Dave Reeve's design mistake!

[mikhailwatt]

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.

[RM112]

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!!

[Tripower455]

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?

[lffish133]

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.

[RETSUB007]

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

[mhuss]

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

[RETSUB007]

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

[OldSchoolDave]

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

[Bryce]

Heyyy, I see what you did there, Dave!

[Zells]

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:

[mhuss]

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

[Dr.HI-TONE]

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.

[mikhailwatt]

However, this mod is much easier to do.

Really? It's almost the same operation.

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."

[mhuss]

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

[mikhailwatt]

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.