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In article , wrote:

John Byrns wrote:

The power transformer I was going to use for the mono block version has a
B+ winding rated for 50 mA DC with a half wave rectifier. This easily
meets the estimated current requirement of 40 mA for a single channel, but
the stereo version will require something on the order of 80 mA. If my
math is correct, the transformer should be good for 70 mA if I use a full
wave bridge, which I planned on using in any case, but that still leaves
me 10 mA short. I would like to use this power transformer so I suppose
there are two options. Option one would be to reduce the quiescent
current of the amplifier to 35 mA per channel, running it slightly into
class AB. The second alternative would be to go with the original 40 mA
bias current and see if the transformer can take it. The 6.3 volt heater
winding will be running at only 40% of capacity, so this will help reduce
the total heat load on the transformer somewhat, possibly allowing me to
get by with an 80 mA B+ draw. I have decided to continue with this
transformer in the stereo version and see what happens.

Go easy on the input cap after the FWB. That will tend to keep the HV
secondary winding VA's under control. And the PS regulation should still
be OK since I get the sense you would prefer to run Class A. From your
description of the PT, sounds like the primary & core will be A'OK for this
application, even at your proposed 40 ma per channel.

Good point about the input cap, I had planned on using a 1000 uF input cap
as in my "Power Amplifier without Power Transformer", but I will revise
that down by an order of magnitude. I hope I don't have to add a choke to
make up for the reduced capacitance.

The main problem area is the output transformers. I had planned on using
a Hammond transformer that I already have in the mono block version. I
was unable to locate this transformer in my transformer vault, I must have
taken it out for potential use in another project, and lost track of it,
hopefully I will eventually find it. This Hammond transformer had a 10k
CT primary and a secondary with taps for 4, 8, 16 Ohms. IIRC-AIMN the
size and ratings of this transformer were similar to the current 1609 with
out the Ultra Linear taps on the primary, and without the series/parallel
secondary arrangement. The fact that the transformer differs from the
current 1609 means that it would be difficult to find a second transformer
to match it for stereo, I suppose I will have to bite the bullet and by a
new pair of 1609s, unless I stumble across something else.

Hammond does seem to offer a cheap little 10k to 4,8,16 Ohms push pull
output transformer, the 125H, but the primary inductance is only 5.6 H,
too low for my taste.

Oops, sorry, I seem to have misquoted the secondary impedance specs for
the 125H, which should read "2,4,8 Ohms", not "4,8,16 Ohms".

The 125H should be OK since the PP E182CC/7119 will not be able to drive
it at full power. I used two of it's big brother, the 125J in the second
Twin-Coupled Amp project. I got excellent results, far beyond what you would
expect from the Hammond published spec. Since you are driving the OPT with
triodes, the OPT primary inductance is much less important than when driving
with pentodes. The 1609 almost seems like overkill, but of course would give
measurably better results. Could we here the difference? I think a lot would
depend on the speaker system used & the source material!

I beg to differ with you about the importance of the primary inductance.
The 125H and 125J both have the same 5.6H primary inductance even though
the 125J is designed to operate at a primary impedance level only 25% to
40% that of the 125H, so the 125H has two strikes to start with. Then
consider that in your amplifier design the transformers are enclosed
within two feedback loops, the output stage cathode feedback, and the
overall negative feedback loop, both of which serve to flatten the
frequency response curve, if not the low frequency power delivery
capability.

In contrast my amplifier has no negative feedback around the transformer,
I figure that the combination of the E182CC and the 125H would yield a low
frequency -3dB point of 70 Hz, too high for even my taste. I suspect that
the difference between the 1609 and the 125H would be plain, even to my
tin ears. The 125H is definitely underkill!

For the phase splitting transformer, my original design called for a
transformer with a 15k primary and 135k CT secondary providing a 1.5X
voltage gain from the input to each grid. While that would be the
ultimate goal I am happy with my plan to forgo the extra gain and simply
use the secondary of a "line to push pull grids" transformer as a center
tapped choke to drive the out of phase grid, leaving the primary flapping
in the breeze.

Lloyd Peppard of Mapletree Audio Design used one of those ITs in a PP 6AS7 amp
which was sold on his web site.

One of which "ITs"?

Suggestions for a pair of small low cost output transformers are welcome,
assuming they have adequate inductance, or if someone has a transformer
that matches my existing Hammond, which they would like to sell, that
would be ideal!

If I were to do it I would go the low cost route, at least for the trial
before spending serious money on exotic, boutique parts. The performance
could be a pleasant surprise!

The Hammond 1609s are hardly "boutique parts" and aren't that much more
expensive than the 125Hs. I like the smaller size of the 125Hs, and lower
cost is always desirable, but the 125Hs appear to come with too great a
performance compromise for this amplifier. Also my 96 dB sensitivity
speakers have "16 Ohm" voice coils, the 1609s have a 16 Ohm output tap to
match, while the 125Hs do not.

Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/