Marwatt Hiwatt channel frequecy response - help me understand

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kermito
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Marwatt Hiwatt channel frequecy response - help me understand

Post by kermito »

I built a Marwatt as per the schematic (only added vvr) about a year ago. I liked the idea as well as some sound clips I heard. I don't like the hiwatt channel -- not as in it isn't my cup of tea, but as in it is difficult to tune in and to get a good sound out of. The high mids and highs are overpowering in an unpleasant way.

This seems to be in line with some what others have reported on the marwatt, on this forum and elsewhere:
- too bright,
- either too much or too little gain as compared to the marshall channel (relates to the tube choice for the hiwatt channel),
- tone stack controls not responsive enough or don't respond as expected.

I think all of this can be traced to one design error in the schematic as published: the 680n cathode bypass caps in both gain stages, which causes a significantly higher gain in high mids and highs than in low mids and lows. Clearly, this makes the amp bright, but it also overpowers the tone stack (which has a similar response). But where does this come from? I am by no means an expert, but in all the hiwatt schematics I have seen, preamp gain stages are either fully bypassed, unbypassed, of have 47n cathode bypass caps (boosting 4k+ plus frequencies, i.e. adding presence, but not high mids). In other words, they add some presence, but are otherwise pretty much flat, as opposed to severely skewed as in the published marwatt design. Would someone care to explain the reasoning behind those 680n caps? Am I missing something?

I also wonder whether these caps have been retained in the production version. I am aware the designer, who hopefully will chime in himself, has said that his engineer has made some changes in the circuit in the production version, but I do not know what those might be.

Btw, I will tweak my amp until it works for me, I am not seeking input on mods. Rather, I would like to understand how anyone can liken this amp to a real hiwatt, when in fact the frequency response is decidedly different.
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Re: Marwatt Hiwatt channel frequecy response - help me understand

Post by JMPGuitars »

Which schematic version are you looking at? The one I see is 19 years old, and a little awkward. The Marshall section looks like the Lite II, but the Hi Lo inputs should say Single Parallel respectively, and parallel would not be lower.

I'm not familiar with the Hiwatt sound or anything, but that side of the circuit is a cascaded preamp tube. That can give some great higher gain sounds, but as the schematic says, have you tried a tube besides a 12AX7? It says 12AU7 as an option, but I'm sure you could try some other similar dual triodes and see how they sound. I've designed amps with cascaded preamps, and for me the issue was a combination of either too much bass, or too much gain that needed to be tamed before the next stage.

Either way, I'd love to hear a sample of the finished amp along with the mods you make to get it to sound right for you.

Thanks,
Josh
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kermito
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Re: Marwatt Hiwatt channel frequecy response - help me understand

Post by kermito »

Yes, that's the one. I'm not aware of any newer schematics. All docs here: viewtopic.php?f=25&t=24420
Others here and elsewhere have expressed praise for this schematic, so me not caring for it is a matter of opinion.


The designer is Jeff Swanson, who produced the amps under the DST Engineering brand I believe. He (username Spentmusic) has said in these forums to the fact that the production version differs from the published schematic (e.g. here: viewtopic.php?t=3103).

I was wondering if anyone could chime in on what *might* have been the reasoning behind the 680n cathode bypass caps, which differs fundamentally from any hiwatt design I'm aware of (whereas tone stack and coupling caps are the same/similar). It would also be interesting to know whether people have built it with the 680n caps and like it that way.
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Re: Marwatt Hiwatt channel frequecy response - help me understand

Post by JMPGuitars »

I don't have the answers to those questions, not sure if anybody currently active will or not. Some less-frequent posters might if you're patient. There are a TON of circuit variations on the hiwatt stuff though: https://hiwatt.org/tech2.html

Personally I would just go ahead and experiment and see what sounds good to you. Like you said, it's all very subjective.

Thanks,
Josh
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