Z-Pre Quesitons

[makinrose]

Hi I'm interested in knowing a bit more about the Z Pre-amp design.  From the description it says it's Soldano inspired?  I'm assuming that it does away with the cathode follower to lower the tube count and make the EQ more effective?   

I'm looking to be build something with the gain level of SLO-100 but smoother and fuller and sweeter sounding---a little less fizzy than the SLOs I've heard.   Will the Z-Pre amp work for that sort of thing?  Are there any suggestions about places in the circuit to tweak?  I'm interested to hear people's experience with it.  Thanks everyone!

[K O'Connor]

Hi Guys

Yes, Z-PRE is the essence of an SLO, which was copied extensively and verbatim by Mesa for all of their "Rectifier" models, by Marshall for the SLX, Peavey and Fender for the 51050, etc. Most amp manufacturers with a high-gain model have copied the SLO with few changes to cover the fact.

Like any other preamp, Z-PRE can have any of its values changed to alter the gain or tone to suite the needs of the individual. Cathode-bypass cap values can be changed,; caps removed or added. Plate bypass caps can be removed or changed or added.

The Gain control can be padded further than it is if you find the maximum setting to be too extreme. This will provide more resolution of control over the range you use. We can take a note from the SLX where the attenuation between the second and third stages is variable, as well, giving two points of control. I did this in the OD section of D-PRE to make it more flexible.

Personally, the aspect I hate about most high-gain preamps is the low-value slope resistor in the tone stack, which sucks out the mids and constipates the tone. A guitar is a midrange instrument, so why would you remove its main frequency range? For me, it makes the tone a bit lifeless, so I prefer 100k there instead of 56k or 33k, but to each his own.

Of course, with the stock circuit you can simply swap lower-mu tubes to have huge differences in smoothness and gain. The original is based on a modified Marshall which begin with 12AX7s or ECC83s. These have a mu=100 per section. Remember: fresh tubes sound harsh and noisy until after their 100-hour break-in period. Mike's SLO was fitted with 5751s, described by GE as "modified 12AX7" with a mu=70. This is the same as a 12AT7, so one wonders if they are actually just 12AT7s? In any case, the 'T' has less mu and gain than the 'X' and will sound smoother. It is generally a lower-distortion tube and is used in a lot of broadcast equipment.

You can take things further with the 12AY7, the preference of harmonica players in Fender amp front-ends, with its mu=50. The extreme is mu=20 of 12AU7. The stock voicing has remained the same with these tube swaps, so you get to hear a unique combination of voicing that may never have been thought of based on the use of these alternate tube types.

Have fun

[makinrose]

I really appreciate your response. It was very helpful.

We are in agreement about slope resistors. I rarely like a 33K in that position. There are some amps that I like the more scooped 56K that Marshall and Fender used in some models but overall prefer the 100K.