Honeytone Tube conversion using subminiature tubes

[blackcorvo]

Recently, I finished a project I've been work on and off over the last year. It's a subminiature tube amp fitted inside of a Honeytone mini amp enclosure. The original circuit board was busted beyond repair, and I had gotten some subminiature tubes to play with, so I decided to put them to good use.

Here's the final result:

https://www.youtube.com/watch?v=B79d7bydqr0

Check the video's description for the schematic, layout, pics, etc.

Or you can see those and more over here:

https://drive.google.com/open?id=1v7-ppCu_mGSNfZdqfjUrZtzhPhdUXQ48

[phatt]

Great stuff, sounds good
A lot of work for such a small amp.
Can I ask what the heat sink is doing?
Phil.

[blackcorvo]

The heat sink was originally on the switching transistor for the High-Voltage board, but since the circuit pulls only about 5 watts for the B+ rail, the thing doesn't even get warm enough to the touch (the board can handle loads up to 45 watts, so 5 is almost a joke for it). I removed it from the transistor because the board fit in the enclosure better that way.

I had to put it back in the new position to work as a shield for the 6N21B tube, because the switching noise from the transformer was being induced onto that tube. The shielding worked perfectly well, being grounded to the common of the circuit.

A better solution would be adding a piece of metal attached to the chassis, sitting between the tube and the High-Voltage board's transformer, that wrapped around the tube a bit, for proper isolation.
It works as a proof of concept, at the very least. But now that I know, I can avoid this issue on future versions of the project, should those come to be. Wink wink 

[blackcorvo]

Just a small update to this project.

To deal with the noise issue I mentioned on my last post, I tried many things but none of them worked. I ended up using a different DC-DC converter board, which is smaller, and that allowed me to place it further from the preamp tube and, thus, avoid inducing noise onto it.

The board in question is this, from eBay seller VFDClock: https://www.ebay.com/itm/141728332617
Still has plenty of power to feed this amp, no problems. I just needed to adjust the bias on the 5902, by increasing the cathode resistor from 330R to 1k. Works great!

[Coop]

blackcorvo,

Thanks for sharing the schematic.  I realize this is approaching a year old but given that only the schematic is available at the shared link might you describe the speaker impedance and output transformer that you used.  I just wanted to get an idea of the 5902 load-line.  The sound is very nice, BTW.

Thanks,
Coop

[blackcorvo]

The speaker I used is 8 Ohms, rated at 2.5 Watts. The output transformer was made to order from a brazillian transformer manufacturer (http://schatz.eng.br/), 3k:8R/4R, with bass response starting at 120Hz (due to the space I had available).
I have a switch on the speaker output to change from the 4R to the 8R tap, but looking back at it, I could probably fit 2x switching jacks in there and a larger O.T., no problem. Specially now that I got some 3" alnico speakers off eBay, with much smaller magnets.

I just need to find another fried Honeytone!