See attached audio file. I recorded this directly in front of the tweeter on my speakers. It's the same in both speakers. Amplifier is an Audio Research D-70 MkII. It does this intermittently. At the time of this recording the amp had been on for maybe 45 minutes. The tubes are all at least 25 years old.
The intermittent part of it is intriguing, and the fact that is on both channels suggests it is not tubes.
Trimmers would be a good place to look, but this could be anything, from dirty tube sockets to noisy resistors or bad coupling caps.
check for your testing points for values shown here:
https://audioresearch.com/new_website/wp-content/uploads/2024/11/D70II_Manual.pdf
Trimmers would be a good place to look, but this could be anything, from dirty tube sockets to noisy resistors or bad coupling caps.
check for your testing points for values shown here:
https://audioresearch.com/new_website/wp-content/uploads/2024/11/D70II_Manual.pdf
That's a good life for 6550 output tubes, and probably all the 9-pin tubes as well.
jeff
Is it exactly the same in both channels (i.e. a mono sound) or just similar? If I remember that circuit right, there is one 12AX7 with a half used in each channel, otherwise only the B+ and bias supplies would be in common.
Edit: I looked up the schematic: https://www.diyaudio.com/community/threads/audio-research-d70-mkii.416845/, this one uses 6DJ8 not 12AX7.
Edit: I looked up the schematic: https://www.diyaudio.com/community/threads/audio-research-d70-mkii.416845/, this one uses 6DJ8 not 12AX7.
What is it with caps all the time? There are a few makes of capacitors (electrolytics and foil) that can't be trusted, like the good old tar bombs, but I went through lots of gear 50 years and older with all caps in good shape not causing any issues. Old tubes being a different story.
The fact that it is common to both channels makes me suspect something other than tubes. Have you confirmed that it is not external to the amp - modern homes are full of all sorts of sources of RF interference.
After that components common to both channels which probably points at the PSU.
Tube age is much less important than the number of hours of use and the conditions under which they were used.
After that components common to both channels which probably points at the PSU.
Tube age is much less important than the number of hours of use and the conditions under which they were used.
I just listened to the sound file. It could be a faulty coupling cap... However, the Audio Research D-70 normally came with excellent quality polypropylene coupling caps (rel caps I think) - So they really shouldn't fail. Have you cleaned all the contacts? The pin contacts on the sockets too?
Kevin made a good point about the source potentially being in the PSU since it seems to come from both channels.
Kevin made a good point about the source potentially being in the PSU since it seems to come from both channels.
Is there anything I could do to test any of these failure paths? Like, I know freeze spray can identify bad resistors. Is there something similar for tubes or capacitors?
Sure sounds like tube noise and farting to me. The manual linked above does not include a schematic, but does mention the screen and bias power supply/supplies are regulated. A 12AX7 and 6550 tubes are listed as regulator driver and regulator tubes. I would be looking at those tubes. The other suspect would be resistor noise in the power supply, but it doesn't really sound like resistor noise to me. Your cold spray will help with diag. here.
Dan
Dan
Sometimes a bad connection develop at one weak point, it could be a diode, a cap, a resistor heating up solder then the connection becomes intermittent.
Start with cleaning all the valve pins and after, trough multiple inserting, the respective tube sockets. If nothing changes, take out all the audio tubes and check all the PSU stages, one by one, according to the schematics from the post #4, and check the respective voltages: fillaments and Hig Voltages. If all good, proceed by checking the audio entry stage by inserting the respective tubes (one channel at the time) and checking each tube parameters. Or, simply, go to a tube guru and enjoy watching him performing wizardry things 🙂
The best advice to save you time is to hook to a cheap loudspeaker, power up , and use a plastic/ isolated tool to tap on each wire/junction, and tube.
Yo, DMT, you did not mention how loud is the 'noise' - can you hear it from the couch with the volume control set at minimum? After re-reading the start of your post, I realize that you recorded just the tweeter. It is only a hiss sound, or has some other lower components. The amp works otherwise, with this troubling noise, in a normal way? if the input volume was not set to minimum, how loud is was when you recorded the proof (at which hour was the volume pot) ? E88CC (6DJ80 is a kind of valve that if is pushed hard, could develop hiss noise in months, not in 25 years, and the input + the splitter is constructed with these tubes. Also, the advise that GABDX gave you is indeed valuable, but for an old tube device, checking the amp from power cord toward the output (PSU, first tube, splitter, drivers, final tubes - parameters + bias ) is mandatory. Old ham-radio school 🙂. If nothing is revealed, then tapping is the way to find a bad contact/component to go further. Also, using a cheap speaker during all this is a gold advice!
Sounds like noise on the heaters or plate.
What kind of gizmos does it have for power supply.
sounds like angry diodes or semiconductors, even sounded like radio interference at some spots.
What kind of gizmos does it have for power supply.
sounds like angry diodes or semiconductors, even sounded like radio interference at some spots.