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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 14 Apr 2019, 03:17 
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Location: australia
very nice amplifier. you should be proud that it’s out there somewhere playing music for someone’s pleasure. there are certain things i like about my pioneer sc1223 theatre amp such as the convenience of wifi, ethernet, internet radio etc, and the sound is respectable if not a little characterless compared to the tube amp. i’ve never opened it up but i read that it uses the ice power class d module. I’m looking forward to putting the tube amp back into service. I have tried using the preamp out from the theatre amp to drive the tube amp but there must be a mismatch as the sound is not quite right. i just use the iphone. if someone comes up with or knows of a good diy dac that has wifi, usb and ethernet capability and would match the tube amp, that might be worthwhile putting together.

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PostPosted: 14 Apr 2019, 06:03 
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Class d normally have low input impedance of 10k ohms


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PostPosted: 15 Apr 2019, 05:47 
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Just can’t find the specs of this amp at the moment but i seem to recall input impedance of 47 k and for the preamp-out impedance was quite low. there is a second preamp-out option with 2k or so impedance. For driving the 6sn7/300b i’m not sure of the significance but the sound isn’t so good.

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PostPosted: 25 May 2019, 22:56 
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Completed the re-wire loosely according to the monoblock vision of @suncalc and @mwhouston. Ran into some space issues and suboptimal layout as I stuck to what I had done for the old pcb amp. I will need to move things like the 5v regs around to get a better and neater layout. There's audible hum which sounds like double mains freq which I will keep at. Musically the sound is beautiful though. Early days but sounds like a big improvement compared to the settings used for the pcb amp. Thanks diy friends for the circuit and the diy site.

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PostPosted: 05 Nov 2019, 16:58 
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Joined: 03 Jan 2019, 13:17
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Woodo wrote:
Regarding the schematic, i have shown a potentiometer but there isn't provision for one in the pcb. I am making provision to fit one in case I don't need a pre-amp. Are there any views on whether it would work in this circuit with the 100k resistor or should this be removed or reduced? Also any comments on the operating points of this amp would be welcome.

Hello mate !

Did you remove that 100k resistor ?

Inspired by your deeds I went ahead and bought this as a fully stuffed PCB.

Got the specified iron. Laid out everything temporarily on a chip board. Plugged in the 300B, 5u4gb and 6SN7's. All bought used from ebay.

Its quite a riot really but it makes music. Great music in fact. I don't think I shall be going back from DHTs now.

But I too am facing the hum/buzzard issue. Its initially kind of a low grumble whilst the volume pot wiper is at minimum. This hum climbs up as I crank the shaft, peaking just before the wiper reaches max. At that point it cuts back sharply to initial level and characteristic. Pot is a 50K Alps Blue Velvet. Would that 100k resistor be messing here leading to this variable hum?

300B filaments are fed by a separate tranny which has dual 5v/3amp wingdings. All power leads to the PCB are short dressed and tightly twisted. Ironware is laid out as per good practices. Prior to this I have built a few SEP and PP amps with P2P approach and achieved nil hum.

Would be great to have your guidance on this specific PCB. I am hoping this gets sorted out so that a proper chassis can be flipped.

And what a smashing job you have done with your chassis. Your posts on this thread and elsewhere have been a great help. :thumbsup:


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PostPosted: 18 Nov 2019, 05:55 
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addressyou wrote:
Woodo wrote:
Regarding the schematic, i have shown a potentiometer but there isn't provision for one in the pcb. I am making provision to fit one in case I don't need a pre-amp. Are there any views on whether it would work in this circuit with the 100k resistor or should this be removed or reduced? Also any comments on the operating points of this amp would be welcome.

Hello mate !

Did you remove that 100k resistor ?

Inspired by your deeds I went ahead and bought this as a fully stuffed PCB.

Got the specified iron. Laid out everything temporarily on a chip board. Plugged in the 300B, 5u4gb and 6SN7's. All bought used from ebay.

Its quite a riot really but it makes music. Great music in fact. I don't think I shall be going back from DHTs now.

But I too am facing the hum/buzzard issue. Its initially kind of a low grumble whilst the volume pot wiper is at minimum. This hum climbs up as I crank the shaft, peaking just before the wiper reaches max. At that point it cuts back sharply to initial level and characteristic. Pot is a 50K Alps Blue Velvet. Would that 100k resistor be messing here leading to this variable hum?

300B filaments are fed by a separate tranny which has dual 5v/3amp wingdings. All power leads to the PCB are short dressed and tightly twisted. Ironware is laid out as per good practices. Prior to this I have built a few SEP and PP amps with P2P approach and achieved nil hum.

Would be great to have your guidance on this specific PCB. I am hoping this gets sorted out so that a proper chassis can be flipped.

And what a smashing job you have done with your chassis. Your posts on this thread and elsewhere have been a great help. :thumbsup:

Sorry for the delay in responding :blush: . I completely missed this post. Actually I moved on from this pcb. While it sounded pretty good to me bar a little hum which I couldn't hear from my sofa I was somewhat motivated by the @suncalc and @mwhouston thread on matt’s design and mark’s build of a 300b set particularly from about page 33 viewtopic.php?f=9&t=2699&start=320#p28729.

I didn’t replace the 100k resistor with the pot in the end as I found i didn’t really need to and then decided later to follow Matt’s schematic for the 300b monoblock. You’ll see that circuit uses a 250k pot. between the first and second 6sn7 triodes. I’m not skilled enough to advise whether on the pcb to replace the 100k with a pot of a particular value. maybe someone with greater knowledge on this could chime in. @ilovehifi ?

Both @suncalc and @gofar have grounding philosophies published on this site. This seems to be of paramount importance for a dead quiet amp. For the monoblock version I went with matt’s version to try not to deviate too far from the circuit (i did recycle the hardware from the pcb version so it is different to some degree). Others also have used Bruce’s method to effect.

I did have some noise issues with the new build which i believe may have been due to the 24va 6v filament transformer being at its limit. I still have the slightest noise at switch on which seems to settle down after a minute or so. a 30va 6v toroidal transformer made a big difference.

I was a bit disappointed with my chassis outcome, i’ll try harder next time. There are others’ builds of various amps and speakers of which I am in awe of their patience and execution of skill. Hope everything has turned out well for your build and the sound is satisfying.

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PostPosted: 20 Nov 2019, 16:24 
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Joined: 13 Jan 2018, 21:33
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Location: australia
FYI: technical library diy resources.
http://diyaudioprojects.com/Technical/

Matts grounding advice
viewtopic.php?f=9&t=4542&p=38008&hilit=Philosophy+Grounding#p38008

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PostPosted: 21 Nov 2019, 01:25 
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Joined: 03 Jan 2019, 13:17
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Thanks mate for your valuable guidance.

I too have decided to abandon the PCB. Its difficult to make that PCB hum proof primarily due to its approach to 300B filament heating. A simple rectifier with 10k of uF isn't going to cut much ice it seems. A shame because that PCB kit quality is really top grade.

Currently I am thinking point 2 point with essentially the same schematic and the suncalc/mwhouston build thread is the right address for now.

Thanks and best cheers !


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