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PostPosted: May 10th, 2020, 11:59 am 
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Grover,

I know I am posting to an old thread, but I have been working on a Williamson amplifier project during the shutdown here in DC. I started thinking about a Mullard circuit, but Charlie was so enthusiatic about building a Williamson, I decided to try that.

Output iron is a pair of Dynaco (DynaKitParts) A-431, and am using the Hafler/Keroes Dynaco Modified Williamson as a model since the feedback network is tailored for the transformers I am using. I am modifying the circuit in two significant ways. First, I am using a ECC99 in the differential driver stage, with a higher current (10ma vs around 4.5mA for the original 6SN7). I am replacing the 47k plate resistors with 20K plate resistors for the sections to keep the same voltage at the plate. Second, I am using a current sink under the differential pair cathodes.

The Dynaco circuit has a 100pF capacitor from the feedback loop to the plate of the inverted side of the driver tube that is there for high frequency compensation to limit response above 80 kHz and provide 12dB of stability margin. I believe this capacitor acts into the impedance at the plate of the driver, being the parallel resistance of anode resistance and plate resistor, essentially forming an CR network high-pass filter. Therefore, using an ECC99 will end up changing this, since the impedance is different. Based on a nifty online filter calculator for a CR high pass circuit, and modeling the original circuit values, the 100pF capacitor is acting into the parallel resistance of 9500 ohms (6SN7 at the op-point) and 47K plate resistor, or 7900 ohms. This results in a pole at around 200kHz. With an ECC99 with an anode resistance of 2300 ohms and plate resistor of 20k results in a parallel resistance of 2060 ohms. The feedback capacitor would need to increase to 390pF to maintain in the same 200kHz pole. Does this sound right to you? (Anyone else please chime in)

One amplifier (building monoblocks as usual) is almost complete, just tidying up the last bits (such as feedback) before I fire it up for a first test.

Hope things are well in Oregon.

David


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PostPosted: May 10th, 2020, 1:24 pm 
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David McGown wrote:
Grover,

I know I am posting to an old thread, but I have been working on a Williamson amplifier project during the shutdown here in DC. I started thinking about a Mullard circuit, but Charlie was so enthusiatic about building a Williamson, I decided to try that.

Output iron is a pair of Dynaco (DynaKitParts) A-431, and am using the Hafler/Keroes Dynaco Modified Williamson as a model since the feedback network is tailored for the transformers I am using. I am modifying the circuit in two significant ways. First, I am using a ECC99 in the differential driver stage, with a higher current (10ma vs around 4.5mA for the original 6SN7). I am replacing the 47k plate resistors with 20K plate resistors for the sections to keep the same voltage at the plate. Second, I am using a current sink under the differential pair cathodes.

As I recall the Dynaco amplifiers were not based on the Williamson design but on the Ultralinear (it had taps for the screen grid) that David Hafler pioneered.

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PostPosted: May 10th, 2020, 1:35 pm 
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Hey David! Cool that you're building a Williamson! Unfortunately, that level of calculation is beyond me. ;-) But 390pF seems like an awfully steep increase in the phase-lead cap. The danger is that you can increase ringing rather than decrease it if you use too much. Do you have a 'scope? If so I guess I'd play around with some values to see how it affects your 10kHz square waves. Also, since you're increasing the gain of driver stage, albeit slightly, I'd check to make you aren't increasing the feedback, which would affect stability. It's probably not significant, though. Capacitive loading across the load resistor can also tell you a lot about the HF stability. If you can get up to .22uF without the 10kHz square wave totally freaking out you're doing good. Hope I'm not telling you anything you already know...


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PostPosted: May 10th, 2020, 1:38 pm 
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SoundMods wrote:
David McGown wrote:
Grover,

I know I am posting to an old thread, but I have been working on a Williamson amplifier project during the shutdown here in DC. I started thinking about a Mullard circuit, but Charlie was so enthusiatic about building a Williamson, I decided to try that.

Output iron is a pair of Dynaco (DynaKitParts) A-431, and am using the Hafler/Keroes Dynaco Modified Williamson as a model since the feedback network is tailored for the transformers I am using. I am modifying the circuit in two significant ways. First, I am using a ECC99 in the differential driver stage, with a higher current (10ma vs around 4.5mA for the original 6SN7). I am replacing the 47k plate resistors with 20K plate resistors for the sections to keep the same voltage at the plate. Second, I am using a current sink under the differential pair cathodes.

As I recall the Dynaco amplifiers were not based on the Williamson design but on the Ultralinear (it had taps for the screen grid) that David Hafler pioneered.


Hey Walt! No, it's still a Williamson, which is characterized by the front end. Ultralinear Williamsons pretty quickly replaced the triode versions.


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PostPosted: May 10th, 2020, 2:34 pm 
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Joined: February 28th, 2013, 1:19 pm
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Grover, Walt,

Just to give you more information, the Ultralinear Williamson circuit of Hafler and Keroes is based on an article in Audiocraft, January 1956 (when I was 4 months old, btw). But Hafler and Keroes were playing around with Williamson circuits for Acrosound before Hafler launched the commercial kits (Dynaco MkII, etc. AFAIK), which was NOT Williamson designs.

http://www.pmillett.com/file_downloads/modernize_your_williamson.pdf

I am sure you all know, but a great resource for Williamson circuits is at per the following:

https://dalmura.com.au/projects/Williamson.html

There is an excellent analysis of the various circuit "improvements"

https://dalmura.com.au/static/Williamso ... 20info.pdf

I do have a scope, as soon as I fire up the first monoblock, I will check the 10KHz square wave response. I made sure to put the feedback network in an easily accessible location in the amp so I can make changes as needed.

Thanks

David


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PostPosted: May 10th, 2020, 5:16 pm 
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Hi David,

Based on the descriptions I'm seeing, such as "Hafler [22] introduced a 100pF capacitor from driver V4 anode to R4 to achieve additional 12dB margin, with bandwidth reduced from 200kHz to 80kHz", I'm thinking that the 'direction' of the feedback is from the plate of the diff amp back to the cathode of the input tube. If I read your analysis correctly, you are positing that the feedback arrangement is from the secondary of the OPT to one plate of the diff amp. (a'la Schade, sort of...).
In my scenario there are two 'sources' (OPT & Driver tube plate) and one 'sink' (Input tube cathode).
In your scenario, there is one 'source' (OPT) and two 'sinks' (Driver tube plate & Input tube cathode).
I can't tell you which one is correct...... ;) I'm not that good.....

Based on the 80kHz pole that is mentioned in the historical text, I believe we are looking for a resistance in the original circuit of approx. 20k that is paired with the 100pF to give a corner frequency of ~80kHz.

Sorry to have muddied the waters, perhaps....

Bruce



David McGown wrote:
The Dynaco circuit has a 100pF capacitor from the feedback loop to the plate of the inverted side of the driver tube that is there for high frequency compensation to limit response above 80 kHz and provide 12dB of stability margin. I believe this capacitor acts into the impedance at the plate of the driver, being the parallel resistance of anode resistance and plate resistor, essentially forming an CR network high-pass filter. Therefore, using an ECC99 will end up changing this, since the impedance is different. Based on a nifty online filter calculator for a CR high pass circuit, and modeling the original circuit values, the 100pF capacitor is acting into the parallel resistance of 9500 ohms (6SN7 at the op-point) and 47K plate resistor, or 7900 ohms. This results in a pole at around 200kHz. With an ECC99 with an anode resistance of 2300 ohms and plate resistor of 20k results in a parallel resistance of 2060 ohms. The feedback capacitor would need to increase to 390pF to maintain in the same 200kHz pole. Does this sound right to you? (Anyone else please chime in)
David


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PostPosted: May 10th, 2020, 6:12 pm 
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Joined: February 28th, 2013, 1:19 pm
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FerdinandII wrote:
Hi David,

Based on the descriptions I'm seeing, such as "Hafler [22] introduced a 100pF capacitor from driver V4 anode to R4 to achieve additional 12dB margin, with bandwidth reduced from 200kHz to 80kHz", I'm thinking that the 'direction' of the feedback is from the plate of the diff amp back to the cathode of the input tube. If I read your analysis correctly, you are positing that the feedback arrangement is from the secondary of the OPT to one plate of the diff amp. (a'la Schade, sort of...).
In my scenario there are two 'sources' (OPT & Driver tube plate) and one 'sink' (Input tube cathode).
In your scenario, there is one 'source' (OPT) and two 'sinks' (Driver tube plate & Input tube cathode).
I can't tell you which one is correct...... ;) I'm not that good.....

Based on the 80kHz pole that is mentioned in the historical text, I believe we are looking for a resistance in the original circuit of approx. 20k that is paired with the 100pF to give a corner frequency of ~80kHz.

Sorry to have muddied the waters, perhaps....

Bruce


Thanks, I hadn't thought of that being the path. This feedback is rather new to me, been a SET and PP triode (no feedback) in all my experience. fee In that case, the original value is probably OK since it is feeding back to the input stage. Though that is at a low impedance point in the circuit (470 ohm), so still not sure.

Well, I have bigger problems, I tried a combined bias level and balance network which is not working properly. On first powerup, the output tubes are redplating and bias voltage is super low. Gotta look at that first.

David


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PostPosted: May 11th, 2020, 12:13 am 
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Posts: 271
Nice post, Bruce! You might be right that there isn't a whole lot of adjustment needed. David, I too was completely new to feedback as of last summer and couldn't understand why things went drastically wrong when I made my first Williamson attempt. You might wan to read Dave Gillespie's superb breakdowns of the early Heathkit Williamsons at Audiokarma, specifically "Regilding the Gilded Lily" and "A New Beginning":

https://audiokarma.org/forums/index.php ... fo.669567/

Dave tends to employ a very aggressive step filter in the first stage to insure stability, and I have to say I agree that the sound is vastly improved in all instances. I have restored both W2's and W3's on hand, and used some of his tips in my recreated "Musician's Amplifiers."

You might want to revert to the original Hafler circuit to establish a base for experimentation. Tweaking Williamson amps is always an adventure in the unknown. You have the tried-and-true key component--the output transformers--so you are in well-established territory and it can all come out right in the end. The trouble arises when people say, "I bought these Edcor trannies etc."

BTW, my friend Jim, who has retired to Rehoboth Beach, has a pair of my "Gilded Lilies," which are the 22wpc "ultralinear" version of the Musician's Amplifier with original Peerless S-265-Q OPTs. He is very pleased with them. I know times are difficult, especially on the East Coast, but I'm sure he'd be happy to let folks hear them on an individual basis, and hopefully later bring them to a meet-up.


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PostPosted: May 11th, 2020, 9:39 am 
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Posts: 484
Grover,

Thanks for the recommendation on "Regilding the Gilded Lily", I will read it now that I am in wait mode until I get in new pair of bias transformers. I am convinced to set the feedback back to the original Hafler design (still keeping the ECC99 driver - I will be driving KT-88s to start)

The bias winding on the power transformers I used (Angela Universal Transformer) seems to be incredibly weak, and cannot bias negative enough for the KT88s. This was causing the output tubes to runaway, which of course draws the bias more positive, just compounding the situation. Everything looked ok functionally in the bias circuit and it produced the correct voltage with the rest of the power transformer unloaded (pulling the rectifier tube). However, on turn-on, voltage increased from -70V at the max setting of the bias pot to -45V. Of course, B+ dropped from 480V idle (tested without output tubes) to around 340V due to the massive current draw. Fun times!

David


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PostPosted: May 11th, 2020, 10:51 am 
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Can you voltage double on the bias winding?

Roscoe

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