First post/First Design!

[Satellite Icarus]

Hello!

I've been working on a tube amp design as part of my undergrad senior thesis, and I'd really appreciate some feedback concerning it. The schematic is available here:

Elephant Space Snowstorm: Electronics

in a few words: low Zin for matching with ipods/soundcards, half a 12AT7 for the preamp, PCL86 triode pentode combo for phase splitter/power amp, ultralinear output configuration, 2 channels, solid state PS, variable negative feedback and about 9-10W of power.

Comments, feedback and suggestions would be more than helpful!

Thanks in advance,

SI

[loudthud]

Returning the feedback to the volume pot is not a good idea because the impedance at that point changes with the pot setting. Better to change the LED to a resistor and return the feedback to the cathode of the 12AT7.

[mooreamps]

If you dump the cathode led, the hiss content will go way up... Also, D4 and D5 ; you don't need them...

-g

[octal]

Since you're new here, I'll take the time to explain that mooreamps is a fellow who for whatever reason is irrationally fixated on LED bias schemes and mistakenly believes that cathode resistors are a major contributor to the noise floor of the amp.

Loudthud has a good point... at lower volumes, you'll be getting less NFB. You could add a series resistor after the wiper of the volume pot to reduce the amount of change, but it would make more sense to use the cathode resistor NFB injection point Loudthud mentioned.

Nathan

[DIY_Guy]

Move D4 in between the power amp node and R22, otherwise it's kinda pointless. I'm assuming you put those extra diodes in to either prevent 'sagging', or are they there because of the power supply feeding 2 channels?

[mooreamps]

Since you're new here, I'll take the time to explain that mooreamps is a fellow who for whatever reason is irrationally fixated on LED bias schemes and mistakenly believes that cathode resistors are a major contributor to the noise floor of the amp.

How bouts he tries it for himself, and let him make up his own mind ?

-g

[Satellite Icarus]

lets not fight about it

I do have to choose between the Led and the nfb loop, the choice is going to be pretty hard.

[redelephant]

The first triode is linear enuff by itself, no need to mess it up with including it in the NFB loop. Inject the NFB into the LTP's inverting input. NFB does lots of good, but when too many stages are squeezed into it, the sound no longer benefits from it. You'll have less open loop gain for the NFB, but tubes are not Opamps, so u shouldn't need that much.

No need for all those fuses either. One on the primary, and one on the heater secondary is plenty.

My 0.02

[Satellite Icarus]

Redelephant: that's what I was considering. But wouldn't injecting NFB in the LTP inverting input be problematic in terms of phase shift/gain difference?

I'd love to make it work though, as I really like the idea of LED cathode bias - visual indicators are pretty awesome.

I as even thinking of trying to implement a few other LEDs, àla Valvewizard:

The Valve Wizard -Cathode Follower - bottom of the page

[Satellite Icarus]

Also (for DIYguy): D4 is a fast recovery diode to prevent switching noise from the two 1N4007 rectifier diodes (D2-3). I could make D2-3 fast recovery diodes and eliminate D4

[redelephant]

Redelephant: that's what I was considering. But wouldn't injecting NFB in the LTP inverting input be problematic in terms of phase shift/gain difference?

I'd love to make it work though, as I really like the idea of LED cathode bias - visual indicators are pretty awesome.

I as even thinking of trying to implement a few other LEDs, àla Valvewizard:

The Valve Wizard -Cathode Follower - bottom of the page

It's because of the phase shift u want as few stages as possible.

[Satellite Icarus]

True. I guess the gain difference part of my question still holds. Where would you inject nfb for the PI gain to remain balanced?

SI

[mooreamps]

- visual indicators are pretty awesome.

yes, on some of the other forums they are known as the "bite me" lamps...

I as even thinking of trying to implement a few other LEDs, àla Valvewizard:

Really ? Well lets see..

1. D1. Mistake.. Makes the sound fuzz when pushed too hard..
2. D2. Will illuminate if using the correct part.. Correct part not specified..
3. D3. Inserted in the wrong part of the circuit for fixed bias in a C/F.
4. D4. Has no effect in smoothing anything..... "I measured that one myself."
5. D5. You cana if you wana.. Personally, just another waste of parts..
6. I don't use D-C coupled stages for that reason..

-g

[redelephant]

Edit: this was meant as a response to Satellite and not gary, well actually both I guess.

The NFB can go like so?

Regarding the diodes, D3 is fine if that's a centertapped tranny. I'm assuming the pin5 is the 55V... I agree with GM that D4 and D5 are a waste. D1 should be fine, remember Gary, this isn't for guitar.

Gary...what's DC coupled? you mean the input?

[mooreamps]

Regarding the diodes, D3 is fine if that's a centertapped tranny. I'm assuming the pin5 is the 55V... I agree with GM that D4 and D5 are a waste. D1 should be fine, remember Gary, this isn't for guitar.

Yes, I see that. The print looks just so much better now...

Gary...what's DC coupled? you mean the input?

The input of DC coupled C/F as referenced....For using fixed bias in this configuration, the drawing on the web site is incorrect...

-g