el34 cathode bias or... need help...
Mikko Kankaanpää
I just got large PA amp (110W, 4xel34, 4xef83, 1xef86, 2xecc85, 4xmic pre
and one line in, made in Finland) that previous owner tried to make himself
a guitar amp but did not finnished the project. So it now quite useless.
Amps has odd power stage design, but output transformer has many taps so It
can be used with "normal" p-p output stage. I measured I can get from 1752K
to 33300K (anode to anode) primary with my speaker settings.
My problem is that is had about 670V on anodes of tubes and about 350V on
screengrids. If I wired it like normal p-p stage it will give me too much
power for my use, even with only two el34. looking el34 data, seems like it
could give something like 80-90W with two el34 with voltages I got.
So I consider that could I wire it as cathode biased output stage? I newer
seen catode biased amps that has so high voltages on anodes. But cathode
bias gives often less output power than fixed bias. How much output do I get
with cathode bias p-p el34 stage with voltages that high? What value of
cathode resistors should I first try? And what is good primary impedance (a
to a) for this kind of connection?
Thanks all!!!
yours,
Mikko Kankaanpää
Peter Völpel
That is a class B amplifier...
You can´t use simple cathode bias I guess.
Use 130 Ohm cathoderesistor for NFB and about - 35V fixed bias.
Power output could reach 150-200W depending on the power supply
Expect output impedance to be around 5k p-p
Alternately drop anodevoltage to 400V for AB mode,
probably it is easy to convert the power-supply,
supply the screens through 470 Ohms each,
use kathoderesistor of 470 Ohms for both,
expect about 70-80 Watts at 1,7k p-p
drivevoltage will be as original
good luck
Peter
Mikko Kankaanpää
and thanks for answer!
>
> That is a class B amplifier...
> You can´t use simple cathode bias I guess.
> Use 130 Ohm cathoderesistor for NFB and about - 35V fixed bias.
> Power output could reach 150-200W depending on the power supply
You mean 150-200W when using FOUR el34, right?
> Expect output impedance to be around 5k p-p
>
> Alternately drop anodevoltage to 400V for AB mode,
> probably it is easy to convert the power-supply,
> supply the screens through 470 Ohms each,
> use kathoderesistor of 470 Ohms for both,
> expect about 70-80 Watts at 1,7k p-p
> drivevoltage will be as original
Again with FOUR el34?
I was planning to use only two of them but getting 70-80W on cathode bias
sounds good to me! Offcourse It´s far more cheaper to buy two than four
power tubes but maybe they last longer when not beat the hell out of them...
Thanks Peter!
Yours,
Mikko Kankaanpää
Peter Völpel
Right, when I mention power out that means for 4 tubes.
I wonder how your mains transformer in the amp is configured.
Many standard transformers use a center-tapped secondary winding
to use fullwave-rectification ( with 2 diodes ).
To get higher ( double ) voltage they are used with a bridge-rectifier
over the full winding.
May be you are lucky and they used such a transformer in your amp,
in that case you only have to ground the center-tap of the secondary
and lift the minus-connector of the bridge from ground to get
around 350V.....
Mikko Kankaanpää
Amp has four different high voltage windings. All about 260Vac. Those are
rectified with bridge rectifiers so that each gives about 340Vdc and then
those are filtered with 50UF cap to 100ohm resistor to 50UF cap. Two of
those are connected almost "normally" (- to cathodes and cathodes to ground
via output transformer) BUT two are connected so that - of bridge rectifier
(and two filter caps) are connected to + of another 340V suplly´s last
filter cap. Those two´s + are connected straight to plates of the power
tubes. So when measuring voltage between ground and plates it is something
like 680V. Those two normally connected supplies are connected to screen
grids.
Like I said that power amp circuit is not normal p-p. Cathodes are connected
to sides of output transformer and output transformers center tap are
connected to ground. I´m not quite sure how this circuit works.
Hope you understood something... I do not have schematics.
So I could use four different 340V HV supply for pre and power amp. With
four el34 output power should be enought fo me.
Thanks,
Mikko Kankaanpää
"Peter Völpel" <qro...@compuserve.de> wrote in message
news:3D4A7007...@compuserve.de...
Shiva
"Mikko Kankaanpää" <mikko.ka...@pp2.inet.fi> wrote in message
news:YVu29.236$fz.1...@read2.inet.fi...
> Hello,
>
> Amp has four different high voltage windings. All about 260Vac. Those are
> rectified with bridge rectifiers so that each gives about 340Vdc and then
> those are filtered with 50UF cap to 100ohm resistor to 50UF cap. Two of
> those are connected almost "normally" (- to cathodes and cathodes to
ground
> via output transformer) BUT two are connected so that - of bridge
rectifier
> (and two filter caps) are connected to + of another 340V suplly´s last
> filter cap. Those two´s + are connected straight to plates of the power
> tubes. So when measuring voltage between ground and plates it is something
> like 680V. Those two normally connected supplies are connected to screen
> grids.
>
> Like I said that power amp circuit is not normal p-p. Cathodes are
connected
> to sides of output transformer and output transformers center tap are
> connected to ground. I´m not quite sure how this circuit works.
>
> Hope you understood something... I do not have schematics.
>
>
> So I could use four different 340V HV supply for pre and power amp. With
> four el34 output power should be enought fo me.
>
>
> Thanks,
> Mikko Kankaanpää
>
Whoa there! Please don't change anything for a bit! What you have is
something rather interesting - the output tubes are running as
cathode-followers. This is not a commonly used configuration, since it
requires very high drive voltage (what's the driver tube, please measure V
RMS coming from it to the grids. A cathode follower 9almost everyone here
could explain it better than I) has an amplification factor <1, thus the
high drive voltage. Output tubes are basically used as current amps, and,
if you double-check your tranny, you'll see that the turn ratio is
significantly lower than that of a typical PP tranny, offerin' up all kinds
of advantages. If you could sketch the driver/OT stage diagram, I'd be
really interested / happy to help. Please don't convert the amp into yet
another trash-sounding EL34 wonder... I'd be glad to send you the parts
etc. to build a 50 or 100W EL34 amp, if you send me *your* chassis. I'd
even build one for you, to your specs... I'd really like to help you get
that thing going, though, and,if you have a digital camera, please send me
some pics of the amp /detailed underside, I could start guessing the
schematics from there...
-dim
Mikko Kankaanpää
> Whoa there! Please don't change anything for a bit!
Previous owner has already made some small changes while he was trying to
convert that to guitar amp. But It could be repaired...
What you have is
> something rather interesting - the output tubes are running as
> cathode-followers. This is not a commonly used configuration, since it
> requires very high drive voltage (what's the driver tube, please measure V
> RMS coming from it to the grids. A cathode follower 9almost everyone here
> could explain it better than I) has an amplification factor <1, thus the
> high drive voltage. Output tubes are basically used as current amps, and,
> if you double-check your tranny, you'll see that the turn ratio is
> significantly lower than that of a typical PP tranny, offerin' up all
kinds
> of advantages.
Well could it be cathode follower output stage cause it gives 110W output
power (as read on schematics). I got schematics on paper but do not have
scanner (or digi camera). Maybe I could redraw it with ivex windraft.
There are two ecc85 as drivers. The ouput transformer is a bit complicated
than usual. There are many taps that are not in use. I maesured impedances
with sine wave signal. But I´m not sure are there really output for speakers
or just 70/100V line output.
It´s made by Teleste (company is still going www.teleste.fi). Teleste made
audio (and telephone) eguipment for schools, hospitals etc. All I have saw
has similar kind of output stage with similar voltages. I paid 20 Euros for
it and have couple of years ago paid 25 Euros (150finmk) for teleste power
amp.
If you could sketch the driver/OT stage diagram, I'd be
> really interested / happy to help. Please don't convert the amp into yet
> another trash-sounding EL34 wonder... I'd be glad to send you the parts
> etc. to build a 50 or 100W EL34 amp, if you send me *your* chassis. I'd
> even build one for you, to your specs... I'd really like to help you get
> that thing going, though, and,if you have a digital camera, please send me
> some pics of the amp /detailed underside, I could start guessing the
> schematics from there...
Like tube amps usually do, these things are VERY heavy so postages will be
more than value of amps itself...
depending where you live... offcourse...
yours,
Mikko Kankaanpää
Peter Völpel
Very well, so two power-supply circuits are serial connercted, thats
fine,
in case you like to convert just connect them parallel for the anodes.
On your first writing you mention the amplifier to have 110W.
Reading your last one it seems to be something else, like Shiva
explained it could be a cathode follower which usually has a gain of
0.9;
But, the output transformer as you descibe would not make sense, as
its impedance should be much lower for a kathode-follower design.
I also would like to suggest, that you take the time to find out the
complete diagram starting at least from the driver + phaseinverter tube
and
also double check the output transformer
Peter
Mikko Kankaanpää
> On your first writing you mention the amplifier to have 110W.
> Reading your last one it seems to be something else, like Shiva
> explained it could be a cathode follower which usually has a gain of
> 0.9;
The original schematics says: 110W power amp schematics (from 1966). Or
those are not schematics for that amp but another teleste amp but have same
circuit than my amp.
> But, the output transformer as you descibe would not make sense, as
> its impedance should be much lower for a kathode-follower design.
I have to measure the output transformer again.
> I also would like to suggest, that you take the time to find out the
> complete diagram starting at least from the driver + phaseinverter tube
> and
I´ll send pdf schematics of power amp soon.
yours,
Mikko Kankaanpää
Mikko Kankaanpää
Here is pdf schematics of power stage. Sorry for very dirty drawning but
maybe you find out what it is all about... note that power supply - is
connected to cathode of power tubes. Two of power tubes have left out ; they
are connected same as others. pin numbers of tubes are wrong, i think...
http://personal.inet.fi/surf/ivan/schem/teleste.pdf
yours,
Mikko Kankaanpää
"Mikko Kankaanpää" <mikko.ka...@pp2.inet.fi> wrote in message
news:xNx29.316$fz.1...@read2.inet.fi...
Peter Völpel
Thank you Mikko,
I look into it tomorrow or tonight, leaving for ham-activities now
Peter
Gregg
<mikko.ka...@pp2.inet.fi> wrote:
>hello,
>
>Here is pdf schematics of power stage. Sorry for very dirty drawning but
>maybe you find out what it is all about... note that power supply - is
>connected to cathode of power tubes. Two of power tubes have left out ; they
>are connected same as others. pin numbers of tubes are wrong, i think...
>
>http://personal.inet.fi/surf/ivan/schem/teleste.pdf
I like that output stage.
Makes my little 12AT7 line driver stage seem not so mad after all :-)
--
Gregg
nntp://rec.audio.tubes
"It's probably useful - even if it can't be spiced."
Peter Völpel
Checking the diagram I can´t find too much sense in using a cathode
follower with the transformer in use as described by you.
The main advantage of using katode-follower audio amps would be in
omitting
capacitors and transformers in the signal path like the circlotron or
futterman OTL or similar.
With 2 parallel EL34 in push-pull the katode impedance should be about
180
ohms, so an autotransformer could be the right choice.
With your transformer with 1,7k minima impedance it hardly works right.
So either someone modified the layout or the transformer data you found
is wrong...
Peter
Mikko Kankaanpää
I measured those impedances asuming that output is for 4ohm speaker (that I
have), but it could be those 100V line that could drive loudspeakers (that
has own 100V to 4-8ohm transformer inside) far away from amplifier. Like in
schools that have amplifier(s) in one room and only speakers in class rooms.
I think normal 4-8ohm output is not capable to drive speakers with so long
speaker cable. I´m I all wrong?
So if i put 1 volt to secondary ands get 40V from primary. so 40/1=40
40*40=1600 and 1600*4=6400ohm so transformer is 6400 to 4ohm. But what is
the primary if secondary is 100volt line?
So Should I leave it like this or make it a istrument amp. It has good
chassis, so it would be very easy to convert.
Thanks!
yours,
Mikko Kankaanpää
"Peter Völpel" <qro...@compuserve.de> wrote in message
news:3D4C7A9A...@compuserve.de...
Patrick Turner
"Mikko Kankaanpää" wrote:
> Hello,
>
> I measured those impedances asuming that output is for 4ohm speaker (that I
> have), but it could be those 100V line that could drive loudspeakers (that
> has own 100V to 4-8ohm transformer inside) far away from amplifier. Like in
> schools that have amplifier(s) in one room and only speakers in class rooms.
> I think normal 4-8ohm output is not capable to drive speakers with so long
> speaker cable. I´m I all wrong?
>
> So if i put 1 volt to secondary ands get 40V from primary. so 40/1=40
> 40*40=1600 and 1600*4=6400ohm so transformer is 6400 to 4ohm. But what is
> the primary if secondary is 100volt line?
>
> So Should I leave it like this or make it a istrument amp. It has good
> chassis, so it would be very easy to convert.
If indeed the votage ratio is 40 to 1, then that's 6.4 k to 4 ohms.
I have come across a few 100 volt output amps.
Some have taps for 70 v and 50 v, and usually,
there is a seperate 4 ohm winding just for monitoring
where the operator with the mic was situated,
maybe a desk girl, whoever.
This means that if you have 6,400 ohms at the primary,
and the power is say 32 watts, then the signal voltage across the primary
is 452 vrms.
The voltage at the secondary will be 100 vrms, so the transformer
would have a turns ratio or voltage ratio of 4.52 to 1.
allowing for transformer losses, say 30 watts comes out of the sec,
then if the voltage is 100v, then the load at the sec is 333 ohms.
Thus you could run quite a few speakers each with their own transformer,
in a say a school, say one speaker of 3 watts to each class room,
and say you had ten rooms, then each transformer
has an input load of 3.3 k, transforming the 100 volts down to
whatever for the speaker, say 4.9 volts for 3 watts into 8 ohms.
The ten lots of 3.3 k in parallel makes the load equal the 330 ohms
load of the amp.
The reason for doing it this way means the losses in long cable runs are low,
since the cable impedance to load impedance ratio is high.
Patrick Turner.
Ken Gilbert
> and the power is say 32 watts, then the signal voltage across the primary
> is 452 vrms.
> The voltage at the secondary will be 100 vrms, so the transformer
> would have a turns ratio or voltage ratio of 4.52 to 1.
> allowing for transformer losses, say 30 watts comes out of the sec,
> then if the voltage is 100v, then the load at the sec is 333 ohms.
i've always wanted to use one of these as a PP driver tranny for a big
amp... you could drive the piss out of a couple of high mu class B
tubes. center tap could be a problem though.
ken