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Beware, this post may offend some of you, hiwatt lovers, or even be considered as heretic by the Great Hiwatt Inquisitors!

To make a long story short, I have a '78 DR504 that is proned to sound fizzy and farty. I have tried to see what's going wrong with my amp since 2005 and explored different boards (vintageamps) asking what's going on, to Professor Huss among others! I've searched for bad tubes, bad solder joints, bad ground connection, parasitic oscillations, etc. But nothing had improved.

In the late 2008, Professor Huss came with his spitty mod, pointing the problem is blocking distortion, as I learned I am not the lonely guy facing farty sounds with Hiwatt. Actually, there is blocking distortion going on in my amp too. So I tried the spitty mod, and it has just slightly worked. It's a band-aid, I still hear farty sounds as I dime the volume.

I've also tried to convert the late 70's preamp configuration to the mid 70's one. It improves the sound futher more, but the level of gain of the amp decreased a lot. That's not what I want, I want to enjoy the classic hiwatt crunch sound. So I'd plugged a clean boost in front of the amp to match the level of gain of the late 70's circuitry, and I hear the exact same amount of blocking distortion going on.

I was totally fed up with that amp, so I've let it alone for one year and a half.
Now I see we have learned a little more. So people seem to face the same symptoms, others not:

mhuss wrote: 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.

I am surprised that some hiwatt fellas have the chance to own a two-holer that don't fizz. Whether they don't hear the problem (How could it be? It's so an unpleasant distortion, that you would know it's not a natural crunch!), or it's not a design mistake. I mean, if it's a design mistake, all the two-holers may face the same problem.

At first sight, I thought it's a design mistake because the first stage of the preamp has a lot of gain. And when the volume is dimed, it sends all the signal to the grid of the second stage tube, without any attenuation nor grid-stopper resistor. So because of the RC constant of time of the normal channel, it is aimed to face blocking distortion problem. I've tried to search solutions that are not only about applying a band-aid, like the two 470K (or even 1M) which doesn't totally cure the problem for me. It implies to radically mod the amp, witch sounds like blasphemy. My technical knowledge is far too limited to find a real solution. And I don't know if I have the guts to hack the amp!

So now, why 20% or so of the two-holers seem to not fizz?
I wonder if those who made the statement that they have a stock two-holer which don't fizz, have tried to peg the volumes, and so if it sounds perfectly correct. Same question to those who are plenty satisfied with the spitty mod, or the conversion to the four-holer type of circuit.
Can the four-holers be really really pushed hard, helped with a clean boost, without becoming spitty?

If so, does the problem really is about blocking distortion? Could it be another problem, like a layout mistake (like some of the connection under the preamp board) which makes most of the two-holers sound spitty (while for some reason some of them don't), or an electronic part, like the volume pot, which becomes defective over the years (while, say, some of the amps are built with long lasting pots)?

It would be great if we can resolve that mystery!
The hiwatt is cool for its clean sound. But I'd really like to enjoy the crunch. As is, it's like owning a Plexi and don't be able to make it crunch!

loylo wrote:I wonder if those who made the statement that they have a stock two-holer which don't fizz, have tried to peg the volumes, and so if it sounds perfectly correct. Same question to those who are plenty satisfied with the spitty mod, or the conversion to the four-holer type of circuit.
Can the four-holers be really really pushed hard, helped with a clean boost, without becoming spitty?

FWIW... My 1980 (PCB) DR504 2-holer (de-modded & converted to resistive mixer configuration) doesn't "spit" at all. Not even with the channels dimed and a clean boost in front, which is how I normally set it up. Believe me, I'd notice if it was the least bit "spitty." Great crunch & lead tones for days! My DR103 4-holer clone doesn't "spit" either when dimed and boosted.

Before I removed the importer gain mod and did the conversion it had loads of spitty blocking distortion when dimed. Not to mention insane amounts of gain.

I have a '77 DR505 2-holer that needs to be de-modded... this time I'll test it before doing the conversion.

There is no question that blocking distortion is going on in the stock two-input preamp, you can measure the grid voltage going several volts negative (when it should normally be zero volts).

I agree that the series grid resistors are not a 100% solution, as I could still hear a little spit post-mod, but it was very much improved.

I think the older (four input) circuit does a little better, since the "other channel" is connected to the grid and helps drain off the accumulated charge. This advantage goes away when you jumper the two channels.

Removing the cathode bypass cap on V2 (as in the "clean machine" early 70s amps) will get rid of the blocking distortion, but will also reduce the gain. You could make up this gain by putting that cap on V3 if there is not already one there, or by using a clean boost in front of the amp.

--mark