Oscillation fixed / now NFB switch hums?

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asd
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Oscillation fixed / now NFB switch hums?

Post by asd »

(i had posted the previous thread about the Heyboer PT help)

The amp sounds KILLER!

Question surrounding the onset of oscillation/whistling when diming everything.

The amp is quiet as can be, ie: no buzz, or hum (aside from the usual speaker sound when an amp is cranked and your volume is down on the guitar.) At this time the chassis is not yet in a cabinet.

If I jump the channels a get a whine (oscillation?) I can actually dial it out if I roll down the treble on the TMB side.

It also presents as such when the channels are not jumpered and all controls are dimed. It starts to onset when the master volume gets up around 85%.

If I unplug the guitar while it is doing it, it stops. It will not do it if the volume of the guitar is rolled back. It does it on any of the 4 inputs.

It will also diminish if I touch the TMB treble knob or the other tone knob. (I have yet to put knobs on, so I touch the bare knob)

I'm gonna pick up some chopsticks tomorrow and start prodding around in it to see if I can narrow it down. Any advice on where to start or focus on? My guess its a dressing issue based on what I have read on the boards?

Thanks,
Scott
Last edited by asd on Mon 03/05/12 10:55 pm, edited 1 time in total.
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Post by asd »

Update. I had caught other messages on the boards with similar issues and it was suggested to try swapping the OT wires on pin 7 on the power tubes. (the blue and brown wires on the heyboer transformer).

http://www.18watt.com/modules.php?name= ... ght=squeal is the thread I had seen that mimicked my symptoms exactly.

I did swap them and the oscillation issue is now gone.

One funny thing that showed up now is the amp literally "Feedsback" when flipping the feedback switch to the other position. Presents as wild hum. The volume of the hum is fixed and remains loud as it is unaffected by either volume or master volume.

Prior to my swapping the OT wires, the switch was fine, although the sound was a bit hollow.

Any ideas? I did double check to make sure I didnt drop any solder anywhere .

Thanks.

Scott
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Post by rock_mumbles »

Sorry but ... NFB on a TMB 18watt ??? why?

So if you have howling with NFB 'on' you now have positive feedback not negative feedback ... so your OT leads are reversed.

If you want NFB you'll have to go back the way you had it ...

If you can live without NFB (a 'proper' 18 watt does not have NFB) leave it as is ...

You have to be really careful with NFB, it can remove all of the 'tone' from an amp
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Post by asd »

Being a noob, I just built the 36 watt Ritchie TMB build straight up from the layout / schematic without any changes and figured I'd go from there.

So basically, if I'm happy with the sound as is, it would be safe to simply remove the NFB portion of the circuit? I wasn't sure if there was something else going on that might need attention... didn't want the amp blowing up... :D

Scott
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Post by CurtissRobin »

When you swapped the primary OT leads you corrected the unwanted feedback condition but that also inverts the signal phase. To correct the phase of the NFB you now need to move the feedback lead to the other OT secondary lead.

Incorporating NFB in an amp smooths (stiffens) the dynamic characteristics. That's something that stereophiles demand but rarely what people want from an 18 watt amp. Those guys are reproducing music while we're making music. Different tools. (We've had a couple people build 18 watt amps with NFB in the past. I'm curious to know if they stuck with that or if they removed it.) After you've played the amp for a few months please let us know what you do.

KennyO
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Post by asd »

Curtis, forgive my ignorance. Would you be able to point me to location where that would be?

Are you referring to relocating the connection from the DPDT swith that runs to the 0.1UF?

I figure I might as well leave the switch in there as option. Plus I'm curious to hear / see how it changes the character of the amp.

Scott
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Post by Plexi »

Sorry, i'm not here often anymore. But to answer the question. The 36 watter, had quite a few changes over its life from the beginning. The Feedback switch was one of those options. It was like a preset presence. And when engauged it would clean up the amp. So one could if they wanted have more clean. If not, flip out the feedback. If you don't want to use the feedback switch, you can leave it out. Ground the tail resistor of the PI.
Many amps do use feedback. lots of marshall and fender amps and many other guitar amps.
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Post by asd »

Ritchie - Thanks for the info. I think I'm just going to remove it. The amp sounds fantastic as it is.

Plus I've been a little confused as to where to move the NFB connection to as suggested in the post that suggested:
To correct the phase of the NFB you now need to move the feedback lead to the other OT secondary lead
I'm not quite up to speed yet on the amp terminology. Lol.

Muchos Gracias,
Scott
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Post by Plexi »

no problem.

the amp lingo will come, like anything, just takes a little time.
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