Poundland 45

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Cressy Snr
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#91 Re: Poundland 45

Post by Cressy Snr »

What!
Garish purple glow tube bling, taking attention from those lovely 45s? Next it’ll be blue LEDs lighting up the undercarriage. :D
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izzy wizzy
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#92 Re: Poundland 45

Post by izzy wizzy »

Great job!

Don't fiddle with it before Owsters ;)
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#93 Re: Poundland 45

Post by Cressy Snr »

Got a revised audio cct to post:
I know Izzy mentioned not fiddling with it before Owston, but I have never been really happy with the bias on the input stage.The LED was used in the first instance, because it was a quick way to provide something that worked because I was in a hurry to experience the wonder of the 45 tube and couldn't be arsed to look for a suitable bias resistor for the input stage.

I don't feel that 1.85V is sufficient to keep the stage out of grid current with a loud input signal. 2.5V is more like the minimum requirement in my view. The long and short of it, is that I now have resistor/bypass cap bias on the front end, with a 2.6V cathode setup, courtesy of 470R Kiwames and 330uF Elnas. There are no ill effects that I can hear, indeed the sound seems bit smoother, bigger and with more solid images.

I think that'll do now. These things take time to optimise, especially when one is dealing with a DHT one has not used before. I'm glad I persisted with the listening and the modifying as it seems to have paid off pretty well.
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#94 Re: Poundland 45

Post by Cressy Snr »

Damn' blast and buggeration happened on Sunday evening!
Sitting there, with me cup oh tea, after the missus had gone to bed, I was enveloped in aural ecstasy, when all of a sudden, a lot of hum appeared behind the music, which was disconcerting, as the amp is normally a smooth and quiet operator. The mains transformer was also buzzing like a banshee. All heaters and filaments were glowing, so, 'feck it!' I thought. Whatever was going on could be left until the morning. The land of nod beckoned, so it was off to bed. I stayed in the present, realised that resistance was futile, messing about at that time of night would be unproductive, and so I turned in.

Monday I got the amp on the bench (yes I've actually got a bench now. Imagine that :) ) and it was clear there was something odd going on with the HT. It was there, after a fashion, but there was a lot of mechanical noise coming from the mains transformer and I was only getting 210V at the first cap.
I suspected the rectifier, so I removed the AC from it and tested the diodes, nothing wrong with either of them, at least according to the diode test facility on the meter. Testing across the transformer secondary, there was no continuity end-to-end, so for a second or two, I was puzzled about how I was getting any high voltage at all. It was then that I remembered the secondary was centre-tapped. Removing the centre tap from the earth busbar and testing to there, revealed that there was continuity from one end to the centre, but not from the other to the centre. IOW, half the winding had gone open circuit somewhere. I was only getting half wave rectification, which explained the noise, both mechanical, electrical and the higher voltage sag at the cap when all the valves were in because of the resulting poor regulation....Sh*t!

This was not good. Although I had a 200VA/230V toroidal isolation transformer I could substitute in, it had no 6V heater winding, so no heaters for the ECC88. Worse, the filament transformer for the 45s lived under the rear cover with the now busted mains transformer, which meant no room for the toroidal replacement, unless said filament transformer was relocated onto the underside of the top plate, but there was a pair of glow tubes right where I needed to put it. There was also the question of a 6V heater transformer needing to also go under the top plate. The chassis is tiny and I'm in serious trouble here because of that.

It has taken me three days of head-scratching, standing there looking, measuring with a ruler, moving stuff about under the top plate, fitting the parts needed, wiring and finally testing it all, just to get back to a working amplifier. Sodding thing! :x

The problem is I think, that I've built so many amps over the years, that the bits I've got are getting extremely long in the tooth and didn't ought to be being used in new builds; not if I want things to be reliable at any rate. The mains toroid is new, so hopefully I won't have any more problems. I had to convert the rectifier to a bridge of course.
Last edited by Cressy Snr on Wed Oct 20, 2021 8:15 pm, edited 1 time in total.
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#95 Re: Poundland 45

Post by Cressy Snr »

Amp interior is now packed to the gunwales. Three quarters of the inside is now taken up by the power supply :shock:
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New power supply diagram; hopefully the last:
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Last edited by Cressy Snr on Wed Oct 20, 2021 8:02 pm, edited 1 time in total.
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#96 Re: Poundland 45

Post by Cressy Snr »

Audio circuit has been modified too, to accommodate the higher voltages being generated by the oversize mains transformer. That took some thinking through too. 45s are dissipating 9W, meaning globes are out of the question, even if I could afford them. The ECC88 is running at 10mA. (0.95W per section) The great thing is that it still sounds great and even more amazingly, given the packed interior, makes no more electrical noise than it did before.
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#97 Re: Poundland 45

Post by IslandPink »

Hmm. lots of trouble you didn't need !
So, was the old mains transformer a vintage IE one , or just an older toroid ?
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#98 Re: Poundland 45

Post by Cressy Snr »

IslandPink wrote: Wed Oct 20, 2021 9:19 pm Hmm. lots of trouble you didn't need !
So, was the old mains transformer a vintage IE one , or just an older toroid ?
It was a vintage IE Oxford Electronics transformer courtesy of Maplins, a very long time ago. It had been in and out of all sorts of stuff.
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#99 Re: Poundland 45

Post by Paul Barker »

Doesn’t sound like a badly wound transformer, more likely a rare event.
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#100 Re: Poundland 45

Post by Cressy Snr »

Aye, rare indeed, but the same thing happened about 15 years ago with one of the output transformers on a WAD Pre II - one of the first things I built. WAD replaced the transformer FoC, apart from the postage to send the defective one back to them to look at.

Guess I have been unlucky or maybe I am too cack-handed to be allowed near a soldering iron. :)
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Paul Barker
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#101 Re: Poundland 45

Post by Paul Barker »

I doubt it was you’re fault either, only takes one knot in fine wire caused by slack in the spool when winding followed by a jerk and a knot forms if not spotted as it happens, years later insulation stress at that point shorts transformer. Nobody is watching attentively the entire time.
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#102 Re: Poundland 45

Post by RhythMick »

Is it worth adding fuses to each transformer secondary?

You could also off board the power supply into a separate chassis with an umbilical if the chassis is tight, though it sounds like you don't need it now. I usually find the power supply dominates the space requirements.
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#103 Re: Poundland 45

Post by Nick »

Is it worth adding fuses to each transformer secondary?
Maybe, but in the case of Steve's TX failure what would they have done. I have had them fail and create a shorted turn, but that would be caught by a fuse on the primary.
You could also off board the power supply into a separate chassis
But to do that properly adds more problems (and fault conditions to catch) than it solves, especially for a power amp.
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#104 Re: Poundland 45

Post by Cressy Snr »

Yesterday and today, I have been mostly throwing more solid state at it, and to very good effect:
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I moved the rectifier to the rear and out of the way. Doing this, made the transformer/rectifier/first cap loop, just about as short as it can possibly be.
I had a 2008 DIYHS DHT low noise supply knocking about, but the heatsink was too tall not to stick out well below the base of the chassis. I changed the sink for a smaller one as we are only supplying 300mA to the ECC88 heater and another 300 to the indicator light. A 9V transformer was subbed in to supply the DC module. The heatsink gets quite warm but not excessively so.

Not really that experienced in feeding IDH valves with DC and the 13 year-old DIYHS module is probably ridiculous overkill. I only have the one, which isn’t a lot of good, so why not at least get it doing something useful I thought. I must say that with both ends regulated, the ECC88 front end has stepped up its own performance and that of the whole amplifier to quite a significant degree; something that to be honest I wasn’t expecting. I’m not complaining obviously.
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#105 Re: Poundland 45

Post by Paul Barker »

Ooh
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