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
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
TVA-1 PSU schematic
TVA-1 PSU schematic :
https://picasaweb.google.com/vbkolsen/HiFiAlbumForTVA1TVA100#5646735769824016018
TVA-1 PSU schematic :
https://picasaweb.google.com/vbkolsen/HiFiAlbumForTVA1TVA100#5646735769824016018
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/vbkolsen/HiFiAlbumForTVA1TVA100#5646739642431944130
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
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 🙂
Central ground point under the bias-pcb
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.
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.
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!
Best regards!
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)
"Factory standard" as a start
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!
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!
TDP designed the TVA-1s circuit but it's not sure he was actually involved in the mechanical and wiring layout. The M&A amps were handbuilt in a small shop in London (not by TDP) and building quality was inconsistent: all the exemplars I worked on were different inside and all that changes indicates M&A had many technical issues they tried to correct during the production run of this amp.
Sure, a properly designed and built amplifier must be dead silent, without any audible hum or noise.
Yes, hum was allways an issue in the TVA-1 and your results are close to what I've found myself. Still, 4mVac is equivalent to a 75db S/N ratio for an amp rated at 70W rms into 8 ohms. Not an outstanding value but not inacceptable neither for a 70W tube amp. (I've seen much worse). Of course, how much perceived noise it will actually produce depend of the efficiency of your speakers and can range from nearly inaudible to very disturbing, but there is really no need to use a 70 watts amp with high efficiency speakers anyway...
I agree, lack of central (star) earthing/grounding point and uninsulated input jacks are the best recipe to get ground loops and hum and that's a major design flaw of the TVA-1. This can (and must) be easily corrected by some re-wiring work but even then you'll get some residual hum, though it will be more bearable. I still suspect there's some kind of magnetic coupling through the stainless steel chassis/transformers and of course there's nothing you can do to fix this.
I never mentioned (or encountered) any stability issues with the TVA-1 but thermal runaway (red plating) and subsequent destruction of the output tubes were very common even with the original Genalex/Gold Lion tubes the amp came with. It looks the designer pushed the tubes very hard to get 70 watts out of a pair of KT88's. With currently produced KT88/6550's the problem will be even worse and reducing the grid return resistors AND plate current (bias) could help protecting your tubes from repeated failures and self destruction. The cathode resistor has no effect on this and is only there to protect the tube and help measuring the cathode current. I use a 10 ohms/2 watts flameproof film resistor at this point which give a good protection and easy way to compute cathode current.
I would be very interested too, though I no longer have a TVA-1
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TVA-1 hum problem
Thanks to a friend in UK who in the seventies assembled amplifiers and worked for mr. A. Michaelson and mr. K. Austin , we managed to find the cause of the 50-60 herts hum (in size order of 7-10 mA), which were generated from the TVA-1 both along with sound reproduction and also by clean idle without sound .
According to my friend, the original ground circuit was connected to a central earthing point under the bias circuit board near the power supply capacitors.
In my TVA-1 version the ground point was located in the middle of the amplifier near the driver PCB.
This explanation makes sense, and at the same time it gives me a lot of work to reorganize the ground connections in my TVA-1.
All my home-built amplifiers have an earthing point near the power supply's main capacitors, but you do not move the central earthing point on a factory-built tube amp .... or do you?🙂
Later on find the rewiring on : https://sites.google.com/site/httpstubeamp/
Thanks to a friend in UK who in the seventies assembled amplifiers and worked for mr. A. Michaelson and mr. K. Austin , we managed to find the cause of the 50-60 herts hum (in size order of 7-10 mA), which were generated from the TVA-1 both along with sound reproduction and also by clean idle without sound .
According to my friend, the original ground circuit was connected to a central earthing point under the bias circuit board near the power supply capacitors.
In my TVA-1 version the ground point was located in the middle of the amplifier near the driver PCB.
This explanation makes sense, and at the same time it gives me a lot of work to reorganize the ground connections in my TVA-1.
All my home-built amplifiers have an earthing point near the power supply's main capacitors, but you do not move the central earthing point on a factory-built tube amp .... or do you?🙂
Later on find the rewiring on : https://sites.google.com/site/httpstubeamp/
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New DIY site available
LS20 has closed the account but the TVA-1 amplifier to be found on a new site at:
https://sites.google.com/site/httpstubeamp/home/michaelson-austin-tva-1
gudmund
LS20 has closed the account but the TVA-1 amplifier to be found on a new site at:
https://sites.google.com/site/httpstubeamp/home/michaelson-austin-tva-1
gudmund
TVA-1 psu centralground point
Just to repeat the answer to me from LS20 :
Efter some research LS20 found one person who actually worked for Michaelson & Austin as technical assembler around 1978. He soldered and wired up the TVA-1, and according to this statement the central groundpoint was located upon one of the nuts which hold the brackets of the main serial power capasitors...under the bias pcb 🙂
Very nice information...it makes sense... and just what I did with the DIY valve amplifiers of my own.
The hum from my TVA-1 is now beyond the audible range of the ear , even with one ear close up to the bass speaker unit.
The AC measured on the op-terminals stays below 0,2 mVac.....
The TVA-1 : https://sites.google.com/site/httpstubeamp/home/michaelson-austin-tva-1
One advice with the TVA-1: Try to use 1000 VDC poly signal capasitors and RCA connectors without direct connection to the chassis.........
Just to repeat the answer to me from LS20 :
Efter some research LS20 found one person who actually worked for Michaelson & Austin as technical assembler around 1978. He soldered and wired up the TVA-1, and according to this statement the central groundpoint was located upon one of the nuts which hold the brackets of the main serial power capasitors...under the bias pcb 🙂
Very nice information...it makes sense... and just what I did with the DIY valve amplifiers of my own.
The hum from my TVA-1 is now beyond the audible range of the ear , even with one ear close up to the bass speaker unit.
The AC measured on the op-terminals stays below 0,2 mVac.....
The TVA-1 : https://sites.google.com/site/httpstubeamp/home/michaelson-austin-tva-1
One advice with the TVA-1: Try to use 1000 VDC poly signal capasitors and RCA connectors without direct connection to the chassis.........
Hi,
I picked up a unit which was sold in 1980 and only was used for less than 10 hours. After that the unit broke down and the owner did not return it under warranty. Instead he moved from one house to another and started his own business. Because of a lack of time he put the amp on his attic and forgot about it until one week ago.
This unit is in pristine condition and in fact NEVER used. The Original MOV KT88 tubes still have the GEC sticker on the glass where this sticker does not even have traces of heating. So in fact this amp is new but did not function. Now I found there was a solder joint of one of the bridge rectifiers missing (or did not make contact).
I repaired this but I have the same problem with the grid to ground resistance. As many of these amps have slightly different schematics, mine does to. It's the version with the large PCB. I've studied many mods, including the one on this page but I was wondering if mine is plausible.
I picked up a unit which was sold in 1980 and only was used for less than 10 hours. After that the unit broke down and the owner did not return it under warranty. Instead he moved from one house to another and started his own business. Because of a lack of time he put the amp on his attic and forgot about it until one week ago.
This unit is in pristine condition and in fact NEVER used. The Original MOV KT88 tubes still have the GEC sticker on the glass where this sticker does not even have traces of heating. So in fact this amp is new but did not function. Now I found there was a solder joint of one of the bridge rectifiers missing (or did not make contact).
I repaired this but I have the same problem with the grid to ground resistance. As many of these amps have slightly different schematics, mine does to. It's the version with the large PCB. I've studied many mods, including the one on this page but I was wondering if mine is plausible.