M-125 Build

Turn On, Cold, 5AR4, Two Tubes, no delay relay, KT120s

Power ON is at zero

Turn On, Cold, WZ68, Two Tubes, no delay  (All tests will be with KT120s unless mentioned)

Turn On, Cold, WS1 w/o thermistor, Two Tubes, no delay

Turn On, Cold, 5AR4, Two Tubes, 15 sec delay

Turn On, Cold, WZ68, Two Tubes, 15s delay

Note:  My delay relay may be setup differently than Bobs.  I have mine using the normally closed contacts.  I did this so that the relay is normally OFF.  So, when the unit is turned ON it takes a few mS for the relay to Open.  This causes the B+ to jump up a bit.  It settles down during the delay.  

Turn On, Cold, WS1, Two Tubes, 15s delay

Turn On, Cold, 5AR4, Four Tubes, no delay

I don't see any point is looking at the WZ68 and WS1 with four tubes and no delay. The B+ comes up long before the current starts flowing.

Turn ON, Cold, WZ68, Four Tubes, 15s delay

It looks like it takes at least 20s for the tubes to start conducting.  My delay is set for 15s.

Turn ON, Cold, WZ68, Four Tubes, 30s+ delay

Interesting...  Now I'm starting to wonder what the delay does for us.  I assumed that this would slow down the B+ rise time and it may but not a lot.  I think I'd rather not have the current jump up that fast.

I was thinking at one point that if the tubes were not conducting the B+ would rise too far and exceed the caps rating but that doesn't seem to ever be the case.

Output sag with a burst signal.  5AR4

1.228Vrms input, about 68W out

You can see the B+ moving up and down with the load and the output amplitude drop a bit during the burst.  There has been talk on this forum that this colors the sound to give it a a traditional tube-like flavor.  I want to see how the different rectifiers change it.

Same but with the WZ68

Same but with the WS1.  

Input Power at Idle

Input Power with 125W out

- Gain is 22.2
- Phase = 0
- With the input shorted to GND the output noise = 350uV.  31.62Vrms out into 8 ohms = 125W.  That's about -99 db SNR.  WOW, not bad!

Thanks for posting all of this info.
I admit it's above my pay grade but I'm sure it's of great interest to the members with more tech know-how.

Frequency Response 10Hz to 30KHz at multiple levels of inputs.

Looks great!  Higher levels drop off faster but where we mostly use it it's good for the whole range

Gain at the same input levels.  Depending on how I measure it I get different numbers for Gain. I believe this one. Looks good.

THD vs Bias Current, For 15W and 60W output

Looks good!

What I mean by "Bias Offset" is when I had the difference in bias current from the left and right sides.  This shows that you really don 't need to be too concerned about how carefully you balance the currents.  I'm assuming that the THD is the best indicator of this.  If there is a better way let me know and I'll take more measurements.

Last but not least, temperatures. I have a 40mm cheap CUI 12V fan blowing air in from back. It's running at 9V to keep the noise down.

I set the output to a constant 25W out guessing that is a good average music output and let it run for 1/2 with and without the fan.

All temps in DegF. Ambient = 70

Fan ON:
Internal chassis = 84
Power Tube Top = 151
Power Tube on glass near Bottom = 198
Driver Mid = 137
WZ68 Top = 171

Fan OFF:
Internal chassis = 100
Power Tube Top = 163
Power Tube on glass near Bottom = 260
Driver Mid = 140
WZ68 Top = 186

Finished Pics.  Finally ready to try them out.  :-)

Thanks a lot for the measurements. Did you measure any difference in THD with different rectifiers ?
The WS1 doesnt seem to sag as much during the burst so it would be interessting to see if it differed in any other way.
I was expecting a bigger difference in startup voltage from the WS68 to the WS1, so....

Fantastic work you have done by the way

Dahlberg wrote:Thanks a lot for the measurements. Did you measure any difference in THD with different rectifiers ?
The WS1 doesnt seem to sag as much during the burst so it would be interessting to see if it differed in any other way.
I was expecting a bigger difference in startup voltage from the WS68 to the WS1, so....

Fantastic work you have done by the way

I didn't measure THD vs rectifiers because I'm reasonably certain that they have no effect at all.

The B+ sag is barely noticeable for any of the rectifiers and I don't think their difference in sag shows up as any difference in the sound. IMHO, any of the rectifiers are just fine. Use the 5AR4 if you want to keep everything old-school and either of the SS rectifiers to save the 10w of heater power. I went with the WZ68 to keep the B+ voltage a bit lower.

Thanks

I spent about 6 hours yesterday listening.  I'm BLOWN AWAY !  I have no idea what to do with the Krell amp because I'm not going back to it !  I'll probably just use it in the office.  I previously installed a speaker switch to make it convenient to compare the amps but I'm not going to bother.  The amp that I did most of the testing on initially came up better than the other.  It took a couple of hours for the amp with less time on it to get to the same volume and settle down.  It also had intermittent crackling noise every min or so that went away.

The M-125 sound isn't in-your-face tube-like but I never hear the harshness that is sometimes a problem with things like horns or cymbals.  It all sounds so very natural and pleasant.  Even my wife noticed and she prefers singing to dance music in her car.  With the Krell after listening at moderately high levels for a couple hours to any music I felt a bit worn out.  With the M125 I wasn't ready to stop after 6 hours and did only because it was late.  I'm using Ohm Welsh 2000 speakers that are rated at 6 ohms and used the 8 ohm output.  Tonight I'll try the 4 ohm tap.

Yes - I made some changes but nothing really that changes the sound of the original kit.  

Knowing what I know now I would have changed a couple things.

- The delay relay doesn't do anything useful and I would have not bothered.  I changed it back to 15 sec.  It may be important on the other ST-x amps when using SS rectifiers.

- The fan seemed nearly silent on my workbench but is noticeable in my listening room.  I have to see if I can lower the voltage more or try a different fan.  (Major surgery)  If that doesn't work I'll just remove it.

It's been a big and not cheap side project but worth it.    :-)