What's this in old Fender amps?

[slideman82]

I've been seeing this configuration in some schematics and never payed attention, but now I wanted to know what does that 12ax7 do before the phase splitter.

http://ampwares.com/schematics/bandmaster_6g7.pdf

[DigitalJunkie]

I'm not a Guitar amp guru,like some here..Buut.. It's an oscillator for the Vibrato function.

[dcgillespie]

It is a "vibrato" circuit that was used before the availability of LDRs. It takes two 12AX7s to operate. Basically, one 12AX7 section is the LF tremolo oscillator. It feeds a common phase splitter to develop two equal, but out of phase LF tremolo oscillator signals. These two signals are then sent to the two 12AX7 sections (one tube) that you are questioning. This tube is where the tremolo action happens. But also mixed into the signal driving this tube are the audio signals that are to have the vibrato effect impressed upon them. But notice, the audio is applied through frequency sensitive networks: the LF signals are applied to one section, while the HF signals to the other. At the plates of these two sections (right before the power amp phase inverter section) the signals of these two sections are combined. The two audio signals are combined back into one full frequency signal, but now have been modulated by the LF tremolo signal also applied to the two sections: Since the LF tremolo signals are out of phase, each section of the second tube is alternatingly boosted or reduced in gain to give the semi vibrato effect, but also since they are out of phase, the actual LF tremolo signal cancels out at the mixed signal, so that only the vibrato modulated audio signal remains.

With a LDR design, all signals are equally boosted or cut at the same time producing a true tremolo effect, so the early approach Fender used was quite unique.

I hope this helps!

Dave

[slideman82]

It realley helped me! Thanks, really! I will sleep well tonight! With the avaibility of LDR's circuits became cheaper, also... I just have to try each tremolo version someday!