Michaelson and Austin TVA 1 Bias Help

[leakstereo20]

A mosfet power transistor; yes I thought about this with slow AC start and a triac. A triac in series with the AC supply to a hifi-amp...no!

I might be considered as being old fashioned, but I like relay mechanics .

[leakstereo20]

Something like this circuit found on the TVA-1 pictures from Japan.

High volt delay relay for valveamplifiers. May be.. this circuit could be used on the primary windings (powertransformer)as softstart for solid state amplifiers.

The circuit is tested with 6,3Vac supply and works on a Vero-board.

The circuit testet with C1, 10uF and without R2, 47R. The AC applied from the heater supply.

The circuit includes a voltage doubler which is:

6,3Vac x 1,41 x 2 = 17,76V - minus the loss from the two rectifiers ( 1,4V approx.) = 16,36Vdc
The R2 placed to reduce voltage to 12Vdc for the relay.

By use of C1, 10uF the maximum delay adjusted on P1 will be approx. 20 seconds.
Larger C1 capacitor increases delaytime.
The PCB layout could be reduced in lenght if the 5 pcs. 10R/7W resistors is removed and exchanged with a 50 Watt chassis aluminum resistor.(Welwyn or similar).

Details:

https://picasaweb.google.com/1031761...27848470641250

Still wondering if the relay contacts can handle the startup transient from the secondary AC windings of 400Vac?

PCB component view.

[leakstereo20]

Originally Posted by trickie_dickie

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.

I like to keep electronics as close to the original layout if possible, and I have never seen the advice with the 330R resistor. Did you mean in series to the bias cap to reduce current flow? ...before the 0,47uF?

rgds.

[trickie_dickie]

Its standard practice in many circuits used by manufacturers where a low voltage DC supply is needed and the space/weight/cost of a transformer isn't an option. Say you wanted a 30v supply from 240v mains. The peak voltage is 340v, if you switch on the supply at this point, you'll get a huge switch on spike of current through the capacitor with major consequences for any following components. Putting a 330R wirewound 2.5w resistor in series with the capacitor will limit this spike to a maximum of 1 amp. Another point is the capacitor should be an X2 type (UL approved) for added safety. Hope this helps.

[leakstereo20]

Yes ...I got your point concerning the voltage spike and the resistor.

Before I received your answer, my engineer friend looked at the circuit and said that a 1000v polypropylene 0,47uF would be sufficiant in this position for AC outlet.
The "C-pump" ,which he called the circuit, shall only deliver a few mA, and if the intention is the mentioned high voltage delay by a relay switch after the ac-outlet for the negative bias, he found the circuit quite safe.

I still have in mind to use the rectifier bridge for the neg. bias. My friend don't have any long time experience at 400Vac as far as relay contacts is concerned, so I might try with some experiments before any delay/relay installation.

Kim

[leakstereo20]

Another friend and valve entusiast has received the bias question about a safe way to achieve approx. -60Vdc to -80Vdc from the high volt 400Vac vinding.

In the meantime before any answers according to the bias issue, the attached Quad 6L6 PPP schematic do have one suggestion for generation of the negative bias . ( someone told me that this is the Quad 6L6 PPP amplifier?)

Notice the 15K/3W resistor connected to the 350Vac wire, and after the resistor a simple 1N4007 rectifier followed by a capacitor.
Looking at Dynaco Mark III schematic the negative bias current draw stays in the region of approx. 20mA for a pair of 6550 or KT88.

By approx. 20mA of current consumption is a "selfhealing" 0,47uF/1000V polypropylene capacitor safe enough with a resistor as currentlimiter before the poly-cap?

[leakstereo20]

Informed by a member. The above 6L6GC valveamp is only mentioned as "quad" because of four outputvalves.

[leakstereo20]

To hi-fi entusiasts with interest in Michaelson & Austin valve amplifiers.

Today some additional TVA-1 information arrived from a friend with years of valve experience and valve circuit knowledge.

The text has partly been translated by Google, so please accept any language faults that appears.

"Hi Kim!
Here are some comments! See in red in your text!

Hi George ... Long time ago ... too much work!

Will you please look at my drawing of the TVA-1 I have mentioned.

It's about the TVA-1 amplifier with the actual resistor values and capacitors soldered in.
Looking at the power supply, you can find a 0.47 uF/600Vdc capacitor connected to one 400VAC outlet wire on transformer. The other end of the 0.47 uF cap connected to the midpoint of the two 1N5408 diodes forming a part of the supply to the negative bias ....- 276V after the 10K resistor.

Answer:
This is quite ok.! Use a good quality capacitor, if any. set of two small parallel or triple, so this does not hurt the amplifier so much if one capacitor fails (interrupts). If one short ends so you get just more negative voltage (no voltage drop across the capacitor if shorted). Your diodes provide as full a negative unidirectional transformervoltage. The amplifier then just closes down completely and if your resistors in the bias circuit is dimensioned for the current ,that will be all right .
I think they are, because through entire bias circuit is approx. 72k Ohm load to be carrying it around. = 450V at least in the short term. E = IxR try to count for and check. 450 = I x 72,000 = 6.25 mA = ca.3 Watt

My questions is: If the 0.47 uF shorted or burned over, will the output valves so just "run with the bias" and burn the two 100R/5watt cathode resistors ? Answer:

ONLY IF THEY ARE INTERRUPTED by the 0.47 MIF!
Question:
There is a 100Vac outlet on the early TVA-1 model no.553. Only later (the model No. 1700-1800 and up) amplifier came with a 100Vac outlet to the bias circuit. There is no mid point outlet at 400VAC winding.
Is there a safer way to create a negative voltage ... example. through the 370Vac outlet you can see there on the trafo.
Can you throttle the voltage down by a diode bridge from the approx.: 370Vac x 1.41 (rod2) = 521Vdc? Proposals received??

Answer:
You can create a voltage divider from the transformer outlet with some dimensioned resistors. In the biascircuit there are a total resistance, and overall resistance to ground. This will load your bias voltage circuit harder, so this voltage divider must have some ”stiffness”.
Try with 50K / 10W in series with 400V ac outlet, followed by 50K / 10W to ground. Then you have 200V ac and ca. 2 Watt to your bias.
Check it with tension and temperature of the resistors (I know that you do not like heat/hot resistors). You can easily use your 370V ac gain you get with above 370 / 2 = 185V ac x1, 41 = 261V = You can easily play with the voltage dividers. I have for reasons of convenience made the choice of half and half resistors.

Question:
I have tried with a delay circuit. The delay circuit works, but can a 250Vac relay with 50 ohms connected to the contact points clearly connect the 400VAC after the glow has been on between 10 to 20 seconds for warm up? see attached diagram.
Answer:
It may, since the voltage drop across the 50 Ohm is not so dramatically after 20 sec. It will probably be very close to The specifications………if you run the valves with a separate non delayed bias windings / voltages so the tubes are closed / muted in their anodevoltage until they get full working voltage.

Hope it goes well with you. Everything about the same old stuff here. Pensioners were always busy. But just give me a call when you get time for it .MvhJørgen

Regards. Kim"

He's one of the "good guy's" !

If we have further questions to the TVA-1 amplifier, he might be able to give us the answers.

Rgds.

Kim

[leakstereo20]

Originally Posted by robin coleman

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 Robin, I would like to "hold you to your word" ...and ask you one more question.

Do you have any knowledge about the size of the main powertransformers used in the TVA-1/Mentmore amplifiers?

The erly model of mine has the "small" transformer type....later TVA-1 models is equiped with a powertransformer about the same size as the large outputtransformers from Partridge.
In my diy valveamps I normally use about 500mA for a stereo version and high voltage...in addition approx. 1,8A times 4 for the OP-valve heaters plus 0,3A times 4 for the driver valve heaters ...in total 9,6A for all the heaters.

I have tried to calculate on the "small" powertransformer in my TVA-1 and it seems to be a bit undersized as far as wattage is concerned. I assume that the transformer should have been some few amperes stronger for the glow?

Do you know something about this issue? Is it possible to buy a reissue powertransformer for my TVA-1 in the UK?

Many regards
Kim

[leakstereo20]

TVA-1 negative - Ug1 supply taken from highvoltage + Ug.

A safe and solid - Ug1 supply to be used for older TVA-1 valveamplifiers where the powertransformer don't have a 100Vac output tag.

The low impedance supply by mr. J. G. need to be changed for the first two 33K resistors because of the shown example of 240Vac input. The TVA-1 runs with approximately 400-440Vac input.

Take care of the dangerous high voltages.

rgds.

[Jac]

Doesn't the 1.5u-250V cap short circuit the signal to ground?

(didn't read the entire thread, but just noticed this)

Jac

[leakstereo20]

Hi,
It's not a capacitor for signals. This is a power supply for the negative bias, and the 1,5uF capacitor decouples to ground after the multiturn resistor and removes noise generated by the resistor.

rgds.

[Jac]

I still have the meaning that it leads the audio signal (coming out of the "signal cap" in the LH bottom of your diagram) to gnd. It (de)couples the hum in the bias supply as well as the audio signal to ground.

You need an extra resistor between your circuit ("0-150V") and the connection of "signal cap"/12k resistor.

Convince me! or better: build it and let's see what happens. The proof of the pudding is in the eating.

Best regards,
Jac

[leakstereo20]

Sorry Jac,
I was sure that the drawing was ok and did not look close enough, but I did not see that the resistor in the schematic between the signal capacitor and the 12K grid resistor was missing.

You are quite right! I apologize for the mistake.

Best regards
Kim

[Jac]

Hi Kim,

No apology required

I only once listened to the TVA-1 very briefly and was impressed by the sound. It must have been close to 30 years ago!
It was at the address of someone I went to to buy his Stax SR-X mk3 headphone.

Best regards,
Jac

[leakstereo20]

Hi Jac,

I did about the same as you, listened to the fabulous beauty from M&A some 35 years ago without being able the buy the amplifier at that time. The sound from the TVA-1 still impresses me after all these years, in spite all the other equipment I have built or other factory amplifiers I have bought.

The TVA-1 in top condition with matched valves beat's many modern constructions as far as music reproduction is concerned.

The circuit made by TDP and the Partridge OP's is a good combination with an amazing soundstage and great musicality in my opinion.

I do not know the Stax' that much, but my friend owns a pair of Stax headphones, and I have lately heard the Cream reunion concert from 2005 in these headphones, and the sound was very convincing, like " I am there " sound.
Hopefully you still have them.

Best regards
Kim

[Jac]

Hi Kim,

We are getting a bit off topic, but yes: I still have that Stax headphone.
What is "worse", I collected several other Stax headphones ("earspeakers" in Stax-talk) to them....

They all have an effortless relaxed high definition sound in common.
The SR-X mk3 has always sounded very neutral to me, and I still have that opinion. Bass could be better though. And that certainly has been realized in later models.
Couldn't live without them (and my ESL63s (and my wife )).

Never owned a TVA1 (or TVA10) though. Would be nice to have one....
Well.... someday...

Jac

[Centrino]

Hi Kim,

I can see you have done a big effort to restore your TVA-1. I have one with a slightly smaller power transformer. Today I'm only using it from 500Hz and above.

But before that I did some modifications. Actually my unit was the one with a very big "mother board" also covering the sockets for kt88 (as far as I remember) bought in '85 as used.

But anyway here is a link to my mod's ;-)

http://www.hifi4all.dk/forum/forum_p...ID=29770&PN=23

[leakstereo20]

Originally Posted by robin coleman

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.

Hi Robin Coleman,

I hope that you are still around here in "Vintage Audio Repair, UK"?

I have a question you might be able to answer:
Did the TVA-1 designer, Tim De Paravicini, actually work together with Kevin Austin and Anthony Michaelson in 140 New Bond street, London, UK, building the TVA-1?....or did Tim De Paravicini design the TVA-1 amplifier "only", and then passed the amplifier construction and cable work to Kevin and Anthony and thus to you?

The question came up because some people claims, that there were never a " standard setup" for hookup wiring and the production procedure of the TVA-1, and that "the missing standard" was the reason why one always could detect audible 50-60 Hertz hum from the TVA-1.

My argument against the above criticism was, that I could imagine, that a amplifier designer wants his first valve amp on the market to be as silent and well sounding as possible, and that a building/ construction standard was the only way to improve and ensure a uniform product.

Could you tell me what's right, and help me on this one?

Hope its ok to go off topic....

Regards Kim

[AC/HL]

Depends on what you mean by off topic. Please confine the discussion to this amplifier or start a new thread.