Hiwatt Presence and Resonance

[Travis_HY]

Hey y'all.
Today as I was looking at a Peavey Windsor schematic and it's application of negative feedback on the first cathode resistor of the phase inverter, which is a cathodyne style. It's a Presence (and Resonance) circuit similar to an Orange. Now the question is: could an amp have two global negative feedback loops that connect to the OT but with different signal "sources"? Meaning, could you have a loop similar to this where the signal being tapped is earlier in the preamp; perhaps the EQ's cathode resistor as well the typical presence controls from the long tailed pair's second input? Is this just a wacky idea? Have I been sniffing too much solder? Can you have two loops like this?

Thanks in advance!

[Malcolm Irving]

In theory you can certainly have two 'nested' feedback loops, but it could be very difficult in practice. Because of phase shift in the forward path, at some frequency the negative feedback can become positive feedback and the amp loses stability. The further back you take the feedback injection point (towards the input stage of the amp) the more phase shift is introduced (by coupling caps) and the greater the problem. This is why amps generally only feedback to the PI stage.

[FUCHSAUDIO]

Yes, what Malcolm said. You likely would run into instability issues.

[Travis_HY]

In theory you can certainly have two 'nested' feedback loops, but it could be very difficult in practice. Because of phase shift in the forward path, at some frequency the negative feedback can become positive feedback and the amp loses stability. The further back you take the feedback injection point (towards the input stage of the amp) the more phase shift is introduced (by coupling caps) and the greater the problem. This is why amps generally only feedback to the PI stage.

Malcolm, you're the man!

Upon reading everything I could about feedback, including local feedback, I came to the conclusion that the phasing is really the issue. My idea was having one feedback loop that controls the resonance circuit in the PI and another feedback loop in the PI that controls the Presence circuit as I don't have the know-how to integrate a resonance circuit into a cathodyne phase inverter without applying the feedback to the cathode resistor of the first stage.

I was trying to find a way to preserve the Presence control in a Hiwatt build but also have a control to add bass to only the phase inverter like a resonance control. I could do a simple variable cathode bypass cap circuit as worst case scenario on the first stage of the PI, but I know that's probably not going to give me what I want without feedback to the OT; it's too subtle. Granted that, I could also omit the Hiwatt Presence circuit (it's not particularly useful, but it's subtly is it's virtue), and apply the Orange/Peavey Windsor idea using the Presence circuits inherit to that design.

Hiwatt Resonance circuits? Am I on the right path? Any thing I am missing?

Thanks for the interest, fellas. I appreciate your wonderful input!

[Bob S]

Do a search on Presonance here.
Martin Manning did some great work on this.
That's why we call it the Martinizer.

[Travis_HY]

Do a search on Presonance here.
Martin Manning did some great work on this.
That's why we call it the Martinizer.

I read everything I could on Resonance circuits on here, and most if it was related to NF in a long tailed pair PI, which didn't really help me all too much because of the weird PI design (now I realize it's not that weird) in a Hiwatt. The Presonance may go into my Deluxe Reverb as that amp has a preset resonance amount and a presence on the back. The Presonacne might add some versatility to those parameters.


I really should rename the thread now to "Resonance and Presence controls for Hiwatt" as I figured out how to do this yesterday. Basically it's the Peavey Windsor Resonance and Presence circuits, which tap off the first cathode of the first PI tube in a Hiwatt. After you remove the stock presence circuit, you also have to remove a few parts of the PI including the 470 ohm and .01 tail that is parallel to the 2.2k tail. The cathode on the last tube should be 22k and the tail is 2.2k with the 10k feedback resistor removed. If you copy the Peavey schematic verbatim, that'll get you close; I changed the resonance cap to .0047uf and the presence cap to .1 as these are typical values. I also changed the feedback resistor to 100k off the 16 ohm tap. I may be adding a switch to toggle the NF between the 16 and 4 ohm taps; that's a great mod for some different poweramp flavor. I also notice that this is where the feedback is tapped in a Hiwatt "Slave Preamp" so I knew it was a possibility.

Now after I'd done the mod, I turned on the amp and got a horrendous squeel because the feedback was positive; taking the feedback from that cathode is out of phase with the previously stock presence circuit. I then had to flip the primaries of the OT to flip the phase and *presto*: Resonance and Presence controls in a Hiwatt that worked relatively perfectly besides needing a reverse audio taper presence control. I was expecting this addition to be unstable but it works flawlessly. I think it sounds better than it did with the stock arrangement without a doubt.

I'm going to try to edit a Hiwatt schematic so you guys can see what I did and some sound clips soon. I have had fevered dreams about this amp over the past few nights so I'm really excited to share it with fellow amp nerds.
Thanks guys!

[Travis_HY]

Alright! Now be easy on me, this is my first ever schematic written in the digital realm so hopefully I got everything right. I think the pots are correct with the 'high' arrows being where they are.

I think i included everything on there and it's pretty easy to read. Let me know if this looks wrong anywhere.

[Gaz]

I got a little confused reading this thread quickly, but if your amp is really going to copy the Hiwatt power amp, then there's no reason you can't add a regular old resonance control in the feedback loop before or after the 10K feedback resistor. Just add a 10uF or bigger blocking cap to keep the control from being scratchy.

Also, just remove the useless 470R/10n to ground, which is just an "always on" presence circuit for super high frequencies. It may have been added for stability or something but it does absolutely nothing but add fizz to the overdrive tone. I always remove it in Hiwatt style builds.

And do not add the typical 47p cap you have across the inverter. That will actually make the amp unstable. Trust me on that one.

[Travis_HY]

I got a little confused reading this thread quickly, but if your amp is really going to copy the Hiwatt power amp, then there's no reason you can't add a regular old resonance control in the feedback loop before or after the 10K feedback resistor. Just add a 10uF or bigger blocking cap to keep the control from being scratchy.

Also, just remove the useless 470R/10n to ground, which is just an "always on" presence circuit for super high frequencies. It may have been added for stability or something but it does absolutely nothing but add fizz to the overdrive tone. I always remove it in Hiwatt style builds.

And do not add the typical 47p cap you have across the inverter. That will actually make the amp unstable. Trust me on that one.

Gaz, thanks for chiming in! Sorry it was confusing; this build has gotten my head spinning a bit and the thread directly shows that.

I had added the typical resonance control to begin with but without a blocking cap with DC on the pot. It didn't work so well because of said DC. Perhaps I'll swap it back now that I have your input.

I removed the "presence tail" before and I liked it without. I did add that PI cap just as a precaution but I certainly take your word for it. Oddly enough, larger values (100pf) actually made the amp "crash" when completely dimed on the higher gain channel (which is similar to a Jose, so lots of gain) probably because of phasing with the new FB network. Old Marshalls non master do the same thing when you play them with all the knobs dimed, 50 watters particularly. Of course, I would never be playing the amp at those settings but it's always good to have things as stable as possible.

Thanks for sharing your knowledge, Gaz. I'll make a new version of the schematic with an "unverified" note as it was a spec I arrived at after some tweaking.

[Roe]

could you elaborate on the problems with the 47/100pf snubber cap?

[Gaz]

For me, adding the PI snubber cap introduced static-like distortion on low transients. Sounded like a blown speaker, and removing the cap fixed. Never investigated much past that, but just assumed it was a phase demon. If it ain't broke...

[Gaz]

By the way, this power amp really does benefit from a resonance control because of the huge amount of NFB it uses. It's actually an effective control, especially if the preamp has lots of gain and you can keep the master low.

[Travis_HY]

I was having similar experiences to Gaz. I don't exactly know why, but that cap in a Hiwatt style PI seemed to introduce some weird stuff on the high end when the amp was really wound up. If you take it out, the "crashing" would go away. Must be Hiwatt specific.

I also tried Gaz's approach to the Resonance control and it works great especially with a preamp with more gain at lower volumes; and it's certainly less invasive. I have a 1M pot in there right now with a .0047 and it sounds really good, but I might back it off to 500k. Not much difference between 1M and 500k except a little bit more flub. Maybe a .0068 to lower the frequency to separate the Bass and the Resonance controls a little more; as they are controlling a lot of similar frequencies. Got a few shematics to write up. Thanks for all the info! You guys rule!!!

[Gaz]

Glad that's working well for you so far. FWIW, I like a 250KA pot best with the stock feedback values. 100KA may be even better.