express pre mods

[castor]

has someone changed Express pre trying to make it closer to a new mk amp? Voltages are different, so even making the pre close to a one of these new Jewels, overall sound probably will be different!
has anyone some experience?

[keithrick]

I just changed my Express to Mark A. Express variation 2, which is basically a Komet 60. I don't know how he does it! But it is dead on and I think his schematic is dated 2004!!!

I left the 1k choke resistor and PS intack. I did add the Cut control, this is a good area to reduce high end hash if that is bothering you guys. I did leave the bright switch in the circuit (since I had the hole!) but changed it to a on/off at 120PF. The 500pf gernerated to much noise for my taste, YMMV.

To further reduce hiss I am going to try something I read in a previous post about adding a small poly cap in parallel with the Electrolytic to reduce hiss. Notice that the Komet60 uses a Mallory 150 .68uf and I believe Allyn mentioned it was very quiet. That cap could be the reason?


It sounds great. Clean tone is warm and the hiss is reduced. I haven't had the chance to put is through it's paces at high volume yet but will tonight at a Jam. Looks promising though!

Ken Fisher did a great job designing this circuit!

[gearhead]

How'd you implement the Cut control?

[keithrick]

I had previously installed a PrePIMV in the back of the chassis so I just replaced that with a 250K pot and .0047 cap. I coild a wire aroung the two leads coming off the 220k PI resistors and gounded one end to a power supply ground (like the split load switch.) The cap goes to the middle wiper and the other lead goes to the input wiper. You could also place it in the second speaker jack. It works well along with the presence cap.


I had a chance to crank it last night. I do not have the same gain level that I had with the Express. But is also quieter as well and the cleans are improved. I am thinking about increasing the 1uf bypass cap on the first stage to 4.7uf-10uf in order to increase the gain a bit.

The distortion is also tighter then the express, that could be because of the lower feedback resistor 47K as opposed to 100K. That might be adjustable too before I am said and done! I do have a 50K pot lying around!

The jury is still out on this one.......I just can't leave my amps alone! Plus I have parts for two more amps sitting hear to be finished!

[tubedogsmith]

Keith, increasing the value of the bypass cap on the first stage cathode won't increase gain persay it will just allow more lows to pass through. Lowering the value of the resistor will increase gain though. same with the cathode resistor on the next stage. the best sounding old marshalls 100 watters I've heard had stock 680 ohm resistors on the first two stage cathodes instead of 820's. That would probably be too low for one of these though.

[keithrick]

Hey Tubedog,

Where you been hiding?

I bet 680 ohm resistors would peel the paint off the walls! I'll try your suggestion and reduce the 2.7K on the first stage maybe to about 2.2K and see how the amp reacts! I just have these giant film bypass caps from Apex.Jr. that I have been dying to use..LOL

If your familiar with Mark A's express variation basically it reverses the first and second stage bypass caps reducing the cap on the 2K7 resistor to .68uf, also it includes an additional voltage divider before the 10K grid to ground.

Is the purpose of this "evolution in design" of the circuit to reduce noise/hiss that the Express's have? .....Then is increasing the coupling cap after the fast/normal (split/load) the way bass frequencies are recovered?

Just curiuos how these things work


One more question I'll put out too everyone.....

What is the effect of reversing the cathode cap/resistor values on the response/gain characteristics the amp. To me, Mark's variation has more of a modern gain flavor.


Happy soldering everyone!!

[Richie]

just leave those cathodes alone... and change the 10k on the second tube cathode to 8.2k 6.8k i wouldn't go any lower than 5.6k

Richie

[castor]

I bought all spare parts from Nik... (I can not find mallory's in Italy) but after a while I replaced all Supply caps with right values according to Francesca scheme. These caps are from ATOM SPRAGUE and I bought all 450V.
Then I use HI-FI ELNA 22 micro 25V.
Overall sound of the amp is not so bright, consider that when I play with the strato I have to use the bright settled on 100 pf, while with les pauls (i prefer knopfler sounds) i use normal bright --> changing something in the PS will let you have a different sound color
As of today I added only the 82-22Kohm split (fast/gradual switch).
Changing values of the 22 micro to 0.68 (and changing res value) will let you have a more marshallesque sound, because you decide only to amplify some frequencies (cutting some lows). I read many books but I love the simple way to explain these info you can find in Ken pages.
I agree that you can higher distorsion/gain lowering 10K res in the 3rd stage.. I never tested it, but i fear that changing it toomuch you will have a different response on the overall dynamics of the amp.

[tubedogsmith]

Keith, Richie's point is a good one, to see if you like the idea just tack another 10k across the one in there ans see what you think. If that's too much but you like the direction you can try some others. Main thing to remember is changes you make at the front of the amp get amplified all the way back through the circuit.

[keithrick]

Thanks Guys! Next time I have this open I'll try reducing the 10K.

[castor]

keith, can you describe the sounds you can have with these variations (pre a la komet)?
in terms of clean sounds, high gain sounds and overall changing in dynamic of the amp!... probably if you have less distorsion, the amp will tend to turn clean better that the express.
I think that doing these mods in the pre section you will have a marshallesque sound, but keeping the same power supply and caps value perhaps the amp loose color and distrosion (I think that the difference in supply between a komet and an express after the 1st caps stage can be more than 100V)
can you share your opinions about the amp sound you have now?

[castor]

about noise... I think that the big difference in having noise or not is the presence of the choke filter... komet amps use choke filters, trainwrecks use a resistor.
Some days ago I was thinking of replacing the big res with a choke filter, but the good one I have at home (an hammond one) is now used in my soldano... and the hamp is completely quiet even with gain at max

[AdmiralB]

The 10K cathode resistor on the third triode controls a large portion of the response and feel of the amp. At 10K, that triode only has a gain of about 7, but it's biased pretty close to cutoff and it acts as an asymmetrical clipper - Marshall did the same thing with the 220x MV amps, although their implementation is less pronounced.

If you start reducing the value of the cathode resistor, the triode moves away from a 'clipper' circuit and starts acting more as an amplification stage - which smacks the PI harder and IMO makes the amp mushy.

The Concorde has a 3.3K resistor here (results in a gain of about 15, IIRC), but it's followed by a direct-coupled cathode follower, and THAT gets thumped instead of the PI (it still gets mushy on 'fast' with the volume over about half).

I have my clone set up to switch between K60 (10K cathode, voltage divider, no CF) and Concorde (3.3K, no voltage divider, CF). IMO the 60 is a much more versatile and tighter circuit.

Something to play with would be to replace the voltage divider with a 500K pot. It's sort of a Hiwatt-style MV (early in the circuit) and you can 'tune' the response of the third triode with it.

[HiGain]

Just a thought... has anyone here tried a 500K pot in place of the 220k/330k voltage divider ahead of the third stage (clipper stage)?

Jake

[AdmiralB]

Did you miss the previous post?