Switching Tube Gain Stage

[idylldon]

I've got a couple Gates tube pres that I really love but they have a TON of gain that I don't usually need.  While using a pad on the input works, I'd like to be able to switch one of the 5879s out of the circuit to lessen the overall gain (and noise!).  I've attached a schematic and would appreciate any thoughts on the best way to do this.  Can I simply switch the lead coming off the volume control of the second stage to the grid of XV3 and connect it to the grid of XV4, ground the grid of XV3, and then open the connection between XV3 and XV4? 

Cheers,
--
Don

 

[emrr]

I have bypassed many 2nd stages in these amps.  I have also left them alone.  You won't remove a stage and suddenly not need an input pad.  Headroom in V1 and V2 is about the same, and the dual volume control does work really well to reduce both gain and noise floor, when you think you might really need 90dB gain.  In terms of gain, I never need so much, so killing a stage makes sense on the surface.  Noise may actually be better with both stages and partial gain control reduction.  I would measure noise in 5 or 10 dB gain reduction increments as-is, so you have benchmarks, then remove a stage and see how it compares.  I haven't gone so far as to do that yet. 

The second largest argument for removing a stage would be so the volume control can stay further up, lessening the chance of high end loss.  In practice I haven't seen any change in highs after the mod. 

A fair number of 1930's broadcast amps provided gain strapping options exactly like we are discussing, so you could alter max gain as needed for your installation. 

 

[idylldon]

Thanks, Doug! 

Does my plan about the switching seem OK or is there a better way I'm missing? 

Cheers,
--
Don

 

[emrr]

I see what you proposed; no.  V3/4 is a feedback looped output stage.  Lose V2 instead. 

Move V3 grid lead from atten sweeper 2 to atten sweeper 1 after disconnecting V2 grid lead from sweeper 1.  Take V2 out, put tape over the socket.  Done. 

 

[idylldon]

Great.  Thanks!

 

[ricardo]

The schematic is a bit confusing so I'm not sure exactly what you are doing.

Check the Phase when bypassing single tube stages.  A single tube stage inverts so bypassing it will change phase.

If a stage has 2 tubes, it is usually non-inverting so OK to bypass.

 

[emrr]

Well, of course it'll flip the polarity.  Goes without saying.  Polarity isn't marked or adhered to much in many of the old wiring departments, you have to check it.