Marwatt Hiwatt channel frequecy response - help me understand
Posted: Tue 08/03/21 4:17 am
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.
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.