Dave Reeve's design mistake!

[OldSchoolDave]

I really like it now. It's going to be my new "Daily Driver"

Whew! I'm really glad to hear that. After the cold solder joint loosened in shipping, I was getting a little nervous about our trade.

I was amazed at the difference the "correction" ("mod" has negative connotations) made and will be doing the same to my '77 DR105 as soon as I find some time (and a hot soldering iron).

Dave

[mhuss]

I'm curious, Doc, did you use the 470k resistors as we were discussing?

--mark

[Dr.HI-TONE]

Honestly I hadn't paid much attention to your earlier posts about the mod as I don't have any HIWATTs that might benefit from it. So later last Sunday afternoon, I looked at your site and it mentioned 470k and 1M, I used the 1m in David's amp.

VHR does plan to offer a kit for this mod, this week or next when I get back into the office..

Nice Work Mark!

PS.
I got a killer new Weller iron!
and 5/8" fuse holders

[mhuss]

I got a killer new Weller iron!

I'm glad to hear that, your old iron was pretty tired! Thanks again for finishing the job for DL.

--mark

[HIWATTuser1]

I'm curious-
Would I hear a difference in the 2 different resisitor values (470k and 1 meg) ? Gain, headroom ?

Please advise.

DL

[mhuss]

470K *might* be a taste less dark, and would allow for a slight bit of fizz.
1M was the best compromise (between fizz and losing brightness) that I ended up using in my original experiments.
There should be no perceptible difference in gain between these two values.

--mark

[OldSchoolDave]

"De-Fizz" Kits are now available on VHR, with both 470k and 1M resistors:

http://www.vintagehiwattrestorations.co ... ucts_id=63

Dave

[loylo]

I am a bit confused with this problem and the "De-fizz" correction.

On my stock '79 DR504, when the preamp volume is crancked, applying a 300mV sinusoïde at the input, I read -8,6VDC (0VDC from 0% of volume to 50%) at the grid of V2, and in the meantime, the cathod of V2 goes from -1,7VDC (volume at 0%) to -0,7VDC (volume at 100%). So it seems there is blocking distortion there, for sure.

Now for the correction, I put a 1M resistor as suggested by Mark, and I read -4,5VD on the grid of V2 when the volume is crancked. And the fizz is still significantly there.
I really don't know how to get rid of this blocking distortion. The correction doesn't work as well for me as it seems it does for you, fellow 2-inputs owners.

[mikhailwatt]

I really don't know how to get rid of this blocking distortion. The correction doesn't work as well for me as it seems it does for you, fellow 2-inputs owners.

Since we're already talking blasphemy here...

You could convert it to the earlier resistive mixer feeding V2A only. I did this on my 2-input DR504 and it sounds much more like a classic 4-input Hylight now.

viewtopic.php?f=2&t=288&p=2406#p2406

[mhuss]

+1

The 'de-fizz' mod is a bit of a band-aid, designed to help a bit via minimal, easily reversible changes. In my experience, it does sound a lot better, but as you've seen, it will still block when cranked to 10.

Converting over to the "half V2" circuit, using a conventional 470k resistor mixer would most likely sound better, and if you can believe mikhailwatt , it does.

--mark

[mikhailwatt]

if you can believe mikhailwatt , it does.

I'll put up a "post-correction" soundclip sometime, but for now let's put it this way... my corrected 2-input DR504 has been running happily >>in stereo<< with my DR103 clone - channel volumes dimed on both amps. No splatty fizz, just lots of HIWATT goodness.

Caveat: I'm guessing that you lose some gain with this mod, since you no longer have the parallel gain stages at V2. I like it just fine with the old 4-input gain structure but YMMV.

[loylo]

Thanks Mikhailwatt!
I've already seen your post and I was considering to do this conversion to mid 70's preamp as a next step if I couldn't find anything else. The problem is that I fear the loss of gain.
But I suppose I can use a clean boost with the mid 70's preamp to achieve this kind of sound: achiev this kind sound: http://sylvain.raulin.free.fr/Download/shinken5.mp3 (yes, it is a boosted hiwatt on the right channel of this clip, a JCM800 on the left! ), whereas I can't use the 2-input "as is" with a booster without aggravating the fizzyness.

And thanks to you, Mark! I understood the "de-fizz" mod was an improvement but not a "miracle" solution. The problem is that in my experience, the sound is merely the same with or without the mod, even when the volume is not cranked. That is why I was wondering if there wasn't another problem , besides the blocking distortion.

I'll try the conversion to the mid 70's preamp and let you know how it sounds.

[loylo]

Here are some news of my DR504.
Well, I tried the conversion to mid 70's preamp and the loss of gain I was fearing is real. With that less gain, there is barely no "fizz". But when I plug a clean boost, the sound is fizzy again, as fizzy as the unmoded late 70's preamp when it is cranked!
I am sorry about the dead link above, but it was proving that a Hiwatt can sound evil with such a drive that it couldn't fear any high gain amp. I really don't know the reason why I can't make my DR504 sound like that, why it sounds so bad.
The voltages I have measured seem to show me there is a problem of blocking distortion, but the cure doesn't seem to work. I am desesperate.

[mikhailwatt]

Something doesn't sound right there... bad preamp tube? Bad solder joint?

My "converted" DR504 has roughly the same amount of gain as my early '70s DR103 clone. It has a fair amount of breakup (using humbuckers) with the channel dimed and the master volume low, and it just gets better and crankier as you turn up the master. I demoed a new (old) HIWATT cab with it last night, cranked it nearly all the way up and I'm pretty sure I heard angels singing. Boosting it with an OCD can put it into high-gain territory, but it doesn't get spitty or fizzy at all.

I can't say for sure how much gain you lose with the V2 conversion - I did the Canadian gain de-mod at the same time, so I never did hear the original 2-input circuit... but I'm so happy with it now that I don't see a reason to backtrack.

[loylo]

No bad preamp tube, no cold solder joint, nothing. That's been many years since I periodicaly try to see what goes wrong in this damn amp.
I've also tried all new components (resistors and caps, not only the power supply caps) without any improvment, so I've put back the original resistors and mustard caps.
Recently I've read the DIY pages of our venerable Mark Huss, and after mesuring the voltage on V2, I first thought he had put the finger on the problem: blocking distortion. But now I doubt the problem lies only there.

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)?