Shango, why is tube stuff so much more willing and tolerant and alive sounding? It seems to me that 10 tube watts sounds like 80 transistor watts. Can you explain? Thanks. Steven
I bought an old 1960 AM/FM tube stereo yesterday. Brand is Roberts, from Roberts Mfg. in Cleburne, Tx. I'll probably just sell it because I'm not a tech. I like this stuff.
Wow, that turned out nice, that whole series was fun to watch. And yeah, the insides of those 'lytics really do reek....I made the mistake of cutting one open in my house, and had to put the can and innards outside pronto.
My mother has an Silvertone chord organ that has an 8" speaker that I fixed back in the early 80's. It had an amplifer similar to this one if I remember correctly, It also had bad resistors, The capaciters seemed ok. The organ still sounds good
I can’t get enough of the comment about you talking about someone smoking that roll of electrolyte paper! Haha put it in a bong, or for the vape enthusiast in your house.
Awesome!!!! I too have gotten really into tube amp repair and design... ...now after seeing johnaudiotech's great channel I want to do some transistor/op amp builds and possibly a class D.
So people know, equipment does not have to be ancient for resistors to go high lol, I just repaired a Sylvania 32" HD Flat LCD set that had two 18K resistors that read up to 4.3 megs when they got hot, this caused the set to shut off after a period of time. Test was done with a heat gun on them out of the set and on the meter. I put in a pair of 21K and the set works fine now. That old amp has a feedback line on it to limit the drive, the original speakers in the unit will make up the volume. Also there should be remote option socket that puts the unit in "mute" mode when its not connected, there could be a shunt in the socket, see if that's bad or if it was even included with the amp..
I think a lissajous analysis between input and output of that amp would prove interesting. Perhaps the output transformers likely have a bit of phase shift. With those compliant speakers that is the recipe for a LF siren. Speakers such as the originals may have higher free-air resonance, and thus would not disclose this problem. I've also seen some cheap OTL amps (such as used in '70s-'80s amps with a blocking cap on the output do the same thing if an unbaffled woofer with low free-air resonance is used in test. It's amusing to watch, until the amp perhaps oscillates itself to death. I had different test speakers for such situations to try to avoid this.
Wow, should you just replace them with NOS & be done with it. I'm recapping my Silvertone 1472 & removing the death cap. Seems like taking resisters out one at a time to test is a pain, just change them, I see 2 that have been replaced, also have 3 Sangomo .01 600v caps & some Aerovox .001 600 in there also. Thanks for the video, nice to learn about these things, I should have been collecting them, I feel more comfortable about working on them. Amp guys are far away here in the sticks.
18ks measuring 19k would have been ok, they are within the tolerance, but as for the 270ks going open, i've come across those and 220ks going high more than any other value!
There is something still wrong with that amp. Tube manual says 4.3 watts for single ended operation. That ought to blow your head off at close range.The instability (absurdly wild infrasonic oscillation with high bass) of the amp is a probable cause. I thought you were going in the right direction checking the feedback path. Too much negative feedback is a bad thing; at the ends of the frequency response of the amp, phase shifts (approaching 180 degrees) can occur which can turn negative feedback into positive feedback. Try cutting the feedback loop open and see if the instability clears. You will probably want to run some feedback; perhaps you can select a higher value resistor in the feedback loop. Given that you can damp the oscillations by gently holding the foam surround of the speaker indicates high amplifier output impedance, thus a poor damping factor. The feedback loop has a great effect on damping factor. Negative feedback reduces gain (easily made up for in other stages), broadens frequency response, and decreases output impedance. What about C49? is it leaky? If problem is not in the feedback loop, perhaps the power supply impedance is to high. How much plate voltage is there at idle versus thumping? You can also hang a scope on the power supply and see if its dancing around while at high volume. High power supply impedance will be reflected to the output impedance. Good luck! Edit: The more I think about this the more I suspect the power supply - both channels weak and unstable - common element is the power supply. Are the new filter caps good (second decoupling filter - this thing almost acts like a motorboating AA5)? Weak rectifier tube?
+RF Burns 6 ohm our of a projection set. I think its the dampening of the speakers. Those just like to bounce. I have speakers out of a similar silvertone console and no feedback and much louder. I could try connecting that feedback loop and i bet it would crank up into distortion
It used "Full Range" speakers. Probably 4 ohm Full Range "tweeter" (not really because it's Full Range) in series with a large 4 ohm "woofer" (also actually Full Range) for a total of 8 ohms per channel. You cannot use those (car?) audio speakers. The system probably only outputs about 10 watts per channel. How many watts were those (car?) audio speakers? When you connected one channel to the "old" full range speaker, the impedance could have been wrong. How many ohms was that speaker? Plus, it was playing at the same time as the (car?) speaker and this is not a "true" stereo system. It "splits" the signal into two channels, but when you overload one channel with incorrect wattage and impedance, doing so _also_ affects the other channel. So, that "older" speaker was never really tested properly. It _may_ have been correct for the system, but that (car?) audio speaker killed both channels.
Come on, when did PCB boards were invented? i mean, what's the point of having all of those components just flying through everywhere? Isn't that risky?
+André Campana Merétika PCB boards didn't come into common use commercially until the mid 1950's, but the idea goes back decades to 1903, and by WWII was in use for some war gear before it became a commercial product post war. By the late 50's and into the 60's, it was commonplace, especially for SS electronics, though many tube electronics may or may not use them, such as this stereo.
I'm jealous! Dang it!! Wish I had tons of cash so I could send half of my projects 3000 miles to see a great and entertaining repair video! I'll be in the money in about two years... Ug can I wait that long?
