Replacing a driver tube in an existing EL84 SE Amp

[Bas Horneman]

Thanks John,

Violins are one reason why I listen so little classical music these days. I have this gut feeling that a violing can sound warm, but on hifi they don't.

But it is this mental picture I have that is quite unrealistic, I realize now. Because after listening to violins "live" that was a let down as well... Maybe something about a couple of them together because if I listen to hillbillies in the bayou there is something very warm about the sound...maybe it is the moisture in the swamps? Or the warmth of the people!!! Hahhaa..

Where do I want to go with this post...ehm I don't know but here you have it anyway..


Guess I just don't feel like working today...

 

[corbato]

Thanks Bas, John and Dave.

I think I jumped the gun early on this one..

After 3 nights Amp is definitely sounding better. Today I rebuilt the same, more carefully and both channels now. Proper shielding, DC heaters and start grounding etc. The major difference I can now notice includes a better sound stage and more easy and spread out Sonics. Sounds very good. At least with kind of music that I listen to. Jazz, Big Bands and Indian Classical and Fusion mostly

Going by the traits, I would expect this Amp to be less punishing on ears.

But I detect a slight rippling kind of hum if pressed closed to the speakers. And the output power still is weak. Could it be because of my power supply? 275-0-275, 250mA while Bonavolta states 300-0-300. Also, is a Choke supply required? I will have difficulty finding that.

Your kind suggestions please..

 

[Bas Horneman]

A choke is not required....but does a better job than a resistor in filtering out rubbish...

 

[corbato]

Bonavolta EL84 SE

After hearing the Bonavolta amp for last few days, I am quite happy now. The sound is very detailed and easy and I'm having much fun with my vinyl collection.

But I'm really getting depressed about the output transformer It just cuts out the real low's and high's from the sound spectrum besides sacrificing a lot of Pout. Can some body help me with detailed construction specs for a good output transformer for EL84 SE operations ? I know a person who winds transformers and has agreed to try Output Trasformer if provided with complete details.

I also want to know if low Pout can happen if +B voltage slightly is less than those specified in the schema. I had a 275-0-275 rated power Transformer, which I used in place of the 300-0-300 volts as specified. In my construction I'm getting a +B of 218 Volts with EZ81 rectification and 227volts if the EZ81 is replaced with a couple of Diodes.


Please.....can someone help with the transforemer spec.

 

[G]

Just download the datasheet for a 6BQ5. It will have the information you need for a SE output transformer. Gap the transformer for about 50mA or so and use a 4.8K - 5K primary. The real key is going to be the primary inductance of the output transformer. That is what dictates the bass response.

G

 

[fdegrove]

POWERXFORMER.

Hi,

which I used in place of the 300-0-300

Using a 275-0-275 VAC xformer will set you 50VAC under the specified 300-0-300 VAC xformer.

The EL84 is usually fed from 250 to 275 VDC.

Cheers,

 

[mig-ru]

hmm, not a big fan of feedback either, but isnt it supposed to increase the amplification somewhat? and yes it could very much be the output tranny, although they tend to under rate them for the frequency range. perhaps you should print a few data sheets for the tubes, and figure a different cathode bias, or see where it stands now. my two cents.

 

[fdegrove]

NFB.

Hi,

hmm, not a big fan of feedback either, but isnt it supposed to increase the amplification somewhat?

It will decrease the amplification ( voltage gain), not increase it.

Cheers,

 

[mig-ru]

ah, well im still learning, my many appologies, i havent done anything with feedback yet.

 

[corbato]

Thanks Gavin, Frank and Dmitry.

However, I'm still hoping that some of you gurus will know the specs- core size, winding gauge, turns etc for a DIY OPT. I'm looking to have modest specs; 20 to 20Khz and rated at max 7 watts, 70mA.

BTW Dmitry...I've had a close thing with the Fulcrums in the past, but since last two years this thrills me