Peavey Classic 30 amp repair question

[Planobilly]

Hi,

New to all this. This amp has power to the tubes. It does not blow the fuse. There is no sound at all. There is no sign of anything burned or out of place looking in the amp. Speaker test good. I have ordered new tubes which will get here next week.

I took the tubes out. All three 12ax7EH pre amp tubes sockets show a 330 to 336 volts at pin 1 and 6. I get no voltage on the heater pins on the 12ax7EH.

The EL34's (4 of them) all read 342 volts on pin 7. All the heaters light up in the EL84's. I did not measure the voltage of the heaters on the EL84's as they seem to be working.

There are two full wave  solid state rectifiers using silicone diode 1N4003 on the low power transformer output which measures apx 15.5 volts AC and 1N4007's on the high output side....I forget what I measured but around 250+ volts.. Point being I thank the voltage coming out of the two rectifiers is correct, more or less.

Here is a link to the schematic     http://www.bustedgear.com/images/schematics/Peavey_classic-30.pdf

Question 1, Can I put the new tubes in to check without fear of screwing up the new tubes.

Question 2, Where should I look next to find the problem?

Sorry for all the detail but I don't know enough to know what is important.


Thanks,


Billy

[picburner]

The 12AX7 filaments are all in series.
If you measure the voltage of the single filament with all valves extracted clearly you measure no voltage.
Keep in mind that the EL84 filaments are AC supplied  while 12AX7 filaments are DC supplied.
Since the EL84 filaments light up, the fuse F1 is intact.
-36V voltage is present between R63, R66 and GND?
If yes probably one of the valves has a melted filament, if no R66 may be open or may be open one of diodes 1N4003.
Good luck.

[Shock]

I wouldn't put in the new tubes until you confirm what the problem is or at least exhaust options before doing this.

It sounds like you don't have an oscilloscope or a signal generator so you could plug a pedals output into the loop receive of the amp, this will bypass the preamp. But don't even consider this if you are missing voltages.

You will need a pedal that causes some sort of noise/oscillation if you do this test and remember to plug a spare 1/8" plug or cable into the pedals input otherwise most pedals will not switch on properly.

For missing voltages if you cannot work out what is wrong, pull the plug straight from the wall and leave the tubes to cool, this will discharge the circuit enough to pull the tubes and allow you to remove the tubes without damaging them, keep track of which one tube was where.

Then when you reconnect the amp you can check your supply voltages to see if a tube was causing an issue add them back one by one if you need to and avoid damaging or red plating your new tubes.

[Shock]

Be careful with safety as well. There are endless ways to get caught out measuring a live circuit.

Plus you should also know by now that the filter caps must be checked and drained before working on a fully depowered amp replacing or measuring components, and it be disconnected from the wall. Here is some essential reading (the whole chapter) that you should learn before messing around.
http://www.allaboutcircuits.com/textbook/direct-current/chpt-3/importance-electrical-safety/

Everyone ages differently but starting late you need to have a rock solid idea on what safe procedures are and what is unsafe.
 

[cvanc]

Hi Billy-

Please clarify the filament situation.  You state the output tube filaments light up, but how about the 12AX7s?  Do their filaments light up?

[oldway]

We used to diagnose and repair tube amplifiers with only one instrument: a 20Kohms/V analog multimeter. (I used an Hioki AF105 and an Avometer
First, you must be aware of safety with high voltages because it is lethal.
Only work with only one hand, don't secure the chassis nor anything with the other hand.
Be sure what you do, check twice.

I would touch C31 (V3A pin 2 GRID of 12AX7 with a screwdriver , eventualy with your finger), and you must ear a hum in the loudspeaker.
If this is OK, you know your power stage is ok.
If not, you should first check connections of output transformer and loudspeaker and replace the tube 12AX7.
If still not working, you will have to measure voltages of the power stage and compare with schematic.

If power stage is working, you have to look at the preamplifier.
Check for bad contact at J2 and J3 jacks.

Again, try to touch V1B pin 7 GRID of 12AX7...you should ear a hum in the loudspeaker if it is allright.
If not, replace V1 (12AX7)

After doing all this, report here.

NB: you can also use a signal injector (signal tracer) in place of your finger, that's safer ...

[Planobilly]

Thanks guys,

I need to go back and read everything said again but this is what I have done while I was away from the forum.

Hi Guys,
Peavey Classic 30 issue...no sound at all

Amp powers up, pilot light stays on, 70watt light bulb in variac normal, does not blow fuse, all three fuse checked good, B+ at 336 (6V high per schematics), all heaters powered up, no visual issues on close inspection, installed new tubes with no change, no open resistors (did not check every value) speaker checks good, checked effects loop inputs, 440hz signal reaches plate per scope clean signal, checked all jumpers from each PCB and found no issue

At this point I need some advice as what to check next or based on the above what other things could cause no sound at all. Going back to read all the post now.

Cheers,

Billy

EDIT: SAFETY... I read the whole chapter three. A good review of "how not to kill yourself" Well worth the time to read.

This is sort of a cross roads for me. I put the amp back together and was going to give up. To hard to deal with the problem, am I smart enough to do this, don't know what I am doing??? All these questions I am asking myself. Well....if I am going to be a repair guy I will never get there if I give up on the hard stuff. So....take the bloody thing apart  one more time and try again. I ask these same questions learning to fly. I ask the same when I crossed from Europe to Florida by myself in my sail boat.

I may have to ask a lot of stupid questions of you guys but I am going to fix this amp!!!!

[john_p_wi]

Shouldn't take more than 5 minutes to find. Check for power, grounds and signal.

Check if you are getting signal past the effects loop send and return jacks. This is a common problem point. Inject a signal into the return jack....  anything?  If not, check the jacks / effects loop. Move towards the phase splitter. Does a disturbance test on the grid of the proceeding gain stage make a pop?

If so start working back towards the input and find where the disturbance test stops making pops... etc etc.

[Planobilly]

To answer picburner question "-36V voltage is present between R63, R66 and GND?"

Yes

Part of the problem is you can not get to anything much on the PCB in the chassis. It is in three sections connected by jumpers folded into a U shape. When you take it out the input transformer leads are to short to reach the PCB and all the output leads are to short. The output transformer is mounted in such a way that the leads rap around the chassis and terminate on three pin connectors that will not go through the hole in the chassis. All this is most likely the cheapest way the MFG could build the amp but makes it super hard to work on.

Getting late and tomorrow is another day....2:20 AM here.

Working on high voltage stuff when you are tired is just plain stupid!! Enough for one day!!

Cheers,

Billy

[Planobilly]

Hi guys,

FIXED

After way to many hours messing with this amp I found the cause of the problem. There was a bad jumper connection between two PCBs in an area I could not get to without taking everything out of the chassis and unfolding the three boards.

After I fixed that everything else was easy. Now the amps sounds very good.
 
Really bad design to make a amp that you cannot get to the back of the tube sockets with tearing the whole thing apart.

The amp drove me a bit crazy for a while...lol

Thanks for all the feedback.

Billy

[Brumby]

... am I smart enough to do this, don't know what I am doing??? All these questions I am asking myself. Well....if I am going to be a repair guy I will never get there if I give up on the hard stuff.

I think everyone here has felt the same at some point - the key is not giving up.  Sometimes stepping away for a bit is the best move.  You can then come back to it with a clearer head.

I may have to ask a lot of stupid questions of you guys but I am going to fix this amp!!!!

Well, you have certainly achieved your goal - and you aren't the first one to have been ripping your hair out for a simple bad connection (and there's lots of variants).

Also, I wouldn't be worried about asking 'stupid' questions, we all have to learn - and sometimes the really stupid questions are the ones that didn't get asked.

[Planobilly]

Thanks for the kind words.

Billy