Dear AdmiralB,
please let me know something more... you made a K60 pre and use the power structure (power section + supply) of the express or made a k60-similar-amp?
in the 1st case: can you explain the difference in sound you found? with tightness you mean that the amp is more bright? logically when changing the pre parts you have to change the pots values...
another thing.. do you replace the last 40 micro in order to have a supply of 80 + 80 micro or keep it as is (80 + 40)?
I built a dedicated K60 clone. Initially it was a 99% clone with the exception of the 500K pot replacing the voltage divider in front of the 'clipper' tube, as discussed above. There are some other differences...filtering is a little different, an extra node on the B+ rail so the first two triodes don't share one, a few other things that make more sense to me.
After a while I used the remaining triode as a CF, driving the tone stack a la 6G6 Bassman. I put it on a switch and nobody who played the amp, myself included, could tell any difference with it in or out of the circuit.
After I got circuit info on the Concorde, I converted it to that, but don't like it as much. I've tried lots of little things - Express-style 25uF cathode caps on both gain stages, different tone stack values...and where I wound up was back to an essentially stock K60 with the fixed voltage divider, and a rotary switch that brings in/out the circuit changes to go to a Concorde configuration.
At less than half-volume (in 'fast' mode) the Concorde sounds great - thicker than the 60, if not as defined. But you can pretty much play a 60 full-up without losing much definition...the Concorde mushes out.
my idea is to change from an express pre to a similar-k one, and change the supply form 80+40 micro to 80+80 micro. Perhaps I'll replace the big resistor with an hammond choke.
my fear is that doing these mods i'll obtain a sound close to the one I have now but I loose some of the express characteristics like touch sensitive and so on.
I can remember where but some years ago someone told me that increasing supply caps let you have a bigger and plenty of bass sound but you loose dynamics
At the end in amps of this kind the power part and the supply have a big role
I read some posts but at the end I was not able to understand what can I obtain changing only the pre (or the pre plus caps in the supply)
My amp is not so bright (after replacing power caps with atom sprague ones)
I have 50uF at the plates and screens (a series pair of 100uF Atoms at each node) and four 30uF nodes in the preamp. I also have a choke, since the amp mirrors the Komet and not the Express.
If anything, the 60 is more touch-sensitive than the Express. Much higher voltages throughout.
I haven't had the possibility to play a k60, but referring to what I can hear you are right. My fear is that keeping the low express voltages you loose a lot of gain of the amp with k60 pre and a touch of dynamics
Castor, You can also try the Komet or Torres diode trick placing a diode in series with the B+ supply to the preamp. This will prevent the power amp from stealing power from the preamp
keithrick wrote:Castor, You can also try the Komet or Torres diode trick placing a diode in series with the B+ supply to the preamp. This will prevent the power amp from stealing power from the preamp
Just beware that it will also keep your preamp caps from bleeding down through the bleeder resistors. (I ALMOST learned this the hard way when working on my Komet.)
ok.. thank you very much fo the info... sometimes ago I was thinking why they placed this diode.
Yesterday after some (4 or 5) months I fired up my soldano and play both express and my clone at the same volume (express with THD at -16 db ad the bedroom level very low - soldano with solo channel at 3:00 and master close to 0).. differences:
- noise (in the express much higher)
- sound.. the express is more defined, the soldano is more round and more distorted but with less dynamic.
at the end I'll have to put a choke to reduce noise close to 0!!! generalli a pi-greco solution requires the same caps vale in the both ends of the choke.. here we have 80 and 40 micro... do you suggest to change to 80 and 80 micro?
as of today I'm still waiting for the pots, but I did the pre mod as per pre variation 2 of M. Abbott .
I found that the amp has less distorsion but it is more dynamic and cleans better... tone is good, but even the normal express is very good.
after receiving some suggestion of not changing the supply,I followed them and not replaced the res with a chocke... in my construction the ratio sound/noise is very good.
as soon as possible I'll put some audio files of the amp
Last edited by castor on Thu Sep 30, 2010 7:10 am, edited 1 time in total.
I like the Express Variation 2 better as well. I also made the V3 10k cathode resistor switchable but have found over time that it is great at 10k. I am thinking about couple other options I might try as experiments and in an effort to futher my understanding of how the circuit works.
1. I am going to make the feedback reisitor variable between 22K and 122K to dial in the feedback I want.
2. Might also play with the value of the .1uf cap to .68uf. Mainly to see how much of the Express's magic is tied up in this area.
3. Still thinking about changing the 220K/330K voltage divider to a 500K pot. Very curious how this will effect the tone.
4. Last is the fast gradual switch.
I'll try to post opinions of these changes in a few weeks.
Can I just change the cathodes to 2.7k/.68uf on V1 pin8 and 1.5k/22uf V1 pin3 along with adding a 33k to the input without doing the voltage divider? What effect would this have ? What is the pupose of the voltage divider and how would you implment it on a turret board without adding turrets? Keithrick do you have any pictures of your amp?
Wow, My simple Trainwreck circuit is going to look like my Dumble when I am finished! 
I have got to get my JTM45 built!! Not that this will take long! All I have to do is wire it up!
(I ALMOST learned this the hard way when working on my Komet.)
