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Old 19th March 2012, 01:33 AM   #1
mbeards is offline mbeards  United States
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Default Transient Responce of Williamson Amp build

Hi everyone,

I am working through my design on the classic Williamson high power amp ala Acrosound with a few modifications.
Here is the specific schematic:

Click the image to open in full size.

I first plan to do the modifications described in the article "Improving the Williamson amplifier" by Wright to get the 6SN7 tubes biased right. I have heard of others doing this with good results.

Later in the artical he talkes about improving the power supply of the amp to improve the transient response. The schematic I am working with above uses a different power supply than the one Wright works with so I am wondering what would be a good way to improve the design I am working with?

I know you have to be careful with capacitance directly connected to the cathode of rectifier tubes so I presume that changing the 10uf cap before the choke is not advisable to keep inrush current down. However, the rest of the caps seem wimpy so im not sure what an advisable amount is to increase them.

Also I am not sure if this effects possible improvements to the power supply, but I plan to loose the ultraliner design and go with some 6146W output tubes and use a regulator to drop the 450v supply to 220-230v for the screen grids. The 6146W does not like screen grids above 250v.

Thank for the help

Matt

Last edited by mbeards; 19th March 2012 at 01:36 AM.
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Old 19th March 2012, 02:11 AM   #2
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What are you using for O/P "iron"? Unless it's absolutely top notch, the phase shifts in Williamson style topology will eat you alive.
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Old 19th March 2012, 02:17 AM   #3
mbeards is offline mbeards  United States
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Quote:
Originally Posted by Eli Duttman View Post
What are you using for O/P "iron"? Unless it's absolutely top notch, the phase shifts in Williamson style topology will eat you alive.
It is a custom wind from a guy who does rewinds for Mcintosh and others. It is very nice indeed. He is the one who turned me onto the build with his excellent results.

Also I know the acrosound schematic improved some of the stability issue with the filter on the plate/grid connection of V1.
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Old 19th March 2012, 02:21 AM   #4
kevinkr is offline kevinkr  United States
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I agree with Eli, and further I suspect that pentode mode operation will make the problem even worse unless the very best iron is used.

I think the Mullard 5-20 design or the Citation II approach might be a little bit more likely to work and be stable.

The charm of the Williamson was that properly implemented it had sufficient open loop gain for a rather prodigious amount of negative feedback to be applied, the price paid for this was the need for an exceptionally good output transformer. The original Partridge transformer used in this design was rather special.
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Old 19th March 2012, 03:50 AM   #5
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Williamson provides some basic OT inductance measurements to make - did they at least compare with the benchmark levels - your winder would have checked them I guess.

You should measure your PT winding resistances, and check the design limits for capacitor input level with the rectifier tube you have chosen. You may be able to increase the first cap a little. You can increase all cap levels downstream of the choke - back in the late 1940's, the range of cap values was limited and cost was a big concern - but not so nowadays.

You can full wave rectify the bias supply, with a soft series resistor, and extra downstream capacitance, so as to lower any residual ripple. Increasing the cap bypass on the heater elevation may also be worthwhile, as its impedance to ground is a hum loop path.
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Old 19th March 2012, 04:20 AM   #6
mbeards is offline mbeards  United States
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Thanks for the tips.

I assume that the cap bypass on the heaters is the .1uf in parallel with the 15K?
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Old 19th March 2012, 05:51 AM   #7
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Yup
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Old 19th March 2012, 10:20 AM   #8
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True orig Williamson uses cathode autobias...as for transient response in near class A, power supply demands is different and softer.
I still beg to disagree to others who find the o/p tranny criteria to be so critical. The LF pole becomes critical due to the absurd values of original concertina interstage coupling caps....Morgan Jones Valve amps 4th ed, page 472.
I have a couple of 1955 des 88-50 o/p trannies made by Parmeko for the 50W Williamson design, and the phase margins are narrower than modern replicas by Sowter & others. (I daren't use the original transformers as it's pretty guaranteed the insulating varnish is peeling, hence the capacitance change).
The proper concertina "out cap" values should be around 10-22nF, and I regulary find the optimised circuit can take another +15dB of global nfb (on top of the 20dB) before instability starts with a step filter in the global nfb path.
In my 40 yrs I've never found it a "flighty" design, probably due to my design insistence of using stabilised power supplies for each section. Also, by putting a CCS in the Williamson driver tails does open up excellent peformance possibilities, bandwidth & top end performance.

I realise that I've optimised many of the disadvantages of these types of amps to make them into excellent runners.

richy
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Old 19th March 2012, 09:25 PM   #9
mbeards is offline mbeards  United States
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So I drew up this schematic with some of the modifications I plan to do. Still a work in progress but it is getting there.

Williamson Amp 3 .bmp

Last edited by mbeards; 19th March 2012 at 09:37 PM.
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Old 19th March 2012, 10:15 PM   #10
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It's important to keep in mind that the lead and lag networks' values are for a completely different amplifier, and cannot be known yet. As others have already mentioned, pentode operation and a large RC timeconstant both exaggerate stability issues. Some Zobels around the output transformer "cain't hoit".

All good fortune,
Chris
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