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Well, #14872 has a TON of gain, especially on the bright channel with both volumes dimed (the normal volume is operational in this mode). And it definitely has the 2-input "spitty/splatty-ness" thing going on.

"Before" sound clip (sorry - slop ahead ): http://home.comcast.net/~going2spain/hi ... ianmod.mp3

My plan is to remove the Canadian import gain mod, then convert the V2 wiring to the earlier resistive mixer circuit, I guess taking Professor Huss' "2-input mod" a logical step further? This mod looks fairly straightforward and non-invasive, so we'll see how it goes.

"After" sound clip to be posted later...

A high-res "before de-modding" pic: http://home.comcast.net/~going2spain/hi ... ainmod.jpg

A couple items of note:
- The added 470k resistor at the input jack was omitted from this specimen.
- The V2 cathodes appear to have been wired opposite from the mhuss 2-input schematic - the 1k cathode resistor + 47nF bypass cap is disconnected, and the other 1k is connected to pin 8.

And... it's done. The über-gain is history, and no more splatty-ness. It's much more like a classic early-mid '70s HIWATT now.

The only issue I'm finding is turning up the normal volume when plugged into the bright channel introduces some hum. Hmmmm... I had the normal channel mixer input cable laying on the heater wires...

Haven't tried it with the inputs y-corded yet.

Resistive mixer mod (not quite as tidy as the Professor's de-fizz mod):


Overall pic w/ gain de-mod: http://home.comcast.net/~going2spain/hi ... 4demod.jpg

It only involved moving the two mixer input wires and adding two 470k resistors. The Canadian importer gain mod had already taken the extra half of V2 out of the picture and, as I mentioned before, the V2 cathode only had the 1k resistor, no bypass cap. I swiped the terminal strip that was used for the 120v-only mod (replaced with solder & heat shrink for now) and cut it down to suit the mixer mod.

"Post-mod" sound clip to follow...

Hi there mikhail,

Would you be able to post a post-demod sound clip?

I have a 1979 DR103 Canadian Import with the high-gain mod, and really, I can't say I dislike the sound, with both normal and bright set at 100% it has a real nice overdriven sound at normal practice volumes. Maybe it's a combination of the pickup that's driving it, but it doesn't sound fizzy as in your pre-demod sound clip. For comparison, though, I'd like to hear what the de-modded sound is like, in which case I might just go ahead and try it on mine to see if it makes the sound better or not.

Also, for the less-technically inclined (me), I'm assuming that your mod is different than M. Huss's at least that appears to be implied in your text? I've seen his detailed step-by-step at his website - could you explain exactly what's different about what you did - It looks virtually the same except Mark appears to have added the blue wire across the V2 pins, and it doesn't look like you did?

Many thanks
The REAL Joe

therealjoeblow wrote:Would you be able to post a post-demod sound clip?

could you explain exactly what's different about what you did

Welcome to the forum!

I'll see if I can find some time to record soon, but don't hold your breath.

Post-mod it sounds pretty much identical to an early-'70s DR504... Perhaps a hair more gain and slightly darker due to couple of different component values and the shared cathode on V1. Way less gain than it had before the de-mod, which is a good thing IMO.

(FWIW - the gain mod on mine didn't include the "flying" 470k resistor at the input jack, which may have contributed even more gain & fizz. The "before" clip was recorded with both volumes up full and the master up halfway, Smit's Classic Coils 8.6k Alnico 2 PAF-style bridge humbucker.)

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.

That's it. If you don't like it, it's really simple to undo.

Thanks for the add'l info.

Mine DR-103 looks exactly the same as your DR-504 with 1/2 of V2 completely disconnected, and the coax cable connected to the input jack (with the resistor that yours doesn't have). It does overdrive very easily at low volumes, but really doesn't have the fizzy sound as in either your's or Mark;'s sound clips. Strange.

However, mine also has the coiled yellow wire that yours doesn't appear to have, but that the ones in the photos on Mark's 'Canadian De-Mod' page and "Defizz" page have.

The differences that I see between what you did and what he did is that he reconnected both the coax cable from the input jack and the yellow wire to the 'currently disconnected' side of V2 to simply apply the Canadian De-Mod; and to carry further and apply the Defizz, he then disconnected both coax cables and reconnected each one to their respective side of V2 thru the resistor and jumpered across V2 with a blue piece of wire.

But you didn't connect a yellow wire (possibly it doesn't exist, or else you just ignored it and didn't show it in the picture; or maybe it's a 504 vs. 103 thing?? I don't know), and you connected both of the coax cables thru the new resistors to pin 7 (one side) of V2, leaving the other side still disconnected.

Is that right? And if so:

1) (again, for the non-technically inclined, sorry), why is yours all connected to V2 pin 7 but Mark's split to either side of V2 (and is it optional, you can do either/or); and is there any particular benefit of one versus the other?

and

2) what would I do with my yellow coiled wire?

Thanks once again!
The REAL Joe

The yellow (cathode) wire had already been removed when I got it. You can just leave it coiled up.

The original 2-input wiring (assuming no gain mod) used both halves (triodes) of V2 to mix the channels prior to the tone stack... in other words, the coax cable from each channel volume pot was connected directly to pins 2 and 7 respectively on V2.

Mark's de-fizz mod simply places a 470k (or 1M) resistor in series with each coax cable to reduce the blocking distortion (source of the fizz). Both halves of V2 are still used, and the plates are paralleled via the blue jumper.

This mod also places a 470k resistor in series with each coax cable, but the resistors are connected together at pin 7 (instead of pins 2 & 7) to create a resistive mixer. One half of V2 as a gain stage ahead of the tone stack - same as the early '70s circuit. The other half is unused (pins 1-3). This gives you the less gainy, classic HIWATT sound. It still breaks up nicely if you crank it... yes, I still owe you a sound clip

In case it helps, the preamp tube pinouts are:

Pin 1 = anode/plate
Pin 2 = grid (signal input)
Pin 3 = cathode
Pins 4-5 = heater
Pin 6 = anode/plate
Pin 7 = grid (signal input)
Pin 8 = cathode
Pin 9 = heater

mikhailwatt wrote:And... it's done. The über-gain is history, and no more splatty-ness. It's much more like a classic early-mid '70s HIWATT now.
The only issue I'm finding is turning up the normal volume when plugged into the bright channel introduces some hum. Hmmmm... I had the normal channel mixer input cable laying on the heater wires...

Thread revival!
Hi Mike
Did you resolve the hum issue? I just bought a totally unmolested '78 DR105, and would like to do this mod. Like yours mine didn't have the resistor at the input jack.
Since I'm a bass player I want this as clean as possible... if it gets closer to my 4-holer I'll be happy.

Thanks mate!
Cheers
Sean