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Old 29th January 2013, 10:54 PM   #1081
eyoung is offline eyoung  United States
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stenks,,,

I think it is a 680r 2w ....somebody correct me if I'm wrong...

Regards, E
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Old 29th January 2013, 11:44 PM   #1082
stenks is offline stenks  Serbia
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Quote:
Originally Posted by Dick West View Post
You should try the search function on DIYaudio. The subject of caps has been discussed many, many times over. For starters check (on this thread) posts 579 and 647 through 650.

My opinion is that the best is the $90 cap package from Musical Concepts.

But, many experts here say most any good quality computer grade cap will be OK for the power supply. Beware of caps whose body are the right size but do not have low profile posts. Also, some experts have commented about also including a 500 uF to 1000 uF quality (low ESR) electrolytic in parallel to the larger caps.

You should also consider installing 6800 ohm 2 watt bleeder resistors across the terminals of each PS cap.
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Old 29th January 2013, 11:46 PM   #1083
stenks is offline stenks  Serbia
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Quote:
Originally Posted by eyoung View Post
stenks,,,

I think it is a 680r 2w ....somebody correct me if I'm wrong...

Regards, E
Sure, thank you.
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Old 30th January 2013, 09:29 AM   #1084
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Quote:
Originally Posted by eyoung View Post
stenks,,,

I think it is a 680r 2w ....somebody correct me if I'm wrong...

Regards, E
If you guys are talking about adding resistors across the main filter caps for discharging, I think 6.8K 2W is more appropriate. 680R across 60Vdc will be dissipating 5.3W.
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Old 30th January 2013, 10:22 AM   #1085
stenks is offline stenks  Serbia
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Quote:
Originally Posted by Stormrider View Post
If you guys are talking about adding resistors across the main filter caps for discharging, I think 6.8K 2W is more appropriate. 680R across 60Vdc will be dissipating 5.3W.


Yes, I talking about resistors serving as bypass for quick discharge capacitors. Found somewhere need to be from 6,8K to 10K 2W.
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Old 3rd March 2013, 01:02 AM   #1086
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Originally Posted by vilfort View Post
New Year - new schematics

Since last post, I have had fun restoring two MOSCODE 300s. Sounds great and has lots of bass slam. For those who know a little about these fun amps, you may remember that they are based on a cascaded 6DJ8 with a 6FQ7 as a white cathode follower driving the MOSFET outputs. The signal has to pass through 4 capacitors. So the idea struck me: why not re-design the cascade and white cathode follower to eliminate at least two capacitors that we did not need? - I still like to keep the two caps between the MOSFETs and the tube section. Sure, a direct coupled output stage could work, - I just do not trust that the tubes will not drift over time. Still, going from 4 caps to 2 seemed well worth it. This also means that one can splurge a bit on the quality of the caps (I went w/ Russian Teflon in glass).

The MOSCODE cascade VAS section yields a lot of amplification. Using to TubeCad and plugging in the 33K and 39K plate values, the compounded open loop amplification came out at ~ 200X. Wow! Also the plate resistors seemed excessively large which made the 6DJ8s operate mostly on the curved section of their gridlines. Great for max amplification - and more distortion... The MOSCODEs reflect some older design ideas popular in the 1980's - lots of amplification and lots of feedback. Great for damping factor and bass - not so good for an airy top end. So lower plate resistors was a design goal.

Speaking of airy top end; here the SE 300B amp w/ zero feedback is a better choice. No bass slam though. This hobby is always about giving something up to get something else. But could a compromise be reached? Could we get good sound-stage and bass slam with a hybrid amp? The earlier SRPP version of my Hafler conversion sounded great. And it had no feedback. Yet, the MOSCODE 300 had better bass...

Since I have a few Haflers sitting around, and I did not wanted to butcher stock MOSCODEs, it was time to build yet another conversion. This time the design goal was:
a) Bass slam - so feedback = yes
b) Clean/warm midrange - so 6SN7s w/ Allen Bradley carbon comp resistors
c) Smooth treble - so as few coupling caps as possible
After some trial and error (as usual), here is what version 6 looks like (see attached).

There is a couple of tricks here I have not come across elsewhere. As you can see from the schematic, the two first triodes look very straight forward: cascaded w/ direct coupling. Each delivering roughly 10X gain so this VAS has only 100X amplification - which in turn meant that we could get by with half the feedback of the MOSCODEs. The 22K plate resistors offer a better plate load to plate resistance ratio for less distortion. Here we have a 3X ratio whereas the MOSCODEs have 10X. Direct coupling also got rid of the first coupling cap. Then there was the coupling cap from the plate of the first triode to the grid of the second triode in the white cathode follower driver. This had to go. Inspired by Broskie and some of his posts on Aikido, it struck me that we really need the bottom triode in white cathode follower to act as an active resistor to the top triodes cathode follower. The white cathode follower normally take this signal from the plate of the top triode via a cap... but since we already had another triode to spare, why not a) lose the plate resistor on the cathode follower (ala Aikido) and b) drive the bottom triode directly with the same signal as the second stage in the cascade? Voila! Done. No more cap and efficient use of the 4 triode available. Simple is best.

I have never seen this configuration before, so I might have "invented" something new. Probably not... But if someone reading this post has seen this approach elsewhere, please post a link.

So how does it sound? Great (as all amp builders will say). Lots of bass slam and a bit more aggressive treble. Midrange - Especially lower midrange is there. It is still breaking in - so perhaps the treble will get sweeter.

If you chose to build this MOSCODE 300 inspired amp, set the BIAS for 300mA (like MOSCODE) and then set the DC offset to 10-20 mV on the output terminals. Also like MOSCODE, you can go AC on the heaters without problems. Just make sure the AC heater is tied to the -60 rail via a 100 ohm resistor so that heater to cathode voltage of the 6SN7s is not too great. This also keeps hum in check. Speaking of hum: use a star grounding scheme and terminate all ground leads between the power supply caps.

Happy 2011

PS: I have posed a .PDF version of the schematic. That should be easier to read than the low res graphic.
Hey Vilfort

How is the amp working? Amy more versions or was this the top of the line? Just got a factory wired unit again for next to nothing and plan to gut and go with tube front end.
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Old 16th March 2013, 05:04 PM   #1087
stenks is offline stenks  Serbia
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Default Before modification

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Old 16th March 2013, 05:21 PM   #1088
stenks is offline stenks  Serbia
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Default After modification

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Old 16th March 2013, 06:15 PM   #1089
stenks is offline stenks  Serbia
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Default DH-200 DH 101 bad boys

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Last edited by stenks; 16th March 2013 at 06:41 PM.
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Old 16th March 2013, 08:36 PM   #1090
fab is offline fab  Canada
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Nice !

Clean job and Good caps upgrade. Specially the Muse cap for feedback.
Can you comment on the sound "before and after " mod (bass probably better tight and defined ?or else).

Thanks
Fab
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