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Old 19th August 2013, 09:07 AM   #1
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Default Michaelson and Austin TVA-1

I am the owner of a TVA-1 , one of the erly amplifier types from Michaelson and Austin with the small powertransformer, and I need a picture of the original ground wiring.

The amplifier was received with some soldering changes made for the ground connection. Its difficult to keep the weak 50-60 Hz hum away from the speakers.

A hole has been drilled in the midle of the TVA-1 for gnd., but the central ground point should be close to the large psu capacitors under the psu-PCB? ...or?

thanks
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Old 19th August 2013, 10:16 AM   #2
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Default TVA-1 PSU schematic

TVA-1 PSU schematic :

https://picasaweb.google.com/vbkolse...35769824016018
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Old 20th August 2013, 08:14 PM   #3
Arno Pf is offline Arno Pf  Netherlands
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Brings back memories ....20years ago I restored one for a friend of mine...but can't remember any excessive hum though
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Old 21st August 2013, 07:43 AM   #4
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Quote:
Originally Posted by Arno Pf View Post
Brings back memories ....20years ago I restored one for a friend of mine...but can't remember any excessive hum though
Hi, ...
The point is that the former owner made some rewiring of the ground wires and moved the central ground to the midle of the amplifier.
One hifi interested person who said that he worked for Michaelson & Austin in New Bond str., London UK told me, that the central ground point was located at the large high voltage capacitors ( under the the "Soundlease services FB1" bias PCB).
Of all the internet pictures taken from the bottom of the TVA-1, the central ground point is not visible.
Could you provide me a picture of the central ground point from the TVA-1 , erly model?
I would like to bring my TVA-1 back to "factory standard" ....as when Kevin checked my amplifier back in 1979.

Thanks in advance.

Link for strange ground point at the TVA-1:
https://picasaweb.google.com/vbkolse...39642431944130
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Old 21st August 2013, 07:47 AM   #5
Arno Pf is offline Arno Pf  Netherlands
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Unfortunately I don't have any photo's

Can you measure with an oscilloscope? If yes, check if there is a "hum" signal between the signal ground of the small tubes and the huge gnd connection on the chassis...for a start
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Old 21st August 2013, 09:15 AM   #6
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Quote:
Originally Posted by Arno Pf View Post
Unfortunately I don't have any photo's

Can you measure with an oscilloscope? If yes, check if there is a "hum" signal between the signal ground of the small tubes and the huge gnd connection on the chassis...for a start
Yes, I have measured approx. 7-8mVac hum on the right channel and 3-4mVac on the left channel after the first attempt to rewire the amp. The mentioned 7-8mVac was reduced from 10 -20 mVac on both channels when I received the amplifier.
If you look at the picture on Picasaweb , a strange "soldering tower" is visible and a center gnd.point between the output tubes, which is not original.
The idea was "TVA-1 back to factory standard"!

...thanks anyway
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Old 25th August 2013, 10:02 AM   #7
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Default Central ground point under the bias-pcb

Quote:
Originally Posted by Leakstereo20 View Post
............ the central ground point should be close to the large psu capacitors under the psu-PCB? ...or?

thanks
By help form another TVA-1 owner it has been determined by photo's, that the original central ground is located under the bias-pcb "Soundlease Services FB1". The groundwire from the two mainpower capacitors is soldered upon the "ground soldertag" on the powertransformer, and from there another wire to the main cap bracketholder-screw on the chassis .
Original Michaelson & Austin chose to connect speaker minus to a chassis point number two near the output transformer, which is located furthest away from both output terminals.
The designers must have preferred this solution rather than floating ground output.

Thanks to the DIY members.
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Old 25th August 2013, 12:12 PM   #8
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I think you'd first select your main goal - factory standard or hum minimizing. For the latter one star grounding really is not the worst choice. Provide a "sub star" for each stage and one main star next to the PSU's filtering caps. And you don't need any photos for doing so :-).

Best regards!
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Old 25th August 2013, 01:59 PM   #9
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Back to "Factory Standard" wiring will bring you back to higher hum level which is structural in the M&A TVA-1 amp and his many variations. Also, I'm wondering what "Factory Standard" really means as there were countless different (factory) versions of this amp and all suffered from the same residual hum problems. Looks like the previous owner tried to minimize the hum by re-routing the ground wiring (star ground) and then probably gave up at some point. I worked on many TVA-1's and tried all the possible hum-reducing strategies (new star ground circuit, complete re-wiring , increasing power supply capacitor values, DC on heaters, better decoupling, etc...) but whatever I tried there was still a small residual hum remaining, allways higher on one channel. I concluded there might be some magnetically induced hum in the metal chassis and total hum cancellation was not possible. You should consider 3-4 mV pp residual hum an excellent performance for a TVA-1 (probably exceeding the original factory spec's) and further lowering this value a real hard battle. My experience is based on 4 or 5 exemplars of this amp I worked on and all suffered from this same hum problem, factory versions or not. Also, the TVA-1 (in his original version) is a troublesome unreliable amp, but this can be corrected. (see the many other threads about the M&A TVA-1)
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Old 25th August 2013, 05:28 PM   #10
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Default "Factory standard" as a start

Quote:
Originally Posted by Tubologic View Post
Back to "Factory Standard" wiring will bring you back to higher hum level which is structural in the M&A TVA-1 amp and his many variations. Also, I'm wondering what "Factory Standard" really means as there were countless different (factory) versions of this amp and all suffered from the same residual hum problems. Looks like the previous owner tried to minimize the hum by re-routing the ground wiring (star ground) and then probably gave up at some point. I worked on many TVA-1's and tried all the possible hum-reducing strategies (new star ground circuit, complete re-wiring , increasing power supply capacitor values, DC on heaters, better decoupling, etc...) but whatever I tried there was still a small residual hum remaining, allways higher on one channel. I concluded there might be some magnetically induced hum in the metal chassis and total hum cancellation was not possible. You should consider 3-4 mV pp residual hum an excellent performance for a TVA-1 (probably exceeding the original factory spec's) and further lowering this value a real hard battle. My experience is based on 4 or 5 exemplars of this amp I worked on and all suffered from this same hum problem, factory versions or not. Also, the TVA-1 (in his original version) is a troublesome unreliable amp, but this can be corrected. (see the many other threads about the M&A TVA-1)

Thanks for your reply.

I can not imagine, that TVA-1's designer TDP, would have accepted less than a minimum of 50-60 Herts hum on the first tube amp he constructed.
Then, of course, there has been a basic form of the way the cabling was performed. That being said, one can not exclude that the current wiring was changed slightly in production line, as you mention when you say that the TVA-1's you worked with had different ways of wiring .

I've built some amplifier copies of known structures like the famous CJ MV75, CJ MV45 & Leak Stereo 50 and my own design with 4 xEL84 / ECC80 / 6SL7gt.

Without sounding arogant , none of my own constructions of DIY had detectable hum on the output terminals....< 1-2mVac.

I believe you when you say, that there was always residual hum back in the TVA-1 regardless of your efforts to avoid hum. I even tried a lot on the TVA-1 myself.

I can't accept 3-4mVac in one channel and up to 7-10mVac in the other channel.

I looked in my purchased TVA-1, and found no central earthing point near the main caps.... that central point I would always consider of placing well in my own amplifiers.
Therefore I would like to hear from another TVA-1 owner, where there was no immediate visual soldering in TVA-1 amplifier. (How did M&A do it?)
One thing I did not understand was, that the RCA input jacks were not isolated from the chassis / shell? I find this as a huge mistake.
Stainless steel has a lower conductivity than, for instance, copper, so one can not exclude induced hum generated magnetically or ground loops due to several ground points, or perhaps the output / network transformers coupling to the chassis.

Finally, I do not agree, that the TVA-1 differs from other tube amps in terms of instability when taking into account, that the KT88 from contemporary production can not tolerate more than about 140K from the signal grid to ground at high B +. At the design time TDP used the UK MO-V KT88 without any instability problems, the strongest KT88 tubes ever made. Even old and used MO-V KT88 tolerates much higher grid to ground resistance than reissue valves. I have changed the four 220K bias resistors to 100K on the bias-pcb, and 100R (instead of 47R) as cathode resistors, there are no sign of instability on the scope.

If something new comes up concerning the TVA-1 hum issue, I am a good listener!
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