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Old 19th June 2012, 10:53 AM   #1
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Default need advises tweaking PSE 300B

hi Folks,
I need suggestion to tweaking my amplifier, which is PSE 300B,
the schematic and real amps has few difference, like r24 in amp is 200 ohm, and r25 only a jumper and r6 is 56k Ohm.
in the OT secondary winding there is a zobel network between 0 and 8 ohm tap, a series network of 1uF and 680 Ohm.
I did try disable the zobel network, and for my ears, the treble is subdued.
better i keep the original or follow the schematic?
thank you, and sorry for my messy english.
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File Type: pdf VP-300BD???12b.pdf (167.0 KB, 140 views)
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Old 19th June 2012, 10:18 PM   #2
tomchr is offline tomchr  United States
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What are you trying to accomplish by modifying your amplifier?
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Old 19th June 2012, 10:45 PM   #3
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hi Tomchr, better detail, and better treble, warmer sound, the last one is the most goal, because i feel dry Sir.
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Old 20th June 2012, 12:58 PM   #4
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Quote:
Originally Posted by sensphoto View Post
hi Folks,
r24 in amp is 200 ohm, and r25 only a jumper .
Are you sure about that? In the schematic, they have the same function, which means that it should be R24=R'24=R25=R'25 . Best thing to do is make them all as in the schematic.

Anyway, something to test would be disconnecting the feeedback loop (lift R23) and see if it improves. It will have more gain this way, but you should concentrate on the sound difference.
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Old 20th June 2012, 02:54 PM   #5
tomchr is offline tomchr  United States
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I would like to remove the feedback loop as well, but with the 12AT7 in there, the gain will be rather large. I'm resisting the urge to suggest to scrap the input/driver stage and redesign it based on a 6SN7 or ECC99...

The amp as-is is DC coupled all the way to the output stage. This complicates "simple" mods. Ensure that the coupling caps C2, C3 are of good quality (polypropylene film caps). Personally, I'd prefer to AC couple into the cathode follower (V2B) rather than into the output stage, but that's hard to do in this design without having to redesign substantial portions of it.

The 300B tube is a tricky tube to drive. It's taken me a couple of years to arrive at a driver I like.

~Tom
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Old 21st June 2012, 01:07 AM   #6
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"Are you sure about that? In the schematic, they have the same function, which means that it should be R24=R'24=R25=R'25 . Best thing to do is make them all as in the schematic."

yes i'm sure, and this one makes me think a lot, and don't ring any bells in my head

one more R6 and R'6 on the schematic are 47K/5W, on my board are 56K/5W, i don't why this R is different and 5W rating, since the dropping R before this stage is only 2W rating...

as you advised, i better follow the schematic.

i don't want to change the topology for this amp, this amp using PCB, and i hasn't a capability to modifying the board. but I'm still open for any idea.

i have we437a and keep those for my future DIY amp, still dunno what to do, c coupling 2 stage amp, or precious 2 stage amp with interstage guys...
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Old 26th June 2012, 11:35 PM   #7
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Default grid stopper resistors

Quote:
Originally Posted by costis_n View Post
Are you sure about that? In the schematic, they have the same function, which means that it should be R24=R'24=R25=R'25 . Best thing to do is make them all as in the schematic.

Anyway, something to test would be disconnecting the feeedback loop (lift R23) and see if it improves. It will have more gain this way, but you should concentrate on the sound difference.
following the schematic provided, which are R24=R'24=R25=R'25, but still on amp original value resistors (220 Ohm) which is on the schematic are 1 k Ohm.
Gurus, what difference if i change the grid stopper resistor from 220 ohm to 1 k ohm?
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Old 1st July 2012, 06:16 AM   #8
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Difference is minimal. Just use 1Kohm
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Old 2nd July 2012, 12:04 AM   #9
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I decided to use R24=R'24=R25=R'25 220 Ohm, try to change it to 1 K ohm and I cant hear any difference.
but I do experimenting with value of coupling cap, from the original value (0.47uF) to 0.27uF and 0.22uF, I keep the 0.22uF, the mids and treble more natural and rich.
with the original value which is 0.47uF, bass very good but the mid and treble are too much subtle.
experimenting with 300B tubes, from the original which are valve art tubes to Full Music 300b Mesh Plate, the sound much better again, but I dont have much money to buy quad pairs so I keep the original tubes from factory for the 300B...
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Old 3rd July 2012, 06:27 AM   #10
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Do not change randomly the coupling caps, they are inside the global feedback loop, and their values have been (we hope) calculated to avoid instability.

Anyway, as tomchr said, it is very difficult to make changes to this design, because it is all DC coupled. So, if it has a PCB (circuit board) and is not point-to-point wired, most you can do is change different tubes and see if it sounds better.
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