A bit confused about V1 and V2 in my F-50

F-30, 50 and 100's

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A bit confused about V1 and V2 in my F-50

Post by Andree88 » Wed Oct 12, 2016 4:21 pm

Hi everyone.
I'm a bit confused about V1 and V2 in my F-50.

I though that V1 was for clean and V2 for crunch, but I saw in the manual that V1 and V2 are "both channel input". And I've red that V2 comes first in signal chain.

So I have a question: I've bought a Shuguang 12AX7B (very cheap, but the seller told me that it has great gain to improve my crunch channel). Should I put the Shuguang in V2 or in V1? I'm a bit confused.

Last question: in V3 I have a TAD 12AX7-C, very hi-gain, but I need a very hi-gain in FX Loop? Or shoult I use a "normal" 12AX7 and maybe put the TAD 12AX7-C in V4 (Where now I have a TAD 7025)? Yeah, I use the FX loop everytime (delay, reverb, booster, phaser etc.)

Thanks all :)

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Re: A bit confused about V1 and V2 in my F-50

Post by elvis » Fri Dec 02, 2016 1:19 pm

It's... complicated.

Note that each 12AX7 (or equivalent) is actually TWO independent amplifier stages. So 4 preamp tubes make 8 gain stages. The two halves are called A and B.

The guitar signal in an F50 in the rhythm (clean) channel goes directly into V2B, which is the first stage tube. In the lead channel, V2A is added BEFORE V2B. So V2 is the input either way, though different halves of V2 make the input stage on different channels. This can be useful in diagnosing, as half of V2 can fail, giving a fully-functional rhythm channel and a dead lead channel, for instance.

Next is V1. In rhythm channel, signal path is guitar -> V2B -> V1B.

For Lead channel, It's guitar -> V2A -> V2B -> V1A

So you can see that both V1 and V2 are used for both rhythm and lead channels. Generally, you want the highest gain tube in front, unless it is also the highest noise (or microphonic). Then it is a tradeoff between gain and noise. Best bet is to try the Shugang in V2. If it is not too noisy, you're done. Noisy? Try it in V2. Unfortunately with this amp, you cannot put a low-gain tube in the clean channel and high gain in the rhythm channel. Nor can you bolster the gain of the high gain channel with a high gain tube downstream that doesn't affect the rhythm channel.

As another twist, the tone stacks (Bass, Mid, Treble) are between V2 and V1. So V1 is AFTER the tone controls. That will also affect how the tubes interact with the circuit.

Also, the gain controls are in different places. Lead gain is between V2B and V1B. Rhythm gain is after the tone stack. This means that you control the total gain of V2B and V1B on rhythm, so if V2B is very high gain, it may push V1B and no matter how low you turn the rhythm gain, it may never be quite clean. For lead, you control how much of V2A goes into V2B and V1A, so you will get however much V2 overdrives V1, plus however much V2A overdrives V2B.

On a side note, there is a version of a 12AX7 that has low gain on one channel and high gain on the other. One of these used as V2 could give you super clean rhythm and heavy lead. Alternatively, a low gain 12AX7 in V2 can give you pristine cleans, but your overdrive will be limited, as BOTH gain stages of V2 will be low.

At this point, both channels converge. They run into V3A and then the Reverb circuit. The Reverb is driven and buffered by opamps. Note that only the tank signal goes through the opamps. The dry signal bypasses the reverb circuit and they are summed on the way into the EQ.

Next is EQ and FX send. The FX send is driven by the EQ, a transistor circuit. The FX return is buffered by V3B. So for V3, gain is not super important, though it does affect total gain of the signal chain. Try it. If you like it, you're good. If it's noisy or microphonic, maybe a long-plate would be better.

Finally, V4 A&B make the Phase Inverter that drives the output tubes. So V4 needs to have matched gain between the two halves. Total gain on that tube is not as important.

Total signal path:

guitar -> V2B -> Tone Stack -> Gain Control -> V1B -> V3A -> Reverb -> EQ -> FX Loop -> V3B -> V4 -> Power Tubes

guitar -> V2A -> Gain Control -> V2B -> Tone Stack -> V1A -> V3A -> Reverb -> EQ -> FX Loop -> V3B -> V4 -> Power Tubes

Good luck!
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