fender 75 amplifier fuse blowing problem

[chris]

Hello,

I would appreciate any help on a problem I'm having concerning my fender 75
export amp, e.g with primary voltage selector. I'm living in Germany so its set to 240 volts.

I recently pulled one out one of the 6L6 output tubes to take a closer look at it.
After plugging it back in, i turned the amp on in standby on low power. It started to
squeal so i switched it to high power and then it blew the 3.15 amp fuse. I then unplugged
the amp and pulled out the 6L6 that i had removed previously. On closer inspection,
I saw that the socket groove and the key on the tube were so worn that it allowed me
to incorrectly plug it in. That explained the squeal.

So i bought a matched pair of 6L6's, new ceramic sockets for the power
tubes and 4 new grid resistors. I also got some new 3.15 amp @ 250 volt slow blow fuses.
However, the fuses look a bit different from the one i took out, e.g. they appear to be
filled with sand. The German manufacturer is ESKA. All other physical characteristics, like
size and length are identical to the original ones.

After installing the new parts, I started to take some in circuit measurements without the tubes
plugged in.

6L6 Anode Voltage on pin 3:
   on low power, 247 volts,
   on high, 490 volts

6L6 Grid voltage on pin 4:
   on low power, 244 volts,
   on high, 493 volts

6L6 Grid bias voltage on pin 5:
   on low power, -22 volts,
   on high -53 volts

The other 6L6 had similar values.

Heater voltage:
   6,3 volts

I made these measurements several times in between of turning the amp on
and off. Around the tenth time, i wasn't able to measure any voltages any more, so
i checked the fuse and saw that it had blown. So i put another one in and again
after some turn on/off sequences, the fuse blew.

So, i measured the amps in the primary circuit of the power transformer.

Usually it pulls anywhere from:
0.1 to 0.24 amps "initially", e.g. soon after turning the power switch on.
Soon afterwards, it goes to to 0 amps.

However if i continue turning it on and off, say around the 30th time, i sometimes get some extraneous values, e.g.
values > 3.15 amps like 3.26 amps.
Here to, it quickly goes back down to 0 amps.

To help isolate the issue, i removed all secondary wires of the power transformer
from the circuit and repeated the above sequence of turning the amp on and off while
measuring the amperage.

Here too, I was able see 3 values > 3.15 amps out of sequence of about 40 iterations.

After all that i measured the resistances and winding voltages of the power transformer.

NOTE: Export Version Transformer with 110-260 volt inputs, fender part nr.: 013917

Primary Windings: (all measurements against the black or common wire)
-------------------------
Black Red (260v) 2.7 Ohms
Black Green (240v) 2.4 Ohms  --> my current voltage setting
Black Yellow (220v) 2.0 Ohms
Black White (127v) 0.8 Ohms
Black Orange (120v) 0.7 Ohms
White (100v) 0.6 Ohms
 
Secondary Windings: (all measurements against same color wire accept where noted)
-----------------------------
Bias Supply:
Brown1 x Brown2
   6.8 Ohms, 54 volts

High Voltage:
Red1 x Red2
   8.6 Ohms, 365 volts

Center Tap:
Red x Red-Yel
   4.3 Ohms, 183.5 volts

Heater:
Green x Yel-Green
   0.2 Ohms, 6.4 volts


Do the above values seem normal?
Is it normal to have such current spikes?


Thanx,
Chris

[PRR]

The iron core in a transformer holds a magnetic charge.

If you turn it off, it may be charged one way depending when in the cycle you turn off.

When you turn it on, it must be charged depending when in the cycle you turn on.

Worst-case, if you turn-off on one peak and turn-on at the opposite peak, the charging current could be twice the peak-to-peak voltage divided by resistance. 280 Amps!! This will decay-down in much less than a half cycle. The average meter will not read the peak value, but may read some lesser value depending on meter speed constants. Normal fuses should not blow on this transient.

You will rarely hit the worst-case turn-off/turn-on timing combination. It is not unlikely the odds are near 1 in 30 for getting near worst-case.

Slo-blo fuses traditionally used a lump of lead to slow-down melting. Lead is now banned. Packing a fast-blow's element in sand would slow blowing, also absorb arcing, and may be the new Slo-Blo.

All your voltages and resistances look plausible.

Your "0.1 to 0.24 amps "initially"....Soon afterwards, it goes to to 0 amps" makes sense if it has solid-state rectifier. The B+ caps charge-up, then (with no heaters or tubes) power consumption drops to negligible leakage.

The full-roar power consumption of a 75W tube amp is 150W-200W. At 240V this is 0.7 Amps. The 3 Amp fuse is ample, maybe bigger than it needs to be.

So I do not see what is wrong.

If the only complaint is occasional fuse-blow, re-check the fuses. If you got sold some 0.3A fuses instead of 3A, it would start 9 times out of 10, maybe 29 times in 30, and even play fairly loud, though not FULL-ROAR.

[chris]

Thanx for your thoughts PRR.

Remember the current measurements i'm taking on the power
transformer are ones with the secondary wires hanging in the air,
so to speak.

So no rectification or cap charging, etc. is happening.

Actually in this state out of a seq. of 10 turn on's / turn off's, I get
at least 2 readings over 1 amp. In fact one of those was 2.75 amps.

Seems a little strange to me that a transformer would pull so many
amps with all its secondaries removed? Also if there is a "minor" short
somewhere in the windings, why do all the voltages and resistances look
somewhat normal?

I have also rechecked the fuses and they are rated at 3.15 amps @ 250 volts.

I assume if i reconnect all of the secondaries and put in the tubes and remeasure,
that the current draw will climb even higher. Prob. the frequency of high readings will also
increase....

[chris]

I made some more measurements. What i did now, is to observe
the secondary peak voltages shortly after turning the amp on and off.

Here's what i got (NOTE: all secondaries are disconnected from the circuit)

Bias Supply:
Brown1 x Brown2
   initially or briefly 80 volts, thereafter settles to, 54 volts, approx. 48% increase!?

High Voltage:
Red1 x Red2
   initially or briefly 370 volts, thereafter settles to, 365 volts, approx. 1.4% increase

Center Tap:
Red x Red-Yel
   initially or briefly 195 volts, thereafter settles to, 183 volts, approx. 6.5% increase

Heater:
Green x Yel-Green
   initially or briefly 8.6 volts, thereafter settles to, 6.4 volts, approx. 30% increase!?


The largest delta is in the bias supply and the heater windings but would this indicate a small short?

In the schematic, these windings appear at either ends of the iron core, e.g they seem to be furthest away
from one another.

Doesn't seem logical, e.g. i would expect a voltage drop if there were shorts in the windings.

Maybe this is all normal transformer behavior, dunno...

[chris]

Ok today i connected the primaries directly to the "wall-socket" with another power cord.

So i bypassed all internal wires, like to the fuse holder, the power switch etc.

I also solder in a 3,15 amp fuse on the new power cord, which was also properly grounded to the chassis.

First time plugging the cord into the socket:
Everything seemed ok. Could here the trany humming.

Second time plugging the cord into the socket:
Fuse blew right away!

Then i decided to meas. the current with my meter.
Hooked it up but nothing happened.

So i checked the 15 amp fuse in the meter and sure enough it was blown too.
However, I'm pretty sure it blew yesterday during one of my other experiments.


I was also able to get some info out of the fuse company here in Germany.

They said: ( for my T 3,15@250, T stands for Träge or Sluggish)
-@ 2x the indicated amps @ 250 volts, the fuse can last a maximum time of 30 minutes.
-@ 2.75x the indicated amps @ 250 volts, the fuse can last a maximum time of 5 seconds.
-@ 4x the indicated amps @ 250 volts, the fuse can last a maximum time of 2 seconds.
-@ 10x the indicated amps @ 250 volts, the fuse can last a maximum time of 0.3 seconds.

It looks like my trany does have a short somewhere. Probably the current readings i made were
also wrong, since the fuse pretty much blows within a 1-2 seconds if the not on the first power up, than
on the second or third.


Any recommendations where i might find an appropriate power transformer.
TAD has one, but its a bit pricey over here at 159 € and it doesn't even support all the input
voltages for the export model, not that i need them all. I also found some guy here
who has some that might be correct, they're about half that and he also makes
custom ones.

[Willabe]

Do you have a lamp limiter for testing/checking for shorts? This is a must have/must use tool for amp builders/repairman.

Here's a link to sluckeys web site that has instructions on how to build and use one.

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


                   Brad       

[chris]

Thanx a lot for the link Brad!
Some other useful stuff there too, I see!

I will try it out, even if its pretty obvious now that my trany is the culprit.

[Willabe]

No thanks to sluckey and yeah good stuff at sluckys.