Any experience with the Mercury Magnetics HI100-P??

[Mike00]

Hi all, this is my first post.

Has anyone here used the Mercury Hiwatt/Partridge transformers? I just got their 100w PT (HI100-P) to replace a blown PT in a friend's '72 DR103. I'd like to know your thoughts on them... I'm in the middle of installing it now.

My one complaint so far is that even though it has the Patridge style mounting brackets, it's not a 'perfect' fit. I had to drill two extra holes. Keep that in mind if you're considering purchasing one. I didn't totally mind drilling the two holes because this particular amp already had extra holes drilled into it for the last (blown) PT; a Marshall JMP style. Still, that kinda sucked.

Thanks for any advice...

[mhuss]

No experience with Mercury's Hiwatt replacements; they're generally well-made but really pricey unless you buy in quantity.

The footprint not matching is a bit disappointing for a supposedly top-shelf brand.

--mark

[Mike00]

quick update... I was emailing Mark for his help (thanks again Mark!). I finally had time to work on this amp.

The MM Hiwatt PT has a 72v bias tap. I had to change the 1k bias supply resistor to 15k. With the 1k, the amp sounded broken... kinda like a distortion pedal with a near dead battery. The 15k made the amp work.

I just don't know what I'm doing with the 15k bias supply resistor. I chose 15k because a Marshall's bias (around 100v) uses a 27k. 15k seemed like a good choice with the 72v and it seems to be working... ??
Any Suggestions welcomed!

On a Marshall (with EL34s), the bias wire connects to a 27k resistor first, then to the diode. On a Hiwatt, the wire goes straight to the diode. Why is that?

I changed the 47k resistor to 33k to get the bias in the right range. (33ma - 530v) I'll add a trim pot later.

I'm also getting a loud buzz from the preamp, like there's a missing ground but everything appears to be grounded properly. ugh (noise disappears when V4 is removed).

[OldSchoolDave]

The MM Hiwatt PT has a 72v bias tap.

Sorry I don't have the knowledge to help you with this.

Only have to comment that, after all the glowing reviews MM gets for their transformers, I am surprised (and appalled) their advertised "Hiwatt" PT is not an identical replacement .

Best of luck in getting your amp running properly.

Dave

[JC103]

Quite the over-site on MM's part. Especially if their other models ARE drop in replacements.

[Mike00]

I'm at a loss here. No matter how I hook it up, it doesn't work. I've used various bias ckts and it doesn't work.
Like this one: http://hiwatt.org/DIY/bias.gif And this one: http://hiwatt.org/Layouts/DR103bias.png

With the the 1k resistor, the amp only farts bits of guitar signal out. With 4.7k and higher (I tried 10k and 15k), I get normal sounding guitar signal with tons of loud hum. Any value lower (like 2k-3.3k) still makes a broken farty signal. It's like something is not grounded... But all the connections are correct. I even installed a second MM hiwatt 100w PT transformer (I got them at cost) in my modified Sound City L100 and it does the same thing.

The bias taps put out 72 VAC. One side goes to ground and one side goes straight to the diode in the bias ckt. There is a center tap but I did not connect that.
I'm going crazy...

[Mike00]

Never mind. I was getting confused with a filament center tap verses a B+ center tap (or whatever it's called). Mercury put a warning label on the filament center tap so I assumed the worst. Never mind, nothing to see here.

[mhuss]

I hate when I do dumb stuff. Glad you got it sorted.

[Mike00]

No matter what, I can't get a 1k resistor to work in the bias supply. It just causes a short (only get a spotty sound when I play hard). I always see a 1k in there whether it's an old one or a DIY. I had to use a 4.7k and 10k with 25k trim pot. (Instead of 1k and 47k). The amp doesn't seem as loud as it should be. What am I doing wrong?