What valve should i use for lower gain?

What valve should i use for lower gain?

A customer recently asked “what valve should i use for lower gain?”

 

It’s a great little combo, nearly perfect and a great platform as I tend to use pedals to get my overdrive sounds. But to my ears the lack of a middle control on the clean channel is reducing the tonal flexibility and the treble is quite shrill and difficult to tame. Having played with the drive channel (which has T, M and B controls) with minimal drive dialled in, its starting to deliver more of the tone I am looking for. However, it’s the drive channel and designed to distort and therefore has a lower headroom, so I am thinking that I might be able to reduce the gain in the pre amp by replacing the 12AX7 with a 12AT 7or even a 12AU7 to help it clean up. Just wondering if you had any thoughts on this.

Replace a 12AX7 with a 12AU7 for the lowest gain

You’ll definitely find the 12AU7 cleans up your gain channel. The amplification factor is reduced from 92 to 17. You’ll also find that there’s a loss of volume, you’ll have to run the volume control much higher than you would normally. I’ve used this trick with Marshall plexi type amps and they end up sounding like a clean funk machine. I like the sound quite a lot!

Replace a 12AX7 with a 12AY7 for lower gain

The 12AY7 is the classic fender tweed valve. It is a very usable effect, midway between the 12AU7 and the 12AT7 in terms of gain. Taking Electro Harmonix valves as an example, the amplification factor is reduced from 92 to 40.

Replace a 12AX7 with a 12AT7 for slightly lower gain

The 12AT7 is a more subtle effect, midway between the 12AY7 and the 12AX7 in terms of gain. The amplification factor is reduced from 92 to 50. This is probably the valve to choose if you’re just looking to slightly clean up the amp

Replace a 12AX7 with a 5751 for a subtly lower gain

The 12A*7 series valves all have paralells in a 4 digit code series. The 12AX7 is also known as a 7025 and has another slightly lower gain cousin. The 5751 is almost a 12AX7 but the gain is dropped. The amplification factor is reduced from 92 to 70. This is another valve to choose if you’re just looking to slightly clean up the amp.

 

Hi Rowan,
Great success, although I’ve not yet managed to get the amp up to full ramming speed. Certainly at lowish levels the 12AU7 has really taken the sting out of the all valve channel, it cleans up beautifully and I think I can probably just about get enough grunt out of the channel for gigging by maxing out the volume on the output side. The clean sound now from the drive channel is very smooth with a different character to the clean channel – which is what I wanted to achieve although the eq on the amp doesn’t quite get me where I wanted. I suspect though when I get the amp up to louder levels and start driving it harder, the top end will expand and it will be near perfect. And it takes pedals beautifully.

A big thanks for all your help.

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