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Old 24th Oct 2013, 6:19 pm   #1
yestertech
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Default Junk box amp - pre amp valves & choices

Whilst attempting a tidy up the other day, I thought it was time I pressed into service some of the more useful junk hanging around in dark corners.
I fancy the idea of an audio amp. I am fortunate to have a pair of reasonable PX4s. I also found a large mains transformer and a Partridge O/P transformer.
These have so far found their way to an old tape amplifier chassis, which has been cleaned up and should be good enough for a "quick 'n' dirty assembly.
Further rummaging produced some B4 valve bases.
So far, so do-able. Good to get back to some honest to goodness metal bashing....
Where I'm undecided is in the choice of preamp/driver valves. I can have either 4v or 6v heaters but I fancy keeping to all 4 volt valves. I could replicate the famous RGD 1046 chassis so loved by audiophiles, but that means MO valves like SP41 and HL41 and I'd prefer a valve without a top cap if possible. I'd also need some decent specimens here otherwise the grey coating makes the amp look scruffy. ( Mmm.. probably being too fussy, given the reworked chassis et al ! )
This also affects the choice of rectifier - If I go down the MO/RGD route I will have to use UU7
If, as I prefer, I stick to nice large 'glass', I can use U18/20 ( aka AZ50 ) - this valve has to be 4V as well with a highish load current rating, as the tranny has no 5V winding.

The tranny HT winding is rather precisely marked 374-0-374 . With a capacitor input filter and the average saggy rectifier, what HT would I get approx ?

All ideas gratefully received.

Andy

PS - just found some nice tall paper in oil caps too
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Old 27th Oct 2013, 12:18 am   #2
Herald1360
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Default Re: Junk box amp - pre amp valves & choices

Quote:
Originally Posted by setsappeal View Post
The tranny HT winding is rather precisely marked 374-0-374 . With a capacitor input filter and the average saggy rectifier, what HT would I get approx ?
Well, it depends on how saggy it all is, but probably around 420V at 100mA dc load. If you're using paper in oil for smoothing (8uF or so?), you'll need a decent inductor to clean up the ripple.
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Old 27th Oct 2013, 2:48 am   #3
kalee20
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Default Re: Junk box amp - pre amp valves & choices

I'd agree with Herald1360. But it does depend on what the transformer is rated to supply, and what you are drawing.

Capacitor input rectifier circuits are rather dependent on limiting resistance in series with the rectifier. With very low load, the output voltage will approach 1.414 x the RMS secondary voltage. And that itself could be greater than 374V (because that itself may be specified on-load). So you could get upwards of 529V at low loads.

On-load, the limiting resistance will reduce voltage by quite a lot, far more than just DC load mA x limiting resistance kΩ. But it's rare that the DC voltage drops below 1 x RMS secondary volts, and also rare that voltage is greater than 1.3 x RMS input voltage. Higher power transformers have lower resistance windings, so you need to load them appropriately.

The choice of rectifier valve also ag=ffects output voltage, because some rectifiers themselves introduce a fair voltage drop. Generally, indirectly heated cathodes sem to be lower resistance. That's not always beneficial because they may need extra resistance deliberately adding, so as not to exceed the peak cathode current rating.
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