Plexi 6v6: 1r Shunt resistor, CLC vs CRC 1st node

[Scaramoucheii]

Hi All,  Long time listener, first time caller ...

I have built a few Guitar amps from Hammond Organ Amps,  and also worked on quite a bit of HiFi gear, mostly Dynaco, as well as some other kits and scratch built phono preamps.

I'm currently working on a scratch built Plexi 6V6 from the M Huss/Hoffman schematic.  I decided on this as I had 90% of the components in my parts boxes including the Transformers. Lots of AX7's and old 6V6's,  and enough Aluminium to make a rather nice, but large chassis.

So the Questions De Jour ... 

1:       I have some 0.68r 1 Watt resistors that I could use for R42, R43 (Cathode Shunt).  so just to confirm that the value of these resistors is just a arbitrary low value to provide the R in E=IxR for measuring the current, and not for any sonic or performance.

2:    I will be wiring up the Power supply over the next couple of days,  but currently don't have a Choke,   is there a big sonic difference in just using a dropping resistor ( from my box full of large power resistors) and having CRC filter in the first node as opposed to CLC?  Or should I just spring for the 30 bucks and get a Choke?  Not that i'm cheap ( i'm just frugal).

Keep in mind that this project started by trying to empty out my box of parts into something useful,  so not buying more components is a bit of a project goal over cost saving.

Regards

[shooter]

1. .68 works, but with calculator rather than direct "conversion"


2. The "L" Vs R is more "mojo" than reality in my world.

[HotBluePlates]

I'm currently working on a scratch built Plexi 6V6 from the M Huss/Hoffman schematic. 

I assume we're talking from the Plexi 6V6 stuff on this site.  If not, post a link to what you're looking at.

... currently don't have a Choke,   is there a big sonic difference in just using a dropping resistor ( from my box full of large power resistors) and having CRC filter in the first node as opposed to CLC?  Or should I just spring for the 30 bucks and get a Choke?  ...

Screen current of the output tubes changes from idle to full-tilt.

Screen Volts affects plate current; low screen volts ---> lower plate current.

The choke keeps the voltage at Node B (C18) of the schematic I linked more-constant as power output changes from idle to full-tilt.  That results in higher total power output and less compression.



You can choose either the choke or a resistor.  The choke gives the filtering effect of a ~3kΩ resistor, but installing a resistor over 1kΩ will probably reduce max output power and increase compression somewhat.

[stratomaster]

I will say that a properly improper choke will add undertones and harmonics due to ringing in the 3rd order filter this creates as they aren't 0 resistance.  These can't be replicated with an RC pi filter node alone.  These undertones are usually undesirable in most contexts but they are part of the character of some Plexi circuits and will affect the overdrive tone.  If you're going for maximum authenticity then a properly improper choke is the way to go. 

[Merlin]

[shooter]

 

[Scaramoucheii]

I assume we're talking from the Plexi 6V6 stuff on this site. 

Yes.

You can choose either the choke or a resistor.  The choke gives the filtering effect of a ~3kΩ resistor, but installing a resistor over 1kΩ will probably reduce max output power and increase compression somewhat.

I'll start with a resistor,   but leave space on the chassis top for the choke,  and then just go ahead and start building.  I need to figure out how the Power transformer I have works out so actually getting it into a working circuit and doing some measurements is job one.

[stratomaster]

Roasted by the Wizard. 

I stand by my statement.  It's a real phenomenon, and can really make the amp sound terrible if you hit a note that resonates--almost like a ring modulator in the worst way. The trick is to have just a little bit of this effect.

I didn't render judgement as to whether it was an integral part to the tone, just that it happens--and the only way to replicate it is to replicate the power supply arrangement if one is so inclined.

An RC will filter adequately and perform better than a poorly tuned LC from a proper design perspective.

[Merlin]

I wasn't roasting you in particular. I agree with you, the difference between a resistor and a choke is real but minor, not something the OP needs to worry about if he wants to save several dollars.

[shooter]

I'll also agree...IF we're in engineering school, out in heathen land i've listened to too many great guitarists make amazing music with equipment that would give a double E a heat attack.