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Old 12th July 2012, 03:24 PM   #1
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Default DIY Guitar Amp Oscillation

Hello everyone.
I built the amp using attached schematic.
The problem is that it oscillates. Frequency of oscillation varies from about 1kHz to 20kHz. I do not hear all frequencies but I can see them with an oscilloscope. I can stop the oscillation and change its frequency and intensity by moving of anode wires of output tubes. However even when I find such place where the amp is not oscillate it is so unstable that I donít like such solution (it is enough to move even a little part of either wire for a couple of millimeters to start the oscillation). It is very interesting that changing the places of the anode wires (changing the phase of the OT) does not change the situation significantly (it only changes the frequency of the oscillation). In all tests Gain was at least in the middle (usually at maximum).
When I put signal input wire to ground oscillation stops.


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(Is there any better place for files?)

The sound of the amp is cool (when I find the place for anode wires at small level of volume).
What shell I check further?
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Old 13th July 2012, 11:58 AM   #2
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Does it oscillate with the volumes turned right down?.

Often it's down to weither a mistake in construction, or poor layout.

With valve amps you can at least remove valves to help isolate the problem.
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Old 13th July 2012, 01:43 PM   #3
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Quote:
Originally Posted by Nigel Goodwin View Post
Does it oscillate with the volumes turned right down?.
No

Quote:
Originally Posted by Nigel Goodwin View Post
Often it's down to weither a mistake in construction, or poor layout.
What do you mean under a mistake in construction? Schematic mistake?
What should I check primarily if it is poor layout?

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Originally Posted by Nigel Goodwin View Post
With valve amps you can at least remove valves to help isolate the problem.
If I put away any tube - oscillation stops. Without output tubes there is also no oscillation.
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Old 13th July 2012, 03:06 PM   #4
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Quote:
Originally Posted by Alexander Baev View Post
No


What do you mean under a mistake in construction? Schematic mistake?
No, a wiring error.

Quote:

What should I check primarily if it is poor layout?
Post a picture of it's construction - but it's essential to keep inputs well away from outputs.

Quote:


If I put away any tube - oscillation stops. Without output tubes there is also no oscillation.
If oscillation stops when you remove any, then it must be feedback from the output to the input - does it do it with no input plugged in (I'm assuming you've used the shorting jack socket on the input?.

But as V1 and V3 are on different channels, only unplugging one of them should stop it, the other (as it's not connected) shouldn't make any difference.
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Old 13th July 2012, 03:12 PM   #5
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May be a feed back via the power or heater lines, too.
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Old 13th July 2012, 03:19 PM   #6
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Quote:
Originally Posted by Nigel Goodwin View Post
Post a picture of it's construction - but it's essential to keep inputs well away from outputs.
I will a little later. (But the input is far from output).

Quote:
Originally Posted by Nigel Goodwin View Post
If oscillation stops when you remove any, then it must be feedback from the output to the input - does it do it with no input plugged in (I'm assuming you've used the shorting jack socket on the input?.
If I put out the cable from the input (in this case the input is connected to the ground) - the oscillation stops. If I leave the cable in (the length of the cable is 4 meters) but put the signal wire to ground on not connected end of the cable at maximum volume level small oscillation is still detectible.

Quote:
Originally Posted by Nigel Goodwin View Post
But as V1 and V3 are on different channels, only unplugging one of them should stop it, the other (as it's not connected) shouldn't make any difference.
The problem appears only on distortion channel (V1,V2). I disconnected everything which has no deal with the distortion channel (the character of oscillation did not changed).
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Old 13th July 2012, 03:20 PM   #7
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Originally Posted by Osvaldo de Banfield View Post
May be a feed back via the power or heater lines, too.
How can I check this?
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Old 13th July 2012, 04:07 PM   #8
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Try adding some low value caps along the wires: a .1uF 600V across supply rail and chassis (or GND if chassis is floating), and see if oscillation frequency varies in frequency and or amplitude. Also add a "snubber" in the output stage. A snubber is a R-C series circuit in parallel to the output transformer, try for start, a resistor of about 1.3 times the load impedance (say, 6500ohm for a 5K output transformer), and a .01uF 2KV or more, and re-chech amplitude/frequency of the oscillation. If oscillation disappears, fix it in the circuit. Use the lower cap and the greater resistor that dampens the oscillation.

Caution: resistor not inductive (Means not wired wound), and be careful with high voltages in the circuit, and the resistor may become hot!!!
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Old 13th July 2012, 05:30 PM   #9
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The first thing to do is get the phase right. The output at the speaker should be the same phase as the input at Loop_out. The phase inverter circuit take care of making the feedback subtract from the input. If the phase is incorrect, reverse the PS1 and PS2 lines between the phase inverter and the power tubes.

There are no grid stoppers on the output tubes. Install a 10K resistor at the power tube socket in series with the pin 5 wires.

There is a mistake the way the schematic is drawn. It shows a virtual ground near F3 and F5 in addition to the ground on pin 4 of the DC supply for the preamp tubes. Remove the 100 ohm resistors. If you have not implemented the DC heater supply, leave the 100 ohm resistors in.
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Old 18th July 2012, 02:00 PM   #10
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Originally Posted by Osvaldo de Banfield View Post
Try adding some low value caps along the wires: a .1uF 600V across supply rail and chassis (or GND if chassis is floating), and see if oscillation frequency varies in frequency and or amplitude.
I tried this - nothing changed.

Quote:
Originally Posted by Osvaldo de Banfield View Post
Also add a "snubber" in the output stage. A snubber is a R-C series circuit in parallel to the output transformer, try for start, a resistor of about 1.3 times the load impedance (say, 6500ohm for a 5K output transformer), and a .01uF 2KV or more, and re-chech amplitude/frequency of the oscillation. If oscillation disappears, fix it in the circuit. Use the lower cap and the greater resistor that dampens the oscillation.
Adding the snubber changes the frequency of oscillation but doesn't eliminate it.
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