JL Sargent Posted May 6, 2009 Share Posted May 6, 2009 Found this unit on the way home from work. I was in an old non-working console at the local thriftstore. It is out of an 60ish Sears console. Service tag revision '64. The amp itself works quite well. I never can seem to get the old FM tuners to do squat. I noticed one of the 4 output tubes glowing a little brighter than the others? I've seen several SETs with the 6BQ5 compliment. Just wondering if the push/pull was as common. Quote Link to comment Share on other sites More sharing options...
thebes Posted May 6, 2009 Share Posted May 6, 2009 From what I gather EL84 amps are very common in both SET and push pull. I have a small Fisher console here that uses them. Sweet sounding things. I'm sure you already know this but don't plug it in without at least a dummy load and some speakers hooked up to it. Pull the bottom off and check for leaking stuff. The one tube is probably running hotter cuase somethings way out of spec or just plain shot. Could be the tube itself, but since you said it was from a non-working console chances are it's in need of tender loving care. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted May 6, 2009 Share Posted May 6, 2009 Here's the Magnavox 9300 series of push-pull 6BQ5 console amplifiers. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted May 6, 2009 Share Posted May 6, 2009 This is a 6BQ5 PP console amplifier pinched from a Admiral console. I reconfigured the circuit to a Dynaco 6BQ5 PP circuit. I didn't like the sound of the stock Admiral circuit using 12AU7 as drivers. I'd go as far to say that the 6BQ5 push-pull console amplifiers are just as common as single-ended 6BQ5, if not more common...... Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted May 6, 2009 Author Share Posted May 6, 2009 Hey Mike, thats a good looking little amp. Guess I could swap out the brighter tube and see what happens. Im gonna install a proper switch, RCA inputs, clean it up. What brand caps are you guys putting in these units? I'm developed a bad habit of looking for consoles in the garbage and at the thriftstore. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted May 6, 2009 Share Posted May 6, 2009 And a fuse................ I would think the orange drops would be fine, unless you want to get exotic. I like the russian K40Y caps for coupling......good cap for the price. Quote Link to comment Share on other sites More sharing options...
thebes Posted May 6, 2009 Share Posted May 6, 2009 You probably really want to get new power supply caps too, especially the filter cap. Orangies are a tubies best friend. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted May 6, 2009 Author Share Posted May 6, 2009 Orange drops and PS caps. Where do you source those? I was thinking parts express on the RCA connectors. I check them on the caps too. OK, found some. Check out the Orange Drop kit at the bottom of this page. http://www.hi-endstore.com/OrangeCapacitor.html Quote Link to comment Share on other sites More sharing options...
thebes Posted May 6, 2009 Share Posted May 6, 2009 This is the point where I step in on my own renderings and say to you, this is high voltage stuff and it can kill you. If you don't know what you are doing you have two choices, send it to Craig, or get a nice fellow like Mike Stehr to walk you through it and, if necessary, attend your funeral. What you have here is a 50/60 old piece of equipment which means it's about 170 year old in people years. The caps are probably shot, some of the resistors are probably out of spec, several of the tubes are probably worn out, and after you fix all that stuff you have to bias the thing so it operates optimally. So think about it, and until you decide, never leave the room while that thing is turned on. It's could go forever, but really it's living on borrowed time. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted May 6, 2009 Author Share Posted May 6, 2009 I always make sure power disconnected and that caps are bled down before surgery, anything else I should do? Of course I have already pulled this one, removed all support wiring, found and connected primary, variaced it, connected a source and listened to music. On electrocution, I have performed high voltage electrical service work for the last 20yrs and was hit by 480v once which was not pleasant but not terminal either. I know of many deaths due to industrial electrical accidents but have never heard of a tube amp death, have you? Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted May 6, 2009 Author Share Posted May 6, 2009 A 25uf barrel cap to the base of the HOT 6BQ5 indicates open. The opposing one (other channel) charged and discharged properly. So maybe that explains the hot tube, but it was making music out of both channels with this bad cap? I have found two Spraque Atom 20uf 50v in my stash of electronics. Wonder if that would work ok? Quote Link to comment Share on other sites More sharing options...
mike stehr Posted May 7, 2009 Share Posted May 7, 2009 A 25uf barrel cap to the base of the HOT 6BQ5 indicates open. The opposing one (other channel) charged and discharged properly. So maybe that explains the hot tube, but it was making music out of both channels with this bad cap? I have found two Spraque Atom 20uf 50v in my stash of electronics. Wonder if that would work ok? Is there a 25uF capacitor in parallel across a resistor, coming off the #3 pins of the push-pull pair of 6BQ5? If so, those are the cathode bypass caps. If the one is pretty much open, it might bias the pair of 6BQ5 a bit harder than the other 6BQ5 pair. The 20uF 50 volt capacitors should be fine. They are more than likely a 10-20% tolerance capacitor, that should be close enough. I would check the resistance of the cathode resistors as well, along with all the others. If you had it in operation, maybe flip it on it's side for access and check the #1 grid pins of each 6BQ5. (Pin 2) Check for DC voltage on each #2 pin. Start with a higher voltage scale with the multimeter, and scale down to millivolts as you check. The DC voltage measurement should be in the millivolts. Any one of the #2 pins measuring over a volt DC indicates a bad coupling capacitor. That can bias the pair of tubes hard, maybe into a runway condition resulting in red plating of the tube/tubes. If you have ramped up the amp with a variac, and the amp has little hum if at all, the power supply filtering capacitors should be ok. But like Marty said, it's a borrowed time kind of thing. A old reformed multisectional capacitor could last for a long time, or it could just die over a short period of time. You never know.....If the main multisectional capacitors seem ok, maybe do the things to the amp like you mention. And if you like what you hear from the amplifier, then maybe go ahead and buy new multisectional capacitors. At any rate, new PS capacitors will be of big help with making the amp quiet and stable. You can get multisectional capacitors and orange drops from Antique Electronic Supply (tubesandmore.com) The Dynakit site has multisectional caps as well. There is more sources out there as well. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted May 7, 2009 Author Share Posted May 7, 2009 Yes, thats exactly the setup. Off #3 pin parallel with a resistor. There are two in the amp and I'll get those swapped after work today. I'll give it a good listen with the bypass caps replaced and test for DC on pin #2 of each. Thanks for the help guys! Quote Link to comment Share on other sites More sharing options...
fini Posted May 7, 2009 Share Posted May 7, 2009 Here's the Magnavox 9300 series of push-pull 6BQ5 console amplifiers. Mike, is that an ipod dock there in the front-right corner? Quote Link to comment Share on other sites More sharing options...
mike stehr Posted May 7, 2009 Share Posted May 7, 2009 Mike, is that an ipod dock there in the front-right corner? It's where the molex plug was for the power take off to the tuner/pre-amp. Quote Link to comment Share on other sites More sharing options...
thebes Posted May 7, 2009 Share Posted May 7, 2009 Ok, that's cool JL. Not knowing your skill set I was just being cautious, since there's probably at least 300 volts running around in there. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted May 7, 2009 Author Share Posted May 7, 2009 Got those two 20uf caps installed. All working well. Tested each pin #2 and had nice low voltages (IE: .02 .01 etc) except one which was at 1.2v This was adjacent to the worst looking coupling cap in the amp. Its really sweatin there so I'm suspecting its kaput. Getting my list together for ordering and one of the smallest caps has no value on it, just a number 475. Anyway, I thought I would just measure capacitance and go with that value? I moved the slightly brighter tube to another socket and the brightness moved with the tube so that one may be a short timer. When I get all the new caps installed I will post a picture. Thanks again. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted May 7, 2009 Share Posted May 7, 2009 Getting my list together for ordering and one of the smallest caps has no value on it, just a number 475. Anyway, I thought I would just measure capacitance and go with that value? Yeah, it's one way to determine the value. 475 probably indicates the voltage rating. I didn't realize that was a solid state rectified amp. You could change out the diodes for more modern quieter ones, unless the old ones work fine as it is. There may be more B+ rise if you swap to modern diodes, which can compensated with more droppong resistance. If the rectifier is a square thing with plate fins, it's a sellenium rectifier and should be swapped out. They willl reek like rotten eggs when they fail, and they eventually will. Quote Link to comment Share on other sites More sharing options...
JL Sargent Posted May 7, 2009 Author Share Posted May 7, 2009 Here is a picture with the two orange Sprague barrel caps already installed. The diodes are barrel type center left in picture. Is the sound of the amp different with SS rectification vs tube. My little SET amp is tube rectified. Quote Link to comment Share on other sites More sharing options...
fini Posted May 7, 2009 Share Posted May 7, 2009 Mike, is that an ipod dock there in the front-right corner? It's where the molex plug was for the power take off to the tuner/pre-amp. Really? Is the ipod dock on the back then??? better do this [] Quote Link to comment Share on other sites More sharing options...
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