Mongrel schematic, please review

mountainhick

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Mostly Gibson with Fender style 12ay7 channel.

Not sure about HV filter cap sizes for the nodes after the 22uf right after the 5y3GT. Gibson used 10uf. pros/cons to using 22s all the way through? I already have them, don't have ~10uf in my larder.

Ground scheme?

Fender's channel mixing resistors are 270K, Gibson had 470Ks, ant benefit either way?

12AY7 is after twin 5E8-A, but that circuit has 2 dual triode channels on the same cathode resistor and bypass cap. is the 250 bypass too much?

What kind of taper for inverter based tone pot?

Anything else?

(I'll build it this way, and leave room for possible extra stage later for proper tone controls and/or EFX)

sparton-proj-schematic.jpg
 
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Lowerleftcoast

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filter cap sizes for the nodes after the 22uf right after the 5y3GT. Gibson used 10uf. pros/cons to using 22s all the way through?
Pros: they will work, more filtering. Cons: maybe the touch sensitivity will be less with the 22uF caps.
More on power filtering: The power handling capability of the choke must be large enough to handle all of the current in this design. Typical Fender designs place the choke after the OT CT. The Fender chokes can be smaller due to this placement.
Many builders increase the reservoir cap to 40ish uF when using a 5Y3 rectifier for firmer bass and quieter ripple current.
Fender's channel mixing resistors are 270K, Gibson had 470Ks, ant benefit either way?
The 470k will scrape off a few more highs.
is the 250 bypass too much?
Depends on what you want. hah. I would probably drop the value to 2.2uF to 4.7uF. OMMV. (Large cathode caps pass all frequencies and provide a quieter environment addressing heater hum. Smaller values lose the extra quietness factor and can reduce some unwanted sub bass/ bass frequencies.)
 
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mountainhick

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Pros: they will work, more filtering. Cons: maybe the touch sensitivity will be less with the 22uF caps.
More on power filtering: The power handling capability of the choke must be large enough to handle all of the current in this design. Typical Fender designs place the choke after the OT CT. The Fender chokes can be smaller due to this placement.
Many builders increase the reservoir cap to 40ish uF when using a 5Y3 rectifier for firmer bass and quieter ripple current.

The 470k will scrape off a few more highs.

Depends on what you want. hah. I would probably drop the value to 2.2uF to 4.7uF. OMMV. (Large cathode caps pass all frequencies and provide a quieter environment for heater hum. Smaller values lose the extra quietness factor and can reduce some unwanted sub bass/ bass frequencies.)


Thanks!

Re: first filter cap. I may also try 5AR4/GZ34 depending on whether the 5Y3GT is too brown. I think it would handle a larger first cap?

Re: choke, I was wondering how the fender spec chokes got by on less amperage. I wonder about the Gibson's higher screen voltage as drawn, and no screen resistors? This is consistently the Gibson design for the schematics I've been looking at.

Re: 12AY7 cath bypass, I'll edit to 25uf for now.
 

Lowerleftcoast

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5AR4/GZ34
Datasheet says it is good for 60uF. 40uF is enough for a design like this, imo. OMMV.
no screen resistors?
A lot of old designs ran without screen resistors. Maybe it was the lower B+ that helped save it from having trouble. I look at the original design as a cost saving measure. Less parts and less labor. I change those designs unless I am building a true part by part clone.
 

mountainhick

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Datasheet says it is good for 60uF. 40uF is enough for a design like this, imo. OMMV.

A lot of old designs ran without screen resistors. Maybe it was the lower B+ that helped save it from having trouble. I look at the original design as a cost saving measure. Less parts and less labor. I change those designs unless I am building a true part by part clone.

I have not studied chokes yet, except I remember they filter well with less voltage drop than a resistor.

If I move the choke between OT and screen, what's a good screen resistor value to start with?
 

mountainhick

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I totalled preamp, PI, and screen currents at 22.2MA, looks like 4H Fender Deluxe 125C3A/Hammond 194A would do the job.

Deluxe screen resistors are 470Ks 470 ohm for 6V6's. Should I expect differences in the sound comparing Fender style to Gibson style schematic?


Gibson Style:

sparton-V1.jpg




Fender style:

sparton-V2.jpg
 
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Lowerleftcoast

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what's a good screen resistor value to start with?
Fender's go to was/is 470R. Watch it... the 470k you mention is a common mistake.
Since this amp design looks very tweed era, I might try it without. Sound wise the difference with/without is just noticeable.
Should I expect differences in the sound comparing Fender style to Gibson style schematic?
Maybe if you had the amps side by side and could switch between the two.
Imo, they would sound very close to the same. Other differences will be heard more.

The Hammond 194A will work.
 

mountainhick

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Alright, can't edit the first post any more so here's the proposed final version. I found just a couple small details to correct.

If anyone finds any circuit errors please say so!

In the meantime I'll start moving on to the layout.

sparton-proj-schematic-V2.jpg
 
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mountainhick

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im not sure what the .005 cap and 1M pot is between the plates of the PI is doing?

It's the "voicing" control, i.e tone circuit. Pretty different eh?

There's also a previous version with the 1M pot going to ground rather than back through the plate resistor and filter cap to ground.
 




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