Kegger
R.I.P. 1/12/1966 - 6/1/2017
Here are the mods/changes I made to this ST-40.
(Again another take on the ASL wave 20 build I did a while back)
First it's not fair to compare the original to the "upgraded" version the original was,
well original/all stock, caps, resistors, everything, BUT just a quick take, it was the
old school "very" vintage tube sound, thick an muddy, loose bass, not the very clear
sharp top end, no sparkle, no air. AGAIN it really is not a fair comparison as a proper
rebuild would have changed things a good amount I am sure.
No I’m not saying all
vintage pieces sound like described above, some do and I was just using that for my
reference to try an put out there what I was hearing an how I would character some
of the old school tube setups that were produced.
(ON MOST TUBES)
As I’ve stated in the past unless I’m doing a regulated screen supply I’m not into the
pentode mode of operation, so here with no UL taps triode it was with a 130ohm res
connecting the plate to screen. In my experience when running triode output stages
feedback really is not needed so I did away with it an removed all the center channel
stuff along with it. The original grid resistors were off and I dropped them to 240K.
(pentode with no regulators does still work on certain tubes quite well though)
(next section help I did a little spice simulation for good values before building)
Others have mentioned this as well, the drive/gain in the output section is more then
is needed so I chose what I thought would give me less but enough drive and little to
change on the circuit, so I went with a 12AT7 same pinout as previous but 60mu both
sections where the original (Most know I’m sure) uses basically a 12AX7 100mu input (12DW7)
and 12AU7 20mu splitter in one bottle. All this took was changing the 2 plate resistors
on the inputs from 330k to 100k and the cathode resistor from 1.1k to 560ohm, I left
splitter values the same. In this config there is a few other tubes that can roll in here
as well with no changes needed like 5965, 6829, E80CC, E180CC ect. Grid resistors in
this section had drifted as well but were replaced with 470K.
(the splitter resistors could be 56k as well if wanted, better drive)
This amp is for a friend who has rather highish sensitivity speakers so power is not the
concern here and so I had no problems redoing this supply for more filtering but at that
cost of available voltage to the outputs. There are 2 RC stages of filtering before those
outputs see the voltage, consisting of 75ohm 12w WW resistor to 47uf cap then 33ohm
12w WW resistor to a 47uf and 180uf in parallel for roughly 230uf, nice good clean clear
power supply to start an amp off with, the next stage resistor was calculated in PSUDII
so the rest of the amp would get original voltages (or at least very close to it), screens
were no longer on the next section an today’s higher wall voltage actually made the 1.8k
resistor increase to 2.2k.
All 8 of the black .1uf coupling caps were changed to the green Russian .22uf K42 caps.
Here’s were I had to make a choice that I really went back an forth on, the 7591’s in this
amp were not very strong and I did not have any in my stash to put in here, but I do have
many 6L6 types around, and the voltage would be low enough that old school and the ST
shaped ones could be used, so yep I rewired the outputs for 6L6, 250 ohm shared cathode
resistor bypassed with 220uf running roughly 104ma per pair and the original grid resistors
had drifted, since I was changing them I opted for 240K, should keep any suspect vintage
tubes in line and since the coupling cap was upped were fine there to. (Running 6L6GC)
Only resistors that really seemed to drift in the amp were the grid resistors on the output
stage and driver stage plus the plates of the drivers.
All the controls cleaned with Deoxit, and I gotta say they are nice and quiet, also did the
loudness mod. I tapped straight into the grid of the 12AT7 driver from the Tape 1 Tape 2
jacks making the unit useable as strictly an amp or still using as an integrated going into
any of the other inputs.
I have left the 4ohm taps as ground and for the time being don’t see a need to change it,
amp performs well and sounds very good. If anyone has thoughts on this (or anything I'd
done here) I would be glad to hear about it.
---------------------------------------
No this is not a rebuild to keep the unit as original design, it was a mod for my preference
and what I felt would give a better performance for where this unit will be used, many like
the stock units performance and sound, I have no problem with that and if the extra power
is needed or original sound wanted then this would not be what someone would go after.
All in all I am very happy with how this unit turned out, it is dead quiet, not a hint of noise
on high sensitivity speakers, the mids are clean and clear, the high’s go out nicely with the
air I was after, overall a more “updated” and HIFI sound than most vintage amps tend to do
and the bass is not loose in the least, has nice detail and plenty there! I wish it was mine.
One kinda nice thing here as well when used as an integrated some of the prior “old school”
sound is sprinkled into the mix and could be quite pleasing at the same time.
Final tube compliment now is (1) 5AR4, (2) 12DW7 (now have 2 spares), (2) 12AT7, (4) 6L6.
(plus the phono section, (2) 12AX7)
New power supply and output section schematic is attached, I got the original voltages from
the build manual when I redid this unit, and these new voltages are marked on this schematic.
(I used the original schematic then made my changes, hopefully it is readable)
Original schem attached as well for reference.
Anyway hopefully someone finds this useful or at the least entertaining.
Keg
(Again another take on the ASL wave 20 build I did a while back)
First it's not fair to compare the original to the "upgraded" version the original was,
well original/all stock, caps, resistors, everything, BUT just a quick take, it was the
old school "very" vintage tube sound, thick an muddy, loose bass, not the very clear
sharp top end, no sparkle, no air. AGAIN it really is not a fair comparison as a proper
rebuild would have changed things a good amount I am sure.
No I’m not saying allvintage pieces sound like described above, some do and I was just using that for my
reference to try an put out there what I was hearing an how I would character some
of the old school tube setups that were produced.
(ON MOST TUBES)
As I’ve stated in the past unless I’m doing a regulated screen supply I’m not into the
pentode mode of operation, so here with no UL taps triode it was with a 130ohm res
connecting the plate to screen. In my experience when running triode output stages
feedback really is not needed so I did away with it an removed all the center channel
stuff along with it. The original grid resistors were off and I dropped them to 240K.
(pentode with no regulators does still work on certain tubes quite well though)
(next section help I did a little spice simulation for good values before building)
Others have mentioned this as well, the drive/gain in the output section is more then
is needed so I chose what I thought would give me less but enough drive and little to
change on the circuit, so I went with a 12AT7 same pinout as previous but 60mu both
sections where the original (Most know I’m sure) uses basically a 12AX7 100mu input (12DW7)
and 12AU7 20mu splitter in one bottle. All this took was changing the 2 plate resistors
on the inputs from 330k to 100k and the cathode resistor from 1.1k to 560ohm, I left
splitter values the same. In this config there is a few other tubes that can roll in here
as well with no changes needed like 5965, 6829, E80CC, E180CC ect. Grid resistors in
this section had drifted as well but were replaced with 470K.
(the splitter resistors could be 56k as well if wanted, better drive)
This amp is for a friend who has rather highish sensitivity speakers so power is not the
concern here and so I had no problems redoing this supply for more filtering but at that
cost of available voltage to the outputs. There are 2 RC stages of filtering before those
outputs see the voltage, consisting of 75ohm 12w WW resistor to 47uf cap then 33ohm
12w WW resistor to a 47uf and 180uf in parallel for roughly 230uf, nice good clean clear
power supply to start an amp off with, the next stage resistor was calculated in PSUDII
so the rest of the amp would get original voltages (or at least very close to it), screens
were no longer on the next section an today’s higher wall voltage actually made the 1.8k
resistor increase to 2.2k.
All 8 of the black .1uf coupling caps were changed to the green Russian .22uf K42 caps.
Here’s were I had to make a choice that I really went back an forth on, the 7591’s in this
amp were not very strong and I did not have any in my stash to put in here, but I do have
many 6L6 types around, and the voltage would be low enough that old school and the ST
shaped ones could be used, so yep I rewired the outputs for 6L6, 250 ohm shared cathode
resistor bypassed with 220uf running roughly 104ma per pair and the original grid resistors
had drifted, since I was changing them I opted for 240K, should keep any suspect vintage
tubes in line and since the coupling cap was upped were fine there to. (Running 6L6GC)
Only resistors that really seemed to drift in the amp were the grid resistors on the output
stage and driver stage plus the plates of the drivers.
All the controls cleaned with Deoxit, and I gotta say they are nice and quiet, also did the
loudness mod. I tapped straight into the grid of the 12AT7 driver from the Tape 1 Tape 2
jacks making the unit useable as strictly an amp or still using as an integrated going into
any of the other inputs.
I have left the 4ohm taps as ground and for the time being don’t see a need to change it,
amp performs well and sounds very good. If anyone has thoughts on this (or anything I'd
done here) I would be glad to hear about it.
---------------------------------------
No this is not a rebuild to keep the unit as original design, it was a mod for my preference
and what I felt would give a better performance for where this unit will be used, many like
the stock units performance and sound, I have no problem with that and if the extra power
is needed or original sound wanted then this would not be what someone would go after.
All in all I am very happy with how this unit turned out, it is dead quiet, not a hint of noise
on high sensitivity speakers, the mids are clean and clear, the high’s go out nicely with the
air I was after, overall a more “updated” and HIFI sound than most vintage amps tend to do
and the bass is not loose in the least, has nice detail and plenty there! I wish it was mine.
One kinda nice thing here as well when used as an integrated some of the prior “old school”
sound is sprinkled into the mix and could be quite pleasing at the same time.
Final tube compliment now is (1) 5AR4, (2) 12DW7 (now have 2 spares), (2) 12AT7, (4) 6L6.
(plus the phono section, (2) 12AX7)
New power supply and output section schematic is attached, I got the original voltages from
the build manual when I redid this unit, and these new voltages are marked on this schematic.
(I used the original schematic then made my changes, hopefully it is readable)
Original schem attached as well for reference.
Anyway hopefully someone finds this useful or at the least entertaining.
Keg
