RH84 SE, pentode driver... advice please

[bigjppop]

I think I'm going to try my hand at the RH84 SE for my first scratch built, point to point wired project:

RH 84 - Tube Audio ...... RH DESIGN

I've read lots of great opinions on this one, particularly that it responds very well to a pentode in the driver position (I've seen the 6AU6 mentioned a couple of times). Can anyone enlighten a newbie on what would be required to swap out the driver for a pentode?

I'm still debating the iron; I've sent emails to both O'Netics and Electraprint for quotes on a complete set of transformers and choke. I would like to try one of their offerings if its not too far out of budget. I can always fall back on a full setup from Edcor.

With the exception of the possible pentode driver, I'll be following the original schematic pretty closely. I'm planning on using a 5V4 for the rectifier, but I am a little curious on the choke. In the 20+ different threads I've read by people that have built this amp, it seems that most use whatever choke happens to be sitting in the parts box. The schematic calls for a 20H choke, minimum of 50mA; that seems to be a little bit of challenge to find. How critical is that value?

Thanks in advance for any thoughts; I always appreciate the assistance I get from this forum.

[Soonerorlater]

To see the effect of playing about with different values of chokes why dont you give Duncan psu simulator a try.


Download


50ma might do for a single channel - is this a single channel project?


I used 10H @ 200ma which gives me the option of using this choke in a future project if the need arises

Brgds Bill

[Tom Bavis]

RadioDaze has a 8 Hy 125 mA choke for cheap... $10.95. Custom Choke As mentioned above, 50 mA is only enough for one channel.

The choke and the output capacitor determine the final supply ripple. Bigger output C costs less than a bigger choke...

[bigjppop]

Originally Posted by Tom Bavis

RadioDaze has a 8 Hy 125 mA choke for cheap... $10.95. Custom Choke As mentioned above, 50 mA is only enough for one channel.

The choke and the output capacitor determine the final supply ripple. Bigger output C costs less than a bigger choke...

Thanks for that link; I think I have seen that choke mentioned somewhere else for this amp, but they mentioned the idea of using two of them in some sort of a pseudo dual mono power supply; I think that might be beyond my skill level (maybe not to implement, but to design).

So, I could go with a smaller choke (minimum 100 mA for a stereo setup) and a larger cap; correct? I know I used a motor run cap in my Tubelab Simple SE for good effect, but that was a situation of "replace this cap with this cap." I'll probably have to do some learnin' to figure out the best way to manipulate cap vs. choke in this setup.

[quikie22]

Originally Posted by bigjppop

Thanks for that link; I think I have seen that choke mentioned somewhere else for this amp, but they mentioned the idea of using two of them in some sort of a pseudo dual mono power supply; I think that might be beyond my skill level (maybe not to implement, but to design).

So, I could go with a smaller choke (minimum 100 mA for a stereo setup) and a larger cap; correct? I know I used a motor run cap in my Tubelab Simple SE for good effect, but that was a situation of "replace this cap with this cap." I'll probably have to do some learnin' to figure out the best way to manipulate cap vs. choke in this setup.

For the RH84 stereo, this is the perfect choke!! 8Hy at 125mA is just nice for a stereo amp without the choke saturating, and at a very good price too. Just directly replace the choke in the original circuit

[mach1]

Can anyone enlighten a newbie on what would be required to swap out the driver for a pentode?

I think it would be a good idea to build it as shown in the circuit diagram first, then decide if you want to change to a pentode.

[carbondated]

I just finished this as my first build a few days ago, and, holy smoke, it not only worked, but sounded great! Have fun. Now, if folks out there could suggest a perfect second project...

[carbondated]

By the way, I could use some advice on the 10uF/350V capacitor connected to ground... how critical is this; can polypropylene be sacrificed for electrolytic?

[Soonerorlater]

Originally Posted by carbondated

By the way, I could use some advice on the 10uF/350V capacitor connected to ground... how critical is this; can polypropylene be sacrificed for electrolytic?

In the schematic of the OP's original link the 10uF cap is labelled as being polarised. As my build was from junk parts I used electrolytic. It's function to me looks like an extra layer of filtering for the ECC81 anode and ( in my very humble opinion ) doesn't have to be too critical.

I just finished this as my first build a few days ago, and, holy smoke, it not only worked, but sounded great! Have fun. Now, if folks out there could suggest a perfect second project...

That's the sort of question that could take us well off thread.


Gingertubes " Baby Huey " would get my vote.........


EL84 Amp - Baby Huey


.

[quikie22]

Am confident low ESR cap would make the sound a little more dynamic. Used a plain jane electro there and was good enough, but when bypassed with a PP cap / oil cap is when it begins to shine.