Skelt

an update on this GA-5 Well its back in the shop, the 1A fuse wont hold. pulled the tubes and the same results. the tubes test within spec a 2amp fuse will hold but the PT begins to get hot and start to sizzle after about 30 seconds to a minute i see no arcing on the chassis, The owner say he use this amp about 18 times in the 3 years since I worked on as described in the top this thread. im about ready to pick one up from mercury magnetics unless im missing something

Its a home brew winter project using parts I had gathered from years of collecting. The PT was taken out of a webcor RR. It has a 6v heater winding and using a 6X5 rectifier there is 450v B+. The 12ax7s heaters are supplied by a 12vdc wall wart. Its built on the lid the RTR the PT came from. If I can quiet it down and like the way it sounds then ill build a better enclosure and use new tube sockets and 1% parts. With this build I want to try different RIAA feed back circuitry. I can see the hum on a scope and it goes up and down with the volume. There is about 50mV ac at the output. I'm going to disconnect the input and output wires to the rca jacks and move the PT far away. Metal sheilds around it and the signal tube hasn't done enough.

I checked the heater voltages 4.97 and 6.45. so no worry about the line being higher today compared to the past. Thank you bluesplayer for your help, this is a very cool circuit. I can see why these are so desired. Single ended with very little circuitry to get in the way. In the future I might build a modified stereo pair to power a dedicated vinyl setup with external speakers. But in the mean time, how do I eliminate the hum from this phono amp?

All the tubes tested ok on a Lafayette KT-209. And changing tubes was the first thing we tried. I had an order placed with digikey for a different amp project (phono preamp). So I bought two 630v. .022 CDE films and installed them. Now there is no dc on the low side of the coupling caps. Last night the owner stopped by with a 79 strat and a Gibson LP with humbucker PUs. The Gibson definitely has the hotter PUs. We tried a .011 uF at the volume pot and the sound was too lean. Then we rolled several tubes thru the 12ax7 position. Now he is taking with him a 12AU7 Sylvania(tame and mellow) 12AX7 GE (sharp and bitey) 12AT7 Sylvania (a little edgy, somewhere in between the others) His old RCA 12ax7 was very dull when compared to the others. After a couple of hours of playing the new caps are settling in. It has fixed the early breakup issue the amp was brought in for. Now he is able to play at full volume on the amp and guitars. Most of the volume comes in the fist half of the volume pot and after the 12o'clock position there is more added distortion than added loudness. Is this normal for this amp? Any thoughts on adding an in rush current limiter to the power cord? The Dynaco forum recommends to put one, a CL-80 in their case, in old tube equipment. Supposedly it has two functions. First it brings the source voltage up slowly so the heaters dont experience full current when cold and it knocks the line voltage down a few volts because today's 120v line is higher than it was in the 50s & 60s when it was more like 110-115.

There's .5v at the screen of the 6v6 and .3 at the volume pot. Ill try to convince my friend they need changed. What effect do the cathode caps have on tone? I put a .022uF 200v cap in at the 6v6 temporarily and it sounds somewhat better. Im using a phone for a signal as I am no guitar player. Ill order some in 600v. Any thoughts on changing the cap at the volume pot to .016 uF? I read another post that the op said it reduced some muddiness.

Hi guys, this question causes fights on the hifi stereo forums I hope it doesn't start one here but here it goes. My friend brought me over one of these amps to work on. The problem is it breaks up too early. I believe the 2 caps(.02uF) in the signal path are leaking current bc an ohm meter shows resistance and never indicates an open. So what are recommended caps to keep it as close to the original sound my friend wants? Or Is there such a thing? 6V6, 12ax7, 5y3, and all resistors test good. The PS was recapped by someone else. The amp has an alnico speaker.