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stickjam

Conversion to PM speaker instead of Field Coil
« on: January 20, 2011, 05:36:30 PM »
While the proper setup for the following amp schematic is using an electrodynamic speaker, I'd like to in interim, connect it to the permanent magnet one I have until I find (and can afford) a proper field coil speaker.  So...

What does a real field coil do for this circuit that replacing it with a 2.5K resistor wouldn't?    It looks like it would draw 30-some watts, would a heatsinked 50-watt resistor do, or should I overrate it more like 100W?



Thanks!
-Bob
« Last Edit: March 14, 2011, 10:33:05 AM by stickjam »


stickjam

Re: Temporary use of PM speaker instead of Field Coil?
« Reply #1 on: January 20, 2011, 07:50:27 PM »
BTW, the B+ is a typical unregulated supply with two 5Y3 rectifiers feeding two 30uF 'lytics with an actual choke between them.  (as opposed to the common application where the field coil is used as the PSU choke)   The 2560 ohm rating on the field coil is DC resistance.
« Last Edit: January 20, 2011, 07:52:38 PM by stickjam »

PRR

Re: Temporary use of PM speaker instead of Field Coil?
« Reply #2 on: January 20, 2011, 08:22:31 PM »
Has to be Hammond or Lesley.

Yes, use 2K5. A 50W part will need BIG heatsink. And if it burns-out, the 6V6es will be under-biased and red-plate fast. Depending how long it must run, I'd be looking at plain dumb ceramic stick rated 100W.

You could instead omit the FC and raise the 75 ohm to 125-150 ohms. The lower supply current will un-sag the B+, so you better compute plate dissipation. And be wary of very-old 6V6 over 330V.

No capacitor on the screen node?

stickjam

Re: Temporary use of PM speaker instead of Field Coil?
« Reply #3 on: January 20, 2011, 08:44:27 PM »
Has to be Hammond or Lesley.

Yes, use 2K5. A 50W part will need BIG heatsink. And if it burns-out, the 6V6es will be under-biased and red-plate fast. Depending how long it must run, I'd be looking at plain dumb ceramic stick rated 100W.

You could instead omit the FC and raise the 75 ohm to 125-150 ohms. The lower supply current will un-sag the B+, so you better compute plate dissipation. And be wary of very-old 6V6 over 330V.

No capacitor on the screen node?

Thanks PRR--I knew I'd get a good no-BS answer from ya!   8)  

Yep - Leslie 21H Series 3 amp.   I'll probably grab that 100 watter.  Who knows--the saggy B+ could be part of the "mojo"  ;)  

It has no screen cap per the original schematic.   The 75 ohm is also the cathode resistor for a triode that drives the tremolo relay, but I plan to leave that tube out and install a separate solid state relay circuit because it's a 2-speed cabinet the amp's going into.

schmidlin

Re: Temporary use of PM speaker instead of Field Coil?
« Reply #4 on: January 20, 2011, 09:32:45 PM »
I'm assuming if you can read the DC resistance that coil is good but the speaker part is not.  I've seen folks rip the coil off the speaker and mount it in the cab, thus leaving it in the circuit.  Then onto a PM speaker.

Just a thought.
perception > reality

radardoug

Re: Temporary use of PM speaker instead of Field Coil?
« Reply #5 on: January 21, 2011, 02:48:46 PM »
You'd be better off to get rid of all that heat production and increase the cathode resistor, or can you
generate a minus suppy and apply it to the grids directly? I am guessing that the idea of using the field coil was to generate fixed bias rather than cathode bias in the first place, so a source of fixed bias would be better. Doubler on the filament line maybe? You only need 19 volts at almost no current.

stickjam

Re: Conversion to PM speaker instead of Field Coil
« Reply #6 on: March 14, 2011, 10:28:59 AM »
OK, I have this amp in hand.  It needs a bunch of work in short order, as I need to get it installed this week for a Saturday recording session.  :P  I'm decades-rusty in tube design calculations, so I need a bit more help figuring out how to get rid of the need for the field coil.

1. With four new Electro-Harmonix 6V6GT tubes and changing the output cathode resistor to 150 ohms, what, if anything do I need to do on the plate side?  The output transformer is 4KCT impedance with a DC resistance of 75 ohms per half.     (PRR: you were referring to a straight cathode resistor to ground instead of the fixed bias, right?)

2. What should I hang on the screen grids?  Assuming a voltage divider, but what kind of current?

3. What effects would unsagging the B+ make on the preamp* section and detract from any tone-desirable distortion characteristics?  What changes would be advised to mitigate that?

*I am considering wiring in a 9-pin miniature socket in parallel with the 6SN7's octal socket to provide optional substitution of a 12A*7A

Thanks!
-Bob  
« Last Edit: March 14, 2011, 10:34:04 AM by stickjam »

abbey road d enfer

Re: Conversion to PM speaker instead of Field Coil
« Reply #7 on: March 14, 2011, 12:51:33 PM »
1. With four new Electro-Harmonix 6V6GT tubes and changing the output cathode resistor to 150 ohms, what, if anything do I need to do on the plate side?  The output transformer is 4KCT impedance with a DC resistance of 75 ohms per half.     (PRR: you were referring to a straight cathode resistor to ground instead of the fixed bias, right?)
You need to check the plate voltage now. As PRR said, if unsagging makes it too high, you're in trouble. The Fender Deluxe Reverb runs with 415V, though...
Quote

2. What should I hang on the screen grids?  Assuming a voltage divider, but what kind of current?
Grid current can be estimated at 7-10mA. Assuming 340V, any value between 470R-1.5k will make the amp work. Now, the max power and breakup may not be the same.
Quote

3. What effects would unsagging the B+ make on the preamp* section and detract from any tone-desirable distortion characteristics?
Almost nothing. By preamp, do you mean just the PI or something else. Unless you want the preamp to run very brown, increasing B+ has marginal effect.
Quote
 What changes would be advised to mitigate that?
If you're fastidious about it, you may increase the value of the filter resistors.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

stickjam

Re: Conversion to PM speaker instead of Field Coil New
« Reply #8 on: March 18, 2011, 07:55:45 AM »
Here is a revised schematic of an attempt to adapt it to a permanent magnet speaker.

Thoughts and suggested changes?

Thanks
-Bob

« Last Edit: March 18, 2011, 08:30:20 AM by stickjam »

 

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