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DCPP - 'notch' in frequency response
DCPP - 'notch' in frequency response
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Old 8th November 2019, 03:36 PM   #51
tristanc is offline tristanc  United Kingdom
diyAudio Member
Join Date: Dec 2015
Location: Oxford
Thanks all for your comments - been unable to do much more on this for the last few days.

Originally Posted by baudouin0 View Post
OK its a sharp notch. Do you think the amp has always had this notch in the right channel.
Yeah - I checked back to my first frequency sweeps and it's there, although I was focussing on 10Hz-30kHz audio range I did spot ringing but thought nothing of it as the frequency response was flat.
Things to check - can you see the 40KHz on any of the supply or screen grid supplies. Your looking for a peak in amplitude as you sweep from say 10KHz to 100KHz. Maybe one of the decoupling caps is not working.
I'll check this. My analogue sig gen is very useful for this - tactile dials beat clicking options on screen!
The other thing could be a PCB tracking issue in that some of the output current has made its way back into the input [snip]
Actually PCB looks good layout.
This got me thinking - could solder flux left on the board allow current leakage? For this build I used some leaded solder acquired from an online auction site. And I recall it did leave quite a residue. I cleaned a bit of it off but quite a bit remains.
Originally Posted by baudouin0 View Post
One other clutching at straws. If you have grounded your input phono connectors to the chassis you could have created a magnetic loop from the output transformer through the cable and back into the input.
Another one I'll check - I used isolating jacks but there's always a chance there's some contact.
Originally Posted by Tubelab_com View Post
1) Is the damping factor / output impedance of the two channels equal. Drive both channels with a sine wave to a low power level (5 watts or so) into a dummy load. Bridge a fixed resistor of about the same value as the load across the load and observe the output voltage drop. Do both channels drop by the same amount? Try this with and without GNFB. The LNFB affects the damping factor. Something messed up here can cause weird effects.

2) Probe each end of the balance pot in the bias circuit (R22 and R23) with a scope. Compare the two channels. There should be only DC here, an open or defective cap can upset the LNFB balance without affecting bias.

A trick that I have often done is to divide and conquer. From what you have said the driver sections function OK without the output tubes present. I have often used drive from the good channel to drive the bad channel (actually both channels). In this case you would lift the plate ends of the coupling caps in the bad channel, and run jumpers from the lifted ends to the plates of the driver tubes in the good channel, thus driving both sets of output tubes from the good channel. That's a bit more complicated in your case since you used "box caps" for your coupling caps, and I used leaded tubulars.
George, this advice is great for anyone - particularly the resistance to ground measurements and comparing channel to channel. I'll do what I can.

That being said, I have just liberated an unpopulated DCPP board from a fellow forum member in the UK (plus valves and sockets). It might be just as easy to order a new set of components from Mouser and start-over. Testing as I go. I'll use screw terminals for off-board connections like you did so removal is easier. This will be a longer-term project over winter if I can't spot and quick remedies with the tests above.

One thing I have noticed - through all this testing and switching on and off frequently, the new power amp valves I inserted in the problem channel have become rather noisy during warm-up. A few bangs through the speaker. Silent as usual after a few seconds. They didn't do this when 1st used...
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