• 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.

SE Amp with Thoriated Tungsten output tube

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#144
Apologies, I should have realized it sooner, but I will not be able to trace A2 curves, it is a limitation of the tracer. A2 for true triodes is viable, there are two positive HV supplies, one must be used for the grid to trace A2. For triode-strapped pentodes, both HV supplies are in use for plate and screen, so it cannot be done.
 
#145
L0rdGwyn said:
Apologies, I should have realized it sooner, but I will not be able to trace A2 curves, it is a limitation of the tracer. A2 for true triodes is viable, there are two positive HV supplies, one must be used for the grid to trace A2. For triode-strapped pentodes, both HV supplies are in use for plate and screen, so it cannot be done.
Click to expand...

Really? How does the machine tell a triode from a pentode rigged as triode at the socket?

minhaj, what load? Whilst looking at DHP tubes, the datasheet for 4E27 says 30W from a single tube at 750V B+( I think ). There is the issue of delivering 7.5A/5V to light its cathode. 75W plate... 🙂

Am going to suggest 5k. I have run these at 30% U-L to a 10k a-a output( the Peerless S265Q )at 350V.
cheers,
Douglas
 
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#147
Here is HY69 with A1 + A2, grid current in green, scale on the right.

HY69 A2.png
 
#153
Now it's time to buy few HY69 and import to my country. I think socket and plate cap is same as 807. It will take some time due to covid19 situation. Anyways, once HY69 reaches my door I will modify my 300B SE and give you update about their sonic qualities.

Regards
 
#155
minhaj - yes, it is the same plate cap as the 807.

This is off topic, but the HY69 A2 curves remind me of some other A2 curves I am very close to at the moment, the 801A.

I have been developing a 801A A2 amplifier for several months, it is getting very close to completion. It needs one last breadboard before the circuit is final.

I thought to myself, well if I am going to take the time to breadboard, I might as well make something I can re-use - so I made a prototyping board on plywood, completed this morning.

All the pieces are now in place, including custom iron from Sowter, I should have the circuit functional by the weekend, very exciting 😀

It goes EF37A > 6BX7 parallel CF > direct-coupled 801A at near 0Vg1 bias with plate to EF37A cathode feedback. When it is done done, I'll make a thread with measurements and photos. It will be 6-7W into 8ohms.

PXL_20210421_184313031-2.jpg
 
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#158
I don't have the data handy to overlay them without retracing, but here are the full 801A curves for comparison. They follow the RCA datasheet very well, this one was made by Taylor.

Taylor_801A.png

Thanks for the idea, that's a good one, but I am already going a little nuts - my next design is based on the HK54 transmitting triode 😀 15W single-ended design, will spend most of its time in A2 with +38V grid bias point, drawing around 10mA at idle and 60mA at peak swing, 25W filament.

PXL_20210327_111210193.MP-2 (1).jpg

3K load line, 425V 105mA bias, will require NFB of course.

HK54 3K LL 425V 105mA.png
 
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#160
I am running a low power 845 amp as my daily driver. They are Psvanes bought cheaply on Amazon. I have about 9 volts on the filament. About 400 volts on the plate and a 6K load. The 845 amp drives horns so I am using DC on the filaments. I have used it for about five years like this. Worth doing if you find 845s cheaper than 300Bs. I am about to likely replace the amplifier with something else and reconfigure for 20 watts instead to drive lower efficiency speakers.

I am looking to configure two competing systems to see who can win. Sort of a medium efficiency versus high.
 
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