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#1 2011-12-12 11:05:54

AEB-1
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Ampeg SB-12 buzzy distortion - Output voltage question

I'm working on a 69-70 Ampeg SB-12. It had a blown Output transformer so I ordered a replacement from fliptops and did a total recap while I was at it. Now the amp is dead quiet and has plenty of volume but there is a buzzy type of distortion that follows the notes. The output is pretty standard 6L6 setup. With no signal and an 8 ohm load, my bias reads about -55v (a little high) the screen grids read +490v (normal) but the plates only read +260v. The schematic says the plates should be +10v higher than the screens (near +500). The transformer center tap (red) has +500v but the blue and brown wires on the plates only read +260. Nothing is glowing or hot. What is causing the plates to read so low? Is that the cause of the background buzzing?

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#2 2011-12-12 18:49:47

hangman
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

That isnt right.

if you pull the power tubes,  and check the blue and brown wire,  do you still measure 260v?

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#3 2011-12-12 20:28:49

AEB-1
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

When I pull the tubes the plate voltage goes to the same as the red wire. I replaced the 6L6's and the 5AR4 with the same result. I did notice that the main b+ rises to about 240v when the standby switch is in standby. The standby switch applies gnd to the center tap of the high voltage winding. when off the center tap is floating. Should the b+ be zero when in standby mode?

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#4 2011-12-13 07:11:38

hangman
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

just to be sure,  pull the OT center tap and measure DC resistance between it and the blue and brown wires.

The amp should have no B+ when in standby.   I would go over your recap work and make sure that nothing got mis-wired. 
thats a circuit board amp... so I can't imagine its too weird. 

Also,  check the AC voltage on the Center tap... maybe your reservoir cap isn't working or isn't hooked up right?

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#5 2011-12-14 05:08:38

AEB-1
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

I had limited time last night but I double checked all the wiring against the schematic. The only deviation is on the replacement OT. The original OT had two black wires connected together in the tansformer while the replacement only had one. The original OT had one black wire connected to the speaker black wire and the other wire was connected to the circuit board ground. I wired the replacement OT black wire to the speaker black wire and ran a jumper from there to the circuit board ground.  Ohms-wise, the primary on the OT is approximately 100-0-95. I checked for shorts to ground. The PT secondary was also reasonable and not shorted to ground. Tonight I'm going to pull the board back up to make sure something isn't pinched. Having 260v B+ when switched to standby seems to be the major clue. Seems like it has to be PT issue. Tonight I'll pull the 5AR4 and take some measurements.

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#6 2011-12-14 07:07:48

hangman
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

260 is odd... but its more strange that you would have 500volts down the line.   it doesn't make sense that you would have a lower voltage at power supply point A than the rest of the amp.... because the rest of the power supply points are taken from power supply point A. 

the only possibility i could think of off hand,  is if the cap isn't working there.. and you're getting a pulsating DC voltage.  in which case your dc measurement wouldn't tell you the whole story.

does your amp have flyback diodes?

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#7 2011-12-14 10:00:24

AEB-1
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

I might mnot have been clear. Point A is approximately 500v when switched on. It is 250-260 when in standby. When I experience the low plate voltage, Point A=500, B=490,etc. The OT center tap=500. Only blue and brown show 260v (plates). When I go to standby, point a=260, b=250, OT center tap and plates are all 260. Bias stays around -55 in all conditions.

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#8 2011-12-15 06:44:44

AEB-1
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

No flyback diodes. I don't think Ampeg ever used them in this model. I'm missing something obvious so I decided to start over. I pulled the output tubes and the 5AR4 rectifier. Measuring from the center tap, the PT secondary reads 400ac-0-400ac. It goes to the correct pins of the 5AR4. the 5vac winding also goes to the correct pins of the 5AR4. With the tube out the 5v reads 5vac. I put the rectifier back in and measure the ponit "a" DC voltage while still in standby. The voltage is initially zero but as the tube warms up the voltage rises to roughly 260vdc. When I flip to "on" the voltage goes to 535dc but returns to 260 if I go back to standby. Still no output tubes. All three OT leads follow the "a" voltage. Could there be a problem with the PT?

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#9 2011-12-15 09:47:06

hangman
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

do you have an oscilloscope?

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#10 2011-12-15 10:47:08

AEB-1
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

Yes I do.

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#11 2011-12-15 23:03:12

hangman
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

if you put a 1khz sine wave into the input,  and plug a dummy load into the speaker output,  can you see this buzzy distortion?
I assume you've also tried another cabinet?

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#12 2011-12-16 09:01:04

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Re: Ampeg SB-12 buzzy distortion - Output voltage question

Hangman, I dug out the O-scope and discovered a massive (hundreds of volts p-p) high frequency oscillation on the plates. I've chased it down to the trebel control. When the treble control is full CCW the oscillation goes away and the voltages return to normal. Once I go about a third CW the oscillation returns. I'm thinking the 470pf cap in the tone module must be bad. I'm looking through my parts to find a spare cap. I'll post the results soon. Thanks for the help. You were definitely correct.

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#13 2011-12-16 11:14:53

hangman
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

the SB-12 likely has the wafer style tone module.  you'll have to replace all the parts.  See my thread entitled
"Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)"

if that doesn't help,  two other potentials:

There are no screen resistors on the SB-12.  I bet putting some screen resistors in would make a great deal of difference.   you could go with 220ohm or 330 ohm 5 watts,  but most folks go with 470ohm.
the grid resistors on the power tubes are rather low in the SB-12 too... I wonder if you boosted them up to like 4.7k or 6.8 k if that would help.
-steve

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#14 2011-12-16 13:55:12

AEB-1
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

I did the tone circuit upgrade and the problem still persists. If I take the signal side of tone stack completely out of the circuit by moving the input wire of the tone stack to the input grid of the phase inverter, the amp plays killer with lots of power and fullness distorion free. But if I just touch the output wire from the tone stack to the grid while the amp is working well, the amp immediately loses volume and becomes buzzy. As a precaution, I removed the sheilded external amp wire from the tone stack to be sure it didn't have a problem and there was no effect. I'm getting cross eyed on this one. Ampeg had two variations of hum balance on this model. The schmatic shows the hum wiper going to the neg bias voltage. My control has no effect on hum like that. If I put the wiper to ground then the hum control works. Any thoughts?

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#15 2012-01-31 18:47:47

AEB-1
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Re: Ampeg SB-12 buzzy distortion - Output voltage question

Steve,
With the holidays over I finally got back to the SB12. Not beinging convinced that the new OT from fliptops was an exact replacement, I decided to reroute and tighten the twist of the OT leads. That didn't help much. Then I replaced all the volume/tone wires with shielded cables. that got the oscillation to a real nice clean sine wave. Next I lifted one end of the 6.8k feedback resistor and wow, the amp is clean and loud, no oscillation. This amp is essentially a B-15 with only one preamp. The B-15 uses a 10k resistor so I changed the 6.8k to 10k. and the oscillation is less but not gone. Seems like the OT is backwards but I have OT blue to V3 and Brown to V4. Any suggestions?

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