Feedback with Battery Bias

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David
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Feedback with Battery Bias

Post by David » Wed Nov 22, 2017 9:06 pm

Hi All,
on the WAD300b pp the cathode resistor is 1k and feedback is applied at the cathode end of a 33r lift resistor. If I'm using battery bias does anyone know if I can connect the battery minus to the 33r lift resistor and apply feedback in the same way?
Thanks

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Nick
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Re: Feedback with Battery Bias

Post by Nick » Wed Nov 22, 2017 11:56 pm

Assuming (without a diagram) its:

Cathode of valve -> +ve of battery, -ve of battery -> 33r resistor -> 0v

Then the feedback link should be ok between the 33r resistor and the -ve of the battery.
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Re: Feedback with Battery Bias

Post by David » Thu Nov 23, 2017 7:55 am

Thanks Nick

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Re: Feedback with Battery Bias

Post by David » Sun Dec 03, 2017 6:56 pm

Hmm, I get screaming feedback connected like this.............

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Re: Feedback with Battery Bias

Post by Nick » Sun Dec 03, 2017 9:59 pm

Is the 1k cathode resistor normally bipassed with a cap?

What is the full circuit?
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Re: Feedback with Battery Bias

Post by David » Mon Dec 04, 2017 7:57 pm

1-300Bcircuit.pdf
(59.18 KiB) Downloaded 29 times
Hi Nick,
here's the circuit. The amp wrks perfectly with or without cathode bypass cap, some preferring the sound without and also with battery bias. I used the original circuit values and all I get is feedback with the switch in the feedback on position, the amp behaving perfectly without feedback.
Thanks

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Re: Feedback with Battery Bias

Post by Neal » Tue Dec 05, 2017 3:23 pm

I've just checked my old 300B PP and the f/b works fine with battery BIAS, it should work if you connected it up as Nick described. The only difference for mine is I have R4 at 100R not 33R. I don't think that will make much difference TBH.

Is this a clone build of the amp or an original? Do you have a picture of how you've plumbed in the battery?
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Re: Feedback with Battery Bias

Post by Mike H » Tue Dec 05, 2017 7:04 pm

David wrote:
Mon Dec 04, 2017 7:57 pm
1-300Bcircuit.pdf
Hi Nick,
here's the circuit. The amp wrks perfectly with or without cathode bypass cap, some preferring the sound without and also with battery bias. I used the original circuit values and all I get is feedback with the switch in the feedback on position, the amp behaving perfectly without feedback.
Thanks
Do you also get the positive feedback if the 1k resistor and bypass cap are used instead of the battery?


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David
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Re: Feedback with Battery Bias

Post by David » Fri Dec 08, 2017 5:17 pm

Hi All,
The amp has worked perfectly for 5 years. Over that time I’ve tried cathode bypass capacitor, no bypass capacitor and then landed on battery bias with no issues and glorious sound. The only issue now and all connections are correct is the feedback squeal which occurs when using battery bias and then also when I did a test moving back to the cathode resistor. I then thought possibly the feedback squeal was caused by the channels being incorrectly connected, so tried reversing. The amp appeared to be working correctly with and without feedback then BANG BANG two loud cracks like bullets and a smell of burning……

On investigation the two 820r droppers that I use before the valve rectifier had exploded firing the end connectors off like bullets, one of which had destroyed the feedback switch in the process. An output transformer also appears to have gone open circuit. Now, I can’t for the life of me work out how this occurred given that the rectifier never looked distressed and that all the shorting current –if that’s what is was-would have had to have gone through the rectifier…..

Anyone proffer a theory?
Thanks

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Re: Feedback with Battery Bias

Post by Mike H » Fri Dec 08, 2017 9:34 pm

Don't understand this, you used "battery bias with no issues and glorious sound." But then: "The only issue now and all connections are correct is the feedback squeal which occurs when using battery bias" :?:

What has changed?

The amp appeared to be working correctly with and without feedback then BANG BANG two loud cracks like bullets and a smell of burning……

On investigation the two 820r droppers that I use before the valve rectifier had exploded firing the end connectors off like bullets, one of which had destroyed the feedback switch in the process. An output transformer also appears to have gone open circuit.
Sounds like a 300B went short-circuit and took out both the OPT and the rectifier resistors. :(


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Re: Feedback with Battery Bias

Post by Neal » Sat Dec 09, 2017 10:01 am

Yup all a bit confusing, a picture of how the battery was implemented may have helped but I fear things have now moved on a touch! :shock:

What are these dropper resistors for? Are you using a GZ37? Is this a clone version?
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Nick
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Re: Feedback with Battery Bias

Post by Nick » Sat Dec 09, 2017 10:57 am

Going back a step, it sounds like you have connected the left output to the right feedback and vice versa, and that may have caused some huge motorboating, Surprised at the damage its caused though. In what way is the output TX damaged?

If I understand what you have said, the feedback happened after you removed the battery and went back, so that would indicate either something else has changed, or something decided to take that moment to fail.
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Re: Feedback with Battery Bias

Post by Mike H » Sat Dec 09, 2017 5:29 pm

He says OPT gone open-circuit.
Neal wrote:
Sat Dec 09, 2017 10:01 am
What are these dropper resistors for? Are you using a GZ37? Is this a clone version?
Make a guess they are in series with the rectifier anodes to add DC resistance to the HT winding.
 
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Re: Feedback with Battery Bias

Post by David » Sun Dec 10, 2017 6:33 pm

Hi All,
There was nothing strange that a photo would have revealed about the way battery bias had been installed. It was in the standard way as was the resistor put in in place of the battery bias as described. The amp is a clone but uses exactly the same circuit as the kit. The only mod done was to add one 15h choke on the ht of V1 which tightened up the front end, along with the battery bias. The two 820r resistors were used on the anodes of the GZ37 to reduce dissipation for the TJ Meshplate 300s which sound great and were very happy at a measured 23.8watts.

No 300b appeared distressed when the resistors exploded and no oscillation was audible when the bangs occurred. It is possible the channels were connected incorrectly at first causing the oscillation but any short would have to have been major to explode the 50w droppers and would have had to have drawn current through the GZ37 which didn't appear distressed. The output transformer on one side has gone open circuit whereas I would have expected a shorted valve or similar to cause a short circuit or even show some degree of burn out of the transformer none of which is there.
David

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Re: Feedback with Battery Bias

Post by Mike H » Mon Dec 11, 2017 12:33 am

David wrote:
Sun Dec 10, 2017 6:33 pm
No 300b appeared distressed when the resistors exploded and no oscillation was audible when the bangs occurred. It is possible the channels were connected incorrectly at first causing the oscillation but any short would have to have been major to explode the 50w droppers and would have had to have drawn current through the GZ37 which didn't appear distressed. The output transformer on one side has gone open circuit whereas I would have expected a shorted valve or similar to cause a short circuit or even show some degree of burn out of the transformer none of which is there.
David
There's no doubt an appreciable current flowed to blow those resistors, also seems like the OPT primary as it is thin wire. A 300B with zero negative grid bias (e.g., grid has gone open circuit) can pass enough current, as can also a GZ37, how do I know? I've done it, fortunately I had fuses. And same as you neither the 300B nor the GZ37 were harmed.

Your oscillation could be the audible manifestation of a much higher frequency instability, in the radio range, I've also had this, also with 300B's. In that case I had two in parallel with grids and anodes joined together, made a perfect VHF oscillator, to stop it I added grid stoppers, that is a small value resistor in series with each o/p valve grid. You could try that also. Values in range of 1k - 4.7k. I see there are none in the schematic.



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