Battery powered radio tube guitar amp

sds1

Friend of Leo's
Joined
May 4, 2017
Posts
2,154
Age
46
Location
Orlando, FL, USA
You need a decent inductor though, low DCR
Cool, I didn't know about the importance of the inductor DCR. I have been buying a Coilcraft 100uH with 220mOhms max DCR so I'm excited to make a possible optimization there. Mostly I've just upgraded the MOSFET and diode to more robust parts. My first iteration failed after not too much on-time, not sure which component blew. But it used more baseline rated components and a 220uH (330mOhms) inductor which may have been too hard on the circuit? I dunno.

How are you load testing, you just lower resistance to ground on the output of the converter until you see the converter output sag? Maybe I can do before/after test on that inductor.
 

tubegeek

Friend of Leo's
Joined
Jan 31, 2020
Posts
3,882
Age
61
Location
Brooklyn, NY
Cool, I didn't know about the importance of the inductor DCR. I have been buying a Coilcraft 100uH with 220mOhms max DCR so I'm excited to make a possible optimization there. Mostly I've just upgraded the MOSFET and diode to more robust parts. My first iteration failed after not too much on-time, not sure which component blew. But it used more baseline rated components and a 220uH (330mOhms) inductor which may have been too hard on the circuit? I dunno.

How are you load testing, you just lower resistance to ground on the output of the converter until you see the converter output sag? Maybe I can do before/after test on that inductor.

Yeah that's exactly how I tested it, I started with maybe 20K (heavy wattage wirewound) and then went lower until it became too saggy.

The amperage rating on the inductor tells the tale - the Tayda is 2.4A spec and the DCR 91mOhm so much less lossy. The amperage rating is a fair substitute for a specified DCR if you are digging through unknown surplus bins. 1A good 2A better. It definitely improves the output behavior, it's the most critical part. Biggest too - this one does NOT let you get quite the postage stamp layout the smaller ones do but you know what they say: "Big iron!"

I think there is a lower loss diode that is fairly key too, come to think of it. UF4004 (fast recovery.)
 
Last edited:

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,640
Location
Canada
- if you needed more current - why not put two of them in? The power tubes can get one (each?) and certainly everything in front could be run off of one unit. I think I tested them and got 40 mA before they started to sag a little?

I was thinking of trying a pair of them together but think a ferrite inline with them might be needed to keep the two from messing with each other. The two feed your OT so hard to do one supply for each tube. The switching modules need higher voltage supply to put out more power.
 

supersonicobr

Tele-Meister
Joined
Jun 6, 2011
Posts
108
Location
Brazil
There's a Canadian guy on the DIYaudio forums, swears by the higher-power (typically 150W), high-frequency square-wave-AC output / 12VDC input converters sold on Ebay and Cousin Ali. They are non-adjustable so you'd have to choose the output voltage by hardwiring into the circuit board and because of the high freq two output AC output we'd have to use a HF (UF4007 or higher amp rating) diode bridge for rectification. I bought maybe three of different form factors, but haven't actually sat down and tried to build something with them. (and it's not for lack of extra time with nought else to do either)
 

tubegeek

Friend of Leo's
Joined
Jan 31, 2020
Posts
3,882
Age
61
Location
Brooklyn, NY
Here are a couple of pics of a tried and true layout, I hope they are helpful. The diode is a UF4004 or equivalent. You want a "fast recovery" type.

I'd say the one at the top of the group photo was one row too few.

20200612_181845.jpg
20200612_181830.jpg
20200612_181759.jpg
 

tubegeek

Friend of Leo's
Joined
Jan 31, 2020
Posts
3,882
Age
61
Location
Brooklyn, NY
I was thinking of trying a pair of them together but think a ferrite inline with them might be needed to keep the two from messing with each other.
Makes sense, or a 10 ohm mixing resistor each and just live with the voltage drop.

The two feed your OT so hard to do one supply for each tube.

Which idiot suggested THAT?

(Oops.)

I'll see if I can test the DIY ones soon and report back. But they definitely do way better than 5 or 6 mA I promise you.
 

radiocaster

Poster Extraordinaire
Joined
Aug 18, 2015
Posts
9,975
Location
europe
There were actually a few tubes meant to work with a 12V plate voltage or thereabout, but they're not that easy to find.
 

sds1

Friend of Leo's
Joined
May 4, 2017
Posts
2,154
Age
46
Location
Orlando, FL, USA
Here are a couple of pics of a tried and true layout, I hope they are helpful. The diode is a UF4004 or equivalent. You want a "fast recovery" type.
Looks like I ended up on UF4004 as well. After I did the first one on protoboard and it blew up, I was like to hell with these protoboards I'm going to China on this:

OamSOCz.png
 

supersonicobr

Tele-Meister
Joined
Jun 6, 2011
Posts
108
Location
Brazil
for reference on what I posted above - this is the best featured one I've purchased

https://www.aliexpress.com/item/32862270409.html

DC 12V in, AC 110/200/220/280V selectable high freq square wave output.

comes with a pigtail for switching on and off - would be equivalent to your HT standby switch, you plug the DC12v source in, switch that on for filaments, then this

also comes with a diode bridged but unfiltered DC output (the four diodes between the green DC and blue AC output terminals). I can't remember the diodes that are used but I looked them up and they are fast recovery and rated for 2A, maybe more.

and finally, jumpered voltage output selection, so no hardwiring required as you could use terminals to tie the HV circuit (filter caps, droppers) up to the green screw connectors.

I haven't tested mine under load yet but I got a quad of 6BM8s burning a hole in my tube box as well as an appropriate push-pull output transformer - I need only be bothered to stake some turrets onto a board and raid the components drawer to run some tests, but going by what the guy I mentioned above says, long as you get enough filtering downstream, these and the higher powered ones should work a charm.
 

Puguglybonehead

TDPRI Member
Joined
Jan 30, 2011
Posts
4
Location
Toronto, Canada
Any more progress with this? Sound samples? Really curious what these beach radio tubes actually sound like.

I was going to try something similar using the schematic from this article:


I was going to try it with a quad of 3S4's at the output and a B+ of 45 volts so I could safely do it off batteries. The idea of using 90 volts outdoors scares me a bit.
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,640
Location
Canada
Any more progress with this? Sound samples? Really curious what these beach radio tubes actually sound like.

I was going to try something similar using the schematic from this article:


I was going to try it with a quad of 3S4's at the output and a B+ of 45 volts so I could safely do it off batteries. The idea of using 90 volts outdoors scares me a bit.
With the amp being a battery powered device the 45 or 90V does not matter too much. It is not like the voltage is referenced to ground. The only danger would be getting dc out to the guitar and touching the chassis of the amp. Put a capacitor on the input and even if the circuit malfunctions the capacitor will keep the dc out of the guitar. Of course it would have to be rated for the voltage.
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,640
Location
Canada
Any more progress with this? Sound samples? Really curious what these beach radio tubes actually sound like.

I was going to try something similar using the schematic from this article:


I was going to try it with a quad of 3S4's at the output and a B+ of 45 volts so I could safely do it off batteries. The idea of using 90 volts outdoors scares me a bit.
And you might be interested in a 1W tube that was designed for portable use after 6.3V became standard, 6AK6. Then there are some people that have used 6AU6 for less power.
 




Top