Thanks, SS! That RCA pair of globes are the only ones I have, every other pair are the ST Hytrons, the most common in my experience too. Wow, never came across a Raytheon 841, I'll have to keep an eye out. There is a fella with some really nice NOS GE globe 841 on eBay, but asking quite a pretty penny for them. Switching everything to TO-247 is probably a good idea, I'll keep that in mind 
On the subject of rare DHTs, this simple design could be implemented with another high-mu DHT option, the Valvo / Siemens Aa postal tubes (mu of 30).
These are actually more common than the 841 and would relax the B+ requirement big time (around 200-250V). They are around $150-200 a pair though.
Here is the datasheet: https://frank.pocnet.net/sheets/082/a/Aa.pdf
These are actually more common than the 841 and would relax the B+ requirement big time (around 200-250V). They are around $150-200 a pair though.
Here is the datasheet: https://frank.pocnet.net/sheets/082/a/Aa.pdf
I recognize this design is somewhat silly with the high B+, impracticality of the 841, heat dissipation, etc., but that is the joy of diyAudio, building things because you can, not because you should
Could you not look at a folded cascode? You'd be able to keep a high headroom without needing a high B+ to give the stack room.
Nice tubes euro21, beautiful Taylor globes, maybe I have seen those once before but I cannot remember where.
Hi Nick - I am not intimately familiar with the folded cascode topology, it is something I have been meaning to look into, thanks for the suggestion. Too late to change course on this amplifier though as the chassis and layout are finalized. The 841 biases at a high plate voltage, which is the primary reason for the B+. In my prototype, plate voltage is 375V with a 460V B+ which gets me right about to the 500mW power limitation of my Sowter 8665 OPT.
Hi Nick - I am not intimately familiar with the folded cascode topology, it is something I have been meaning to look into, thanks for the suggestion. Too late to change course on this amplifier though as the chassis and layout are finalized. The 841 biases at a high plate voltage, which is the primary reason for the B+. In my prototype, plate voltage is 375V with a 460V B+ which gets me right about to the 500mW power limitation of my Sowter 8665 OPT.
Should have the chassis for this amplifier any day now, I've attached some photos from the gentleman who machines them for me, powder coated in a metallic gray.
I also made a big purchase the other day, the etracer curve tracer. Attached are some A1 and A2 curves for the 841. Some interesting applications at a positive grid A2 bias point perhaps, although this amplifier is A1 only at a very modest bias point, the tubes should last a very long time.
I also made a big purchase the other day, the etracer curve tracer. Attached are some A1 and A2 curves for the 841. Some interesting applications at a positive grid A2 bias point perhaps, although this amplifier is A1 only at a very modest bias point, the tubes should last a very long time.
Attachments
I'm a little jealous of the curve tracer...
That 841 has really pretty curves. I run mine with a 10mA CCS load, about 400V on the plate, and about -2.3V bias. It swings about 440V pk-pk. Looks like a good zone to operate. Solid state driver supplies the necessary grid current.
Amp is beautiful so far by the way.
That 841 has really pretty curves. I run mine with a 10mA CCS load, about 400V on the plate, and about -2.3V bias. It swings about 440V pk-pk. Looks like a good zone to operate. Solid state driver supplies the necessary grid current.
Amp is beautiful so far by the way.
Hey thanks, SS! The etracer is a pretty cool little unit, but not cheap by any means :/ luckily I have been working overtime lately.
Wow 440Vpk-pk, need to go back to your blog and refresh my memory on how you are using them, they can swing some serious voltage, think I'll find another circuit for them some day. My little headphone amp is sort of an odd use I admit, but it sounds great. I have them CCS loaded as well, 6mA, 400V on the plate and roughly -3.75V on the grid. They'll swing something like 150Vpk-pk which is plenty to drive any headphone to ear-destroying volumes with the output transformers I'm using.
Here are just the A1 curves up to 15mA.
Wow 440Vpk-pk, need to go back to your blog and refresh my memory on how you are using them, they can swing some serious voltage, think I'll find another circuit for them some day. My little headphone amp is sort of an odd use I admit, but it sounds great. I have them CCS loaded as well, 6mA, 400V on the plate and roughly -3.75V on the grid. They'll swing something like 150Vpk-pk which is plenty to drive any headphone to ear-destroying volumes with the output transformers I'm using.
Here are just the A1 curves up to 15mA.
I'm using the 841 as a driver for a Unity-Coupled (50% cathode feedback) output stage. It's a really linear output stage but it's basically a 1/2 cathode follower so takes a lot of voltage to drive.
I did experiments with the few smaller tubes I could find that could take a 400V idle voltage. I compared 841 to 6SN7 and 6BL7. 841 had the lowest distortion but 6BL7 was pretty good. 6SN7 doesn't do all that great for that much swing. The lower distortion and bright filament of the 841 really sold it for me.
I was also considering some of those smaller transmitter tubes you are looking at, but I wanted to avoid exposed voltages if I could.
I did experiments with the few smaller tubes I could find that could take a 400V idle voltage. I compared 841 to 6SN7 and 6BL7. 841 had the lowest distortion but 6BL7 was pretty good. 6SN7 doesn't do all that great for that much swing. The lower distortion and bright filament of the 841 really sold it for me.
I was also considering some of those smaller transmitter tubes you are looking at, but I wanted to avoid exposed voltages if I could.
Finally making some headway on this amplifier. Prepped the chassis last night - sanded away powder coating overspray on the top plate and mating faces of the side panels to get a good ground connection.
Started mounting parts as well. My recessed socket design turned out well! Looks really nice with a pair of globe 841 in place. Should be able to finish the amp by this weekend.
Started mounting parts as well. My recessed socket design turned out well! Looks really nice with a pair of globe 841 in place. Should be able to finish the amp by this weekend.
I like the recessed mounting with plates under the chassis. I've got something similar planned when I make my 826 amp, so that I can have a hole in the chassis and different sockets that mount under the hole for different tubes. I also probably have to put a little fan under for extra airflow.
I really like the chassis and I might have to try Landfall out for my next project.
Where do you get those 4-pin sockets? Those look really nice and I will need some sockets for my 100THs.
I really like the chassis and I might have to try Landfall out for my next project.
Where do you get those 4-pin sockets? Those look really nice and I will need some sockets for my 100THs.
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