Mark wrote: ↑Fri Aug 16, 2024 4:24 am
No I haven’t, I can try it but the amp sounded buzzy before I put it in. In its sitting state it is biased off, the link discusses the theory behind this mod.
I suggest you read it again. It says without the diode you get more even order distortion that warms up the sound and reduces fizz. ...
From Merlin's DCCF page -
the added resistors and diode are protections components and do not affect tonality
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Mark wrote: ↑Fri Aug 16, 2024 4:24 am
No I haven’t, I can try it but the amp sounded buzzy before I put it in. In its sitting state it is biased off, the link discusses the theory behind this mod.
I suggest you read it again. It says without the diode you get more even order distortion that warms up the sound and reduces fizz. ...
From Merlin's DCCF page -
the added resistors and diode are protections components and do not affect tonality
yes merlin's circuit here works well. I've never had any problem with it although I use a 47k, not a 10k resistor. I'm suspecting that metal oxide resistors are causing unpleasant distortion here
LOUDthud wrote: ↑Fri Aug 16, 2024 11:38 am
Where did the unusual values in the tone stack come from ? Do they possibly emphasize the buzzy distortion ?
seems like a standard marshall 1987/2204/1959/2203 tonestack, which works well usually
LOUDthud wrote: ↑Fri Aug 16, 2024 11:38 am
Where did the unusual values in the tone stack come from ? Do they possibly emphasize the buzzy distortion ?
seems like a standard marshall 1987/2204/1959/2203 tonestack, which works well usually
Sorry, I was looking at the ax84 schematic with 100K slope resistor and 0.015 bass and mid caps.
LOUDthud wrote: ↑Fri Aug 16, 2024 11:38 am
Where did the unusual values in the tone stack come from ? Do they possibly emphasize the buzzy distortion ?
I removed the tone stack from the circuit and the amp sounds even more buzzy than before. The tone stack values are typical Marshall values, 33K slope resistor, and 470pF, and 22nF caps. They are a departure from the P1 circuit, I assume the original builder was after more Marshall like response from the amp.
As far as I can see the first stage remains undistorted and I suspect the bad distortion is coming from the second stage. I will try changing out the metal oxide resistors. They are cheap to replace and I have the components.
LOUDthud wrote: ↑Fri Aug 16, 2024 7:03 am
I suggest you read it again. It says without the diode you get more even order distortion that warms up the sound and reduces fizz. ...
From Merlin's DCCF page -
the added resistors and diode are protections components and do not affect tonality
yes merlin's circuit here works well. I've never had any problem with it although I use a 47k, not a 10k resistor. I'm suspecting that metal oxide resistors are causing unpleasant distortion here
Why do you use 47K instead of 10K suggested by Merlin?
Having spent a fair amount of time with circuits like these, I would guess you've got some blocking distortion. You're pushing a ton of signal directly into the power tube. A normal JCM800 style circuit has a phase inverter and some other coupling circuits (coupling caps, voltage divider, master volume, etc etc) that compress/tame the signal before hitting the power tube. Also, there is no negative feedback in this amp. You can tame some of the harsh frizzes by installing a Fender Champ style NFB circuit. That alone will go a long way. A master volume might help as well.
Another thing before heading out the door... The original circuit is an anode fed tone stack. You've added a cathode follower, which prevents some of the signal loss introduced by the tone stack. In other words, you've made the signal hotter than the stock circuit was designed for as well. The power tube is just getting slammed. A couple years ago, I developed a mod for a Jet City Custom 5, which is a JCM800 style front end into a single ended power section. Very similar to this. The only way I got it to sound good and not blatty/shitty was to add negative feedback.
Here is a clip. Note, I also added a ton more gain as well. Mindless noodling alert.
Gainzilla wrote: ↑Sat Aug 17, 2024 3:00 am
Having spent a fair amount of time with circuits like these, I would guess you've got some blocking distortion. You're pushing a ton of signal directly into the power tube. A normal JCM800 style circuit has a phase inverter and some other coupling circuits (coupling caps, voltage divider, master volume, etc etc) that compress/tame the signal before hitting the power tube. Also, there is no negative feedback in this amp. You can tame some of the harsh frizzes by installing a Fender Champ style NFB circuit. That alone will go a long way. A master volume might help as well.
Another thing before heading out the door... The original circuit is an anode fed tone stack. You've added a cathode follower, which prevents some of the signal loss introduced by the tone stack. In other words, you've made the signal hotter than the stock circuit was designed for as well. The power tube is just getting slammed. A couple years ago, I developed a mod for a Jet City Custom 5, which is a JCM800 style front end into a single ended power section. Very similar to this. The only way I got it to sound good and not blatty/shitty was to add negative feedback.
Here is a clip. Note, I also added a ton more gain as well. Mindless noodling alert.
Thanks for your reply, but the distortion is at its worst when the master volume is set low, if I turn the master volume up, the signal becomes compressed, the bass and lower mids become more dominant.
The hand drawn circuit diagram is the actual circuit implemented. The version 11 diagram shows where it deviated from.
As far as I can see the problem is in the second stage, but that’s just a hunch.
You know, Mark, I think I tried that follower arrangement as per Merlin's website. There was something weird about it that I felt negatively influenced the tone. I pulled it out after only about 5mins in favor of a standard CF arrangement, and all was well. Apologies if this was discussed already, but looking at your schematic, the only thing that looks odd is that follower. I guess my thought is nearly every amp I've ever seen with a cathode follower uses the standard. I don't think I've ever seen one of the Merlin mods anywhere but on his site. Maybe try the standard way and see if it helps. Again, sorry for not following the thread and potentially repeating other's comments. I have no excuse. lol
Oooh one more thought. Did you check for leaky caps in the tone stack? I didn't see if you had a coupling cap between the master volume and the power tube, but if there is DC getting through it could cause some ugly noise.
pdf64 wrote: ↑Fri Aug 16, 2024 9:30 am
From Merlin's DCCF page -
yes merlin's circuit here works well. I've never had any problem with it although I use a 47k, not a 10k resistor. I'm suspecting that metal oxide resistors are causing unpleasant distortion here
Why do you use 47K instead of 10K suggested by Merlin?
Just curious.
Merlin himself suggested 47k to be sure the diode doesnt affect sound at full distortion. I think the value is found is his book
I did replace the metal oxide resistors and didn’t hear and difference. I replaced the 1.5K cathode resistor and 1uF cap with the typical 2.7K and 0.68uF and it reduced gain but it didn’t do too much.
Gainzilla wrote: ↑Sat Aug 17, 2024 5:16 am
Oooh one more thought. Did you check for leaky caps in the tone stack? I didn't see if you had a coupling cap between the master volume and the power tube, but if there is DC getting through it could cause some ugly noise.
Thanks for your input. I did have a situation where there was a small amount of DC voltage on the second triode and I thought that was a leaky cap but it was the low rail causing the mV DC voltage.
I’m thinking of putting a 330pF across the plate to cathode on the first stage. I heard it limits the top end of the signal to 10Khz.
