LOW VOLTAGE 845 SE

zygadr · « **Reply #20 on:** 2 Sep 2009, 07:57 am »

Anyone care to comment on the attached first stage of this amplifier schematic : in particular the return path to the grid of the 300B from the cathode of the 6SL7?........seems strange for what seems to be a regular cascade stage?

smbrown · « **Reply #21 on:** 2 Sep 2009, 05:32 pm »

Yep, if I'm following this correctly wouldn't that be applying positive feedback to the first stage? (= big oscillator)?

zygadr · « **Reply #22 on:** 3 Sep 2009, 02:30 am »

Yes, I actually got on to the author's site (shoud have in the first place

) and you are correct smbrown.

I don't think I like it - feedback of any type. So, I'll just stick to a straight forward 6SL7 cascade which seems to work fine.

I'll be buying Lundahl interstages before the end of the year to replace the Hammond 124D 's......that should make a huge difference

zygadr · « **Reply #23 on:** 6 Sep 2009, 11:24 am »

Well, this is intersesting..........

.......It was suggested and decided that the Hammond 124D interstage was not producing enough bass.

I have just listened to the mono breadboard connected to some proper speakers (wired in series for mono) and have BASS

.......speaker impedance is about 11-12 ohms on an 8 ohm output transformer.

I could easily hear quite low notes in jazz pieces on FM broadcasts and drums were not bad at all.

What's happened???

JoshK · « **Reply #24 on:** 9 Sep 2009, 05:31 pm »

I need to ask. Doesn't this have way too much gain? A single 6SL7 should do the job, especialy since the 845 is being operated at low voltage/low current levels.

Too much gain is a recipe for excess distortion and noise. The cascade stage was apparently to get gain up to use feedback. You also have to admit another cap to add the cascade as shown. I'd loose the second stage and if that is not enough gain, put a CCS or plate choke on the single 6SL7.

About the positive feedback. I am not so sure. This is AC feedback right? It would seem that the 120K resistor between the two cathodes would limit the positive feedback aspect. Enough, I don't know. (side note, why the 120K in the first place? weird way to bias) It looks as though he is injecting the feedback into the cathode of the first, equals gain plus same phase into the grid of the second 6SL7 to get gain and flipped phase.

zygadr · « **Reply #25 on:** 10 Sep 2009, 04:21 am »

JoshK, My circuit is not actually the same as the one posted.
I am using the SAKUMA 845/845 SE schematic, but have replaced the input transformer with a simple cascaded 6SL7 stage. I was only curious on the posted schematic due to the way the 6SL7 cascade was designed.

At the moment the circuit sounds very good apart from the possible downfalls of a cheap interstage.
There is ample loudness before audible distortion eventually kicks in as far as I can tell.

I must confess that the sound of this amp is like no other SE amp I've ever heard

......highly transparent, toe tappingly musical/lively and no ''veil'' that is often encountered with other designs(my last 300B SE

).

Gain is important as I'm not using ultra high efficiency speakers (been there and done that!).