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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 20 Mar 2012, 23:00 
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Hey y'all!
I was looking at a schematic posted in the DIY Audio Schematics section of DIYAudioProjects;
Single-Ended (SE) 6V6 and 6V6GT Tube Amplifier Schematic

I have a couple of questions regarding this design. :confused:
1.) Looking at R8, which is across the primary of the OPT, is that a "PTC" device and what benefit does it serve. (Some European symbols give me fits).
2.) It looks like some global NFB is being used, but what is going on with the circuit containing C6 and R6 (from the plate of the 6V6 to the driver side of the grid resistor in to the 6V6)?

I've found I have a NOS 6V6GTA, and a 5U4GT and a suitable PT and maybe OPT from an old Sears Silvertone AM/FM/Phono chassis. I know the amplifier worked at one point in my possession, but I don't have any wishes to rebuild it. Most of the PIO caps have wept and I can not identify the particular model to find schematics. Sears sold several models under the Silvertone brand. I know the tubes are NOS because I bought them brand new when I obtained this chassis back in the 80's (they're Realistic branded, which is a Radio Shack brand). Just checking out some ideas for these and this particular schematic made me say "What the.....??". Since I only have the one tube and its OPT, I have to stick with SE. Though, I am not married to the OPT, I could toss that if need be.

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The key to a successful build is to keep the smoke IN the circuit.
-Les

Science without religion is lame, religion without science is blind. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board
Current project: 6V6 "pseudo Champ" prototype
Still to come (On hold): 6CG7/12AX7 Guitar amp (modified FireFly)


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PostPosted: 21 Mar 2012, 11:14 
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Hi Les, An interesting schematic. The part across the output transformer is a trim pot. Apparently there is some impedance issue with the design that makes it sensitive to the output loading. (if someone else knows for sure, please jump in). The R and C off the anode is a negative feed back loop. Fairly common. There is also some global NFB as you guessed. The design is not novel, just not all that common. The use of a direct coupled voltage amp section to a power output tube is OK, just requires a bit more care in design than most folks want to do. Any voltage change from the input (it will respond to dc offset in the source) to variation in the AC mains can alter the current in the output stage. It does get away from having coupling caps though. My personal thought is that a single coupling cap is a better solution and far less likely to cause problems than the cap free design (just my 2 cents). When good caps were hard to come by, the design made sense, now not so much. Overall the circuit should work. How well is up for grabs. A SE 6V6 is not going to shake the world for loudness. I would guess the max out with less than 2% distortion to be about 2 watts, perhaps less. You could update the design with an EL84 and need only adjust the cathode resistor in the output stage to get the proper current and have a better amp. :)

Good listening
Bruce

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PostPosted: 21 Mar 2012, 12:02 
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Thanks Bruce!
gofar99 wrote:
The part across the output transformer is a trim pot. Apparently there is some impedance issue with the design that makes it sensitive to the output loading.

Improper matching of the OPT to the power tube, perhaps?

The circuit off the 6V6 plate I referenced, I kinda had an idea that was what was going on, but wanted to be sure.

Since I have (had) a working amplifier in that Silvertone, I may just ferret out the actual circuit use in that amp and reconstruct it. It was no powerhouse, its OPT is tiny, but with a good high efficiency speaker, it should be loud enough for a boutique guitar amp.

Thanks again, Bruce!
:beerchug: Cheers!

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

Science without religion is lame, religion without science is blind. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board
Current project: 6V6 "pseudo Champ" prototype
Still to come (On hold): 6CG7/12AX7 Guitar amp (modified FireFly)


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PostPosted: 21 Mar 2012, 20:58 
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WTH...? 250V for C6 is suicide! DC+Vp-p will take that to 600V easy! :o

It doesn't need to be 2u2 either. For a 2Hz rolloff, 15n will do it.

Cheres!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 22 Mar 2012, 13:23 
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Great point, Gregg! The DC component at that capacitor, assuming the specified voltages and assuming that is at idle, is 222VDC. Add the AC component, which will be present from the plate of V2 and at the plate of V1 and which should be about 180 degrees out of phase, you could easily get way over that 250V rating, anyway :eek: .
Wonder how many firecrackers have been built from this schematic?

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

Science without religion is lame, religion without science is blind. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board
Current project: 6V6 "pseudo Champ" prototype
Still to come (On hold): 6CG7/12AX7 Guitar amp (modified FireFly)


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PostPosted: 31 Mar 2012, 12:28 
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Ok. I finally got around to tracing out the audio amp section of that old Silvertone chassis. Seems pretty straight forward, though there are a couple of odd (to me) things being done.
This amp section uses a 6SQ7GT as a driver and 6V6GTA in SE topo. I have drawn up the schematic and will try to post later today.
The odd things I see are that the cathode of the 6SQ7 is directly grounded and the plate's B+ is derived from the UL biasing of the 6V6GTA. Like I said, odd. :confused:
Anyway, in the mean time I am posting files relevant to the 6SQ7.
Attachment:
6SQ7GT.jpg


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_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

Science without religion is lame, religion without science is blind. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board
Current project: 6V6 "pseudo Champ" prototype
Still to come (On hold): 6CG7/12AX7 Guitar amp (modified FireFly)


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PostPosted: 31 Mar 2012, 14:16 
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Les wrote:
The odd things I see are that the cathode of the 6SQ7 is directly grounded....


That's not cool.

The 6SQ7 is too sharp of cutoff for fixed bias in that day and age.


Quote:
....and the plate's B+ is derived from the UL biasing of the 6V6GTA. Like I said, odd. :confused:


Oh, hey.... cool way of anode following :D

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 31 Mar 2012, 15:36 
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Attachment:
6V6 SE Silvertone trace.JPG


Audio-in comes from the VR (Volume). I haven't completed ferreting out up to the volume. It's a real rat's nest up there around the VR, it's a gang and also has tone VR. There is a rudimentary slide switch arrangement that spans the width of the chassis. I expect I'll find the pre-amp circuit in that mess, possibly involving a 12AX7. That whole area also contains the AM & FM circuitry - like I said, a rat's nest.


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_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

Science without religion is lame, religion without science is blind. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board
Current project: 6V6 "pseudo Champ" prototype
Still to come (On hold): 6CG7/12AX7 Guitar amp (modified FireFly)


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PostPosted: 31 Mar 2012, 19:25 
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OK, so the B+ to the pre is bypassed.... I thought they were using it as FB.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 31 Mar 2012, 21:10 
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Location: South East US - Tennessee
Like I said, odd. It is what it is. I must say that it worked quite well, as I remember (that was over 20yr ago). Another interesting item (not shown) is that the power supply has minimal filtering. No chokes, no RCR, just a single 40uF electrolytic coming off a 5Y3. Granted the unit did have some hum, it was far less than I would have expected given the limited filtering.

This definitely is going to get built, probably sooner than later since I have everything but a case. It should make a really nice boutique guitar amp. I'm not going to be using any of the passives from the old chassis. The resistors have aged well out of tolerance. This is the one time I had to rely solely on recognizing the colors of the bands. Every resistor I checked with DVM was better than 20% out of tolerance (ie, 220k banded was reading 290k). And I don't have that warm fuzzy feeling with all those PIOs covered in a waxy substance. I don't remember them being that way before. Maybe they're ok, but I'd rather put new stuff in the build.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

Science without religion is lame, religion without science is blind. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board
Current project: 6V6 "pseudo Champ" prototype
Still to come (On hold): 6CG7/12AX7 Guitar amp (modified FireFly)


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 Profile  
 
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