Amps: retubing Marshall 1923C 85th Anniversary Combo

Introduction

Of my three amps, the Marshall 1923C 85th Anniversary Combo is the most beloved. '70s Hard Rock is what pumps my heart, I guess and, early Marshall's vibe is what I breathed from the cradle. Even that I love so much a clean Fender or a clean or crunchy Vox sound (as many other outstanding sounds), Marshall is were I feel at home, usually.

Paradoxically, I've done extensive tube tests in all my amps (the ones I own know and, the ones already sold) except for that Marshall Combo. For your tone reference, the 85th Anniversary is some kind of DSL50 with a better wood and well paired speakers (one V30 and one Heritage).

This one came from a German store and, even that it was sold as brand new, I am 100% positive, it was hardly used in the Store (blame you!) for demo, testing or whatever. But, it was so nice and good and, a limited run that I wasn't consider to send it back, because was hard to find this amp and, worst to find it at a good price tag.

It sounded like heaven during first weeks then, the first pre-amp tube started to fail so, I had to swap this tube. I hadn't stock of tubes that days so, I've just thrown there a TAD 7025 Highgrade tube (because of low microphonics and noise) and, even, I've found it always piercing, I was so Lazy to swap it, since this means to remove the back metallic grid to access the tubes and, flip the amp to see the pin's gap.

But, after I've totally wamplerized my pedal board, that was time to fine tune that loved amp.


Goals

Certainly, there are a bunch of tube models (few makers) for pre-amp tubes of family 12A or ECC so, first step was to define my goals. What I wanted this amp to do for me.

Respect of the classic / clean channel what I wanted is a nice clean channel with sparkling trebles (but not piercing ones), silk mids and defined basses. Just a nice platform to shape any sound with the help of pedal effects.
Respect of the ultra gain channel / gain 1, I wanted a crunch overdrive in the ball park of the Marshall 1959 (SuperLead). For sure, I cannot get the exact tone but, I wanted be as close as possible.

Both channels should take pedals nice and, the overall loudness should be moved down but retaining the tone. I am not gigging with this amp crank (that would be a dream) but, practicing at home and, I want the tone but without compromising my ears.


Pre-amp tube tests

First attempt

Experience says that a tube that sounds awesome in certain model of amp, shouldn't necessarily sound good in other amp but, I had to start in someway so, during my first try I thought that to put a NOS tube (an RTF ECC83) in V1 would improve the sound of this amp. Mullard 12AX7 reissues are my preferred new production 12AX7 tubes so, I thought in to put a Mullard in V2 (the second gain stage). Since V3 isn't a tone shaping position, I was ok leaving there the stock Marshall tube (probably a JJ ECC83S). For V4 (the PI), I wanted to check how a NOS 12AT7 would work and, therefore, I've put there a RTF ECC81 tube.

So, that was the configuration for this first try:

V1: NOS RTF ECC83
V2: Mullard 12AX7 reissue
V3: Stock Marshall (probably, JJ ECC83S)
V4: NOS RTF ECC81

I was a bit disappointed with the results. The sound had perfect definition note by note but, with a total lack of sustain and, without some "glue". No mojo there. Too much hi-fi and not so inspiring, in my honest opinion. Trebles were also a bit harsh, overall.

I thought: "well, I think those were my best weapons so... what can I do now?".


Second attempt

This was the time to review my notes corresponding to the intensive tests I've done for other amps and, the list of tubes that I've got with comments coming from Myles Rose, Watford Valves and myself.
I've found an interesting note about tube type 12AY7 and Marshall DSL, saying that this tube in V2 will bring the DSL to earlier Marshall tones (JTM45 - 1959) so... I've recalled that I had one EH 12AY7 that I bought some time ago just to test exactly this (but, that I've forgot later).
So, ok, I had a candidate for V2.

This time, I wanted to go over secure with the PI, since I've found the 12AT7 as weakening somewhat the rest of preamp tubes, since it drains considerably higher current (about 10 times the current of a 12AX7) and, the sound was resulting very thin to my taste. I've got two tubes that work specially well in this position (apart of the well known NOS JAN/Philips 12AT7). The best one is the Sovtek 12AX7-LPS and the second one is the Mullard 12AX7 reissue.
Since the LPS tends to go microphonics in combo amps, I am usually leaving it as a PI tube but, I recognize that works really nice in other positions (while it works). LPS I had in some high gain combos lasted very few but, I love how it works as PI so, I've used my last LPS for V4.

V3 is a cathode follower position type and, not directly related to the basis tone of the amp but, I discovered in previous tests that, has a clear impact in how the reverb works or how the tone stack behaves so, I wanted to thrown there a specifications-correct tube, without caring about the tone. This position is typical for cheap Chinese tubes with high gain but, I wanted to put there a Sovtek 12AX7-WC, a tube that Myles Rose identifies as very close to 12AX7 specifications and that, tone-wise, sounded very plain to me (not bad, not exciting).

Well, V1 is the key tube for tone so, I was thinking about how to get a darker tone but, while preserving an even an harmonically rich sound. The TAD 7025-S HighGrade came to my mind. I've remembered this tube as very linear, harmonically rich (just slightly worst than the Mullard re-issue) and, with an overall nice tone and darker than the Mullard (and way darker that the piercing TAD 7025 HighGrade). So, that's what I've chose for V1 this time.

So, the preamp configuration was as follows:

V1: TAD 7025-S HighGrade (Mullard like)
V2: Electro Harmonix 12AY7
V3: Sovtek 12AX7-WC
V4: Sovtek 12AX7-LPS

Overall, the gain of the whole amp went considerably down (I had to raise the gain control between 2 and 3 numbers) to had more or less same sound I had before. This is a direct result of the 12AY7 in V2 (and, it covers one of my goals). The piercing trebles were completely removed and the overall sound went darker and warmer (for tests, I am leaving tone controls always at noon).
The sound was good enough, a tad bumped in mid-lows range, with good representation of trebles (but, without excessive high end) and good representation of basses (but, without going boomy). Everything sounded well glued (as a difference respect of the previous test) but, the clean channel was lacking some of the beauty and silknesss I was after.
Thanks to the 12AY7, it was easier to crank the amp while maintaining acceptable loudness and, that was really good for the Ultra Gain / Gain 1 channel, that is sounding now to '70 Hard Rock.


Third Test

I was absolutely satisfied with the work of tubes in V2, V3 and V4 so, I am in the way of determining the right V1 for this amp. The RTF EEC83 sounded too hi-fi and a bit trebly for this amp. The Mullard is in same ballpark and, it can sound even trebly. JJ ECC83S is what probably was there in the beginning and, even that I like it, it's not my preferred tube usually because it can go very honky, with a very pronounced bump in mids-trebles. I wanted to leave that one for the end. But, I remembered that the Svetlana 12AX7 is somewhat a JJ ECC83S with smoother EQ and silky (even heavenly) cleans.
Even that this is not the most note-by-note defined tube, I liked it in all the tests I've done before and, cleans sounded like heaven in some amps.

The preamp configuration was as follows:

V1: Svetlana 12AX7
V2: EH 12AY7
V3: Sovtek 12AX7-WC
V4: Sovtek 12AX7-LPS

The overall sound was "in-the-spot", I mean, the sound wasn't dark or bright but caught in an even EQ respect of this amp. Good representation along the full frequential range, defined basses, mids and trebles, without a clear bump in any of them.
Gain is lower than in the TAD 7025-S HG. Well, it's a highgrade, what means that is a specifications-correct tube (while the JJ is over specifications, respect of output).
I had to raise one or two numbers gain to be in same gain level.

In the clean channel, the sound isn't so harmonically rich as with the TAD but, sounded more silky and balanced. Maybe, still lacks something or needs just a touch in the EQ controls to be perfect.
But, the sound of the crunch channel (Ultra Gain / Gain 1) was pure heaven. Pure '70 Hard Rock!.
Palm mutes are impressive, defined basses, nice crunch and enough trebles (without going piercing). A tad more undefined note-by-note than in the case of RTF, Mullard reissue or the TAD 7025-S but, the overall sound is excellent.


To be continued

By now, totally satisfied with the work of tubes in V2, V3 and V4. I think those I've choose work  perfectly well for my goals. Now, I still have some tubes that I want to check in V1. Up to now, it seems that the Svetlana 12AX7 is giving me what I was looking for but, let see how other tubes work there, because you never know where a certain model of tube will shine by itself in a certain amp and spot.

Tubes that I am planning to test in V1 are: EH 12AX7, JJ ECC83S, Tung Sol 12AX7 reissue and, probably, the Mullard 12AX7 reissue (tested before in V2 and PI positions).

Will post a new entry with the results of those tests and my final election