Canadian Mod

[ServiceTech]

Guys - Does anyone have a schematic of the so called Canadian Mod? I'm also curious what year(s) this mod was performed and if there were any variations.

Thanks,

Ross

[OldSchoolDave]

http://hiwatt.org/ca_mod.html

Should tell you all you need.

Dave

[Dr.HI-TONE]

I am pretty sure it was a mid -late 1970's deal.
edit:
I just checked Mark's page on it and it is pretty conclusive.

[ServiceTech]

Thanks Guys, I'll try to reverse it on a schematic.

[therealjoeblow]

I have a question regarding the Canadian Mod, if done in conjunction with Mark's mod to address the Blocking Distortion:

I've read both of his pages and understand pretty well as *summarized* below (please correct me if I'm wrong):

1) The Canadian Importer Mod pulled the bright channel volume control's output off of V2 and daisy-chained it to the input jack for the normal channel, so this boosts the gain for the bright channel, right? And de-modding this simply involves moving the coax cable that connects to the bright volume center tap back to V2 pin 2 (and reconnecting the yellow wire back to pin 3, and jumpering pins 1 and 6 with blue wire for best tone!)

2) The Blocking Distortion mod can be done in conjunction with this de-mod, and instead of the normal volume and bright volume center-tap coax wires connecting directly to V2 pins 7 and 2 respectively, they should both be connected to pin 7 through 470k resistors, and the jumper from pins 1 and 6 should be removed (or not installed in the first place if doing all this at once)

I know I simplified Mark's detailed instructions for the 2 procedures, but that's not my real question, if I do this I can follow his instructions exactly.

My question is:

Can I leave the Canadian Importer Mod with the bright channel daisy chained to the normal channel input, and just add a single 470k resistor to the normal channel coax lead where it connects to V2 pin 7; and if so, will this get rid of the blocking distortion, but still leave the amp with slightly higher gain? Or is this an all-or-nothing deal, where to get rid of the blocking distortion I need to do exactly as per Mark's instructions with both the normal and bright channels.

Many thanks
TRJB

[mikhailwatt]

It's been awhile and I no longer have this stuff memorized... but I think this might be similar to what you're thinking:

What I did with my DR504 is convert it to a resistive mixer circuit (ala early-mid '70s):

1. Disconnect the shielded cable (and the flying resistor, if there is one) from the input jack.
2. Disconnect the shielded cable from V2 pin 7.
3. Install the 3-position terminal strip as pictured.
4. Install (2) 470k resistors as pictured (I used 1/2 watt, but 1 watters "look better"), one end at each respective terminal, the other ends landed together on V2 pin 7. The grounded terminal (the one attached to the chassis) is not used.
5. Terminate the two shielded cables, one at each respective terminal, as pictured.

[sorry - the pics have gone AWOL - imagination required]

The result is something like a mid-'70s Hylight but with a bit more gain due to the hotter preamp component values.

[therealjoeblow]

Yes, I understand that is the Canadian de-mod combined with the blocking distortion fix, all-in-one.

HOWEVER - my question is: If I do the blocking distortion fix for the normal channel (add the 470k resistor to the normal channel coax lead at V2), but leave the bright channel coax lead at the normal input where the Canadian importer moved it to, will this result in fixing the blocking distortion throughout, but still be even higher gain as the 2 channels are daisy-chained... or can I not do that?

[mhuss]

Are you experiencing bad blocking distortion currently? I wouldn't change anything unless there's a problem to fix.

However, yes, adding a large value series resistance (470k or even 1M) will reduce blocking distortion no matter what's happening with the upstream preamp.

--mark

[therealjoeblow]

Thanks Mark, I appreciate your help and advice.

Yes, I have a problem with blocking distortion when the normal and bright volumes are cranked. My amp is a 1979 2-input Hylight that was imported into Canada and also has the importer mod...

I know I'm going against the grain with contemplating mods, but I like to play some higher-gain stuff, and so would like some higher gain from my amp. And no, I don't want to sell it and buy a Marshall...

I think what I would like to do is one of the following, which I think are all pretty non-intrusive versions of one-another, but I could certainly use your advice on which you think would perform better:

Options:
1) Remove the Canadian mod, and add the 2x470k resistors to replicate the earlier resistive mixer, and use a Bad Monkey in front of the amp to push the gain; or

2) Leave the Canadian mod, but add the 470k resistors to the normal and bright pot outputs to address the blocking distortion (I'm not sure how this would sound as it was never part of any "standard" configuration); or

3) Do #1 above, but also at the same time ADD the hi-gain extra stage that is found in the early 80's Biacrown OL models by using the extra half of V2 that would become unused by implementing option 1 above. This looks pretty non-intrusive, it's just adding 2 resistors and a capacitor, all of which I can do right at the valve and easily remove and return to stock down the road if I don't like it.

As I said, I would very much appreciate your advice on what you think would work or sound best to get more gain out of the amp, but without any major modifications. I am leaning towards #3 personally, as this seems to be not really a radical "mod" but just implementation of an alternative factory configuration, and therefore, one would think is likely to sound good and achieve my goal (again, not to completely turn the amp into something it isn't).

Thanks
TRJB

[OldSchoolDave]

However, yes, adding a large value series resistance (470k or even 1M) will reduce blocking distortion no matter what's happening with the upstream preamp.

I used this technique to clear blocking distortion on my '80 SA112.

The only thing I might change on mark's documentation:
...would be to keep the "downstream" side of the resistors as short as possible.

Those leads tended to increase microphonic sensitivity in my amp. YMMV.

Dave

[therealjoeblow]

So finally, after *months* of mental labouring over what to do with my amp, I did what I posted as Option #3 above:

1) removed the "Canadian Mod"
2) added the 470k x 2 resistors (resistive mixer) for Normal and Bright channels
2) installed the extra 2 resistors and capacitor at V2 to add the "Biacrown OL" extra gain stage

Photos:

Code: Select all

http://postimg.org/image/5ozyggwtf

http://postimg.org/image/bs2a3rm2b

And the tone... In a word, FANTASTIC!!

My amp never sounded great before, it was always too tinny, trebbly and harsh for my liking. Even with pedals, everyone else said what a great pedal platform it should be, but everything just sounded like bees-in-a-box. I was never happy with this, so this morning, I figured, what the hay, lets give the mod a whirl, it's pretty unobtrusive, and if it doesn't sound better, I can just take it off and sell the Hiwatt and move on.

With the "OL" mod, now everything has smoothed out so much, and the harsh trebbly spike is gone. The drive from the amp itself, while not high gain by any stretch, is nice and crunchy, and with a Bad Monkey set with level high and drive off, it pushes to a real smooth classic rock grind. For higher gain stuff, I have a Carl Martin Plexitone, a Wampler Pinnacle Deluxe, and a Hardwire SC-2 Valve Distortion. Like I said, before the mod, none of these sounded very good, too trebbly and harsh and fizzy. Now they all sound awesome, just like they should when you hear other guys on Youtube getting awesome tones from them. (I'm using a 2x12 with Eminence Legend V128's FYI).

So now with a couple of hours of work this morning and <$10 in parts, I have the most awesome amp and tone combination that I always wanted, but never understood why this DR103 wouldn't provide. I still have the classic Hiwatt cleans, the Hiwatt crunch, and now 3 different variations of Marshall all the way from Plexi to JCM800 to Hotrodded 80's JCM tones.

Man, am I glad I finally got around to doing this. Last week I was actually writing up the ad to sell my amp and was going to look for something completely different, now I'm so pleased I'll likely never sell it and probably quit with the GAS for quite some time!

Cheers and thanks for all the input and advice!

The REAL Joe

[OldSchoolDave]

And the tone... In a word, FANTASTIC!!


Man, am I glad I finally got around to doing this. Last week I was actually writing up the ad to sell my amp and was going to look for something completely different, now I'm so pleased I'll likely never sell it and probably quit with the GAS for quite some time!

Glad to hear it! Enjoy your "new" HIWATT!

Dave

[therealjoeblow]

Having continued to play with the "OL" version of my DR-103 for a few weeks now, while it is vastly improved IMO from the pre-mod version, I'm still finding it a bit brighter than I would like, and without adding any radical additional mods, I think I understand from some research that by increasing the bypass cap value on V2a from the stock 47n I can shift the gain of that stage from acting primarily as a treble booster to being a bit more full-range, which should allow me to tame the highs a bit more, and also add a bit more lower midrange grunt?

Marshalls seem to use a 2k2 cathode resistors & 680n bypass cap with 100k plate load resistors on their gain stage valve; the Hiwatt uses a 1k cathode resistor with the 47n cap, and a 220k plate resistor.

Can anyone suggest an appropriate value to add to the 47n capacitor to shift the frequency in the gain stage a bit more to the lower midrange? I don't think that just adding a 680n is the right thing to do since the cathode and plate resistor values are significantly different.

Or is this on the wrong track?

Many thanks
TRJB

[mhuss]

Try .47 or 1.0 in parallel to see if you like it better.

--m

[therealjoeblow]

Thanks Mark, that's what I was thinking... I plugged the Marshall values into the online cathode bypass calculator at ampbooks.com, and then the Hiwatt values, and it looked like a total capacitance in the range of 1000n to 1500n would give me a frequency curve and gain very similar to the the Marshall curve, so that's what I figured I would try (that's what you meant by 1.0, right? 1.0 uF = 1000 nF?). But your expertise and advice as a backstop are always appreciated!

TRJB