#1
Hey guys, a friend dropped off an AC15CC1 with no sound.

It's fairly odd for me. I changed all the tubes, and I do have hum/noise from the speaker when I turn up the reverb, and the tremolo works (it modulates the hum, lol). Simple pop test gets a pop from the speakers on each side of each preamp tube except pins 6 & 7 on v1. But V1 does not light up or get warm. Swapped v1 and v2, and still, only V2 lights up. Power tubes light up as well.

Heater connections test for continuity as well.

any ideas? Not much of an tube amp tech, but I know some of the basics of electrical work, built pedals, discharging, etc. simple stuff.
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Last edited by Reincaster at Apr 2, 2013,
#2
Measure your plate voltages.
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#3
V1: Pin 1 504V
V1: Pin 6 504V

V2: Pin 1 463V
V2: Pin 2 463V
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#4
OMG. testing the plate voltage, and the wire to pin 6 V1 just came off. going to get that fixed now.
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#5
Done, plate voltages are still the same.
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#6
But is it working now? The poor connection to pin 6 isn't the culprit?


Hang on. Plate is pins 1 and 6 on a 12AX7. That's a typo above, no?
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Cathbard Amplification
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Last edited by Cathbard at Apr 2, 2013,
#7
Quote by Cathbard
But is it working now? The poor connection to pin 6 isn't the culprit?


not working yet. Reverb controls increase the hum, and tremolo controls modulate the hum, , but no sound from the regular input. No response when I try the pop test on pin 6 and 7 on V1. Resistors and caps from the triode to ground test and look okay. There is continuity from the input board to the main pcb.

EDIT: Yes, that's a typo on V2 Pin 2. I meant pin 6.
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Last edited by Reincaster at Apr 2, 2013,
#8
i am not totally sure about this, but i don't see why you couldn't.

could you run the effects loop out to another amps return, you would narrow it to see if it was the preamp is the problem. make sure you have proper load.

please confirm before doing what i say. i have personally done it, but don't want to pass bad advice.
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#9
Well if the voltages are all ok to the tubes you're just gonna have to chase the signal path looking for breaks and faulty caps (and maybe resistors). Make sure the signal caps are blocking DC. They normally go short.
Also check tube sockets are actually making a proper connection to the tubes.
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#10
I have tried plugging in my mixer into the RCA return for the amp (cc1 has no effects loop) and have got sound, I'm fairly sure something is lost in the preamp, so I'm going to check there. I think it's suspicious that V1 has no temp or glow, so I'm going to go through the preamp. Luckily it's not as complicated as a doublesided pcb on the mesa boogies, lol!
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#11
I found tapping on the capacitors helped isolate the faulty ones on 2 of my amps.

Check your heater voltages on V1.

Good luck.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#12
Quote by R45VT
I found tapping on the capacitors helped isolate the faulty ones on 2 of my amps.

Check your heater voltages on V1.

Good luck.


How does the capacitor tap test work? I found one that transfers the tap to the speaker, right in between V1 and V2, which is the PI.

Heater voltage on V2 is the same as V1, they are linked at Pin 9, and a DMM test confirmed the same voltage and continuity. Pin sockets look good as well, can't retension the ones in this amp, but I thought it was worth a second look.

There is definite sound when I dropped the reverb tank taking it out of the leather holder thing, so I believe the poweramp section (V2, V3, V4) is working just fine. I think I just have to bite the bullet and trace the paths from each V1 pin and confirm each component is working.
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#13
If it makes a noise when you tap it, it's f***ed.
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#14
Quote by Cathbard
If it makes a noise when you tap it, it's f***ed.

Nice, one down, gonna double check to see all of them. I'm guessing this one was enough to prevent the preamp from getting through. It's C11 in this schematic:

EDIT: found C12 also bad.

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Last edited by Reincaster at Apr 3, 2013,
#15
Ridiculous plate voltages on those pins and the V1 tube doesn't light up? I'd say something is borked in the heater circuit. There's no current flowing through the b side of V1 at the very least. You shouldn't have voltages that high on the pins, which indicates no voltage (or very little voltage) is dropping across your plate resistors.

Check voltages on both sides of R17 and R18 as well as the plate resistors for V1 to see if there is any difference.

Example: Check voltage on the HT side of R17, and then compare that to the voltage you get at the pin 6 side. Check the HT side of R18 and compare that to the voltage at the pin 1 side. Etc., etc. Do that for all plate resistors. Should be a sizable voltage drop in each case. If not, it is an indication of very little (or none at all) current flowing through the tubes.

Your mention of V1 not lighting up or getting warm points to the likely culprit: heaters.
#16
Thanks so much! I've been researching similar amps and ac15/30 clones, and I notice one thing, both sides of each tubes heater circuit (Pin 4+5 and Pin 9) are tied to power. There is nothing on the V1 Pin 4 and 5. Should that be tied to Pin 4+5 of V2? Otherwise, it seems like the heater circuit is not being completed.
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#17
Quote by Reincaster
Thanks so much! I've been researching similar amps and ac15/30 clones, and I notice one thing, both sides of each tubes heater circuit (Pin 4+5 and Pin 9) are tied to power. There is nothing on the V1 Pin 4 and 5. Should that be tied to Pin 4+5 of V2? Otherwise, it seems like the heater circuit is not being completed.


You're welcome.

Pins 4, 5 and 9 on a dual triode are the heater pins. Pins 4 & 5 are the opposite ends of the heater filament (pin 4 is the 1-2-3 side and pin 5 is the 6-7-8 side)and pin 9 is the center tap. In an AC heater circuit, pins 4 and 5 are tied together and connected to one leg of the filament supply while pin 9 is connected to the other leg. In a DC heater circuit, pins 4 & 5 are tied together and connected to the positive DC supply while pin 9 is connected to ground.

If pins 4 & 5 are connected to nothing, then that is the problem. Connect them to 4 & 5 of the other tube.
#18
Quote by CECamps
You're welcome.

Pins 4, 5 and 9 on a dual triode are the heater pins. Pins 4 & 5 are the opposite ends of the heater filament (pin 4 is the 1-2-3 side and pin 5 is the 6-7-8 side)and pin 9 is the center tap. In an AC heater circuit, pins 4 and 5 are tied together and connected to one leg of the filament supply while pin 9 is connected to the other leg. In a DC heater circuit, pins 4 & 5 are tied together and connected to the positive DC supply while pin 9 is connected to ground.

If pins 4 & 5 are connected to nothing, then that is the problem. Connect them to 4 & 5 of the other tube.


DING DING DING. Amp is working now. Still replacing those caps.

I guess I didn't catch it before because they don't usually include the heater circuit on amp schematics ( I guess that's Amp Building 101, lol)

Thanks guys! Learned a ton here, going to approach one of those MOD 101 tube amps now!

PS. I thought that 500v was pretty high for preamp tubes, but I was measuring from the pin to chassis ground, is there another, correct method to measuring plate voltage?
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Last edited by Reincaster at Apr 3, 2013,
#19
You're shitting me? The heater wasn't connected. How the **** did that happen?
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#20
Quote by Cathbard
You're shitting me? The heater wasn't connected. How the **** did that happen?


I got the amp open already, it was sitting for a few years until my friend just asked me if I could take a look.

I want to assume the heater wire broke off at some point (terrible little solid core wires in here, like the ones in cat5 plenum), and it wasn't obvious to me.
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#21
Hmmm.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#22
Buddy rewire that whole thing... sounds like a can of worms otherwise. I think even touching anything could cause another break or poor connection. Voltage drops a plenty...


Glad you found it. CEC, Cathbard, ECistheBest and Mmolterax are pretty smart and helpful guys. I learned a ton from them.


Do you have pictures of this beast?


Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#23
Quote by R45VT
Buddy rewire that whole thing... sounds like a can of worms otherwise. I think even touching anything could cause another break or poor connection. Voltage drops a plenty...


Glad you found it. CEC, Cathbard, ECistheBest and Mmolterax are pretty smart and helpful guys. I learned a ton from them.


Do you have pictures of this beast?



Do a ptp conversion? haha. I think I want to rewire the wires coming off the board to the tubes, otherwise it looks fine. Fairly thin wires doing the job here. What is usually used? Solid core? Threaded?

I didn't mean to say they were the actual wires from cat5 cable, just that they look like it, lol. Amp is completely stock and un-modded, but seeing how that wire just detached, and the others, seems like that's where they cut corners!
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#24
Maybe one of the other guys will jump in here and comment.

http://tubedepot.com/wire.html

Typically stranded wires are needed were a bit of flexibility is required. Solid would break in no time.

Solid has the advantage of being able to be shaped. It can look much neater. However the amp isn't on display and that should not dictate how you repair it.

What gauge wiring is used now? Do you want to keep as original looking as possible? Does it have cloth insulation? Just a few thoughts before you replace the screwy wiring.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#25
Quote by Reincaster
DING DING DING. Amp is working now. Still replacing those caps.

I guess I didn't catch it before because they don't usually include the heater circuit on amp schematics ( I guess that's Amp Building 101, lol)

Thanks guys! Learned a ton here, going to approach one of those MOD 101 tube amps now!


No problem, glad to be of help!

Quote by Reincaster
PS. I thought that 500v was pretty high for preamp tubes, but I was measuring from the pin to chassis ground, is there another, correct method to measuring plate voltage?


That is correct. When measuring those voltages, you want to be referencing to 0v DC. The chassis should always be at 0v DC.

You can reference to anything, but the meter will only give you the difference in potential. So for example if you referenced to 300v DC and measured a true 500v point, the meter would only read 200v.

That said, when taking voltages, always reference the chassis which can almost invariably be assumed to be at 0v DC.
#26
^Yep, and if the chassis isn't at 0 Vdc then you stand an excellent chance of being electrocuted since you will usually be grounded and thus be at roughly 0 Vdc, thus the difference in potential might flow through you.
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#27
Quote by Kevin Saale
^Yep, and if the chassis isn't at 0 Vdc then you stand an excellent chance of being electrocuted since you will usually be grounded and thus be at roughly 0 Vdc, thus the difference in potential might flow through you.


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