Stumped on a 7868 amp

[Davidg]

Started messing with an amp I got recently.Its a Sound X505 with 7868 power tubes.Supposedly did not work when I went to buy it but turnred it on and all was well owner almost backed out of the deal-brought home played several times all was well took chassis out to clean pots and would not turn on and has been on the backburner.Whats stumping me is I have all voltages where they should be but tubes or pilot light or nothing lights up.There is a hum coming from the speakers and u can even hear the meter when u check plate voltage but none of the tubes or pilot light will come on-all PT secondaries are OK-have heater,plate and screen voltage but nothing is happening, ANY IDEAS?

[Geezer]

I would suspect a loose/intermittent connection in the filament/heater string.

I'm guessing that the pilot light is run off the 6.3v w/ the heaters, correct? If yes, then there's the indicator.

1st, with the power off (unplugged from the wall) & caps drained, trace & closely inspect the filament string (all wires starting @ the 6.3v supply wires from the PT, thru all the pin connections @ the sockets & the pilot light) for bad solder joints, loose wires & such.

If you can't find anything visually, I would (be very careful here IF you do this) power the amp up & use a non-conductive stick (chopstick) & carefully move 6.3v wires around while watching the pilot light for signs of life. Use ONE HAND only if you do this, keeping the other hand in you pocket.

Do you have a schematic?

G

[Davidg]

 Thank You Geezer, I dont know what I was doing last night-filament is 3.5 volts on each string but I have no current- 0 ma.

Looks like the filament on the PT is shot, would u agree or could I be missing something.I used a chopstick to move wires and connections and there were no changes in volts or current.

I have a hand drawn schem without part values-these are very rare amps and that is all I could find. Dont think I will  be able to find a PT for it either.Its a shame cause it was a great sounding amp, best Tremelo I ever heard.

Looks like it was kept in a wet basement most of its life very dirty and corroded, there is some rust on the PT and OT-both handwound from the 60s,I will check filament secondary only in a bit to confirm it is the PT -Thank u for the help-alwys appreciated

[drgonzonm]

It is my understanding
the x505 and x305 are very similar. 

design is similar to ampeg designs of the era. 

There are copies of schematics on this website http://music-electronics-forum.com/t12511/

Additionally, there are some pics which may allow you to read some of the resistor values.  at  http://www.premierguitar.com/Magazine/Issue/2011/Sep/Souping_up_a_Sound_X_305_R.aspx

There is some additional infor at  http://www.collectorsweekly.com/stories/15014-sound-electronics-corp-x305r

I hope this leads you in right direction.    the collectorsweekly also gives an abbreviated history
 

[Davidg]

Thank you drgonzo, from what I could tell the X305 and X505 are identical amps with different speaker configurations. Dont know why I didnt think of this till last night but could I not get a seperate filament tranny to power filaments only or would that be more trouble than she is worth. Is thare any pitfalls to having an xtra tranny in there other than limited space and possibly noise/interference?

I found one rated at 20 amps,6.3 volts CT. Would this be overkill or should I try to get the smallest I could get by with and still have alll the current I need?
I figure the cooler I can keep the filament PT the better to reduce heat in the amp and the larger current rating would run cooler-Am I crazy,stupid or is this my first good idea since starting this little obsession with the tube?

[HotBluePlates]

Your amp is probably delivering proper filament voltage.

Thank You Geezer, I dont know what I was doing last night-filament is 3.5 volts on each string but I have no current- 0 ma.

If you measured from one side of the filament string to ground, and got 3.5v for each side, that is the same as saying your filament winding is delivering 7v. That's a believable reading if there is no current being drawn.

I don't know why no current is being drawn. You'd have to look for issues, like loose wiring, loose socket pins, possible broken wire leads, intermittent connections, etc.

It might be worthwhile to unhook some portion of the filament circuit (a few tubes perhaps), and connect them temporarily to your on-hand filament transformer. You're just looking to see if you can get signs of life. If the amp is in trashed condition, cleaning and revamping some wiring may be in order anyway.

The 6.3v 20A is massive overkill... that transformer should be HUGE! I've got a couple of 6.3v 10A transformers that are bigger than most power or output transformers. But, this filament transformer is good for some testing.

[Davidg]

Thanks Hot Blue Plates! That is what I was just thinking while I was reading Six-El-Six problem with the Carvin Legacy. I will try to do so tonight but my girl is still up and I dont work on any live voltage while she is in the house (cant be too careful).I think that corrossion and dirt may be my problem after all-I didnt realize that 3.5 was 1/2 of the voltage.I will report back when I have an answer.

[drgonzonm]

I would check the resistance on the pilot light.  If you can pull the lamp, check for resistance.  If you don't get a reading, then the lamp might be bad.  I would also do the same on each tubes heater. 

I am having a dense moment, how are you reading the current draw the heaters?  Seems to me that its a little tricky when using parallel heaters.  (The solution is probably easy). 

[SoundmasterG]

If it makes you feel better about the filament voltage, then instead of measuring from one side of the string to ground, measure from one pin to the other. So for a 12AX7, measure from pin 4,5(tied together) to pin 9. You should get 6.3v or a bit more if the string is unloaded.

I wonder if they may have used series heaters for some reason? If one of the filaments in the string was bad that could explain the problem. Start with the filament bulb and see if that is bad. Check over the wiring for the filaments too. You should be able to follow it from the transformer through to each socket on each half of the winding. Every point should have a good connection to the socket, and the socket should be tensioned correctly so that each socket makes contact with the pin of each tube. If not, then you would need to retension the sockets. Make sure the amp has been drained of all voltage and unplugged from the wall before you do this.

7868's sound great...you will like it once you get it going again.

Greg

[Davidg]

I know this is an old post but I wanted to thank all of you that replied and helped me get this thing going again!Turns out (if I am right) it was a series filament string and the wire going to the first power tube had a bad connection-the wire had actually cracked apart right at the solder connection on the second pin.I could power each tube and lamp individually with external power source but could not get the string to work.Went to reflow the pins snd found broken wire-resoldered and she is better than ever.Still the SMOOOTHEST Tremelo I ever heard Really amazing!Thanks again to all of you that helped and Doug for making it possible.

[rafe]

Could you post a photo or two?

[Davidg]

I will try to-it may be a day or so.Ive actually never posted or uploaded any pics but have been meaning to figure it out.My wife does all of that and she is gone tonight because our central HVAC went out today.I have a bunch of stuff I have been needing to sell and need to figure it out anyway so I will try @ get some pics of it in a day or two-Let me get cool again first!It really sucks when its 10 at night and  your sweating in your living room