Stereomour 2A3 LED cathode bias

[sharpi31]

I'm three quarters through building my Stereomour and have been considering the best upgrade for the standard 220/270uF cathode bypass capacitors. From experience of other builds I'm not a great fan of electrolytic cathode bypass.

One option would be to install large film caps. I have both 400uF and 600uF MKP caps that would do the job, but neither would fit nicely within the chassis.

On this basis I'm considering replacing both the stock caps and resistors with an LED array. Best results are achieved with LEDs that have low dynamic resistance, and I'd previously identified the OPE5594A as being very good in this regard - see: http://docs-europe.electrocomponents.com/webdocs/0325/0900766b80325c6f.pdf

As the stock cathode resistance is 1.2Kohm, bias point 60V and plate current 50mA I calculate that a chain of 44x OPE5594A would suffice for each 2A3 (on the basis that the forward voltage at 50mA is approx 1.38V, so 43.48x series LEDs would be required to reach the 60V bias point).

Before I take the plunge I'd be very grateful if others could sanity check this idea. Please feel free to point me in the direction of better LEDs, if I've not selected the best option.

Many thanks in advance.

[sharpi31]

Alternatively could a single 60V zener diode be used in place of an LED array? In other words, omit the 270/220uF cap and 1.6K & 4.7K resistors and use the 60V diode in isolation.

Based on 50mA at 60V the diode would need to be able to handle 3W dissipation (probably best to use a 5W rated diode to provide a safety margin).

I'd be keen to know if (a) the single zener is viable, & (b) whether you'd expect better/worse sound than the parallel resistors and electrolytic cap.

[Paul Birkeland]

Alternatively could a single 60V zener diode be used in place of an LED array? In other words, omit the 270/220uF cap and 1.6K & 4.7K resistors and use the 60V diode in isolation.

Based on 50mA at 60V the diode would need to be able to handle 3W dissipation (probably best to use a 5W rated diode to provide a safety margin).

I'd be keen to know if (a) the single zener is viable, & (b) whether you'd expect better/worse sound than the parallel resistors and electrolytic cap.

I would expect horrible noise performance out of this arrangement.  Though the 1.2K resistor dissipates only 3 watts, if you used a 3 watt part there, it would get hot enough to burn itself to the point of turning black, discoloring the aluminum around it, and scorching other parts, so at least a 9 watt resistor should be used. Zener diodes are even worse at actually cooking off any heat.  I would think you would need a 50 Watt zener diode for it not to explode on you. (It would fail shut, and you would cook your plate chokes in the process) Zener diodes also tend to only hit their rated voltage at a very narrow current window, so you would need to find a zener that would have 60V across it with 50mA going through it.   The zener will also require a (hefty) bypass capacitor.

The diode question is a little more difficult to answer.  I would want to built the Stereomour stock first, sweep the amp, then clip lead in the LED array and sweep the amp again.  There's a very good chance that instead of needing a ~220uF cathode bypass cap, you may need a 1000uF cathode bypass cap to combat degenerative feedback (not sure how well this would work).

I'm sure PJ will weigh in once he's had his morning coffee. 

[Paul Joppa]

OK, coffee has been downloaded.

The reason for using a cathode resistor is that the bias adjusts itself for variations between tubes and as a tube ages. It's sometimes called "automatic bias" for that reason. With a fixed bias voltage and plate voltage, the plate current may vary over a very wide range. That's why amps with fixed bias have a current meter and a bias voltage adjustment on the front panel.

On "...experience with other builds..." I will note that the Stereomour returns the signal current current loop to the cathode rather than to ground, so that signal current (outside the deep bass) does not flow through the cathode bypass capacitor. This makes the capacitor less audible. Not that you won't still hear a difference, but it's less than what happens in more usual designs.

On replacing the capacitor, I used the 220uF in Stereomour because we use that value extensively so we have tons of them on hand, and can get them at a good price. Personally I would not hesitate to go as low as 27uF, at least as an experiment - and I can say with confidence that 47uF (as used in Paramount)  is plenty good enough.

There is some room adjacent to the cathode resistors; I did that specifically so that the cathode bypass capacitor could be easily bypassed or replaced. You might notice that the parafeed and interstage capacitors are also fairly accessible and have a bit of room around them, and that there is a spare hole in the PSU board making it easier to bypass the final power supply capacitor. I have not actually done any of these mods so I can't advise anyone on whether or how, but I did at least allow for the possibility.

[sharpi31]

Many thanks both for the excellent advice.

As suggested I completed the amplifier using the stock parts, and have been enjoying great sound over the weekend.

As and when the upgrade bug bites, I'll look at alternative bypass caps.

[galyons]

On "...experience with other builds..." I will note that the Stereomour returns the signal current current loop to the cathode rather than to ground, so that signal current (outside the deep bass) does not flow through the cathode bypass capacitor. This makes the capacitor less audible. Not that you won't still hear a difference, but it's less than what happens in more usual designs.

On replacing the capacitor, I used the 220uF in Stereomour because we use that value extensively so we have tons of them on hand, and can get them at a good price. Personally I would not hesitate to go as low as 27uF, at least as an experiment - and I can say with confidence that 47uF (as used in Paramount)  is plenty good enough.

Paul,
Does the Paramour return the signal current current loop to the cathode rather than to ground? Would the lower capacitance work for the Paramours, as well?

Cheers,
Geary

[Paul Joppa]

...
Does the Paramour return the signal current current loop to the cathode rather than to ground? Would the lower capacitance work for the Paramours, as well?

Cheers,
Geary

Paramour I does not; Paramour II does, and Stereomour does.

You can certainly try the smaller capacitance and see whether you hear a difference in the deep bass. But the three circuits are slightly different, and only the Stereomour is similar to the Paramount in that way, so that's the only one for which I have confidence and knowledge.

[galyons]

Thanks Paul! I always find it helpful to hear "deeper drills" into the designs.  I may try a lower cathode bypass cap and see if it positively impacts the sound without a negative impact on bass.

Cheers,
Geary