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Step 8Mod that amp!
A rebuild is the perfect time to mod your amp!
Here's a great link: Tubes for Dummies! :-) It outlines how each component in a tube circuit functions, and explains their purpose. Note: designing viable circuits really is more complicated than this...
Search for schematics of similar amps. I found at least 7 schematics of amps that use the 12AU6, 35Z5 and 50L6 tubes. Compare those schematics to your amp. Having similar examples is the best way to start, especially if you are unfamiliar with tube circuitry.
Note that the schematic drawing for the 12AU6 looks different--this is a pentode valve, rather than a triode, which is normally used for preamps. But the component functions are analogous.
Make one change at a time. Really large changes will probably be...unsuccessful (unless you really know what you're doing, or the design was flawed to begin with.) Here are some examples:
--Change coupling caps. They'll effect the tone. Smaller values will be brighter, larger values will add bass.
--Adjust the " grid leak resistor ," which effects the impedance and the gain of the stage.
--The initial input attenuation/RC network can be adjusted (resistors directly after the jack.)
--Cathode bypass caps on both the preamp and the power amp stage can be adjusted. This effects both the response and the tone (to a lesser extent.)
--Changing the values of the Plate Resistor and the Cathode Resistor have a direct effect on the gain, headroom and distortion. Don't do this without a thorough understanding of the tube's characteristics, however.
--Changing your speaker is another modding possibility. Be sure to match the speaker's impedance to the transformer.
--Output transformers are harder to replace, unless the schematic is well documented. If it's not, look up the power amp tube data sheet and it's 'load resistance.' For instance, load resistance for the a 50L6 @ 100V is 2000 ohms, and @ 200V, 4000 ohms. This amp uses a plate voltage of 135V, so 2500-3000 ohms for the primary is close. Choose the secondary impedance based on the speaker...
Here's a great link: Tubes for Dummies! :-) It outlines how each component in a tube circuit functions, and explains their purpose. Note: designing viable circuits really is more complicated than this...
Search for schematics of similar amps. I found at least 7 schematics of amps that use the 12AU6, 35Z5 and 50L6 tubes. Compare those schematics to your amp. Having similar examples is the best way to start, especially if you are unfamiliar with tube circuitry.
Note that the schematic drawing for the 12AU6 looks different--this is a pentode valve, rather than a triode, which is normally used for preamps. But the component functions are analogous.
Make one change at a time. Really large changes will probably be...unsuccessful (unless you really know what you're doing, or the design was flawed to begin with.) Here are some examples:
--Change coupling caps. They'll effect the tone. Smaller values will be brighter, larger values will add bass.
--Adjust the " grid leak resistor ," which effects the impedance and the gain of the stage.
--The initial input attenuation/RC network can be adjusted (resistors directly after the jack.)
--Cathode bypass caps on both the preamp and the power amp stage can be adjusted. This effects both the response and the tone (to a lesser extent.)
--Changing the values of the Plate Resistor and the Cathode Resistor have a direct effect on the gain, headroom and distortion. Don't do this without a thorough understanding of the tube's characteristics, however.
--Changing your speaker is another modding possibility. Be sure to match the speaker's impedance to the transformer.
--Output transformers are harder to replace, unless the schematic is well documented. If it's not, look up the power amp tube data sheet and it's 'load resistance.' For instance, load resistance for the a 50L6 @ 100V is 2000 ohms, and @ 200V, 4000 ohms. This amp uses a plate voltage of 135V, so 2500-3000 ohms for the primary is close. Choose the secondary impedance based on the speaker...
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