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Biasing tubes?

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SalsaNChips

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I am interested in replacing the tubes in my late 80’s-era ADA T100S tube head. It still has the original Siemens tubes (EL34 and 12AX7) which are 20-something years old. They are working fine except the DC hum out of the amp when I take it off standby has grown louder as of late. I’m thinking it would quiet down and perhaps sound better with new tubes and a proper biasing. Which brings me to the point of my post.

To bias tubes, my understanding is you have to put a sine wave input into the amp and measure the peak to peak amplitude and signal quality pre/post gain on each tube, adjusting the input voltage to the tube (anode/cathode?) to get it as clean and accurate as possible. That requires an oscilloscope and a sine wave generator, both of which I don’t have (closest I have 40 year old Techtronix scope that doesn’t work). Please correct me if I am wrong on this or over simplifying.

I wanted to see if anybody here can provide insights into a quick and dirty (but reasonably effective) method of biasing, or if biasing is even really necessary (suspect it is).

Thanks for any info.
 
Hmmm.  25 year old amp.  Excellent quality original tubes.  Hum louder over time.

It's not the tubes - it's the power supply decoupling caps.

You need a cap job, my friend.  Fairly simple job, even for a PCB amp like the ADA.

 
Your biasing method isn't over-simplifiying, it's over-complicating, unless you're an audiophile. In that case, you'll need many thousands of dollars worth of test gear just to start...

But, Mayfly's right - you've got primary supply filter problems. The electrolytics have dried up and don't have much capacity to them anymore, so they're not filtering the DC from the rectifier. It's a common problem in older amps. Fortunately, as he says, it's not a complicated job to replace them.

Also, I wouldn't use the amp until you fix it. Once electrolytics start to dry up, they get somewhat conductive and can overheat and explode. Sometimes it's pretty violent, too. They're built into metal cans, and the heat buildup turns 'em into low-yield hand grenades.
 
If I ever get back into making amps, I'm not going to use electrolytics anywhere. 

Solen makes some great high capacity film types.  A little pricey mind you....
 
Technology is moving right along for those things. They're looking at "super caps" for energy storage sufficient to replace batteries in some circuits.

I remember when I was in school, there was no such thing as a 1 farad cap. Now you can buy 2 and 3 farad parts for sourcing surges in the mega-power automotive sound systems the ghetto cruisers love so dearly.
 
This is great info direct from the amp experts -- appreciate it :)

I was fortunately able to locate the complete schematics and service notes for the ADA T100S online.

On the power supply PCB, C1 & C2 are 220/350V, C3 is 10/100V and C4/C5 are 100/250V.

There are also caps on the tube PCB that I should probably replace.

At work now, I will print them out once I get home, open the amp and identify the cap manufacture and if avalable, part/sku numbers. Then start sourcing them online. May need some advice on best places to go for that.

Desoldering and soldering should be no problem for me, I used to work in a DoD contracter board shop, manufacturing (20 years ago that is). Still, hope I don't lift a pad desoldering those big power caps :\
 
If you use a solder sucker, you should be able to get on and off those pads fast enough that you won't damage them.
 
This evening I opened up the ADA T100S head to replace the 80mm fan with a quieter 26db unit (details on another thread).

While I was in there I looked over the PS PCB and verified the cap type, dimensions and lead spacing. They are 220uF 350V and 100uF 250V (pair of each). I was not able to find a manufacture logo but they look exactly like this:
http://www.futurlec.com/Capacitors/C220U400E.shtml

and this:
http://www.futurlec.com/Capacitors/C100U250E.shtml

Question -- on the 220uF 350V, I was not able to locate a 350V, closest I could get is the 400V linked above which looks identical manufacture, same form factor with 10mm lead spacing, Electrolytic, radial type, etc. Is there any reason why a 400V cap of the same specs and capacitance rating would not work in place of a 350V cap? My limited electronics sense tells me it's a non-issue but wanted to ask.

Below are a few pics of the board and the PCB layout, FYI. Note the overheated area on the back of the PCB opposite the 5W 39K ceramic resistor and that MPSU60 transistor. Nothing I can do about that I guess :\

ADA_01.jpg

ADA_02.jpg

ADA_03.jpg

ADA_04.jpg


Appreciate your input and time :)
 
SalsaNChips said:
Is there any reason why a 400V cap of the same specs and capacitance rating would not work in place of a 350V cap? My limited electronics sense tells me it's a non-issue but wanted to ask.

No. The working voltage is simply the point where the device will possibly fail. It's a DNE (Do Not Exceed) rating. The closer you get to it, the greater the likelihood of failure. So, higher numbers are better, and usually cost more. As technology improves that specification keeps rising and the size of the device keeps diminishing. So, don't be surprised to see higher ratings in smaller devices for less money.
 
I ordered all the Radial Electrolytic Capacitors and also the mylar caps on the tube board from Furturlec. Around $20.00 and that was including fresh solder, new lead snips and a heat sink for that hot transistor on the PS PCB (not sure if it will help but doesn't hurt). Also ordered new power and standby switches cause the switch illumination is out and I want different colors.

I have thought about getting one of the new Rocktron solid state rack amps, they are supposed to sound real good but I just don't know. I tried a MosValve years ago and didn't like it. Maybe go try one out at GC.

Hopefully this takes care of it and my old ADA will be all fresh and new sounding. Hope so cause that amp has a deep, very reactive tone with my Marshall cabinet that is just unreal.
 
You might want to consider one of Carvin's DCM series amps. They're very reasonable price-wise, very robust electrically and are surprisingly light. They're using an aluminum chassis, switching power supplies and class D circuitry. Check the specs and pricing on the power level you'd like - I think you'll be surprised. Lotta amp for the money.
 
Now there you go -- got me thinking about a new amp!

Seriously, if I continue to have noise problems with my ADA after this cap update, I'll put it on eBay and go shopping for a stereo rack tube amp. The Carvin TS100 looks pretty good, spec-wise and can't complain about the price.

What I would really like is something like this:

rack-front_cropped_750.jpg

http://www.3rdpower.com/product_hlh_sv3015.htm

Very $$$ though and I really have a million and three other things to spend money on...

I'll follow up next week after I get my caps in. Maybe the ADA has some life left in it yet :)
 
I just sold a TS100 because it was too heavy. They're like 25lbs or better, while the DCM200L I replaced it with has twice the power and only weighs 4lbs. I'm getting too old to be grunting around equipment like some kind of army mule. Didn't lose anything in the switch, as the resale value of the TS100 is pretty high, and it was so clean it sounded like a solid state amp anyway. Plus, now I don't have to worry about tube maintenance. For the bass, the DCM1000L will do 650W into 8Ω and only weighs 9lbs. That's some real headroom.
 
Like Cagey, I too have tooled around on Carvin DCM 1000 and just bought a TS100.  I actually have (2) DCM 1000s, (1) DCM 2000, and an HD 1500.

I've found the DCMs to be much more suited for what you'd think a dedicated power amp would be used for, a PA.  Using the HD 1500 and DCM 1000 as the power section of my bass rig, neither could handle a good boosted bass/treble scooped mid sound.  They both struggled to do so.  Running the DCM in bridged 4 ohm mode to a 4 ohm bass cab, trouble.  Any other sound, they worked fantastic, especially in the PA.  Could not ask for a better weight/power/price package either.

I've only had the TS100 for a week and have the first gig with it tomorrow.  I'm using it as a bass rig with a Sansamp as the preamp.  "Clean" is not how I'd describe it so far.  There is a bit more gritty warmth than with my other Carvin power amps, cleanest with the presence knobs at full cut.  The Sansamp isn't so much marketed as a modeling amp, mainly because it has knobs instead of labeled presets, but it is.  I took a gamble with it, betting whether the tube warmth was on the pre or power end of things.  It's in the power section, and the sound I would describe as tank like.  It sounds heavy and indestructible.

 
Update -- I received the parts from Furturlec. Took about 2 weeks since it shipped out of Malaysia.

Last night I replaced all the filtering caps on the power supply PCB. That would be the cluster of 5 caps in the picture below (shown after I finished). Also fabricated a heat sink and attached it to a IC on the PCB where it was showing evidence of heat damaging (delaminating) the PCB, removed, cleaned and reseated the EL34 and 12AX7A tubes and replaced the power and standby switches.

Powered it up no problem. Took it upstairs put it back in my rack, works fine. Except -- the hum is still there. MAYBE a little quieter, not much.

Oh well, it probably needed those caps replaced anyway. I anticipated that might not fix it when I placed the order with Furturlec, so I also ordered all the can and mylar caps that go on the tube back plane board. Which I can replace next. But at this point I am thinking I may not bother as a.) the hum is not that bad and b.) I am going to be replacing this ADA head in about 3 months anyway, most likely with a 3rd Power SV30/15 (pictured above). I have been in touch with Jamie Scott at 3rd Power and he is re-doing that head design now and will be re-launching it in October/November and I am pretty sure that is going to make a fine replacement for the ADA. We'll see.

FYI, the de-soldering and re-soldering was pretty easy, the entire process took under 3 hours. Having a good hot iron, a solder sucker and a vise to hold the work stable is essential (and a bright work light!).

ADA_05.jpg
 
Okay, have to follow up and report that I found and fixed the source of the hum my ADA tube head was producing.

It was a DC ground loop between the ADA head and my Rocktron Piranha pre-amp. Should have checked that first. Actually I did, but it was a while back and apparently this problem has developed since then. Maybe a bad ground sleeve contact on one or more connectors, either on the ADA head, the pre-amp or both (it is *not* the cables).

For now, I am using a couple of passive direct boxes with a ground lift which has solved the problem completely. It does attenuate the signal slightly, which I compensated for by turning up the output volume up at the pre-amp. 

At least the 3 hours spent removing and replacing 5 power capacitors in the ADA tube head didn’t hurt anything and was actually a good thing in that they would have needed it eventually anyway :)
 
COOL!  Glad to see that it's all resolved.

and the cap job is just good maintenance  :)
 
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