Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 22, 2013, 04:23:36 pm

Title: Is this an NFB or a Tone Control ? (Dynacord KV10)
Post by: kagliostro on March 22, 2013, 04:23:36 pm

The circuit betrween the "CT" of the secondary winding of the OT and the screen of V2a

is an NFB circuit or a Tone Control ?

Thanks

K

Title: Re: Is this an NFB or a Tone Control ? (Dynacord KV10)
Post by: overtone on March 22, 2013, 05:12:06 pm

I have a few 50's amps that use the NFB in combination with capacitors to act as a tone control.
Below is a gut shot of my KV10. Hope the image loads.
Ignore the addition on the left side, someone had added another channel so that a four piece band could run through it.
The control is rather crude compared to the usual tone stack, I think of it more as a dirt control.

Title: Re: Is this an NFB or a Tone Control ? (Dynacord KV10)
Post by: HotBluePlates on March 22, 2013, 11:16:35 pm

Quote from: kagliostro on March 22, 2013, 04:23:36 pm

is an NFB circuit or a Tone Control ?

Yes.





Which means, it is a negative feedback circuit which provides tone-shaping.

Title: Re: Is this an NFB or a Tone Control ? (Dynacord KV10)
Post by: jazbo8 on March 23, 2013, 03:02:42 am

NFB & Tone control.

Title: Re: Is this an NFB or a Tone Control ? (Dynacord KV10)
Post by: kagliostro on March 23, 2013, 09:16:38 am

Thanks for answering to All

I was thinking to NFB circuit firstly as having a cathode intake

this grid intake sounds unusual to me

K

Title: Re: Is this an NFB or a Tone Control ? (Dynacord KV10)
Post by: HotBluePlates on March 23, 2013, 10:26:40 am

Quote from: kagliostro on March 23, 2013, 09:16:38 am

I was thinking to NFB circuit firstly as having a cathode intake

this grid intake sounds unusual to me

No, it's not exactly unusual. Negative feedback is applied to a point in the circuit where it will have an opposite polarity to the guitar signal.

So you ask why not apply feedback directly at the (split-load) phase inverter, like you might see with a long-tail inverter? It would probably have a tendency to have a major impact on one of the outputs and almost no impact to the other. So the feedback is applied to the pre-gain stage ahead of the split-load inverter.

Why grid instead of cathode? Well, at that point in the circuit, application of feedback to the cathode would result in positive feedback after each of the circuit elements that invert polarity. So the feedback is applied at the grid to give an opposite feedback polarity than if feedback were applied to the cathode.

Title: Re: Is this an NFB or a Tone Control ? (Dynacord KV10)
Post by: kagliostro on March 23, 2013, 10:55:43 am

Many Thanks HotBluePlates

Very interesting

K