NAD - thoughts on the circuit??

[fusedbrain]

So I just picked up a new amp. It's a 100watt Marshall style single channel amp. It's well built on a turret board, with high quality iron, and I paid less than the cost of the parts. ( which is why I bought it in the first place )
It kinda sounds alright I guess, but I think it could sound a lot better with a few "adjustments", and just wondered if you guys had any thoughts on this particular circuit?

First of all, the amp was stupid loud. The master vol was like an on/ off switch, so I took out the linear pot and replaced it with a log pot and did the Friedman master vol thing with the 220k and 1M resistors. Much fuller sounding at lower volumes, and much more controllable now!!!

So I still have a couple things I'd like to address:
1) the amp has a TON of bottom end, and the gain seems a little fuzzy. I'm thinking changing 1 or both of the first 2 coupling caps to .0022uf from the .022uf may help. I've found in the past that too much bass too early in a higher gain amp can make the amp sound like a Big Muff. I'm also wondering about the lack of any grid stoppers on the last 3 gain stages?

2) The total amount of gain available is just stupid. Nothing past about 4 on the gain pot is usable. I think the gain pot is also a linear pot, so I'm going to change that to audio taper and see if I get a bit more usable sweep.
I'm also looking at the 68k / 680k voltage divider after the first gain stage, and wondering if I should dump more signal to ground here.

The preamp scheme is attached below. The rest of the amp is typical Marshall PI / power section. 500V on the plates and is loaded with a quad of super fresh and matched =C= el34's ( another reason it looked like a good deal )

Curious if anyone has any thoughts or ideas? It's my fist time messing with a plate fed tone stack design.

Edit: I was hoping the scheme would display in the post without having to download it. Not sure how to do that.
Edit 2: added a png file that displays in the post.

 

[SpiderWars]

You could add a 470pf cap across R8 to open it up but that first stage is a good place to start to reduce gain and bass.

I'd prob make that first cathode more standard (like 2k7//0.68u) and then adjust that 68k-680k voltage divider to reduce gain (I'd seriously consider reversing them!). Just my first crack at it.

 

[fusedbrain]

I'd prob make that first cathode more standard (like 2k7//0.68u) and then adjust that 68k-680k voltage divider to reduce gain (I'd seriously consider reversing them!). Just my first crack at it.

Yeah, I'm wondering if that voltage divider is a mistake, and the 68k is supposed to be to ground. HBE stage in a Friedman is 1meg / 68k to ground.

 

[CornNutz]

68k/680k is that what I'm seeing? You should verify that. If it is change it to 68k/68k

Try a .0022uf on c2

See if you like r3 at the normal 2.7k rather than that 10k

Maybe after that add a 68k, or 33k grid stopper to V1B

 

[nico22]

this pre is rocca/rocco mod
first cathode should be 100n, not 1u
470p snubber should also be 1000p

You have lots informations about it over metroamp

 

[fusedbrain]

68k/680k is that what I'm seeing? You should verify that. If it is change it to 68k/68k

Try a .0022uf on c2

See if you like r3 at the normal 2.7k rather than that 10k

Maybe after that add a 68k, or 33k grid stopper to V1B

Everything on the scheme I posted is 100% whats in my amp.

 

[fusedbrain]

this pre is rocca/rocco mod
first cathode should be 100n, not 1u
470p snubber should also be 1000p

You have lots informations about it over metroamp

I was hesitant to put the amp brand in the thread title and then rip it, but yeah it's a Roccaforte. Not sure what model, but it says "custom 100" on the back panel.
So here's the what I've been able to kinda figure out so far:
Looks like in the beginning Doug was making the amps himself, and was signing them. Then later on he was farming them out to be fabricated. ( may or not be true )
***Edit: link for reference to Doug farming out the amps : viewtopic.php?t=206299#p2259517

Most of the Roccaforte amps I've found pictures of on-line are signed, and have Orange drop caps. My amp is not signed, and the caps look like yellow Mallory 150's, but the only marking on them are a "C" in a circle. The amp does have the Roccaforte Mercury Magnetics iron and the overall layout looks like an HG100. I'm starting to think that my amp was one of the later ones that was made by a fabricator, and it has some mistakes in it. I bought the amp off the original owner, and he said he bought it in the states about 12 or 15 years ago. The circuit board looks un-touched / un modded, and I'm convinced whatever is in the amp is how it was made.
***Edit: as stated below, it seems like the only issue was the wrong value cap on V1 cathode. My amp had a 1uf & info was provided indicating it should be .1uf. I made that change, and the amp sounds incredible
The amp looked pristine and un-touched when I got it, so not sure how that 1uf cap ended up in my amp.

Roccaforte amps have a pretty good rep, but unfortunately my amp doesn't sound that great, and you can probably see why from what I posted.
If I can tweak it to sound good - that's cool!
If not, I can easily replace the turret board and do a different design.
Like I said, I couldn't buy the parts for what I paid, so it's all good.
Open to any and all suggestions

 

[fusedbrain]

Update 1: installed a log gain pot, a 33k grid stopper on the second gain stage, and tacked in an 82k parallel to that 680k to ground. First voltage divider is now 68k / 73k ( close enough for R&R )
This makes a lot more sense to me. Gain now goes from Plexi-ish @ 2-3 to crushing @ 8-9. All of it totally usable.

Amp still has Waaaaaay too much bass. I think I'm gonna swap that 1uf on the first cathode for a .1 like Nico22 was saying, and see what happens. Should def cut some bass.
Possible that may be how the amp was intended. Curious to see what it'll sound like........

 

[fusedbrain]

Update 2:
Did the full Nico22 mod :
.1uf on the first cathode, and just for shits and grins, upped that snubber on V1B plate to 1000pf.



Night and Fukking day!!!

No idea how the amp ended up in the configuration it was, but it's kickin' major ass now!

Just because I can, I'm gonna rip out the rest of those cheap yellow caps and replace them with Orange drops of the same value, because it seems like that's how the original amps were.

This is a super cool amp now, and it doesn't really sound like anything else I own.

Thanks everybody for the input and advice

It's the people on here that make this place so cool.

 

[nico22]

good news that's cool
what's the nfb? 100k and 8ohm tap, 100n and 5k pres?

Also iirc someone put a linear bass pot in the ts to get a better bass sweep....

 

[fusedbrain]

good news that's cool
what's the nfb? 100k and 8ohm tap, 100n and 5k pres?

Also iirc someone put a linear bass pot in the ts to get a better bass sweep....

Yes, that is correct, to all values you quoted on the NFB and presence.
No idea about the bass pot, but the balance of bass to the rest of the tone is very good, and the bass control has a good sweep.

 

[nico22]

and finally did you go back to the 68k-680k divider along with the 100n cathode and 1000p snubber?

 

[fusedbrain]

and finally did you go back to the 68k-680k divider along with the 100n cathode and 1000p snubber?

Ha Ha funny you ask that. When I swapped out the 1uf cap for the .1uf, I forgot to put the little 82k jumper back in ( the cathode cap and the voltage divider are grounded to the same turret)
The amp sounds stellar now, so it looks like the main issue was the 1uf cap on the first cathode. No idea how that got in there, but it's fixed now anyway.
The larger value in the voltage divider has a lot less impact with the proper value cathode cap in there.
The master volume sweep is really nice now with the little mod I did there, and I think the log gain pot evens out the gain sweep too.
Thanks again for the help

 

[V2a]

The Rocca HG was based on a schematic in the first book in Kevin O'Connor's "the ultimate tone" (TUT) series. I talked to KOC about it, and it was just something he did for one customer. The 68k/680k voltage divider should be reworked. Also, in my opinion, the 100n/10k cathode circuit is pretty silly, since there is a HUGE difference between gain levels for bass and treble frequencies. The HG gets really ice picky when the third stage has the same bypass. You can try adding a series resistance (10k for starters) to the bypass cap, or you could try values that don't lead to such a big jump in treble frequencies (3k3 || 470n works well). If there is too much bass after that, you could adjust the value of the first coupling cap.

 

[nico22]

A little clip?

 

[Devin]

Pics?

 

[fusedbrain]

The Rocca HG was based on a schematic in the first book in Kevin O'Connor's "the ultimate tone" (TUT) series. I talked to KOC about it, and it was just something he did for one customer. The 68k/680k voltage divider should be reworked. Also, in my opinion, the 100n/10k cathode circuit is pretty silly, since there is a HUGE difference between gain levels for bass and treble frequencies. The HG gets really ice picky when the third stage has the same bypass. You can try adding a series resistance (10k for starters) to the bypass cap, or you could try values that don't lead to such a big jump in treble frequencies (3k3 || 470n works well). If there is too much bass after that, you could adjust the value of the first coupling cap.

Thanks for the info!
I got the idea to re-work the master vol from a post of yours that popped up while researching this amp.
To me, the 680k on the voltage divider works now with the change in first stage cathode cap. I know the whole front end looks kinda strange on paper, but I'm telling ya, the amp sounds really really good now. Yeah, it's on the bright side, but it's not harsh and it cuts like a mofo. Good iron and good tubes help. The third stage cathode is NOT bypassed in my amp, and I did add the 33k grid stop to the second stage and the 1000pf snubber probably helps too.

I'm on the road today and tomorrow, but I'll see what I can do about pics and a clip later in the week.

 

[nico22]

would you share the link to that post about master volume please?

 

[fusedbrain]

would you share the link to that post about master volume please?

This thread, very last post:
viewtopic.php?t=146739#p1729991

Damn master vol on my amp was like an on / off switch.
I checked mine and sure enough....
Friedman style master vol fixed that right up
Good to know....

 

[nico22]

Yep pics and clip would be great!