Amps: retubing Marshall 1923C 85th Anniversary Combo - Part 3

Introduction

This is my first retubing project for my beloved Marshall 1923C 85th Anniversary Combo (based on DSL50) and, a continuation of the first and second entries related to this topic.
The tests of Part 2 seemed enough but, the the truth is that, playing a bit more I've realized about two issues:

•  The 12AY7 in V2 was changing the distortion character, what was nice for cleans but, not so good for crunch and high distortion.
• The 12AY7 in V2 was making the pre-amp to break up later, what obliged me to increase the gain control to much, so introducing more unwanted noises and, I had to raise the overall volume a tad more to get better results.

So, as it happened with the swap of a 12AX7 tube for a 12AT7 in V4, to lower the gain in those two positions doesn't seem to help to the tone of this particular amp.


More pre-amp tests

Test 7

This time, I just wanted to confirm that a change in V3 (tone stack / reverberation) will have no impact in the tone and neither in the way as EQ and reverberation work. Since I swapped it with a Sovtek 12AX7-WC, I wanted to bring back the original tube and make some comparison.
I've put there the original Marshall tube that was in position V3 that, in fact, is just a JJ ECC83S relabeled.

So, the pre-amp configuration was:

V1: Mullard 12AX7 reissue
V2: EH 12AY7
V3: Stock Marshall
V4: Sovtek 12AX7-LPS

I didn't noticed any change in tone or in the way the EQ and Reverberation controls were working. So, it seems to effectivelly have no impact in the tone so, a specifications-correct tube here should be enough.

Test 8

Reviewing the notes of the previous tests, I liked the silkiness of the Svetlana and, I liked more the results with the TAD 12AX7-Cz (a verified JJ ECC83S) even. I've started a new battery of tests removing the not 12AX7 tubes from the equation, since they doesn't seem to work well (to my ears) in this amp.
I've swapped in V2 the EH 12AY7 with a TAD 12AX7-Cz, that is equivalent to the stock Marshall tube (both are just JJ ECC83s verified for their goodness and relabeled).
For V1, I wanted to recheck the Svetlana, once those foreigner tubes (12AY7 and 12AT7) were removed.

And this was the configuration of the pre-amp:
V1: Svetlana 12AX7
V2: TAD 12AX7-CZ
V3: Stock Marshall
V4: Sovtek 12AX7-LPS

It resulted in a good combination, with a sound that was warm and with a very even EQ. Good gain and takes pedals perfectly in both channels. The resulting sound is a TAD darker, overall.

Test 9

Well, that 12AX7 in V2 made a lot of differences so, time to test again that Mullard 12AX7 reissues. I've thrown a pair in V1 and V2, to check this tube in both channels.

This was the configuration of the pre-amp:

V1: Mullard 12AX7 reissue
V2: Mullard 12AX7 reissue
V3: TAD 12AX7-Cz
V4: Sovtek 12AX7-LPS

Respect of the previous combination, the clean channel had a tad more gain, while the crunch channel had less gain so, I would say that the Mullard has more gain than the Svetlana but less than the JJ.
In clean, very detailed tone and took all pedals without any issue.
In crunch, very detailed also but, made me to rectify the EQ to give some body to the sound. Respect of the previous test, the sound was more open but also more edgy. In crunch channel, the Plexi Drive was working a bit constipated, not so good as with the tubes in test 8.

What always happens with those Mullard reissues is that they work really awesome, when the gain is at 1:00 to 2:00 o'clock, when they deliver a rich crunch sound but, at lower levels, they can sound a bit weak, specially in complex front-ends.


Test 10

Well, the more I try, the more clear to me is that this amp seems to be designed around JJ ECC83S tubes so, it's time to come back to stock configuration. For this test, I've thrown three TAD 12AX7-Cz in the preamp and, the configuration was:

V1: TAD 12AX7-Cz
V2: TAD 12AX7-Cz
V3: TAD 12AX7-Cz
V4: Sovtek 12AX7-LPS


Definitively, this amp seems to had being designed around the JJ ECC83S tube. With a couple of TAD 12AX7-Cz in V1 and V2, everything is on its right spot. Cleans are now beautiful detailed, with a nice body and not piercing or edgy. Crunch is smoother than with other tubes but, the overall sound is really good and, surprisingly, while the other tubes made the crunch channel way darker than the clean channel, the JJ makes a warm an smooth clean channel and a brighter crunch channel.

Still more good things, the amp is now the quietest of all tested configurations. JJ's are usually working with low floor noise and low microphonics issues but, since those are verified JJ's by TAD, they should work even better than any random JJ tube. The results are quieter than with stock Marshall tubes (well, once again, relabeled JJ's)!!!. Both channels are now very quiet and, that's awesome.

I don't feel in the need of further tests, related to V1, V2 and V3, since in my previous tests, was very clear that the only two tubes more defined than the JJ ECC83S were the Mullard 12AX7 reissue and the RFT ECC83 but, those delivered and edgier and weaker sound under low volume settings. I don't believe neither that a Tung Sol in V1 can add anything else, since it sounded way less defined and blurred under high gain conditions.


So, for V1 and V2, best tube is JJ ECC83S, without any kind of doubt.
For V3, it makes not noticeable difference to get one or the other but, since JJs are cheap enough and, the result is really quiet in noise, to leave there one more JJ seems a good thing.
For V4, I still want to run one more test and, see what happens when swapping the LPS with a Mullard reissue, just for fun.


Test 11

The Sovtek 12AX7-LPS is a very good tube for PI positions but, during my tests in other amps (high gain combos, specially), it lasted very short. In paper, the Mullard reissue should have a better transconductancy value than the LPS and, this would make it a better candidate for that PI position (once I saw that helps nothing in tone positions: V1 and V2).
Higher transconductancy means faster response of the tube and thus, better definition note by note when playing fast riffs. Without any doubt, a 12AT7 tube has way more transconductancy than any 12AX7 but, in my previous tests, didn't liked the results, because the 12AT7 has side effects: firstly, it drops the gain of the PI so, it gives more headroom to the power tubes to remain clear at louder volumes and, it want those to break up early, to move down the overall loudness; secondly, it drains ten times the current of a 12AX7 what, at the end, seems to affect weakening those 12AX7 tubes in the rest of positions.

The results in test 10 are fully satisfactory at all levels so, this test is just to satisfy my curiosity and see if the Mullard reissue can do a better job in this position, while maintaining everything as quiet as it is right now.
Configuration was:

V1: TAD 12AX7-Cz
V2: TAD 12AX7-Cz
V3: TAD 12AX7-Cz
V4: Mullard 12AX7 reissue

Mixed results and some surprise here.
In one side, it's absolutely clear that the notes are better defined, specially noticeable with speedy riffs and, even more the more you raise the gain level (in your amp and with gain pedals).
The drawback is that this tube, respect of the LPS, makes the sound thinner, weaker and edgier and, the clean channel looses some of its previous body and silkiness.
So, clearly, this tube has less gain and less output than the LPS and, probably gets harder to pump up the power stage. The higher transconductancy defines better speedy riffs but, to get a nice tone, you need to push the amp way harder than with the LPS. If you can krank your amp then, probably the Mullard reissue will be a better option, because will maintain an awesome note-by-note definition, even under high gain settings.
I am surprised because this change had more impact in the overall tone (not just the immediateness or responsiveness of the amp) than expected. Probably that weaker output and gain is removing part of the natural harmonics of the EL34s and, this clearly affects the overall tone.
So, if the LPS makes a difference respect of that Mullard reissue, will a JJ ECC83S make a difference respect of that LPS?. Seeing that this amp seems to be designed around JJ ECC83S tubes, it worth my last tube swapping try.


Test 12

Time to check if the JJ ECC83S in the PI position will enhance even more what it's already a nice sound with the Sovtek LPS in such a position.

The configuration was:

V1: TAD 12AX7-Cz
V2: TAD 12AX7-Cz
V3: TAD 12AX7-Cz
V4: TAD 12AX7-Cz
The clean channel is just perfect. It has the high definition of that Mullard 12AX7 but, with more punch and body, and some kind of warm pad in the bottom. Seems to have less gain than the Mullard but, way more output so it works very well in the clean channel.
The crunch channel, instead, sounds more darker than with any of the other two tubes, with lots of mid-bass and bass frequencies. Also slightly less gain than with the Mullard, but more punch and low end.
It seems as if the LPS had the right EQ for better balance both channels but, the JJ has more punch and body so, my final decision for that position will be very related to which better cuts the mix.
I have to re-EQ the tone stack and try all this with backing tracks.


Final Conclusions

Don't waste time, energy or money. The preamp of this amp (based in the line of DSLs) loves JJ ECC83S tubes. No other tube works better, neither NOS tubes. Just be sure to take verified tubes from a well known tube dealer (as TAD, Watford Valves, Eurotubes, etc...), because you will get a crystalline clear JJ with very low levels in noise and microphonics. Fortunately, this is one of the cheaper tubes so, thanks Marshall to design this amp around that tube. It makes it easier and cheaper to maintain this pre-amp in a good condition.

The only pre-amp tube that, maybe, makes some sense to upgrade is the one in V4 (PI position). Here a tube with higher transconductancy can make the difference to achieve a better note-by-note definition and immediateness when playing fast riffs. For this, the Sovtek 12AX7-LPS is a great candidate but, we aware that occasionally can last very short in combos. The JJ will work perfectly, also but, darkening the tone in the crunch channel and providing more punch and low end.