Below are two schematic excerpts from an AMI R-145 tube amplifier. I want to replace the audio output transformer with a newer one. The new transformer does not have the taps going back to pin 4 of the 6L6 output tubes. I have modified the schematic in #2 below to reflect what I've found. Does anyone see any potential problems with this mod before I make the change?
Attachments
You will loose 40% power by converting to triode
You also might need to adjust the NFB components for stability and performance.
Not enough information to be certain about the differences of the UL versus the non-UL output transformer to know the results, you need to try it.
But, I think you are probably set for Plug and Play; not Plug and Pray:
Primary impedance, same or different.
Primary inductance, same or different.
There is global negative feedback that has 2 different RC coupling circuits (either the same pole frequency and phase, or different pole frequency and phase), and the low frequency pole of the output transformer. That might need adjusting to prevent low frequency oscillations.
The fact that you are switching from Ultra Linear to Triode Wired mode is to your advantage:
Triode Wired mode has plate impedance, rp, about 1/2 of rp in UL mode.
Triode Wired voltage gain has about 1/2 the gain in UL mode.
So that eases the job of keeping the global negative feedback stable with a different output transformer.
The output power will be reduced because of the switch from UL to Triode Wired mode.
The distortion Without global negative feedback will be lower at low power levels (triode wired mode is lower distortion than UL);
The gain with global negative feedback is about the same for UL and Triode wired mode, gain is dictated by the negative feedback resistors.
But the ratio of open loop to feedback loop is greater for UL than the gain of Triode Wired mode, so the distortion at low power may be about the same for both UL and Triode Wired..
Caution:
I did not look over the complete schematic for any changes, I assumed that all that you changed was connecting the screens to their respective plates through a 100 Ohm resistor, instead of connecting the screens to their respective UL taps.
Any other changes to the rest of the circuit might change the answers I gave above.
But, I think you are probably set for Plug and Play; not Plug and Pray:
Primary impedance, same or different.
Primary inductance, same or different.
There is global negative feedback that has 2 different RC coupling circuits (either the same pole frequency and phase, or different pole frequency and phase), and the low frequency pole of the output transformer. That might need adjusting to prevent low frequency oscillations.
The fact that you are switching from Ultra Linear to Triode Wired mode is to your advantage:
Triode Wired mode has plate impedance, rp, about 1/2 of rp in UL mode.
Triode Wired voltage gain has about 1/2 the gain in UL mode.
So that eases the job of keeping the global negative feedback stable with a different output transformer.
The output power will be reduced because of the switch from UL to Triode Wired mode.
The distortion Without global negative feedback will be lower at low power levels (triode wired mode is lower distortion than UL);
The gain with global negative feedback is about the same for UL and Triode wired mode, gain is dictated by the negative feedback resistors.
But the ratio of open loop to feedback loop is greater for UL than the gain of Triode Wired mode, so the distortion at low power may be about the same for both UL and Triode Wired..
Caution:
I did not look over the complete schematic for any changes, I assumed that all that you changed was connecting the screens to their respective plates through a 100 Ohm resistor, instead of connecting the screens to their respective UL taps.
Any other changes to the rest of the circuit might change the answers I gave above.
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Thank YOU for the quick replies. I'm just going to leave it as-is at least for now. It works fine, just wanted to use some
transformers I bought a long time ago.
transformers I bought a long time ago.
What is the unused winding on the OPT? I have an AMI Rowe amp with a 100VCT winding for line output which I am sure I can use as a tertiary winding for screen grids if I can find the right tube.
M1K35R,
The easiest thing to try . . . is to wire the screens to the plates through 100 Ohm resistors.
You can keep the original UL transformer in the circuit, just change from UL mode to Triode Wired mode.
You might like the sound of the modified amplifier, even though it is lower power.
What are the specifications of the extra transformers you purchased?
Output transformer?
Power transformer?
Perhaps you would like to build another amplifier.
A second amplifier in the bedroom or garage might be nice to have.
The easiest thing to try . . . is to wire the screens to the plates through 100 Ohm resistors.
You can keep the original UL transformer in the circuit, just change from UL mode to Triode Wired mode.
You might like the sound of the modified amplifier, even though it is lower power.
What are the specifications of the extra transformers you purchased?
Output transformer?
Power transformer?
Perhaps you would like to build another amplifier.
A second amplifier in the bedroom or garage might be nice to have.