A Bodger's Tale

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setting son
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Re: A Bodger's Tale

Unread post by setting son »

I built a pair of the Bottlehead 2A3 monos some years ago, very nice they were too.

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SteveTheShadow
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Re: A Bodger's Tale

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I modelled the look of the mono amps in the pic, on the Bottlehead and Welborne labs amplifiers
Somebody’s telling me the latest scandals.
Somebody’s stepping on my plastic sandals. Joe Jackson (1979)

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Dr Bunsen Honeydew
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Re: A Bodger's Tale

Unread post by Dr Bunsen Honeydew »

More story progress please.

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SteveTheShadow
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Re: A Bodger's Tale

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After having built the Metronome loudspeakers and the 2A3 monoblocks, I started messing with EL34 single ended amplifiers, designing a parallel 6SL7 driven EL34 circuit I named "Rocky" because it kicked arse big style on my soul, rock and R&B music. This was a great sounding amplifier, easy to live with and it went LOUD through my Metronomes with twin subs.

At the time I built the amplifier, World Audio Design had become World Designs, and was run by British designer Peter Comeau. The forum changed hands, but otherwise it was business as usual. The DIY meets continued at Eggborough and I took along "Rocky" and my Metronomes and subwoofers.

At that time the organisers of the Eggborough meets which became known as "Eggfests" used to present a trophy. The trophy usually consisted of a dud transmitting valve mounted on a base with a gold coloured engraved plate. The June 2006 trophy was an 845 tube and to my astonishment I was declared the winner.
The award was for best newcomer to the DIY scene and I was chuffed to bits. I'd gone from a suicidal wreck the previous year to a fairly competent DIY hi-fi builder, made new friends and had gone back to work as a supply teacher. It was a massive salary drop, but I was free of politics and was my own boss.

More amps came and went over the next few years, all developing and adding to my knowledge and skill, until one day I got just a teensy weensy bit too cocky......
Somebody’s telling me the latest scandals.
Somebody’s stepping on my plastic sandals. Joe Jackson (1979)

mikeyb48
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Re: A Bodger's Tale

Unread post by mikeyb48 »

Confuscius say "man with hand in pocket feel cocky all day, but man with both hands in pockets no feel two cocky"

:mrgreen:

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SteveTheShadow
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Re: A Bodger's Tale

Unread post by SteveTheShadow »

Indeed.

By around 2009 I had developed an unhealthy fascination with output transformerless valve amplifiers and was devouring literature on them. One book by a famous USA OTL guru piqued my interest and among the projects in the back of the book such as "Single-Ended With Slam" "The Grounded Grid Preamp" "Super Compact 150WPC Amplifier" all of which I would have been far better off building, there lurked a "Low Cost 15WPC Output Transformerless amplifier"

Now having recently built three output transformerless tube amplifiers perfectly sucessfully: two for one of our Ant's clients, one for myself, and having heard the direct descendent of the low cost OTL from the aformentioned book, through my own Fane speakers and at Owston last year, I can say that the thing is a fabulously good sounding amplifier. Utterly spellbinding stuff and unerringly reliable.

But back in 2009 I thought I was the bee's knees and believed that I knew a lot more than I actually did. I set about building the "Low Cost OTL" I ordered a load of expensive mains transformers from Sowter and four big hefty 2200uF capacitors, a load of zener diodes and other sundry stuff. To a load of hubris driven fanfare on Audio-Talk I commenced the build, proudly showing every step of the construction in glorious Technicolor photographs and marvelling to myself how clever I was.

Now there were both positive and negative power supplies in this thing, plus a negative bias supply for the tubes.
A conventional output transformerless amplifier has a sort of white cathode follower output, which means basically one tube configured as a cathode follower (output taken from the cathode where the cathode "follows" what is happening on the tube's grid) sitting on top of a conventionally configured tube outputting at its anode. The two signals are therefore out of phase so that one pushes and the other pulls, so we get a push pull output at the centre connection between the two tubes but without an output transformer being required. Balance is critical at that centre point because the thing is directly coupled to the speakers. Now transistor amps do this as a matter of course, but not with 340V between the outer ends of the two halves (neg 170V and pos 170V)

Off I merrily went and finishing the amp, I installed the eight EL509 triode strapped, television line output pentodes, four on the positive bank, four on the negative bank, and went to adjust for zero volts at the output. Suddenly a terrible screaming noise came from the cheap speakers I had connected to the output. This made me jump, so much so that the meter probe slipped out of my hands into the interior of the electronics.

Reaching into the live chassis, one hand behind my back as always, I saw a brief flash that seemed to come from inside my head. It was only after a few seconds that I realised I was on my back and on the floor. The speakers were still screaming their heads off but the noise seemed somehow distant; further away than it should have been. I tried to get up but my legs didn't seem to be working properly and still the screaming continued from the speakers. It can't have been that long, but it seemed to take forever for my legs to recover, but there now came a strong smell of burning, then a bang, then silence. The fuses in the lines to each tube bank had blown. I went to pull out the plug from the extension lead, and burned severely the inside of my right forearm on one of the tube banks. The pain was incredible and I rushed to the bathroom, and flooded the injured forearm area with cold water for at least fifteen minutes.

Jesus Christ I'd been lucky not to have been killed. I'd committed the cardinal sin of of firing up a newly built project on my own, with no-one else in the house. NEVER EVER work on a valve amp unless there is someone else there with you. I'd touched a 2200uF cap, charged to 170V and the discharge into my arm had thrown me onto my back. This is defibrillator stuff! At least I'd only had the one hand in the thing. A DC charge like that across the chest could have been curtains. As it was, I had superficial burns on my right forearm and on my right palm, where the electric discharge had entered the skin. It hurt for days.

The post mortem revealed an error in the feedback resistor values on both channels that had turned the circuit into a power oscillator. I'd misread the colour codes, and the resistor values I'd put in there were ten times lower than they should have been, giving ten times the design feedback amount....OOPS!

It was years before I dared go near an OTL again. And it was the first and last time I ever fired up a new project with only me in the house.
Somebody’s telling me the latest scandals.
Somebody’s stepping on my plastic sandals. Joe Jackson (1979)

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Dr Bunsen Honeydew
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Re: A Bodger's Tale

Unread post by Dr Bunsen Honeydew »

I had similar from guitar amp in about 1969, I don't remember the make, and it was switched off, it still threw me across the room into the wall where I lay for a couple of minutes. The screwdriver that created the short had lost about half it's length in vaporised metal and melt on the carpet.

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SteveTheShadow
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Re: A Bodger's Tale

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Aye, excess electricity and the central nervous system are not easy bedfellows.
Somebody’s telling me the latest scandals.
Somebody’s stepping on my plastic sandals. Joe Jackson (1979)

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Dr Bunsen Honeydew
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Re: A Bodger's Tale

Unread post by Dr Bunsen Honeydew »

:epopc:

_D_S_J_R_
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Re: A Bodger's Tale

Unread post by _D_S_J_R_ »

My six months with a Croft 30WPC OTL put me off for ever I'm afraid, although I know a little more now about the needs of such things. This was the last *unregulated* sample Glenn had made (now I know why he regulated future ones). I used it at first with the hideously insensitive ATC 20's and the Epos 14's with absolutely no issues I can recall. It clipped gracefully and had a wonderfully delicate 'cut glass' kind of presentation, the lack of bass 'slam' never an issue as there was little need to ever want to thrash it. I moved and began to use a prototype Celef three way speaker (10" Volt bass driver, 6" SEAS or Peerless white cone as mid and matching tweeter with concentric cut-outs in the faceplate) with a warm smooth tone. I suspect the impedance was cruel for this amp but didn't understand at the time, why at random, any one of the four line fuses would pop with no fanfare. One Saturday evening with friends round, the system was playing very quietly and we were greeted by a sudden firework display inside the mesh top case (eight? 509 output valves with connectors on top of each). I ran to the fuse box and shut the entire house-mains down before disconnecting this amp. I never used it again, as it went straight to Glenn for checking and was sold by pal HiFi Dave immediately after. A recently rebuilt Quad 405-2 followed on (sou8nded frine to me) and then a wholesale switch to AVI and ATC for a few years followed. That was my last dalliance with valves until I was given (to look after) the Croft rebuilt Quad II's and these are a different, gloriously coloured yet so addictive to listen to thing entirely.

It looked like this - https://hifiwigwam.com/forum/topic/1305 ... 3-otl-amp/

All I can remember was very little indeed underneath and Glenn's elegant 'military' straight wiring paths. No pics of mine sadly.

Sorry, as you were Steve :D
Hanging on in quiet desperation is the English way...The time has gone, The song is over, Thought I'd something more to say...

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