bluetube

The theory.

Build a low cost valve amplifier.

I plan to use it with my Nokia N810 tablet.

But how to do it?

Good unused audio valves are generally pretty expensive these day (well I reckon £20 per valve is expensive). There are a few projects on the web where either very cheap valves have been used, e.g. types originally not intended for audio, or where valves are mixed with semiconductors.  However used valves with plenty of life in them are fairly cheap, especially types that aren't used in guitar amplifiers or high end hi-fi.  Valves from old radios and gramophones (record players) are cheap enough.

Having said that, the valves aren't usually the most expensive component.  The iron components, power and output transformers, take that place.  There are several reasons for this - they were actually more expensive in their day than the valves, they haven't been saved by junk hoarders as much as valves, they are heavy so postage is costly. Again, old radios are a suitable source if only modest output power is required.

Stereo pretty much doubles the cost and isn't really such a big deal.

So there's the plan.  Re-use components from an old radio to make a small mono valve amplifier.

The practice.

First find you amplifier.  I bought this one on ebay.  It cost £4.99 including the valves plus £7.00 postage  (it weighs about 1kg). Apparently it was working when removed from the gramophone - the same day it was put up for auction.  It should be is easier to modifty than starting with a valve radio, as there's no need to remove the unwanted tuner.

Before buying it makes sense to try and learn a little about what you're getting.  The service sheet for mine is available for free download at RadioManuals.info  (Collaro Microgram)

The unit isn't exactly as shown in the service sheet.  One very obvious difference is that a small auto-transformer is used to provide the heater supply for the 3 valves rather than a large tapped resistor.  This is a definite plus, also there's a modification which seems to be the addition of a  negative feedback circuit, which if it's right should improve fidelity.  So far, very good - NOTE I've not tried to apply power to it yet.

 
Things I already know I'm going to have to change even if everything is working -
    Isolate from mains supply.  The direct feed of HT from the mains and LT through an autotransformer isn't very safe.
    Replace wax capacitors  (for these to remain good for 60 years is very unlikely)

Things I might have to change -
    Variable resistors (including mains switch) - these do fail, or become very noisy - though turning them back and forth many times can improve them greatly.
    Electrolytic capacitors.
 
Things I won't change but others might -
    The 3 valves in this amplifier have series connected heaters.  The first valve EF36 has a 6.3V heater drawing 0.2A, the output valve CL33 and rectifier CY31 also have 0.2A heaters but with very different voltages as each needs a different power to the heater.  There is a 6.3V equivalent of the CL33,  the better known EL33/KT61/6M6G/SP25 so replacing with one of these and rewiring for parallel heaters is an option. 
 
Touching the wax capacitors reveals they are sticky, so the amp has been powered up recently and that they're leaky, and hence getting warm. This is no big deal, but it also means the electrolytic caps have had high voltages applied after many years unused, so they might have been damaged.

Circuit diagrams

The Collaro Microgram circuit as published.  This isn't what I found when examining the amplifier.

The Wireless World Quality Push Pull Amplifier of 1945.  Using three capacitors and two chokes for HT smoothing seems to have been standard Wireless World, no expense spared, practice.   By using negative feedback the WW amplifier avoids the need for tone correction components.  


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Michael Saunby,
1 Dec 2008, 15:15
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