A Bodger's Tale

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SteveTheShadow
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Re: A Bodger's Tale

Unread post by SteveTheShadow »

If your amp sounds good to you, don't spend all your waking hours wondering if it could sound even better, just enjoy the music. All too often I have fallen prey to that bug and believe me, it is debilitating. Yes, tweaking can be rewarding, but if your amp is constantly in bits, you're not going to get much out of this hobby.

Now then, valve amps aren't rocket science; actually they are more complicated than that. OK, so their circuits might be a lot simpler than a computer, but they work towards outputting a signal that affects both the senses and the emotions. The sound of a valve amp, or any amp for that matter contains a complex blend of objective and subjective elements that's very difficult to unravel, even if you have an electrical engineering degree and a lab full of test equipment.

Sometimes, you nail it as I did a few years ago with a pair of monoblock SET amplifiers based on a pair of new old stock, Sylvania 6B4G direct heated triodes. The 2A3 valve runs on 2.5V filaments. The 6B4G is the same valve but has 6V filaments and it is far cheaper to by than new old stock 2A3 valves.

At the time I built this pair of monoblocks, the Audio Talk crew were well into driving direct heated power triodes with the high transconductance pentodes that were once common as video amplifiers in old TV sets. These valves such as the 12GN7, EF184, C3g etc are (apart from the German C3g) cheap and plentiful as new old stock, can run high currents up to 30mA without breaking sweat and with a low value anode resistor, they put out a decaying spectrum of even order distortion harmonics, rather than the odd order harmonics that result when pentodes are used conventionally in ultra high gain mode. They make excellent two stage DHT amplifiers a realistic proposition.

Pentodes have five electrodes. The two extra electrodes are called the screen grid and the suppressor grid. The screen grid does what it says, and "screens" the anode from the grid, effectively preventing the grid from "seeing" the anode. This has an important effect on the Miller capacitance of the valve itself. If the grid can't see the anode electrically, then it can't see any capacitance either. This has the effect of greatly increasing the bandwidth of the device, up into the MHz region; waaay out of the audio band, so slewing distortion at audio frequencies is banished, which should result in far better top end response from the amp, provided the output transformer is capable of showing the difference.

The other electrode is the suppressor grid, which suppresses secondrary electron emission from the anode, basically by throwing any stray electrons emitted by the hot anode (which is only supposed to collect electrons) back to where they came from. It does this by interposing itself between the screen grid and the anode and being at an earthy potential with respect to the said anode. Normally this earthy potential is achieved by internally connecting the supressor to the valve's cathode, although, in some pentodes such as the EL34 the suppressor is brought out to its own terminal. There is a danger of it attracting electrons on their way perfectly legitimately to the anode, so to avoid this, the suppressor grid has as coarse a pitch as practically possible and it is arranged so that its wires are aligned exactly with those of the screen grid. In this way, the supressor doesn't 'get in the way.'

In order for a pentode to work at its best, the screen grid needs to be at a fixed potential relative to the anode, as its voltage has a huge effect on the operation of valve in comparison to that of the anode. To this end it can be locked by a zener diode, or much more prettily be held by a gas discharge tube. These lovely devices, are filled with an inert gas and glow orange or purple according to what gas they contain. They are usually available in 75V, 108V and 150V versions and can be strung together in series to drop higher voltages eg a regulated 300V can be obtained by stringing two 150V tubes together.

So I built this 6B4G amp, with a 12GN7 pentode driver, with 108V on the screen, regulated by a 108V glow discharge tube per channel. Also I did something to each power supply that I had never done before....and the improvement in the music quality was not subtle.........
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SteveTheShadow
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Re: A Bodger's Tale

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Now this pair of monoblocks was not exactly pretty. They were built onto a pair of aluminium guitar amp chassis, and were, in the true spirit of the guitar amp, filled with carbon composition grid stopper resistors, Sprague Atom and JJ can, power supply caps and Sprague 716P "orange drop" coupling caps. Not an audiophile cap or resistor anywhere in the circuit.

A first for me at this point: the power supply was series regulated, using a 6080 parallel heavy duty, twin power triode as the pass element. The error amplifier that kept the voltage at the output of the regulator constant, was a 6AU6, 7 pin miniature pentode, with its screen regulated by a single 108V glow discharge tube. The output voltage was adjustable via a miniature trimmer pot, meaning both amps could be set up with identical, HT voltages.

So we had 6 valves on each chassis including the two glow tubes: 3 for the power supply regulation and 3 in the amplifier circuit.
It was the first time I had considered the power supply and it's influence on the performance of an amplifier. The result of this was a pair of amps that, when played through my 5ft high Metronome speakers sounded utterly captivating. I showed them at that year's Owston meet and they went down very well.

I was captivated by these amplifiers and now appreciated the importance of power supplies. Fantastic!
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SteveTheShadow
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Re: A Bodger's Tale

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So after learning about power supplies, I went beserk for the best part of five years and disappeared up my own arse with ever more complex circuits involving balanced push-pull pentodes festooned with regulators and all kinds of crazy feedback schemes. They were powerful amplifiers on the right speakers, but on anything resembling a commercial speaker, they were wheezing geriatrics. This was a huge wake-up call as far as I was concerned. I had a big pair of 50WPC EL34 balanced monoblocks that cost me over a grand to build and on any pair of speakers other than mine, they were utter shite. It was humiliating and dissapointing.
The previous stereo pentode amp I'd built was wonderful, so much so that a mate off Audio Talk bought it off me and still uses it today. But as always, you think you can do better. The reality was that the "improved" versions were anything but better. They are an episode I'd rather forget, but I suppose I learned something; you always learn something.

Then I tried doing semi-omni speakers and that was a great project as far as I was concerned. I adapted my Metronomes into a design resembling the Larson Pyramid omni speaker of the late 60s/early 70s and it sounded superb. I could see why the Doc raved about semi-omnis. I made a flatter backed version to get the up firing drivers nearer the wall. They looked a bit strange but sounded excellent.

Then I ended up in a massive forum fall out as the dormant Generalised Anxiety Disorder re-activated partly by my retirement, came back with a vengeance and I became completely paranoid. Exact same pattern: frightened of the postie, frightened of the phone/computer, scared of my own shadow again, and couldn't cope with even the slightest criticism.

Inevitably all hell broke loose, just as it had in 2005, and I said some things and upset a lot of people, both forum wise and at home. Again I was brought out of this unfolding horror by Mrs Shadow, who thrust "The Power of Now" by Eckart Tolle into my hands, told me to damned well read it, and also packed me off to the Doctor, who referred me to talking therapy, where I was introduced to MBCT: Mindfulness Based Cognitive Therapy.

I found, in Waterstone's on a bus trip to Scarborough, "Mindfulness - Finding Peace in a Frantic World" That book and it's accompanying CD of guided meditations plus the GP referred MBCT course was the catalyst that changed everything.

For the first time in my adult life, I'm at peace. It has taken twelve months of dedicated and sometimes difficult and frustrating practice, but I kept at it and I'm now a reasonably competent meditator, live in the present moment most of the time and have managed to embrace "less is more" both in terms of my hi-fi, which with the Fane, sealed full range single driver speaker and the just finished, single-ended, single valve, output transformer less amp, has never sounded better, and in the more general aspects of life itself.

Niiice :angelic-cyan:

Long live DIY Hi-Fi.
Somebody’s telling me the latest scandals.
Somebody’s stepping on my plastic sandals. Joe Jackson (1979)

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SteveTheShadow
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Re: A Bodger's Tale

Unread post by SteveTheShadow »

And so endeth the bodger's tale.
Keep things simple.
The best amplifier tone comes from simple signal circuits and good power supplies.
The best speaker tone comes from simple enclosures and simple driver arrangements.
Complexity is your enemy.

Take care Grasshopper, because the ego's nature abhors simplicity as there's nothing to brag about when you keep things simple and minimal. ;)

Oh and enjoy the pastime and stay sane.
Somebody’s telling me the latest scandals.
Somebody’s stepping on my plastic sandals. Joe Jackson (1979)

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karatestu
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Re: A Bodger's Tale

Unread post by karatestu »

A great read. Thanks Steve and so glad you are in a better place now.

Yes, simplicity is the key to everything. :guiness;
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guydarryl
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Re: A Bodger's Tale

Unread post by guydarryl »

As Karatestu said above; thanks and well done :guiness;
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Re: A Bodger's Tale

Unread post by CN211276 »

guydarryl wrote: Sun Mar 31, 2019 8:48 am As Karatestu said above; thanks and well done :guiness;
+1
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Re: A Bodger's Tale

Unread post by terrybooth »

Yes, thanks for sharing.
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Re: A Bodger's Tale

Unread post by Dr Bunsen Honeydew »

Some bits seem to missing from the story.

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SteveTheShadow
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Re: A Bodger's Tale

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Dr Bunsen Honeydew wrote: Sun Mar 31, 2019 10:46 am Some bits seem to missing from the story.
Oh yes! And then there was the time the Doc sent me a pair of NVA amp boards at cost, and I managed to build a clone of, near as dammit, an A60; but not until I'd blown up both, yes both, boards with my hairy arsed valve amp bodger gigantic soldering iron antics! I got a right bollocking for that but he did send two replacements! :lol:

After buying a more appropriate tip for the iron and simplifying the over complex, twin carb power supply, I succeeded in building one of the best transistor amps I've ever heard. So much for the myth that tubes are better than transistors in all cases. No they're not! And so much for the myth that transistor amps have grainy treble in comparison to tube amps.....er not this transistor amp.

The NVA clone is something I'd never part with and I'm sure Stu feels the same about his breadboard NVA efforts. He's a much better transistor bodger than I am and he'd have a good story about solid state bodging and how to build isobaric cubes with nothing more than a chainsaw, a few six inch nails and a big hammer. ;)
Last edited by SteveTheShadow on Sun Mar 31, 2019 11:30 am, edited 2 times in total.
Somebody’s telling me the latest scandals.
Somebody’s stepping on my plastic sandals. Joe Jackson (1979)

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