(Yet) Another question about taming the PRS MT-15 drive channel

McToot

I feel this has probably all been answered before but I'm struggling to find all the relevant threads. I've certainly seen a recommendation from @notonlybutalso which I'm considering, as @jaymenon is offloading a load of valves at the moment. 

So first of all I am going to replace the loop valve for an ECC82/12AU7 as recommended by a tech from PRS when I first moaned about the slightly noisy loop. 

Then I would like to put in some ECC81s/12AT7s in the drive side to take some of the edge off and make the gain sweep a little more forgiving. This is something I've seen suggested before.

My question is this: according to the following schematic would I have to replace V1, V3 AND V4 or just V1 and V3 (or even V3 and V4)?

jasonbone75

This is what I did with mine:

V1 becomes Tung Sol ECC83/12AX7

V3 becomes JJ ECC83/5751

V6 becomes BALANCED JJ ECC81/12AT7

You get less overall gain on lead and more of the gain control becomes usable. I didn't use the loop on mine so left that alone but the recommended V5 swap sounds right to me.

The 5751 has about half the gain of a 12AX7 and I would suggest swapping just V3 first and seeing how you like it as it may tame it enough.

I'm not sure the 12AT7 is your best bet in the lead positions - @ICBM or @ecc83 might chime in here as there is something I remember about not being always a drop-in replacement for the higher gain ones.

ecc83

Hi Jason, I have two objections to the 'ad hoc' swapping of the 83 for other double triodes.

The first is, without intimate knowledge of the anode load ratings, fitting a valve with a lower mu will cause it draw more HT current possibly putting said loads in danger of burning out. IF you can be sure they are rated at 1/2W or more, no problem but some circuits are not so rated* You might get away with the ECC81 but not the '82.

The second objection is more 'hi fi' but is general. The wrong valve will be wrongly biased and therefore its headroom and distortion level will be compromised. That means the circuit might sound 'rough' at all levels.

*The original Blackstar HT pedals necessarily used wee, but perfectly adequate anode loads. I doubt they would cope with an ECC82. I know nothing of the latest incarnations.

Dave.

Neilybob

Buy a MT-100 instead?! 

StefB

Neilybob said:

Buy a MT-100 instead?! 

Even better, a H&K Trilogy if you can find one. 

snowblind

You might want to take a look at Psionic Audio's yt channel, specifically the clip about the PRS Sonzera. His comment is that the loop design, at least in that model of PRS, is inherently noisy. I haven't had chance to watch some of his other PRS clips but the man does know his way around an amp circuit. If the MT-15 uses the same basic layout as the Sonzera then the indication is there is not much to be done to cure the loop noise short of a fairly major rework.