Adding resistance to OT circuit.

Printer2 · 23rd March 2011, 03:27 AM

I have a push pull output transformer with a 66oo to 8 ohm ratio (http://www.hammondmfg.com/pdf/EDB1760H.pdf) . I would like to load the tubes at 8k in a Class AB amp. Of course I would get a dc voltage drop, 45mA at idle would give about 30v drop.

More curious about the AC characteristics of putting a resistor in the circuit. I would guess there would be some compression at full output due to a greater voltage drop. Also I would figure while in Class A the drop should stay the same but increase in Class B operation. Come to think about it, maybe a single resistor on the centertap would be simpler, Still trying to wrap my head around the concept. I would appreciate any input from those much brighter than me here. And yes that casts a wide net.

DF96 · 23rd March 2011, 11:48 AM

Leave it as it is. Adding a resistor would reduce gain more than using a slightly wrong impedance. If you really do want to go ahead, then the place to add the resistor is on the secondary, not the primary. About 1.8ohms should do. Totally unnecessary, though.

aardvarkash10 · 23rd March 2011, 06:19 PM

given that any speaker you attach it all to has an impedance curve that ranges across upward of 50% deviation from nominal, it seems a bit pointless...

Printer2 · 23rd March 2011, 06:48 PM

Yeah I was thinking along the lines that it was not important but some people swear it really maters using 8k for 6BQ5's. I thought might as well just put a two ohm resistor in line with the speaker. Not really too concerned with a drop in efficiency, this is just a sound generator (guitar amp). Thinking a little outside the box I was wondering what would happen to the signal if you did put a resistor in the primary circuit. As I said before, I thought it might have a compressive effect in Class B while leaving the signal unmodified in Class A. Maybe even a larger R value may be needed to be audible. One day I will have to learn a Spice program to model these ideas, for now I just try them.

23rd March 2011, 03:27 AM	#1
Printer2 diyAudio Member Join Date: Apr 2010	Adding resistance to OT circuit. I have a push pull output transformer with a 66oo to 8 ohm ratio (http://www.hammondmfg.com/pdf/EDB1760H.pdf) . I would like to load the tubes at 8k in a Class AB amp. Of course I would get a dc voltage drop, 45mA at idle would give about 30v drop. More curious about the AC characteristics of putting a resistor in the circuit. I would guess there would be some compression at full output due to a greater voltage drop. Also I would figure while in Class A the drop should stay the same but increase in Class B operation. Come to think about it, maybe a single resistor on the centertap would be simpler, Still trying to wrap my head around the concept. I would appreciate any input from those much brighter than me here. And yes that casts a wide net.

23rd March 2011, 06:19 PM	#3
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	given that any speaker you attach it all to has an impedance curve that ranges across upward of 50% deviation from nominal, it seems a bit pointless... __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012

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23rd March 2011, 11:48 AM	#2
DF96 diyAudio Member Join Date: May 2007	Leave it as it is. Adding a resistor would reduce gain more than using a slightly wrong impedance. If you really do want to go ahead, then the place to add the resistor is on the secondary, not the primary. About 1.8ohms should do. Totally unnecessary, though.

23rd March 2011, 06:48 PM	#4
Printer2 diyAudio Member Join Date: Apr 2010	Yeah I was thinking along the lines that it was not important but some people swear it really maters using 8k for 6BQ5's. I thought might as well just put a two ohm resistor in line with the speaker. Not really too concerned with a drop in efficiency, this is just a sound generator (guitar amp). Thinking a little outside the box I was wondering what would happen to the signal if you did put a resistor in the primary circuit. As I said before, I thought it might have a compressive effect in Class B while leaving the signal unmodified in Class A. Maybe even a larger R value may be needed to be audible. One day I will have to learn a Spice program to model these ideas, for now I just try them.

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