Re: 'PC Speaker' Tube Amp

In article <KKLUj.38101$AL4.35754@[EMAIL PROTECTED]
>,
 "Ian Iveson" <IanIveson.home@[EMAIL PROTECTED]
> wrote:

> Thanks. One or two details might be worth discussing; I'm
> always interested to know how ppl make decisions.

Hi Ian, I hope you don't mind my jumping into your discussion.

> Why did you decide not to direct-couple the voltage amp to
> the concertina, and to share the valve halves by channel
> rather than by function?

Conversely why would one want to direct couple voltage amp and the 
concertina stage?  By avoiding direct coupling the design of each stage 
can be optimized.  The only down side is that somewhat greater care must 
be taken in the choice of the three low frequency time constants.

> What's going on with the connection of the concertina's
> cathode load to the cathode of the voltage amp, rather than
> to ground? How well is the splitter balanced, given the
> different total loads?

Flipper explained that it is positive feedback used to increase the open 
loop gain of the amplifier.  This is common in phono preamp stages, and 
not unheard of in power amps.  As far as precise balance of the 
concertina loads goes, aren't you being a bit anal, they are close 
enough.

> I can't see how the output valves' bias is adjusted.

I assumed it isn't adjusted, why should an adjustment be provided?

> Why do Zobels go to anode supply, rather than to screens? I
> have read quite a lot of waffle around this subject but
> nothing very convincing. What, exactly, are those Zobels
> for?

When I use Zobels I connect them to the anode supply as flipper did, why 
would you want to connect them to the screens?

> Using a resistor in series with headphones isn't ideal, to
> my mind. My opinion is that a head amp should be have an
> output impedance of no more than 5 ohms, or one tenth of the
> 'phones' impedance.

I'm not into headphones and hence don't understand how to properly 
interface them.  That said I was surprised that flipper didn't find some 
attenuation necessary beyond that provided by a series 51 Ohm resistor?  
I would think that a different circuit could be found that would meet 
your 5-Ohm impedance criteria, perhaps by simply tapping the headphone 
output doen on the 8 Ohm load resistors and eliminating the 51 Ohm 
series resistor.  Not understanding headphone circuits I would be 
interested in learning what flipper's criteria were for the design of 
this part of his amplifier?

> Nice pictures. Speakers are beautifully done. Amp looks
> functional. Valves look a bit gloomy. Transformers look
> cheerful but very small. How's your bass?

The transformers do look "cheerful", but I don't understand why you say 
the "Valves look a bit gloomy", they look quite bright to me?


Regards,

John Byrns

-- 
Surf my web pages at,  http://fmamradios.com/