Members Barry Wuthrich Posted August 5, 2019 Members Share Posted August 5, 2019 I built a new cab for an amp I bought from an builder in 2013. I jostled the amp around a bit when fitting the cab. When I finished the install and powered it up, it blew a cap and a resistor. The preamp tube tested bad, the rectifier tested marginal. The rectifier socket is labled EZ80, but the tube in it is a 6202, which is not a replacement. I can't seem to get hold of the builder. I've read that an EZ81 can substitute for an EZ80, but if the 6202 is correct then that would be a mistake. Is there a way to be sure which is the correct rectifier? The builder sold it as a 'Vox AC4-style clone'. This amp was once in the shop for over a year, but never got fixed, so it is possible the tube may have gotten switched. It was playing and sounding really good with the 6202 in it right up until I built the cab. Link to comment Share on other sites More sharing options...
Members Barry Wuthrich Posted August 5, 2019 Author Members Share Posted August 5, 2019 Ok, I just checked the 'properties' on the dialog box for the first pic, and the pic is from 2014, so that 6202 is the tube that came with the amp. So now my question would be, is that what is causing me problems with this build? Of the 4 amps I bought from this builder, this is the only one I've had problems with. I have blown 3 different resistors and 1 electrolytic cap in this amp. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted August 5, 2019 Moderators Share Posted August 5, 2019 My first thought is, not enough information. If I were in your position, I'd open up the amp and trace the circuits to make sure that everything is wired the way it should be. Link to comment Share on other sites More sharing options...
Members Barry Wuthrich Posted August 5, 2019 Author Members Share Posted August 5, 2019 My first thought is' date=' not enough information. If I were in your position, I'd open up the amp and trace the circuits to make sure that everything is wired the way it should be.[/quote'] Thanks Isaac. Unforunately, I'm not a tech, so I don't know how 'it should be' wired. . But I have a friend that is fairly knowledgeable. I am wondering if we could measure the current going to the plate and determine if that is correct for one tube or the other. Or maybe some other simple test I can do with a multimeter. that would indicate that the amp is set up for a 6202 rectifier and not an EZ80. My AC 15 clone has an EZ81 in it, but I'm reluctant to plug it into the circuit if the 6202 is correct. I don't know if that could lead to some catastrophic failure. The cap that blew up is the small one that is wired into and sitting on top of the only white rectangular resistor shown in the 3rd pic. And the resistor that blew is the Red/Red/Black/Silver just to the left of it that connects the standby switch to the rectifier tube. If that helps...... Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted August 6, 2019 Moderators Share Posted August 6, 2019 I have to correct myself. In a tube circuit, the first thing to be done is always to verify that the tubes are working properly. If you have access to a tube tester, that's the way to go. If not - and few of us do, anymore - then the best thing is to use a known good tube in place of the suspect tube(s). Do you have any other amps that use the same tubes that are working properly? Link to comment Share on other sites More sharing options...
Members Barry Wuthrich Posted August 6, 2019 Author Members Share Posted August 6, 2019 found something. I googled GL6202 tube basing diagram, and, EZ80 basing diagram. Turns out the EZ80 is a nine pin tube and the 6202 is a 7 pin tube. So that was easy. But the 6202 shows no connection at pin 2 and 5. My 7 pin socket has connections at pins 2,3 and 4 only. Pin 3 and 4 supply the heater, so pin 2 must have and internal connection to Plates? Still confused here. I'll keep digging. Link to comment Share on other sites More sharing options...
Members Barry Wuthrich Posted August 6, 2019 Author Members Share Posted August 6, 2019 I have to correct myself. In a tube circuit' date=' the first thing to be done is always to verify that the tubes are working properly. If you have access to a tube tester, that's the way to go. If not - and few of us do, anymore - then the best thing is to use a known good tube in place of the suspect tube(s). Do you have any other amps that use the same tubes that are working properly?[/quote'] A friend tested the tubes on a simple 'pass/fail' tester. The preamp tested bad, I have others. The rectifier tested marginal. I have read that a rectifier tube failure can take out the power tranny, so that's what started this mess, along with the cap/resistor explosion. I have eliminated the EZ80 possibility, as that is a 9 pin tube and I have a 7 pin socket, so the chassis is mislabled. Still confused by the way the 6202 socket is wired as per my previous post. Still digging. I am hoping the preamp tube failure caused the cap and resistor to blow. Also the new power tube tested as shorted, so I have a back up EL84 already. I guess I'll replace all three tubes, the resistor and capacitor and cross my fingers. lol Link to comment Share on other sites More sharing options...
Members Barry Wuthrich Posted August 6, 2019 Author Members Share Posted August 6, 2019 I've been playing on my Tweed Deluxe clone while this AC4 is down. Thank goodness for Carl's Custom Guitar's speaker soak, or I'd be deaf by now. What the use of playing a tube amp if you can't drive the power tubes, right? Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted August 7, 2019 Moderators Share Posted August 7, 2019 found something. I googled GL6202 tube basing diagram' date=' and, EZ80 basing diagram. Turns out the EZ80 is a nine pin tube and the 6202 is a 7 pin tube. So that was easy. But the 6202 shows no connection at pin 2 and 5. My 7 pin socket has connections at pins 2,3 and 4 only. Pin 3 and 4 supply the heater, so pin 2 must have and internal connection to Plates? Still confused here. I'll keep digging. [/quote'] Nope. 3 and 4 are for the heater, but pin 2 is NC (no connection) on the 6202. 5 is also NC. 1 and 6 are the plates, and 7 is the cathode. See the data sheet here: https://www.amplifiedparts.com/sites/default/files/associated_files/6202.pdf Link to comment Share on other sites More sharing options...
Members WRGKMC Posted August 12, 2019 Members Share Posted August 12, 2019 I suggest you bring it back to the builder or find someone knowledgeable enough to figure out what's going on. You at least have a chassis photo. This looks to be a simple single ended amp so there cant be too many causes to your problem. The amp consists of a power transformer, Output Transformer, Preamp tube, Power tube and Diode. The rest of the components provide DC power and balance the gains staging. This is most likely the schematic used with whatever modifications added. [ATTACH=JSON]{"data-align":"none","data-size":"full","title":"000100 el84_8a.gif","data-attachmentid":32568194}[/ATTACH] The diode can be rewired depending on the tube you have available. So long as it fits the socket and the heater voltages are correct. As far as having problem. Two things that stand out like a sore thumb. First the tubes look to be used. That means you have no idea how many hours the tubes have on them and mounted in a small combo can the speaker vibrations can rattle the crap out of the tube elements and cause them to short/blow. Second. Its obvious the power transformer is used/scavenged from some other piece of gear. It may be correct, but given its size, my guess is that transformer provides higher currents then this circuit likely needs. When you read about caps blowing, possible tubes arching Its important to check the ratings of the components being used. Guitarists love to push small wattage amps like this to their limits and you need top quality parts and tubes if you expect to have them hold up, The EL84 tubes are known for blowing out in small combo's with as little as 6 months of use or less. My suggestion would be to spend the $20 needed for all new tubes and see where that gets you. If you still have issues, making sure electrolytic caps are properly connected with their polarities in the proper direction. Link to comment Share on other sites More sharing options...
Moderators AMZ-FX Posted August 21, 2019 Moderators Share Posted August 21, 2019 I will first test voltages with no tubes in place. If that looks basically okay, then I will put in the rectifier tube and check voltages again. Once the supply voltages are confirmed to be okay, I move on to inserting the other tubes and checking them (preamp first). Best regards, Jack Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.