Ampeg SVT3 pro bias problem

[boz_j]

Hi guys,

In first, english is not my first language, so I hope write best as possible to be understandable and not boring

It's my first post on this very interesting forum, I have read few posts about svt3 problems and I have not found exactly what I'm looking for.

Here a short description of my issue.

Since few months, the sound of my svt3-pro was not realy good, there was like a "fuzz" above the clean sound.
I have read that this problem can be generated because of a wrong bias.
So, I have open my amp and try to adjust the "AP1" resistor to set a correct bias but AP1 don't change the voltage on all 5W resistors (R32, R34, R36, R38, R33, R35, R37, R39 near output mosfets).
So, I have change the 12AU7 tube (and all the 12AX7) and try again to adjust the bias but the problem stays present.
AP1 won't adjust the bias voltage but it's value change...

I have some little skills on electronics but here I don't know what can I do to fix my issue...

Some guys here seems really good and I hope found help from you.

Thanks for reading me.

Regards,

Julien

(here, the complete shematic found on this forum)
Ampeg SVT 3.pdf

[diydidi]

Hi guys,

In first, english is not my first language, so I hope write best as possible to be understandable and not boring

It's my first post on this very interesting forum, I have read few posts about svt3 problems and I have not found exactly what I'm looking for.

Here a short description of my issue.

Since few months, the sound of my svt3-pro was not realy good, there was like a "fuzz" above the clean sound.
I have read that this problem can be generated because of a wrong bias.
So, I have open my amp and try to adjust the "AP1" resistor to set a correct bias but AP1 don't change the voltage on all 5W resistors (R32, R34, R36, R38, R33, R35, R37, R39 near output mosfets).
So, I have change the 12AU7 tube (and all the 12AX7) and try again to adjust the bias but the problem stays present.
AP1 won't adjust the bias voltage but it's value change...

I have some little skills on electronics but here I don't know what can I do to fix my issue...

Some guys here seems really good and I hope found help from you.

Thanks for reading me.

Regards,

Julien

(here, the complete shematic found on this forum)
[ATTACH=CONFIG]42241[/ATTACH]

Is the trimmer's wiper still intact? Could be a faulty pot.

[boz_j]

Hi diydidi,

Thank you for your message.
I have check the pot AP1 and it seems working fine.
I have a doubt on Q2, I'll change it and check again.

[MarkusBass]

Tubes have nothing to do with bias settings in this amp. Check that turning the trimmer changes voltage on the base of Q2. Be careful not to change the voltage to much - this may cause failure of the output transistors.

Mark

[Enzo]

You mentioned that you heard bias could cause this problem. Did you ever measure the bias to find out? In my experience, one of the most common problems with these amps is wrong tubes. Ther are both 12AX7 and 12AU7 in this amp, and it latters that they are in the proper sockets. Wrong tubes don;'t affect bias, but they do affect teh sound and balance of the amp.

[Dave H]

Since few months, the sound of my svt3-pro was not realy good, there was like a "fuzz" above the clean sound.
I have read that this problem can be generated because of a wrong bias.
So, I have open my amp and try to adjust the "AP1" resistor to set a correct bias but AP1 don't change the voltage on all 5W resistors (R32, R34, R36, R38, R33, R35, R37, R39 near output mosfets).

Sounds like it's crossover distortion. What are the voltages measured across the 0.47R 5W resistors? Disconnect the load (speaker) before measuring.
There's a thread about setting the bias current - here Check #11 and #17

[nhbassguitar]

https://www.talkbass.com/threads/svt...#post-18963040

Might want to read the whole topic, but that permalink is where the real meat is.

[boz_j]

Tubes have nothing to do with bias settings in this amp. Check that turning the trimmer changes voltage on the base of Q2. Be careful not to change the voltage to much - this may cause failure of the output transistors.

Mark

Hi Mark,
I have measured the voltage on Q2 base and it change when I turn AP1.
Just to test, I have changed Q2 but it's the same. I'm always not able to set the voltage on 5W resistors.

Julien

[boz_j]

You mentioned that you heard bias could cause this problem. Did you ever measure the bias to find out? In my experience, one of the most common problems with these amps is wrong tubes. Ther are both 12AX7 and 12AU7 in this amp, and it latters that they are in the proper sockets. Wrong tubes don;'t affect bias, but they do affect teh sound and balance of the amp.

Hi Enzo,

I try to measure the bias by measuring the voltage above all 5W resistors but I found 0,0mV on each one.
Saldy, all tubes are new and in the good soket.

[boz_j]

Sounds like it's crossover distortion. What are the voltages measured across the 0.47R 5W resistors? Disconnect the load (speaker) before measuring.
There's a thread about setting the bias current - here Check #11 and #17

Hi Dave,

I have get the voltage above all 5W resistors but I have 0.0mV on each.

[boz_j]

Thanks guys for your replies.

I have measured the voltage on each transistors grid and I can see something around 1.5V or -1.5V (with minimun AP1) if I'm on N or P chanel.
If I trun AP1, this voltage change but I don't understand why nothing change above the 5W resistors.
The grid voltage that I get seems good for you?

[Dave H]

I have measured the voltage on each transistors grid and I can see something around 1.5V or -1.5V (with minimun AP1) if I'm on N or P chanel.
If I trun AP1, this voltage change but I don't understand why nothing change above the 5W resistors.
The grid voltage that I get seems good for you?

There's no voltage across the 5W resistors because +/-1.5V is not enough to turn on the FETs. The FET's Vgs(th) is in the range 2V to 4V so Vgs must be greater than that. With minimum AP1 Q5 should be off and the current through R10+R11 and R16+R17 will generate a bias voltage of about 10V i.e +/-5V per FET. The low voltage could be caused by an open circuit AP1 or a short circuit Q5 but you have changed it. Measure the voltages across R14 and R19 so we can calculate the current through the bias circuit.

[boz_j]

There's no voltage across the 5W resistors because +/-1.5V is not enough to turn on the FETs. The FET's Vgs(th) is in the range 2V to 4V so Vgs must be greater than that. With minimum AP1 Q5 should be off and the current through R10+R11 and R16+R17 will generate a bias voltage of about 10V i.e +/-5V per FET. The low voltage could be caused by an open circuit AP1 or a short circuit Q5 but you have changed it. Measure the voltages across R14 and R19 so we can calculate the current through the bias circuit.

Hi Dave,

Here th voltage across R14 & R19:
UR19=2.20V so 1mA
UR14=2.25V so about 1mA

I'm sorry but I have wrote something wrong in my previous post. I have wrote "with minimum AP"1 but it was "with maximum AP1" so for the minimum bias voltage (if I'm not wrong). Sorry for this mistake.

What can I do to help you know?

[Dave H]

Here th voltage across R14 & R19:
UR19=2.20V so 1mA
UR14=2.25V so about 1mA

I'm sorry but I have wrote something wrong in my previous post. I have wrote "with minimum AP"1 but it was "with maximum AP1" so for the minimum bias voltage

Yes, 1mA looks about right. I calculated 1.2 mA by subtracting 0.6V (Q3 Vbe voltage) from 3.3V (D2 voltage) and dividing the result by 2.2k (R14). I calculate 0.4mA to be flowing through R12 so that leaves 0.6mA through the bias chain which is enough to generate about 4V per FET (if Q2 will turn off). I calculated 4V as follows – if AP1 is at zero and Q2 is off the 0.6mA must flow through (R10+R11) in parallel with (R16+R17) = 13.7k. 13.7k * 0.6V = 8V, that is 4V per FET.

Yes I think you are now correct, maximum AP1 is minimum bias voltage. To set the bias current you need to turn AP1 towards zero resistance while monitoring the voltage across the 0.47ohm source resistors of the output transistors.

EDIT: From post #11 in the thread I linked -

"The adjustment MUST be done with NO LOAD. Any load will draw a slight current due to small DC offsets, and messes up the reading. Just about any SS biasing should be done no-load.

Then also, it is 25mv, not 10 mv, measured across individual source resistors. That is the average per "set" of N-channel or P-channel mosfets.

This is for a unit with graded and matched mosfets. There should be a color dot on each device, and all N type or all P type devices should have same color, although the N and P may be different."