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Posted by dogwan (A) on November 29, 2009 at 17:27:13

In Reply to: RE:
Eli....Zobel network....? posted by Eli Duttman on November 29, 2009 at
16:47:44

Yeah,
that's what got me going on this project. I finally got a hold of some panel
mount IEC connectors. That pic is from when I brought them home. Spent last
night and this morning cutting in and installing a proper grounded 3 wire
power supply, a power switch, proper binding posts, better RCA'a and a pilot
light. That 4 pin connector has been removed and that's where I put the
power switch but it can easily be moved if I need to add another tube in
that locale.

Like I said the previous owner had started the mods but I kind of had to
start over. Some of the caps had been replaced but not all. Also a lot of
the resistors are really out of spec. and are some really old style. Oh
wait, I think you responded to my old post about that type of resistor.

Now that I feel safe about plugging them in I'll start on the resistors
and order up the caps.

Thanks for more ideas. What if I added another 6SN7? I thought I've seen
a 6L6 AMP using 2 6SN7's but can't find it again.

-Dogwan

Edits: 11/29/09

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Follow Ups:

RE:
old pic... - Eli Duttman 18:45:20 11/29/09 (12)

In Reply to:
RE: old pic... posted by dogwan on November 29, 2009 at 17:27:13

2X 6SN7s as the small signal complement is
the "classic" Williamson style circuit. It works well, IF
the O/P trafo is absolutely top notch. Otherwise you get (
YUCK!! ) a phase shift oscillator.

Mullard style circuitry
is "classic" too, but it is much less prone to stability trouble than
Williamson style is.

Please provide a photo of the underside that
includes the O/P transformer.

Eli D.

under
side.... - dogwan 18:55:21 11/29/09 (11)

In Reply to:
RE: old pic... posted by Eli Duttman on November 29, 2009 at
18:45:20

Here ya go, again this is an old pic before I started
work on it.

RE:
under side.... - Eli Duttman 19:56:21
11/29/09 (10)

In Reply to:
RE: under side.... posted by dogwan on November 29, 2009 at
18:55:21

That O/P trafo does not look
particularly hefty. I strongly suspect the tubes installed in the
"beast" now are 6L6GCs as the "finals" and a 5U4GB as the rectifier.
My "educated guess" is that OEM was (respectively) 6L6s (metal
envelope/19 W. max. plate dissipation) and a 5U4G. There's no problem
with the later variants being in situ, as they will "loaf".

IIRC, you previously indicated that O/P trafo replacement was a
possibility. Edcor's model CXPP60-MS-6.6K seems suitable. The
ultra-linear (UL) taps are a blessing. Full pentode mode requires a
lot of NFB of 1 kind or another to obtain a satisfactory damping
factor. UL mode requires less help from NFB to be acceptable in the
damping factor dept.

Eli D.

Edcor CXPP60-MS-6.6K Specifications and Pricing
(Open in New
Window)

Hahaha...now
you got me thinking! - dogwan 21:29:07
11/29/09 (9)

In Reply to:
RE: under side.... posted by Eli Duttman on November 29, 2009
at 19:56:21

If I want to add another driver
stage and beef up the OPT I think I would pretty much have to scrap
the chassis. At which point the only thing worth salvaging (aside
from the I/O parts I've added) would be the power tranny.

Might be a fun project. Maybe go with a different output tube at
that point.

The learning experience is why I picked these
amps up in the 1st place. A scratch build would definitely challenge
me.

-Dogwan

RE:
Hahaha...now you got me thinking! - Neff 05:15:21
12/05/09 (0)

In Reply to:
RE: Hahaha...now you got me thinking! posted by dogwan on
November 29, 2009 at 21:29:07

Those chassis have a sweet
appearance. Remove the can cap & use seperate caps under the
chassis. A better audio transformer should be able to fit in the
new space. Edcor has some interesting products and many of the
audio transformers are only 3.2" wide. Smallest Edcor high quality
full bandwidth PP transformer is rated 25-watts and superior to
the existing audio transformer.

RE:
Hahaha...now you got me thinking! - Eli Duttman
19:34:14 11/30/09 (7)

In Reply to:
RE: Hahaha...now you got me thinking! posted by dogwan on
November 29, 2009 at 21:29:07

Before you start a scratch build
project, do something constructive with the Wurlitzer
monoblocks.

Let's leverage the 30 W plate dissipation
max. of the 6L6GC compared to the 19 W. max. of a metal envelope
6L6, while allowing for lack of heft in the OEM O/P trafo. SS
rectify the B+, to increase the rail voltage. Triode wire each
6L6GC by connecting g2 to the plate via a 100 Ω Carbon film
resistor. A small toroidal trafo will be installed to energize a
bias rail (C-) for the "finals". "Fixed" bias will allow for the
greatest possible plate to cathode differential. Power O/P will
be less than that of the OEM setup, but strain on the O/P "iron"
is lowered and loop NFB requirements are reduced.

It will
not be the end of the earth if a Mullard circuit's LTP is a 6SN7,
as gm is still reasonably high and R P is reasonably
low. LTP performance will be enhanced, if the tail resistor is
replaced by a 10M45S CCS. Mount a 7 pin mini socket for a 6AB4
voltage amplifier. 200 to 220 V. on the plate and I B
= 3 mA. is an excellent set of operating conditions for
the voltage amplifier. The 5 VAC winding will be used to energize
the 6AB4's heater. Hopefully, regulation effects will bring the
actual voltage within 6.3 VAC +/- 5%. It's not inconceivable that
a small series resistance will be needed to prevent an overvoltage
condition. Remember, a 5U4's filament draws 3 A., while a 6AB4's
heater draws 150 mA.

Eli D.

Oooops...
pop then fuse blew. - dogwan 22:39:17
11/30/09 (6)

In Reply to:
RE: Hahaha...now you got me thinking! posted by Eli
Duttman on November 30, 2009 at 19:34:14

Think one of the multi-cap cans gave out. I had
the bottom open so I could double check the power tranny spec's.
Heard a crinkling popping sound then the power fuse blew.
Replaced the fuse then turned on for just a 1/2 sec and got blue
flames in the 5U4GB.

Nice thing about mono-blocks is
you can compare readings. Was worried it was the PT so I
compared resistance readings across all windings between the 2
PT's and they matched up. But when I compared C8's I couldn't
get any resistance or capacitance readings from the suspect one.

Anyway I did find the spec's for the PT online:
380,0,380 @ 150ma, 5v @ 3a, and 6.3v @ 3a.

I was
trying to see if these would support a Dynaco MKIII or IV
clone. If I move just one of the tube sockets I should be able
to squeeze the Edcor OPT in on top.

Take a look at the
attached schem. Do you think this could work on the chassis with
the existing PT's and those Edcore's?

Thanks for all
your help,
-Dogwan

RE:
Oooops... pop then fuse blew. - Eli Duttman
19:44:37 12/01/09 (5)

In Reply to:
RE: Oooops... pop then fuse blew. posted by dogwan on
November 30, 2009 at 22:39:17

"Take a look at the attached
schem. Do you think this could work on the chassis with the
existing PT's and those Edcore's?"

No go! :>
( 2X EL34s draw all 3 A. the 6.3 V. winding can supply. 2X
6L6GCs draw 1.8 A. from the filament winding.

3 A. is
enough to power 2X 6L6GCs (1.8 A.), an ECC99 (0.8 A.), and a
12AT7 section (0.15 A.). :> ) The total is 2.75 A.

Look at the RCA 6L6GC data sheet. Notice the triode
plate curves show upwards of 50 V. as the grid bias for
rational "idle" currents. More than ever, "fixed" bias is
(IMO) the obvious way to go. SS rectifying the B+ will yield
a rail of approx. 490 V. A 50 mA. "idle" current seems
sensible, as anode dissipation will be under the 30 W. max.
allowed.

While it is possible to derive the bias (C-)
rail from the 5 VAC winding, I would not do so. A (SIC) 16
stage voltage multiplier occupies lots of space and is not
inexpensive. Just tie the 5 VAC wires off. An AnTek
(www.antekinc.com) model AN-0124 toroidal trafo costs $10.
Connect the 2X 24 V. secondaries in series and Greinacher
("full wave") voltage double the 48 VAC with 2X 100 PIV
Schottky diodes to get the negative bias rail. Notice that
you will have the option to run the "finals" at a small "idle"
current, should you so choose.

Eli D.

RCA's
6L6GC Data Sheet   (Open
in New Window)

Starting
to lose me.....I think I understand...... -
dogwan 20:54:32 12/01/09 (4)

In Reply to:
RE: Oooops... pop then fuse blew. posted by Eli
Duttman on December 01, 2009 at 19:44:37

But I would need to see it laid out in a
schematic.

Re: the Mullard EL34 circuit I forgot to
mention that I knew I would have to add a small cheapo 6.3v
"helper" transfo. for the heaters, probably a 4a just for
the EL34's.

Found this design that I think could
work. What do you think?

-Dogwan

RE:
Starting to lose me.....I think I understand...... -
Eli Duttman 18:53:11 12/02/09 (3)

In Reply to:
RE: Starting to lose me.....I think I understand......
posted by dogwan on December 01, 2009 at 20:54:32

"Found this design that I
think could work. What do you think?"

That's
an "interesting" hodge/podge. A paraphase splitter
driving a differential amp. Unfortunately, for good
cause, paraphase splitters are out of favor. So, forget
that design too.

It seems that you want a well
proven "road map". OK, I've got a real good 1 for you.
It's the Harman-Kardon Citation V. "Steal" the Cit. 5's
small signal circuitry and use "fixed" bias triode wired
"finals" into the O/P trafos currently in situ. SS
rectify the B+ and use the "5" VAC winding to energize the
heater of the 12BY7 voltage amplifier, in a "6" V. setup.
The 6CG7 shown in the schematic is 100% electrically
equivalent to the 6SN7 now in place. ;> ) AA sponsor
Jim McShane is THE expert on H/K tubed Cit.
units. Send him an EMail about the project. Undoubtedly,
Jim will have parts you can use at a fair price.

Eli D.

H/K Cit. 5 Photofact Reference
(Open in New
Window)

Now...where
was I? - dogwan 00:50:35
12/05/09 (2)

In Reply to:
RE: Starting to lose me.....I think I understand......
posted by Eli Duttman on December 02, 2009 at
18:53:11

Hey Eli,
Hope I'm not testing your
patience too much.

You're right I am leaning
towards a proven design I can follow easily. I like the
Cit.V idea as well as the 6AN8 idea from the Dynaco
mkIII.

So while I was out for a run in the park
today I was thinking about this project. With so many
options on the table I realized I should list out my
goals and prioritze from there.

1. Use existing
PT's and chassis, structural modification if necessary.

2. Go UL and upgrade OPT's.

3. Achieve
25-30w output.

4. Cathode bias for simplicity,
Or if fixed bias add toroidal xfmr. as per your
suggestion. (More power would be nice but I like the
more lush tube sound)

5. If possible retain 1
tube for driver/phase splitter (6AN8?) to stay within
capacity of PT filament circuit. If needed could add
helper small helper xfmr., maybe combine with toroidal
for C- rail if fixed bias.

6. Stay within 6L6
family for output stage. 6L6GC, KT66, KT77 ?

7.
No preference for SS or tube rectification. Tubes are
more romantic, SS could free up some mA's if needed.

I don't want to run this thread on forever but
I did find this out there on the internet. It was on a
blog site, most of it was in Japanese. To my uneducated
eyes it looks similar to the Dynaco driver stage. Of
course I would need to make sure it is a viable design.
But I'm intrigued.

-Dogwan

RE:
Now...where was I? - Eli Duttman
08:04:14 12/05/09 (1)

In Reply to:
RE: Now...where was I? posted by dogwan on
December 05, 2009 at 00:50:35

Stay away from the
KT77 and KT66. The '77 is like an EL34, while the '66
is electrically similar to the 6L6, it draws more
heater current.

If you are going to go UL,
buy Edcor CXPP100-MS-6.6K trafos. The extra magnetic
headroom is insurance against core saturation from the
GNFB loop. Also, you keep the option of Class "AB2"
operation open, down the road. Big "iron" never
hurts.

I don't know where you got 290 Ω for
the cathode resistor value. Remember, upwards of 50
bias Volts are needed. 50 V. and an "idle" of 50 mA.
works out to 1 KOhm. Can you say heat ?
Dude, go combination bias. Build the C- supply. Put
a shared 100 Ω/330 μF. network under the cathodes.
That's a single grid voltage adjustment and very
stable. 10 V. across the 100 Ω resistor = 50 mA.
"idle" per tube. :> D

SS rectify the B+
with what I call a "cockeyed" bridge. Construct
series wired pairs of UF4007s. Parallel each pair
with a 10 nF. high WVDC snubber. Connect
the cathode ends of the assemblies to the ends of the
rectifier winding. Connect the assemblies' anode ends
to ground. Connect the anode of a 1200 PIV Schottky
diode to the CT of the rectifier winding. Connect the
Schottky's cathode to a CL150 inrush current limiter.
Connect the CL150 to the PSU. That setup is QUIET and
has a slightly softened start. Use 525 WVDC twistlok
(FP) caps. in the PSU filter. Never ignore start up
surge, regardless of rectification style.

Finally, install some protection for the screen grids
of the "finals". Put 100 Ω resistors in the lines
leading to the UL taps.

Eli D.

RE:
Now...where was I? - dogwan
09:48:40 12/05/09 (0)

In Reply to:
RE: Now...where was I? posted by Eli Duttman
on December 05, 2009 at 08:04:14

"I don't know where
you got 290 Ω for the cathode resistor value"

- That wasn't my doing. Thanks for the
catch. Like I said I mined this from someone else
(copied a JPG.) while looking for PP 6L6GC ideas. I
would definitely verify all values and layout before
building.

"buy Edcor CXPP100-MS-6.6K
trafos"

-Do you mean the CXPP60-MS-6.6K?
Wouldn't that be more than adequate? Oh wait, as I
type this I realized why you recommend.

So
I'm liking this 6AN8/6L6GC UL PP direction. I'll
start saving up for the the trafos. and acquiring
some hardware. Also I'll also draw up the schem's a
little better and start verifying values.

I like some of your ideas. Won't be starting right
away. Have another project (vintage SS, aaargh!)
that I've been putting off. I will re-post and ping
you when I get closer to launch.

-Dogwan

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