[Patrick Turner]

abra cadabra.

[Engineer]

On Apr 30, 5:58 am, Patrick Turner wrote:
abra cadabra.

Patrick, I'll take this opportunity to ask you a question... if I may.
I find myself with a couple of Hammond 1650F OPT's to hand. My plan
was to build a couple of p-p EL34 amplifiers (monoblocs or same
chassis), likely next year when I retire. The OPT's are 7.6K p-p. Do
you have a preferred operating point for the o/p tubes so that this
impedance is roughly right for around 20 watts per channel? My
reference design will be the Mullard 5-20 for old times sake (I built
two 5-10's in the 1960's.) I do not have a B+ p/s designed yet (It
depends a lot on what I have in the way of power transformers lying
around), so I'm not sure how much headroom there is for cathode bias,
per the original 5-20.
Thanks, very best, and cheers,
Roger

[Patrick Turner]

Engineer wrote:

On Apr 30, 5:58 am, Patrick Turner wrote:
abra cadabra.

Patrick, I'll take this opportunity to ask you a question... if I may.

Just imagine the silent boredom of a world without questions.....


I find myself with a couple of Hammond 1650F OPT's to hand. My plan
was to build a couple of p-p EL34 amplifiers (monoblocs or same
chassis), likely next year when I retire. The OPT's are 7.6K p-p. Do
you have a preferred operating point for the o/p tubes so that this
impedance is roughly right for around 20 watts per channel?

If you only want 20 watts, then they should nearly be all class A
and cathode bias with Ea = 375V, Ia = 60mA.

My
reference design will be the Mullard 5-20 for old times sake (I built
two 5-10's in the 1960's.) I do not have a B+ p/s designed yet (It
depends a lot on what I have in the way of power transformers lying
around), so I'm not sure how much headroom there is for cathode bias,
per the original 5-20.

For class A UL, and Ea at 375V, you'l need about 30V for cathode bias,
so you need about 410V at the CT of the OPT.
The minimum unloaded ac voltage needed for this is about 320Vrms,
and silicon diodes will give a working 432V at the output of a cap input
filter.
If a tube rectifier is used the acV needs to be much higher,
probably about 360Vac, but anything up to 400V is
easily used and a series resistor can limit the B+
which won't need regulation because class A circuits
have little B+ current input variation with music signals.

The Mullard 520 was a poor man's hi-fi solution.
EF86 and 12AX7 belong in preamps.
The Mullard suited low source input signals of the 1960s,
but now we have cd signals far higher so
"reference" what you build to ancient history of 1960?
I would never just do what i did back then, or wanted to do,
but couldn't. I'd aim a bit higher.

With very little extra effort, you could build the power amp section
of the integrated amp shown at

http://www.turneraudio.com.au/5050Integrated.html

Scroll down 25% to the schematic.

And all of what i said above about biasing can be addopted
with KT88 if you wish; these tubes will do slightly better than EL34,
and if switched to triode will perform to give more triode power than
EL34.

I recommend the paralleled twin triodes for each half of the LTP
it makes the amp more dynamic and "alive" sounding.

If you have not got enough HT voltage on a PAIR of trannies lying around
for monoblocs, maybe you have to buy trannies, or if you have one
big one then consider the stereo two channel chassis, or one PS chassis
feeding
the two audio channels in a second chassis.

I have a couple of ancient trannies with quite high HT meant probably
for
some amateur radio transmitter; I bought them from a ham who retired to
a coastal town. Maybe I wind a choke and make a choke input filter one
day.

But one day will never come, there are too many customers
hammering at my door.

Patrick Turner.








Thanks, very best, and cheers,
Roger

[Engineer]

On Apr 30, 11:06 pm, Patrick Turner wrote:
Engineer wrote:

On Apr 30, 5:58 am, Patrick Turner wrote:
abra cadabra.

Patrick, I'll take this opportunity to ask you a question... if I may.

Just imagine the silent boredom of a world without questions.....

(snip)

But one day will never come, there are too many customers
hammering at my door.

Patrick Turner.



Thanks, very best, and cheers,
Roger- Hide quoted text -

- Show quoted text -

Patrick, many thanks. I've saved all this to ripen for a while...
Will likely build in 2008.
Cheers,
Roger