Valve Gurus... 5751 in P.I.?

AlanRatcliffe

I just managed to get my hands on a NOS GE 5751 to experiment with on my Vox AC15. I'm planning on trying it in V2 (first valve in the "Top Boost" channel, which seems to be the standard place to try them.

I got to wondering if the lower gain of the 5751 might make it a good choice for the phase inverter, bringing the overall volume of the amp down. On the other hand, in a non-master volume amp like the Vox, would it not kill some of the output stage's tone? I realise the only real way to find out with any amp is to try it and see, but are there any other considerations that would make it a good/bad idea?

Gearhead

I surely do not qualify as a tube guru, but for what it's worth:
I guess it comes down to whether with the 5751 in the V2 position you still drive the phase splitter into clip. If you do, putting the 5751 in the phase splitter would change the sonic nature: you would get more clip on the 5751 (purely triode distortion, unless you turn the preamp down) and less drive onto the EL84's (with their pentode distortion). If you never get to clipping the phase splitter with the 5751 in the V2, it makes little difference imho.
So there's two questions: which distortion do you like better and how do you know at what stage it goes into clip?

AlanRatcliffe

Thanks. I think in either V2 or PI it will bring down the drive from the EL84s, the difference being that in the PI position it will also affect the EF86 channel (which I use as my crunchy edge-of-distortion channel).

It's very nice in V2, lets me turn up more and, as I use the Top Boost channel as my clean it works really well. A lot more headroom and clarity with a slightly sweeter edge. Still, it'll be worth a try in the PI too (who knows? I may end up trying to find another so I can use it in both).

Gearhead

Don't you think a modern 12AU7 would have a similar effect on the relationship between the channels whilst being cheaper and easier to get?

AlanRatcliffe

This is the Heritage Hand-wired. Only a few months old... Kinda my version of a practice amp, LOL!

Don't you think a modern 12AU7 would have a similar effect on the relationship between the channels whilst being cheaper and easier to get?

You mean a 12AT7 (AU7 has a gain of only 20)? At the risk of sounding like I know what I'm talking about: The gains are similar between a 5751 and a 12AT7, but there's a big difference in transconductance and current output:

ValveGainCurrentTC
12AX71001.2 mA1600
5751701 mA1200
12AT76010 mA5000

So you can see that a 5751 is quite similar to a 12AX7 aside from the gain reduction.

Schematic here, BTW:
http://ratcliffe.co.za/sundry/vox.gif

Gearhead

I did mean 12AU7 but you're right, the AT gain is closer.

Transconductance must be seen in conjunction with plate load - together they will make voltage amplification. I have yet to see a schematic with a 12AX7 at mu of 100. My point? it's down to how the particular characteristics of the tube will work out in that particular part of the circuit. I invite you to name the AC15 stage where you want to put in a 5751 and I'll do the calc vs. 12AT7. :goodtimes:

After going through the AC15 power supply thoroughly I'd say it will hardly notice the change in current consumption.

In general, for others: you can replace any 12AX7 for a 12AT7 and get less gain, without running the risk of f'ing up your amp. Checked it with Karel.

AlanRatcliffe

I couldn't find any reason why I shouldn't try it (in fact I found a few people who recommended it highly for Blackface Fenders). So I did it the old-fashioned way - swapped out tubes and used my ears. The 5751 in the P.I. position took a lot of sparkle out of the whole amp, so I went back to the Philips Miniwatt 12AX7. In V2 (first valve on the top boost channel) it's magical - lots of Vox chime, even at lower volumes) and more clean headroom.

I managed to lay my hands on some TAD EL84s and a NOS Tesla EZ81 and the amp is really singing now, with less mushiness. Mojo JoJoe has a couple more NOS preamp valves he picked up for me a while back, so I should have some more to fiddle with sometime in the near future (when I can get into town) - but the amp's already making me happy.