Fusing -- ST70

[dcriner]

The original Dyanaco design called for a 3-A, slo-blo fuse. I think the fuse rating was based on pre-amp power being supplied from the ST70 via the front sockets?

Call me neurotic, but I'm big on fusing my equipment at the minimum rating that will still avoid spurious fuse blowing. I've been running my clone ST70 with a 2-A, slo-blo fuse and, so far, so good.

I have also installed a CL-80 current inrush limiter which gives a soft start. Often, an inrush limiter will allow use of a fast-blo fuse -- which I may try.

Given the possibility of rectifier tube flash-over, I think a 2-A fuse may provide some good peace of mind. I recently had my 5AR4 rectifier tube flash over at turn on. It took maybe 3-5 seconds for the 3-A, slo-blo fuse to blow -- during which time there was a sizzling sound (probably coming from the tube) and the table lamp on the same 120-V circuit noticably dimmed. No harm done, but scarey.

Doug Criner

[TomMcNally]

I recently had a JJ Firecracker 5AR4 arcing and blowing the fuse in my 832A amp. I threw caution to the wind and put a 5 amp slow blow in, and guess what ... the tube fixed itself for awhile. The next time it arced inside, I tossed it in the trash and am trying a Sovtek now. I put a smaller fuse back in.

[EWBrown]

Just stay away from those "generic" Sino branded Chinese 5AR4s, they're barely up to sub-standard 5Y3GT capabilities... Especially avoid those marked "Made in Chian" and "Election Tube". I have one which has both mis-spellings, it serves just fine in a little whimpy 350VDC, 70 mA bench supply, built onto an 8X8 cake pan chassis. So far, this light duty service hasn't sent it into firecracker mode - yet. Yellow_Light_Colorz_PDT_07


I've had no J/J or Sovtek 5AR4 ST-70 failures, yet... Adding an inrush current limiter is definitely a good idea, and even more so with those heavy duty "Triode" PA060s, they're far more "ballzy" than the original Dynaco trannies, the ICL can act as an electrical "shock absorber" and help protect everything downstream from the power trannie. A 2A mp or 2 1/2 Amp slo-blo fuse should work well, 3A is more than is really needed.

A good rect tube plug-in substitute is to get an empty octal tube base (or take one off a dead "plinker") , and install a couple of (at least) 1200V, 6A HEXFREDs, connected to the appropriate pins. Anodes each to pins 4 and 6, and both cathodes to pin 8. Definitely use an ICL with this combo, to avoid the Big Power-on Slam Yellow_Light_Colorz_PDT_09

Yeah, the B+ is going to be a bit higher, so be sure that your electrolytics have sufficient voltage ratings...


/ed B in NH

[erichayes]

Hi All,

Back in my Watkins Johnson days, one of the most worn out manuals I had was the Littelfuse applications guide.

http://www.littelfuse.com/data/en/Data_ ... 12_318.pdf deals with the 3AG family of "normal" blow fuses we're most familiar with. I'm happy to say that I've finally literally forgotten more than most folks have ever known about fuses.

The Stereo 70 was always way overfused. Contrary to popular belief, a fuse's primary job is not to protect the circuitry; it's to prevent fire. It's not on the sheet, but a 3A slo blo can withstand a 15 amp overload for up to 5 seconds--a lot of serious damage can occur in that amount of time.

Typical sizing is 300% of operating current, not starting current. If a properly sized IRCL is used (75~100% of operating current, depending on what class and how loud for power amps), the fuse can be valued at 150 to 160%. Bear in mind that these values and formulae hold true only for legitimate 3AG or AGC fuses. If you're using Asian wannabes, all bets are off.

I made a useful discovery about IRCLs when I was designing the 1773, and Ed's mention of a solid state plug-in sub jolted my memory. I was experimenting with using a pair of 6AX5s for rectifiers, so I had a transformer wound to put out an extra 30 volts to make up for the Efwd loss I was going to incur. After the experiment was over--they worked--I wired the '5408s back in, along with a larger dropping resistor on the first leg of the filter. The amp sounded okay, but I'd lost the transient response in the low end that I'd had with the tubes and, especially, with the original solid state lash up. So I tried intentionally mis-sizing the IRCL so that it would never get up to its spec.'d hot resistance. That got my high voltage down where it needed to be and, because of the IRCL's thermal inertia, put the punch back in the low end. You guys using Hammond iron might try this instead of using resistors on the secondaries to lower your voltages.

[EWBrown]

Hmmm, I never figured that I would see the words "Watkins-Johnson" on this site... I use some of their vintage gear in my line of work (RS-125, 8617B, 340-A7, and lots more. Even have some WJ goodies at home, and I have used an RS-111 as an FM mono tuner, talk about overkill Yellow_Light_Colorz_PDT_02

Now for sonething completely different, and way OT:

Speaking of tune substitutions, years and decades ago, I had a military surplus Collins R-392 HF receiver which I snagged at a now long-gone purveyor of such goods (John J Meshna) for chump change. Actually I bought two at the time, and re-sold the second one, recouping mu expenses...

This receiver ran off 24-28VDC, and the B+ voltage for all tubes inside was the same low DC voltatge. The audio output tube was a weird dual pentode in an octal bottle, a 26A7. Back then, these were hard to find, so I designed a simple plug-in transistor amp replacement, which actually worked and sounded quite good. Years later, I sold the set to a fellow ham, who still has it, along with a T-195 transmitter of the same vintage. Nowadays, I wish I'd kept that radio, the going price is over %650 -750 for one in reasonably good shape. 26A7s are sometimes available through AES and others.

http://www.mif.pg.gda.pl/homepages/fran ... 2/26A7.gif

http://www.mif.pg.gda.pl/homepages/fran ... 26A7GT.pdf

(edit: a better, more informative data sheet for these little cuties) Yellow_Light_Colorz_PDT_02 Check out the minimalist circuit on page 6 :o


Another receiver which used this tube, was the ARN-6 radio direction finder, which made an extremely sensitive and selective AM broadcast band receiver, with a few simple mods. This used two of the 26A7s in a PPP output stage, and had amazingly good, low-powered (1W), audio (and fairly deep bass) with a simple OPT change, to one which had a 4 or 8 ohm secondary. This RX was another all 24-28VDC design.

Perhaps a new design, low-powered, low B+ voltage audio amp could be created using these and other LV tubes. They can run on 48VDC B+ just fine, get a whopping 500 mW out of a PP pair :o

/ed B in NH