New chassis day

AndyD

Well-Known Member
Joined
Sep 13, 2018
Messages
723
Reaction score
715
Location
UK
This superb chassis, circuit board and PT frame arrived in the post this morning from @StuC. The nights are drawing in so I fancied another build. I have a Drake transformer set (B+ 470V ish) from a Carlsbro TR60 which I intend to use as a base for a high headroom amp along the lines of a 50 W Pig ( piglet?!)/Orange/Hiwatt/Carlsbro. Haven’t quite decided yet. Looking forward to making a start.
p61KygZ.jpg

8ceptpS.jpg
 

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,831
Reaction score
2,032
Location
Staffordshire UK

AndyD

Well-Known Member
Joined
Sep 13, 2018
Messages
723
Reaction score
715
Location
UK
@Pete Farrington just had a quick look at the Sound City schematic. Very interesting. I notice the PI balancer that Dumble famously implemented. I will give it a good amount of study!
Btw, any idea what the B+ was on the SS amps?
 

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,831
Reaction score
2,032
Location
Staffordshire UK
The HT is up around 460-470V, and pretty stiff due to the FWB rectification (lower copper loss), see https://www.marshallforum.com/threads/hiwatt-dr504-build.121746/post-2237884
If you value your EL34's operational life, it's probably best to avoid cranking an amp like this super hard :)
KT88 may be a better bet.

Might you be confusing the independant bias for a phase splitter balance? It can do a similar thing I suppose.

Be warned that in quiet settings, those tone controls can be a little hissy, maybe be due to the high grid circuit resistance at V3 pin7. That may be why the sensitivity switch was added. A master volume may have been a better idea.
 
Last edited:

StingRay85

Well-Known Member
Joined
Aug 18, 2019
Messages
2,367
Reaction score
2,141
I actually managed to make a Sound City L120 Mark 4 into a usable amp, free from hiss. The mid control behaves like a volume pot. It's extremely loud. I wouldn't recommend building one. I do the opposite, I gut them to build something different like SLO or 2204.


Oh wait, I did change the first gain stages to 2204 layout, but the "active" tone control is still in place.
 

Amadeus91

Well-Known Member
Joined
Apr 9, 2019
Messages
279
Reaction score
488
This superb chassis, circuit board and PT frame arrived in the post this morning from @StuC. The nights are drawing in so I fancied another build. I have a Drake transformer set (B+ 470V ish) from a Carlsbro TR60 which I intend to use as a base for a high headroom amp along the lines of a 50 W Pig ( piglet?!)/Orange/Hiwatt/Carlsbro. Haven’t quite decided yet. Looking forward to making a start.
p61KygZ.jpg

8ceptpS.jpg
"A job well begun is half done." Superb looking components!!!
Look forward to seeing pics documenting your journey to tonal nirvana.......
 

AndyD

Well-Known Member
Joined
Sep 13, 2018
Messages
723
Reaction score
715
Location
UK
@Pete Farrington This is the power supply and out put stage of the Carlsbro TR60 with a HT of
470V. I note the 10K GS resistors. OT is apparently 4K.
KtjTWsr.png

rZSgHby.png
 
Last edited:

StingRay85

Well-Known Member
Joined
Aug 18, 2019
Messages
2,367
Reaction score
2,141
The power supply is indeed a bit different than a typical Marshall 50 W. The choke is positioned before the HT supply to the center tap. So the screens are not separately filtered. A Sound City mark 4 50 plus has a similar HT supply.
 

AndyD

Well-Known Member
Joined
Sep 13, 2018
Messages
723
Reaction score
715
Location
UK
The power supply is indeed a bit different than a typical Marshall 50 W. The choke is positioned before the HT supply to the center tap. So the screens are not separately filtered. A Sound City mark 4 50 plus has a similar HT supply.
Thats an interesting observation Dom. What difference does that make to operation?
 

StingRay85

Well-Known Member
Joined
Aug 18, 2019
Messages
2,367
Reaction score
2,141
I would assume it's better practice to do it the Marshall way. Have a LC or RC filter in between the two. I would probably copy the 2204 schematic when it comes to PT and OT arrangement.

Notice that in this case, the bias supply is also taken from the secondary of the HT supply. So you can really do it in a similar way. Only your voltages will be a little bit higher.

2204.jpg
 

AndyD

Well-Known Member
Joined
Sep 13, 2018
Messages
723
Reaction score
715
Location
UK
I would assume it's better practice to do it the Marshall way. Have a LC or RC filter in between the two. I would probably copy the 2204 schematic when it comes to PT and OT arrangement.

Notice that in this case, the bias supply is also taken from the secondary of the HT supply. So you can really do it in a similar way. Only your voltages will be a little bit higher.

View attachment 116823
I will move the bias supply to before the standby switch, though!
 

StingRay85

Well-Known Member
Joined
Aug 18, 2019
Messages
2,367
Reaction score
2,141
I will move the bias supply to before the standby switch, though!
Can't find an example where this is applied. I need to read again the section in the Merlin book on this part. It's good practice to have a good negative bias supply upon firing up of the amp, not to overload the power tubes during start up. By having a separate bias winding, this is never a problem, but in this case, it might be.
 

AndyD

Well-Known Member
Joined
Sep 13, 2018
Messages
723
Reaction score
715
Location
UK
Can't find an example where this is applied. I need to read again the section in the Merlin book on this part. It's good practice to have a good negative bias supply upon firing up of the amp, not to overload the power tubes during start up. By having a separate bias winding, this is never a problem, but in this case, it might be.
The way I see it is that unless the bias supply comes directly off the HT supply, ie before the standby switch, there would be no need for the standby. As it is wired in the scheme above, the standby isn't doing anything.
 

AndyD

Well-Known Member
Joined
Sep 13, 2018
Messages
723
Reaction score
715
Location
UK
The standby interrupts the HT though
Yes, of course you are correct! I forgot that the filaments are unswitched! My main point is that the bias supply should go on at the same time as the filaments, before the standby switch. The Marshall schematics show the bias feed after the standby switch, buts its considered to be bad practice.
 


Top