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Volume loss in B52 AT100...

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Postby AndyM » August 17th, 2011, 6:08 pm

New here, not sure if this is the right place to post this.

Before anyone tells me, yes I know this is a very problematic amp. I've heard it before.

So during practice about a week ago, just starting up the most epic clean sounding fingerpicked riff I know, my amp stopped working. I don't remember if there was a pop or anything. But I've lost volume on all three channels. I think I may have blown a powertube, but I'm not entirely sure (I've been asking around, and I've gotten at least twenty different answers as to what may be wrong, along with a lot of "why did you buy this, you should have spent $3000 on a marshall or mesa instead").

Anyway, I've already tried switching out guitars and cables (including the speaker cable). Nothing. I'm certain it's a problem with the head. The rectifier tube is fine (I think...); I tried switching to the solid state rectifier (the amp has switchable rectifier modes): nothing. No improvement. No improvement on the all tube rectifier either, so the SS mode is fine.

I already tried replacing all the preamp tubes. Nothing.

I was told one of the screen resistors may be open; so I pulled out my DMM and checked them: no problem. They're fine.

Anyway, I'm still getting sound, but it's very low; when I crank the master and channel volumes, I get a very low powered, overdriven sound.

As for the tubes themselves, the heater filaments are still glowing. But when I switch from "standby" to "on," I can't see the voltage jump into them (if you don't know what I'm talking about, watch your power tubes very closely when you switch on. I'm not talking about the glow from the heater filaments, I'm talking about the actual voltage). Upon mentioning this, I've had several people tell me they have no idea what I'm talking about.

So anyway, any ideas?

Also, I've been putting a BOSS ME70 through the effects loop. I know a lot of multi-effects units don't go well with effects loops, so could this be harmful to my amp's health?
Delay and wah... that's almost one pedal too many...
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Postby Blue Jay » August 19th, 2011, 8:25 am

AT-100 features and specifications

• All-Tube Signal Path
• Tri-Mode Rectifier: Tube - Class A or AB,
• Solid-State - Class AB
• Overdrive Channel: Gain 1 and Gain 2
• Each with Independent Master Volumes,
• 3-Band EQ, Assignable Contour Control
• Clean Channel: Master Volume, 3-Band
• EQ and Bright Switch
• Global Controls: Volume, Low-Res
• (Resonance) and Reverb

• Effects Loop with Level Controls
• (Footswitchable)
• Line Out with Level Control
• Selectable 4 / 8 / 16 Ohms External
• Speaker Jacks
• 4-Position Footswitch: Channel, Gain,
• Reverb and Effects Loop
• Ground Lift Switch
• Dimensions: 27" x 11" x 11"
• Weight: 53lbs
The AT-100 features rock-solid 13-ply Baltic birch construction

AT-100 Tube Chart

Power: 4 – 6L6 / 5881

Preamp: 4 – 12AX7

Phase-Inverter: 1 – 12AX7

Reverb: 1 – 12AX7

Effects-Loop: 1 – 12AX7

Rectifier: 1 – 5AR4

Not a power tube issue since you have 4 and could probably run the amp on a specific choice of two only, but that's not it.

Maybe the biasing circuit went, but maybe something in the speaker cab went on the fritz! Try another, or blame the amp?

So, use another amp with your speaker/cab, or try the amp itself with a different speaker or cab & look inside cab too? :shock:

I see from specs that you can do without a reverb tube for awhile, and swap it to phase inverter or the "push-pull" function circuit, see if it works. I only presume that the amp could work with no effects loop tube too, if you damaged it, switch off.

You're new, but you seem to have been asking around. Anyway, I can't speak for the forum, just my own advice and what I would earnestly, or honestly do myself. I'm glad you can use the Digital Multi Meter, but don't touch anything hot or "live"!!!
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Postby AndyM » August 19th, 2011, 8:58 am

Blue Jay wrote:You're new, but you seem to have been asking around. Anyway, I can't speak for the forum, just my own advice and what I would earnestly, or honestly do myself. I'm glad you can use the Digital Multi Meter, but don't touch anything hot or "live"!!!


Don't worry, I've opened up amps before. I know to drain the caps before touching anything.

I've tried switching heads; the cab's not the problem. Mainly it's the voltage not jumping into the tubes that makes me think it's the head.

I'll check the bias and see what I can do. Other than that I'll probably end up taking it to a tech.
Delay and wah... that's almost one pedal too many...
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Postby kent_eh » August 23rd, 2011, 6:36 pm

AndyM wrote:
Blue Jay wrote:You're new, but you seem to have been asking around. Anyway, I can't speak for the forum, just my own advice and what I would earnestly, or honestly do myself. I'm glad you can use the Digital Multi Meter, but don't touch anything hot or "live"!!!


Don't worry, I've opened up amps before. I know to drain the caps before touching anything.

I've tried switching heads; the cab's not the problem. Mainly it's the voltage not jumping into the tubes that makes me think it's the head.

I'll check the bias and see what I can do. Other than that I'll probably end up taking it to a tech.


Do you have the capability to *safely* check the voltage(s) coming from the power supply section?
Is the plate voltage getting to the standby switch? (*! caution 400-500 volts*!)
Is the standby switch faulty?

That's where I'd start.


By the way, do you have a copy of the schematic? That's always helpful when you're troubleshooting.
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Postby AndyM » August 30th, 2011, 6:27 pm

kent_eh wrote:Do you have the capability to *safely* check the voltage(s) coming from the power supply section?
Is the plate voltage getting to the standby switch? (*! caution 400-500 volts*!)
Is the standby switch faulty?

That's where I'd start.


By the way, do you have a copy of the schematic? That's always helpful when you're troubleshooting.


Yeah, I printed off the schematic before opening it up. Checked the voltage and it's reading just fine. Standby switch is fine too.

I finally found an amp tech in the area. I'm just gonna take it to him. This is getting a bit out of my comfort zone.
Delay and wah... that's almost one pedal too many...
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Postby kent_eh » August 30th, 2011, 7:29 pm

AndyM wrote:I finally found an amp tech in the area. I'm just gonna take it to him. This is getting a bit out of my comfort zone.


Let us know what he finds. I'm a bit curious.

If nothing else, it'll help the next guy that comes along.
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Postby bluefish750 » June 18th, 2012, 7:47 pm

I know this is an old post .. but just had the same thing happen to me .. after some tinkering , I found the main fuse BLOWN .. replaced & worked great again (1A-250V) .. yeah i know , sometimes your overlook the obvious !!!

:lol: :lol: :lol:
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