Step 2 on Fisher Iron

[Peter Wieck]

I did a little bit of hunting about, based on some suggestions received
here and elsewhere. At this point, after looking hard at the Fisher
500B schematic, the B+ available out of the power transformer I have,
the load headroom I have if I reduce the number of tubes, and other
factors, I am leaning towards 5881 PP output tubes driven from a single
12AX7. I will investigate some further on whether the output
transformers are appropriate for this configuration, but on first
blush, it looks OK and has the virtue of simplicity. I ought to be able
to get about 20 very clean watts or so. Nothing that will set the world
on fire (at least one certainly hopes not), but could be very pleasant.


This also fits my existing tube inventory nicely. Parsimonious, not
cheap.

So, solid-state rectification, significant filtration, and as few parts
and pieces as possible thereafter.

Thoughts?

Thanks in advance.

Next, start sketching actual circuits...

Peter Wieck
Wyncote, PA

[Peter Wieck]

Bret Ludwig wrote:

> You have reinvented the Cary SLA-70, I think. It uses a single 6SN7 to
> drive a pair of cathode bias 6L6s or similar. it also used tube
> rectifiers, no big deal. It's actually a good beginner circuit,
> performance is not earthshaking but listenable and easy to
> troubleshoot. Beats most any SET all to hell. I'll try to find a
> schematic.

I have heard of Cary units. There is a modern Chinese version out
lately that I discovered when searching using 5881 & 12AX7 that appears
to show SS rectification:

http://www.divertech.com/aslwaveav20dt.htm

This came up this morning, funny that it should be exactly what I had
in mind, and even showing 6L6 outputs. As to "beats most any SET", my
answer would be "at least that". What is even more of a coincidence is
that there are a pair of these beasts at my friend's shop nearby
(www.tsto.com). Maybe he would lend me one for reverse engineering....
hmmm....

Shameless Endorsement: If anyone in the Southeastern PA (or beyond)
region is looking for an honest Audio Dealer, Mike Leshner is one of
those very rare individuals.

Peter Wieck
Wyncote, PA

[Mark S]

"Peter Wieck" > wrote in message
oups.com...
>I did a little bit of hunting about, based on some suggestions received
> here and elsewhere. At this point, after looking hard at the Fisher
> 500B schematic, the B+ available out of the power transformer I have,
> the load headroom I have if I reduce the number of tubes, and other
> factors, I am leaning towards 5881 PP output tubes driven from a single
> 12AX7. I will investigate some further on whether the output
> transformers are appropriate for this configuration, but on first
> blush, it looks OK and has the virtue of simplicity. I ought to be able
> to get about 20 very clean watts or so. Nothing that will set the world
> on fire (at least one certainly hopes not), but could be very pleasant.
>
>
> This also fits my existing tube inventory nicely. Parsimonious, not
> cheap.
>
> So, solid-state rectification, significant filtration, and as few parts
> and pieces as possible thereafter.
>
> Thoughts?
>
> Thanks in advance.
>
> Next, start sketching actual circuits...
>
> Peter Wieck
> Wyncote, PA
>
Peter,
Check out this schematic of an Eico 87courtesy of Triode Electronics. (1)
12AX7 and (2) 12AU7's (in place of the 6SN7's in the schematic) instead of
(2) 12AX7's. For that you get a pretty decent LTP phase splitter which is a
good choice, especially if you triode connect the 5881's.
http://www.triodeel.com/eicohf87.gif

Best Regards,
Mark

[Peter Wieck]

Mark S wrote:

> http://www.triodeel.com/eicohf87.gif

Thank you. One more hat in the ring. Yes, I will look it over. Five
tubes... (10 in total for both channels). Not the end of the world.

Peter Wieck
Wyncote, PA

[Sander deWaal]

"Peter Wieck" > said:

>
>Mark S wrote:
>
>> http://www.triodeel.com/eicohf87.gif
>
>Thank you. One more hat in the ring. Yes, I will look it over. Five
>tubes... (10 in total for both channels). Not the end of the world.


http://img508.imageshack.us/img508/1751/wkschemod0.jpg

One of my latest efforts in doing something nice with my stash of
KT88s.

I've demonstrated this amp and various other pasrt that I built last
saturday for an audience of audiophools, they liked it. ;-)

For those of us who can read Dutch:
http://zelfbouwaudio.nl/forum/viewtopic.php?t=2613&postdays=0&postorder=asc&start=255
and further.
My nickname there is "KT88".
Some pictures are here:
http://www.circuitsonline.net/forum/view/38940/6
And there I'm also known as "KT88".


Peter, I'm still looking for that 2A3 schematic, it's very
straightforward but I did something tricky with the filament supply,
and can't remember what it is anymore <shamed>

If necessary, I'll take it apart and reverse-egineer it ;-)

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[Bob H.]

> http://img508.imageshack.us/img508/1751/wkschemod0.jpg

Hi Sander.

I was wondering how you came to having 66k and 68k in the anode and
cathode legs of the cathodyne phase splitter? Junk box parts
selection? Or did you have the partial feedback leg on the cathode,
and later removed it?

thanks,
Bob H.

[Sander deWaal]

"Bob H." > said:

>
>> http://img508.imageshack.us/img508/1751/wkschemod0.jpg
>
>Hi Sander.
>
>I was wondering how you came to having 66k and 68k in the anode and
>cathode legs of the cathodyne phase splitter? Junk box parts
>selection? Or did you have the partial feedback leg on the cathode,
>and later removed it?


Intentionally generating some 2H into the amp, to compensate for the
cancelling in the push pull power stage.
Because there's no loop feedback, the 3H would have the most effect if
I hadn't done that.
The local feedback loop from the KT88 anodes to the 12BH7 cathodes
linearizes the power stage somewhat, while at the same time lowering
the output impedance even more.

That way, it sounded more "balanced" to me.

Yep folks, this is one of the cases where I intentionally introduced
distortion to satisfy my preferences.

And judging from the comments of the listeners (which you, sadly,
can't read I presume, because it's all in Dutch) , I succeeded ;-)

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[Bob H.]

..
>
> Yep folks, this is one of the cases where I intentionally introduced
> distortion to satisfy my preferences.
>

There's someone else who builds amps who intentionally mismatches the
sections of the driver or output tubes in order to generate the same
effect. I don't remember who it is, but he may be in this newsgroup.
It's the theory that it's more important that each order of harmonics
be in a natural descending order in relation to each other than the
total amount harmonic levels (to some degree). Another reason for me
to use triodes naturally low in odd-order harmonics for PP.

Bob H.

[Sander deWaal]

"Bob H." > said:


>> Yep folks, this is one of the cases where I intentionally introduced
>> distortion to satisfy my preferences.


>There's someone else who builds amps who intentionally mismatches the
>sections of the driver or output tubes in order to generate the same
>effect. I don't remember who it is, but he may be in this newsgroup.
>It's the theory that it's more important that each order of harmonics
>be in a natural descending order in relation to each other than the
>total amount harmonic levels (to some degree). Another reason for me
>to use triodes naturally low in odd-order harmonics for PP.


You might have noticed that my KT88s are triode-strapped ;-)

But your analysis is correct.

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

[Ruud Broens]

"Sander deWaal" > wrote in message
...
: "Peter Wieck" > said:
:
: >
: >Mark S wrote:
: >
: >> http://www.triodeel.com/eicohf87.gif
: >
: >Thank you. One more hat in the ring. Yes, I will look it over. Five
: >tubes... (10 in total for both channels). Not the end of the world.
:
:
: http://img508.imageshack.us/img508/1751/wkschemod0.jpg
:
: One of my latest efforts in doing something nice with my stash of
: KT88s.
:
: I've demonstrated this amp and various other pasrt that I built last
: saturday for an audience of audiophools, they liked it. ;-)
:
: For those of us who can read Dutch:
:
http://zelfbouwaudio.nl/forum/viewtopic.php?t=2613&postdays=0&postorder=asc&start
=255
: and further.
: My nickname there is "KT88".
: Some pictures are here:
: http://www.circuitsonline.net/forum/view/38940/6
: And there I'm also known as "KT88".

ja, heel aardig. ben natuurlijk weer net op vakantie bij de volgende demodag :(
pimp led's met krimpkousen ... the dutch way
hehehe

R.

[Peter Wieck]

For the record, I am collecting all this wisdom and advice for further
action. Through October, we are dealing with closing and winterizing
our summer house, getting the main house ready for winter, and doing
some last-minute heating-system installations for the kids' houses. I
will post a couple of proposed schematics in November for critique and
suggestions. And likely start breadboarding around American
Thanksgiving.

I have checked the transformers, the outputs match and show no apparent
defects, the power transformer will sit (unloaded) for hours and barely
get warm. Output voltages were as-expected, so it appears superficially
that the iron is good.

Peter Wieck
Wyncote, PA