Radford STA25 Mk III -- a lot of questions.

[brecklandboy]

One enthusiast showed off his full active system in one of the Hi-Fi magazines way back in the 70's, comprising of a Radford STA100 for bass duties, a Radford STA25/3 for mid-range and a Radford STA15/3 for HF.

Loud Speakers were the IMF Professional Monitor IV.

The STA15/3 has the sweetest sound reproduction of all the Radford STA's.

I cannot comment on the ultra rare Radford STA7, as I have never heard or seen one in the flesh.

Having owned several Radford amps in my time, I would say that the 'sweetest' sounding of the lot is the original STA12. According to a contemporary review from 1961, the '12 ran its EL34 valves at a significantly lower plate voltage than was the 'norm' for manufacturers of the time. This obviously reduced output but had the upside of contributing to an exquisitely detailed and beguiling sound which the later and more powerful Radfords don't quite manage to achieve IMHO.

Try listening to a 'sorted' STA12 with Mullard valves through a pair of original Quad ESL57s if you get the chance. Truly, the 'forgotten' Radford of the model range.

 

[scotty38]

A Series 3 original or a "Radford Revival" reproduction?

A very good choice in any event.

I would trade one of my STA25/111 for a RR STA15 c/w Mullard GZ34 mains rectifier valve if RR produce them.

A Radford Revival MKV but to be fair, to my eyes, there’s not a lot in it with regards to a III, cosmetics aside around power switch etc.... As you’ve since heard the STA15 is available too

 

[mandryka]

I am now a dab hand at adjusting the bias, I love doing it, it makes me feel like a techie.

But what is bias? Why does it need to be adjusted?

 

[Yank]

Bias applies a negative voltage to the grids of the output tubes. The grids need to be at a lower potential than the cathodes. Since the cathodes are grounded, that means applying negative DC to the grids to pull them down. This controls the idle current flowing through the outputs. The more negative voltage you apply to the grids, the less current flows. If the grids aren't biased enough, too much current will flow and the plates will glow red. The designer of the amp will have determined a sweet spot for the adjustment to get the idle current to the desired result.

 

[Arkless Electronics]

I am now a dab hand at adjusting the bias, I love doing it, it makes me feel like a techie.

But what is bias? Why does it need to be adjusted?

In a fixed bias amp such as the Radford (yeah it's a daft counter-intuitive name as fixed bias amps can have the bias adjusted but cathode bias have no adjusters at all) a negative voltage is applied to the control grids of the output valves to lessen the amount of current they draw at idle and thereby basically set "how much class A" is used. This voltage is set for each valve by the appropriate bias adjust pot. More negative voltage = less current drawn/less class A. Too much class A = output valves wear out tout sweet. Too little = valves last a long time but the sound is degraded. Hence a manufacturer decides on an optimum bias CURRENT that give a nice compromise with those output valves, output transformers and PSU voltage and the negative bias VOLTAGE is set to achieve this current.
The voltage to give a specific current varies depending on the individual valve, the make, how much wear it has on it etc hence it must be made adjustable as it will change over the life of the output valves.
For the pair of valves in each channel they also need to be set to match each other or the "push" valve could draw more current than the "pull" valve or vice versa. The OPT is arranged so that in effect the two currents cancel in the OPT and hence do not cause any magnetisation of the core of the transformer at idle. This prevents core saturation and gives the lowest THD, especially at low frequencies.

 

[snowman_al]

Basically bias (grid bias) is the voltage that sets the 'static' current trough the valve. It can be a steady negative voltage applied to the input grid of the valve (used in the STA25) and is called fixed bias. Or derived by a cathode resistor (used in the STA15) and called cathode bias. You do not need to adjust cathode bias amps, they set automatically by the choice of resistor.
You need to set fixed bias amps either to the manufacturers voltage or current setting. Because each valve is slightly different it needs to be adjusted or it may pass too little or too much current. Too little looses you output power. Too much results in the valve running much hotter and causing failure by red plating or thermal runaway.

When you set the Radford bias at 2 volts, you are adjusting the current through each EL34 to 51mA (milli amps). You are actually measuring across a 39 ohm resistor and 2 volts across 39 ohms gives 51mA by Ohms law.

PS there are several easy to understand explanations on the web. Google what is grid bias or similar.

 

[mandryka]

Thanks gents. So I guess I wasn't misleading myself to think it actually sounds better after the bias is set to 2V. It certainly sounds good -- anyone wondering whether to buy one of these amps should stop wondering and buy one -- all other things being equal of course.

 

[Seanm]

Thanks gents. So I guess I wasn't misleading myself to think it actually sounds better after the bias is set to 2V. It certainly sounds good -- anyone wondering whether to buy one of these amps should stop wondering and buy one -- all other things being equal of course.

Can I ask what pre you're using with it? Might have asked before...

 

[mandryka]

Can I ask what pre you're using with it? Might have asked before...

None. I only have one source. There's a direct link to the amp from a Theta DAC. Out to JR149 Mk 2.

 

[Yank]

It certainly sounds good -- anyone wondering whether to buy one of these amps should stop wondering and buy one -- all other things being equal of course.

They don't exactly grow on trees on this side of the Atlantic, or I would.

 

[George J]

Having owned several Radford amps in my time, I would say that the 'sweetest' sounding of the lot is the original STA12. According to a contemporary review from 1961, the '12 ran its EL34 valves at a significantly lower plate voltage than was the 'norm' for manufacturers of the time. This obviously reduced output but had the upside of contributing to an exquisitely detailed and beguiling sound which the later and more powerful Radfords don't quite manage to achieve IMHO.

...

Now I am getting somewhere with amplifiers to investigate.

Obviously I am only interested in a mono power amplifier, and I have always preferred the idea of "less' power with better quality.

The notion of driving valves less hard also appeals for durability and so less fuss.

I have added the Radford MA 12 to my list of amplifiers to research.

I have an ESL, which is more or less the idea speaker to derive the best from a low power high quality amplifier, given my aversion to loud replay. I tend to listen rather near field [sitting on my haunches - not a chair - about four or five feet away] and more or less use the speaker like a huge earphone rather than attempt to fill the room with concert or gig levels of sound pressure. The method really means you listen "into" the music, rather be presented with a big objective to listen to from afar.

My favoured amps [for investigation] include the Leak TL 12 [family] and Quad II so far, but a lower power Radford would be a fascinating find and potential audition.

Thanks for this nice thread.

Best wishes from George

 

[Arkless Electronics]

/\ The supposition is wrong. Longer valve life and reliability yes. Better sound? No.

 

[George J]

Dear Jez,

I would profess almost no real understanding of the inner workings and principals of electronic amplification, so accept your point with gratitude. However, I do regard long service life and serviceability as being absolutely equal in terms of my priorities with sound quality as such. I doubt that any of the amplifiers I mentioned two posts up are worse than my Quad II Forty, which is certainly not new, and I bought second hand about five years ago, though it is a currently available product.

What I fancy is another functioning amp that can either become top dog or as stand in when one or the other needs a bit of TLC.

Essentially I am a pragmatist when it comes to replay. The best I can have is fine for me, even though it may be far from the best there is!

Best wishes, and thanks, from George

 

[brecklandboy]

George, if you're looking for a medium output, high quality vintage EL34 mono amp, then you may also like to add the Armstrong A10 Mark 2 to your shopping list. Very nice indeed !

 

[George J]

George, if you're looking for a medium output, high quality vintage EL34 mono amp, then you may also like to add the Armstrong A10 Mark 2 to your shopping list. Very nice indeed !

Thanks,

I have heard of Armstrong. I am not necessarily concerned to have a Leak to brand match my lovely Trough-Line radio, or a Quad for my ESL. ESLs go well on the end of valve amplifiers with output transformers.

Thanks and best wishes from George

PS: EDIT, It may be supposed that my ESL from 1957 and my 1957 Leak Trough-Line radio are non-negotiable! I'll leave the house feet first before they do. It is just about the power amp that connects the two. Then I might be brave and reconnect my digital music library in AIFF Mac mini iTunes to the ESL again ... Currently headphone only ...