Hi everyone,

Well my old Kriesler has been put through a workout lately as I get through my vinyl collection.

But maybe a bit too much work for the old girl as she has an annoying new mains hum - even though it’s been recapped.

It is unaffected by volume so I assume it’s introduced via the output transformers voltage supply from the rectifier valve?

There’s no visual issue from inspecting the chassis….i swapped over the rectifier valve just incase but no change.

Any tips on where to start with this issue? I’m bit of a novice at fault finding…if the main transformer was on the way out would it possibly cause a hum? Or should I just try replace the rectifier caps & try it again?

Thanks for any tips!

Hey guys,

I got so darn annoyed with the humming I went hunting for another filter cap in my shed & found an old one off my last Kriesler….cut it out & soldered it in…..FIXED!!!

Absolute silence now….i guess we’ll never know if it was the previous (although new) cap or a bad solder on it….but it’s totally gone just from that replacement.

Happy listening!

This if it has not been done, is indicative of a set that needs a birthday.

If its still full of paper caps and old electrolytic caps, they have to go before they cause more issues. You don't just replace the filter electrolytics you replace all of them and do not touch the Micas.

I only put new electros in this one as the remainder are mustard caps - Which I understand are pretty reliable? Think there was 5 electros from memory that I replaced.

I may change the remaining later on anyway then - surprised the filter cap only lasted roughly a month! Was a 450v one too…but anyway it’s running perfect now with the recycled one (which is probably only a year old so should be fine to leave hopefully!).

Did you change the original electros?

A Kriesler new enough to be fitted with Philips Mustards (beyond reproach unless physically damaged) would have had blue Ducon electros and/or Elna electros. They will be more reliable than many new electros you might source these days.

Yes I swapped out all the electros…but they were grey still like my other (older) 11-97 which had paper caps.

But I don’t know the history of this chassis so it could have been tinkered with although it doesn’t look like it.

I have not looked at the derelict 11-97in my container, but Kriesler used to use green inspection paint on their terminations. If that was damaged that was a tell tale sign it had been monkeyed with.

With electrolytics, I have a 1938 and a home built reformer. The latter I now use to test the electrolytics, before I use them. How long they have been unused, is becoming an issue and its better to reform them, than have them fail in service, which has happened. That is not all I use the reformer for as its regulated output is stepped from 25V - 400V DC.

I stopped using 450V electrolytics years ago as they were proving unreliable and tended to fail quickly, which is another reason for the reformer, which can also step power "B" for fault finding.

The lowest I will try to use is 500V to keep the inventory down. That, often, just covers Silicon Diodes, Metal rectifiers & filament tubes like #80 / 5Y3 as they produce a surge. I prefer 600V for them. The heater rectifiers and filament rectifiers with cathode sleeves, do not produce a significant surge, and that's where I will use 500V & 450V if I have to.

As noted ELNA and some of the later caps can be good, but I will make a point of checking them first. I have had new (to me) Electrolytic caps in service, get to a voltage under their rated voltage, & then break down and start conducting at a hyperbolic rate.

That is smart work - it’s time consuming enough recapping & solving current faults that the last thing we need is to introduce new faults via faulty components.

I wish I put an order through for 600v caps before recapping this one now - the filter cap I grabbed from the shed was a 600v 50μF from just radios, still works perfectly compared to the “new” 450v electro from Jaycar that failed already.

I’ll order some 600v for the remaining electros that I used 450v for….theres not many really just 3 others in an 11-97 - excluding the 25v cathode caps on the 6gw8’s.

Would you sell & post the old 11-97 chassis you have? Always nervous I won’t be able to source spare parts someday…

At the moment, it all there so I would not like to break it up. There is not much in them that cannot be replaced, or made.

Watch the 6GW8's. A while ago I had a repair where one went short in a Thorn stereo. That destroyed the rectifier (Metal) and melted down the HV secondary. Fuse ineffective.

Cathode and back bias electrolytic caps rarely exceeded 40 WV. I tend to use 50-63V types and use the higher temp ones.

Cathode bypass caps in valve TV vertical OP stages work hard. A TV vertical OP stage is like an audio amplifier being driven to full power 100% of the time.

A common original part is 50μF 25V and it's about 40mm x 20mm, or it's part of a twist-lug can multi. Either way, as Marc would say, they have a high attrition rate.

A modern 47μF 25V is seriously tiny! A typical TV would kill one of these in 6 months.

So I replace them with 220μF 63V to get some size and some better ripple current ratings.

No worries Marcc, they will probably be worth a bit in several years when there are barely any left, especially if it’s in good condition!

I have heard a few stories of the 6gw8s…seems like there’s not much that can be done preventively other than crossing your fingers. Do you recall what brand valve it had?
I have had a few of these now though & never come across a shorted 6gw8 so can only hope it’s a rare fault.

Ian that’s fascinating about the modern cap, I knew how much smaller everything is from recapping several sets but didn’t know they were less durable! That’s a significant increase in capacitance does it affect anything by upping that much?

I upped the bypass caps on this by about 10% to add a little more bass - although the old ones were physically small anyway at only 25v rating. I’d up some coupling caps too for a bit more bass but the hum when listening to the radio is quite significant already, I fear anything to add more bass will ruin listening to the radio!

The only time you need to worry about too much C is the input to filter cap, the one on the cathode(s) of a valve rectifier.
Too large a cap there stresses the rectifier by increasing the peak current that flows on the charge cycle and shortens its life.

Apart from that, within reason, most things improve with more C. It always improves the vertical linearity on old tele's!

The tolerance of electrolytics, especially older types, is very broad. The originals were often rated at -20%, +200%.

I think the tube was a Philips. Somehow I have a Philips data sheet on them. Where possible I normally run the tubes through one of the testers. Did not do that recently & got a NOS 6U& with an unsoldered pin.

The thing with modern caps is advances. The introduction of polymers being one. They do not absorb water and the polymer quality control allows a more consistent and higher voltage rating dielectric than paper, volumetrically.

I do make a point of modifications can have ramifications. The first filter cap does have an effect on the voltage developed and those caps, as noted can tear a cathode to pieces. Another thing to note, that if the tube is being run near maximum voltage, it may have a choke input filter and no first cap.

Beware of the Plate bypass cap especially if it goes to ground. In old sets they had an attrition rate and were often rated in KV to stop the audio transients from killing them. Do not use an under specified voltage.

The rarity reminds me of a conservators opinion in respect of "Franklin Mint" investment crockery. They would be valuable after all the others were broken.

"Watch the 6GW8's. A while ago I had a repair where one went short in a Thorn stereo. That destroyed the rectifier (Metal) and melted down the HV secondary. Fuse ineffective."

It certainly can be a game of luck to avoid collateral damage for a bad output stage valve failure. I recently looked at that issue during restoration of a Playmaster PM110 stereo power unit chassis for their tape project from EA in April 1965.

The chassis has solid-state doubler rectified B+ feeding each OPT with its SE 6GW8 output stage (screen to B+). Failure mode such as internal 6GW8 or socket short-circuit or full conduction would be cathode limited by cathode bias 180Ω and anode limited by OPM1A OPT 390Ω, to about 280mA (160V on B+ and ~50V on cathode) through OPT primary (~31W) and cathode resistor (~14W), with PT secondary current rising to 1.2A (3x multiplier of 0.4A PT secondary side fuse if added). A faulty screen conduction only limited by cathode, so up to 380mA, and 1.6A from PT secondary. Screen terminal #3 is next to heater #4 and cathode #2, so some risk of socket arcing if not kept clean.

The 180Ω 1-2W cathode resistor could see a nominal class A peak current of 2x 36 = 72mA (1Wpk) with an average of 36mA (0.24W). Given a fault of 280mA, the dissipation rises to 14W, so retrofitting a modern 0.6W resistor may help that part act like a poor mans fuse. Alternatively, given class A operation, a PTC may be practical but needs a trip current level lower than ~280mA, and a hold current level higher than 40mA. An RXEF005 has a 50mA hold that may be ok if ambient temperature is not onerous; and the 100mA trip exceeds the likely peak nominal current; and the hold resistance is likely about 10Ω and could vary by a few ohm.

So adding a simple PTC into the cathode circuit, and/or replacing the cathode bias resistor with a modern mf resistor of the lowest suitable power rating, may provide some future proofing against collateral damage, and seems to be lower risk than relying on even a secondary side HT fuse (which can be more of a pain to retrofit and requires some design skill).

This follows in the light of AWA with 6X5's. where there were 100 ohm low watt resistors in the plate circuit that slowed inrush current, however they had potential for a fusistor. Some early 6X5's had a notorious habit of going short.

Same with Back Bias resistors to ground. Keeping them sailing with the wind, gave the hope that on overload, they would fry and not the transformer. I never recommend that they be heavied up with a high watt resistor.