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Mystery OPT



 
 
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  #1  
Old August 5th 05, 05:40 PM
Don Pearce
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Default Mystery OPT

On Fri, 05 Aug 2005 11:15:17 -0500, flipper > wrote:

>I just received a pair of SE OPTs that were claimed to be from a
>stereo console with 6BQ5 outputs. So far so good as that's precisely
>what I wanted to build.
>
>Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
>ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
>from what little I know of 6BQ5 characteristics that doesn't make any
>sense to me. I mean, 8K would be good but a console with 2 ohm
>speakers?
>
>Physically, the core is gapped with (actual core) dimensions of 2.25
>inch wide by 1.74 inch high and .75 thick. Mounting is 2.75 centers
>with 10A15-10 marked on the frame. Primary and secondary, 4 wires
>total. No taps.
>
>Does anyone have information, or an educated guess, on what these
>transformers may really be or what I'm missing about 6BQ5s with such a
>high OPT impendence?


These transformers sound an awful long way from optimum for a 6BQ5. I
think you were fed some duff gen.

d

Pearce Consulting
http://www.pearce.uk.com
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  #2  
Old August 5th 05, 07:45 PM
Eike Lantzsch, ZP6CGE
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flipper wrote:
> On Fri, 05 Aug 2005 16:40:30 GMT, (Don Pearce)
> wrote:
>
> >On Fri, 05 Aug 2005 11:15:17 -0500, flipper > wrote:
> >
> >>I just received a pair of SE OPTs that were claimed to be from a
> >>stereo console with 6BQ5 outputs. So far so good as that's precisely
> >>what I wanted to build.
> >>
> >>Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
> >>ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
> >>from what little I know of 6BQ5 characteristics that doesn't make any
> >>sense to me. I mean, 8K would be good but a console with 2 ohm
> >>speakers?
> >>
> >>Physically, the core is gapped with (actual core) dimensions of 2.25
> >>inch wide by 1.74 inch high and .75 thick. Mounting is 2.75 centers
> >>with 10A15-10 marked on the frame. Primary and secondary, 4 wires
> >>total. No taps.
> >>
> >>Does anyone have information, or an educated guess, on what these
> >>transformers may really be or what I'm missing about 6BQ5s with such a
> >>high OPT impendence?

> >
> >These transformers sound an awful long way from optimum for a 6BQ5. I
> >think you were fed some duff gen.

>
> That was my first impression too but I wondered if maybe there's some
> 'secret' I'm unaware of.
>
> Is there some other tube these things would be a fit for?
>

ECL80 Ra~17k, B+=250V, but it only produces 1.4W
It can be had cheap however.

Kind regards, Eike
  #3  
Old August 5th 05, 09:29 PM
Eike Lantzsch, ZP6CGE
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flipper wrote:
> On Fri, 5 Aug 2005 14:45:06 -0400, "Eike Lantzsch, ZP6CGE"
> > wrote:
>
> >flipper wrote:
> >> On Fri, 05 Aug 2005 16:40:30 GMT, (Don Pearce)
> >> wrote:
> >>
> >> >On Fri, 05 Aug 2005 11:15:17 -0500, flipper > wrote:
> >> >
> >> >>I just received a pair of SE OPTs that were claimed to be from a
> >> >>stereo console with 6BQ5 outputs. So far so good as that's precisely
> >> >>what I wanted to build.
> >> >>
> >> >>Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
> >> >>ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
> >> >>from what little I know of 6BQ5 characteristics that doesn't make any
> >> >>sense to me. I mean, 8K would be good but a console with 2 ohm
> >> >>speakers?
> >> >>
> >> >>Physically, the core is gapped with (actual core) dimensions of 2.25
> >> >>inch wide by 1.74 inch high and .75 thick. Mounting is 2.75 centers
> >> >>with 10A15-10 marked on the frame. Primary and secondary, 4 wires
> >> >>total. No taps.
> >> >>
> >> >>Does anyone have information, or an educated guess, on what these
> >> >>transformers may really be or what I'm missing about 6BQ5s with such a
> >> >>high OPT impendence?
> >> >
> >> >These transformers sound an awful long way from optimum for a 6BQ5. I
> >> >think you were fed some duff gen.
> >>
> >> That was my first impression too but I wondered if maybe there's some
> >> 'secret' I'm unaware of.
> >>
> >> Is there some other tube these things would be a fit for?
> >>

> >ECL80 Ra~17k, B+=250V, but it only produces 1.4W
> >It can be had cheap however.
> >
> >Kind regards, Eike

>
> Thanks. I made a note of it.
>
> I'm wondering, wouldn't the 6BQ5 do about the same thing with the same
> Ra and B+?
>
> I'm also wondering if I could run the 6BQ5 with a much higher B+ and
> correspondingly lower Ia through that transformer. Say 550 and 17ma.
>
> (Actually 5V6 or 12AQ5 is what I had in mind as I can get those for 2
> bucks and I already have the sockets)


Well, here we have to draw a line! - a loadline that is ;-)
AFAIR 6BQ5 can be used with far higher Ra and up to 400V. Less Pout
less THD. That is left as an exercise for the reader. At least until
I have time to draw the line.
If it *sounds* good - I can't tell.

Kind regards, Eike
--
Lions go to absurd lengths to retain the posession of their skins.
- Stewart Edward White 1913
  #4  
Old August 6th 05, 03:14 AM
Mark S
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Default

Hi Flipper,
Didn't the old console stuff have speakers with lower impedence VC's? As I
recall, 3.2 was somewhat of a standard..maybe this console was that or
lower.
Mark S

"flipper" > wrote in message
...
>I just received a pair of SE OPTs that were claimed to be from a
> stereo console with 6BQ5 outputs. So far so good as that's precisely
> what I wanted to build.
>
> Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
> ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
> from what little I know of 6BQ5 characteristics that doesn't make any
> sense to me. I mean, 8K would be good but a console with 2 ohm
> speakers?
>
> Physically, the core is gapped with (actual core) dimensions of 2.25
> inch wide by 1.74 inch high and .75 thick. Mounting is 2.75 centers
> with 10A15-10 marked on the frame. Primary and secondary, 4 wires
> total. No taps.
>
> Does anyone have information, or an educated guess, on what these
> transformers may really be or what I'm missing about 6BQ5s with such a
> high OPT impendence?



  #5  
Old August 6th 05, 05:03 AM
[email protected]
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Default

The model # & make of the console wld solve the info dilemma quickly,
as most schematics are freely available.

6BQ5 has also been used with rather high load Z's for lower-fi
applications at lower pwr, like radios. And a consumer console might
run 2x4z spkrs in parallel.

If someone gave or sold you 2 carbs from an unknown Italian car, would
you expect 'em to fit your Alfa? ;-)

  #6  
Old August 6th 05, 05:17 AM
Phil Allison
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"flipper"
>I just received a pair of SE OPTs that were claimed to be from a
> stereo console with 6BQ5 outputs. So far so good as that's precisely
> what I wanted to build.
>
> Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
> ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
> from what little I know of 6BQ5 characteristics that doesn't make any
> sense to me. I mean, 8K would be good but a console with 2 ohm
> speakers?
>



** In the 50s and early 60s, 2 ohm speakers were not uncommon in tube radios
and budget stereos.

Also, there may have been 2 speakers wired in parallel.

Fender made a number of tube guitar amp models in the 1960s that were
matched to a 2 ohms load.



........... Phil





  #7  
Old August 6th 05, 03:57 PM
Fabio Berutti
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Default

A quick solution is to have it connected to a couple of 4Ohms LS in
parallel, a more involved one is to use it as it is, a 16K load, at a high
B+ (up to 500V using Russian 6P14P or eq.) and max. 300V G2 in pentode
connection, with a robust feedback, to get some 20W out of a measle couple
of EL84s, if the transformer bears'em...

Ciao

Fabio


"flipper" > ha scritto nel messaggio
...
>I just received a pair of SE OPTs that were claimed to be from a
> stereo console with 6BQ5 outputs. So far so good as that's precisely
> what I wanted to build.
>
> Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
> ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
> from what little I know of 6BQ5 characteristics that doesn't make any
> sense to me. I mean, 8K would be good but a console with 2 ohm
> speakers?
>
> Physically, the core is gapped with (actual core) dimensions of 2.25
> inch wide by 1.74 inch high and .75 thick. Mounting is 2.75 centers
> with 10A15-10 marked on the frame. Primary and secondary, 4 wires
> total. No taps.
>
> Does anyone have information, or an educated guess, on what these
> transformers may really be or what I'm missing about 6BQ5s with such a
> high OPT impendence?



  #8  
Old August 6th 05, 05:44 PM
BFoelsch
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Posts: n/a
Default


"flipper" > wrote in message
...
>I just received a pair of SE OPTs that were claimed to be from a
> stereo console with 6BQ5 outputs. So far so good as that's precisely
> what I wanted to build.
>
> Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
> ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
> from what little I know of 6BQ5 characteristics that doesn't make any
> sense to me. I mean, 8K would be good but a console with 2 ohm
> speakers?
>
> Physically, the core is gapped with (actual core) dimensions of 2.25
> inch wide by 1.74 inch high and .75 thick. Mounting is 2.75 centers
> with 10A15-10 marked on the frame. Primary and secondary, 4 wires
> total. No taps.
>
> Does anyone have information, or an educated guess, on what these
> transformers may really be or what I'm missing about 6BQ5s with such a
> high OPT impendence?


Hmmm.

Well, if these are really rotten output transformers, which are down 3 dB at
60 Hz, your calculations would be way off, no?

Measure them again at 400 or 1000 Hz. Connect them backwards to the output
of a small amp fed with1 kHz and report back. I'll bet the calculated ratio
is different at 1 kHz than it is a 60 Hz.

Also advise your test equipment. Many old VTVMs and VOMs are horribly
inaccurate on the low VAC ranges. They may also be bad at frequencies other
than 60 Hz.


  #9  
Old August 7th 05, 03:50 AM
Phil Allison
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"BFoelsch"
>
> flipper
>
>> Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
>> ratio of 63.5/1,

>
> Well, if these are really rotten output transformers, which are down 3 dB
> at 60 Hz, your calculations would be way off, no?



** No.

Voltage drive to a transformer will overcome the usual low frequency 3dB
down point experienced with drive via a tube or other high source impedance.

120 volts at 60 Hz should be within core saturation limits of the tranny the
OP has described - a 6BQ5 would likely supply it with more AC volts.



> Measure them again at 400 or 1000 Hz.



** No need and no point at all.

The OP has the right numbers now.


> Connect them backwards to the output of a small amp fed with1 kHz and
> report back. I'll bet the calculated ratio is different at 1 kHz than it
> is a 60 Hz.



** You will lose your dough.


> Also advise your test equipment. Many old VTVMs and VOMs are horribly
> inaccurate on the low VAC ranges. They may also be bad at frequencies
> other than 60 Hz.



** How asinine.

1.9 volts AC is NOT a low voltage .

The OP has a digital meter.

He used 60 Hz for his test.



............. Phil







  #10  
Old August 7th 05, 03:02 PM
Patrick Turner
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Default



Phil Allison wrote:

> "flipper"
> >I just received a pair of SE OPTs that were claimed to be from a
> > stereo console with 6BQ5 outputs. So far so good as that's precisely
> > what I wanted to build.
> >
> > Problem is, when I put 120.7 AC on the primary I get 1.9VAC out for a
> > ratio of 63.5/1, which comes to roughly 32k/8 ohms, or 16k/4 ohms, and
> > from what little I know of 6BQ5 characteristics that doesn't make any
> > sense to me. I mean, 8K would be good but a console with 2 ohm
> > speakers?
> >

>
> ** In the 50s and early 60s, 2 ohm speakers were not uncommon in tube radios
> and budget stereos.
>
> Also, there may have been 2 speakers wired in parallel.
>
> Fender made a number of tube guitar amp models in the 1960s that were
> matched to a 2 ohms load.
>
> .......... Phil


That's quite correct Phil.
Many early radios and grams had 2ohms speakers,
so the ratio could well be 8k:2.

Now the range of voltages for B+ for a 6BQ5 to operate
in pentode is about 250V to 450V, and for above 350V,
the screen voltage should be kept at no more than 350V.

But let's suppose we had Ea = 350V.
To get the max class A power we must also have the max reliable amount of idle
dissipation for the tube
which would be 12 watts so if Ea = 350V, Ia = 12 / 350 = 34 mA.

Now we could expect a max plate efficiency of say about 40%, and so the Ia swing
will be about
+/- 30mA peak, and Ea swing approximately +/- 320V peak,
and ohm's law tells nme the RL which would accept what the tube could do
is 320 / 0.03 = 10,666 ohms, and the power output
is Vrms squared / RL = 226 x 226 / 10,666 = 4.78 watts which is 39.9% of 12
watts of dissipation,
so we know what to expect when we test the OPT in the circuit.
if the Z ratio of the OPT is 8k : 2, then that's also 10.66k : 2.7 ohms.
So tests using Ea = 350V should have 2.7 ohms as the sec load.
Remember Ea isn't the B+. Ea is anode to cathode voltage potential
excluding the cathode bias of about 7V and the voltage across the OPT primary.

BUT, and there always is a at least one but, the po at the speaker secondary
into 2.7 ohms will **never** be 4.8 watts since these crummy little OPTs used in
radios and grams typically
had winding losses up to say 25%, but let's assume its 20% like many are
and this means the tube sees the load of 10.66k **plus** the winding losses
of 20% of 10.66k which is more, or about 2.13k + 10.66k, = 12.8k.

So therefore we could afford to drop the secondary RL by 20% from 2.7ohms to
2.16 ohms and the primary load will be Z ratio x 2.16 = 4,000 x 2.16 = 8.64k.
But plus the 20% losses the tube sees a load of 8.64k plus 20% of 8.64 =
10.368k,
which is close to what we calculated would be a good load for Ea = 350V

PO will simply be 4.8W x 80% = 3.84 watts.

Total HD will be about 13% of 2,3,4,5,6,7,8,9,10H at 3.8 watts.
Bloody awful, and not very good even at 1 watt.
But if 20dB of global NFB is used the thd will be acceptable at a watt or two


If the Ea is reduced to say 300V, then the RL must also be reduced to suit the
Ea.

But in practice the maximum po is only for the theoretical load on the tube,
and when RL = twice or half the ideal RL, the pentode po drops away alarmingly,
and the useful po is only a maximum of around 1.5 watts for the range of RL.

This was found to be fine when most speakers in old radios and grams were about
95dB/W/M and there was little bass below 200Hz.
People sat around their radios, and were still able to talk to each other.

To measure the actual turns ratio and then find the impedance ratio
by squaring the TR the transformer must be tested without a load
and with any F between 60Hz and 2 kHz, which any DVM will read quite accurately.

Using a variac with the mains is ok and even the full US mains voltage
of 120V across the primary is OK.
The mains is a low impedance source, ie, the secondary load connected won't
cause a change to the mains input voltage.
The winding resistance of the tranny can be found by measuring the secondary
voltage with no load,
and then measuring the sec with 2ohms, and recording the voltage change between
unloaded and loaded.
The effective or total winding resistance as measured at the secondary
= difference in loaded/unloaded sec voltage / sec loaded current, and the units
are ohms.
This value of ohms should be about 20% of the secondary load .

But as RL is raised, the measurement and calculation of the WR will give the
same WR,
but the WR / RL ratio becomes more favourable, ie, the % losses are lower.
The word "losses" mean that 20% of the power output is dissipated as heat in the

OPT windings, in both sec and pri windings.


The higher the RL, the higher the F at which the OPT will saturate,
since saturation is a voltage related phenomena.
So high RLs with a given tranny mean poorer bass performance.

When connected to the 40kohms of plate resistance which is typical of 6BQ5,
the primary inductance will probably have an impedance at 60Hz which
will reduce the value of the load seen by the tube.
If RL = 10k, and Lp = 12H, a typical value, the Ra in parallel
with RL will be 40k //10k = 8kohms.
The LF pole will be at 8,000 / ( 6.28 x 12 ) = 106Hz.

At 1 khz, the Lp has an impedance which is so much higher than RL that
the tube sees an RL = 10k.

So using a tube as the device for tests will reveal different test voltages at
60Hz and 1 kHz,
when a load is connected, and its a lot harder to calculate what's happening
because the winding resistance also is involved.

Load line analysis will also allow calculation of the load that is ideal
for the 6BQ5 and its tranny.

Patrick Turner.




 




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