Author Topic: 200S bias  (Read 4621 times)

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Offline george

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200S bias
« on: July 14, 2004, 09:20:20 pm »
200S with 6550s in the output.  I'm assuming that the standard bias setting will work for these tubes? They are old RCA stock with minimal usage.

Anonymous

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200S bias
« Reply #1 on: July 16, 2004, 05:24:46 pm »
I think he schematic calls for -55VDC. If you want to protect your tubes and extend their life a little, you might want to raise the bias current to -58 to -60VDC. I don't think you'll head any differance but you will not be working the tubes quite as hard.

Tom

Offline Isaac

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200S bias
« Reply #2 on: July 17, 2004, 01:28:42 pm »
Isn't that backwards, Tom? I thought higher bias increased the quiescent current through the tubes, moving them further into class A, making for nicer distortion but lower power and shorter life.
Isaac

Offline JoeArthur

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200S bias
« Reply #3 on: July 19, 2004, 04:10:21 pm »
No... increasing the bias, meaning providing a higher negative voltage to the control grid will reduce the current flow through the tube.  Reducing the negative voltage (i.e. moving it towards zero) will increase current flow.

Offline bentbolt

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200S bias
« Reply #4 on: August 01, 2004, 05:52:26 pm »
silly questions to add:
does this apply to KT88's also?
can I bias by ear?
what is the "best" tube for a low and rumbling overdriven sound for these 200s heads?
I get a great sound with JJ 12ax7 and GZ34 and KT88's, but wonder if I'm going in the right direction?
also- what make is best for the driver tube 6an8a? can I make a substitution?
when will I need to have the caps done?
hope this ain't too much to ask all at once!
THANKS!!!!
deru kugi wa utareru

Anonymous

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200S bias
« Reply #5 on: August 06, 2004, 07:54:46 pm »
yeah, well... uh...
help me out?

Offline Soundmasterg

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200S bias
« Reply #6 on: August 20, 2004, 12:21:50 am »
The KT88 has a higher plate dissipation rating...I believe it is 42 watts instead of 35 watts, so you could bias it a bit hotter. Generally, people recomend to bias somewhere between 60% and 70% of the rated dissipation. What current you actually set it to depends on what the plate voltage is. Notice I said current and not voltage. If you set it to a particular negative voltage, you will never know what the actual current is, because the relationship isn't linear.....meaning they don't track each other exactly. Also, every tube is different and will have a different amount of bias current for a given voltage.

I'll use my 200S as an example. It has about 565v DC on the plates. I've added 1 ohm resistors to ground, hooked up to tip pin jacks that I added to the back panel so I can measure the actual bias current for each tube. I also added a seperate bias pot for each tube. If I set each tube to 41ma of current with the bias pot for each, then that works out to about 23.165 watts of dissipation. (565*.041=23.165) If I take the max dissipation value of the tube I am using, and multiply by 70%, then you can see I am just under 70% with the 6550, but I could still go quite a ways with the KT88. (35*.70=24.5 for 6550) (42*.70=29.4 for KT88) Each time you adjust the bias voltage with the pot, the plate voltage will change, as will the current through the tube. You can use the Sunn schematic's suggestion and just set the voltage at the grid of the power tubes to -55v, and call it good, but you don't really know how much current the tube is dissipating that way. If you have a bias probe, you can use that to find out what the actual current is, as I did with the 1 ohm resistors. Incidentally, I have a bias probe, and it was quite accurate as compared to the resistors. I just did mine this way so that I won't have to take the chassis out of the head to measure voltages. I guess I'm lazy that way!

You can also bias by ear, and set it where it sounds good....but make sure if you do it this way that you watch the tubes for at least 15 minutes in a dark room after you set it to make sure the plates aren't glowing orange or red. When you set the bias, whether by ear or by meter, or even a scope, you should always have all the controls at zero on the amp, nothing plugged in, and the output of the amp should be hooked to a resistor load, NOT a speaker. If you hook it to a speaker, your bias setting will not be accurate. If you are doing it by ear and don't have the necessary equipment, then you can use a speaker, but again, watch it for glowing plates!!

Your tube setup in yours sounds like a pretty good choice and it should sound pretty nice for bass. I've found that having the original Sunn 2x15 cabinet with the original JBL D140's is VERY important to the sound. It will work with other cabinets, but just doesn't sound as good. Hope that helps?

Greg

Offline Soundmasterg

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200S bias
« Reply #7 on: August 20, 2004, 12:33:10 am »
A couple more things to add that I didn't answer in the other post.

You can use a 7199, a 6AN8, and a couple others. The socket may have to be rewired however. I prefer the 6AN8 as it sounds good and is MUCH cheaper than a 7199, and is more common than the others.

When do you need your caps done? That is opening a can of worms....some people say to never replace caps on a vintage amp until they fail, because then the amp isn't original. Other say any caps older than 10 years, replace them. They often get into pretty big arguements about it.

I suggest to measure the caps for DC leakage and ESR, and if they are leaky or have high ESR, then replace them. This of course requires some special equipment, which I admit I don't have yet myself. I also don't know how to do it yet completely, but an engineer friend of mine suggests it and it sounds good to me. I completely rebuilt my 200S and replaced every cap in it, and every resistor also, except for 2 resistors. I also used isolated jacks, and wired up an isolated ground buss, hooked to a ground lift switch, as Kevin O" Connor recommends in his TUT1,TUT2, TUT3, etc series of books. It made the amp MUCH quieter, and punchier and tighter. When I got mine, there were some bad caps somewhere in the power supply and I couldn't even get the bias up past 20ma without the plate voltage dropping to about 390v. Replacing the caps fixed that, and since I rebuilt the rest of the amp, I now know I won't have to do anything to it barring tube replacement for at least 10 years, AND it sounds as it did when it was new. It cost me maybe $50 and a couple weeks of work, but it was worth it! If you don't know when the power supply caps were replaced, and they don't look like they have been replaced, then I suggest to do it. It can only improve the tone and reliability, especially if you intend to use it for bass.

Greg

Offline bentbolt

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200S bias
« Reply #8 on: August 21, 2004, 11:19:36 pm »
well, a LOT of that flew right over my head, but I get the general idea...
thanks for the reply. I'll have an experienced SUNN amp tech look into the cap situation soon.
Would love to find one of the 200s cabs with the JBL's, but I'm fairly pleased with the SUNN 215 (don't know what its actually called- sealed box with SUNN speakers- silver grill cloth of the second design, Fenderish looking...)
I punish this amp regularly with both guitar and bass, so getting it fresh and reliable is more important to me than ALL ORIGINAL (sorry, vintage junkies- I'll rub it in a little and say that it's a LOW FOUR DIGIT serial number, too :o )
one more thing, I've heard two sides to the impedance on this amp...
it's got an 8 ohm and a 16 ohm tap, is it true that I can only use one of these at a time? or does it switch down to 4 ohms if I use both taps? would love to put this through more speakers like I've seen in old Noel Redding photos! he was running each head through THREE CABS, if I see it right...
PROST!
deru kugi wa utareru

Offline loudthud

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200S bias
« Reply #9 on: August 22, 2004, 10:13:18 am »
Ok, both sides of the impedance story can be true depending on when your amp was made. The transformers used on all the Tubed Sunn amps I've ever seen (except Model T) had 4, 8 and 16 ohm taps but only two of the taps are connected to the speaker jacks.The remaining tap is there, but it's not connected to the speaker jacks. In your case, the 4 ohm tap just goes to a blank terminal on a terminal strip. Your amp could be modified to use the 8 and 4 ohm taps, it's a matter of moving two wires. (Note that the 16 ohm tap has a couple of connections internally that should be left connected.)

Here's how the jacks are wired. They are bassically in parallel so both are connected to the same tap. When you plug into the Extention jack, a mechanism on the jack switches to the lower impedance tap. On your amp, if you had just one 8 ohm cabinet, you would simply plug it into the Extention jack and the mechanism would switch to the 8 ohm tap. The way your amp is wired now, your options are one or two 16 ohm cabinets or one 8 ohm cabinet. If modified, your amp could run one or two 8 ohm cabinets or one 4 ohm cabinet. With an external Y box (hooked to the Extension jack), or by daisy chaining, you could run four 16 ohm cabinets, my preference.

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