[bcowen]

Found a much cooler vintage bakelite volume knob. The ones I purchased I worry are too small, might look / feel too dinky on the front panel of a 3.25" thick chassis.

The skirt of this one is 2" and I like the aesthetic 10/10 projected knob feel.

Love it! And those transformers look pretty sweet too. They're the outputs, I presume?

 

[L0rdGwyn]

Love it! And those transformers look pretty sweet too. They're the outputs, I presume?

Yessir! Output transformers, custom wound by Sowter for this design, 5K:8ohm single secondary rated for 60mA on the primary. I may bias at 55mA for tube longevity, we will see.

Here is the a 320V 55mA load line, around -3V bias on the grid. In a "typical" single ended class A1 design, the positive portion of grid swing is not accessible and the 0V grid line represents a barrier where clipping will occur due to grid current. However, if you supply the requested current, and the tube is made for A2 operation, you can drive the grid positive and increase output voltage swing. At saturation (maximum swing to the left), the grids will draw 15-20mA provided by a DC-coupled cathode follower. 801A in class A1 can do about 3W with a 600Vish B+. 801A in A2 can do 7W with a 320V B+ but this type of design is not easy, which is why I have been working on it for so long, and local NFB is a must (can also use a high primary impedance OPT, but that has its own problems, compromises, compromises...).

 

[Galapac]

Yessir! Output transformers, custom wound by Sowter for this design, 5K:8ohm single secondary rated for 60mA on the primary. I may bias at 55mA for tube longevity, we will see.

Here is the a 320V 55mA load line, around -3V bias on the grid. In a "typical" single ended class A1 design, the positive portion of grid swing is not accessible and the 0V grid line represents a barrier where clipping will occur due to grid current. However, if you supply the requested current, and the tube is made for A2 operation, you can drive the grid positive and increase output voltage swing. At saturation (maximum swing to the left), the grids will draw 15-20mA provided by a DC-coupled cathode follower. 801A in class A1 can do about 3W with a 600Vish B+. 801A in A2 can do 7W with a 320V B+ but this type of design is not easy, which is why I have been working on it for so long, and local NFB is a must (can also use a high primary impedance OPT, but that has its own problems, compromises, compromises...).

Now if only you could design your own tubes to "rectify" the compromises...

 

[L0rdGwyn]

I was taking some measurements of my 841 amplifier and came across an intermittent RF oscillation gremlin, better get rid of it. I snuffed it out, but had to add some components to the source follower boards. So they are getting an overhaul, here are the new ones.



Alright, that's it for PCB designing, now the wait.

 

[bcowen]

I was taking some measurements of my 841 amplifier and came across an intermittent RF oscillation gremlin, better get rid of it. I snuffed it out, but had to add some components to the source follower boards. So they are getting an overhaul, here are the new ones.



Alright, that's it for PCB designing, now the wait.

If you don't mind me asking, once you've drawn the board about how much does it cost to get one made (for a small one like this)?

 

[L0rdGwyn]

If you don't mind me asking, once you've drawn the board about how much does it cost to get one made (for a small one like this)?

Depends on if you go with HASL vs ENIG surface finish. For example, for a set of five of one of the boards above, going from HASL to ENIG the price jumps from $2.00 to $18.00. Either way though, since these are manufactured in China through JLCPCB, the real cost is the shipping!

The cost isn't huge, but designing and laying out the boards is time consuming.

 

[L0rdGwyn]

Here is a little overview of HASL vs ENIG.

https://www.ourpcb.com/hasl-vs-enig.html

 

[bcowen]

Here is a little overview of HASL vs ENIG.

https://www.ourpcb.com/hasl-vs-enig.html

Thanks!!

 

[L0rdGwyn]

I'm basically just messing with tube stuff all day since I don't have to work, very surprising I know.

Picked up yet another pair of 6A5G. Don't be fooled by the branding, these were only ever made by Sylvania and Visseaux (under Sylvania license). I have several pairs but never hurts to have one more right? Especially at a good price. Got this pair for $53 + shipping.



I'm just going to list off what I have on the way for the 801A amp, I can't wait, here are the parts from Hificollective and a few extras I haven't mentioned yet.

Jupiter copper foil coupling caps. There will be two coupling caps in this amplifier (at least as currently planned), there is an element of battery bias on the pentode gain stage (it's sort of a mixed bias situation), necessitating an input capacitor, then a coupling cap between the pentode and cathode follower, so 0.22uF and 0.1uF will be used.



Removeable anode cap cable for the EF37A / 6J7G - nickel-plated top caps (might seek out something fancier), Audio Note silver-plated RCA paired with Audio Note silver-plated RCA plugs. A removeable grid cap connection will be made using these and will connect to the chassis via RCA, if that makes sense.



KLE Innovations Naked Harmony binding posts. Thought these looked cool and wanted to try them out, work with the aesthetic I am going for.



TKD 2CP-2511 50K pot. Decided to go with a conventional potentiometer this time around as opposed to a stepped attenuator. Of course it will pair with my new bakelite knob from above



I will be RC decoupling the pentode stage from the 6BX7 cathode follower / 801A output, both the positive and negative supplies. For that, I will be using Clarity Cap ESA caps.



Amphenol RCA jacks. I am going to do something a bit unconventional and put them on the front of the chassis near the pot.



I will use my usual Switchcraft 152B headphone jack.



For the color, I want something vintage-ish, not flashy, not sleek, sort of boring lol contrasted by an eclectic array of parts.

One that I have had my eye on is a glossy "burnt brown" from Prismatic Powders. I know it won't please some, but I think it would work well with what I have in mind. Might get a small sample to spray at home so I can see it in real life before committing, still thinking this one over.



Another option might be a gloss gray (Edit: actually liking this idea more and more as I think about it...).



Part of the aesthetic inspiration is this line of Asano SET amplifiers.



That's all I can think of at the moment...should be an interesting build! Fingers crossed on the prototype.

 

[whirlwind]

Those 6A5G's look nice.

 

[L0rdGwyn]

I bought a pair of tubes I've be interested in having on hand for future experimentation, RFT EC360. Like the 6080 / 6AS7G, the EC360 is a low mu, low Rp regulator triode. However, it is a single triode unit as opposed to dual triode with a package similar to that of the EL34.



So let's trace some curves and see what we've got.



At the operating point listed in the datasheet, the numbers add up with a plate resistance of around 120ohm. However, at what I would consider to be a more likely operating point (say 125-150V, 125-150mA) plate resistance is around 200ohm. Could make for an interesting OTL headphone amplifier.

 

[bcowen]

I bought a pair of tubes I've be interested in having on hand for future experimentation, RFT EC360. Like the 6080 / 6AS7G, the EC360 is a low mu, low Rp regulator triode. However, it is a single triode unit as opposed to dual triode with a package similar to that of the EL34.



So let's trace some curves and see what we've got.



At the operating point listed in the datasheet, the numbers add up with a plate resistance of around 120ohm. However, at what I would consider to be a more likely operating point (say 125-150V, 125-150mA) plate resistance is around 200ohm. Could make for an interesting OTL headphone amplifier.

Pretty tubes! My German is a little rusty, but at least we speak the same language in Triode.

It looks like you can run this at 12.6v (with half the current draw) on the heaters as well?

 

[L0rdGwyn]

Pretty tubes! My German is a little rusty, but at least we speak the same language in Triode.

It looks like you can run this at 12.6v (with half the current draw) on the heaters as well?

Yup, similar to a 12AU7, 12AX7, etc. the heaters can be run in series or parallel for 12.6V or 6.3V, respectively.

 

[Zachik]

At the operating point listed in the datasheet, the numbers add up with a plate resistance of around 120ohm. However, at what I would consider to be a more likely operating point (say 125-150V, 125-150mA) plate resistance is around 200ohm.

I am curious: why do you say that 125V to 150V is a more likely operating point for this tube?

 

[L0rdGwyn]

I am curious: why do you say that 125V to 150V is a more likely operating point for this tube?

Operating point from the datasheet is 60V 200mA with -7V on the grid for the tube's intended use as a DC regulator. In an audio circuit as a cathode follower, the grid will need to swing the output signal from the driver, so -7V on the grid isn't going to work. If biased at -7V, the grid will only be able to swing max 14Vpk-pk before clipping, which will make for a pretty crummy amplifier! Needs to be biased at a point where the grid can swing the full peak-to-peak signal from the input tube.