Classic amp topologies made with submini tubes

[PRR]

> ...directly heated cathodes, the cathode follower would be really difficult to implement  ... So for the cathode follower stage I used a lower gain/ higher current stage, with a 20k plate resistor. Not sure if this is the best approach. What would you do?

I would do Anode Follower. Grounded cathode (filament battery), Plate-loaded, with ~~unity-gain NFB from plate (after coupling cap) to grid to source. Since real work happens, I would run it as Pentode (G2 to 45V), and looks like 47k plate load for best swing.

[thomasha]

Thank you PRR! I just tested a NFB 500k pot in the existing circuit and it sounded better, less hairy.
The output dropped a little bit, but that's because I have the voltage divider before the 80k grid stopper.
The cathode still was at -1V, but bypassed with the 100uF capacitor.

Is DC coupling a better option than AC? I don't think I can use a DC coupling scheme, because my grid is elevated to be at the same voltage as the cathode in the filament string, which is at around 6.25V at the negative side.

I imagine I could bypass the upper 470k resistor and have the full blast of the previous stage at the grid stopper. This way I could decrease the gain of the anode follower and still have a voltage swing big enough to achieve the full output of the power stage (65mW),  with it's grid at -6.25V (12.5V p-p for full power).

[thomasha]

Hi,
After some tests, the final result

1 - I moved the tonestack one stage forward, so that there is still a driver stage for the 5672. The cathode follower has no gain anyway, and I didn't want to add another 5678 to the circuit, 6 tubes seems enough.

2 - I added a clean switch, to bypass some stages and get a nice clean or overdrive with the gain maxed.

3 - The SMPS I used was based on the 555, with some minor changes, my layout for the MAX1771 was too noisy.

4 - Some stages have a bias trimpot. The bias is calculated between the negative side of the filament and the grid voltage. Since the filament voltaeg is fixed by the series string, I adjust the grid voltage, as in a fixed bias amplifier. Third stage is biased at almost 2.25V, close to cut-off, a in the mesa. The 5672 is biased at the same time, and has a cold bias, as specified in the datasheet.

Here is the board



I fixed the metal pin of the socket directly to the board. A slightly undersized hole keeps it in place.

Here, another board with an extra RC filter for the 70V.


I adjusted the trimpots before mounting it in the box. So I could also make some measurements.

The tonestack was monted in the box. Since I only had a PCB potentiometer for the mids I had to use a small board and solder it to the neighbours. I thought about doing a tonestack board, but there is no space for resistors or capacitors between the pots, so it makes no sence. This way I can still test different tonestacks


I had to sand the lid a little bit, so that it would fit


The etch was inspider by Homer and Scylla, the six headed monster. (six tubes ...)



The SMPS would not fit in the box, so I butchered it!


I had to cut it, and turn it 90 degrees, so that it would still fit.

The tubes were soldered to the sockets to make things easier


And the final result

[thomasha]

Here a not so short video, in case you're wondering how it sounds.

[turbobork]

Hey Thomas, your amps are inspiring! I was looking at the Bassman Micro project recently (https://robrobinette.com/Bassman_Micro.htm#Bassman_Micro_LTP_EF80), as the idea of building a baby bassman/plexi type amp is very appealing. The designer Rob has recently added an EF80 output stage version of his design to emulate pentode overdriven sounds.

I was wondering if I might ask you if you thought combining the preamp of the Bassman Micro with the output section of your high gain/5672 output design might be possible? Essentially I would like to build a Bassman/Plexi with an output of way under 1/2W, but I'm not concerned about small packaging! Cheers.

[printer2]

If not concerned with small packaging why not use regular tubes? Just use a low power output tube like a 6AK6 and run it at lower voltage?

[turbobork]

Yes I am open to any solution. 6AK6, EF80 or subminiature.

Part of my attraction to submini tubes was I suppose an expectation that there may be an option out there to run a pair of tubes in push pull closer to (or a bit past) their limits, and still have a nice low audio output. Rather than running larger tubes capable of a greater output colder to achieve similar audio output.

Ideally I'm looking to build a power amp section of 1/4W or less, perhaps 100mW is my ideal, and pair this with the Plexi style pre amp section from Rob Robinett's Bassman Micro.

[Marcos - Munky]

If you really want a pentode power amp, you can give it a try on the EF184 tube. It's cheap, and I believe you'll have a low output power from a pair of them in PP.