Hi

Can anyone tell me looking at the circuit diagram for an audio note
conquest he
http://www.drtube.com/schematics/an/conquest.gif
what the HT should be?
I'm piecing a pair together slowly, output tubes still on order. Tried a
couple of 10k power resistors in place of the 300b's and get an HT of
500V, which seems way too high.
Looking at the 825ohm cathode resistors i guess I'll be looking at
roughly 50v bias? seem low? As the choke is rated 150mA none of this
seems to fit! I dont really want to fry a couple of 300bs the day they
arrive.
Looking at the P4 he http://www.drtube.com/schematics/an/p4mono.gif
it has an HT of 420V, similar circuit, so why the hell am i gettign 500V?

Any help would be apreciated

Thanks, Chris

Hi RATs!

It may be clear to you by now that things are not as simple as you thought.
There is more to be done than spending money.

Read some books, build some simple circuits using cheap tubes.

Don't come in here screaming rape when the meter confuses you.

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Chris Parkin wrote:

Hi

Can anyone tell me looking at the circuit diagram for an audio note
conquest he
http://www.drtube.com/schematics/an/conquest.gif
what the HT should be?
I'm piecing a pair together slowly, output tubes still on order. Tried a
couple of 10k power resistors in place of the 300b's and get an HT of
500V, which seems way too high.

Well, first off, do you have the 47ohm resistor coming out of the
rectifier? That is supposed to mimic a *choke input* filter. Note in
the second schematic, one version *is* a choke input filter. Use
the choke if you can. It will definitely reduce the DC voltage
compared to a capacitor input filter and improve the reduction
of B+ ripple.

Secondly, there is often a *big* difference between *no load* voltage
and voltage with a load. Draw the requisite current with your test
resistors and see where the B+ ends up. Your 10k resistors ought to
pull 50ma each - are you using one for each tube? Does the B+
drop at all when they are in??

ALSO, check your line
voltage accuracy and the secondary AC volts of your transformer.
it all can add up to a fair amount of voltage... 10% variation @ 500vdc
is 50 volts, eh?

Regardless, the 300B will likely handle the 500 volts...

AND, the voltage that the tube most cares about is the voltage between
the Plate and the Cathode, not the voltage between the Plate and Ground,
which is something to be considered here... unless of course you can push
the Cathode voltage down to near ground potential while the B+ stays
too high and draw current at the same time. Hard to do in a self biased amp though.

As mentioned, it might be a good idea to dork with some triode strapped
cheap tubes, like 6L6, 807 or similar to get your feet wet without having
to worry about blowing up 300Bs... even chinese ones.

_-_-bear





Looking at the 825ohm cathode resistors i guess I'll be looking at
roughly 50v bias? seem low? As the choke is rated 150mA none of this
seems to fit! I dont really want to fry a couple of 300bs the day they
arrive.
Looking at the P4 he http://www.drtube.com/schematics/an/p4mono.gif
it has an HT of 420V, similar circuit, so why the hell am i gettign 500V?

Any help would be apreciated

Thanks, Chris

--
_-_- BEAR Labs
- Custom Audio Equipment, Cables, Mods, Repairs -
http://www.bearlabs.com

Chris Parkin wrote:

Hi

Can anyone tell me looking at the circuit diagram for an audio note
conquest he
http://www.drtube.com/schematics/an/conquest.gif
what the HT should be?
I'm piecing a pair together slowly, output tubes still on order. Tried a
couple of 10k power resistors in place of the 300b's and get an HT of
500V, which seems way too high.
Looking at the 825ohm cathode resistors i guess I'll be looking at
roughly 50v bias? seem low? As the choke is rated 150mA none of this
seems to fit! I dont really want to fry a couple of 300bs the day they
arrive.
Looking at the P4 he http://www.drtube.com/schematics/an/p4mono.gif
it has an HT of 420V, similar circuit, so why the hell am i gettign 500V?

Any help would be apreciated

Thanks, Chris

The 300B is often set up with 400v between anode and cathode,
and about 90 mA of plate current, which give a dissipation of 36 watts at
idle.
The 90 mA flowing in the cathode resistors which total 825 ohms on your
schematic
give a cathode voltage of 74v, so the B+ supply has to be about +475v.
So two tubes draw 180mA, and I worry that your choke is rated well enough.

So one tube and a cathode R is equivalent to 5.278 k ohms, and two
such tubes in parallel are equal to 2.638 k ohms.

I suggest that if you place a 100 watt rated 2.7k across your B+ power supply

then the voltage will sag to about 475v, ie, about right for what you want.

The 1k25 value for the primary load of the OPT seems way to low
for these tubes, 2k5 : 5 ohms for the pair of tubes would be more
appropriate.
The 2.5 k tranny will give an Rout of about 1.8 ohms, with no nfb loops,
including winding resistances.
A load of 2.5k for the two tubes will give about 13 watts,
but if the load drops to 1.25 k, which it probably will for
with many speakers along the band, then the power will be 16 watts,
but the thd is a lot higher.


The other thing that looks not quite right, ( imho ), is the 6SN7 direct
coupled driver stage.
To power the 300B, you need about 50vrms applied to its grid,
and for two tubes, a single 6SN7 with both halves paralleled would be a
better option
than one lone half tube.
Without the direct coupling shown on your schematic,
you could get more voltage swing at the 6SN7 anode.

You could take a look at the circuit at
http://www.turneraudio.com.au/htmlwe...hemseul22w.htm
This isn't a 300B circuit, but it does have a driver circuit you could use,
if you are able to follow the general principles.
A 6SN7 can be used instead of the ECC32.
I have nfb applied in this amp of mine since the Rout of the UL amp
is too high without it.
In your case, if little or no NFB is to be used, then a 6SN7 could be used as

the input tube, since the sensitivity will be about right.
With a 12SL7/6SL7 input tube, and 6SN7, there is no
real need to have a pramp, even with nfb applied as I have it in my
schematic.

I have heard many very good 300B amps.

Happy listening,

Patrick Turner.

Thanks for the advice guys, certainly given me food for thought. I'll
probably increase the cathode resistors a little, and maybe put a 500 ohm
after the choke to drop the ht a little (also a little worried about the
6sn7's). And I have a mate with a variac, so i can crank it up slowly.
I have built a couple of kits in the past, a 5881 pp and the audio note
pre amp kit, just soldering stuff together as per the instructions didn't
give me much of a feeling of input though, so this is more like it!
I wasn't expecting a walk in the park, though maybe i should have gone for
something a little more run of the mill. I have been picking at Morgan
Jones' book for the last 6 months, though he doesn't dwell much on either
single ended or power triodes, yet alone zero nfb. Ploughed through a lot
of theory on the net which has been useful.
I would spend more time experimenting, but i haven't really got the space
to leave things set up, got the guts of a valve electronic organ in the
cupboard, ht is about 750v unloaded which scares the crap out of me!
though looking at the size of the choke I think it's probably designed as
a choke input, which should bring it down a little. Might dig it out
again soon.
Anyway thanks again Patrick, bear and Andre.

Cheers, Chris

Chris Parkin wrote:

Thanks for the advice guys, certainly given me food for thought. I'll
probably increase the cathode resistors a little, and maybe put a 500 ohm
after the choke to drop the ht a little (also a little worried about the
6sn7's). And I have a mate with a variac, so i can crank it up slowly.

The problem with a variac and tubes is that the cathode emissions
rely on adequte heating, and only full supply voltage conditions allow you to
realistically examine the working signal voltages.
If you are nervous about applying full voltages, then get the
driver stage working separately, without the 300B in their sockets.
The driver circuit you have is a bit flexible with B+.

When you are happy with the drive circuit, plug in the 300B,
and then measure the rise in voltage
across the cathode resistors right after turn on, and if it exceeds
what it should be, then turn off, and check your circuit.
After that, and you have it operational, and without motorboating
oscillations, you can fiddle with supply voltages and plate current,
to best suit the load you have.
But just remember, that the load your amp will see is a speaker load, which
means that the refelected load of the tube varies as much as the speaker load,

and while 1.25k might be the nominal load given to the tubes via the OPT,
the load may dip to 800 ohms, and rise to 8 kohms.
Choice of operating point for a load takes some understanding,
but there is little difference in principle
between a 1/2 12AX7 or a 300B.


I have built a couple of kits in the past, a 5881 pp and the audio note
pre amp kit, just soldering stuff together as per the instructions didn't
give me much of a feeling of input though, so this is more like it!
I wasn't expecting a walk in the park, though maybe i should have gone for
something a little more run of the mill. I have been picking at Morgan
Jones' book for the last 6 months, though he doesn't dwell much on either
single ended or power triodes, yet alone zero nfb. Ploughed through a lot
of theory on the net which has been useful.
I would spend more time experimenting, but i haven't really got the space
to leave things set up, got the guts of a valve electronic organ in the
cupboard, ht is about 750v unloaded which scares the crap out of me!
though looking at the size of the choke I think it's probably designed as
a choke input, which should bring it down a little. Might dig it out
again soon.
Anyway thanks again Patrick, bear and Andre.


Just be careful with the B+,
we don't want to read about you in the orbiturary columns.

Patrick Turner.


Cheers, Chris

Chris Parkin wrote:

Thanks for the advice guys, certainly given me food for thought. I'll
probably increase the cathode resistors a little, and maybe put a 500 ohm
after the choke to drop the ht a little (also a little worried about the
6sn7's). And I have a mate with a variac, so i can crank it up slowly.
I have built a couple of kits in the past, a 5881 pp and the audio note
pre amp kit, just soldering stuff together as per the instructions didn't
give me much of a feeling of input though, so this is more like it!
I wasn't expecting a walk in the park, though maybe i should have gone for
something a little more run of the mill. I have been picking at Morgan
Jones' book for the last 6 months, though he doesn't dwell much on either
single ended or power triodes, yet alone zero nfb. Ploughed through a lot
of theory on the net which has been useful.
I would spend more time experimenting, but i haven't really got the space
to leave things set up, got the guts of a valve electronic organ in the
cupboard, ht is about 750v unloaded which scares the crap out of me!
though looking at the size of the choke I think it's probably designed as
a choke input, which should bring it down a little. Might dig it out
again soon.
Anyway thanks again Patrick, bear and Andre.

Cheers, Chris

In addition to my last advice, if you test the drive amp without the
output tubes plugged in, the B+ will rise to quite a higher value
than the normal operating value.
With only 20 mA of current supplied to the drive amp,
its supply voltage will be up around +550v perhaps.
I assume the PS seriesed electros are each rated for +350vwkg
The 6SN7 should cope with that B+, but I would
place in some high powered resistor to replace the current draw of
the output tubes.
I sometimes use a couple of 25,40, or 60 watt 240v lightbulbs in series.
For the US, 4 x 110v bulbs are used.
If two 40 watt bulbs result in a voltage between the OPT and cathode pin
equal to say 380v, and the cathode R indicates say 170 mA,
this means that the power in the resistor is 64.6 watts, and the value of the
R in the lamps is 2.235 k. An R of 2.2 k could be used as a check,
but ithas to be a rugged R for long period testing, which is why I like lamps,

and after trying a couple of different values one can guess about what is
possible
to get with the tubes plugged in.
If the tubes are inserted, the cathode bias R value could be adjusted to
produce the same
current, or near it.

BTW, a 500 ohm series R after the choke will cause a B+ drop of 75 volts
at 150 mA.
That value could be fiddled with, to trim the final working point of the tube.

One can always place 500 ohms in, and shunt it with other R later,
or remove it.

I don't think you should have too much trouble with the circuit
as it is, although I think the driver stage with half a 6SN7
having to produce around 50vrms is driving the tube too hard.
Its total signal load is 19.5k plus the 100k in parallel,
which is 16.6 k, and 70 peak volts makes a 4.2 mA peak current change,
and thd will be quite high, maybe 6 %.
The supply plate voltage for the second 1/2 of the 6SN7 driver tube
is not from the HT of the PS to 0V, because there is 140v across the
19 k cathode R of the driver. The anode current is around 7.2 mA,
so the effective B+ of the 2nd triode is the HT-140v, approx, which
is probably around 310v working.
My load line analysis reveals the driver barely makes
65 peak volts swing before driver grid current flows.
But the designer may have deliberately chosen the working
condition to cause some 2H cancelation when the 2H of the driver stage is fed
into
the 300B, which are not without their thd at full output,
perhaps about 7%.
Theoretically, if both stages had the same % of 2H,
then the output should be free of 2H, but rarely
does the 2H increase of each stage occur to make perfect 2H cancelation
from low power to high power.
Sometimes one gets more 3H and 4H after trying to achieve
distortion cancelling.
In my SEUL amps, I designed the driver stage for reasonable
linearity, and a large v swing ability, with little regard for 2H cancelation.

But in my latest SE amp with CFB windings, the 2H in the output stage is
only 2% at clip, so the 2H from the driver gives significant
2H cancelation, without having to set up a driver triode
with a high Ia and low value RL.
I found it was difficult to make the 2H track each other
exactly over a wide power range, or with different value speaker loads.

The bottom line is how it all sounds, and 12 watts max from two 300B
should be possible, and at 2 watts, with sensitive speakers,
the music should be excellent.
Its not too hard to change the circuit around, and I have a friend with a GM70
SET,
with 2A3 driver, 417A input, and he changes tubes, caps, loads, chokes et all
almost daily, with a breadboard arrangement spread all over the lounge room
floor.
SE is so simple, it is all doable with aligator clipped leads.
I get a frown when I say,
"Geez Doug, those alligator clips sound a bit leathery and swampy."

Some say the 300B is bass shy, and top end lazy,
but one dies for the midrange.
In a comparison recently with a renowned SS amp
of 100w per/ch which was made locally, a chinese tubed ValveMark
amp with a single 300B/ch was superior in its bass response,
and had no trouble with the highs.
300B amps are not for the head bangers, the SS is right for that,
but for easy listening, 300B is OK.

Patrick Turner.