Newbie's 1st Amp, comments please.

[dhaen]

James,

It's not too clear anyway.

The lowering of Ra that I hinted at is the source impedance of the KT88s' anodes.

But this is digressing. I'm only here for the ride

 

[Joel]

Robert,

Your initial choice of tubes is fine. Several million linear amps have been built with 12AX7 preamp stages. Audiophiles are biased against any tube that happened to be used in guitar amps, or in large quantities.

 

[dhaen]

As long as you don't post diagrams resembling your avatar we'll manage.

OK, I promise.....

Joel,

I haven't asked others, but the reason I've not used 12AX7, is the gain. They've only been suitable in designs with lots of feedback. Could that be the reason?

Cheers,

 

[Joel]

don't fear the gain...

I use a 6F5 in my 71A SE amp, and it has a distortion around .1%, with no NFB, and passes beautiful sine waves. That tube has a mu of 100.

 

[dhaen]

Joel,

OK, to clarify, I didn't mean that a valve with high gain is a bad thing. It's just that in the designs I've done in the last 8 years, I have not found a place for such a device.
Those amps have been low or no feedback.
I was speculating that others who use similar design lines might have found the same thing.

I have no grudge against 12AX7's

Cheers,

 

[Joel]

John, I agree with you. It becomes useful when you need lots of negative feedback, or - as in my case - need to swing a big grid voltage with only a single stage.

 

[dhaen]

Phew!

 

[halojoy]

A NON-COMMERCIAL DIAGRAM

A NON-COMMERCIAL DIAGRAM
Hi,
Just what the doctor ordered?
Ciao,

Thanks for that diagram! I have saved it on my disk for eventual future use.

nice with tube schematics
you don't get a severe headache by looking at

 

[SHiFTY]

Robert, a good Aussie site for valves is www.evatco.com.au

I have got several orders from them, they seem to be pretty economical and speedy.

~h~

 

[Robert-V]

Hi everyone,

I like this forum idea!
Much better response than I expected.
I don't even have to ask questions and I get answers to them.

fdegrove - Frank is it?
The ciruit you provided is exactly what I had in mind.
Where did you find it?


I will check again later, then I will go home for the weekend and ponder these suggestions.
And I will see if I can come up with some sensible questions for next week.

 

[fdegrove]

GLAD YOU LIKE IT.

Hello,

Where did you find it?

It's one of those circuit I helped out of the door but that's a long story.
No idea how it landed on the web.

Here's where you can find it + some more info:

PP KT 88 UL

It's pretty mature but everything can be improved when the dollar is no object.

Enjoy,

 

[Robert-V]

Some questions

Hello


Power Supply

There are many varied opinions.
I think that the general consensus is that if you have a power supply capable of supplying the demands of the amplifier, that is suitably filter with a good LC filter, it doesn't matter what you use.

So I think I will use diodes, a pi type filter (CLC) with a reasonable capacitor on the output.

I have read in another post about 'stand-by' or 'delayed turn-on'
and I get the impression this is not good.

What about a slow start circuit?



Tubes

Matched pairs - pros & cons?

Are they nessesary with fixed bias?

I intend to use fixed bias on the last stage.

 

[G]

This might interest you.

http://www.pmillett.addr.com/push-pull_kt88_class_a_amp_with_universal_driver_pcb.htm

G

 

[fdegrove]

DELAYED RESPONSE...

Hi,

I have read in another post about 'stand-by' or 'delayed turn-on'

When you choose silicon rectification a delay on the B+ wont hurt
(say 10 secs).

A tube rectifier does that automatically though.

However,what is just as important to preserve the tubes it to provide for a quick discharge path of the PSU when turning OFF.
Otherwise the same problem occurs!

That is assuming you want to preserve the tubewildlife....

Cheers,