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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