scottosan
Well-known member
Looks like a Roccaforte HG100 circuit with plate fed tone stack.
N
nico22
Member
IIRC long time ago at metroamp i saw an HG100 handrawn layout. The first cathode was switchable : 10k/100n or 100k/680n 
you could try that second style cathode?
you could try that second style cathode?
scottosan
Well-known member
I reversed the amp.and shared it in 2005. Here was the quick mockup I did thennico22":2c827g11 said:IIRC long time ago at metroamp i saw an HG100 handrawn layout. The first cathode was switchable : 10k/100n or 100k/680n
you could try that second style cathode?
N
nico22
Member
scottosan
Well-known member
Apparently there were 2 revisions to the HG100. I can validate the 10k/.1uf on v1a for the one I had in the bench.nico22":251ft2mt said:
fusedbrain
Well-known member
Another update: First of all, I'm not really sure whats up with the different versions of this amp. In this thread, second post: viewtopic.php?t=124211#p1449475
you can see gut shots of a Roccaforte HG100 that is loaded with OD715 caps with 3 yellow box caps used as cathode bypass caps. The date codes on the tranny's and the sig indicate October 2004.
The first pic below is my amp. This pic is after a few changes, but at this point, the majority of the amp is what I believe to be stock. The date codes on my amps tranny's are SEPTEMBER 2004. The changes seen in the pic are: Audio taper gain and master pots with Friedman style master volume, first cathode cap changed to a NOS ERO .1uf cap, snubber on V1B to 1000pf, PI fizz cap upped to 100pf, and I removed the passive insert loop because the switching jacks were funky.
As you can see, ALL the caps are a generic axial film type cap, and in it's original state before the changes, it sounded kinda lame. The biggest factor in the shity sound was a 1uf on the first cathode, where it looks like there should be a .1uf cap. As I stated earlier, the amp looked completely stock when I got it, and I have no idea why there was 1uf cap and a 470pf snubber in my amp. All the other cap and resistor values were correct vs the other schemes that have been posted.
After the changes suggested by nico22, the amp sounded really good, but I was curious to see what it would have sounded like with OD715's. So..... I swapped all the caps for OD's of the same value.
Yeah, that was a big mistake
Amp sounded thin and lifeless.
Next step, as shown in pic #2, was to go back to axial polyester film caps.
I don't have any of those little yellow generic film caps, so I loaded it up with Synergy next gen's + Vishay MKT1813's on the cathodes.
Amp sounds stellar now! Even better than before I switched to the OD's.
The only thing left to do is install a Metro loop ( the existing holes in the chassis are the right spacing )
This is a really great sounding circuit. It has that bright Marshall kerrang, but it's not harsh. Very similar to a JCM800, but way more gain and maybe just a hair more low end.
I also posted the complete scheme of my amp, and included voltages as well
Enjoy!
Edit: Forgot to mention I added a 33k grid stop on V1B - it just seemed like there should be one there
Also, on the scheme, I didn't draw the bias supply, the power tube cathodes aren't grounded, I didn't show the balancing resistors on the screen cans blah blah blah....
All the good stuff is on the drawing.....
you can see gut shots of a Roccaforte HG100 that is loaded with OD715 caps with 3 yellow box caps used as cathode bypass caps. The date codes on the tranny's and the sig indicate October 2004.
The first pic below is my amp. This pic is after a few changes, but at this point, the majority of the amp is what I believe to be stock. The date codes on my amps tranny's are SEPTEMBER 2004. The changes seen in the pic are: Audio taper gain and master pots with Friedman style master volume, first cathode cap changed to a NOS ERO .1uf cap, snubber on V1B to 1000pf, PI fizz cap upped to 100pf, and I removed the passive insert loop because the switching jacks were funky.
As you can see, ALL the caps are a generic axial film type cap, and in it's original state before the changes, it sounded kinda lame. The biggest factor in the shity sound was a 1uf on the first cathode, where it looks like there should be a .1uf cap. As I stated earlier, the amp looked completely stock when I got it, and I have no idea why there was 1uf cap and a 470pf snubber in my amp. All the other cap and resistor values were correct vs the other schemes that have been posted.
After the changes suggested by nico22, the amp sounded really good, but I was curious to see what it would have sounded like with OD715's. So..... I swapped all the caps for OD's of the same value.
Yeah, that was a big mistake
Amp sounded thin and lifeless. Next step, as shown in pic #2, was to go back to axial polyester film caps.
I don't have any of those little yellow generic film caps, so I loaded it up with Synergy next gen's + Vishay MKT1813's on the cathodes.
Amp sounds stellar now! Even better than before I switched to the OD's.
The only thing left to do is install a Metro loop ( the existing holes in the chassis are the right spacing
)This is a really great sounding circuit. It has that bright Marshall kerrang, but it's not harsh. Very similar to a JCM800, but way more gain and maybe just a hair more low end.
I also posted the complete scheme of my amp, and included voltages as well
Enjoy!
Edit: Forgot to mention I added a 33k grid stop on V1B - it just seemed like there should be one there
Also, on the scheme, I didn't draw the bias supply, the power tube cathodes aren't grounded, I didn't show the balancing resistors on the screen cans blah blah blah....
All the good stuff is on the drawing.....
Attachments
N
nico22
Member
Thanks to Novosibir (Larry amps) i tried and found also vishay 1813 cathode and IC MPW caps and murata pf values to sound pretty good...
Could you quickly try that first cathode switch please and tell us how it sounds?
Thanks!
Could you quickly try that first cathode switch please and tell us how it sounds?
Thanks!
fusedbrain
Well-known member
Yes I got the idea to try the MKT1813's from Larry over at the Metro forum. They are my go-to cathode bypass cap. Never tried the IC MPW, but it's on the list......nico22":2697uohz said:
I prefer a certain brand NOS ceramic cap found on ebay ( thanks again to Novosibr) for peaker and tone stack caps. I use the BC high voltage caps for pf value snubber applications, but only Y5P dielectric. Murata's do sound good in all applications as well.
I tried a number of combinations on the cathode of V1A, but found nothing that would sound good without changing other parts of the amp. I also tried bypassing the 10k on V2A with a few different resistor / cap combo's, but original configuration is best.
I did NOT try 100k / .68uf on V1A as shown in the above layout you posted. I thought maybe it was a typo.
N
nico22
Member
ah yes the idea of typo or bad reading of the color code is good...
Check your pm please
Check your pm please
fusedbrain
Well-known member
This would be my assumption as well based on the values, and it's been bugging me that this amp doesn't SOUND like you would expect.V2a":3d4qwnl3 said:
I think I may have found the answer.....
In the Duncan Tone Stack calc graphic below, you can see the frequency response for a Marshall style tone stack fed from a cathode follower vs fed from the plate of the preceding stage. The stock input impedance on the calculator for a Marshall style cathode follower tone stack is 1.3k. My limited research suggests that the input impedance to the tone stack fed from the preceding plate stage would be around 39k, so that's what is used here. Nothing else was adjusted for the 2 plots.
As can be seen, it looks like the plate fed TS has an imbalance of bass vs treble that compensates for the opposite imbalance from the wacky cathode values used on V1 in my amp.
I've never played around with plate fed tone stacks before, so this has been a good learning experience, and the amp does sound really good.
Thought some of you guys might find this interesting
Attachments
V2a
Active member
Yes, it is all a balancing act. The 22n coupling caps throughout the preamp also let through whatever low end will be generated by the gain stage (while other amps have more low-end gain and then attenuate it with a 2n2 CC).
311splawndude
Well-known member
I don't know what the hell you guys are talking about but it sure is sexy to read 
congrats on your amp fusebrain and the fact that you are resurrecting it. or transforming it I should say.
congrats on your amp fusebrain and the fact that you are resurrecting it. or transforming it I should say.
V2a
Active member
311splawndude":1nlqtro2 said:I don't know what the hell you guys are talking about but it sure is sexy to read
well this is the tech corner
fusedbrain
Well-known member
Thanks man!311splawndude":vqwa3xij said:I don't know what the hell you guys are talking about but it sure is sexy to read
congrats on your amp fusebrain and the fact that you are resurrecting it. or transforming it I should say.
My motivation for starting this thread was to try and figure out how this amp was supposed to be configured, and to try and bring it back to that.
These amps have a good rep for a reason, and now that I have the amp sorted, I understand why.
I'm really happy with the amp, and it sounds f'n great!
And I couldn't have done it without the input from the guys on this forum!
There are some really good dudes on here who are always ready to help out!
This place kicks ass
