SEM Mod on a 5f6-A

[harpdog123]

I'm a harmonica player and I just received the SEM mod kit for my 59 Bassman and I was wondering what other modifications would be necessary aside from the pot and capacitor that are added to the output stage.  Should I add grid stoppers and increase the value of the screens?

Best Regards,
David Pearce

[K O'Connor]

Hi Guys

As far as the SEM goes, it contains the only parts needed for that specific modification.

Grid-stops are missing from the output tubes and Fender later adopted 1k5 as standard. The 470-1W screen-stops should be 1k-5W wire-wounds.

Many harmonica players find that a lower-mu tube in the front end helps give a smoother tone. The stock 12AX7 has mu=100; 12AT7 has mu=70; 12AY7 has mu=50; 12AU7 has mu=20. X and T are widely available; U is inexpensive but may be too big a loss of gain; most harmonica guys like Ys which are a bit harder to get.

The original schematic specs a Y in the input section followed by two Xs. Using a T for the splitter gives a cleaner sound, however...

if you want a warm sound the SEM set just a bit off full loudness may give you the harmonics you want. Were you to modify the bias circuit so each tube had its own control and monitoring, say using BMK2, this would allow you to sub a 6V6 or other pin-compatible tube in one socket and still be able to hum-balance the output stage. To mix in anything that takes more than 900mA of heater current would require adding an auxiliary PT to supply current just to the output stage.

One convenient thing about the 6V6 is that it is fully voltage compatible with 6L6 and you can simply pull out a 6L6 and insert a 6V6 without worries. The dissimilar tubes provide a complex texture not available with same-type pairs.

Since the amp is fixed biased, you can pull either tube and run it that way safely forever if you like the sound. You may get about 1W clan and 5W distorted.

The Schmitt splitter inherently cancels some of the even harmonics - not all of them, as it is not perfect. The earlier amps with concertina splitters have a warmer sound.

[harpdog123]

Hi Guys

As far as the SEM goes, it contains the only parts needed for that specific modification.

Grid-stops are missing from the output tubes and Fender later adopted 1k5 as standard. The 470-1W screen-stops should be 1k-5W wire-wounds.

Many harmonica players find that a lower-mu tube in the front end helps give a smoother tone. The stock 12AX7 has mu=100; 12AT7 has mu=70; 12AY7 has mu=50; 12AU7 has mu=20. X and T are widely available; U is inexpensive but may be too big a loss of gain; most harmonica guys like Ys which are a bit harder to get.

The original schematic specs a Y in the input section followed by two Xs. Using a T for the splitter gives a cleaner sound, however...

if you want a warm sound the SEM set just a bit off full loudness may give you the harmonics you want. Were you to modify the bias circuit so each tube had its own control and monitoring, say using BMK2, this would allow you to sub a 6V6 or other pin-compatible tube in one socket and still be able to hum-balance the output stage. To mix in anything that takes more than 900mA of heater current would require adding an auxiliary PT to supply current just to the output stage.

One convenient thing about the 6V6 is that it is fully voltage compatible with 6L6 and you can simply pull out a 6L6 and insert a 6V6 without worries. The dissimilar tubes provide a complex texture not available with same-type pairs.

Since the amp is fixed biased, you can pull either tube and run it that way safely forever if you like the sound.  You may get about 1W clan and 5W distorted.

The Schmitt splitter inherently cancels some of the even harmonics - not all of them, as it is not perfect. The earlier amps with concertina splitters have a warmer sound.

[harpdog123]

Hi Guys

As far as the SEM goes, it contains the only parts needed for that specific modification.

Grid-stops are missing from the output tubes and Fender later adopted 1k5 as standard. The 470-1W screen-stops should be 1k-5W wire-wounds.

Many harmonica players find that a lower-mu tube in the front end helps give a smoother tone. The stock 12AX7 has mu=100; 12AT7 has mu=70; 12AY7 has mu=50; 12AU7 has mu=20. X and T are widely available; U is inexpensive but may be too big a loss of gain; most harmonica guys like Ys which are a bit harder to get.

The original schematic specs a Y in the input section followed by two Xs. Using a T for the splitter gives a cleaner sound, however...

if you want a warm sound the SEM set just a bit off full loudness may give you the harmonics you want. Were you to modify the bias circuit so each tube had its own control and monitoring, say using BMK2, this would allow you to sub a 6V6 or other pin-compatible tube in one socket and still be able to hum-balance the output stage. To mix in anything that takes more than 900mA of heater current would require adding an auxiliary PT to supply current just to the output stage.

One convenient thing about the 6V6 is that it is fully voltage compatible with 6L6 and you can simply pull out a 6L6 and insert a 6V6 without worries. The dissimilar tubes provide a complex texture not available with same-type pairs.

Since the amp is fixed biased, you can pull either tube and run it that way safely forever if you like the sound.  You may get about 1W clan and 5W distorted.

The Schmitt splitter inherently cancels some of the even harmonics - not all of them, as it is not perfect. The earlier amps with concertina splitters have a warmer sound.

Thanks for responding to my questions so quickly!  I've already got the Ay7 in the first preamp position.  Funny that you mentioned the balanced adjustable bias, because I was just considering doing that myself.  It would be a lot of fun experimenting with different tube combinations.

[K O'Connor]

Hi Guys

The bias should not be a 'bias+balance'; rather, two independent bias controls, one for each tube.

Have fun