[Brian]

Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B

[Phil Allison]

"Brian"


My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?


** Which version is it?

Tube rectifier or diodes ?

Local model or export with multiple AC voltages ?

Any mods done ?


........ Phil

[PeterD]

On 27 Feb 2007 05:17:38 -0800, "Brian" wrote:

Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B


Start by checking current drain on the power supply lines. I'd guess
(just a WAG at that) maybe a leaky filter cap?

[Phil Allison]

"PeterD"


Start by checking current drain on the power supply lines. I'd guess
(just a WAG at that) maybe a leaky filter cap?


** It would be running very hot or probably have exploded by now.

When the only symptom is a hot tranny - there is a short on one of the
windings.



........ Phil

[David R Brooks]

François Yves Le Gal wrote:
On 27 Feb 2007 05:17:38 -0800, "Brian" wrote:

1968 Fender Princeton reverb

Is it a late blackface (AA1164) or an early silverface (AB868)? I've seen
both, even if the SF's were supposed to have replaced the BF's by late '67.

had a power tranny smoke out

The PS is pretty simple, with three secondary windings : 5 V (valve heaters,
with either a 5U4GB or a 5AR4/GZ34 ), 6,3 V (heaters) and 340 - 0 - 340 V
(B+).

Smoking a couple of trannies means that there's a short somewhere, with
current demands above the max that the PS can handle.

If the amp works, we can presume that the heaters are OK, making the B+ the
prime suspect.. Check the voltages before and after rectification/filtering,
the wiring, the layout as well as the components - IMO you could be in for a
bad surprise with the electrolytic filtering caps.


Assuming it has a conventional split-phase rectifier, a short on one
side of the HT would still let the amp run, with the HT running as a
half-wave rectifier.
Put a voltmeter or CRO on each of the rectifier tube anodes. The
readings should be substantially equal.

[Peter Wieck]

On Feb 27, 8:17 am, "Brian" wrote:
Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B

I can understand replacing the tranny once, but not twice without
identifying the problem.

Please define "working". What have you done to verify that state other
than the fact that the amp makes noise? Have you checked:

B+ output?

Filter caps?
Tubes for shorts?
Carbon tracks in the sockets?
Anything else?

Peter Wieck
Wyncote, PA

[Bob H.]

Hi

Also, check for wires shorting to the chassis. Sometimes it looks
like discoloration on the wire. With the amp powered down and
unplugged, physically lift all the wires associated with power from
the chassis and check for holes, burnt spots, etc.
In addition to all the other suggestions here.

Bob H.

[[email protected]]

On 27 Feb 2007 05:17:38 -0800, "Brian" wrote:

Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B

Start by running it for a while with NO tubes installed - see if it gets hot -
report back...

[Patrick Turner]

Brian wrote:

Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B

Brian, If you've replaced the tranny twice,
only to find smoke continuing,
have you checked to se what rectifiers for the B+ supply are present?
Are there silicon diodes? If so, are they OK? if one of two is shorted,
it could be causing the problem.
If the rectifier is a tube, it could have a short inside it.
Try the amp without any tubes in it.
If you measure the input current and it is still high without any tubes
in the amp
you must have a short somewhere between a winding and 0V, maybe
something pinched
or a shorted filament winding to the chassis or shorted HT winding to
chassis.

Are all the pin grippers in the old tube sockets tight? when you remove
the tubes, were the tubes
kinda loose, and did they nearly just fall out of the sockets?
If so, turn off the amp, remove the mains plug, and tighten the socket
pin grippers
with an awl and careful bending of the grippers.
With tight pins, it will now be unlikely that you have bad connections
and if the grids
of the output tubes are not connected the output tubes will conduct too
much current
and overheat the power tranny after some time, a few minutes.
But this is a later check, first you attend to :-

Measure the input current you need to have some way
to do it, either with a variac with a current meter on it,
or with a 1ohm resistor in series with the mains Neutral line into the
power tranny
primary.
Read the ac voltage across the 1 ohm, and that is also the Current input
to the amp.
With no rectifiers or heater supplies with no tubes, current should be
low at
less than 100mA ( 100mV across the 1 ohm R ).

If there is a silicon bridge rectifier, or pair of silicon diodes,
these should operate without the tubes and produce a B+ voltage
which will maybe be +500V, so care is needed measuring.
If the B+ voltage isn't present or remains low say +20V after turn on
then something is shorted, maybe an electro cap
or a diode.

If the rectifier is a tube, plug in the tube rectifier and you should
see the B+ come up to about +500V
and then the input current will reduce to beloe 100mA because there will
not be any load
on the power tranny.
If the B+ remains low with the tube rectifier plugged in, and with all
the pin grippers
in the socket tightened, there is a short in the tube rectifier or a
cap.
Try a replacement good rectifier, and if B+ doesn't come up within 20
seconds, there could be a shorted cap.

If the B+ does rise and stay high at 500V, then all may be well with the
rectifier.

To measure the anode current in each output tube, you need to have a
10ohm resistor between cathode and the chassis
for each output tube, so if it is fixed bias connect a 5 watt wire wound
x 10 ohms between each cathode of the output tubes
and chassis
This will allow easy measurenet of the dc current for each tube, just
divide the Vdc by 10, and this is the
current in Amps dc.

Plug in one output tube, and turn on the amp.
After 20 seconds, the B+ should come up, then go down a little if the
output tube begins to conduct.
Its current should be below 70mA, and you should read less than 0.7Vdc
across the 10 ohm R.
If you have a higher voltage of say 5V appear across the 10ohm, you have
a shorted tube.
Do the same test on the other output tube.
There should be about 0.5V read across each 10 ohm R.
The B+ will have dropped to about 470V from the 500V without a load.


I don't know if the Fender has 6L6 or 6V6 outpus, if 6L6, expect B+ of
500V, if 6V6, expect
400V or less, unloaded B+.

If the tube currents and B+ are OK, then does the amp remain OK after a
minute, 3 minutes, 10 minutes?
30 minutes? Old tubes or open grid circuits can give rising dc currents
with time
and also failing coupling caps.

Food for thought.

Patrick Turner.

[[email protected]]

On 27 Feb 2007 05:17:38 -0800, "Brian" wrote:

Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B
What vaule fuse is in there? Tin foil !!!!

[Ernst]

On Feb 27, 7:17 am, "Brian" wrote:
Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B

1. Put the proper value fuse (use a slo-blow unit) back into the amp.
2. Pull all the tubes.
3. Apply power. If the fuse gives up, there is likely a bias supply
problem (silicon rectifier or associated filter caps).
4. Reinstall the rectifier tube, apply power. If the fuse goes away,
then the rectifier tube, filter caps, output transformer, or other
component in the B+ supply chain has failed.
5. If so far is so good, plug the output tubes back in. If the fuse
fails, an output tube (or tubes) is bad; or, the bias supply is
misadjusted or has failed. Red plates on the output tubes preceeding
the fuse failure point toward a deficient bias supply or open grid
resistor in the bias supply.
4. Next, plug in the support tubes by stage. Obviously, a popped
fuse would indicate a defective component within, or associated with
that stage (associated means a feedback cap or some such thing).
5. Chafed wires and hard shorts usually cause immediate departure of
the fuse.
6. Delayed departure of the fuse is sometimes caused by filaments that
work themselves into awkward positions as they heat up, unruly caps as
they charge, or current runaway promoted by weak components (like
resistors) in a control function.

Just as an aside, I've saved a couple of hard to replace antique radio
power transformers by putting the unit in series with a light bulb
(that's right, line cord, series wired 250W lamp, outlet box, radio
plugged in).

If the lamp lights up bright, there is a short, but the equipment is
protected.

A couple lamp flashes that settle down to a nice glow is good. That
means there was an expected inrush current when power was applied to
the tube heaters, and then the filter caps charged as advertised when
the rectifier tube heated up and conducted.

Gradual increase in lamp brightness means something is giving up.

One more thing: Hum that increases as the amp initially warms up (with
the input properly loaded) causes me to to suspect the filter caps.

Hope this helps,

Ernst

[Brian]

Hi, OP here,

The fuse is a 1 amp, the rec is tube "gz34" The filter caps checks
good [replaced 6 months ago]
Thanks so much for all the advice, I will try to see what current is
being drawn. I have been over the amp very closely looking for any
shorts, out of spec componants and can't find a thing! This is a vety
early silver face.


Thanks again!

Brian

[PeterD]

On Wed, 28 Feb 2007 00:41:31 +1100, "Phil Allison"
wrote:


"PeterD"


Start by checking current drain on the power supply lines. I'd guess
(just a WAG at that) maybe a leaky filter cap?


** It would be running very hot or probably have exploded by now.

When the only symptom is a hot tranny - there is a short on one of the
windings.


I was thinking more along the lines of all of them... g And maybe
more.

I think the OP stated he was on his third transformer, so I wonder if
it is likely that he got a string of bad ones? (Yes! it is possible,
I've seen runs of transformers where 'something' has gone wrong and
they are all bad...) g

[Patrick Turner]

Brian wrote:

Hi, OP here,

The fuse is a 1 amp, the rec is tube "gz34" The filter caps checks
good [replaced 6 months ago]
Thanks so much for all the advice, I will try to see what current is
being drawn. I have been over the amp very closely looking for any
shorts, out of spec componants and can't find a thing! This is a vety
early silver face.

Thanks again!

Brian

If what the others and myself suggested leads no place fast, then rather
than stuff things more yourself,
consider doing what most folks do when some damn thing breaks, take it
to a technician.

I took my old PC No2 to the technician today. Fixed lotsa things and
replaced a CDwriter
for $30, while I waited. I'd have been there for a day and stuffed it
up.....

Patrick Turner.

[Patrick Turner]

PeterD wrote:

On Wed, 28 Feb 2007 00:41:31 +1100, "Phil Allison"
wrote:


"PeterD"


Start by checking current drain on the power supply lines. I'd guess
(just a WAG at that) maybe a leaky filter cap?


** It would be running very hot or probably have exploded by now.

When the only symptom is a hot tranny - there is a short on one of the
windings.


I was thinking more along the lines of all of them... g And maybe
more.

I think the OP stated he was on his third transformer, so I wonder if
it is likely that he got a string of bad ones? (Yes! it is possible,
I've seen runs of transformers where 'something' has gone wrong and
they are all bad...) g

Nah, summink simple is staring at him from that cab, and he just can't
see it...

It ain't as if he's stranded on the moon with a rocket that won't start,
an he's got a girl waiting for him here.....

Patrick Turner.

[Peter Wieck]

On Feb 28, 8:24 am, "Brian" wrote:
Hi, OP here,

The fuse is a 1 amp, the rec is tube "gz34" The filter caps checks
good [replaced 6 months ago]
Thanks so much for all the advice, I will try to see what current is
being drawn. I have been over the amp very closely looking for any
shorts, out of spec componants and can't find a thing! This is a vety
early silver face.

Thanks again!

Brian

Do you have the capacity to test tubes? Chinese 5AR4/GZ34s are
notorious for crapping out and causing down-line melt-downs... if that
is what you have.

Peter Wieck
Wyncote, PA

[PeterD]

On Wed, 28 Feb 2007 14:40:33 GMT, Patrick Turner
wrote:



Nah, summink simple is staring at him from that cab, and he just can't
see it...

It ain't as if he's stranded on the moon with a rocket that won't start,
an he's got a girl waiting for him here.....


Maybe like he's using the wrong transformer? bg

I long ago lost track of the simple things that took hours (and days)
to figure out.

[Phil Allison]

"Brian"
Hi, OP here,

The fuse is a 1 amp, the rec is tube "gz34" The filter caps checks
good [replaced 6 months ago]
Thanks so much for all the advice, I will try to see what current is
being drawn. I have been over the amp very closely looking for any
shorts, out of spec componants and can't find a thing! This is a vety
early silver face.



** Local model or export with multiple AC supply voltages ?



........ Phil

[Patrick Turner]

PeterD wrote:

On Wed, 28 Feb 2007 14:40:33 GMT, Patrick Turner
wrote:



Nah, summink simple is staring at him from that cab, and he just can't
see it...

It ain't as if he's stranded on the moon with a rocket that won't start,
an he's got a girl waiting for him here.....


Maybe like he's using the wrong transformer? bg

I long ago lost track of the simple things that took hours (and days)
to figure out.

Well, maybe if you lost track of so many times why you took so long,
you don't learn by experience, so next time takes too long as well.

I have a friend like this, and he's the local ACT chess champion,
a devil to play either on lightning games or games that take all night.

But if ever i see him with a spanner, a screw driver in his hand, I
remove it,
and lead him away, because it always leads to more trouble.

He's now 60 and battling to keep his chess rating high and to beat the
young
ppl only too eager to see him loose.....

Anyway, Phil could be right, maybe the guy has the amp set for the wrong
mains input voltage.

But if the guy in un the US, with 117V, then any other setting to say
240V won't hurt the amp.
But in Oz, where mains are 240V, products bought on E-bay arrive here
from the US set for 117V,
then arrive here for a repair because ppl don't reset the mains V,
or change the mains fuse for the lower input current we have,
and or the PS caps or diodes expire.
This is the minority, because usually a 117V tranny fed with 240V
will saturate badly, and the high current will blow the input mains fuse
immediately.

Patrick Turner.

[Phil Allison]

"Patrick Turner"


Anyway, Phil could be right, maybe the guy has the amp set for the wrong
mains input voltage.


** Never posted any such thing.

Do not presume you can read my thoughts.



........ Phil

[Patrick Turner]

Phil Allison wrote:

"Patrick Turner"


Anyway, Phil could be right, maybe the guy has the amp set for the wrong
mains input voltage.

** Never posted any such thing.

Do not presume you can read my thoughts.

....... Phil

OK, so you didn't post any such thing.

You did post

** Local model or export with multiple AC supply voltages ?

A leading question?

I am never preoccupied with trying to read your thoughts,
and would never assume I had such limited abilities.

Patrick Turner.

[Phil Allison]

"Patrick Turner"

OK, so you didn't post any such thing.

You did post

** Local model or export with multiple AC supply voltages ?

A leading question?


** One only the OP can answer.



........ Phil

[Patrick Turner]

Bret Ludwig wrote:

If what the others and myself suggested leads no place fast, then rather
than stuff things more yourself,
consider doing what most folks do when some damn thing breaks, take it
to a technician.

I took my old PC No2 to the technician today. Fixed lotsa things and
replaced a CDwriter
for $30, while I waited. I'd have been there for a day and stuffed it
up.....


I put my PCs together from parts so I know how they go together. It's
faster and no more expensive to buy them complete, but the time
savings is on the back end.
Car maintenance is the same. Yes a pro can do the job faster BUT when
you add in making appointments, waiting, driving there and back, and
listening to the excuses DIY is faster overall.

I have a pickup which is what is known to stock car (not NASCAR but
the smaller series of semipros and amateurs at local dirt and paved
tracks) people as a Carolina Hauler. It's a three-quarter ton longbed
with essentially a small school bus ("retard bus") driveline put
underneath. Its purpose is to dodge paying the taxes and insurance
that a medium duty truck involves while pulling Class 5,6 or even 7
loads. Mine has a Clark five speed and a full floater one ton rear
end, others have Allison slushboxes and/or two speed truck rearends.
Clutch goes out so I figure I will pay the outrageous $680.00 quote to
replace it rather than take on the project. Well, after explaining to
the service writer exactly what I have, and explaining there is a
exact parts list and manuals for the rearend and transmission with
parts breakdown in a book on the passenger seat, THREE DAYS LATER I
get a call they can't work on the truck since "someone has modified
the truck and the parts do not fit".

Yes, I TOLD you it was and the BOOK is on the SEAT did you look at
it???

No.

"Well,we don't use any kind of aftermarket part and we don't have the
service manuals for it."

ARE YOU STUPID????? I TOLD you what you need and YOU agreed that was
NO PROBLEM. There is NO reason you cannot do this job and since you
have HAD IT THREE DAYS I want you to tow it free back to my house.

"Well, we didn't know the vehicle was altered from factory design."

What part of my TELLING you that UP FRONT did you not comprehend?

"Well, we must not have understood you. We do not work on vehicles
substantially modified from factory design because of the liability.
Additionally, we do not have the tools to do this job."

Well, you AGREED you could and then DIDN"T CALL ME THREE DAYS. Don't
you think you were negligent?

Bottom line, they towed the truck to my house and I called a guy I
know who is a race car mechanic and paid him $200 cash to come to my
house, and we did the job with a rented transmission jack. But what a
pain in the ass caused by "The Pro" NOT LISTENING.

Yes, we have pros like that here too. Purdy soon you figure if they are
either pro, and on your side, or glorified amateurs, and against you.
You gotta get to know them with a small job first, before trusting them
with a big one.
Then if they don't give value you expect, its not so likely to cause a
heart attack.

PCs I will not touch, but yes, i saved heaps on cars by keeping
pros away, a couple quoted $550 for alternator replacement
and I did it for $77 plus 1/2 a day. Now I got some other problem
causing it to
stall in traffic at low speeds. The pros don't have a clue when I ask
them what may be the trouble.
The manual isn't helpful enough....

I have to figure it out myself.

Patrick Turner.

[Patrick Turner]

Bret Ludwig wrote:

On Feb 27, 8:41 pm, Patrick Turner wrote:
Brian wrote:

Hi guys,
My 1968 Fender Princeton reverb had a power tranny smoke
out. I checked the amp over and could not find anything else wrong so
I bought a new tranny and installed it. Amp worked great. About 1/2 hr
later back in the cab the new power tranny smoked!. Well, I have yet
another installed and the amp is working but I have not put it back in
the cab and after about 20 min the tranny gets way hot! and I unplug
it. I have searched everywhere for some kind of shortmo0r any other
explanation and can't find any. Really don't want to cook another
tranny, and ideas?

B

Brian, If you've replaced the tranny twice,
only to find smoke continuing,
have you checked to se what rectifiers for the B+ supply are present?
Are there silicon diodes? If so, are they OK? if one of two is shorted,
it could be causing the problem.
If the rectifier is a tube, it could have a short inside it.
Try the amp without any tubes in it.
If you measure the input current and it is still high without any tubes
in the amp
you must have a short somewhere between a winding and 0V, maybe
something pinched
or a shorted filament winding to the chassis or shorted HT winding to
chassis.

Are all the pin grippers in the old tube sockets tight? when you remove
the tubes, were the tubes
kinda loose, and did they nearly just fall out of the sockets?
If so, turn off the amp, remove the mains plug, and tighten the socket
pin grippers
with an awl and careful bending of the grippers.
With tight pins, it will now be unlikely that you have bad connections
and if the grids
of the output tubes are not connected the output tubes will conduct too
much current
and overheat the power tranny after some time, a few minutes.
But this is a later check, first you attend to :-

Measure the input current you need to have some way
to do it, either with a variac with a current meter on it,
or with a 1ohm resistor in series with the mains Neutral line into the
power tranny
primary.
Read the ac voltage across the 1 ohm, and that is also the Current input
to the amp.
With no rectifiers or heater supplies with no tubes, current should be
low at
less than 100mA ( 100mV across the 1 ohm R ).


Inductive metering works best for 50/60 Hz AC or a current measuring
xfmr. One ohm is a restrictive shunt.

Yes, but I don't have a clamp meter for current measuring.
Hell, 1 ohm is almost nothing in series with the mains.
and if too much I flows, maybe R fuses.

Patrick Turner.

[Ernst]

On Mar 2, 9:15 pm, "Bret Ludwig" wrote:

Just as an aside, I've saved a couple of hard to replace antique radio
power transformers by putting the unit in series with a light bulb
(that's right, line cord, series wired 250W lamp, outlet box, radio
plugged in).

You absolutely positively need a current limited AC supply, for this
type of thing.

In this scenario, the lamp is the current limiting device. My primary
choice is a variac, but I suspect the OP does not have one. In a
pinch, the lamp or a combination of lamps in series will protect the
equipment. One caveat, the total wattage of the equipment must be
covered by the rating(s) of the lamp(s).

[Patrick Turner]

Ernst wrote:

On Mar 2, 9:15 pm, "Bret Ludwig" wrote:

Just as an aside, I've saved a couple of hard to replace antique radio
power transformers by putting the unit in series with a light bulb
(that's right, line cord, series wired 250W lamp, outlet box, radio
plugged in).

You absolutely positively need a current limited AC supply, for this
type of thing.

In this scenario, the lamp is the current limiting device. My primary
choice is a variac, but I suspect the OP does not have one. In a
pinch, the lamp or a combination of lamps in series will protect the
equipment. One caveat, the total wattage of the equipment must be
covered by the rating(s) of the lamp(s).

Light bulbs have much higher resistance when burning bright.
A 250W bulb with a 117V supply has 2.13Arms of flow, and has 55ohms of
resistance. e
But while its unlit, the R will be quite low while ever the current flow
is low.
A radio drawing 50 watts from 117V mains has only 0.42Arms of input
current
and it gives a load on the mains = 278 ohms.
If something goes wrong with the radio, and a short occurs somewhere,
then the input current might rise to 2.1A which is plenty to turn on the
bulb
and cause almost no voltage across the mains primary input coil,
so the power dissipated in the mains tranny is very low, and it will
survive the experience.

Having a 1A slow blow fuse would also protect the transformer from a
short in the rectifier
section or output tube section.

But if only it was that simple. Trouble is that a 6V6 or some other
output tube might
suffer bias failure and sit there with Ia = 100mA instead of 35mA
and this does not cause the bulb to light up, or a mains fuse to blow so
heat in the thin primary wire
of the ancient OPT can develop shorted turns or fuse open, despite fuse
prtection, light bulbs, or a vaciac
or whatever, except having an active dc over-current detector to turn
off the amp
if the PS dc current goes above 1.5 x the idle value.

Active is the ONLY way, all the rest are BS, although since the radio
has a class A SE output tube
most often, then a fuse between cathode and 0V is a good idea, say 75mA.
But such low value fuses are not stocked by anyone easy to go to.

Its easy to have a small 5VA tranny to make a +12V supply to power an
SCR and relay which
latch on to interrupt the mains to the old original mains tranny if the
Idc goes high.

Having said that, I bet nobody takes my advice.

OK, then get used to re-winding old trannies.

I quite often use a 40W or 100W bulb in series with an amp I suspect is
faulty.
At turn on the lamp flashes with the input surge, then fades to dull red
with the heater power, then brightens as the Ia goes up.
The bulb sheds light on the condition of the amp.
With SS gear, the lamp flashes at turn on then goes right out
because most SS amps are class B and draw SFA current at idle.

But I have never used a 250W bulb, or a number
of equal power 40W bulbs.

I do have 5A circuit breakers to the power points
at my bench though; we have 240V mains here; in the US
10A breakers would be right.


Patrick Turner.

[Ernst]

On Mar 3, 8:41 pm, Patrick Turner wrote:

But if only it was that simple. Trouble is that a 6V6 or some other
output tube might
suffer bias failure and sit there with Ia = 100mA instead of 35mA
and this does not cause the bulb to light up, or a mains fuse to blow so
heat in the thin primary wire
of the ancient OPT can develop shorted turns or fuse open, despite fuse
prtection, light bulbs, or a vaciac
or whatever, except having an active dc over-current detector to turn
off the amp
if the PS dc current goes above 1.5 x the idle value.

Point is welll taken. Remember, the variac or series lamp fixture is
strictly for troubleshooting, and is of course supervised by the tech.

Its easy to have a small 5VA tranny to make a +12V supply to power an
SCR and relay which
latch on to interrupt the mains to the old original mains tranny if the
Idc goes high.

This arrangement is also interesting from a OTL protection aspect.
Perhaps a comparator referenced to monitor voltage drop across the
cathode resistors and trigger an SCR and relay if required.

-Ernst

[Patrick Turner]

Ernst wrote:

On Mar 3, 8:41 pm, Patrick Turner wrote:

But if only it was that simple. Trouble is that a 6V6 or some other
output tube might
suffer bias failure and sit there with Ia = 100mA instead of 35mA
and this does not cause the bulb to light up, or a mains fuse to blow so
heat in the thin primary wire
of the ancient OPT can develop shorted turns or fuse open, despite fuse
prtection, light bulbs, or a vaciac
or whatever, except having an active dc over-current detector to turn
off the amp
if the PS dc current goes above 1.5 x the idle value.

Point is welll taken. Remember, the variac or series lamp fixture is
strictly for troubleshooting, and is of course supervised by the tech.

Its easy to have a small 5VA tranny to make a +12V supply to power an
SCR and relay which
latch on to interrupt the mains to the old original mains tranny if the
Idc goes high.

This arrangement is also interesting from a OTL protection aspect.
Perhaps a comparator referenced to monitor voltage drop across the
cathode resistors and trigger an SCR and relay if required.

-Ernst

There are plenty of active protection schematics at my website which are
used to
stop smoke in the loungeroom.

Patrick Turner.