Merrell SA-30, National Kits STA-24, Radio Shack R90LX066

After years of miserly waiting, I have acquired my first tube integrated amp from ebay, (Since the one who shall remain nameless) threw away my "The Fisher 400" Receiver while I was at college in the 80's. This one arrived Friday 10-19-2013

It was listed as an "unknown amp" I did some sleuthing and found that it was either a Radio shack, National Radio, or a Merrell kit amp.
We'll call it a Merrell SA-30 for now.

Auction pics

 Gratuitous glowing artsy tube picture.

I went to my local surplus place and found the ONLY 5u4gb they had. I paid $6 for it. I had one 6v6, and scrounged 3 more from my Newcomb B-100 Radios. I got 2 - 12at7a's from the B&K television analyst I parted out. I got 1 - 12ax7a from the same B&K and I had another marked Silvertone. At the suggestion of Tube Radio (an Antique Radios Forum member), I also replaced all the ceramic capacitors with other non ceramic caps. It is up and running and I am playing my MP3 player through it now. It needs some tweaking and maybe some matched tubes.

Things I found wrong with it. There is a large 25 watt? 200ohm resistor that (whoever wired it originally) had used WAY too thin wire. The wire was scorched on either side of the resistor. I replaced it with some larger wire. There is a fluttering/static noise when the selector is on Lo Phone/Tape. The Lo Phono connection has more gain than the other inputs. This means it works real well for my Mp3 player. I can drop the volume on the player down to 50% when hooked up there.

In the picture above of the back of the amp, there are 4 connections for each speaker. I determined that the far right connection is common, the other connections are from left to right. 4ohm, 8ohm, 16ohm. I input a sign wave in to the unit and noted that the amplitude of the wave increased as I went from right to left. The 2 RCA connections below the left speaker connectors. They are not Left and Right Tape out.

The unit supposedly does NARTB equalizing on the phono inputs (I cannot tell).

 After about 2 hours listening the Power transformer gets to be about 140°f.

I need to:
get matched tubes for it, though it plays pretty good with the hodge podge that I have in it.
make a cage/cover for it, it had one at some point and there are screw holes for one.
Make a wood base for it.
Properly label everything on the back and remove all the marking pen marks on it.

The filament voltage for the 5U4 on this this amp was over 7 volts A/C, it should be 5. I did some research and calculations that to drop 7.16 volts to 5 volts at 3 amps would take about .76 ohms 6.5 watts. I knew from past experience that it would probably not be that exact a resistor. Most calculators I found said it would take a 1 ohm 6.5 watt resistor.

I picked up 2 - 1 ohm 10 watt resistors. I found that 1 - 1 ohm resistor dropped the voltage down to about 3.5 volts. I tested the 2 resistors in parallel for .5 ohms. That brought the voltage down to 5.07 volts :) They do get pretty hot.

For now they are mounted sub chassis on a standoff in free air.

For anyone that is interested here is the Schematic.
Right click and open in a new tab or window. Then save the jpg. The notations on the drawing are not mine. The Filter Capacitors are not marked on the schematic and I do not know the correct values. However my amp had 40uf 350v and 20uf 350v multi section caps installed. I would replace them on both channels with 40uf 450v electrolytic caps. There is also a 200 ohm 25watt resistor coming off of the 5u4 pin 8, and before the first filter cap. I am thinking that someone put that in there to limit the voltage because without it the screen grid voltages on the 6V6 tubes is 32 volts to high.

Here are the original catalog ads for the amp.
Here is the schematic after I changed it to match my amp and making the mods that improve bass.


TheLazyComic said...

I've owned the National version of this amp. I replaced the caps and power tubes right after I bought it, and that was in October of 2011. It sounds great, I use it constantly, sometimes it's on for days at a time, straight. If you want to help it out thought, aim a small computer fan onto the transformer to blow on it to keep it cool. My transformer was getting kind of hot. I put the fan on it 5 years ago, keeps it nice and cool. I'm playing it right now. Randy McDaniels, TLC.

Unknown said...

I just completed a total overhaul of this same amp (National STA-24). Including new Genelex tubes, all components replaced except for a handful of ceramic caps that still tested in spec. Both preamp sockets were noisy/had broken filament connections and replaced as well. I followed you schematic, mostly. Voltages a little high but not out of spec for Genelex; they are rated a little higher for plate and screen voltages. I added parallel .1 ohm resistors to filament circuit to drop filament voltage to 6.1 volts from 6.6 which I felt was slightly on the high side for the Genelex tubes rated at 6 volts. I am a triode man listening to a 300B amp mostly but I find the ultralinear push pull output of this little amp very warm and inviting - this has been a labor of love as it took me over a year to complete during these trying times we live in. Thank you for the schematic as there is not a lot of documentation out there. Regards Kevin