DR103 fuse values (slight confusion)

HIWATT amps from the Hylight Electronics era

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DR103 fuse values (slight confusion)

Postby therealjoeblow » Thu Oct 15, 2015 4:24 pm

I have a 1979 2-input DR103, Canadian Import. Back in '79, the distributor removed the voltage selector and hardwired it internally with a jumper across the selector pins for the 117v option, applied the orange "Ontario Hydro" approval sticker, and wrote "375W" on the back near the fuse holders, and "4A" for the mains fuse and "1A" for the HT fuse; which is what it has in it (4A and 1A fuses).

I have never really cranked this amp up really loud despite owning it for 30+ years (since I was a teenager!). This time last year I took it out of retirement, played it at bedroom levels for several weeks with no issue whatsoever, but when I cranked it up it blew the HT fuse. I replaced that, and it worked again, but based on input from Mark Huss and others here, I ended up getting the cap's all replaced by a professional tech, along with a new set of JJ EL34's and a bias job because it hadn't been used in 20ish years.

I figured that would cure the fuse blowing problem, but yesterday I cranked it to about 1/4 volume (which is deafening loud!), and it worked for about 30 seconds at that level while playing some power-chord riffs, but after a bit, the HT fuse (1A) blew. I replaced it, and lowered the volume to about 1/5th, ie, still much more than bedroom level, but not quite as deafening, and it worked fine at that level, but when I cranked it another notch back to deafening level, the HT fuse blew again.

Now here's the question - are these fuse values even correct? Based on review of the info at Mark's web page, and looking at the 1978-1979 catalog that I found a copy of online, and other references here and on the vintageamps board, I thought that all of the DR504's were supposed to have 3A mains (for 220v) and 1A HT's and the DR103's were 3A mains @ 220v with 3A HT's (and that for 120v operation the HT's stay the same but the mains are 2x). So for mine I should be using 6A mains and 3A HT.

But my confusion comes from Googling "Hiwatt DR103 fuse values" and reading the info in the following threads:

1) http://vintageamps.com/plexiboard/viewt ... =3&t=35642

...last post at the bottom from Mark Huss Wed Jun 29, 2005 5:16 pm says "1A for HT worldwide."

but then:

2) http://vintageamps.com/plexiboard/viewt ... =4&t=12616

... second post, from Mark Thu Aug 26, 2004 1:53 pm (predates the above post), says "My theory is 3A HT, 3A@240v/6A@120v mains, both slow-blowing."

3) and finally, all of the reference info posted on Mark's site points to 3A@220v and 3A HT.

This is where my confusion lies...

-Why did the Canadian importer mark 4A/1A - did he possibly mistake the DR103 for a DR504, which I assume from the reference material available should use a 1A HT fuse, and simply mark mine incorrectly?

-Why did Mark Huss say in 2004 that the DR103 should use a 3A HT fuse and then in 2005 say it should be 1A, and then more currently host reference material that says 3A?

-Is there anything different about the late-70's DR103 that would make it use a 1A HT Fuse? For info, mine has the original Partridge #TH7548 power transformer and the Partridge #TH7549 output transformer if that makes any difference? These numbers are stamped on my transformers and appear to match the part numbers shown on the schematics at Mark's site that show 3A HT fuses.

I suspect that I have the wrong fuses in here and that the values originally marked by the Canadian importer are simply wrong, and that's why I keep blowing the HT fuse, but before upping it by a factor of 3, I would feel more comfortable if some of the experts concurred that 6A mains @ 120v and 3A HT are the correct values for all of the Hylight DR103's regardless of year of construction.

Many thanks

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Re: DR103 fuse values (slight confusion)

Postby mhuss » Thu Oct 15, 2015 5:41 pm

1A HT is way more than enough for a 50 or 100. The actual current (when cranked) is more on the the order of 250-300mA.
3A is good enough for a 100, again the normal consumed current is way less (except a surge at switch on when the heaters are cold and the caps are discharged).

I suspect there's a problem with your amp if the 1A HT fuse keeps blowing as you describe. Try removing the two outside power tubes and repeat the experiment. If that works OK replace the outer two, remove the inner two and try again.


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Re: DR103 fuse values (slight confusion)

Postby OldSchoolDave » Fri Oct 16, 2015 1:32 pm

mhuss wrote:I suspect there's a problem with your amp if the 1A HT fuse keeps blowing as you describe. Try removing the two outside power tubes and repeat the experiment. If that works OK replace the outer two, remove the inner two and try again.

In addition to the above, I'd suggest removing the impedance selector plug and making sure the contacts are not oxided (or loose). A short there can also pop the HT fuse.

...Hey! I played that song when it was new...


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