Most folk here seem interested in altering the sonics of this wonderful amp in one way or another.
For those that don`t feel inclined or experienced to get involved with a soldering iron, there`s still plenty of scope to play with shaping the OTS tone character.
This does include removing the chassis from the cab, but that`s no big deal.
Just keep in mind being safe around internal components & electricity...!!!

Locate the small cap on the back of the master volume pot.
There`s diagrams elsewhere on this forum that shows the layout of this amp.
Snip one side of the wire from the cap very close to where it is soldered to the master vol pot. This easy mod will even out the highs at lower vol levels & works very nicely..!
The mod can be easily reversed with a hot iron, but chances are you`ll probably stick with it clipped as the results are pleasing.

The 12AX7 pre tubes that come stock in the OTS can easily be substituted
with compatible tubes of lesser gain factor & warmer character.
I personally do like a 12AX7 in the first position (V1) but find the stock JJs produce a very focused  but narrower band of tone in (V1).
I like a Blackface clean tone and a 12AX7LPS (Long plate spirial) in (V1) does the job nicely for producing a broader & warmer sound for the clean channel.

With the O/D channel, a 12AX7 (V2) drives the amp with a hard edged tone that alot of people on this forum seem to be trying to tame one way or another.

Substituting the 12AX7 for 12AY7(60%) of the 12AX7 gain factor, or even a
12AU7 (20%) really warm up the O/D channel nicely, but these lower gain tubes also allow for more scope when fine tuning and dialing in your prefered O/D tone.

This is where the internal gain trim pot needs to be adjusted to suit.
Most folk on this forum seem to agree the best O/D tones come from the trim pot being set somewhere between  9 o`clock & 11 o`clock positions.
(Keep in mind this is with a 12AX7 in V2)

When using  lower gain pre tubes like the 12AY7 & 12AU7, the trim pot will require adjusting up to compensate for the subsituted tubes lesser gain character.
This is where the fun starts and a 12AY7 will probably require a small lift in gain on the trim pot, but you have more scope to find your sweet spot/s.
The 12AU7 will require even more gain adjustment to compensate, but once again
you`ll have a broader range of gain to play with and both these two tubes are much warmer sounding to me.

I`ve just tried a 12AU7 with the trim pot set way up in the 3 o`clock position & that was VERY GOOD. The level & ratio knobs worked together with a broader range for fine tuning your overdrive tone & with more flexability to the user.
If anything, I think the tones are more D-mble like & less of a narrow band gain tone from the use of a 12AX7.
12AX7s are good in the right application, but the down side is the high gain factor
drives harder and can often sound grainy.

I can recommend trying these easy mods, especially if your looking for more
D-mble than Marshall, as i would assume most of us here, are..!

I have tried a 5751 in (V2) but feel it sounds less warm than the 12AY7 or 12AU7.
It was a bit sterile to me.

I hope this might be of help to those looking to experiment and achieve different tones.
Cheers.

Many Thanks for you tone tips. I have a later one from Nik that has the zucker mods but it is more gain and not as clean as I would like.   I think I will give your suggestions a try. It sound great with my Les Paul but I am having a harder time with the Strat Tones. I really do not want to alter it too much. Do you have any before and after sound clips?

Thanks,
Danny

This is a great topic!!

I have a small problem I want to put forward. It seems I get a nasty (fizzy) endge on my OD when I turn up the gain. Even more so when I use humbukcers. I have the latest lay-out. My OD trimmer is set at about 11 o'clock. (I also use the Kleinulator)

I don't realy need a very high gain. Sounds very nice set at 4. But still it's a shame it starts to sound bad when I turn it up. maybe this is common with this amp?

One more thing. Would it be bad to use one tube of a different barnd? I have JJ's, they don't make the LPS tube as far as I can tell.

Cheers
Steven

 

Will I need to rebias the amp after changing tubes?

If you take the chassis out and place it carefully on some supporting blocks, you are then in a position to access the internals.
Connect the chassis up to your speaker cab and away you go... it`s time to experiment.
Just remember though... (amoung the enthusiasm to explore tone).... the amp is alive when plugged in and turned on...!!!   Always be very cautious.!

With the chassis sitting supported & the tubes facing down.. (the same way it is mounted into the head cab) you can easily locate the internal trim-gain pot and adjust as you go.
It will always pay to switch the amp off before putting your small flat-head screw driver in there to adjust the gain trimmer.

It is also very accessable to change the pre tubes in this position, but once again switch the amp on standby & then off at the main to change any preamp tubes.

This way you can try new pre tubes of your choice, fire up the amp & try things.... shut it down again & adjust the trimmer as required.
Fire the amp back up and away you go to repeat the proceedure until you`ve fine tuned to a point that you`re happy with.

Always be careful and take your time when dealing with the amp internally.

I would suspect a 12AY7 will be the one for most guys looking for that warmer, less grainier tone.
12AY7s are usually it little more expensive than a 12AU7, but with the extra gain value it has and the warmth it supplies, it should help you achieve a more refined O/D channel tone with (in my opinion) more D-mble like character.
But you should also try a 5751 as tone is very subjective and you might just love what the 5751 has to say..!

Sovteck does a 12AX7LPS & this is the only sovteck tube i`ve ever liked.
The character of the LPS is warmer than other 12AX7s and a little more open sounding and vintage toned.
Good for (V1) i feel.

You just might be surprised at the difference a change of pre-tubes can bring.

Cheers & goodluck with your tone quest.

I looked at some information on 12AY7 tubes and there seems to be some differences in the types of Tubes. For example, The GT 12AY7 says that there is only a slight drop in gain where the EH 12AY7 decribes a much lower drop in gain with smoothness. Please let me know what 12AY7 you recommend?

Thanks,
Danny

V3- is closest to the power tubes. This is the PI.(Phase invertor)
V2- middle tube. Overdrive channel pretube
V1- is the furtherist away from the power tubes...or firstt tube in line.

12AT7 tubes are often found in blackface fenders & used for functions like the P.I. or somewhere in the reverb driver or revcovery stage... but not so much as a tone generator.
I have tried 12AT7s in other amps before & think these are never as warm or alive sounding as the likes of the 12AY7.
You could try one but i think you`d probably be wasting your time in the OTS as this amp really asks for 12AX7s or very similar tubes.

The stats for most 12AY7s shouldn`t vary too much. I don`t think you should get too concerned about one brand of 12AY7 to another.
Just get hold of something decent like the EH or similar and try one to see if you like tne sonic result..!

I`ve tried all these tubes at one time or another in different amps & the 12AY7 is a really good choice for tone character, warmth and the ability to still have enough gain to deliver.

The 12AY7 is actually about (45%) of the 12AX7s 100% gain factor.
I had written earlier that it sits at 60%, but need to correct that.
The 12AT7 sits at 60%
5751 70%
12AU7 20 %

Thanks for the info, Bluestone.

I'm currently using a JJ 12AX7 in V1, a NOS JAN 12AT7 in V2, and a NOS 5751 in the PI position with JJ 6L6s.  Sounds good with the internal trimmer around 10:30.  I'm very happy.

BTW, the Speaker Warehouse ET65 (their version of the venerable Celestion G1265) sounds REALLY good with this amp.  I have it housed in a supposed Fuchs 1x12" closed-back, front-ported cab.  I gotta take some pics of this rig!