• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

using a plate choke?

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I have been thinking of replacing the bias resistor in this circuit with a high inductance choke (rather than a SS CCS). Can someone please tell me what sort of choke I need, and whether or not it needs to be custom wound, or if any of the shelf types are available like tango etc?



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Don't know about chokes, but...The 3A5 really should have battery powered filaments, espescially in this application. I've used these tubes and they sound great, but are very microphonic, and sensitive to filament induced noise. One D-size rechargeable battery per tube will last 10hours or so before recharging is needed. And these tubes can handle the voltage of fully charged batts.
I agree with Tim regarding the single bias resistor. What's up with that? Save 5cents? An upgrade to the given circuit would be one 300ohm bias resistor per channel, and separate batteries for the filaments.

Actually the designer of the stage has been asked the question of the shared resistor, basically 2 reasons,

1) the stage is actually intended for differntial operation

2) it actually sounds better with the shared resistor, I've tried both, the shared resistor sounds better to me..............

I've had no problem with hum at all, and agree that the tubes are senistive to the filament supply quality, maybe something I'll consider for the future. Though that does mean finding extra space for a charging circuit of some sort. The risk is a run down battery whilst the B+ is still applied......

so I can't use a choke in the tail?????
How could the circuit be adapted to do so???

Hi Raja,

I didn't mean to critisise;) I'm just curious.

If the cathodes of a long-tailed or push-pull pair share a cathode resistor, then degeneration is theoretically negated by cancellation. This is why you find some push-pull designs with no cathode bypass cap. However is practice, this cancellation is imperfect because the 2 sides can never be perfectly matched. It is though, pretty good in class A.

Depending on which camp you are in, you can believe that triodes are pentodes with degeneration. Although this models well, it's a concept I can't accept. In understanding something I start with the simple and work towards the complex, not the other way round.

I hope you can follow my reasoning.

Thanks for the help,

I tried the battery bias last night using fully charged aa batteries, 800mah capacity each in series.

It worked, however I notice quite a bit of 'grain' in the sound, right accross the spectrum, from bass to highs.

I measured the B+ and it reads 10v lower at 94 volts or so. With r7 (resistor bias) I get around 105v. Is the extra voltage drop due to the higher current demand/ or inadequate power supply?

Or is this the difference such biasing would make to the circuit?

The schematic is exactly as overleaf apart from a 47k bleeder placed at the end of the b+ supply.

Thanks for the help.

also a second experience is using d cells for the filaments, the sound is 'cleaner' than the lm317 in current regulator configuration.

I was thinking about regulating the 317 at 2.8vdc, and also attaching the battries in parallel. Using a switch when listening to switch off the 317, allowing the batteries to heat the filaments. And then switching on the 317 so that it can charge the battries and also perhaps keep the filaments powered up at the same time, is this ok?


You should check the current through each valve, before and after the changes. Best to aim at the original specified current as the iron will be optimized for that.
If you can't find a battery voltage suitable to get that current, then you could drop the HT voltage slightly by modifying the power supply.
As for current fed heaters, and battery fed heaters, I'm a sceptic. By all means do what your ears tell you to, but check that your operating conditions are correct.
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