http://music-electronics-forum.com/ en Fri, 20 Nov 2009 21:50:57 GMT vBulletin 60 http://music-electronics-forum.com/images/misc/black/rss.jpg http://music-electronics-forum.com/ http://music-electronics-forum.com/t16223/ Fri, 20 Nov 2009 17:51:14 GMT Just got done with a SVT classic that had bad solder joints in the pre-amp, dead screen grid resistors, and a bad switch. Also retubed it. Now... Just got done with a SVT classic that had bad solder joints in the pre-amp, dead screen grid resistors, and a bad switch. Also retubed it.

Now that its back together and working, it has a fair amount of 60/120 cycle hum. A lot of it I dialed out while biasing, but its still there and audible, just below conversation level. Thats with the pre-amp turned all the way down at gain and master, and as quiet as I could get with biasing.

Is this normal for these amps? I've never owned one so I cant tell. If it was a 100W Plexi I'd say it was normal and call it a day, but I'm not sure what is "normal" level on an SVT.

Thanks! ]]> wizard333 http://music-electronics-forum.com/t16223/ http://music-electronics-forum.com/t16219/ Fri, 20 Nov 2009 14:05:33 GMT I have a Mesa Boogie Heartbreaker head with a reverb issue. Both reverb pots are acting like volume controls. I have checked all components in the... I have a Mesa Boogie Heartbreaker head with a reverb issue. Both reverb pots are acting like volume controls. I have checked all components in the reverb circuit, subbed another tank and cables, subbed tubes, replaced both pots. Problem persists even with reverb tank removed, so I assume a recovery circuit problem. Any help at all greatly appreciated. ]]> jvm http://music-electronics-forum.com/t16219/ http://music-electronics-forum.com/t16218/ Fri, 20 Nov 2009 09:10:50 GMT hi am completely new here so please excuse my ignorance I have built a single input ab763 clone without the trem circuit , hasmmond transformers... hi
am completely new here so please excuse my ignorance
I have built a single input ab763 clone without the trem circuit , hasmmond transformers JJ valvesand a hammond reverb tray
I have about 5.9 volts dc to ground to heaters, 330v ac to pins 4 and 6 of rectifier tube (GZ34)
all voltages from here seem a little high (but i have a cheap meter)
as follows

first preamp tube (nearest to input) 2.3 v to cathode, (sschematic says 1.3) 187 tio plate1 (schematic says 170 ) similar to pin 6 , 1.3 v to pin 8
reverb tube is 450v to pin1 (schematic 410)and only 4.2 v to pin 8(schematic says 8.7)

the BIG DIFFERENCE in second preaamp trube - reading 316 to pin 1 (schematic 170), 5.2 v to pin 3 (schematic dsays 1.3)and 330 to pin 6 - schematic says 180 v

last 12ax7 readings before 6v6 are 190 to pin 1 and 6 and 80v to pin 3 and8 so are ok I think
450 v dc to pins 3 of each 6v6 (schematic suggests 415)
200v to pin one of last 12AT7 (schematic says 170)
I AM GETTING NOTHING BUT A SLIGHT BUZZ from speaker and when guitar goes thro its just a faint distorted noise BUT I CAN HEAR REVERB TRAY VIBRATING A TREAT WITH VOL UP, nothing to tray with vol down

please help It is driving me to drink
Chris (sheffield uk)

no
about 190 to pin 6 of same tube and
82 volts dc to pin 3 (schematic says 77v)


I have po ]]> chris shaw http://music-electronics-forum.com/t16218/ http://music-electronics-forum.com/t16216/ Fri, 20 Nov 2009 08:03:18 GMT I guys, I'm hoping you guys can point me in the right direction here. I have an Ampeg SVT4Pro bass head which has seen much better days. Unfortunately I had a house fire recently and lost nearly everything. Sad story aside, the amp itself was not IN the fire but did have a TON of water dump through it from the firefighters (I have NOTHING but respect for these heros!)

The amp was not plugged in at the time. I removed the lid (it's the rack mount version) to aid in it air drying and allowed it to dry completely and then some.

I cleaned it as best I could but when I plugged it in and turned it on - nothing. It didn't even blow a fuse. Just no signs of life

Everything on the inside looks all and all pretty good - no rust or contamination though clearly something is not good here.

I have an ok understanding of electrical repair and am comfortable with a soldering iron (i.e. I've replaced power connectors on a laptop motherboard, replaced cap's on a pinball machine, modded my xbox, rebuilt the electronics in a number of guitars etc). However, I have very little experience with amplifiers. I feel very comfortable replacing parts but don't know where to begin testing.

I know I should probably have it repaired by a professional but as I said, unfortunately the house burnt down - no insurance money and I have lots of things that need to be replaced before this and not the money to do it. The way I see it at the moment is the amp is blown and I will not have the money for a LONG time to have it professionally fixed. I feel comfortable enough in the skills that I would not make things any worse and really need a working amp.

Thank you so much for any help you can give!

Chy ]]> chymmylt http://music-electronics-forum.com/t16216/ http://music-electronics-forum.com/t16209/ Thu, 19 Nov 2009 17:22:46 GMT Embarrassed to have to ask this, but I have forgotten how to adjust the bias on my '74 SF Super Reverb (AA270 schematic). I got lazy and just kept using similar Groove Tubes sets.

I think that I used to attach some extra resistors (maybe 1 ohm?) to the power tubes and read voltages someplace. Can anyone fill me in on an adequate procedure to set bias? Don't have a scope.

Thanks ]]> pbabadoo http://music-electronics-forum.com/t16209/ http://music-electronics-forum.com/t16207/ Thu, 19 Nov 2009 15:52:36 GMT Hello, I was given an old *Crate VC-3112W* as a gift to my son who plays the guitar. My Amp has one of the four EL84 tubes broken and also the... Hello,

I was given an old Crate VC-3112W as a gift to my son who plays the guitar.

My Amp has one of the four EL84 tubes broken and also the only 10,000uf/16V capacitor is swollen. Those are the "visible" problems.

I would like to ask if anyone knows whether the two pairs of the EL84 tubes are matching pairs and where the best place that I can order a full set of new tubes the SOVTEK EL84 & the SOVTEK 12AX7W or 12AX7LPS for better sound ;)

I have the schematics found on the internet.

If I manage to have this Amp up & running by Christmas I would make a big surprise for my 15year old son !

Many Many thanks in advance to anyone who can help. ]]> Pitsi http://music-electronics-forum.com/t16207/ http://music-electronics-forum.com/t16206/ Thu, 19 Nov 2009 15:14:54 GMT I'm in the process of learning electronics and amps. Slow process due to time available, but it's enjoyable. I have a problem that's cropped up and am hoping someone can help me understand what's happened.

I bought a 1974 Fender silver face Champ at a guitar show a couple months ago. It worked fine for a few hours, then started malfunctioning.

At the time I was told about three capacitors that where replaced (and given the originals). These turn out to be the three cathode bias caps. I've since found out the 1Meg volume pot has also been replaced. That's where the trouble started, I think.

For the first couple hours, the amp worked fine. At some point I started to get volume drops, it started making some popping and cracking noises, and I shut it down because I could tell something was wrong.

So, I pulled the chassis and had a look. This is when I could see the volume pot had been replaced. Whomever did it was pretty sloppy. They'd used a (I assume) non stock shielded single conductor wire to run from pin 7 of the 12AX7 to the volume pot's center lug. They did not connect the sheild at the tube end, but soldered it to the ground lug at the volume pot end. There was such poor lead dress that the sheild wire effectively shorted the input and the ground lugs of the pot.

I fixed this and put it all back together and fired it up once more. The popping and crackling was still there, then I heard one loud pop and saw a bit of smoke coming from the unused input jack. This was a "oh shit" moment for sure. I was getting no signal shortly afterwards.

So, for reasons I can't explain other than idle stupidity, I installed a new rectifier tube and fired it up. The rectifier and the pre-amp tube both glowed (heater's working), but not the power tube. I replaced the power tube with a known good and now the amp worked. It did not sound it's best, but there was no noise that shouldn't have been there (like popping and cracking). I played through it for a bit and noticed the power tube was red plating, especially when I wasn't playing, or playing at low volumes. When I cranked it up and wailed on some barre chords, it wasn't redplating.

Hmm, so I got out my Weber biasrite and plugged it in between the 6V6 and the tube socket. It read 272 volts and 140mA! Geez, no wonder it's red plating and the previous (new) power tube had failed.

So I took the chassis back out and started inspecting. It was then I noticed the 22µF/25volt cathode bias cap had literally blown up and shot it's stuffing out one end. The plastic wrapper was also somewhat melted on the same end. I removed this and the 1 watt 470ohm resistor and measured them both. The resistor measures 516 oms and shows no evidence of being overheated. The capacitor actually measures 0.5 ohms. So effectively, my cathode bias went from around 500 ohms to 0.5 ohms.

I think I can now understand why my Weber was showing 140mA. But, did that cap blow because of the shorted volume pot, or should I be on the lookout for other issues as well?

]]> lowbrow http://music-electronics-forum.com/t16206/ http://music-electronics-forum.com/t16204/ Thu, 19 Nov 2009 14:10:16 GMT I need to replace the power transformer in this Spectra 212-T amplifier for a friend. It's definitely shorted out, as shown by using a current limiter. The amp uses 4-6L6 and 4-12AX7. Any recommendations/sources for a good replacement? ]]> TheEHMan http://music-electronics-forum.com/t16204/ http://music-electronics-forum.com/t16201/ Thu, 19 Nov 2009 03:16:45 GMT So I got most of the hum out of this Ampeg M15 I'm playing with.
I was going to shield the input wires to the first stage. Should I attatch the shield to ground or the plate?
Which do you guys prefer? ]]> drewl http://music-electronics-forum.com/t16201/ http://music-electronics-forum.com/t16196/ Wed, 18 Nov 2009 23:04:56 GMT Today's problem, a Taylor acoustic with a Neve dynamic pickup preamp system with no output.
Anyone have a schematic? Ha! i didn't think so.
So it uses a voltage doubler or tripler it looks like, it's got like 4 pickups/transducers that go to the preamp board.
I've got voltage on the op amps.
What literature I did find says it has a "string ground fuse" which was open. I jumpered that and nothing so far.
Any ideas before I take another look? ]]> drewl http://music-electronics-forum.com/t16196/ http://music-electronics-forum.com/t16195/ Wed, 18 Nov 2009 21:32:51 GMT I'm looking at a SF vibro champ today with something strange going on with the treble and bass controls. The amp is loudest with both controls on 1. If I turn either the treble or bass control to 10 there is no volume. The wiring looks original except the 6V6 cathode resistor and bypass cap have been replaced. There is also a small cap across pins 5 and 8 of the 6V6 socket that I don't see on the schematic. I've checked it against a schematic and layout, looking for shorts or mods, and have reflowed the solder joints on the tonestack and pots with no sucess. Has anyone run into this before? ]]> thumbs http://music-electronics-forum.com/t16195/ http://music-electronics-forum.com/t16179/ Tue, 17 Nov 2009 21:35:33 GMT I just replaced a volume pod on my Marshall MG100HDFX head but the wires that run from the transformer to the board and plug in to positions W4 W5 W6... I just replaced a volume pod on my Marshall MG100HDFX head but the wires that run from the transformer to the board and plug in to positions W4 W5 W6 W7 W8 and W9 got mixed up. Now it pops fuses, I have read alot of posts that point to the same problem whit the TD7293 chip on the back of the heat sink going bad as well. The amp worked fine other than alot of bad noise and weird response from the overdrive volume pod (that is untill I got my hands on it). I have looked all over the internet and called a few shops around here with no luck of finding which order these wires go. There are 2 Red,2Blue,a Black and a Brown wire. If anyone out there could send me information or a picture of the wires from the transformer to board of a good working amp I would greatly appreciate it. Thank you for any and all help. Jeremy ]]> dimefan0908 http://music-electronics-forum.com/t16179/ http://music-electronics-forum.com/t16177/ Tue, 17 Nov 2009 19:00:37 GMT I'm a newbie on this forum. I came across it in searching for answers to my drri which is driving me crazy.
Problem 1 -- after playing for about an hour, the sound changes and is muted/dirty. It stays that way until the next session. Turning off and letting it cool for 15 minutes or so during a break doesn't seem to make a difference.

I've had it back to the shop for warranty repairs where they re-soldered a tube socket claiming it had a hairline crack around the solder. It cleaned it up somewhat but still does the same thing only not as noticable. I took the unit back into the shop where one of the guys played on it for about an hour and he claimed it was all good.

Problem 2 -- from day 1 I've felt the unit was microphonic (although I'm not sure if that is the correct description). We play in a basement that is relatively small however, the drri always seems to be affecting anything that will vibrate. The snare on the drums, a light fixture...anything that will vibrate. The other guys in the band have no problems with this. 1 plays a Rivera Chubster, another plays a Peavey Classic 30, and Fender Bass.

Problem 3 -- the vibrato and reverb sometimes works off the switch and sometimes it doesn't. I've checked the switch and it's fine (it's very simple).

I've changed the rectifier tube and the pre-amp tubes with no change in the above symtoms. I have not changed the power tubes however do not think it is a tube problem. While I love the 6v6 sound I'm looking at options as I don't want to sink in any more money into the amp unless I know it's going to work.

So, question is... is this a bad amp from the factory.

Sorry for the long winded description. Again appreciate your feedback. ]]> lorand55 http://music-electronics-forum.com/t16177/ http://music-electronics-forum.com/t16175/ Tue, 17 Nov 2009 17:34:40 GMT Hello everyone,

I am looking for some clues to get that Selmer T & B 50R SV working properly.

For some reasons after I first dismantle the amp and putted back together the clean channel doesn't work any more, valve is fine, wiring is fine... probably an open solder somewhere...

And the main thing is as soon as I switch on the standby the amp starts to crackle, I have been advised to check if there is resistors on the 5th pin of the OT valves and if not to stick in some 1.5k resistors.

do you have any other ideas?

cheers ]]> http://music-electronics-forum.com/t16175/ http://music-electronics-forum.com/t16173/ Tue, 17 Nov 2009 15:35:25 GMT This is more of a feeler thread right now. In my infinite wisdom, I attempted to read my 560VDC B+ voltage with my Fluke 87 III set to resistance. ... This is more of a feeler thread right now. In my infinite wisdom, I attempted to read my 560VDC B+ voltage with my Fluke 87 III set to resistance. After the sparks flew (literally at the probe to B+ contact point), the meter still reads, but voltage readings are low by about two orders of magnitude; I haven't diagnosed the other settings. I've found a schematic on Fluke's site, but I'm not making much sense of it. Is there a Fluke repair group out there (Yahoo group or forum), or would one of you be willing to hold my hand while I attempt to repair this (if possible)? ]]> defaced http://music-electronics-forum.com/t16173/