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loylo wrote:Did you convert the whole preamp to the mid 70's one, or just the resistive mixer on V2? I wonder if I'll change the resistor and caps on the cathode of V1 from 1K5+150µ to 2K2+47n...

I'd also like to know when you plug the OCD whether you put a bit of drive with the pedal or just boost the amp olumewise (clean boost)?

The only changes were:

1) undoing the gain mod (V2B was already unused due to this mod)
2) adding the 470k resistive mixer.

No components were removed or replaced. V1 is shared cathode with 1k5 + 150µF. V2A has a 1k cathode resistor, no bypass cap - same as the early '70s circuit.

I usually set the OCD as a clean boost (clock face references): volume 3:00, drive 7:00-8:00 (barely up), tone 12:00, LP setting. If I'm playing someplace where I can't crank the master volume I'll occasionally kick in slightly more boost/overdrive for leads from a Robin Trower Overdrive or Jekyll & Hyde pedal.

But if I'm able to wind the master way up it doesn't need any boost pedals, except perhaps for higher gain solo stuff. I can't imagine ever needing more gain than that.

I just had the mod done to my bulldog. The difference is substantial. There is some blocking still there but not near as bad. i find it's almost unoticeable now if I'm running some kind of OD infront of it. Super happy!!

Try as I might, I can't hear this issue with my stock '78.... I can dime the jumpered input channels and while it does get quite overdriven, I don't get that splatty, cutting out sound.

I haven't checked the component values, but the amp is a bit dark overall compared to my 4 hole clone, so maybe some of them had different values from the factory?

I thought I'd chip in my two cents here. I have a 78 505, Canadian modded which I have never been a super big fan of, mainly because I don't think the bright channel sounds as good as it should. So I took the mod out and switched V2 back to the stock late 70's preamp using both sides of V2, I also added in Mark's fix for the blocking distortion. It sounded really good, but I still wasn't super happy with how the bright channel sounded. So I used mikhailwatt's idea and converted V2 to the resistive mixer from the mid 70's pre-amp. Oh what a difference that made! Both channels now sounds amazing. It did clean the amp up quite a bit tho, if you are into alot of gain. I really have to crank the preamp to get it to drive, but it is worth it, the tone is so sweet. Props to mikhailwatt for the idea.

ARE YOU GUYS REALLY PLAYING YOUR HIWATTS ON 10!! are you guys married?? where ?when?,how? i got questions!!!!

Yes, I am married, and my wife doesn't like loud guitar noise. An attenuator (search here for discussions/opinions) is a useful compromise.

--mark

funny my wife did when we were dating , now not so much

RETSUB007 wrote:funny my wife did when we were dating , now not so much

Keeping with the PG-13 Rating of this board, I'll refrain from stating other examples where that might be a universal truth ...

Dave

Heyyy, I see what you did there, Dave!

I first read about Mark's de-fizz mod over a year ago and just now finally took the time to do it. I used two 1M ohm resistors. This amp is now much more articulate when pre-amp volumes are dimed. I expect to plug straight into this amp more often from now on rather than using pedals for drive.

De-fizzed '78 DR103 2-holer

Before:


After:

For what it's worth, all other things being equal, I like MW's fix (change to the early 70's resistive mixer) better. However, this mod is much easier to do.

I still haven't figured out why, say, 20% of the two-holers don't fizz. Any of you folks with stock ones that sound good, I'd love to see some gut shots of your amp.

--mark

I don't recal any 50 watt amps with the issue???? Is it just limited to DR103s?

My '78 DR504 doesn't do it. I will have to dig up some pics.

mhuss wrote:However, this mod is much easier to do.

Really? It's almost the same operation.

Dr.HIWATT wrote:I don't recal any 50 watt amps with the issue???? Is it just limited to DR103s?

My '78 DR504 doesn't do it. I will have to dig up some pics.

My '80 DR504 did it big time. But that was also with the gain mod still in place.

My '77 DR505 needs a de-mod. I'll check to see if it "fizzes."

mikhailwatt wrote:

mhuss wrote:However, this mod is much easier to do.

Really? It's almost the same operation.

I need to revisit your story, then. You don't change out the 100k->220k and 1k5->2k2 for V2?

--mark

mhuss wrote:I need to revisit your story, then. You don't change out the 100k->220k and 1k5->2k2 for V2?

Nope... V2a already has the same 100k plate/1k + 47n cathode as the "regular" early '70s circuit. I suppose you could swap them out if you wanted the S/N 2130 "Clean Machine" variant.

Speaking of #2130, I wonder how many "Clean Machines" were produced. All the 4-holer gut shots I've seen have 100k/1k + 47n @ V2.