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PostPosted: 05 Oct 2020, 10:30 
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Joined: 22 Sep 2020, 11:44
Posts: 10
Location: Prince Edward Island, Canada
Hi from PEI in Atlantic Canada.
Rebuilding an Eatons' 60's Viking am/fm stereo amp. The system has a pair of 7027A driving a pair of 6BQ/EL84 in SET configuration. I imagine first half of the 7025A accounts for the loss in the Bandaxall base/treble circuit. I still have the original tubes, which I intend to try before plugging in the new ones.The console (cabinet) has been lovingly restored, and the chassis has been stripped of everything except for the power tranny and the heater wiring. All new components ordered, and I am rebuilding according to the original schematic, (even) including the Bandaxall circuit. The system has two separate secondary 6.3V windings for the fiaments, wired one for right and one for left chanels, The filament windings have no center taps, but there are two hum balance pots firmly riveted, and soldered to the chassis. I've checked the filament voltages, and the pots still fully operational. Twiddling the pots produces the expected change in the balance.. I'll be implementing a "proper" grounding scheme according to the many "learned" articles and posts on this forum.
One question I do have.. If I leave the filament pots as they are, then they could (in all probability) cause a ground loop, as they are not returning the filaments to the "power" ground at first filter cap. The 7025 has a center tap to the heaters. Can this be used to provide the return path to "power" ground, or it best to create the filament wiinding 'artificial' center tap using a couple of 100 ohm resistors across the windings, and dispense with the trim pots. It has been a long time since I was building tube amps in the early 60's as a young engineer at the BBC in the UK. Removing wristwatches, rings, and keeping a hand in one's pocket, is deeply ingrained when working on electrical stuff, even after all these years. BTW, we were never successful in our attempts to reverse engineer the QUAD gear !!!
Sorry I rambled on.. Forgive an old man !
Regards


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PostPosted: 06 Oct 2020, 05:21 
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Joined: 22 Sep 2020, 11:44
Posts: 10
Location: Prince Edward Island, Canada
Excuse my dyslexia .. "Baxandall"... Giving it further thought, I've answered my own question... Disconnect trim pots, and create a center tap with 100 ohm resostors... I also want mention an acknowledgement to Matt Renaud and his article in DIY Audio projects, on designing a power supply, using the fictional :Ghost" amplifier. I have based my filtering scheme on this article.
Regards

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PostPosted: 06 Oct 2020, 09:20 
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Joined: 22 Sep 2020, 11:44
Posts: 10
Location: Prince Edward Island, Canada
Some pictures of the Viking amp after I extracted it from the console. The knobs, and amp tubes are already removed (except for FM section - top right in chassis picture) . There are also input jacks for an external stereo-FM multiplexor..
The unit has never been removed from the console, since it was put together in early 60's. The speaker cones have unfortunaltely crumbled to dust, but at least the iron can be used as a load for testing purposes. It is set up for 8 Ohms.
It has not had any power for at least 15 years, and despite that, the chassis is in surprisingly good shape. The console had been sanded and re-varnished when I took the pictures. The console looks strange because of the perspective from the wide angle lens. The included record player is a 'Studio by Collaro' model number C-60. Intact, but I really don't think I'll be playing any of my treasured vinyl from 70's and 80's on this....


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PostPosted: 07 Oct 2020, 04:14 
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Joined: 13 Jan 2018, 21:33
Posts: 402
Location: australia
Hi and welcome. An interesting project. Love the cabinet and looks like a sturdy chassis. My 6sn7s filaments were powered by 6.3vAC too and if I recall correctly there was a centre tap from the edcor power transformer I connected to the ground bus. I’ll check that.

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PostPosted: 07 Oct 2020, 08:59 
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Joined: 22 Sep 2020, 11:44
Posts: 10
Location: Prince Edward Island, Canada
Woodo wrote:
Hi and welcome. An interesting project. Love the cabinet and looks like a sturdy chassis. My 6sn7s filaments were powered by 6.3vAC too and if I recall correctly there was a centre tap from the edcor power transformer I connected to the ground bus. I’ll check that.


Thanks from "Down Under".... The chassis is anything but flimsy, and the power tx'former is a serious lump of iron. I've just finished the physical layout of the components (on paper). I'd forgotten what a job it is to translate a tube schematic into an 'orderly' physical layout. It's been over 40 years.
To mount the components I'm cutting small boards, for grouping components by stage. I'm using pieces of hardboard that came from the original large perforated board, that was the back of the cabinet, How and where to mount the boards on the chassis, is another question.
The power transformer checks out, filament voltages are nicely balanced, I plugged in the original preamp 7025A tubes, to check filaments, and the tube warms up nicely. Voltages check out. Difficult to see a glow,, because the tube is in the orignal metal shield (which appears now, to be a permanent fixture after all this time).. Power transformer has center tap for secondary, but not on the filament windings. I am debating whether or not to rewire the filaments, but at the moment, it seems to be fully functional.
Hope to begin soldering very soon. I'll post a photo, when the project is nearing completion.
BTW, I was born and brought up in South Wales, and I remember the song from the 60's, when Welsh rugby was at an all time high,
" Who beat the wallabies, but good old sospan fach !!"
Rgds

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PostPosted: 11 Oct 2020, 16:34 
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Joined: 13 Jan 2018, 21:33
Posts: 402
Location: australia
More of an Australian Rules fan (based on Gaelic football IIRC) but don’t mind watching a good Rugby game. That usually means watching New Zealand play anyone. Australia has a NZ coach now and the future looks promising as we just drew a match with NZ. yeah!

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PostPosted: 12 Oct 2020, 13:24 
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Joined: 22 Sep 2020, 11:44
Posts: 10
Location: Prince Edward Island, Canada
Woodo wrote:
More of an Australian Rules fan (based on Gaelic football IIRC) but don’t mind watching a good Rugby game. That usually means watching New Zealand play anyone. Australia has a NZ coach now and the future looks promising as we just drew a match with NZ. yeah!

Hi John,
We had that game live at 12:00 am in the early hours of Saturday morning.. It was worth staying up for... The All Blacks don't like losing, especially to their nemesis "The Wallabies".
We also get the AFL games on the TSN sports channel.... Those players are so fit, and they do play hard.. Took a while to figure out the rules, especially as the ball is live most of the time..
I'll keep updating the restoration project..
Rgds
Richard

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PostPosted: 11 Dec 2020, 23:53 
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Joined: 11 Dec 2020, 22:57
Posts: 1
Hello,
I just inherited this exact cabinet from my uncle and wanted to learn more about it which lead me to your thread here. By the way your cabinet looks beautiful! I would like to get mine working better, it's very quiet and crackles, and was just curious what troubles you ran into, where you got your parts, and if there were any modifications or alterations that should be done to it to make it better.

Thanks very much,

Nathan.


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