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 Post subject: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Fri 14, 2019 2:55 am 
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Hi, I've been working on radios for years, but this is my first attempt at a stereo tube amplifier. Any education would be appreciated. I installed new filter caps in this SCA-35 today. The rest of the capacitors look like orange drops but they're turquoise. I left those alone.

I tested all the tubes. One of the 7199s was good on the strength meter, but showed a couple shorts. One of the four 6BQ5s tested weak.

I did a dim bulb test and things looked good. I decided to go ahead and fire the amp up, just as a test to see if it was worth moving forward and investing the $$$ in new tubes. One channel worked for a minute or so, in between some squealing. At one point I removed all the tubes from one channel, thinking I had one set of good tubes and maybe I could try one channel then the other with good tubes. Not sure if that was a mistake.

Eventually the amp stopped producing sound. I tested the tubes again to find all four 6BQ5s were totally ruined... the meter went backwards in the strength meter, and grid emission was very high. I'm curious if one weak tube and one bad tube could blow all of this stuff, or if I have a problem elsewhere.


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Fri 14, 2019 7:45 am 
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Now that I thought about this more, at one point I unplugged the record player from the phono input with the unit on. That caused a momentary loud buzz and that's when the sound stopped. I've done this on transistor units with no problems... is this a no/no on a tube unit?


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Fri 14, 2019 9:01 am 
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You should never use tubes that test shorted.

Did you check your bias voltage? This is THE most important thing in tube amplifiers.


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Fri 14, 2019 2:38 pm 
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Did you replace the electrolytic caps with the factory design values - or did you increase the capacitance of the input capacitor in the power supply filter ?
-Chuck


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Fri 14, 2019 3:11 pm 
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Location: Rochester NY USA
The SCA-35 uses one cathode bias resistor for all four output tubes - running it with two removed will nearly double the current in the remaining two. Bet they glowed real nice for a while... probably didn't last long enough to burn out an output transformer. Dynaco was already running them at max ratings in order to get 35W from a quad - many will increase the cathode resistor to get a more reasonable operating point; only a minor loss in output power. And a matched set of 6BQ5s is required.

A shorted 7199 or 12AX7 wouldn't harm the amp but could damage a tube tester (though probably wouldn't hurt a simple emission tester). A shorted 6BQ5 would likely blow the line fuse (assuming it hadn't been changed to an oversized one). Amp is worth fixing if transformers are good - all parts and replacement PCBs are still available.

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Fri 14, 2019 7:13 pm 
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Suggest checking all resistors and replacing all that are out o tolerance or just simply replace all carbon comp resistors.

For the tubes I suggest getting the Russian equivalent of the 6BQ5.

Since this is a stereo amp, you can take the cathode resistor and double its value and add a second with two 6BQ5 tubes per resistor.

That will ensure both channels are further separated far as audio is concerned.

Would suggest checking those capacitors unless someone here knows they are the type that will be good.


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Sat 15, 2019 7:33 am 
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Thanks for the tips.
As far as filter capacitors go, I used the closest newer values I could find to the old values. Some are slightly over one is slightly under, but is that enough to do harm?

68mfd for 60
47mfd for 40
47mfd also for 50
22mfd for 20

If anyone could give me any idea on how to check for bias voltage that would be great. I'm a little lost on that.

I bought some replacement tubes, along with a couple adapters so I can use a 6gh8 tube instead of those expensive 7199s... hopefully the stability issues won't be too bad.


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Sat 15, 2019 7:42 pm 
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Location: Rochester NY USA
Bias is developed from cathode current - nothing to adjust - if all four tubes are conducting equally, it'll be correct. If you replace the common cathode resistor with two of twice the value (as mentioned above), you can use two matched pairs instead of getting a matched set of four. I think the original is 90 Ohms - instead of 180, go up to 200 or 220 Ohms per pair. Separate bypass cap across each one.

Electrolytic values aren't critical - this has a solid-state rectifier, so larger values won't cause problems.

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Sun 16, 2019 3:04 pm 
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Check those coupling capacitors for leakage! Any tube amp that comes past me gets new capacitors, and any selenium rectifiers get replaced with diodes. Those parts are much cheaper than tubes.

Some manufacturers ran the output tubes right up to their maximum ratings, or even beyond. It's a good idea to look in the tube manual to see if the bias and plate/screen voltages are correct. If not, changing some resistor values will make the tubes live longer, at the expense of a little less output power.

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Mon 17, 2019 4:33 am 
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Thanks once again. I'm understanding what you're saying about separating the cathode resistors. The current bypass cap is 100mfd, do I keep this value the same as well for both?

Also, if you don't mind, could someone point out to me where the coupling capacitors are? I think they are C19 and C20 but I'm not sure. I could only find info about coupling caps for transistors. The schematics/manual are here. I apologize for asking so many questions/being clueless, but I'm using this amp as an opportunity to try to learn more.


http://www.thehistoryofrecording.com/Ma ... SCA_35.pdf


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Mon 17, 2019 11:29 am 
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You can use a 100uF capacitor for each of the cathode resistors.


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Mon 17, 2019 8:42 pm 
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i've seen motorola, fisher, magnavox, and sometimes zenith just beat the snot out of those poor little 6BQ5 tubes in many instances. the ran them like 6L6gc tubes :lol: .

rarely have i found any barn fresh 6BQ5 tubes worth anything. often, the tubes are mismatched b/c someone had to replace one/them during the set's lifetime.

i never was a fan of the 6BQ5. i've seen so many from magnavox, zenith, motorola, and fisher that literally had the inside of the glass scorched black and brown. sometimes the plates were warped and looked well heated.

i had one fisher that peaked 35 watts out of a pair of 6BQ5 tubes when measured with an 8 ohm load. when doing les maths on the static dissapation, they were driven well beyond their max. it was a wonder they did not cherry up. i guarantee none of the new production tubes would have been able to handle that circuit w/o doing a cherry job. those 6BQ5s in that fisher were little tubes driven like a 6L6gc :shock:.

this stereo motorola, for example, pulled four 6BQ5 tubes thru one cathode resistor. the original resistor really gave off some heat and definitely showed some visual thermal stress.

i replaced the original resistor, kept all four cathodes pulling thru it, and used two 10 watt resistors in series. each resistor was half the resistance of the original so the power and heat would be dissapated evenly between the two. it worked out well, they get just warm, and everyone is happy. cool parts are happy parts.

i too considered pulling each channel (two tubes) thru their own cathode resistor and bypass capacitor, but went the original route instead by upgrading to two resistors.

the original thread is here on the motorola stereo project if one has some time to dive into 5 pages..

viewtopic.php?f=13&t=341427

cathode resistor situation is on page 4.

steve

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Tue 18, 2019 3:36 pm 
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On a cathode biased amp like this, it's a good idea to monitor the grid voltage on a new set of tubes. If it's anything but 0v, you either have a leaky coupling cap, or the tube is leaky, shorted, or has grid emission.


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Tue 18, 2019 5:46 pm 
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I think the real answer is above (don't run it without all tubes in place). The bias was wrong for *some* reason.

But, if it is otherwise working, but you have short tube life on your EL84/6BQ5, try to find NOS 7189s from a major manufacturer. They are probably just "selected" from the EL84 line but they are rated for more of everything and generally last much longer in these "edge of the envelope" applications. The Rabbit is correct, many manufacturers thrashed the EL84 (and even more so, the 7591a) mercilessly, since they were relatively compact for their output. This was swell when your 7591a came from Westinghouse and the same assembly line they used to build MIL-Spec 7591a for the SAGE radar missile defense program, but not when they come from Russia "salon audio" manufacturers in the 21st century.

The 7189 was more-or-less the same "super-rated" EL84 for defense and audio applications.

Brett


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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Tue 18, 2019 8:46 pm 
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i have used matched pairs of russian or 6P14P something tubes that were made for the soviet military. they are supposed to be extremely rugged 6BQ5 tubes that could handle a lot, perhaps radar as said above ?

here is the guy from whom they were purchased.

perhaps you can send him an email and find out if he has any more.

https://www.ebay.com/itm/vacuum-tube-se ... XQbjxRBd3W

i bought mine a good 10 years ago from him.

steve

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Tue 18, 2019 8:53 pm 
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i have also increased the cathode resistor a bit to just "take the edge off" the poor little tubes.

although the idle current will decrease along with the static dissapation, plate voltage will rise a bit. it is one of those things that if you want to lower the bias with a larger valued cathode bias resistor, a happy medium can be found between the cathode current and plate voltage.

personally, i'd rather have the plate voltage up a bit rather than beating the current thru the cathode and watching the plates cherry and warp before my eyes.

when i restored a few zenith, mags, and motorola amps that *really* drove the tubes beyond their max, i increased the cathode resistor from something like 33 to 47 or from 68 to 82 ohms.

the tubes were much happier and when doing the maths, things looked better. the difference in output and distortion were minimal.

steve

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Tue 18, 2019 8:55 pm 
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Joined: Aug Sat 01, 2009 2:23 pm
Posts: 1653
Get a quad of these:
https://tubedepot.com/products/sovtek-e ... acuum-tube

They're super rugged tubes, and I've used them in many of my EL84 component amplifier repairs. No question those amps were hard on the tubes, but I've never had a problem using this one. Not cheap, but a quad of NOS 7189's is going to cost a lot more. Another choice, if you can wait for delivery from Russia would be a set of these: https://tinyurl.com/y4ewod5d
Basically the same tube. Here's the parameters. Note the much higher plate voltages:
https://www.thetubestore.com/lib/thetub ... -Sheet.pdf

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Thu 20, 2019 2:09 am 
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In a tube amplifier, the coupling capacitors are connected between the driver tube and the grids of the output tubes. Any leakage in these capacitors will upset the bias, and cause the tubes to draw excess current.

Some amplifiers, especially very early ones, use transformer coupling.

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jun Sat 22, 2019 11:17 am 
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Tim Tress wrote:
In a tube amplifier, the coupling capacitors are connected between the driver tube and the grids of the output tubes. Any leakage in these capacitors will upset the bias, and cause the tubes to draw excess current.

Some amplifiers, especially very early ones, use transformer coupling.


Re leaky caps, I had an Ampex "suitcase" amplifier that started to redplate one of its 6V6 output tubes. Cause was a leaky cap from the phase inverter to the 6V6's control grid. No test I could come up with would show anything wrong with the cap out of circuit. So I replaced the handful of old caps in the amp. All is well.

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 Post subject: Re: Dynaco SCA-35 Destroying Tubes
PostPosted: Jul Tue 16, 2019 8:51 am 
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Okay, so I have re-capped this amp. I got a new set of tubes, including using 6GH8 with adapters instead of the hard-to-find 7199s. I've also modified the circuit by separating the two channels... each has its own capacitor and 220ohm resistor.

I'm having similar issues as I was before I did all of this. One channel seems to work okay, but the other channel makes a high-pitched squealing noise as soon as the tubes warm up. It's loud enough to make me worry I'm going to blow a speaker, so I quickly shut the thing off as soon as it happens. It seems to be the same no matter the volume level, or input source. Am I going through all the resistors in this thing to check for drifting values, or is there something else I should look at? Something is very not-happy in the circuitry.


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