I would like to get some spare tubes for V2 and V3 in the GPDI/IR. According to the manual they should be Shuguang 12AX7A, which seems to no longer be in production.
At the same time, my GPDI/IR came with Ruby 12AX7AC5 HG+ in both V2 and V3 positions.
I know they are perhaps interchangeable, but what tubes should I get?
Can I use the same tubes for V1, V2 and V3?
Long&McQuade (I am in Canada) sells three different variants of the Ruby 12AX7AC5 HG+ - which one is best to get for V2 and V3? (see below)
From what I understand the difference between the 123 and 678 is which side measures higher gain(?) - left / right, not sure about the first variant the HG+ only). Does that matter for V2 and V3 positions?
Or should I look for ShuguangB ?
From my search online Ruby rebrands both JJ and Shuguang tubes, so below tubes may very well be ShuguangB.
Honestly, I think any decent quality tube would be OK. We should update the user manual if those are no longer available.
Tube rolling is fine; you can’t really make an error or break anything with preamp tubes, so I would say feel free to experiment. Steve would have had a point about why he liked the Shuguangs, but I’m not sure about the effect he liked. I think V1 is the most important anyway.
Feel free to post data sheets if you can find them for the tubes you are considering. Can help with at least understanding what you are buying.
Does it make any difference which side has more gain (I guess it doesn’t) and if I should go with two that match - 2x HG+ 123 or 2x HG+ 678 or it doesn’t matter?
And is there any general rule for the tube stages in terms of gain? like V2 > V1, V3 > V2 etc.
I’d say it really depends on the circuit the tube is in. But this whole thing bothers me.
Basically, there is one spec for 12AX7: with 250V on the anode (with the cathode grounded) and -2V on the grid a current of 1.2mA should flow. With a small AC voltage injected, the voltage gain should be 100 and the transconductance 1.6 S (or an AC resistance of 62.5k Ohms).
That is it, that is the spec.
The thing that annoys me is that they are measuring these numbers and making small differences seems like an important choice. They should simply quote the Ia, mu and ra above, and tell you how close each is to the spec.
Honestly, I don’t think you will be able to tell a difference.
Anyway, to answer your actual question. Generally, if early in the stage, add harmonics that will be amplified and further distorted by the other tubes in the chain.
money for nothing.