JohnH Posted August 28, 2008 Share Posted August 28, 2008 Recently got a loud sqeal when turned the amp on. Now once every couple of weeks when playing sound starts to cut on the amp. Goes between low and no sound for about a minute before it comes back. Had amp for over a year with daily use. Tubes failing? If I have to replace, can I just go ahead and buy say a "red" Groove tube preamp tube and "blue" Groove tube power tube? Do the colors have to match? This is a first time replacement for me so I am in the dark!! Any help would be appreciated! Link to comment Share on other sites More sharing options...
cGil Posted August 28, 2008 Share Posted August 28, 2008 If you have a voltage meter, you might want to measure the AC at the wall when the amp drops out like that. Might be a brown out dropping your filament heater below operational levels. Or, it may simply be a tube. They're like light bulbs and won't last forever. Don't know anything about blue labels or red labels on Groove Tube tubes. I doubt it'll matter much unless one color happens to make you jump faster and run higher. Stuff like that happens I guess. I don't see the point in paying an extra $10 for a GT re-brand of Sovtek or JJ tubes. So I just buy JJ (a tube manufacturer brand) el84's. JJ also makes an excellent 12ax7. Gil... Link to comment Share on other sites More sharing options...
TWANG Posted August 28, 2008 Share Posted August 28, 2008 Two JJ's makes an awfully nice sound.. and I've got a wall socket at home that wont let any amp I've built sound right or work right. and I've had funky voltage drops, too, so yea to the brown out chance.. what about that loud squeal though.. doesn't seem indicitive of tube or juice...? Link to comment Share on other sites More sharing options...
cGil Posted August 29, 2008 Share Posted August 29, 2008 what about that loud squeal though.. doesn't seem indicitive of tube or juice...? I've heard that before. One time it was a faint squeal from a bad solder joint on a quick connect lug for the heaters. The other time it was loud, and was a quick connect lug for the OT. It was probably heating up while being vibrated, which only served to complicate the issue of the bad solder connection even more. There are other potential squeal sources in a stock amp... such as from a bad pot-to-chassis ground at the volume pot. Or not being neat and tidy when routing the OT primary wires. Gil... Link to comment Share on other sites More sharing options...
JohnH Posted August 29, 2008 Author Share Posted August 29, 2008 Thanks for the info. Will try the amp at another outlet then try new tubes. I've heard about biasing. That something needs to be done w/ a Vavle Jr.? Link to comment Share on other sites More sharing options...
layboomo Posted August 29, 2008 Share Posted August 29, 2008 Recently got a loud sqeal when turned the amp on. Now once every couple of weeks when playing sound starts to cut on the amp. Goes between low and no sound for about a minute before it comes back. Had amp for over a year with daily use. Tubes failing? If I have to replace' date=' can I just go ahead and buy say a "red" Groove tube preamp tube and "blue" Groove tube power tube? Do the colors have to match? This is a first time replacement for me so I am in the dark!! Any help would be appreciated![/quote'] The other thing to think about John is replacing the volume pot......the stock pot's are pretty cheap and a worn pot could cause the kind of drop out's you are getting. Link to comment Share on other sites More sharing options...
strangedogs Posted August 29, 2008 Share Posted August 29, 2008 Thanks for the info. Will try the amp at another outlet then try new tubes. I've heard about biasing. That something needs to be done w/ a Vavle Jr.? biasing is really not necessary on the VJ... Link to comment Share on other sites More sharing options...
JohnH Posted August 29, 2008 Author Share Posted August 29, 2008 So is biasing something that is done w/ amps that use more than one power tube? Link to comment Share on other sites More sharing options...
cGil Posted August 29, 2008 Share Posted August 29, 2008 Okay, here's the short (Ha!) oversimplified semi-non-techno-centric version... Every tube needs to be biased for it to operate. Think of it as the idle setting on your car. With amps, the "bias" is part of the circuit's functional design. There are two popular methods of designing a tube circuit's bias. Fixed bias, and cathode bias. "Fixed bias" means the bias is set by applying a negative voltage to the control grid. "Cathode bias" means the bias is set by elevating the cathode above ground potential. Amps with a "fixed bias" generally do require resetting the bias for new power tubes. Big Mesa amps often use a fixed bias that's set with a specific value of resistor, but they get around the need to constantly rebias (by swapping resistors) for power tube swaps by selling you overpriced Mesa re-brand tubes that were specifically selected because they happen to fall within the general range of the amp's preset bias circuit. And by threatening your warranty if you don't go along with the deal. What they don't tell you is that the majority of average off the shelf tubes fall within this favorable response curve category. Speaking of curve balls... A fixed bias amp can use a pot instead of a fixed resistor to make that part of the circuit adjustable and easier to rebias. But that doesn't change the fact that it's still a "fixed bias" amp. The 22watt Fender Deluxe uses an adjustable fixed bias as do most high power Marshall and Fender amps. Not all amps with multiple power tubes use a fixed bias. Some use a cathode bias. This method is inefficient and won't let the amp generate as much power as a fixed bias design. The tone and feel is quite different, too. The Marshall 18watt, Orange Tiny Terror, and Vox AC 15 & AC30 are all cathode biased amps. Salesmen often refer to this as "self-biasing". Many (but not all) single ended amps like the VJr also use this cathode bias method. The end result is that the power tube's bias really shouldn't need to be adjusted for a tube swap. Ever. However, in the case of the VJr, the transformers were designed for 115v and the power tube circuit was designed and biased with that supply voltage in mind. Unfortunately, most of us get 120v to 125vAC from the wall, which causes the power tube to run too hot, which subsequently shortens the power tube's expected lifespan. Hence the need for some of us to rebias the power tube for it to live happily with local power company voltages. Preamp tubes are most often cathode biased, and don't need rebiasing for a tube swap. And thanks to the design of the amp's power supply, they don't have a problem with higher than expected voltages from the wall, and therefore they don't need to be messed with. Unless you just want to have fun tweaking the tones. If you're interested in hearing more about the technical details from the experts on this, I highly recommend the following resources: http://www.aikenamps.com/WhatIsBiasing.htm http://www.aikenamps.com/TI_Aiken_int.htm http://www.aikenamps.com/TechInfo_2.htm http://www.valvewizard.co.uk/ Gil... Link to comment Share on other sites More sharing options...
JohnH Posted August 29, 2008 Author Share Posted August 29, 2008 Thanks for the info on bias. Been trying to figure it our from various sites. Your explanation has helped me get the idea. Appreciate it. Also appreciate all the input I am getting here from all. Link to comment Share on other sites More sharing options...
strangedogs Posted August 29, 2008 Share Posted August 29, 2008 some of these boys are smart as whips... that cgil is one of em'. I'm old & dumb as a rock as far as electronics but am learning a lot from all these brainiacs in these forums. Now if I could just learn another song cause "Wild Thing" is getting old (the Troggs Version as I'm old too) Link to comment Share on other sites More sharing options...
cGil Posted August 29, 2008 Share Posted August 29, 2008 Don't kid yourself. I'm creeping up on 55, and if I had half a brain cell left I'd, I'd umm, well I'm sure I'd think of something to do with it. I answer questions because when I see a question, I can't stand waiting around to find out what the answer is. It's a neurosis for which I shall eternally blame the Navy. But, it's how I learn stuff, and I think a few fragments are beginning to sink in, in spite of the complete and utter destruction of all remaining brain cells that I'm told I accomplished many years ago. Just wish I could remember some of it, cuz it sounds like it must've been some kinda bodacious fun. Gil... Link to comment Share on other sites More sharing options...
Dr_Satan Posted August 30, 2008 Share Posted August 30, 2008 dain brammage laughing matter no! think smart with rok in head can't do. smart guy ansewr ? good. Link to comment Share on other sites More sharing options...
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