70s or 80s Marshall blows,
since it is wired in the same place.

Explains a lot of arc burnt valves and octal sockets.


..... Phil







Thanks, Phil, that makes a little more sense now.
This switch is connected to the main filter caps, as in the Bassman
schematic, though I don't think it has a choke, it's a pretty sparse 50W
EL34 design (Roost, made in UK).
One Mullard EL34 is toast, the other is OK. It also had a shorted cap in
the tone stack totally crippling the whole amp.

I haven't yet managed to get the switch out of the chassis, it's a tight
bastard, was going to see if I could repair the contacts, but to do so it
needs to come out intact, or I need to source a replacement.



Cheers,


Gareth.

 

[Phil Allison]

Chris Jones wrote:

http://www.andy-duke.co.uk/roost/Non_reverb_schematic_2006_05_14.png

Maybe when you replace the switch, rewire it in series with the HT
fuse - that should allow arcs to extinguish at switch off.


.... Phil


Also, if you can get a 2-pole switch that fits ok, then wiring the two
sets of contacts in series ought to improve the ability to extinguish
the arc, (for a given contact separation etc.).

** If you look at the schem, the HT fuse is in the rectifier ground path where the voltage and current levels drop to zero at twice the supply frequency.

So for a couple of mS each 10mS, there is not enough of either to sustain an arc.


.... Phil

 

[Chris Jones]

On 11/08/2016 18:50, Phil Allison wrote:

Gareth Magennis wrote:



** You have missed the fact that the breaking capacity of a switch depends
on the current flowing at the time. That rocker switch will break 24V DC and
250V AC all day long as long as the current does not exceed 16 amps.

In the Standby position of a valve amp, it maybe has to break 0.4 amps at
most - so the voltage rating goes way up and it can be expected to do the
job fine.

Plus any 250VAC mains switch is insulated to way more than the nominal
voltage to allow for spikes etc - so that is OK too.

The best idea is to have the switch in the transformer wiring, breaking the
AC voltage rather than the DC after the rectifier. Marshall did this in most
models, using a double pole rocker.

But Fender got it *horribly* wrong in a great many models - like the Bassman
100 - connecting the switch in the one spot it should never be.

http://www.thevintagesound.com/ffg/schem/bassman_100_schem.gif

Imagine the switch is operated when the amp is on and running at high
volume - maybe because of a sudden acoustic feedback.

Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now
in SERIES with the filter choke as the main electros are disconnected. Audio
frequency current flows in the choke and generates a very large AC voltage
pulse - making the switch contacts arc and the same voltage also appears on
the output valve plates.

I have measured this voltage at more than 3kV rms.

A similar thing happens when the HT fuse in a70s or 80s Marshall blows,
since it is wired in the same place.

Explains a lot of arc burnt valves and octal sockets.


Thanks, Phil, that makes a little more sense now.
This switch is connected to the main filter caps, as in the Bassman
schematic, though I don't think it has a choke, it's a pretty sparse 50W
EL34 design (Roost, made in UK).
One Mullard EL34 is toast, the other is OK.


** The bad EL34 may be responsible for the burnt switch.

The owner may have tried to use the standby switch when the valve was passing a large DC current - like a couple of amps. At that level, a continuous arc would form and quickly destroy the contacts.

I see there is no DC choke in the schem but the fusing would allow that sort of current to flow.

http://www.andy-duke.co.uk/roost/Non_reverb_schematic_2006_05_14.png

Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off.


.... Phil

Also, if you can get a 2-pole switch that fits ok, then wiring the two
sets of contacts in series ought to improve the ability to extinguish
the arc, (for a given contact separation etc.).

 

[Chris Jones]

On 11/08/2016 23:54, Phil Allison wrote:

Chris Jones wrote:

http://www.andy-duke.co.uk/roost/Non_reverb_schematic_2006_05_14.png

Maybe when you replace the switch, rewire it in series with the HT
fuse - that should allow arcs to extinguish at switch off.


.... Phil


Also, if you can get a 2-pole switch that fits ok, then wiring the two
sets of contacts in series ought to improve the ability to extinguish
the arc, (for a given contact separation etc.).


** If you look at the schem, the HT fuse is in the rectifier ground path where the voltage and current levels drop to zero at twice the supply frequency.

So for a couple of mS each 10mS, there is not enough of either to sustain an arc.

Yes, I noticed that. It is much better than the original arrangement. If
C23 and C24 are in good condition and the B supply rail doesn't get
shorted, then the fuse (and new switch) should only be carrying current
for a quite small fraction of the time, so arcs should extinguish well,
perhaps even better than with a resistive load on AC.

 

[Gareth Magennis]

"Chris Jones" wrote in message news:vz%qz.663818$p74.660064@fx06.fr7...

On 11/08/2016 23:54, Phil Allison wrote:

Chris Jones wrote:

http://www.andy-duke.co.uk/roost/Non_reverb_schematic_2006_05_14.png

Maybe when you replace the switch, rewire it in series with the HT
fuse - that should allow arcs to extinguish at switch off.


.... Phil


Also, if you can get a 2-pole switch that fits ok, then wiring the two
sets of contacts in series ought to improve the ability to extinguish
the arc, (for a given contact separation etc.).


** If you look at the schem, the HT fuse is in the rectifier ground path
where the voltage and current levels drop to zero at twice the supply
frequency.

So for a couple of mS each 10mS, there is not enough of either to sustain
an arc.

Yes, I noticed that. It is much better than the original arrangement. If
C23 and C24 are in good condition and the B supply rail doesn't get
shorted, then the fuse (and new switch) should only be carrying current
for a quite small fraction of the time, so arcs should extinguish well,
perhaps even better than with a resistive load on AC.







Thanks Chris and Phil, that's very interesting.

Why wasn't this practice adopted decades ago? Were Fender (Bassman) really
that clueless?
Beggars belief.



Cheers,

Gareth.

 

[Adrian Tuddenham]

Gareth Magennis <soundserviceleeds@outlook.com> wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.
[...]
Now, this switches the rectified HT winding from the transformer, which is
around the usual 500v with no valves in place.

Don't make the mistake which comes around time and again* of switching a
charged capacitor directly onto a discharged one, the switch won't last
long. Use a resistor to limit the surge current.

* The AR88 communications receiver and a Philips transistor car radio
(40 years apart) spring immediately to mind.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

 

[Phil Allison]

Gareth Magennis wrote:

** If you look at the schem, the HT fuse is in the rectifier ground path
where the voltage and current levels drop to zero at twice the supply
frequency.

So for a couple of mS each 10mS, there is not enough of either to sustain
an arc.

Yes, I noticed that. It is much better than the original arrangement. If
C23 and C24 are in good condition and the B supply rail doesn't get
shorted, then the fuse (and new switch) should only be carrying current
for a quite small fraction of the time, so arcs should extinguish well,
perhaps even better than with a resistive load on AC.


Thanks Chris and Phil, that's very interesting.

** Nice example of how so often, the devil is in the detail .....

Why wasn't this practice adopted decades ago? Were Fender (Bassman) really
that clueless?

** Fender originally placed the Standby switch after the main filter electros in consideration of the valve rectifiers being used. Placing it earlier creates a large current surge every time the switch closes while the electros charge. Such surges well exceed the maximum ratings for 5U4s or GZ34s and will drastically shorten their life spans.

When Fender changed to using strings of silicon diodes, the problem vanished and they ought to have reconsidered the standby switch scheme as Marshall did.

Ferder's "The Twin" model ( 1993) finally has a double pole switch in the AC like Marshall.


..... Phil

 

[Phil Allison]

Adrian Tuddenham wrote:

Don't make the mistake which comes around time and again* of switching a
charged capacitor directly onto a discharged one, the switch won't last
long.


** You mean like the Mesa Boogie MkI model does ??

http://bmamps.com/Schematics/Mesa_Boogie/Mesa_Boogie_Mark_I_Schematic.pdf

Note the 60F, 500V electro fitted AFTER the standby switch !!

The main electro bank is inadequate ( 2 x 60uF, 350V in series )so someone decided to add an extra cap. There is a loud *snap* whenever the standby is closed and it soon burns the hell out of it.

Later versions used 2 x 220uF in the main bank and a 0.1uF 600V film cap after the switch ( so from OT centre tap to ground ) which sometimes fails short.

For sheer stupidity it is hard to beat early 70s Marshalls that switched OFF the bias supply with the standby.

http://bmamps.com/Schematics/marshall/Marshall_1987_MKII_JMP_50watt_6550_year_1970.gif

While the 400V HT come up instantly, output valve grid bias take 3 or 4 seconds to arrive !!


..... Phil