Sunn 200S amp

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Sunn 200S amp

Postby coolhandjjl » Thu Sep 14, 2017 6:32 pm

Hi,
I got a Sunn 200S bass amp recently. 1969/70 perhaps? Apparently, it's essentially a Dynaco amp, not sure which one. It has been modded with the SDS Labs Mk3 Capacitor Board from Triode Electronics to replace the large can cap and solid state rectification has been added as well
http://www.triodeelectronics.com/sdslabmk3cap1.html
I have bias probes with a meter and I biased the KT88s to within .5ma of each other. (I got a pair of matched Gold Lion Re-issues from Jim McShane).
Are there some voltage points I should be checking to make sure all is operating up to spec here?
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Re: Sunn 200S amp

Postby cdmawolf » Sun Sep 17, 2017 5:39 pm

Pretty sure it is a Dynaco MKIII with a 12AX7 preamp.
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Re: Sunn 200S amp

Postby Geek » Tue Sep 19, 2017 12:07 am

I see a way of varying bias with no way of measuring it. Do they expect one to turn the knob until it redplates, then back off until it stops? (???)

With Gold Lion tubes, up those grid stoppers.... 22K at least! =:o
(they LOVE to make RF)
-= Gregg =-
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Re: Sunn 200S amp

Postby EWBrown » Tue Sep 19, 2017 2:50 pm

It should have a 10 ohm cathode resistor for each KT88, in order to properlymeasure the cathode / plate & Screen current, and set the grid bias correctly.

Perhaps they just go by the "set the bias to -55 volts, and call it good enough for government work" philosophy :-/ And keep a CO2 fire extinguisher handy (666)

/ed B in NC
Last edited by EWBrown on Wed Sep 20, 2017 9:25 am, edited 1 time in total.
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Re: Sunn 200S amp

Postby Geek » Tue Sep 19, 2017 3:13 pm

EWBrown wrote:Perhaps they just go by the "set the bias to -55 volts, and call it good enough for government work" philosophy :-/


That's what Mesa, Fender and others do, but they don't include a control to twaddle by (usually) untrained guitarheads.

That's why when you drop in hot-biasing tubes (usually at deep discounts), dey go *BOOM!* (sick)
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Re: Sunn 200S amp

Postby coolhandjjl » Tue Sep 19, 2017 7:23 pm

That schematic looks correct. Just to the left of the two KT88s there is a pot where I adjust to get the smallest ma difference between tubes. I have a set of bias probes with a multi-meter, and got them within .5ma of eachother. I don't recall what the actual ma was. I guess I can check this weekend. I just wondered if there were some places to measure voltages to make sure everything was running to spec.
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