Sunn 200S drawing too much current at idle

[ddm]

Hi everyone..

I just picked up a 200s and rebuilt all the busted parts and put new electrolytics in..

Everything is fine except for some reason, the tubes cannot idle less than 95ma per tube.. if i turn the bias pot from min it only gets higher..

Every other part is stock except the electrolytics..

anyone have a clue?

Thanks
Dennis

[52 Bill]

Everything is fine except for some reason, the tubes cannot idle less than 95ma per tube.. if i turn the bias pot from min it only gets higher..

Could be too little bias voltage available from the bias supply. Or perhaps leaking coupling caps form the phase inverter pulling down the bias voltage.

[ddm]

Hi Bill

What would cause too little bias voltage? It says -55 for bias voltage and i get that..

Thanks
Dennis

[Enzo]

DO you get that at ALL grid pins? Pull the power tubes one at a time to see if one is drawing a ton more current than the others.

[ddm]

I have 2 bias probes and they are on the money with their readings.. Both tubes draw the same.. fluctuates within 1 or 2 mA.

Thanks
Dennis

[Mars Amp Repair]

Say,
do you get the -55V at the grids on the tubes themselves?

Also be certain the amp has a load on it as it might be unstable without a load.

As far as oscillating goes, the amp might be actually oscillating at a very high inaudible frequency that is causing the tubes to draw the high current. IOW, the amp may not be idling at all but putting out major power trying to amplify the high frequency.
If you have a scope, you will be able to see that on the plates as well as the speaker outputs. glen

[ddm]

Hi There Mars

I just checked everything again.. im getting max negative bias voltage of 49.7 and the tubes are at 98ma each at this setting..

If i turn the bias pot the bias voltage goes more positive and the the bias current increases... I have 488v at the plates...

I looked over the schematic and my bias circuit in the amp and the bias circuit looks wrong in the amp.. its defintely not what the schematic says..

Does anyone have detailed pictures of the inside of a sunn 200s?

Does anyone who has a 200s have a 250uf bypass cap on the input tube?

Anyone have pics of what the stock transformers look like?

Here are some pics of my internal bias circuits

Any help would be greatly appreciated..

Thanks

[Enzo]

Do your transformers have numbers on them like 3-60T and 3-6PT?

The bias supply photo doesn't look stock to me, where does the AC for it come from? The drawing calls for a bias tap on the power tranny and that appears to provide not much more than -55v. Perhaps someone rewired it for using the high voltage winding? They might have done that to give the bias adjustment greater range.

DO you have a scope? Can you scope the outputs for RF?

What tubes are in it now? KT88s?

[ddm]

Hi Enzo

Your right about the bias stage.. Its taken off the rectifier tube and its at 433v.

The pt is not stock as it doesnt have the bias tap on it..

The ot looks like a sunn part number..

kt88s are in it now.

I dont know how i would get the bias voltage more negative.. when i turn the bias pot the voltage goes more positive and it draws more current.. The min current now is 98ma per tube..

Do you have a hi res picture of the underside showing all the components?

Thanks
Dennis

[52 Bill]

the bias stage.. Its taken off the rectifier tube and its at 433v.

I dont know how i would get the bias voltage more negative..

Your first photo shows the replacement bias supply circuit. The last resistor on the left side, 47K is what is limiting the voltage going into the bias pot.

Try removing your output tubes, and then measure the bias voltage range available at pin #5, when you adjust the bias pot. I'll guess that the maximum negative voltage will be around -60v or less.

To get more negative voltage to the bias pot, increase the value of the 47K resistor to maybe 68K. Once this resistor has been changed, see what the range of the bias pot is. I would guess that you would need to have at least -70v as a maximum number.

Once you get enough negative voltage available, reinstall your output tubes and see if you can get the current draw down.

[ddm]

I had to put in a 100k resistor to get it doewn to -55 but the tubes still draw 75ma..

I tried pluggin in a bass and its very low volume even when cranked all the way up... and its distorted..

Anyone have an idea?
Thanks

[52 Bill]

I had to put in a 100k resistor to get it doewn to -55 but the tubes still draw 75ma..

I tried pluggin in a bass and its very low volume even when cranked all the way up... and its distorted..

Check the 2 phase inverter coupling caps for leakage

[ddm]

would leakage cause it not to have volume?

[J Martin]

Leakage can cause all sorts of problems like offsetting the bias in the power amp or pre amp. Sometimes it builds slowly until the tubes redplate other times in happens imediately. I had a Dynaco stereo 70 that had that problem and it turned out the caps feeding the outputs were leaky which fooled me for a while since the caps were new. An apt comparison as it turns out since the Sunns were based on the Dynaco circuit. I own about 8 Sunn heads and they have never been a maintenance problem.

You need to get the bias voltage up. My amps seem to need something in the mid 60's to bias correctly. Are the tubes fresh or are they old and drawing too much current? I have repaired Sunns with non original power transformers and they worked fine. Change the voltage divider again so you can get around -70v. at the high side of the pot.

[ddm]

To check for leakage.. Do you just measure for dc on one side of the cap?

Since im already at -55... do i need to go lower.. I know on the schematic it says -52 but i guess for different tubes etc stuff changes..

I went to 100k where the 47k resistor was on the bias network. should i change the one before the 47k resistor?

Tubes are fresk kt88s valve are..i use them in 2 master volume marshall heads.. The tube makes the marshall so dark and evil.. Its great.