Diyhifisupply Billies - dead OPTs?

[analogueman]

I have a pair of Diyhifisupply Billie mono blocks. I suspect they need new OPTs - as they make no sound whatsoever - but I'd very much appreciate opinions on the matter.

The Billies are somewhat modified (MU-driver stage, ultrapath connection, better components and dc-regulated 300B filament supply) and have been playing in that configuration for several years now. The amps power on just fine and all voltages are OK ... but no sound.

I've measured the resistance from the B+ rail to the 300B plate (61.8 Kohm) and to chassis ground (178.4 Kohm) as well as from the OPTs primary lead to chassis ground (116.6 Kohm). Both OPTs show the same values. As far as I know a reading much over 1 Kohm between B+ and the plate of the output tube is a pretty sure sign that the transformer winding is burned open. The other readings support my suspicion as they - as far as I know - should be several hundreds of Kohm and preferably Mohm for a sound OPT.

So, do I need new OPTs?

Any help or comments are much appreciated. Thank you for your time and effort!

[Bas Horneman]

I can't believe the 61.8 K Ohm. Are you sure it is not 61 Ohm?

[azazello]

You can disconnect every primary coil of output transformers and measure their resistance /about 60-100 ohms/, also and of secondary coils /about 0.2 - 0.3 ohms/.
Is there cathode resistors and capacitors for 300B?

[Frank Berry]

If the plate voltage is normal on the 300B tubes, the primary of the transformer is probably good (or shorted).
If both amplifiers are doing the same thing, I would suspect something other than the output transformers.

[kevinkr]

Some digital meters have trouble measuring the dcr of high inductance transformer primaries, and give wildly inaccurate readings.

Do you have a scope? Ac measurement with output load resistors connected and a sine generator on the input would be revealing.

Normal dc plate voltage readings indicate that the primary is either OK or shorted end to end.

One of my friends recently replaced the output jacks on his amplifier and inadvertently shorted the secondaries of both channels directly to chassis which provided exactly the results you describe.

Note the transformer readings to ground are not indicative of any sort of fault and could be leakage in the PSU capacitors or even bleeder resistors in the supply if fitted.

Changed anything lately? Even tubes?

[analogueman]

I thank you all for your replies.

The 61.8 Kohm reading should be OK but I'll redo the mesurement asap. I don't have access to a scope, so I can't perform a measurement with output load resistors and a sine generator on the input. Maybe I should bring my amps to a specialist/qualified technician ...

I'll disconnect the primary coil of the OPTs and do some mesurements as suggested by azazello.

In short (pun intended!) I'm uncertain what has happened to my amps. They where on loan to a friend, and when I got the amps back tubes were reinserted into their sockets. When the amps were powered on in my system they were buzzing and humming and I suspect one of the rectifiers shortened - I have a resistor in series after the tube rectifier and before the Pi-filter and it burned open. However, nothing of that kind happened to the other amp - it's just silent.

I'm surprised that both amplifiers have failed equally and so catastrophically. However, if the OPTs are dead I'm of course a bit hesistant replacing them before I have tracked down what could have caused their failure in the first place.

[kmaier]

Seems like the amplifiers are improperly fused if a resistor burned, any chance they've been replaced with the wrong ones? Chances are the amp which has the burned resistor may be open.... and the tubes from that channel also toast. Perhaps you have switched the tubes when re-inserting?

In any case, your primary resistance is far too high to be correct... or both OPTs are open (the result of putting the bad tube which burned the resistor on one amp into the other?). I would suspect the high resistance to be from other components in the plate circuitry. If there's no known components in the 300B plate circuit you can simply unplug the tube and measure the OPT primary resistance from the tube socket to where the primary gets B+. Get a known resistor value and measure it to ensure you know what your (meter) accuracy is.

Regards, KM

[lt cdr data]

do you want tol sell them, I will buy if you do ?? thanks

[analogueman]

Hi lt cdr data,

Thank you for your buying offer.

I will take the amps to a qualified technician tomorrow. I'll await his verdict beore I decide to keep or sell the amps.

BTW, the amps contain a lot of nice parts: Riken Ohm, Kiwame and Holco resistors, and Mundorf Supreme and silver/oil capacitors. If I decide to sell that will be reflected in my selling price.

Shipping costs will be quite steep as the amps weigh more than 15 kg a piece. I have the original double shipping boxes, so safe shipping is no problem.

[kstagger]

it's rare for OTs to burn out - at least I've never been able to (accidentally) do it even with 50+ year old gear.

For _both_ OTs to go out on a pair of monoblocks would be even harder to do.