Adding preamp bias-vary tremolo

[funkgang49]

I'm looking for examples of preamp tube bias-vary tremolo to add to a cathode-biased single-ended amp build.

I would like to use a single twin triode 6SL7/6SN7 octal base preamp tube to drive it.

Any circuit examples would be greatly appreciated.

[tubeswell]

The vibrochamp trem is a nice cathode-wiggler

[bal704]

Might look at some of the Supro Trems.

[funkgang49]

I was just looking at the layout/schem. for the Vibrochamp. It looks like it would be a pretty easy insert in my amp after the Cathode follower V2 tube and before the Phase Inverter.

Are there any component value adjustments that I should be aware of in using an octal based twin triode 6SL7/6SN7 tube as opposed to the 12ax7 tube the Vibrochamp's trem circuit was designed around?

Might look at some of the Supro Trems.

I'll check that out - I love the trem sounds that I've heard in Leon C.'s videos of his Valclone builds.

[tubeswell]

Are there any component value adjustments that I should be aware of in using an octal based twin triode 6SL7/6SN7 tube as opposed to the 12ax7 tube the Vibrochamp's trem circuit was designed around?

No - you can use the values in the vibrochamp circuit

[martin manning]

Are there any component value adjustments that I should be aware of in using an octal based twin triode 6SL7/6SN7 tube as opposed to the 12ax7 tube the Vibrochamp's trem circuit was designed around?

6SL7 is very similar to a 12AX7, so no change required there. 6SN7 is close to a 12AU7.

[Phil_S]

I don't think you'll get it to oscillate with a 12AU7 or 6SN7. Usually a tube with high internal plate resistance is required. That is why it is usually done with a 12AX7. AFAIK, no tone runs of the tremolo, so you can use a sturdy and inexpensive Russian 6n1P.

Good reading here: http://www.lynx.bc.ca/~jc/tremoloCircuits.html

[BLT]

6SQ7 in the GA-40 Trem.

[gingertube]

The 6SQ7 octal is very like the 6AV6 seven pin. It is a 12AX7'ish triode section with 2 detector diode plates.

Doing a GA40 Les Paul Clone right now but as I am re-purposing an old Philips PA Amp, the 6SQ7 didn't physically fit so I'm substituting 1/2 a 12AX7 for the trem oscillator. This amp varies the pentode input anode voltage as well as its cathode. (I'm also subbing in 6BR7 for the 5879 pentodes and 6DQ6 for the output tubes for 30Watts output).

Other "historic" amps used 6AU6 pentode (7 pin) for trem oscillator (eg Danelectro).

As posters above say you need high gain for reliable oscillator, that means high mu triode (12AX7, 6SQ7, 6EU7, 6AV6 etc.) or a pentode.
Slightly lower mu triodes (70'ish) such as 6SL7 (octal) and 6at6 (7 pin) will also work but are more critical on component values.
Cheers,
Ian

[funkgang49]

As posters above say you need high gain for reliable oscillator, that means high mu triode (12AX7, 6SQ7, 6EU7, 6AV6 etc.) or a pentode.
Slightly lower mu triodes (70'ish) such as 6SL7 (octal) and 6at6 (7 pin) will also work but are more critical on component values.
Cheers,

I know the gain factor of the 6SL7 is lower than a 12AX7, closer to a 5751 but not as low as a 12AT7 or 6SN7. Which components and what values might I need to tweak in order to compensate for the slightly lower gain factor gingertube?

[tubeswell]

For a 6SL7 you can use the same R/C network values in the LFO as for a 12AX7.

Generally, anything between 10nF to 22nF caps and 470k to 1M resistors (higher resistance/lower capacitance and vice versa)

A 6SN7 won't work because it doesn't have enough gain to get the stage to oscillate.

Take a look at Merlin's page: http://www.valvewizard.co.uk/trem1.html

Aiken's site goes into more depth in expelling the math. http://www.aikenamps.com/index.php/desi ... o-circuits

[funkgang49]

Thanks everyone! As always TAG and its community is an invaluable resource!

[funkgang49]

And as follow-up to gingertubes 6SQ7 suggestion:

Any advantages/enhanced tone and/or qualities that using this tube would bring to a preamp bias-vary tremolo circuit over using a 12AX7/6SL7?

[tubeswell]

And as follow-up to gingertubes 6SQ7 suggestion:

Any advantages/enhanced tone and/or qualities that using this tube would bring to a preamp bias-vary tremolo circuit over using a 12AX7/6SL7?

What's important for LFO is the voltage swing you can put into the signal that affects the volume. The difference between different types of tubes that can achieve ballparks similar gain is nix.

[tubeswell]

a single twin triode 6SL7/6SN7 octal base preamp tube

FWIW, these tubes are not equivalent substitutes. A 6SL7 has lower mutual transconductance and higher gain than a 6SN7, and the latter is a higher-current tube, and vice versa. (I didn't spot the fact that you had presented these two types as alternatives in my earlier response.)