Harman Kardon Commander DA-35 tube amplifier with odd problem.

Rare I get a item that has me stumped but given it's a tube amplifier made some 60+ years ago and I haven't played with tubes for over a decade this one seems worth asking about.

I don't even know where I got this one from exactly but I have had it a long time and never so much as turned it on until today.

It powers up but makes all sorts of fine crackling sounds and as the tubes first warm up it puts out fair sound but as the continue to warm up after about a minute the sound gets distorted and fades out. The cracking stays but no further sound comes through.

I've swapped out all the tubes with good spares from other old amplifiers that do work and they all checked out good so what am I looking for that would cause this?

I'm guessing I have a tube bias voltage drifting up or down on one but I can't seem to catch it since I don't know what tube it would be or what would be normal to begin with since I have yet to track down a correct online schematic so far.

12ax7 pre amp, 6AV6 ?, 7247 output tube driver and 2 6L6 outputs.

 

Minor crackling: Fender had humidity problems with their early tar paper circuit mounting substrate. I have seen minor crackling in a dirty tube socket. A cracked carbon resistor could do that.

Fading to zero after time: You know that's a DC problem, or possibly it's going into oscillation, and you know what I'm waiting for.

Get your can of cleaner out and do the pots and sockets. Check/replace the electrolytics. Do DC measurements. Check with a 'scope for oscillation.
But you already know that.

 

Hence why I need a refresher on tube amplifier troubleshooting.

It's been blown clean and the sounds are not pot, socket or general wiring related. I did the wiggle tests to confirm that much.

I'm guessing its a capacitor issue at this point. I can pull the 12ax7 preamp tube out and the crackling stays the same which says it's beyond that but that's all I have so far.

What it sounds like when it fades out is like on a simple push pull solid state amplifier when the feedback loop gets broken, but with a diminishing output gain. Like the audio is being partially clipped on one half of the output signal until the whole signal is swamped by it.

If I had a schematic I could track its most likely sources down but without it I have to reverse engineer the the thing and draw one to which vacuum tube schematic symbols and pinouts are on longer an active part my memory any more.

 

Looks a lot like a 12AX7
http://tubedata.milbert.com/sheets11.html

 

Have you tried searching for Sam's Photfacts? Looks like there are still a number of places that sell them. You might find it in their database that is not searchable via Google.

Also Harmon Kardon is still in business. Bet they have it in their archives.

 

Just one place

http://www.theschematicman.com/photofact-h.htm

A ton of Harmon Kardon schematics

 

All of the tube primary and intermediate voltages stay stable as it warms up but one of the output 6L6 tubes starts to pickup a noticeable coronal glow inside and the metal plating turns as dull red a bit after that so that one's being ran way to hard for some reason.

 

I once had the job of rebuilding old ships 1MC tube power amps. The second thing we did after cleaning and washing out decades of nicotine and dirt was to replace every old wax-impregnated coupling and filter capacitor (Excessive leakage or low insulation resistance) and any electrolytics.

 

http://www.dogpile.com/search/web?f...e=True&q=howard+sams+photofacts&ql=&qlnk=True
https://www.samswebsite.com/

 

It's Fixed!

Replaced both coupling capacitors (.05 uF 600 VDC) between the intermediate stage and output stage tubes. All problems went away.

One tests at about 600K and the other 850K. apparently just enough leakage at the ~ 200 VDC coupling differential to make for an issue.

 

I did a Magnatone like that a couple of months ago. The DC coming in the grids of the output tubes was so high that the 25V electrolytics on the cathodes were at 35 volts and both of the 6V6 tubes were overheating.

What a mess! When I got done with that one, it had about 14 original parts in it, and I'm including the pots, the tube sockets, and the front panel jacks.

Anyway, I found the problems with DC measurements and the old percussion test. A tap-tap here and a tap-tap there and I never turned the 'scope on. It passed the ear test, the bandwidth was correct, the power output measurement, and it went back to California.

 

Got it hooked up to the junkyard rescue Heathkit FM-4 I picked up a few weeks ago. Once the mouse nest and chewed up wiring were cleaned out and fixed it works rather well too! 1950's FM mono at its best!

I put a spare computer 3 channel speaker system on the old amplifier for the time being so I have 3 channel mono music going in the shed right now with 'Dakota Damnation' a sort of midwest monsters of rock/small bands review show one of the local station runs Sunday nights.

The heathkit FM-4 audio quality is not all that great by modern day HD stereo standards but for what I am playing with, being ~ 60 years old all vacuum tube tech it's not bad!

Thinking that since the little workshed is going to be sort of semi retro mid 1900's tech themed I might set it up out there for my music source. Maybe set up my old Dukane 15 watt tube amp for the main speakers and put the old Harman Kardon 35 watt on a old subwoofer and see what comes of it!

 

All you need to finish the job is a Fonzerelli to start the juke box for you.

 

Not going that retro. Tube amps will be set up on a shelf. That's all the 14' x 20' work shed has room for in a sound system.

I noticed the FM-4 unit tuning drifts a bit once it has warmed up for a while.

Gotta move the tuning dial a tiny bit once it gets warmed up good then back next time it is turned on or leave it slightly off station for awhile until it heats up again.

 

14 x 20? I only have a 14 x 10!
I have to let the other end of stuff poke out the door while I'm fabricating.

Oh well, check strantor's Thread about property taxes and you will understand why.

As for the frequency drift? I guess that's why progress kept happening since 1960.
In today's world, nobody has to adjust their FM tuning or their horizontal hold.
I keep seeing Threads about how to avoid pushing a single button, like garage doors that know when you're coming.
Then there is the opposite: "Please tell me how to connect everything in my house to the Internet so I have lots of virtual buttons I can press from anywhere I happen to be (and any government agent or nosey hacker can derive the intimate details of my life)."

Sorry, I'm out back, in the shed, rocking out to my baddest beats and making stuff that earns money.