Iain Churches wrote:
>
> "Patrick Turner" <info@[EMAIL PROTECTED]
> wrote in message
> news:481D792C.577AAEA3@[EMAIL PROTECTED]
> > Today I had a few friends around to share some musical moments.
> >
> > The 845 SET amps i have running were fine.
> >
> > We tried all sort of music and up to loud levels.
> > My voltemeter told me we rarely exceeded a couple of peak volts
output.
> >
>
> Patrick. A couple of questions.
> What tube did you choose for the front end.
> What is the voltage of the B+ rail?
>
> Regards
> Iain
Sorry if I'm late replying, but I couldn't download r.a.t since
saturday.
The rails are set as follows...
B+ at C1 in CLRCRC filter = +640V-.
B- at C1 in CLRCRC filter = -600V approx, depending on mains
fluctuations.
Ea for the 845 = +1,060Vdc, and Ek across each 2.15k Rk of each parallel
845 = 145V
for approx Ia = 70mA.
The +ve rail feeding the OPT has very low hum of less than 6 mV,
and feeds 36mA through 3 parallel EL84 in triode through a 60H choke and
series 7k resistance.
This LR load is at least 7k at extremes of F due to Csh and Lp.
But at 700Hz the LR load > 500k or like a constant current source.
The parallel anodes of EL84 power 845 grids through 3 series 2.2uF,
each with 220k to evenly divide the Vdc between B- and Ea of the '84,
ie, from -585V to +300V.
Although the coupling caps are rated for 1,000 vdc at least,
I ain't taking any chances.
Rg for the pair of 845 grids = 23,5k.
Total load on the '84 = 20k, and VO max is a possible 166Vrms,
but only 127Vrms max is needed, and HD of the driver is 1.4% 2H, which
entirely cancels
output tube 2H.
Input tube is a paralleled 6CG7, with Ea = 155Vdc, and dc load = CCS
with MJE350 for 6mA.
It has to make a maximum of about 9Vrms, when HD < 0.4%
The output tubes are thus set up with a "split" rail.
This avoids having 1,200 volts across the insulation on the OPT.
There are two equal HT windings with many taps to adjust B+ and B-.
Voltage doublers rectify using very normal 1N5408 Si diodes.
There are 14 x 470uF x 400V rated electros used in the B+/B- filters
and for the two cathode bypass caps.
The chokes on the B+ and B- rails = 4H at 150mA.
There are also series 100 ohm R, to act as damping resistors
and filter elements to reduce the Q of the PS resonance at approx 5Hz
which was a problem with the two rails.
At very low F the cathodes are not grounded well at all
and only held stady at AF by the 470uF, but that's
enough C, because 470uF = 34 ohms at 10Hz,
and good enough.
The series coupling caps feeding 845 grids act as a filter
of 0.71uF and 23.5k giving a low F pole.
Very low F at the B- are allowed to appear at the 845 grids
so Vg = Vk at LF, so slight variations of rail voltages
don't produce slow slight variations in tube current and hence
LF noise below 7Hz is less than 1 mV at the output sec of the OPT.
All that took some careful work to get right.
In hindsight, its easier to have one rail of +1,200V.
Easier to use a choke feed to 845 anodes, I suggest
a core of 51 tongue, 70 stack, and 5,400 turns of 0.45mm wire for the
150mAdc.
Gapping material about 0.3mm right across the core.
Cap drive could be 3 x 450V rated 20 uF polypropylene
motor start caps in series with 220k across each to
allow only about 1.5mA dc flow to equalise Vdc cap voltages.
Air gap on choke careful adjustment.
The OPT may then be any suitable 70 watt PP 6k:4,8,&16.
Maybe something from Hammond might even do OK.
The driver stage with choke plus R works well.
My OPT has 2,700 turns 0.45 wire for P winding in 4 sections
of 5 layers each od 135 turns.
Secs are 5 sections in 5 layers each with 72 turns of 0.9mm wire.
The interleaving is SPSPSPSPS, or 5S x 4P.
The last windings on is the last S which is divided into 4 x 18t
windings.
Insulation between P layers = 0.05mm, and between P&S layers = 0.7mm
So you can have S = 5 x 72, all parallel for S = 72t, so this is near 6k
: 4 ohms.
or 4 x ( 72t + 18t ) in parallel for S = 90t.
The PSU and OPT is designed to allow paralleling of P sections
and paralleling of PSU to give 1k5:4 ohms and suit a quad of parallel
SEUL KT90
for about the same PO as the 2 x 845.
Alternatively, a pair of 13e1 may be used and the secondary voltage
used for cathode FB.
The choke feed input may not allow such design flexibilty in the event
that 845 are no longer avalailable.
DC is applied to both the 845 cathodes using 14V ac windings and diode
bridges on HS
feeding 21mH chokes with 0.3 ohms dc resistance and with C = 2 x 15,000
uF caps at 25V rating.
Hum is thus 24mV, and each side of the cathode has 22 ohms to a point
taken to the
bypass C and Rk so that even this low amount of hum is balanced and does
not
modulate the Ia.
Noise at the outputs with 7dB maximum global NFB < 0.3mV.
Rout with 6k:4 ohms and 7dB NFB = 0.35 ohms
BW at full PO = 20Hz to 40kHz approx.
At low levels used for all listening, BW is 5Hz to 55kHz.
All iron wound components on the audio chassis are potted
except the 20mH solenoid core choke filter in a CLC filter for the 12.6V
x 1.6Adc suply
to both input tubes abd 3 x driver tubes.
PT are potted on separate PS chassis.
PT have 51T x 70S GOSS cores and T rise is less than 10C after 4 hrs.
I have metal work to complete and active protection against
bias failure of the output tubes.
At each 845 anode, I do have 0.5A slo blow fuses.
The 845 can conduct a huge current if it is turned on hard.
Patrick Turner.
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