[g0lgul]

Hi all,
have picked up a pair of bell howell projector amps a while back and
have been working on rebuilding them as mono amps. Have given the circuit
listed on J Byrns site a go. http://fmamradios.com/pics/ST-25L6.gif The
only modifications I have made to the circuit was adding some r in the
heater string to alow for mono operation and am using a 6sl7 instead of
the 12sn7 as designed. the only reason for that being that was what came
with the amps.
One of the amps is complete and working!

Am currently wanting to finish the second amp but have not been able to
find a match to the input transformer used. It is a chicago cic - 1.
These are 600/150 ct primary and 100k ct on the secondary. The trouble is
after looking for several months a second one has not turned up....

Have been looking at the utc transformers for a while and think the a12
or ls 12 series would perform the best... these are a bit out of my
budget tho. The question is there is a pair of a11 that are aforadable
but are 50k ct would this be too low? The transformers used on the
website are a10's these are 50k split on the secondary, not really sure
if this means 25k + 25k for a total of 50k or if it is 50k in 2 sections
for a totak of 100k. Is there anyone here famaliar with these
transformers?

My other idea after looking at Patrick Turners site was to canabolize
some of these 120 to 12v transformers that are overflowing their bin and
atempting to wind one. Not really sure about winding an input transformer
tho without some sort of metal can for shielding... ideas???

thank you,
Gerald

[maxhifi]

"g0lgul" wrote in message
...
Hi all,
have picked up a pair of bell howell projector amps a while back and
have been working on rebuilding them as mono amps. Have given the circuit
listed on J Byrns site a go. http://fmamradios.com/pics/ST-25L6.gif The
only modifications I have made to the circuit was adding some r in the
heater string to alow for mono operation and am using a 6sl7 instead of
the 12sn7 as designed. the only reason for that being that was what came
with the amps.
One of the amps is complete and working!

Am currently wanting to finish the second amp but have not been able to
find a match to the input transformer used. It is a chicago cic - 1.
These are 600/150 ct primary and 100k ct on the secondary. The trouble is
after looking for several months a second one has not turned up....

Have been looking at the utc transformers for a while and think the a12
or ls 12 series would perform the best... these are a bit out of my
budget tho. The question is there is a pair of a11 that are aforadable
but are 50k ct would this be too low? The transformers used on the
website are a10's these are 50k split on the secondary, not really sure
if this means 25k + 25k for a total of 50k or if it is 50k in 2 sections
for a totak of 100k. Is there anyone here famaliar with these
transformers?

My other idea after looking at Patrick Turners site was to canabolize
some of these 120 to 12v transformers that are overflowing their bin and
atempting to wind one. Not really sure about winding an input transformer
tho without some sort of metal can for shielding... ideas???

thank you,
Gerald

Hi Gerald,

I don't have a source for transformers, but I'll give my two (Canadian)
cents.

This circuit is not high fidelity, as it uses beam power tubes without any
negative feedback to control their high output impedance. It looks like it
was a quick project put together with parts found "on hand" rather than
something deliberate. The 6SL7 is a completely different tube than the
6SN7/12SN7, so you will need to change the values of the resistors and
capacitors in order to account for this.

My advice, is to redesign the amplifier circuit a bit, eliminating the need
for an input transformer. Copy the circuit from an old radio. If you want,
I can email you a suitable schematic. To make it safe, buy a pair of
isolation transformers, and power the amplifiers through these. In the
spirit of using things found "on hand", put two of those 12:120V
transformers "back to back", and here is an isolation transformer for free!

[g0l[_2_]]

sure, up for ideas! I do have 1:1 input transformers for isolation and 1:1
transformers to limit the current available from the wall. The amps as
original had 5879 tubes for the first stage. and there are two more octal
sockets on the chasis. It was recomended to me to use the third 256l6 as a
regulator for the screen on the output tubes. There is also a feedback
winding on the output transformer like on a quad amp but without knowing
what the values on the transformer are I really do not know how to redesign
this feature.

[John Byrns]

In article ,
g0l wrote:

sure, up for ideas! I do have 1:1 input transformers for isolation and 1:1
transformers to limit the current available from the wall. The amps as
original had 5879 tubes for the first stage. and there are two more octal
sockets on the chasis. It was recomended to me to use the third 256l6 as a
regulator for the screen on the output tubes. There is also a feedback
winding on the output transformer like on a quad amp but without knowing
what the values on the transformer are I really do not know how to redesign
this feature.

If I were doing this project, and I probably will be eventually as I
have one of these waiting to be modified, I would proceed as follows.

1. I would retain the original amplifier circuit incorporating the 6SL7
and the two 25L6s unchanged, including the negative feedback from the
tertiary winding on the output transformer.

2. I would delete the photo tube and the 5879/6AK6 preamp circuit.

3. I would replace the two selenium rectifiers with two silicon diodes
and add a common ballast resistor to restore the plate voltage to it's
original value.

4. I would use the third 25L6, originally used to power the exciter
lamp, as screen ³regulator², as you suggest above, to replace the
original screen dropping resistor.

5. I would change the 25L6 cathode resistor to compensate for the fact
that the cathode resistor no longer carries the current from the third
25L6 which originally used as an oscillator to power the exciter lamp,
and the fact that the 5879/6AK6 heater is no longer in the cathode
circuit.

6. Finally I would add a 1:1 input transformer between the input jack
and the volume control to isolate the input from the power line.


Regards,

John Byrns

--
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