| Author |
Message |
Chedomir Jokanovich
Guest
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 Posted:
Tue Sep 13, 2005 4:45 am Post subject:
12au7 as SE output |
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Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
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Bret Ludwig
Guest
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| Posted:
Tue Sep 13, 2005 4:45 am Post subject:
Re: 12au7 as SE output |
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Fender guitar amps with reverb do this every day. I think they put
about 300mW into the reverb tank driver. |
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John Stewart
Guest
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| Posted:
Tue Sep 13, 2005 4:45 am Post subject:
Re: 12au7 as SE output |
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Chedomir Jokanovich wrote:
| Quote: | Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
|
Typical specs in the RCA tube manuals suggest -8.5 volts bias while running
a 250 volts on the plate for a total of a 258.5 volt plate supply, giving a
plate current of 10.5 ma for each section. If you use cathode bias the
proper cathode resistor would be (8.5 / 10.5) K or about 810R. But if you
run both sections of the 12AU7 in parallel the cathode resistor for the
pair would be (810 / 2) R or about 405R. Using a cathode resistor of 390R (
a standard value) would raise the plate dissipation marginally & reduce the
distortion a bit.
Low voltage electrolytic caps are cheap so use a 100 microfarad cap for the
cathode bypass. That gets that part of the circuit down to 4 Hz.
Cheers & Good Luck with you experiment. John Stewart |
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Patrick Turner
Guest
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| Posted:
Tue Sep 13, 2005 2:20 pm Post subject:
Re: 12au7 as SE output |
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Chedomir Jokanovich wrote:
| Quote: | Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
|
The load should be coupled via an OPT woth 8k:8 ohms.
Perhaps an old radio transformer will do.
You won't get more than about 1/5 a watt in class A1.
Maybe the tube would be good for headphones or a horn speaker...
Patrick Turner. |
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Chedomir Jokanovich
Guest
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| Posted:
Tue Sep 13, 2005 2:57 pm Post subject:
Re: 12au7 as SE output |
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I am going to use a litlle transformer with selectable primary
impedance (8k to 22k into 8Ohm)
The point of experiment is to use it as an interstage in a guitar amp,
when switched in, it would act as overloaded output stage, possibly
into reactive dummy load, and voltage from the secondary would be fed
to the normal phase inverter of a bigger amp (normally attenuated a
little). That way I am hoping to get that "crunchy" tube overloaded
sound at any volume level.
Also I would make an speaker output from that stage so I can use it
for practise at home, I recon half a watt (even more when overdriven)
into a pair of celestions with 97dB sensitivity will make nice practice
volume
thanks for your help |
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flipper
Guest
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| Posted:
Tue Sep 13, 2005 6:42 pm Post subject:
Re: 12au7 as SE output |
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On Tue, 13 Sep 2005 09:20:17 GMT, Patrick Turner
<info@turneraudio.com.au> wrote:
| Quote: |
Chedomir Jokanovich wrote:
Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
The load should be coupled via an OPT woth 8k:8 ohms.
Perhaps an old radio transformer will do.
You won't get more than about 1/5 a watt in class A1.
Maybe the tube would be good for headphones or a horn speaker...
|
I'm curious. Why do you say 1/5 of a watt is about all? Is it
predicated on a particular distortion level?
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Patrick Turner
Guest
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| Posted:
Tue Sep 13, 2005 8:36 pm Post subject:
Re: 12au7 as SE output |
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Chedomir Jokanovich wrote:
| Quote: | I am going to use a litlle transformer with selectable primary
impedance (8k to 22k into 8Ohm)
The point of experiment is to use it as an interstage in a guitar amp,
when switched in, it would act as overloaded output stage, possibly
into reactive dummy load, and voltage from the secondary would be fed
to the normal phase inverter of a bigger amp (normally attenuated a
little). That way I am hoping to get that "crunchy" tube overloaded
sound at any volume level.
Also I would make an speaker output from that stage so I can use it
for practise at home, I recon half a watt (even more when overdriven)
into a pair of celestions with 97dB sensitivity will make nice practice
volume
thanks for your help
|
Use about 220V across the tube and about 8-10mA per half,
so adjust the bias R to get the Ia.
then with the dummy load make the final adjust so it clips
on + and - peaks at about the same level.
Using a wire wound pot at the dummy load can change the sound a bit.
Patrick Turner. |
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Patrick Turner
Guest
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| Posted:
Tue Sep 13, 2005 10:44 pm Post subject:
Re: 12au7 as SE output |
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flipper wrote:
| Quote: | On Tue, 13 Sep 2005 09:20:17 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
Chedomir Jokanovich wrote:
Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
The load should be coupled via an OPT woth 8k:8 ohms.
Perhaps an old radio transformer will do.
You won't get more than about 1/5 a watt in class A1.
Maybe the tube would be good for headphones or a horn speaker...
I'm curious. Why do you say 1/5 of a watt is about all? Is it
predicated on a particular distortion level?
|
Its a rough guess based on typical triode efficiency.
If you have 2 watts of idle plate dissipation in an average triode,
then maximum plate efficiency in SET class A1 and limited by
3% thd is about between 20% and 30%.
EL34 can have a rated Pda = 28 watts, so expect a maximum
7 watts in triode, A1, 3% thd.
Patrick Turner.
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Chedomir Jokanovich
Guest
|
| Posted:
Wed Sep 14, 2005 3:20 am Post subject:
Re: 12au7 as SE output |
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Actually, from my experience with el34, you can get only 5-6w and stay
bellow 5%thd in triode conection. That's what i got from EH el34's.
But I biased them around 24-25W at idle, to keep tubes running longer.
Chedomir Jokanovich |
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flipper
Guest
|
| Posted:
Wed Sep 14, 2005 10:44 am Post subject:
Re: 12au7 as SE output |
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On Tue, 13 Sep 2005 17:44:18 GMT, Patrick Turner
<info@turneraudio.com.au> wrote:
| Quote: |
flipper wrote:
On Tue, 13 Sep 2005 09:20:17 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
Chedomir Jokanovich wrote:
Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
The load should be coupled via an OPT woth 8k:8 ohms.
Perhaps an old radio transformer will do.
You won't get more than about 1/5 a watt in class A1.
Maybe the tube would be good for headphones or a horn speaker...
I'm curious. Why do you say 1/5 of a watt is about all? Is it
predicated on a particular distortion level?
Its a rough guess based on typical triode efficiency.
If you have 2 watts of idle plate dissipation in an average triode,
then maximum plate efficiency in SET class A1 and limited by
3% thd is about between 20% and 30%.
EL34 can have a rated Pda = 28 watts, so expect a maximum
7 watts in triode, A1, 3% thd.
|
OK. So my guess about a distortion factor wasn't outlandish. I presume
you could increase that with some NFB?
What about positive grid drive? |
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John Stewart
Guest
|
| Posted:
Wed Sep 14, 2005 3:37 pm Post subject:
Re: 12au7 as SE output |
|
|
flipper wrote:
| Quote: | On Tue, 13 Sep 2005 17:44:18 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
flipper wrote:
On Tue, 13 Sep 2005 09:20:17 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
Chedomir Jokanovich wrote:
Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
The load should be coupled via an OPT woth 8k:8 ohms.
Perhaps an old radio transformer will do.
You won't get more than about 1/5 a watt in class A1.
Maybe the tube would be good for headphones or a horn speaker...
I'm curious. Why do you say 1/5 of a watt is about all? Is it
predicated on a particular distortion level?
Its a rough guess based on typical triode efficiency.
If you have 2 watts of idle plate dissipation in an average triode,
then maximum plate efficiency in SET class A1 and limited by
3% thd is about between 20% and 30%.
EL34 can have a rated Pda = 28 watts, so expect a maximum
7 watts in triode, A1, 3% thd.
OK. So my guess about a distortion factor wasn't outlandish. I presume
you could increase that with some NFB?
What about positive grid drive?
|
You would need to use +ve grid drive if the tube were a high mu triode such as a
12AX7 while in SE operation in order to get significant output. That is called
Class A2 operation. But be careful of the plate dissipation that results!
The subject 12AU7 amp wherein both section of a 12AU7 are to be paralleled
doesn't need +ve grid drive in order to deliver 350 to 400 mw after losses in the
OPT.
Either 12AU7 or 12AX7 in pushpull with +ve grid drive will deliver significant
output. Some amateur transmitters from the 50's used 12AX7's running as Class B
zero bias triodes, where the grids are driven +ve. They claim 5 to 7 watts while
the plate supply is at 300 volts. In that kind of application, distortion is not
an important consideration. You could do something similar with a 12AU7. On peaks
it is a sure thing that plate dissipation would be exceeded.
Cheers, John Stewart |
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John Stewart
Guest
|
| Posted:
Wed Sep 14, 2005 3:45 pm Post subject:
Re: 12au7 as SE output |
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John Stewart wrote:
| Quote: | What about positive grid drive?
You would need to use +ve grid drive if the tube were a high mu triode such as a
12AX7 while in SE operation in order to get significant output. That is called
Class A2 operation. But be careful of the plate dissipation that results!
The subject 12AU7 amp wherein both section of a 12AU7 are to be paralleled
doesn't need +ve grid drive in order to deliver 350 to 400 mw after losses in the
OPT.
Either 12AU7 or 12AX7 in pushpull with +ve grid drive will deliver significant
output. Some amateur transmitters from the 50's used 12AX7's running as Class B
zero bias triodes, where the grids are driven +ve. They claim 5 to 7 watts while
the plate supply is at 300 volts. In that kind of application, distortion is not
an important consideration. You could do something similar with a 12AU7. On peaks
it is a sure thing that plate dissipation would be exceeded.
Cheers, John Stewart
|
I should have mentioned that if you were to run a 12AU7 in PP Class B you would need
to set the grid bias to something like -20 volts for 300 volts plate. The bias would
need to be fixed. JLS |
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Patrick Turner
Guest
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| Posted:
Wed Sep 14, 2005 5:54 pm Post subject:
Re: 12au7 as SE output / SET EL34 set up. |
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Chedomir Jokanovich wrote:
| Quote: | Actually, from my experience with el34, you can get only 5-6w and stay
bellow 5%thd in triode conection. That's what i got from EH el34's.
But I biased them around 24-25W at idle, to keep tubes running longer.
|
You can set the EL34 up with Ea = 450V, and Ia = 50mA, then Pda = 22.5W,
which includes both anode and screen dissipation.
Then if you load the tube with 6,400 ohms, you should get a class A1 anode
swing of
+/- 320V peak = 226Vrms which is 8 watts into 6.4k.
Thd at 5 watts will be less than 5%, and the ration between RL and Ra =
6,400 / 1,250 = 5.12,
which is a very nice damping factor.
Max efficiency = 35%
Now try setting the EL34 up with Ea = 360V and 63mA for the same Pda =
22.5 watts
and you'll find that RL would ideally be 3,800 for symetrical clipping,
high power and medium thd.
Max efficiency = 23%
V swing = 200Vpeak = 141 vrms giving 5.23 watts, class A1,
and thd will far greater than the case for Ea = 450V and RL = 6.4k.
The reason the lower efficiency as Ea becomes lower is that the anode
swing is limited by the
Ra boundary as shown on the data sheets for Eg1 = 0V.
With tubes like KT88 or KT90 the situation
gets better because Ra is lower than EL34, so the slope of the
Ra line for Eg1 = 0V is steeper, which encourages a wide anode swing.
Tubes like KT66 in triode have Ra = 1.6k, and thus anode swing is more
restricted.
EL36 or 6CM5 have Ra = 600 ohms, less than a 2A3 or 300B,
so Ea can be 375V, Pda = 18 watts, and po into 5.4k is 7 watts at
clipping.
And this brings around to the 300B, and then we use Pda = 32 watts, Ea =
400V, Ia = 80mA,
and RL can be as low as 2.7k for 8.1 watts, but
its better to use Ea slightly higher, Ia lower, RL higher and not worry
if the tube clips asymetically because for hi-fi the first few watts are
all important.
The same principles apply to using 12AU7, 6SN7, 12BH7 for low po amps.
Patrick Turner.
| Quote: |
Chedomir Jokanovich |
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Patrick Turner
Guest
|
| Posted:
Wed Sep 14, 2005 7:37 pm Post subject:
Re: 12au7 as SE output |
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flipper wrote:
| Quote: | On Tue, 13 Sep 2005 17:44:18 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
flipper wrote:
On Tue, 13 Sep 2005 09:20:17 GMT, Patrick Turner
info@turneraudio.com.au> wrote:
Chedomir Jokanovich wrote:
Hi, first of all, I am new to this group (infact this is my first
post). I really enjoy reading it, and it helped me so much, so many
times in the past. I'm glad to be here.
I am contenplating a little experiment of my own:) which may prove
usefull to me.
I was thinking of using a 12au7 as an output tube in a SE amp. Driving
around 8-10k load, with 250v supply. I figure it will give at least
half a watt that way. Now I'm not sure how to bias it propertly.
I was thinking maybe a 1kOhm bias resistor with a 10uF bypass cap. That
way it will dissipate around 2w idle. Is that ok?
Thanks
Chedomir Jokanovich
The load should be coupled via an OPT woth 8k:8 ohms.
Perhaps an old radio transformer will do.
You won't get more than about 1/5 a watt in class A1.
Maybe the tube would be good for headphones or a horn speaker...
I'm curious. Why do you say 1/5 of a watt is about all? Is it
predicated on a particular distortion level?
Its a rough guess based on typical triode efficiency.
If you have 2 watts of idle plate dissipation in an average triode,
then maximum plate efficiency in SET class A1 and limited by
3% thd is about between 20% and 30%.
EL34 can have a rated Pda = 28 watts, so expect a maximum
7 watts in triode, A1, 3% thd.
OK. So my guess about a distortion factor wasn't outlandish. I presume
you could increase that with some NFB?
What about positive grid drive?
|
EL34 doesn't like being pushed by grids forced into running +ve.
When i have tried it i didn't thiunk much was to be gained.
5881, 6L6, KT66, EL36, 6550, KT88 all don't mind
class A2, and also 12AX7, AU7, AT7, BH7, etc all go well in class A2
Tim DeParavicini sold a 25 watt amp with 20 x 12AX7 within for the
output stage, using "enhanced" triode mode which is really class A2,
and in his case it was push pull, whith very little class A1, but a lot of AB2,
using a a "unity coupled" OPT like you see in a McIntosh amp.
This is a case of a shirtload of local NFB which reduces thd Rout.
This guy also designed the EAR509 amp which also has unity coupling with a pair
of PL509s
which would give 120 watts. It had a total of 44 dB of NFB in 3 loops,
but it measured no better than using a pair of 6550 in a far simpler UL AB1
circuit.
But getting back to your idea of an SET amp with a lone 12AU7,
its doable, but you need to analyse the effects of load by means of load line
analysis
to get best results.
Ppl used to use a 6SN7 in preference to a 12AU7, even though the two tubes can be
very similar.
Some 12AU7 have large long plates rather like the 6SN7 which can dissipate more
power than a 12AU7.
The again some 12AU7 I have seen have very small plates, and obviously can't
dissipate
as much power as the larger plated versions.
Triodes with a given Ra, Gm, and µ can be made quite large, or quite tiny,
and for small signal use the small plate versions can offer advantages of
lower material costs and also lower electrode capacitance.
In your case you should only use the largest anode versions you can find.
Patrick Turner. |
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Bob
Guest
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| Posted:
Wed Sep 14, 2005 11:43 pm Post subject:
Re: 12au7 as SE output |
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6CG7 is quite similar to the 12AU7, and can handle higher power levels.
It's really a 6SN7 in a miniature package (though the max plate
voltage is a little lower though). 8CG7s can be had for quite cheap,
and you can easily power its heater from a 12.6V AC source if you
connect the 8CG7's heater in series with a rectifier diode (like a
1N4004) without any filter cap. Remember, power is volts
squared/resistance. |
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