Re: Frankenchamp fixed bias circuit dilemna


"flipper" <flipper@xxxxxxxx> wrote in message
news:47c973lvrncoobmd197etbo6s463cfaqh2@xxxxxxxxxx
On Sat, 16 Jun 2007 11:24:55 -0400, "Phil S."
<psymonds_no_spam@xxxxxxxxxxx> wrote:


"flipper" <flipper@xxxxxxxx> wrote in message
news:ahn573hlno4lnstaviutopvb6r9pvob2ic@xxxxxxxxxx
On Thu, 14 Jun 2007 22:43:37 -0400, "Phil S."
<psymonds_no_spam@xxxxxxxxxxx> wrote:


"flipper" <flipper@xxxxxxxx> wrote in message
news:9dk1731nkcj1c678rk6354gqh9phbojlgt@xxxxxxxxxx
<snip>
What you need to do is lower the impedances in the filtering so you
can use a lower dropping R before your pot. And you need a lower value
pot as well.

If your guess of -20V is right, and it seems high, you're going to
need a rework because 12.6 volts won't provide it. Double it.

<snip idea about PNP Darlington>

Otherwise you have to use large value capacitors with low value
resistors to get enough filtering while keeping the impedance low,
which should be on the order of 5k (max total) for 100k grid
resistors.


Honestly, I'm trying to get my arms around this and would like to make
it
simple (LOL!).

I have two 250uf 25v electrolytic caps. If I put those in with a 1K
resistor between them, and drop the fixed resistor to 47K+50K pot, will
this
give me something I can use?

Well, your 47k+50k pot is added to the 100k grid resistor, putting
grid impedance above the limit again.

I'm thinking that 1K/220uf isn't a far cry from 15K/10uf.

That's fine. Now, adjust the R and pot to match. I.E. a 10 to 1, or
so, reduction (exact values depend on the bias adjust range you want),
so the whole thing is low impedance.

I hate like heck
to try to shoehorn in a doubler. The space inside is tight and there
are
no
spares to hang more parts on the board. I'd rather drop the plate to
250v,
which might help some, too.


Well, dropping plate V will make a difference in that you don't need
to lower idle current to stay inside max plate dissipation but the
bigger gain, from the bias -V standpoint, would be lowering screen V
since *that*, along with -bias, is what sets idle current.

That's the point behind a pentode; current is, within limits,
independent of plate V.

Now, if you lower B+ to get the lower plate V you'll get a lower
screen V too, since it's RC derived from B+, but I thought it
worthwhile to explain what's doing what.

I've got some results. I thought I'd post and take a bit of time to
consider the next step. In the meanwhile I might benefit from any advice
or
comments you are willing to offer.

I changed the bias network to consist of two 250uf 25v separated by a 1K.
The R to ground is 47K + 50K pot. The grid leak is 15K. I am trying to
keep the total grid R under 100K; got pretty close, as max is now 110K
(the
pot meters at 48K max) -- good enough.

If I read that right you're miscalculating the grid resistance. If
you've got -V on one side of the R+pot and the other end going to
ground, for a -V to ground resistor divider, the impedance is from
wiper to one side in parallel with from wiper to the other end...
that, then, in series with the grid resistor.

I'd guess you're overloading the driver tube's plate.


I found a 39 ohm 5W sandblock on the
used parts pile that I placed between the 6X5 rectifier cathode and the
first filter cap; I don't understand what happened to plate voltage; more
below. Modified the filter ladder with a 2K2 to drop screen voltage,
which
is now around 6-7v below plate. With 2K it was same as plate.

Why are you trying to get it lower than plate?


Grid voltage is between ~16.3 and 16.6 and varies some by tube This range
is too narrow, but it's not a bad number and higher (more negative) than I
expected. Voltage on the DC side of the diode is neg 17.2.

Are you saying you can only adjust it .3 volts? Because that doesn't
make sense from what I thought you described.


With the CSF Bull tube (lowest current draw), plate voltage was 285-286.
With the Sylvania tube (highest current), plate voltage was 267. The
reading with the Sylvania is down from the 275-276 I was seeing. The GE
(middle tube on current) had plate voltage of 275-6. I find this quite
odd
and I checked the CSF tube several times.

Tube data follows, min and max bias voltage (min being the more neg
number,
ie -16.6, max -16.3):
CSF Bull
Ia 37.7 38.2
Va 285 286
Dis 10.74w 10.95w

Sylvania
Ia 56.8 57.7
Va 267 267
Dis 15.17w 15.41w

GE
Ia 47.1 48.3
Va 276 275
Dis 13.0w 13.28w

Overall, this is a better result than previous.

Well, without doing math, especially since I forgot your OPT DC
resistance, those numbers make sense. Lower current leads to higher
plate V, and vice versa, because there's less drop across the OPT
winding and also less power supply droop (PT winding resistance,
rectifier resistance, etc).


I am inclined to try a 250 ohm resistor (what I have here) between the
rectifier cathode and the filter network to try to reduce B+. I am
getting
a 2v drop across the 39 ohm resistor. This should "fix" the GE but maybe
not the Sylvania. That Sylvania was respresnted to me as NIB and I think
it probably is and maybe I don't use it in this amp. I can still keep it
in
there long enough to test it.

It shouldn't make much difference which tube because you should end up
adjusting bias so they're pulling the same current, which will mean
the voltage drops, OPT, PT, rectifier, etc, should, then, end up being
about the same for all of them 'once adjusted'.

You're seeing different numbers because you're not adjusting them to
'spec' (your amp's spec) but letting them run 'wherever' they feel
like.

What can I do to increase the range on the bias adjustment pot? Should I
reduce the R+Pot to 10K+50K pot and raise the grid leak from 15K to 47K
(or
I could series 2x 15K and mind the 100K limit)?

I must be misreading how you wired it up because I can't figure out
why you don't have a HUGE adjustment range.

The problem with trying to have high impedance in the bias pot as
'part of' the grid resistance is if you adjust the pot to one end it
is ZERO impedance so you're left with just 15k grid resistance and
that, for sure, will overload the previous plate.

IMO, you need to get your bias V adjust low enough impedance so that
it's 'negligible' compared to the grid resistor, like in the 5k to 10k
range I previously mentioned with a 100k grid resistor. That way you
can adjust -biasV without significantly affecting your grid resistance
and loading up the previous stage's plate.


Regards,
Phil



Oh boy, this one is making my brain work hard. I lack a fundamental
conceptual understanding here, but all this back and forth is starting to
bring things into focus. I'm learing by doing. I've made more than one
gaff here and once I sort them out, I am convinced that it will work as
desired. I will try to restate some things here to be clear about where
this amp is. Also, I made some further changes last night (home alone for
the weekend).

First, new info on the current configuration. Grid leak resistor is 15K.
Rebuilt bias supply: 12.9vac (as measured) source through one diode puts
out -16.7vdc. Filters are two 250uf with a 1K between them. The "fixing"
resistors (what is this really called?) are 10K + 48K pot (actual value) --
these go to ground at one end and connect to the second cap/1K junction.
Bias voltage range is now between (neg) 15.04v and 16.42v. Incidentally, I
am happy to have more range, but suspect it would be desirable to get the
grid voltage to move closer to -13.

As for the plate/screen thing, maybe I'm operating under the wrong
impression. I am looking at the AA764 schematic at it show plate = 342 and
screen = 340. Of course this is for a 6V6, so the numbers are "wrong" for
my amp, but I am thinking there should be a 2v drop between plate and
screen. That is what I was trying to achieve but didn't get there. I was
seeing screen above plate with a 1K in between the first two main filter
caps, so I changed for a pair of 1K in series (total 2K), which put plate
and screen equal. I reasoned that going to 2k2 would give me the 2v drop,
but now I've got a 12v drop! I'm inclined to return to the 2K value and
move on. Is this contributing the amp sound like poo? BTW, I think there
is something inconsistent with the relationship of the resistors and the
voltage drop, as 12v drop was way more than I expected. Both the 2K2 and
the 1K+1K were checked on the meter.

Presently the amp has that high testing Sylvania in it. BTW, it has the
cool blue glow! Plate current is min 48mA and max 52mA. This 4mA spread is
the best I have achieved so far. Plate voltage on this tube is 254, getting
me to about 12W with Ia=48. This is within spitting range of the desired
result of 11.25. Tone is improved, but bass is still farty. Speaker is a
Celestion G10L-35, so it is not the speaker's fault. This might be the best
sounding speaker I've got and as good as the Weber Blue Dog.

Questions:
What should I do about plate vs screen voltage? Make them equal?

I'm not getting it about "overloading the previous plate". Are you
referencing the 12AX7 plate? There is a coupling cap in place, so I thought
the cap acts as a gatekeeper. Based on the schematic values, I judge the
plate voltage on the 12AX7 to be too high. On V1 (older readings) A1=176v
and A2=251v. Schematic calls for 205 and 200 respectively from B+ at 320v,
but this amp is lower than that. B+ supply after the third filter cap is
about 258v. I'd also note that on V1, K1=1.35v and K2=2.99v. K2 is double
what it should be. I've got the feeling I need to be paying attention here,
but don't know where. I built the V1 section according to the AA764
schematic without modification. I'm wondering if the NFB is making a
contribution. NFB is tapped from the 4 ohm lug and build according to the
schematic.

Perhaps you can elaboate on overloading a bit? Well, maybe I am getting it.
Tell me. I should raise the grid leak to maybe 68K. I should make the bias
adjustment total about 30K. This puts the grid leak R above the bias R.
This is what I need to do? I don't have a pot of appropriate value, but
leaving in the 10K+48K isn't so far from what I just suggested and I can try
to keep from maxing the pot. Should I lower the bias adjustment to 4K7+48K
pot? I'm trying to work with things I have as a trip to the store isn't in
the cards today (closed) or even the early part of the week, and mail order
will send the price of parts through the roof.

I have the fixed 10K R going to one outer leg of the bias pot. The wiper
is grounded. I'm thinking you said this is backwards, but I don't see what
difference it would make. The other outer leg isn't wired to anything.
Should I ground the other outer leg? Should I swap the ground and fixed R
so the outer leg is grounded? Should the 10K fixed R go to the outer leg
first and then to the wiper lug (it isn't presently this way)? Should this
just be left as is? I think I have built this correctly but, if not, please
tell me.

Should I try dropping the bias voltage (make it closer to -13) by putting a
resistor either before of after the diode? Or does this count (after the
diode), too, in grid R load?

Can you elaborate on plate voltage? I don't understand what you say:

"It shouldn't make much difference which tube because you should end up
adjusting bias so they're pulling the same current, which will mean the
voltage drops, OPT, PT, rectifier, etc, should, then, end up being about the
same for all of them 'once adjusted'.

You're seeing different numbers because you're not adjusting them to 'spec'
(your amp's spec) but letting them run 'wherever' they feel like."

How do I prevent them from running wherever they feel like? I'm obviously
missing a critical piece here. I did not expect plate voltage to keep
changing.

BTW, primary winding on the OPT is 232 ohms.

I took pictures, and once up, I will post the link. I will post this now
rather than risk losing it.

Regards,
Phil



.

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