300k Volume and 2M Tone Pots

[scstill]

I am adapting a '52 Packard/Buick Radio into a guitar amp (my 4th custom build)
The design is essentially a Fender 5C3 shown in the draft schematic with a tone and volume pot
 
The existing chassis uses a stacked pot for volume and tone shown in the picture
The values measure 300k for the volume and 2M for the tone
By staying with the existing stacked pot (w/switch) the amp is controlled from the original controls

BUT...
What effect will values of 300k for the volume and 2M for the tone do to guitar amp design?
Any suggested circuit changes to accommodate these values?

[shooter]

300k seems fine, 2M isn't unreasonable, If it bothers you, strap a 2M in parallel n ya got 1M, typical in that TS

[HotBluePlates]

... Fender 5C3 shown in the draft schematic with a tone and volume pot ...

What effect will values of 300k for the volume and 2M for the tone do ...

The 300kΩ Volume control will load-down the input gain stages (which will also be loaded-down by each other by the way they're mixed without isolation/mixing resistors).  So your input stages will have weak output signal.

The Tone control (drawn with both caps = 0.022µF) isn't right; usually the cap to the Volume pot wiper is small (around 500pF, give or take) as it will act like a bright cap on the Volume pot to create your "treble" end of the control.  Using a Volume control 1/3 the size implies a cap 3x bigger to maintain the same response, so ~0.0015µF or 15nF or 1500pF.

If 0.022µF was surely-right for the "bass" side of the Tone control, a pot that's 2x resistance implies 1/2-value cap for same-response.  You'd be looking for a 0.01µF or 10nF or 1000pf cap.

Are these the "right value" for your amp?  No idea, as you'd need to try and see if you get enough Mud & Screech when you turn the Tone control all the way to either end.  A well-stocked parts bin reigns supreme in situations like this.

[scstill]

The 300kΩ Volume control will load-down the input gain stages (which will also be loaded-down by each other by the way they're mixed without isolation/mixing resistors).  So your input stages will have weak output signal.

The Tone control (drawn with both caps = 0.022µF) isn't right; usually the cap to the Volume pot wiper is small (around 500pF, give or take) as it will act like a bright cap on the Volume pot to create your "treble" end of the control.  Using a Volume control 1/3 the size implies a cap 3x bigger to maintain the same response, so ~0.0015µF or 15nF or 1500pF.

If 0.022µF was surely-right for the "bass" side of the Tone control, a pot that's 2x resistance implies 1/2-value cap for same-response.  You'd be looking for a 0.001µF or 10nF or 1000pf cap.

Are these the "right value" for your amp?  No idea, as you'd need to try and see if you get enough Mud & Screech when you turn the Tone control all the way to either end.  A well-stocked parts bin reigns supreme in situations like this.

This is great. Many apologies because the draft schematic has incorrect values.
But I think I understand your tone stack suggestions:
   Bright: if .0005uf for 1M; .0015 for 300k
   Bass: if .005uf for 1M; .0025 for 2M
BTW - coupling caps are .05

A little concerned about the 300k loading down the input stage and having weak output.
So....  could that me made up with the 2nd pre-amp? or is there something else that could be considered?

[shooter]

did you measure the 300K with original wires still intact?  If so you might be measuring a parallel condition.


I'd still "make it work", then solve for unknowns after.

[scstill]

did you measure the 300K with original wires still intact?  If so you might be measuring a parallel condition.
I'd still "make it work", then solve for unknowns after.

Everything off when measured.  Like "make it work"

[HotBluePlates]

The 300kΩ Volume control will load-down the input gain stages (which will also be loaded-down by each other by the way they're mixed without isolation/mixing resistors).  So your input stages will have weak output signal.

... A little concerned about the 300k loading down the input stage and having weak output.
So....  could that me made up with the 2nd pre-amp? or is there something else that could be considered?

Do you have a schematic of the radio as it existed originally?  What did it have before the Volume control?

Cause the secret is if you're going to convert some existing electronic item, and you're gonna live & die by keeping some key part (like the concentric Vol/Tone knobs), you're probably also committing to using that feature the way the original item did.

________________________________

What do I mean?

Rule of thumb for getting decent gain from a tube is the plate load resistor should be 2-5x the internal plate resistance of the triode (pentodes have slightly different rule, but you might start with it's triode characteristics).  And then any/all parallel resistance to ground after the coupling cap should be 2-5x the value of the plate load resistor.  Otherwise, voltage-output from the triode is reduced (get less amplification than you expected).

The radio had an audio output section, and this Volume control probably sat in that audio section.  Look at what was driving it in the original article, because as a commercial product it almost certainly followed good engineering practice.

• I'd be willing to guess that original Volume control was driven by a triode with a plate load of 47-100kΩ, and the Volume pot's wiper probably went to a single-ended output tube.  Unless as Shooter is thinking, that the pot is higher resistance & you accidentally measured other resistance in parallel with it.

[scstill]

Here is the original schematic.
6v6 push pull. Lots of large value resistors.

[PRR]

> Lots of large value resistors.

You do know the "M"s are "1000" not "Meg"? I only see two MEGs in there.

....I'd be willing to guess that original Volume control was driven by a triode with a plate load of 47-100kΩ, and the Volume pot's wiper probably went to a single-ended output tube.

It's a car radio.

1) And an AM radio, post-1935. The Volume normally goes after the diode detector. For AM reception there are some rules on AC/DC impedance ratio etc. Also the diode and the IF stage driving it has real impedance. 500k would be typical. 300k could be tolerance and age. The 2MEG tone pot is intended to be very little loss until you damp the tone way down. This will need some source impedance. Indeed in the range of "47-100kΩ".

2) Buick/Packard were HIGH-class cars and got a push-pull output. Indeed an SE output is only for very cheap cars; 3 Watts does not cut it on the highway.

[shooter]

Is this build going back in the car?


are you planning on "shoe-horning" a new board inside, or just re-wire what's there?


I surf long shaft dual gang, got bored but the are out there
https://www.ebay.com/i/383488594808?chn=ps

[scstill]

Thanks for clarifying M is 1000;
This will not go back into a car. Will get a handle and rubber feet
I have two of these radios. One is gutted for guitar amp. One will stay a radio.

The guitar amp will be point to point wired two channel (triode 6sc7a pre on one, pentode 6sj7 pre on the other)
1/4 guitar channel inputs mount in the antenna and trimmer holes
EAch pre channel will combine into a 2nd preamp 6sc7b.
Will drive a 6sc7 phase inverter into the car 6V6 PP; will use car OT (why not?)
Pwr Supply uses old webster reel recorder (also 6V6pp) PT into 6x5 rectifier (the webster fits perfect in Car PT space)
All tubes from the car radio except 6sj7.
Will substitute beefy Jensen Organ 6x9 for car radio speaker; will get a switched output jack to drive external cab

[sluckey]

Weber makes a 6x9 just for guitar. Sounds good.

[shooter]

use the 2M for volume, go Vox, (rotary cap switch), and use the pushbuttons as your "treble"   

[scstill]

use the 2M for volume, go Vox, (rotary cap switch), and use the pushbuttons as your "treble"   

Using the pushbuttons for tone is a great idea. Would I just use each to switch in a different fixed resistor?
If that works, I could use the 2nd pot for a master volume before PI. Would the 300k work here?

I have several designs where a 6x9 is stuffed into a smallish vintage radio cab made for a 5-6". The entire cab front is speaker. Sounds good to me. Couple of pics. I have considered the 6x9 weber in the past (especially for the dual "Goldeluxe") but haven't yet tried one.

[shooter]

I'll leave the "how"  to you 




here's the "idea", I'm better in that pasture 

[AmberB]

Now I'm wishing I still had the old tube radio that was in my 1963 Ford Falcon when I bought it way back when.  I kept that radio for many years after I pulled it to put in an aftermarket radio...

[PRR]

Now I'm wishing I still had the old tube radio that was in my 1963 Ford Falcon...

http://www.dplivingston.com/1962-mercury-comet/radio-and-dash

Apparently 1962 1963 Falcon and Comet were the same radio frame with transistors sneaking in place of tubes. I had a '64 Ranchero and I sure can see Ford saving the cheaper (older) tech for the Falcon badge. FWIW the 1966 Galaxie radio had the same transistor on the face casting but a different shape on the nose?

[PRR]

...an SE output is only for very cheap cars; 3 Watts does not cut it on the highway.

Here's a 1954 Chevy (GM's cheapest). ONE 6V6 at 250V; 3 Watts only if the regulator was out of whack.

[scstill]

...an SE output is only for very cheap cars; 3 Watts does not cut it on the highway.

Here's a 1954 Chevy (GM's cheapest). ONE 6V6 at 250V; 3 Watts only if the regulator was out of whack.

to me these old car radios are a perfect canvas for guitar amp.
I have restored two as radios. The issue I have is its really tough to get the 6v at 9amps they need.

[shooter]

next time you're out and a bout pick up a 3 pack of Trojans .......2 volt cells