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SE Amp with Thoriated Tungsten output tube

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M

minhaj

Member
Joined 2004
#1
Dear All,

I use a 300B SE DIY amp for long time. I wish to build this amp within some limitations viz

1. Low cost
2. Outout transformer should be 3k or 5k
3. B+ 400volt max
4. Class A1 only (no grid current)
5. Triode or shade feedback output

3 watt output is good for me. I thought 865, Hy69, 2E25 (triode connected) might be good candidates. But I find little info about them. Please help to realize my long time dream come true. I will be most obliged to you.

Regards
 
#2
A lot of those thoriated tungsten tubes glow as bright as a regular light bulb. Plate resistances tend to be very high in them, which makes for poorly performing high turns-ratio OPTs, and B+ voltages near 1100Vdc are often needed, which qualifies them for widow-maker duty. They may be cool looking and all, but they are grossly inefficient and don't sound all that good, either, since they were usually designed for RF, not audio use. I'd stay with tubes designed for audio use, especially at the low power you need.
 
#4
egellings...what you are telling is partially true. I briefly ran vt25 in se and the sound was much superior but 1.5watt is not enough for my speakers. Please check all the 811, 809, 814 A2 Se amps they all run with 350-450 volt B+.

Regards
 
#5
Hi euro21...I can go for 801a with 550-600volt B+ and 10K-12K opt and stay in A1 but that will be costly for me.

Triode data/plate curve for 865, hy69, 2e25 anybody. Then I could design/consider if it fits my criteria.

Regards
 
#6
If you give up "A1" condition, most of TT filament transmitter tube working perfectly at "zero bias" mode (it was the radio station transmitters basis mode).

But no free lunch, in this case must be push to grid few -or few ten !!- mA current, so driver must be at least as beefy, than power tube.

The PSU devours the power, the driver as costly than power section, the VAS stage must have a large enough gain, and its output impedance must be enough low to drive the beefy driver section.

So, that's all, just not cheap solution.
 
#10
But minhaj O.H Schade Inverse Feedback gives you your the "triode" .This has been discussed to death on here.
 

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#11
minhaj said:
I heard that Hy69 is like 807 if so I could run it triode connected with 400 volt B+ 5K opt and done with it. What is the non DHP equivalent of 865? 807, KT 88, KT 66?

Regards
Click to expand...

I am not sure there is an equivalent. It's also a low mutual conductance tube, I would expect rather poor efficiency in triode, worse than a 801a.

If you don't need much power, want triode with thoriated filament running at 400V with 5K transformer, I think the 814 is more convenient than all other tubes mentioned in this thread. The 814 pentode in triode mode has approx. 2K plate resistance and 4 mA/V mutual conductance.
Run at 400V/45-50mA with 5K transformer you should get 4W easy in class A1 with low distortion. Only 33-35V peak driving voltage for full output.
 
#12
I am ready to accept/test schade feedback problem is I have no measuring equipment here that's why I need your help. Like the following: 2e24 DHT – Bartola(R) Valves

My understanding is that H3 component is higher in schade feedback. By the way how you measure 10% or 20% feedback? Please enlighten me.

I am very interested to try hy69 or 865 in triode or schade feedback mode.

Regards
 
#15
Hi Andy...I am not very technically inclined but I think Ale's 814 is good for A2 operation, I want to stick in A1 region. Since Ale has extensive experience with 814 if he chimes in it could be great.

Otherwise I may follow 45's suggested OP and try to find out how it sounds. You have any idea where to buy 814 from?

Regards
 
#16
#17
euro21 said:
As you can see in Ale measuring, the 814 triode A1 region is limited, if you use it at low (below 500V) anode voltage.

https://i0.wp.com/www.bartola.co.uk/valves/wp-content/uploads/2014/05/814-SE-class-A1.png

The anode current must be higher, if you want any appreciable power.

At 540V, 100mA (big OPT!) the predicted power about 3+ W, at not so low load.

814 SE Amplifier: measurements – Bartola(R) Valves
Click to expand...

You get up to 5W in A1 with 5K and 500V supply.

They amp Ale built used 600V plate voltage and was driven in positive grid field and, from memory, it developed 10-11W with very low distortion (less than 1%). The A1 power was 3W but that is because of the high quiescent current for optimal A2 drive. But if you bias at lower current you get more power in A1. I designed the transformers that replaced the Lundahls...😉

The only real disadvantage if the drive is limited to A1 is that one has to consume 32.5 VA to light up the filament for just 4-5W (clean, at least) output power. As always, there is no free lunch...
 
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#19
Anyway, looking at the HY69 (which I don't personally know) it might do the job as well. In triode mode should have 2.2K plate resistance and 3 mA/V gm. So it should work ok with 5K. It requires a lot less filament power but I am not sure how much power you can get out of it. Probably 3W are possible.
 
#20
If you have 5K transformer and 400W then with 40mA anode current you could get up to 4W. A1 or A2 will depend on bias. So what you need to do in practice is start using self bias and adjust the cathode resistor until you find the best solution. I am just guessing that for 400V anode voltage and 40-45mA current you bias should be around 40-45V (negative). So you can start with 1K cathode resistor and bypass with a good capacitor. The if your current is too high (low) you increase (decrease) the resistance until you find the solution that gives you max Pout (also keep an eye on distortion...).
 
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