Philips EL6401-02 popping mains fuse

[Jennings]

Finally got round to getting the iron heated up on renewing the Philips amp, and making the fixed bias adjustable.  I replaced the filter caps, moved the AC live feed to the 240VAC tap (from 220) on the PT, moved the fuse (0.5A slow blow) between the mains switch and PT (from the neutral return line), replaced the corroded anode caps and replaced the bias diode, cap and resistors with the network as shown in the schematic.  Also added 1 Ohm cathode resistors.  I haven't replaced the self-contained "GR1" rectifier block (a metal covered oblong unit), and all else is stock currently.


On first power up (valves in) the amp came on fine, albeit with a reasonable low hum, for about a minute...no funny noises, crackles, smells etc...I was just gearing up to start taking voltage readings...and then it blew the mains fuse.  Now it just blows on initial flip of the switch.  So something's not happy somewhere.  A quick review of internals look OK to the schematic (I will recheck), and there are no obvious visual signs of failure in caps, resistors etc.  Power valves test fine, and the amp does the same thing with a fresh known good set in.  Grounds seem to test fine.  I'll look deeper when I get time over the rest of the week, but that's as far as I've checked so far.


Naturally I'll trace out my connections, check values and joints etc fully.  Can anyone suggest any top candidates to check/look into?  Or perhaps suggest (or direct me to) an order to my checks after the basic trace and checking, please?  This is my first fuse popper!

[shooter]

with all tubes out does it still blow fuse?
IF NO
what are the un-loaded volts at the PS taps?
can you adjust the NEG bias?
IF YES
can you measure the same voltage on the pin-side of the PA tubes?

[pdf64]

Always use a light bulb limiter for initial power up testing.
A low wattage bulb for ‘no valves’ check, higher wattage if that checks out ok and valves get fitted.

[Jennings]

Always use a light bulb limiter for initial power up testing.
A low wattage bulb for ‘no valves’ check, higher wattage if that checks out ok and valves get fitted.

Great advice…I’ll build one 👍

[Jennings]

with all tubes out does it still blow fuse?
IF NO
what are the un-loaded volts at the PS taps?
can you adjust the NEG bias?
IF YES
can you measure the same voltage on the pin-side of the PA tubes?

A quick test confirms it blows without any tubes fitted.

[sluckey]

Divide and conquer... Disconnect PT lead to bias supply. Still blow?

[Jennings]

Divide and conquer... Disconnect PT lead to bias supply. Still blow?

Yep, still blows…at least it’s not me goofing the bias supply doing it. So maybe PT, rectifier unit or a duff 50+50uf cap maybe?

[sluckey]

Divide and conquer... Disconnect the filter caps from the bridge. Still blow?

[Jennings]

Divide and conquer... Disconnect the filter caps from the bridge. Still blow?

Fizz pop! Another blow, and I think a bit of a foul smell perhaps coming from that rectifier unit 🤔

[sluckey]

Divide and conquer... Disconnect the PT leads from the bridge. Still blow?

[shooter]

^^^ what he said

foul smell perhaps coming from that rectifier unit

if so, replace it, save your fuses lives, BUT find out where you mis-wired, otherwise history tends to repeat

[Jennings]

Ah-ha!!! I think we found our culprit…


I have two last fuses left, so figured it was worth one last shot…and this time no blow after disconnecting the rectifier.


Thanks chaps…really appreciate the help 👍 If I remove the old one I’ll have a couple of fixing holes for a modern screw down enclosed type. What should I look for spec wise? The partial manual I have doesn’t mention what the spec is of the original unit.

[sluckey]

A 1000V 1A bridge would be fine. Or consider just building a bridge with 4 1N4007 diodes.

BTW, a light bulb limiter would have saved a handful of fuses in this divide and conquer procedure. 

[Jennings]

A 1000V 1A bridge would be fine. Or consider just building a bridge with 4 1N4007 diodes.

BTW, a light bulb limiter would have saved a handful of fuses in this divide and conquer procedure. 

Thanks Steve…my next project is a light bulb limiter 😂👍 I did quite enjoy our little Russian roulette light show though!

[pdf64]

Divide and conquer... Disconnect the filter caps from the bridge. Still blow?

Maybe I'm a werrit, but I'm somewhat uneasy about this method of troubleshooting being suggested, as it intentionally and repeatedly subjects the PT to fault current.
Yes PTs are tough, but they're also the most expensive and difficult to replace thing in an amp.

[shooter]

 
hence all the fuses that saved the day

I think we found our culprit…

don't believe I've ever had a new bridge bad.  was this an old one that was un-tested?
if not, try and figure out why it failed

[pdf64]

hence all the fuses that saved the day

My perception is that stuff has a limited budget for the number of fault current incidents it can withstand before it fails.
Bear in mind that it's an old PT, who knows what it's already been exposed to, how many more it can cope with.

[shooter]

I agree as a "best practice", engineer, you should be able to find a short without power.
as a practical "return on investment" troubleshooting method, it's served me well over my career.


guesses
a .5A fuse blows with a 1A constant load, in seconds
the PT heating effect is near zero since the heat vs cooling cycle between fuse blows is probably 1:999 duty cycle.


The E9 2star (just shy of GOD:) was my last instructor, He passed out steal fuses at graduation, with the comment;
"Use sparingly" 

[Jennings]

don't believe I've ever had a new bridge bad.  was this an old one that was un-tested?
if not, try and figure out why it failed

It's the original bridge...the service manual for the amp is dated 1956, so the amp's definitely been a round a while.  Plus the stains I found when I first opened it to clean indicated that someone spilt their coffee through the top grill...which is where the bridge rec is mounted, rather than inside the chassis itself.  Probably got a soaking with Columbia's finest!

[pdf64]

That’s a horrid selenium rectifier package. If it emitted any fume, I hope you got them vented out.
A blanket replacement policy should apply to them.
Check that rectified voltages aren’t too high when a silicon replacement is fitted.

I agree that as a field tech where time is money, quick and dirty may be the only feasible way forward.

But we can take things at a gentler pace, a more considered approach, with this stuff.

[sluckey]

But we can take things at a gentler pace, a more considered approach, with this stuff.

True, but it often involves 10X as much typing.

[Jennings]

That’s a horrid selenium rectifier package. If it emitted any fume, I hope you got them vented out.
A blanket replacement policy should apply to them.
Check that rectified voltages aren’t too high when a silicon replacement is fitted.

It did whiff a bit in the room for a while...nothing too bad, as the fuses were preventing anything more than a fraction of a second of power.  Now I know what these things look like I'll keep an eye out and swap them in future.  Thanks for the tip too on the silicon replacement...I'll measure and adjust if necessary with a series resistor.

[Jennings]

So the crusty old selenium rectifier is gone, and a new TS6P07G is installed in its place.  Thanks for the help on that one...the result is that the amp powers up fine, idles well and no obvious immediate big issues.  Noise floor is great, and it is stable enough to both take voltage readings and play.


Onto voltages...using the manual (excerpt attached) as a guide, with measured mains at 234VAC at the time of checking, HT voltages look OK, but at the high end of spec. I get the following:


Point    Measure       Spec
  1      = 292 V     (275-295)
  2      = 254 V     (230-250)
  3      = 194 V     (170-190)
  4      = 48.5V     (47.5-48.5)


All filaments are on spec.  ECC40 plates measure in the ranges shown (Va = 38.1, Va' = 64.5). 


I used an ECC83 in the ECC81 PI position...the plate voltages measure higher than manual spec at Va = 154.5 and Va' at 218 (ECC81 manual spec is 140-180 and 95-123 respectively.  Resistance checks on the plate resistors (R11 & 12) shows them to measure 379k & 39.9K respectively.


EL81 plate and screen test OK, but top end of spec: 294VDC both plates (manual spec 270-290V), 288 both screens (manual spec 267-287).  Philips implemented the 1K R18 screen resistor by paralleling two resistors together, presumably to achieve the wattage handling required in that position.  They paralleled a 2K2 and a 1K8, giving a calculated resistance of 990r...I measured this to be 958r.  I'm calculating the idle current to be 19.8mA, so running at around the 70% top end of dissipation according to the Mullard datasheet (says max plate diss' is 8W).


So, how does it sound?  It sounds lovely with a guitar going into the mic input (via a suitable 1/4" jack to XLR three pin adapter).  Much better than I thought it would actually.  Equally nice via the pickup input with a 14" jack to twin banana adapter, just quieter as the mic input has that extra triode.  No real hum or hiss to speak of.  Plenty of tone adjustment via the single high/low control, loads of volume and a cool breakup as you crank it.  Really pleased with that as I was expecting maybe to need to tweak it more for guitar.


Only one thing stands out now to maybe try and track down...to my ears there's a very slight/faint distortion or fuzz, most noticeable at the end of a note.  Not really terribly obtrusive, but definitely there.  Does anyone have any ideas as to what this kind of distortion/oscillation might be, and what to target to tackle it?