Dieselpunk Preamp of Doom and the Beast with Two Backs

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Thermionic Idler
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Mon Nov 04, 2019 11:56 pm

Well poo.

So I got the other amplifier done, and doubled checked everything before plugging in and checking the bias.

Initially all seemed ok, I was able to stabilise the DC with not too much trouble and no fuses blew. I was a bit concerned when I switched to AC on the voltmeter, which seemed to suggest I might have about 0.1V of hum going on.

I then discovered the test resistor (10W) was bloody hot, which seemed to indicate something a bit more serious. The oscilloscope told me I had probably a good 20 volts or so of high frequency oscillation. Good job I didn’t connect the speaker.

So now begins the debug. I’ll probably post on the Transcendent forums as well but any helpful suggestions are, as always, gratefully received. I realise not being permitted to publish the circuit might hinder that a bit.

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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by izzy wizzy » Tue Nov 05, 2019 7:47 am

You can do valve swaps between good and bad amp.

Being an OTL, can you reduce the op valves down to a pair to ease the fault finding?

Cheers,
Stephen

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Mike H
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Mike H » Tue Nov 05, 2019 11:53 am

^ Like.

Are there grid stopper resistors? I've had this problem too with power valves in parallel (300B). Had no grid stoppers so got a nice (sarcasm) RF power oscillator. First clues were, very strange cathode Voltage readings.
 
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Thermionic Idler
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Tue Nov 05, 2019 7:40 pm

izzy wizzy wrote:
Tue Nov 05, 2019 7:47 am
You can do valve swaps between good and bad amp.

Being an OTL, can you reduce the op valves down to a pair to ease the fault finding?

Cheers,
Stephen
Hi Stephen - if a bad valve could cause something like this, that might be an option. Yes, the pairs can be reduced down to 4 in total per channel, that's the minimum number to still obtain balance (the same number as in my Son of Beast amp).
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Thermionic Idler
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Tue Nov 05, 2019 7:43 pm

Mike H wrote:
Tue Nov 05, 2019 11:53 am
^ Like.

Are there grid stopper resistors? I've had this problem too with power valves in parallel (300B). Had no grid stoppers so got a nice (sarcasm) RF power oscillator. First clues were, very strange cathode Voltage readings.
Hi Mike - There are, only on the output stage valves. In fact I specifically used Arcol carbon composite resistors in these positions and made sure the resistor body was close to the valve socket pin per what I understood to be best practice. Triode strapping resistors are 5w Kiwame's.
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Thermionic Idler
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Tue Nov 05, 2019 7:52 pm

I'm probably going to properly come back to this on Thursday, I need a couple of nights off. I've been working very intensively for the last several weeks either doing my day job or trying to get these things completed. More later, stay tuned.
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Ray P » Tue Nov 05, 2019 9:16 pm

Take your time Dave, no point in hurrying after all the time you've already invested. Good luck.

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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Mike H » Wed Nov 06, 2019 7:16 pm

Often helps. Often after a lot of wiring something up, I won't try turning it on just yet but wait 'til next time, then go over it all again. Fresh eyes can spot silly mistakes which previouly had stubbornly refused to be noticed. :D
 
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Wed Nov 06, 2019 8:20 pm

Definitely very sensible Mike.

With this though, a lot of the circuit is on the PCB, it's only the output stage that's hard wired, and that is quie a simple config repeated 6 times. There are only 6 wires that go between the PCB and output stage - 2 feedback lines from the speaker positive, +drive (to positive bank grids), -drive (to negative bank grids) and positive / negative 170V bias. I've just checked the high-rez pic I took of the innards before bolting the cover on and that all looks correct. At the other end it's just wiring to the transformer secondaries, ground buss and bias pot.

My current "armchair" suspicion is something amiss in the global feedback loop, I can't think of any other way the circuit could self-oscillate like this.
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Cressy Snr » Wed Nov 06, 2019 9:20 pm

I would check the value of the feedback resistor on the faulty monoblock. Comparing it with the feedback resistor on the working amp, so that both are actually *visually* compared. That way, if one is different to the other, it’ll stand out.
With an OTL like this one it’d be impossible to get the feedback the wrong polarity, I would have thought. The only other way it could be wrong, would be if it was connected to the wrong place at the input stage end of the wire.

Also what about the positive feedback from the upper bank that matches the gains of the upper and lower tubes? that might be worth a look.

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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Thu Nov 07, 2019 2:45 pm

Thanks Steve, yes I was looking at the positive feedback loop as well, I guess if that was feeding back more than it should then we'd get the same issue. The oscilloscope should be able to tell me how far back the oscillation goes in the circuit.

As you say, the first thing will be an inspection of the components and measuring them with the power off. The good thing is that I have two identical builds (notwithstanding minor layout differences to allow the mirror-imaging), one of which works and one which doesn't, so I have a reference that I can work from. Radio silence from the Transcendent forum so far.

Let's hope I can get it cleared up this weekend...
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Thermionic Idler
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Thu Nov 07, 2019 3:15 pm

A thought has just occurred to me (as I finish my late lunch break).

The bias was measured with nothing connected to the input for both amplifiers. Now the input goes via a Jensen line input transformer so that I don't get any nasty ground loop issues - this one : https://www.jensen-transformers.com/wp- ... 1p4-11.pdf and I've connected it up to the ECC83 first stage per the recommended circuit in that PDF.

Because of the way I had to 'turn everything around' for the mirror imaging, on the faulty amp there is a foot of shielded twisted interconnect running from the input XLR socket at the back, to the transformer at the front. So it's like a very small capacitor was added across the primary.

I wondered if that would be enough to set things off?
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Nick » Thu Nov 07, 2019 3:46 pm

I would have thought not. It's likely the higher gm valves ecc83 don't tend to sing. Did you measure the frequency of the oscillation?
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Thermionic Idler
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Thu Nov 07, 2019 5:27 pm

Nick wrote:
Thu Nov 07, 2019 3:46 pm
I would have thought not. It's likely the higher gm valves ecc83 don't tend to sing. Did you measure the frequency of the oscillation?
Not yet, I need to get it back on the bench and do some proper measurements. This was just a very quick diagnosis to find out why my load resistor had got so hot, unfortunately due to my need for new glasses, I couldn't make out in the low light of the living room what the scope scan frequency was set to! :oops: I am pretty sure it was well up into five figures though.
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Thermionic Idler
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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Post by Thermionic Idler » Sat Nov 09, 2019 4:56 pm

OK here are some more data points.

All the resistors in the global feedback network seem to check out okay. I saw nothing that looked visibly burned out when examining inside.

All the voltages that I was able to measure (i.e. where the circuit wasn't bouncing up and down in oscillation) were within spec, so I think all the power supplies are functioning as they should.

Now, when I put the voltmeter probe on V1A pin 6 (the anode of the first stage) the oscillation stopped dead, and I got a 105V measurement against a spec of 100V. Could that have any significance?

I've posted a screenshot below of the input and phase splitter stage. The feedback to the first ECC83 stage also has a 3K (well, 2 x 1K5) resistor in parallel with a 820pF capacitor, between R2/R3/C11 and the speaker.

I calculated the oscillation to be 116.2kHz, with the output amplitude at 20V peak-to-peak.

At first I thought R39A might have failed as I couldn't measure anywhere near 1Megohm with power off, but I then realised that the meter was being upset by the residual charge across C10.

The oscillation happens regardless of the position of the ground lift switch.

Image
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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