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The two-input late 70s amps have a flaw, which I just noticed while de-Canadian-modding an amp for a customer tonight.

Basically, you have the first stage coupling caps going directly to the preamp volume controls, and the wiper of the vols going directly to the grids of the second stage amp/mixer.

When you peg either volume, it gets really fizzy. Turns out that you get grid rectification going and you end up with -15v on the grid -- almost total cutoff! When the volume is pegged, the coupling cap is connected directly to the grid.

This is the same phenomenon that causes the 18watt "buzz" problem.

I always liked the 470k resistive mixer better, now I know why.

To be fair to DR, I don't think he was planning for people to run his amps on '10'.

--mark

mhuss wrote: When you peg either volume, it gets really fizzy.

I've often described my '77 DR105 with that very word. Certain preamp tubes lessen the fizz, but it's always there to some extent.

So, what's the remedy?

Dave

Two resistors at the grids of the second stage, probably 100k to 220k. This will limit the grid current, preventing the blocking distortion. If I get permission, I'll fine tune this suggestion on the amp I have currently.

--mark

mhuss wrote:If I get permission, I'll fine tune this suggestion on the amp I have currently.

Keep us posted! (pix would be good, too)

Dave

Blasphemy!



This thread should be deleted for such ludicrous remarks!

I suppose some might view this as a "feature" rather than a "flaw".

mikhailwatt wrote:I suppose some might view this as a "feature" rather than a "flaw".

If you heard it, you definitely would not think so!

--mark

mhuss wrote:If you heard it, you definitely would not think so!

I'm certain I'd agree with you.

However, some Triple Rect-O-Fizzer types might think it's a good "feature".

I think I know that customer, great guy btw, he says "Go for it". Go forth with resistors and solder gun in hand in the name of Hiwatt wisdom. The fizz must be gone

All I had to do was 'drop the hint.'

--mark

During the convention, you can demonstrate on my amp how to solve this problem! I'm here for you guys.

I added the resistors, and I think it sounds much better. It sound perhaps just a taste darker, which is easily compensated for with the Presence control. I took pictures and sound clips, I'll post them when I get some time to sort them out...

--mark

Is it different tone and gain-wise than playing the OEM circuit at less than 10?

mikhailwatt wrote:Is it different tone and gain-wise than playing the OEM circuit at less than 10?

Tone is pretty close, perhaps a bit darker.
More gain cranked then possible, without blocking distortion, as per stock. Since these resistors are in series with the (much higher impedance) grid, they drop very little voltage even at 1M.

--mark

Mistake indeed.
Modification executed last week.
I will be running my "79 DR-103 Haloween amp with 2 SE-4122's next week for basic tracks.

I can't wait for the playbacks. I cooked it after the work was done(thanks Mark and Doc).

Just like a 4 holer, but punchier (sp?). I really like it now. It's going to be my new "Daily Driver"

Cheers,

DL