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bigwig bigwig is offline
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Default 5B 255M amplifier?

Hello,
Im new to the group. Has anyone built an amp. with or played with 5B
255M?. I have a load of these valves and would like to put them to
use. The curves seem the same as the 807/6L6 types. I would like to
know if anyone has had any experience with them before I go and build
something. I was thinking PPP class AB1 ultralinear.
Thanks Matt.
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bigwig bigwig is offline
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Default 5B 255M amplifier?

On Jul 12, 11:28*pm, flipper wrote:
On Sat, 12 Jul 2008 07:02:27 -0700 (PDT), bigwig

wrote:
Hello,
*Im new to the group. Has anyone built an amp. with or played with 5B
255M?. I have a load of these valves and would like to put them to
use. The curves seem the same as the 807/6L6 types. I would like to
know if anyone has had any experience with them before I go and build
something. I was thinking PPP class AB1 ultralinear.
*Thanks Matt.


Well, Brimar said it's electrically similar to the 807.

http://www.mif.pg.gda.pl/homepages/f.../5/5B-254M.pdf

The STC data sheet is more comprehensive

http://www.mif.pg.gda.pl/homepages/f...0/5/5B254G.pdf
*(612k)

RCA 807 for comparison.

http://www.mif.pg.gda.pl/homepages/f.../084/8/807.pdf
(1.7Meg)


Ok great thanks guys,
I have a few 7N7s so I think I will build something along the lines
of a Williamson. Should look nice with all Loktal valves. PPP output
stage, 3.3K output transformer.
Thanks for your help with the datasheets.
Matt.
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bigwig bigwig is offline
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Posts: 118
Default 5B 255M amplifier?

On Jul 13, 3:51*pm, flipper wrote:
On Sun, 13 Jul 2008 06:14:42 -0700 (PDT), bigwig





wrote:
On Jul 12, 11:28*pm, flipper wrote:
On Sat, 12 Jul 2008 07:02:27 -0700 (PDT), bigwig


wrote:
Hello,
*Im new to the group. Has anyone built an amp. with or played with 5B
255M?. I have a load of these valves and would like to put them to
use. The curves seem the same as the 807/6L6 types. I would like to
know if anyone has had any experience with them before I go and build
something. I was thinking PPP class AB1 ultralinear.
*Thanks Matt.


Well, Brimar said it's electrically similar to the 807.


http://www.mif.pg.gda.pl/homepages/f.../5/5B-254M.pdf


The STC data sheet is more comprehensive


http://www.mif.pg.gda.pl/homepages/f...0/5/5B254G.pdf
*(612k)


RCA 807 for comparison.


http://www.mif.pg.gda.pl/homepages/f.../084/8/807.pdf
(1.7Meg)


Ok great thanks guys,
*I have a few 7N7s so I think I will build something along the lines
of a Williamson. Should look nice with all Loktal valves. PPP output
stage, 3.3K output transformer.
Thanks for your help with the datasheets.
* * * * * * * * Matt.


You're welcome.

I'm building a 'midget' Williamson (6 Watts from 13FD7s) and Patrick
Turner had an excellent suggestion. Long tail pair the post phase
splitter triodes.

If you use spice to simulate it won't look like that does much, unless
you simulate tube mismatch where it makes a world of difference.- Hide quoted text -

- Show quoted text -


Sounds good,
I need to wind my transformers next weekend(if the wire turns up).
These will be the first OPTs I have wound and my first large valve
project. I built a small EL84 based amp a few years ago. It sounded
very good to my tone deaf ears. I have built numerous MOSFET amps,
some with stupidly good specs. but they all sound a bit harsh to me.
So I need a reasonably sized valve amp, hence this project. Perhaps we
could trade notes as we go.
Matt.
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bigwig bigwig is offline
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Posts: 118
Default 5B 255M amplifier?

On Jul 14, 4:32*am, flipper wrote:
On Sun, 13 Jul 2008 14:37:05 -0700 (PDT), bigwig





wrote:
On Jul 13, 3:51*pm, flipper wrote:
On Sun, 13 Jul 2008 06:14:42 -0700 (PDT), bigwig


wrote:
On Jul 12, 11:28*pm, flipper wrote:
On Sat, 12 Jul 2008 07:02:27 -0700 (PDT), bigwig


wrote:
Hello,
*Im new to the group. Has anyone built an amp. with or played with 5B
255M?. I have a load of these valves and would like to put them to
use. The curves seem the same as the 807/6L6 types. I would like to
know if anyone has had any experience with them before I go and build
something. I was thinking PPP class AB1 ultralinear.
*Thanks Matt.


Well, Brimar said it's electrically similar to the 807.


http://www.mif.pg.gda.pl/homepages/f.../5/5B-254M.pdf


The STC data sheet is more comprehensive


http://www.mif.pg.gda.pl/homepages/f...0/5/5B254G.pdf
*(612k)


RCA 807 for comparison.


http://www.mif.pg.gda.pl/homepages/f.../084/8/807.pdf
(1.7Meg)


Ok great thanks guys,
*I have a few 7N7s so I think I will build something along the lines
of a Williamson. Should look nice with all Loktal valves. PPP output
stage, 3.3K output transformer.
Thanks for your help with the datasheets.
* * * * * * * * Matt.


You're welcome.


I'm building a 'midget' Williamson (6 Watts from 13FD7s) and Patrick
Turner had an excellent suggestion. Long tail pair the post phase
splitter triodes.


If you use spice to simulate it won't look like that does much, unless
you simulate tube mismatch where it makes a world of difference.- Hide quoted text -


- Show quoted text -


Sounds good,
* I need to wind my transformers next weekend(if the wire turns up).
These will be the first OPTs I have wound and my first large valve
project.


Ah, in that case you might want to talk with Patrick Turner because
he's the OPT expert in here.

I built a small EL84 based amp a few years ago. It sounded
very good to my tone deaf ears.


My PP 6GK6 (EL84 clone) amp is my favorite. So far anyway

I have built numerous MOSFET amps,
some with stupidly good specs. but they all sound a bit harsh to me.
So I need a reasonably sized valve amp, hence this project. Perhaps we
could trade notes as we go.


Be glad to. Not sure there'd be much correlation with my little pip
squeak but I suppose you never know. Ironically, I was thinking about
a 6BG6 '807 variant' amp when I switched gears and decided to build
this small 13FD7 feller for my sister.

The 'idea' grew from my 'PC Speaker' amp but that one is 6AW8s, using
the same basic topology as the 6GK6 amp, and I took a completely
different approach with this one.

Got pictures of the 'PC Speaker' amp (and others) on my homebrew web
server at *http://flipperhome.dyndns.org/*but the 13FD7 isn't there
yet because it isn't done yet There's a picture of the first
component fit on alt.binaries.schematics.electronic, though.



* * * * * * * * Matt.- Hide quoted text -


- Show quoted text -- Hide quoted text -

- Show quoted text -


Hi,
Sorry Im so late replying. You have some interesting stuff on your
pages. I have printed all of Patricks transformer and matching pages,
this is what I have based my plans on. I also have RDH4 and some other
transformer related books although these are mainly aimed at
industrial uses. Im hoping to wind somewhere near 6K or so. This will
let me configure it for half that for push pull parallel and also play
about with pure triode, pure tetrode and ultra linear. Im in two minds
at the moment I might even add cathode feedback windings but this
limits my options somewhat.
What transformers do you use on your amps? I would like to build a
smallish amp for a friend to play with. He seems happy with the 8-10W
class A transistor amp he built so I think he will like valves more.
Only trouble is he has really ineficient Mission bookshelves so push
pull is the only way (for a reasonable outlay). Im not a fan of single
ended anyway.
Matt.
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bigwig bigwig is offline
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Posts: 118
Default 5B 255M amplifier?

On Jul 16, 8:17*am, flipper wrote:
On Tue, 15 Jul 2008 13:09:14 -0700 (PDT), bigwig





wrote:
On Jul 14, 4:32*am, flipper wrote:
On Sun, 13 Jul 2008 14:37:05 -0700 (PDT), bigwig


wrote:
On Jul 13, 3:51*pm, flipper wrote:
On Sun, 13 Jul 2008 06:14:42 -0700 (PDT), bigwig


wrote:
On Jul 12, 11:28*pm, flipper wrote:
On Sat, 12 Jul 2008 07:02:27 -0700 (PDT), bigwig


wrote:
Hello,
*Im new to the group. Has anyone built an amp. with or played with 5B
255M?. I have a load of these valves and would like to put them to
use. The curves seem the same as the 807/6L6 types. I would like to
know if anyone has had any experience with them before I go and build
something. I was thinking PPP class AB1 ultralinear.
*Thanks Matt.


Well, Brimar said it's electrically similar to the 807.


http://www.mif.pg.gda.pl/homepages/f.../5/5B-254M.pdf


The STC data sheet is more comprehensive


http://www.mif.pg.gda.pl/homepages/f...0/5/5B254G.pdf
*(612k)


RCA 807 for comparison.


http://www.mif.pg.gda.pl/homepages/f.../084/8/807.pdf
(1.7Meg)


Ok great thanks guys,
*I have a few 7N7s so I think I will build something along the lines
of a Williamson. Should look nice with all Loktal valves. PPP output
stage, 3.3K output transformer.
Thanks for your help with the datasheets.
* * * * * * * * Matt.


You're welcome.


I'm building a 'midget' Williamson (6 Watts from 13FD7s) and Patrick
Turner had an excellent suggestion. Long tail pair the post phase
splitter triodes.


If you use spice to simulate it won't look like that does much, unless
you simulate tube mismatch where it makes a world of difference.- Hide quoted text -


- Show quoted text -


Sounds good,
* I need to wind my transformers next weekend(if the wire turns up).
These will be the first OPTs I have wound and my first large valve
project.


Ah, in that case you might want to talk with Patrick Turner because
he's the OPT expert in here.


I built a small EL84 based amp a few years ago. It sounded
very good to my tone deaf ears.


My PP 6GK6 (EL84 clone) amp is my favorite. So far anyway


I have built numerous MOSFET amps,
some with stupidly good specs. but they all sound a bit harsh to me.
So I need a reasonably sized valve amp, hence this project. Perhaps we
could trade notes as we go.


Be glad to. Not sure there'd be much correlation with my little pip
squeak but I suppose you never know. Ironically, I was thinking about
a 6BG6 '807 variant' amp when I switched gears and decided to build
this small 13FD7 feller for my sister.


The 'idea' grew from my 'PC Speaker' amp but that one is 6AW8s, using
the same basic topology as the 6GK6 amp, and I took a completely
different approach with this one.


Got pictures of the 'PC Speaker' amp (and others) on my homebrew web
server at *http://flipperhome.dyndns.org/*but the 13FD7 isn't there
yet because it isn't done yet There's a picture of the first
component fit on alt.binaries.schematics.electronic, though.


* * * * * * * * Matt.- Hide quoted text -


- Show quoted text -- Hide quoted text -


- Show quoted text -


Hi,
*Sorry Im so late replying. You have some interesting stuff on your
pages.


Thanks.

I have printed all of Patricks transformer and matching pages,
this is what I have based my plans on. I also have RDH4 and some other
transformer related books although these are mainly aimed at
industrial uses. Im hoping to wind somewhere near 6K or so. This will
let me configure it for half that for push pull parallel and also play
about with pure triode, pure tetrode and ultra linear. Im in two minds
at the moment I might even add cathode feedback windings but this
limits my options somewhat.


I see. So you're looking at the 400V plate, 300V screen mode and the
UL taps for PPP half impedance.

* What transformers do you use on your amps?


Well, the 12AX7/6GK6 amp can use any 8K PP OPT, of course, and I
originally had a pair of HKs I got off ebay on it but lately I've
become interested in using line transformers (and the cathode bias
current mirror).

First reason is obviously cost and I've been doing 'PC Speaker' type
amps where you're not running a grand or two worth of speakers so, as
the theory went, you're not going to get 20Hz even if the amp does.

As it turns out they do better than you'd think, judging from the raw
specs, because average program power is much less than peak so you
don't really 'need' full power at the extreme low end for most
material (that I listen to anyway). Best performance should be with
triodes, because of the low plate impedance (and plate feedback),
which is why I'm trying the 13FD7s in this latest amp.

I would like to build a
smallish amp for a friend to play with. He seems happy with the 8-10W
class A transistor amp he built so I think he will like valves more.
Only trouble is he has really ineficient Mission bookshelves so push
pull is the only way (for a reasonable outlay). Im not a fan of single
ended anyway.


Well, that 6GK6 PP amp is a classic design for around 15 Watts,
depending on where B+ comes out, and Edcor makes some reasonably
priced OPTs in 3 different bandwidths but I imagine you'd wind your
own.

The 13FD7 amp I'm working on would do 8 Watts, *maybe a tad more, if
it had a larger B+ transformer but I wanted to keep the thing small
and that limited me to 50VA at around 230V B+. I also thought about
using the Edcors too they wouldn't fit either.

I know what you mean about speaker efficiency, and it makes a
difference what the listening expanse is. Those 'PC Speakers' I built
aren't very efficient either but, then, they're not intended to be
'room' speakers since, in theory, you'd be sitting rather close to
them. Still, I was rather surprised at how much volume they do with
just 3-4 watts and I routinely use them, with the 'PC Speaker' amp,
for background music (got Pirates of the Caribbean playing right now).
But ya know what the 'problem' is? It sounds so good you want to turn
the volume up. It's almost like a teaser for "man, I need to get a
bigger one of these things." hehe

Which is not necessarily a 'bad thing' because you can build the small
ones for not terribly much and then decide of you want a 'biggie'.



* Matt.- Hide quoted text -


- Show quoted text -- Hide quoted text -

- Show quoted text -


Hello,
Your site is great!. I like the trick with a 7805. Years ago I used
to work in a TV repair shop owned by an ex RAF RADAR guy. He taught me
that little trick, you must be careful not to push it too far though
otherwise you end up with quite a good saw tooth oscillator. I saw a
design from an Indonesian guy that used this to make a linear supply
at low inputs that switched to a switcher at higher inputs. It was
quite ingenious.
I checked out Edcors site, they have an extensive range!. I think I
will wind these trannys and then for my friends buy some. By the time
I purchased c-cores, wire insulation etc. plus the laour intensive
task of winding them it will work out about twice the price of a pre-
wound set. Mind you I will probably get the bug of winding them and
after the initial outlay I guess it gets cheaper. Also at least I will
get exactly what I want (providing my maths are OK).
Your use of TV tubes is good, they are plentifull and cheap. I have
seen a lot of people put these type of tubes down but they must be
using them wrong. The reason I chose 5B255M (apart from the fact I had
some) is because they are very cheap, plentifull and as far as I can
tell a later ruggedised version of the 807.
Have you thought about using mains trannys? I know it sounds crap,
but they are cheap and could be worth playing with. Steve Bench used a
couple on an amp he built, check out his web site http://members,aol.com/sbench101/.
As for my power supply Im going to use an industrial control
transformer in reverse (415-230V). I experimented with this with a SS
bridge and I was getting almost 600V! plus a massive switch on surge.
If I was to use this I would be mad, I would have to go for AB2 I
guess. What I plan is to play with a half solid state half vacuum
rectifier bridge and a regulator. Any suggestions? perhaps a full
vacuum bridge with damper diodes would drop it a bit.
Matt.


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bigwig bigwig is offline
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Posts: 118
Default 5B 255M amplifier?

On Jul 17, 5:17*pm, flipper wrote:
On Wed, 16 Jul 2008 14:51:15 -0700 (PDT), bigwig

wrote:

trim



Hello,
* Your site is great!. I like the trick with a 7805.


I presume you mean the heater voltage bump on the PC Speaker Hybrid
amp.?

Years ago I used
to work in a TV repair shop owned by an ex RAF RADAR guy. He taught me
that little trick, you must be careful not to push it too far though
otherwise you end up with quite a good saw tooth oscillator. I saw a
design from an Indonesian guy that used this to make a linear supply
at low inputs that switched to a switcher at higher inputs. It was
quite ingenious.
* I checked out Edcors site, they have an extensive range!. I think I
will wind these trannys and then for my friends buy some. By the time
I purchased c-cores, wire insulation etc. plus the laour intensive
task of winding them it will work out about twice the price of a pre-
wound set. Mind you I will probably get the bug of winding them and
after the initial outlay I guess it gets cheaper. Also at least I will
get exactly what I want (providing my maths are OK).


Yeah. Btw, where do you get your c-cores?

I keep thinking that 'one of these days' I might try winding some
myself.

* Your use of TV tubes is good, they are plentifull and cheap. I have
seen a lot of people put these type of tubes down but they must be
using them wrong.


Hard to tell where attitudes come from and I doubt most critics
actually built anything with them. More likely passing on 'what they
heard', from someone passing on what they heard.

The 13FD7 is a 13EM7 in a similar sized bottle but with a 9 pin base
instead of octal and there's plenty of 13(6)EM7 amps out there. Octal
pin spacing might be better for flybacks but I'm only running 250V B+
and 9 pin is plenty good enough for that, plus the advantage it fits
the hole punch I've got That and I picked up a handful of them for
75 cents.

The reason I chose 5B255M (apart from the fact I had
some) is because they are very cheap, plentifull


Where you are I guess. They be 'rare foreign' stuff over here.

and as far as I can
tell a later ruggedised version of the 807.
* Have you thought about using mains trannys? I know it sounds crap,
but they are cheap and could be worth playing with. Steve Bench used a
couple on an amp he built, check out his web sitehttp://members,aol.com/sbench101/.


Yes, I saw his work on those but when I checked prices they were more
than Edcors.

I did use a power transformer for OPT on the 6CW5 guitar amp, though,
where there's no need to pretend a 20kHz top end and I didn't need
gobs of NFB with low output impedance because speaker 'color' is part
of the 'sound'. Even with pentodes I only wrapped 11dB around it and
they all love it.

*As for my power supply Im going to use an industrial control
transformer in reverse (415-230V). I experimented with this with a SS
bridge and I was getting almost 600V! plus a massive switch on surge.
If I was to use this I would be mad, I would have to go for AB2 I
guess.


More like B (or B2). Can't get much A at that voltage.

I looked at the same thing because I have a whole box of similar
control transformers, except 'US' 230/460-115 and I, too, operate them
'backwards' for my 300V B+ supplies, parallel winding (230VAC), but
can get 600V if I use the windings in series.

All the 'wallwart' powered jobs use reverse 115/230-12 (or 10) VAC
transformers too and you may have noticed the note I made about the
voltage being lower, in reverse, by the 'regulation' factor (twice the
number, actually).. That's because the winding ratio is 'bumped up' to
compensate for full load losses but in revere that translates to a
bump 'down', and then you still have the winding losses on top of that
at full load.

The ones I use are typically 18% load reg so using 10VAC on 12VAC is
just about right to get back the 'bump down' but there's still the
winding losses so it's still 18% less than one would guess from just
looking at the label thinking they'll get '230' from the primaries.

The B+ tranny in the 13FD7 amp is running backwards too, because it's
really a 115-115/230 isolation transformer, but it's also only 11
bucks.

Speaking of which, I just did a marathon session and got the right
channel done. Hopefully the left channel will go a little quicker but
it's dern tight in there and that makes for slow going.

Had me worried there for a sec or two when I rushed to put audio
through it and.... well, LTPs don't work real well when you forget to
connect the tail. hehe

I think this is going to turn out to be a nifty little amp and now I'm
beginning to wonder if I really want to give it way.

What I plan is to play with a half solid state half vacuum
rectifier bridge and a regulator. Any suggestions? perhaps a full
vacuum bridge with damper diodes would drop it a bit.


Well, you're going to burn a lot of power doing it by brute force and
my first thought, if the goal is to keep the control transformer, was
to add a 115 (store bought) in series with the 300V wiring. Or do the
Class B 600V/300V version.

Mine are 100VA so the 11 buck 50VA, at half the voltage, is a proper
match and at 115 out it can operate forward. Or backward if B+ needs
to be nudged down. Kinda kludgey but it would work.



* * * * * * Matt.- Hide quoted text -


- Show quoted text -


Sorry I have not responded earlier, been busy, work, kids you know the
drill.
I got my c-cores from ebay, I know it was silly but they dont seem
bad. The guy I got them from dissapeared from ebay. He or they seem to
be back now as JLW tools. The cores arrived from China in five days to
the UK!!. Be carefull though if you want some they seem a bit
optimistic with the specs. I went for the next size up from what I
needed and Im glad I did. I know you dont need as much iron with c-
cores but the ones I have seem to be about the same size/weight as the
core of a 150VA torroid so im thinking they should be fine. They are
their "core 40" size. They came with bobbin tubes which I will use
with added side cheeks. The cast ally (if you can call it that) base
they supply is very poor. It could be cleaned up but I think it is too
rough to be worth the effort. The cores on the other hand seem OK.
I agree about the control tranny. I just have a few lying about as
Im an industrial electrician by trade. I might wind my own on torroids
after reading a few posts on winding them. Your use of variouse
trannys to get the correct voltage/power is a good way to get what you
need at a reasonable cost. I have been thinking of building a line
stage(eek) to drive some long cables. It will need plus and minus 100V
or so and 4 10VA 0-18-0-18V transformers work out perfect.
Mat
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[email protected] kimtanwoo@gmail.com is offline
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Posts: 1
Default 5B 255M amplifier?

On Saturday, 12 July 2008 22:02:27 UTC+8, bigwig wrote:
Hello,
Im new to the group. Has anyone built an amp. with or played with 5B
255M?. I have a load of these valves and would like to put them to
use. The curves seem the same as the 807/6L6 types. I would like to
know if anyone has had any experience with them before I go and build
something. I was thinking PPP class AB1 ultralinear.
Thanks Matt.


Do you still have a lot of STC 5B255M? Can sell a matched quad of them to me?
  #8   Report Post  
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Patrick Turner Patrick Turner is offline
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Posts: 3,964
Default 5B 255M amplifier?

Sorry I have not responded earlier, been busy, work, kids you know the
drill.
I got my c-cores from ebay, I know it was silly but they dont seem
bad. The guy I got them from dissapeared from ebay. He or they seem to
be back now as JLW tools. The cores arrived from China in five days to
the UK!!. Be carefull though if you want some they seem a bit
optimistic with the specs. I went for the next size up from what I
needed and Im glad I did. I know you dont need as much iron with c-
cores but the ones I have seem to be about the same size/weight as the
core of a 150VA torroid so im thinking they should be fine. They are
their "core 40" size. They came with bobbin tubes which I will use
with added side cheeks. The cast ally (if you can call it that) base
they supply is very poor. It could be cleaned up but I think it is too
rough to be worth the effort. The cores on the other hand seem OK.
I agree about the control tranny. I just have a few lying about as
Im an industrial electrician by trade. I might wind my own on torroids
after reading a few posts on winding them. Your use of variouse
trannys to get the correct voltage/power is a good way to get what you
need at a reasonable cost. I have been thinking of building a line
stage(eek) to drive some long cables. It will need plus and minus 100V
or so and 4 10VA 0-18-0-18V transformers work out perfect.
Mat

Good luck with your OPT winding. If it all comes out right, you'll deserve a medal because so few ppl are brave enough to discuss their OPT winding activities here.

C-cores can be deceptive because when you see a pair of O's on the bench the pair may look bigger than an equivalent core using E&I. But on close examination, the Afe of the double C-cores can be much smaller than the Afe on E&I lams. The rules about Afe and turns must be maintained on any sort of core you use. Usually, the ratio of window size to Afe for C-cores favors using more turns and thicker wire than for E&I, thus making it possible to reduce winding resistances. The idea is that most transformers are for mains and the makers want the weight and winding losses to be minimal while driving the iron to Bac = 1.4Tesla. This makes C-cores compete with toroids. But with OPTs, The greater winding traverse width slightly reduces leakage inductance. The higher window height allows thicker P-S insulation which reduces shunt C.
Using my OPT design methods should give you an OPT that's better and bigger than most amp company accountants will allow into their products.

The other good thing about OPT C-cores is that they allow air gapping.
Maybe you think this strange because air gaps belong on SE amps only.
Well, the permeability, µ, of a pair of C-cores can be up to about 12,000..
This would be for fairly good GOSS material used. Coming from China, maybe the µ is unknown, and you'd have to measure it after assembling the tranny and applying say 100Vac across primary at 50Hz and with 10r0 in series to measure the primary coil current, all with no loads connected.
The XLp reactance = Vac across coil / Current flow.
From that, the Lp = XLp / ( 6.28 x 50Hz )
Then you need to look at the inductance formula at my site and use that to find what µ must be to give you the Lp you have measured. Now µ may anywhere from about 3,000 to 12,000 depending on the C-core material used, which should be cold rolled GOSS, well glued up, and with cuts polished. I have measured 4,500 in one lot of C-cores made in Oz which I bought in 1996. Obviously, the grade of GOSS was lowish, but it did not matter one bit because those C-cores were used in a pair of SE32 amps with 13E1, and then the gap reduces µ to under 500.
However, for PP, having very high µ is of little benefit because any Idc imbalance can cause core saturation with the net Idc flow in one direction. The DC magnetization, Bdc, is proportional to turns x µ x Idc.
So, its best practice to keep the µ down to around 3,000, and if you have followed all my rules on Fsat 20Hz, then µ = 3,000 should give you plenty of primary inductance. To adjust the µ downwards you may need a very thin air gap, thinner than an average sheet of paper which is 0.05mm. I have used some shopping bag which is thinner, but just use a small bit for the gap, and leave enough space for varnish to run in around edges of C-core cut surfaces. At F below 20Hz, the core will act as an inductance rather than a saturated reactance and stray VLF signals have less chance of causing saturation, and recovery after overload is better and without the severe effects of possible RF bursts generated in OP tubes when faced with trying to drive a signal into a short circuit, for part of the wave cycle.

Patrick Turner.
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