91a 300b conversion to 2a3

[gtmcgar1]

Can anyone tell me if I'm operating this converted 300b-2a3 amp at the right operating point. It sounds a bit dull but that might be my low Eff speakers, never heard another 2a3 amp so no baseline. I didnt change all components 880R to 750R for example. Measured voltages attached on the current schem as it stands now, thx
The circuit attached is the best the software can do.

 

[kmtang]

Hi,

You can reduce the value of the 1K dropping resistor to around 250 ohm. Also, the 1uF capacitor to 15-22uF for reducing of hum ripple on the B+. By doing so, the B+ should now read +330-340Vdc which will give you more brightly fast sound.

The max plate volt of 2A3 is 300Vdc. Therefore, you are still operating it rather safely.

Johnny

 

[ThorstenL]

Konnichiwa,

Can anyone tell me if I'm operating this converted 300b-2a3 amp at the right operating point.

You do not.

1) Bump up the Supply voltage for your driver stage.
2) Bump up the anode voltage of your driver valve. You want around 1/2 of the supply voltage.

I didnt change all components 880R to 750R for example.

This makes not such a great deal of impact, but I'd recomemnd you adjust things.

Sayonara

 

[gtmcgar1]

1) Bump up the Supply voltage for your driver stage.
2) Bump up the anode voltage of your driver valve. You want around 1/2 of the supply voltage


I read your information on pentode drivers and was hoping you'd give me some pointers thx

Please confirm if I have this correct,..

change r4 to 750ohm for 2a3
change r1 to 250ohm to inc supply v
change c1 to 15-22uf for better ripple, dont know if i have room.
change r6 to 50-60k for increased anode driver v

thx for the help this is my first diy amp build,

 

[ThorstenL]

Konnichiwa,

change r4 to 750ohm for 2a3

Yes.

change r1 to 250ohm to inc supply v

No, simply adjust it to give 295V on the 2A3 Anode.

change c1 to 15-22uf for better ripple,

That will increase the supply voltage too. Only use this if you have problems with hum.

change r6 to 50-60k for increased anode driver v

No, absolutely not.

Reduce R2 to 3k3 or therabouts.

That should give around 290-300V for the supply to the driver stage.

Change R7 to 75k
Change R8 to 30K
Change C6 to at least 2.2uF

Adjust R9 in circuit for an anode voltage around 60V higher than where the screengrid settles, which I'd expect to be in the 70V region.

The result should work a lot better.

Ciao T

 

[gtmcgar1]

thanks

Thanks for all the help guys. I'll let you know what she sounds like once I get the voltages sorted out, thx Again.

 

[gtmcgar1]

changes were made

Updated as you suggetsed and much much better, thx
driver supply settled at 295v
anode 140v with screengrid at 75v (r9 1.2k)

Thanks again, photo is pre upgrade.

 

[kmtang]

Really interested to know how it sounds for the 2A3 with 6SJ7 driver.

I have built the 2A3 monoblocks based on the design of JE Lab's 2001 edition. That is with use first triode of 6SL7 as gain stage and the 2nd one as cathode follower. I used all Hammond's power transformer, choke and 1627SE OPT.


Johnny

 

[gtmcgar1]

reply

This was originally my first 300b amp, I converted because I picked the wrong Hammond PT which wasn't really up to the job. I also built a 300b version with Angela PT/1627 output which sounds great. The plan now is to try biamping (since i have 2 sets of amps) but I can't decide on what speakers I need or the best way to do the xover. I've been listening to the 2A3 amp for about a week on b&W602's and its surprisingly detailed, I haven't felt the need to go back to the bigger amp yet. I'm pleased with the result so far, now that I have it operating in the sweet spot.