Problem with my Darling!

[philipbarrett]

well at least my amplifier Darling! I have recently finished a Darling with the schematics on Bob Danielak's website. This is the first thing I've ever built except for a few headphones amps & such so please excuse any newbie howlers here. The amp is finished but has very low gain (only enough to drive headphones to a moderate level) and I've really run out of ideas.

Here's the facts Ma'am (with a 400Hz test tone from my iPod line out);

1) input to 6J4 = 88mVAC (100K resistor to ground)
2) output from 6j4 (pin 7) = 34VAC
3) input to 1626 (pin 5) = 34VAC
4) output from 1626 = 10.41VAC

yes Virginia it's an attenuator!

Now all my other voltages read correctly, the 6J4s are seing about 160VDC and the 1626s about 248VDC, the heaters read 12VDC across the pins.

These readings are the same for both channels but being a rank newbie I'm reluctant to assume that both tubes are funky. At this point my limited skills and experience are tapped out and I throw this to the group in the hope that someone has seen this before.

Thanks in advance for your time.

Philip

[John Fader]

Philip,

What is the voltage at the cathode of the 1626? Any possibility that the value of the cathode resistor is wrong on both Channels?

Also, tell us if it is a stereo amp or monoblocks.

Cheers/The Fader

[John Fader]

BTW,,congratulations on your first amp and good on ya' that you chose to build SE !!!

You are most probably on a road that you'll sepnd a lifetime travelling. I build my first circuit in 1956, as a Boy Scout "denner" for a Cub Scout den. Believe it or not, in those days, each level of CS Handbooks had an electonic project.

I was tagged to build a one-tube radio as an example to the cubs. A local Radio--TV repair shop provided the parts and allowe me to sit there and build it with lots of help and encouragement from the proprietor. The radio worked just fine after I'd mowed enough lawns to buy the batteries!!!

BTW, the next level of project was a two-tube redio, which I built the next year ann I wound some coils for short-wave (not in the book, these were just guesses as to the turnsm, I had no idea that this could be calculated!!!) and actually heard some hams talking and some morse code. I eventually became a ham radio operator and built all sorts of stuff including transmitters and receivers.

I sincerely hope you enjoy your Darlings after we get you amplifying :-) That's a marvelous amp.

Cheers/Fader
now listening to Willie Nelson, Red Headed Stranger on vinyl; AWESOME!

[philipbarrett]

What is the voltage at the cathode of the 1626? Any possibility that the value of the cathode resistor is wrong on both Channels?

Thanks for your help. I'm reading just under 27VDC at the cathode (pin 8) of both 1626s, the cathode resitor (I'm presuming it's the one that ties 8 to ground) is a 500R/5 watt.

The amp is stereo not monobloc.

'Preciate it!

P

[martinab2]

Probably a silly question... but have you checked that the output transformer windings are the right way round? Maybe its only me who would do something silly like getting the input and output mixed up

[philipbarrett]

Thanks for the idea. I agree, the transformer looks like a likely suspect. I checked the Hammond specs on the 269EX, primary is blue/brn and secondry (for a 5K prim/8 ohm sec) is black/yellow which is as I wired it. but maybe I'm misunderstanding something.

Thanks again for helping...

P

[jeff mai]

Originally posted by philipbarrett

3) input to 1626 (pin 5) = 34VAC
4) output from 1626 = 10.41VAC

yes Virginia it's an attenuator!

How are you measuring number 4? At the transformer secondary? At the 1626 plate?

[philipbarrett]

Thanks for your post. I am measuring at the 1626's plates (pin 3) using my 400Hz test tone. If I experiment with the transformer secondries I can get a little more gain (still not enough to be called an amplifier) to the speakers but at the expense of gross distortion.

P

[jeff mai]

Originally posted by philipbarrett
I am measuring at the 1626's plates (pin 3) using my 400Hz test tone.

The question had to be asked.

There's still something funny about your measurements. Otherwise your first stage has a gain of over 300! If the measurement at the output of the first stage / input of 1626 was instead 3.4VAC, everything would look pretty normal. 88mV of input isn't going to result in much power output and I believe the amp itself has very little overall voltage gain anyway. 1 Watt referenced to 8 ohms is only 2.8Vrms. My CD player has as much output! (Of course, it'd never source enough current to drive a pair of speakers.)

[philipbarrett]

I just re-confirmed, those voltages are correct.

P