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First shot at a battery DHT preamp stage
First shot at a battery DHT preamp stage
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Old 18th November 2019, 06:11 PM   #61
piano3 is offline piano3  United Kingdom
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Mondo, my figure of a milliamp, or so, was for 1j24b. Since this one seems to be more like an output type of valve (the triode gain suggests this) , it may want a bit more current; after all, it's good for a bit over one watt.
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Old 18th November 2019, 07:20 PM   #62
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I agree, this is what I thought. (Actually didn't read the 1J24B...)

I have run 10k anode load, and gain suffered somewhat, but I didn't spend much time with such a low load, so have few notes.

I'll have to try again.
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Old 18th November 2019, 07:27 PM   #63
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Since they come out with a Voltage gain of 5 or 6 V/V, with diode self bias, that mu is might more than that, when triode wired.

So perhaps good for a line stage, but I dont think they have the "guts" for a headphone amp!
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Old 18th November 2019, 07:48 PM   #64
JamieMcC is offline JamieMcC  United Kingdom
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I have a friend who has made a nice sounding DHT headphone amp by modifying a battery powered Bottlehead Quickie DHT preamp with constant current source he added a couple of inexpensive speco transformers.

The circuit which uses one 3S4 tube per channel its a very simple and nice sounding pre I have one myself but have not converted it to a headphone amp as yet.
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Old 18th November 2019, 10:34 PM   #65
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Well, if you raise the anode voltage to about 120 V, the current to 8 mA and use output transformers, then it will surely work as a headphone amp. You could use mains toroids and capacitor couple to them (parafeed). I would definitely use a CCS rather than a resistor on the anode. You can use 2 stages to get enough gain.
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Old 19th November 2019, 07:37 AM   #66
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Using what I have laying about, I rigged up a single BJT as CCS, using TIP47 (PNP Vce of 100V) and red LED, so I have retained the B+ 90V, to stay within the limits of the BJT.

Crude, but it seems to work OK.
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Old 19th November 2019, 09:19 AM   #67
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If I were you, I would set up a differential pair and use your 250 V supply. Then you can vary the tail current whilst using appropriate anode resistors for that current; note what the bias is at 5, 6, 7 and 8 mA.
Of course you'll need a negative supply of about 10-15 V but any old NPNs will do in the tail.
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Old 19th November 2019, 09:29 AM   #68
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You mean a long tailed pair input stage with 2 valves?

Agreed a higher B+ would ease things somewhat WRT THD, but at the moment, without buying some transistors, I am at the limits of the silicon I have.

I could probably switch to a CC Sink for the cathode circuit, using what I have to hand.
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Old 19th November 2019, 09:39 AM   #69
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That's why I suggested a LTP; the cathode CCS only needs low voltage transistors, BC547, BC337, anything really. Constraining Va, Ia and measuring the resultant bias, then you have your valve characterised.
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Old 19th November 2019, 10:26 AM   #70
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I have to admit it would be the first differential stage, that I have built with Valves.

I'm struggling to see the advantage, it will take me a while...

I'd guess, for the same input signals and desired output differential signal, then gain required in each triode, is halved.

Thus THD would be reduced accordingly.

But...if I'm only using one of the differential outputs, then gain is halved & THD is the same as for a common cathode stage, with the same gain and swing.

Being as I have only just set up the CCS anode load, I might see how far I can get with that.

First tests, checking quiescent V and I, look encouraging.

Using BC237/557/107 for cathode CC sink should be easy enough to build too.

my scope has gone for repair, so FFTs will have to wait
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