Hi,
I've been building a 6L6 PP amp for the past few months. I finished it a few days ago, checked out all the points for correct voltages, etc. But as soon as I plug in the output tubes and turn it on, I hear a loud buzzing at the OPT's (Hammond 5k 30w). The frequency differs for each channel, and changes if I switch the tubes.
I went into the circuit and saw that with the output tubes in place, I measured an oscillation at the output tubes' first grids (after R9 in the schematic). So I thought that it must be coming from elsewhere in the circuit. I took the output tubes out and measured again at the same point. No oscillation.
Now I thought it must be bad output tubes/incorrect OPT wiring. Re-checked the wiring, and it's fine. Ran the output tubes without the 6SL7 inputs: no oscillation.
Now I'm thinking it must be R15, the feedback resistor. I measure at the output: no oscillation. I don't get it.
The schematic is attached.
I've been building a 6L6 PP amp for the past few months. I finished it a few days ago, checked out all the points for correct voltages, etc. But as soon as I plug in the output tubes and turn it on, I hear a loud buzzing at the OPT's (Hammond 5k 30w). The frequency differs for each channel, and changes if I switch the tubes.
I went into the circuit and saw that with the output tubes in place, I measured an oscillation at the output tubes' first grids (after R9 in the schematic). So I thought that it must be coming from elsewhere in the circuit. I took the output tubes out and measured again at the same point. No oscillation.
Now I thought it must be bad output tubes/incorrect OPT wiring. Re-checked the wiring, and it's fine. Ran the output tubes without the 6SL7 inputs: no oscillation.
Now I'm thinking it must be R15, the feedback resistor. I measure at the output: no oscillation. I don't get it.
The schematic is attached.
Attachments
Hello!
Try to swap the output transformer secondary connections: swap the grounded wire with the one to the feedback resistor and see if it cures the oscillation.
Something that happenend to me at least once: check if you have connected each UL tap to the screen grid of the PROPER tube. I sometimes swap then and heavy oscillation occurs.
A note about the amp design: please note that C5 isn't absolutely needed at all
Try to swap the output transformer secondary connections: swap the grounded wire with the one to the feedback resistor and see if it cures the oscillation.
Something that happenend to me at least once: check if you have connected each UL tap to the screen grid of the PROPER tube. I sometimes swap then and heavy oscillation occurs.
A note about the amp design: please note that C5 isn't absolutely needed at all
Ok, trying to remove R15. It's very difficult to make changes now because I insulated everything very well (all covered in heat shrink and tape). My father decided to come to my aid and tried to measure the resistance between the cathode of VL1.1 and the hot output. Naturally, he got some strange number (768ohms), and told me to check R15 again.
Well, removing R15 allows you to divide and conquer. Your feedback, through R15, could be negative (good thing) or positive (bad thing - oscillator). This all depends on the polarity of ypur OPT secondary... very easy to get it backwards!
With R15 removed you can rule out other causes of oscillation.
With R15 removed you can rule out other causes of oscillation.
dmitriy167
768 ohms for R15 sounds about right. Remember both sides of R15 have a path to ground.
Path 1 -> OPT secondary.
Path 2 -> R5 1k.
That's because you've removed or opened the NFB path.
First I would do as poobah suggested. leave all the tubes in place and disconnect one side of R15. If the oscillation stops swap the secondary connections i.e. R15 and ground. Power up the amp (with R15 connected) and if the oscillation is gone...hooray!
Cheers
Wayne
768 ohms for R15 sounds about right. Remember both sides of R15 have a path to ground.
Path 1 -> OPT secondary.
Path 2 -> R5 1k.
That's because you've removed or opened the NFB path.
First I would do as poobah suggested. leave all the tubes in place and disconnect one side of R15. If the oscillation stops swap the secondary connections i.e. R15 and ground. Power up the amp (with R15 connected) and if the oscillation is gone...hooray!
Cheers
Wayne
From the input signal (HOT) to ground or input ground to chassis ground?
Do you live close to a TV tramission tower? Could it be 30Hz? The reason I ask is that it could be RFI. Check your grounding scheme.
Placing a 0.01uF across the input is not a good idea. That's 10,000pf of input capacitance! Your HI's will most likely suffer greatly! OTW you've made a low-pass filter.
Wayne
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