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Old 22nd September 2015, 03:22 PM   #1
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Smile single channel 12ax7 , F-2B, cathode follower using 5670

I've finished building a single channel F-2B using a 12ax7, I have some 5670 tubes laying around and I was wondering, if I could use one of these as a cathode follower/buffer? What would be the best cathode follower design using a 5670 tube that will buffer the output a bit and increase the gain a tad.

Thanx so much
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Old 23rd September 2015, 10:21 AM   #2
DF96 is offline DF96  England
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What is an F-2B? If it is single channel, should this thread perhaps be in the Instruments & Amps section?
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Old 23rd September 2015, 01:25 PM   #3
mike567 is offline mike567  United States
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Alembic F2B is used frequently used as a bass guitar preamp
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Old 23rd September 2015, 03:12 PM   #4
JMFahey is offline JMFahey  Argentina
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Yes, why not
Lower output impedance?
Yes.

Increase gain?
Hardly.

What do you use that preamp for?
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Old 28th April 2016, 05:04 PM   #5
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It is used for a bass pre amp running into a solid state power amp with a input impedance of 20K
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Old 10th May 2016, 07:24 AM   #6
JMFahey is offline JMFahey  Argentina
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Oh, it can drive it, no big deal.

Although the power amp input impedance is lower than last triode plate load , so you *will* have some signal attenuation (I'm not denying that), the fact is the preamp has both lots of headroom anyway and way more signal swing than needed.

In practice it works.

How do I know?
I have seen lots of homemade Bass amps, made out of "a Fender channel" (what F2B is) straight driving some ready made or kit built generic transistor amp, with excellent results.

That said, if you want to do things the right way, you should add a buffer at the F2B output, at least so it can drive most anything better, including longish (more than a couple ft) cables.

If space allows, almost any tube connected as a Cathode Follower will do.

Or a MosFet Follower (google some schematics) which need no filament and can be fed same +V as the 12AX7 and take minimum space ... just remember that although "a transistor" you still have high voltage there, there is a tendency to consider anything "made out of plastic" a harmless thing

Or you could attenuate signal and then buffer it with a low voltage transistor or Op Amp ... but then you will need to feed them.
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Old 10th May 2016, 07:58 AM   #7
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I have done the cathode follower/buffer, using one stage of a old NOS 12au7 with excellent results. it improved the overall tone a whole lot, the headroom also improved a lot especially when playing in low. Tuning's drop A and B standard , it was still clipping the power amp tho so I installed a 10Kohm pot right after the output of the CF, it is sounding absolutely killer, using a boss Bass equalizer pedal in front to boost the signal almost into overdrive, and to dial out the nasty's that come with low tuning in bass.
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