Mesa Boogie MK III - Reverb 12AX7 Gain Influence

[Greenback25]

I have a 100W Mesa Boogie Mark III Reverb head and the overall gain level that the amp delivers seems quite low to me, so I'm going to replace the preamp tubes.

Is it worth changing the V4 12AX7, i.e. the tube that is associated with the reverb of the amp, if my goal is an increase of the overall gain level? I usually set reverb to zero and only use it occasionally when the clean channel is active.

Also, is it worth going for the Mesa SPAX7 for V1 if I want a tight and aggressive tone? Which alternatives would you guys recommend instead of the SPAX7, just in case it isn't available in Europe?

[Stevem]

First some info about preamp tubes and in particular the 12AX7 / ECC83 might help you.

These tubes are said to have a gain factor of 100.
Having a gain of 100 taking place hinges to a large extent mainly on two things.

1) the tube needs to meet its design specifications.

2) the component values in the circuit the tube is in that allow it to amplify and the level of power supply voltage placed on the tube also have a large effect on if you get a gain factor of 100 out of a ideal 12AX7.


A note here.
Amplifiers be they guitar or for audio file use never have circuits designed to use the full design spec gain factor of 100, so keep that in mind also.

Today most new off the shelf tubes that I test ( and yes even new Mesa tubes ) are lucky to have a gain factor in the low to mid 80% range to at best 90%.

As for your question about V4, NO, it is strictly for the function of the reverb, so V1 V2 and V3 are the preamp tubes that effect signal gain levels in your amp.

The strength ( if it meets design specifications ) of V1 in particular is very important.

V1 drives the passive tone stack in this amp.
Passive tone stacks take a lot of current to push signal thru them.

So not only the gain that your preamp tube has, but its current output level is very important in regards to how much signal level you have left on the output bound ( down stream end) of the tone stack that then feeds V2.

In every tube amp the ability of V1 to perform as close to its design specifications as possible will have a big effect on not just the overall gain of the amp, but due to what it has for current output also the tonal nature of the amp.

Since you’re in the EU I would suggest to you to go to the Watford valves site and read up on what they offer.

Also note that in terms of a tube to use for V1 that NOS tubes from the 50s and 60s met there requirements in terms of design specifications far more often and the materials they where made from last longer in terms of the tubes performance too.

If you can afford it and find an audio file source for tested NOS preamp tubes in the EU then that would be my first choice for a replacement 12AX7 to use in the V1 spot.

I don’t know if Watford still offers NOS 12Ax7s / ECC83s.

Another option for tubes is here in the US with Euro tubes in the state of Washington.

They deal only in tested JJ electronics tubes.

I would email them for a recommendation and see what they say about your application.

I have used Mesa tubes on repairs and while they are better in terms of not being microphonic and there gain level is more consistent, they are expensive even here in the US and the bottom line is they are still Russian made tubes so there life span is not always the best.

The worst are Chinese made tubes because once they start to go weak they nose dive off the cliff really fast and some folks for the type of music they play like the sound of them, but I have found them to do a good job only in certain parts of a given amp.

Only Chinese made preamp tubes look like this on the inside with with shinny steel reinforcement strap across each plate .

I would also like to say that having a tight and aggressive sound / tone can be in large part due to the condition of your output tubes.

The weaker they get the more tightness goes away since they will be driven into clipping sooner for any given level of signal input level out of the preamp section.

On a side note what effect on gain does having your graphic EQ in or out of circuit have to your gain level when it’s set flat?

[martin manning]

I have a 100W Mesa Boogie Mark III Reverb head and the overall gain level that the amp delivers seems quite low to me, so I'm going to replace the preamp tubes.

Are you referring to actual gain as in voltage or power or to distortion level?

These tubes are said to have a gain factor of 100.
Having a gain of 100 take place hinges to a large extent mainly on two things.
1) the tube needs to meet its design specifications.
2) the component values in the circuit the tube is in that allow it to amplify and the level of power supply voltage placed on the tube also have a large effect on if you get a gain factor of 100 out of a ideal 12AX7.

12AX7 has a nominal Amplification Factor A of 100 at a specified Va-k and Vg-k, but that is just one point on the characteristic curves where A = 100. Gain depends on the choice of Ra, Rk, and Ck, but is typically ~60.

[Stevem]

Yes, but just trying to keep to a simple basis.

Thank you though!