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7A5 is a good driver for the 300B ?

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aldovan

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Joined 2005
#1
Friends

I have a lot of 7A5 tubes and I like of this pentode sound.
This tube is sufficient for driver one 300B in single ended mode ?
My idea is use 150V in the plate and screen.
This tube work with 150V and 7W dissipation without problems.

Thanks,

Aldovan
 
#4
814 in triode mode

Vinylsavor said:
Hi!

A driver tube with only 150V on the plate and screen will not be able to produce enough voltage swing to drive a 300B to full power let alone some headroom.

Best regards

Thomas
Click to expand...

Thomas

I have other project with a 814 in triode mode.
The curves are these: http://www.tubebooks.org/file_downloads/814 curves.gif
In this case the swing is 80V, them the 7A5 is ok ?

Thanks,

Aldovan
 
#5
Hi Aldovan,

it depends on the circuit of the driver tube and your op point how much voltage swing you can get out of it.

I generally prefer driver stages which have lots of headroom. I shoot for 6dB headroom in the driver, which means I chose a driver which can theroetically swing twice the voltage which is needed.

Best regards

Thomas
 
#6
Vinylsavor said:
Hi Aldovan,

it depends on the circuit of the driver tube and your op point how much voltage swing you can get out of it.

I generally prefer driver stages which have lots of headroom. I shoot for 6dB headroom in the driver, which means I chose a driver which can theroetically swing twice the voltage which is needed.

Best regards

Thomas
Click to expand...

Thomas

But the current in the driver is important, no ?
This power tubes have big input capacitance.

Aldovan
 
#7
aldovan said:
Thomas

I have other project with a 814 in triode mode.
The curves are these: http://www.tubebooks.org/file_downloads/814 curves.gif
In this case the swing is 80V, them the 7A5 is ok ?

Thanks,

Aldovan
Click to expand...

You need 160Vpp min to get 10W out of this valve with a 6K5 OT. This is A2 operation. Grid current will peak about 27-30mA in A2. So guess what type of driver you need then 🙂
cheers,
ale
 
#8
Driving a 300B is not an easy task. The driver will need to swing nearly 200 Vpp with low distortion. It will also need to be able to source a bit of current. In my driver circuit (see my website below) I wanted a single stage amplification followed by a cathode follower for the current drive capability. This puts even more stringent requirements on the tubes.

For the common cathode (amplification) stage, you need a tube with medium to medium-high mu. Somewhere in the 20~40 range is about right. This tube needs to be able to generate 200 Vpp swing at low THD.

For the cathode follower, you want a high gm tube for low output impedance. Gm > 10 mS is nice. This will also need to be able to swing 200 Vpp.

On top of that, if you - like I - want to use a dual triode driver tube, you need a tube that can handle a half-decent voltage from cathode to heater.

So... Medium/medium-high mu, high gm, high Vhk, ability to swing 200 Vpp at low distortion. I went through quite a few tubes and those I found fit the bill were: JJ ECC99, 6N6P, 12BH7A, and E88CC/ECC88/6DJ8/6922. You can use pair of triode coupled d3A as well.

~Tom
 
#9
tomchr said:
Driving a 300B is not an easy task. The driver will need to swing nearly 200 Vpp with low distortion. It will also need to be able to source a bit of current. In my driver circuit (see my website below) I wanted a single stage amplification followed by a cathode follower for the current drive capability. This puts even more stringent requirements on the tubes.

For the common cathode (amplification) stage, you need a tube with medium to medium-high mu. Somewhere in the 20~40 range is about right. This tube needs to be able to generate 200 Vpp swing at low THD.

For the cathode follower, you want a high gm tube for low output impedance. Gm > 10 mS is nice. This will also need to be able to swing 200 Vpp.

On top of that, if you - like I - want to use a dual triode driver tube, you need a tube that can handle a half-decent voltage from cathode to heater.

So... Medium/medium-high mu, high gm, high Vhk, ability to swing 200 Vpp at low distortion. I went through quite a few tubes and those I found fit the bill were: JJ ECC99, 6N6P, 12BH7A, and E88CC/ECC88/6DJ8/6922. You can use pair of triode coupled d3A as well.

~Tom
Click to expand...

Tom

Thanks for the information, but I prefer pentodes for driver the 300B.
About the triode tubes, I prefer the 6N6P and 6BL7..

Aldovan
 
#13
Hi Aldovan,

as mentioned above, there is much more than just the driver tube. How do you want to use it? Pentode or Triode? RC, LC or transformer coupled? Or with CCS, or cathode follower or .....

I am not familiar with this tube so cannot share experience about it, but I'd recommend that you first clarify some more of what you intend to do, then a sensible advice can be given.

Best regards

Thomas
 
#14
Amplifier Sketch

Vinylsavor said:
Hi Aldovan,

as mentioned above, there is much more than just the driver tube. How do you want to use it? Pentode or Triode? RC, LC or transformer coupled? Or with CCS, or cathode follower or .....

I am not familiar with this tube so cannot share experience about it, but I'd recommend that you first clarify some more of what you intend to do, then a sensible advice can be given.

Best regards

Thomas
Click to expand...

Thomas

This is a sketch of my project:

Aldovan
 

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  • 814 triode SE.jpg
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#18
Hi!

aldovan said:
Thomas

I see in your site and you make experiences with the beam triodes , too.
See my prototype with the 6HZ5, this tube sound very fine:
6HZ5 A2 class SE amplifier - YouTube
I have about 200 6HZ5 for experiments, is a cheap, fine and forgotten tube !
Click to expand...

Yes! It is a great tube. I mostly used the 6HS5 which is very similar to 6HZ5 it just has a bit less max plate dissipation and therefore also needs less heater current. I even used it in phono stages.

I will post about the 6HV5A, another variant of this beam triode family as tube of the month soon.

Best regards

Thomas
 
#19
Vinylsavor said:
Hi!



Yes! It is a great tube. I mostly used the 6HS5 which is very similar to 6HZ5 it just has a bit less max plate dissipation and therefore also needs less heater current. I even used it in phono stages.

I will post about the 6HV5A, another variant of this beam triode family as tube of the month soon.

Best regards

Thomas
Click to expand...

Thomas

Yes, this tubes have a big gain, about 300 and is ok for phono stages.
Do you have a schematic of a phono stage with the 6HS5 ?
In my project I use a transistor in emitter follower for driver the 6HZ5, but a CF work fine, too. Or you can use without driver, because the gain is high.

Aldovan
 
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