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Radford bias: options for ultralinear output
Radford bias: options for ultralinear output
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Old 14th July 2015, 02:23 PM   #1
billshurv is online now billshurv  United Kingdom
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Radford bias: options for ultralinear output
Default Radford bias: options for ultralinear output

One of my daft questions. I really shouldn't try and think when been up all night changing nappies, but thoughts turned to the Radford STA-25 that self destructed one chicago winters evening in 1996 and really needs to be bought back to life. Every so often I do some research, come to a decision, park it through lack of funds and 6 months later pick it up again.

So last night's thinking was about bias and I decided to start by looking at LED bias by reading SY's fine red light district articles. Looks good, but runs into problems with ultralinear connections (I rewired the radford to triode back in the 80s when running efficient speakers, but on the rebuild want to go back to UL). So possibly back to drawing board...

At this point thought I had better check how Radford did it back in the 60s and this is where I really should have waited until I had had some sleep. This is very confusing as it appears to use BOTH cathode and fixed bias. On the cathodes is a 39Ohm resistor bypassed with 250uF, 1M grid leaks to ground AND a negative bias on the control grids. Biasing is done by setting 2V across the cathode resistors (to give 50mA). Never seen this before so can only assume Radford didn't trust users to set the bias accurately and put a belt with the braces. But unless someone can illuminate me does seem to be worst of both worlds?

The later renaissance redesigns by woodside DO use a standard fixed bias setup (and 6550 instead of EL34), but have a screwy measurement setup that requires someone to have correctly set pots in the factory! I have seen references to a fully active bias setup that Morgan Jones did, but don't have his book yet to see if that has wings.

Which leaves me back at square one and wondering which way to go. The PCBs are too flaky to resuse so the restoration will be new PCBs which gives me the freedom to do anything I want as long as it fits in the case and doesn't require new iron.
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Old 14th July 2015, 04:21 PM   #2
DF96 is offline DF96  England
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Originally Posted by billshurv
But unless someone can illuminate me does seem to be worst of both worlds?
One could argue that mixed bias gives the best of both worlds: stable bias, but some compensation for ageing and pair mismatch and some limited protection for faults.
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Old 14th July 2015, 04:28 PM   #3
billshurv is online now billshurv  United Kingdom
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Radford bias: options for ultralinear output
might be very limited protection, but I have never worked out what exactly went wrong.
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Old 15th July 2015, 01:52 AM   #4
Keit is offline Keit  Australia
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If you want the lowest distortion figures, you need to use fixed (grid) voltage bias. But that will not self compensate for tube aging as cathode resistor bias will, leaves the tubes liable to self distruct if the bias supply fails, and can result in a tube ran-away condition.

Resistor bias is safe and reliable, but as the bias then tends to increase with signal level (due to grid voltage/anode current transfer curvature), results in a little more distortion and poorer overdrive characteristics. Whether you can actually hear the increased distortion in a carefully designed amp is another matter.

However, tube manufacturers always recommended that some cathode resistance be included. A volume manaufacturer will always comply with tube-maker's recommendations. Then if a batch of amplifiers have an unusually large number of tube failures, there is no debate about whose fault it is, and why it happened. In large scale manaufacturing, and engineer who thinks up an innovation using a tube in an unusual way would always ask the tube manufacturer to comment. Otherwise he leaves himself open to getting the sack if failure rates are high. Or lawyers sucking out the profits of everyone.
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Old 15th July 2015, 08:37 AM   #5
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by billshurv View Post
I decided to start by looking at LED bias by reading SY's fine red light district articles. Looks good, but runs into problems with ultralinear connections
Why does LED bias not work satisfactorily with UL?
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Old 15th July 2015, 10:08 AM   #6
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Radford bias: options for ultralinear output
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Originally Posted by Keit View Post

However, tube manufacturers always recommended that some cathode resistance be included. A volume manaufacturer will always comply with tube-maker's recommendations.
Understood, but this is all the elements of cathode bias (cathode resistor, bypass cap and grid leak to ground) with fixed bias on each grid. Not seen it before so assumed it was something special to Radford. The Mk2 was almost a direct copy of the williamson so used cathode bias.
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Old 15th July 2015, 10:22 AM   #7
billshurv is online now billshurv  United Kingdom
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Radford bias: options for ultralinear output
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Originally Posted by Merlinb View Post
Why does LED bias not work satisfactorily with UL?
You still need to adjust bias current by another means. SY used g2 on a pentode for this, which isn't available, so you need to have a control grid bias on each tube, which means you have to have fixed bias anyway. So there is the question of if the benefits outweigh the complexity. I've not done the sums but am assuming if done right you use the LEDs to get to a safe low bias then only have a couple of V adjustment needed.

Or I have missed something and got confused which is most likely!
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Old 15th July 2015, 10:44 AM   #8
Keit is offline Keit  Australia
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Originally Posted by billshurv View Post
Understood, but this is all the elements of cathode bias (cathode resistor, bypass cap and grid leak to ground) with fixed bias on each grid. Not seen it before so assumed it was something special to Radford.
True - all the parts for cathode bias and all the parts for fixed grid bias. Not that common but certainly not unique to Radford.
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Old 16th July 2015, 11:57 AM   #9
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Radford bias: options for ultralinear output
Been thinking about this a bit more and looking at some of the autobias solutions offered such as the Curcio and tentlabs offerings. Also much better understanding why some swear by cathode bias!

The big negative for grid bias other than parts count (all cheap parts tho) is that there is a nasty failure mechanism. Did get me wondering. If you used LEDs to bias at a higher potential than the nominal operating point (i.e. a safe operating level with low Ia) you could then use a relatively small positive grid bias to get you to the operating point.

At first inspection this seems awful, say biasing to -40V with the LEDs then reducing that bias voltage with the grid, but at least it has some degree of failsafe. Although a protection circuit that trips power on bias fault would be less components...
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Old 16th July 2015, 12:34 PM   #10
Keit is offline Keit  Australia
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The thing is - what do you hope to GAIN by adopting a more complex biasing scheme (counting any protection/backup/auto shutdown circuit as part of the complexity)?

What do you hope to gain, specifically:-
1) Less harmonic distortion?
2) A less objectionable overdrive characteristic?
3) Complete freedom from post-overdrive paralysis?

Clearly, you LOOSE in regard to reliability, serviceability and cost.

Let me be clear: I'm not dead-set against fixed bias, or shutdown schemes. What I'm saying is that you should be clear in your thinking about why you want to go for something more complex than simple cathode resistor biasing.

1. Distortion: While the lower distortion possible with fixed bias is measurable on instruments, many people cannot hear the difference. And the difference can be made up elsewhere, or lost, by subtle design issues with negative feedback (zobel networks), by not using an optimal UL tapping for the tubes, etc.

2. Overdrive characteristics: Again, carefull attention elsewhere can improve this, and non-careful design or ignorance can make it worse. In guitar and PA amps whereoverdrive characteristics are vitally important, carefull attention to the driver can do wonders.

3. Post-overdrive paralysis: Even with cathode resistor bias, sensible design can make this a complete non-issue.

So, what do you hope to gain?


There have been amplifier designs that employ a microprocessor to sense the long term drift in tube characteristics, and adjust grid bais voltage accordingly, with a rate of change restricted to mV per hour. And once you have a micro, you can design the system so that any loss of the bias supply or microprocessor function shuts down the amplifier.

You then have the ultimate: The distortion and overdrive advantages of fixed grid voltage bias, plus automatic compensation for tube aging and tube unbalance, without the risks.

But have I ever considered doing this? No - it's just not worth my while. I'd rather spend the money on a better output transformer.

Last edited by Keit; 16th July 2015 at 12:42 PM.
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