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Old 24th Oct 2011, 8:27 pm   #21
robin coleman
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Default Re: Michaelson and Austin TVA 1 Bias Help

Hi Kim you can ask as many questions as you like .The chassis made of stainless steel from a company called Lordgate engineering of STives Cambridgeshire and the other metal work was done by either mysesf or Camlit engineering of royston.

Hi Kim, you can ask as many questions as you like. The chassis is made of stainless steel from a company called Lordgate Engineering of St. Ives, Cambridgeshire, and the other metalwork was done by either myself or Camlit Engineering of Royston.
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Old 24th Oct 2011, 9:17 pm   #22
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

Hi Robin, I appreciate your cooperation very much.

I find the thick stainless steel chassis from Lordgate Engineering of Stives, Cambridgeshire very stiff and well suited for the 40Kg heavy amplifier.

The bias PCB size in my TVA-1 has been messured, and the two attached pictures shows the schematic of the original component values and the re-built PCB with resistor values changed for the KT88 reissues....Which keeps the KT88's "relaxed" with the fixed cathode bias of approx. 48mA and approx. 530V on the plates.

Do you by any chance have documentation on the presize negative bias value (B - ) in the point between the two 150K resistors for the ECC83 differential phasesplitter?

I shall return with more information.

Rgds. Kim

Ps. Magnification possibility on Picasa:

https://picasaweb.google.com/1031761...48124854041538
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Last edited by leakstereo20; 24th Oct 2011 at 9:26 pm.
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Old 24th Oct 2011, 9:46 pm   #23
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

Just some pictures of TVA-1 serial no. 553.

The amplifier needs more re-built, but the output valves runs quiet and steady, and the musical sound from this TVA-1 is just one of the best in my collection of different valve-and solid state amplifiers.
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Old 25th Oct 2011, 9:02 am   #24
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

In post 22# a minor correction.

Should have been: The fixed bias ( could be something around -50V to -65V)leaves 2,29V across the 47R which calculated is current draw of approx. 48mA.
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Old 25th Oct 2011, 2:33 pm   #25
robin coleman
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Default Re: Michaelson and Austin TVA 1 Bias Help

We spent a lot of time building them and every thought went in to leaving a bit more cable for further repairs in the future.
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Old 25th Oct 2011, 2:35 pm   #26
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Default Re: Michaelson and Austin TVA 1 Bias Help

We always tested using Classical and Roy Orbison.
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Old 25th Oct 2011, 3:55 pm   #27
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

Yes ..details ...voices , piano and classical music for test. For amplifier test I also use Chris Jones, Roadhouses & Automobiles, Phil Upchurch, Love is strange and of course Roger Waters with a lot details and music explosions. One thing is messured data...but how does the beast sound?

I really understand that a lot of time was spent building the amplifiers! Most of my valve - and solid state amplifiers were built upon the kitchentable and in my depository. ( My wife agrees )
When you say " leaving a bit more bable", you mean keep a bit more information for future repairs? ....need to improve my english.

Did you keep the way of hardwiring on paper ?... are you able to remember some of the wiring stuff? I would like to know the fundamental basic groundplane wiring used in the TVA-1...if you can remember some from the M100, the older TVA-1 wiring should be pretty much the same...I think. Would it be possible to make a drawing by hand of the hardwiring?

The attached pictures shows the driverboard:
Soundlease Services, TVA1/A, ISS.3

1. driverboard photo taken inside the amp.
2. First drawing of the driverboard , cobberside view.
3. Driverboard mirrored, component view

Note. Driverboard size and precision of placed cobberlines not finished!

Quick messurement of the driverboard today : 21,0cm x 10,3cm . By exact messurement of the TVA-1 , M100 chassis holes for ECC81 and ECC83 it should be possible to make new driver PCB's.

Picasa for details:
https://picasaweb.google.com/1031761...42596524964130

Rgds. Kim
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Last edited by leakstereo20; 25th Oct 2011 at 4:24 pm.
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Old 25th Oct 2011, 8:51 pm   #28
robin coleman
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Default Re: Michaelson and Austin TVA 1 Bias Help

I will try an d remember the pcbs were made by Anglia pcb company in st ives most of the time we copied a workinh sample to wire up.I did 12 hour shifts at papworth but also took valve assembleys and who units to make at weekends wish id built myself some up
Regards
Robin

I will try and remember. The PCB's were made by Anglia PCB company in St. Ives most of the time. We copied a working sample to wire up. I did 12 hour shifts at Papworth but also took valve assemblies and who units to make at weekends. Wish I'd built myself some up.

Regards,
Robin.
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Old 25th Oct 2011, 9:53 pm   #29
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

Thanks Robin, if you will give it a try and remember some parts of the wiring and PCB work it would be nice, even that I know its difficult to memorize such work after few years.

Among several pictures I found two advertisements from M&A, but I can't say anything about the year of printing. I have only seen the M&A preamplifier TVP-X from this advert paper.

Details: Note: use magnification at Picasa.

https://picasaweb.google.com/1031761...28630732682258

Best rgds. Kim
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Last edited by leakstereo20; 25th Oct 2011 at 10:13 pm.
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Old 25th Oct 2011, 10:08 pm   #30
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

There must have been a lot of work at Papworth at that period, with 12 hour shifts, and that you worked at weekends to complete valve assemblies and amplifier units.

I liked what you wrote, even though it must have been hard work

Rgds. Kim
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Old 26th Oct 2011, 4:31 pm   #31
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Default Re: Michaelson and Austin TVA 1 Bias Help

Picture number 1:

Shows the schematic of Michaelson & Austin serial no. 17.

The drawing made by another entusiast .

One thing which is odd compared to the TVA-1 schematic in Jean Hiraga's book "Rörförstärkere"( valveamps) looking at this drawing serial no.17, is the two capacitors in parallel the KT88 cathode resistors?
2200uf parallel with cathode resistor of 100R might be changes made by the owner?

Picture number 2:

Shows the schematic of Michaelson & Austin serial no. 553, left channel

Picture number 3:

Shows the PSU of Michaelson & Austin serial no. 553

Details:

https://picasaweb.google.com/1031761...22659114274338

Wants to add that the messurement for no. 553 was done without any change of resistors or capacitors on the driver PCB....all components original exept for the valve sockets which was changed for ceramic types.

After the bias regulation changes the KT88 cathode resistor values was raised to 100R/5W for better startup control. 47R might be the best value?
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Last edited by leakstereo20; 26th Oct 2011 at 4:53 pm.
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Old 26th Oct 2011, 8:38 pm   #32
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Default Re: Michaelson and Austin TVA 1 Bias Help

Well ...this is almost what I know about the TVA-1, handmade by Anthony Michaelson and Kevin Austin in their hi-fi shop long time ago......

.....and later on the Mentmore amp handmade by Robin! Thanks Robin for your contribution.
If you write back on the wiring issue, then I am a very good listener.

Another solid state amplifier for re-built is on the workbench right now....after this repair , some of the TVA-1 have to be rewired and a most of the resistors and capacitors shall be renewed. Pictures of the renewed driver-and bias PCB's shall be uploaded later.

If anyone got some additional information or ideas to the words here, they are most welcome.

Rgds
Kim
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Old 28th Oct 2011, 12:17 am   #33
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

TVA-1 no. 553 attached schematic....messured without rebuilt.

Details:

https://picasaweb.google.com/1031761...13571683468162
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Old 28th Oct 2011, 11:59 am   #34
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Default Re: Michaelson and Austin TVA 1 Bias Help

High voltage delay relay suggestion wanted!

....anyone who knows whether the 250Vac/10A Omron relay contacts can handle the startvoltage of 400Vac / approx. current 200 - 225mA.....with a start current limiter of 47Ohm across the contacts?

rgds.
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Old 28th Oct 2011, 8:51 pm   #35
robin coleman
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Default Re: Michaelson and Austin TVA 1 Bias Help

The Omeron relays used to be sold by RS Components. They may still have the specifications in their catalogues.
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Old 28th Oct 2011, 9:29 pm   #36
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Default Re: Michaelson and Austin TVA 1 Bias Help

The 0.47mF and the 1N5408s in the Bias circuit would cause concern for me. ANY failure here and your amp could blow. There should be a good NON fusible resistor (say 330ohm) in series with the cap for "switch on surges". This advice is based on many years of experience, some involved UL in USA.
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Old 28th Oct 2011, 9:37 pm   #37
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Default Re: Michaelson and Austin TVA 1 Bias Help

Quote:
Originally Posted by leakstereo20 View Post
High voltage delay relay suggestion wanted!

....anyone who knows whether the 250Vac/10A Omron relay contacts can handle the startvoltage of 400Vac / approx. current 200 - 225mA.....with a start current limiter of 47Ohm across the contacts?

rgds.
Just a guess, could you put two contacts in series with two 1meg resistors across them like we do with capacitors in series to equalise the voltage?
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Old 28th Oct 2011, 10:08 pm   #38
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Default Re: Michaelson and Austin TVA 1 Bias Help

How about a big chunky MOSFET across the HT line, with a fuse upstream of it obviously. Source to 0V, drain to fused HT. Gate biased so the HT would turn it on, but it is held off by the negative bias - this will always come up (or down?) before the HT, thanks to its silicon rectifier; so the MOSFET should never be conducting in normal operation. If you lose the negative bias supply, it conducts and takes out the fuse before the output transformers suffer.

Edit: Oh, just noticed HT rectifier is silicon. This may not work, then.
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Last edited by julie_m; 28th Oct 2011 at 10:10 pm. Reason: Spotted something I missed.
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Old 28th Oct 2011, 10:33 pm   #39
leakstereo20
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Default Re: Michaelson and Austin TVA 1 Bias Help

Quote:
Originally Posted by trickie_dickie View Post
The 0.47mF and the 1N5408s in the Bias circuit would cause concern for me. ANY failure here and your amp could blow. There should be a good NON fusible resistor (say 330ohm) in series with the cap for "switch on surges". This advice is based on many years of experience, some involved UL in USA.
Yes..I know you are right...its dangerous, but unfortunately only later TVA-1 models had a 100Vac supply tag on the powertransformer for the bias .
This erly bias circuit was made by M&A, and I did try this way to get the negative bias, because a full bridge rectifier taken from the 370Vac outlet tag caused even more 50-60Hz hum, than the shown bias schematic done with the 0,47uF cap.
After rewiring and change of the central ground close to the power capacitors, it's the intention to make the neg. bias through a full wave bridge from the 370Vac supply.

...but thanks for the warning...appreciate that!
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Old 28th Oct 2011, 11:08 pm   #40
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Default Re: Michaelson and Austin TVA 1 Bias Help

Two 1meg on each contacts and the contactpoints in series...I see what you mean...some sort of splitting up the AC voltage in two half values....have not thought about this. Could be a possibility.

Robin...after I mentioned the 250Vac/10A Omron relay in the erlier post, I suddenly remembered an old amplifier of mine with a large 380Vac / 10A / 19Vdc coil, relay used for speaker protection.
I suppose that this relay type can handle the startup transient, but I do not have any experience with long time effect on the relay connection terminals by high voltages. Implementation of such delay,relay circuit have to last for a long period of time...I think.
I would very much like to have the heaters warmed up in lets say...at least 10 seconds, before the high voltage is fully applied. I have the time adjustable circuit for this, but need to know the best suited relay type (most rugged). Remember that we have 47-50 Ohm in series with the 400Vac by turn on....which means that the power capacitors get some AC through the 47-50 Ohm resistors before the relay contacts bypasses the limiting 47-50 Ohm.
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