Bias issues ST70

Got the hum issue resolved on my SY70 with new caps, two new tubes, bias caps and quad cap....could bias it pretty well but seemed to drift, also changing one side altered the other, back and forth could never get it quite right but finally got it fairly balanced. Went in yesterday to fire it up and bias was way way too hot, could not bring it down into range. Did I see a suggestion to parallel a couple of resistors on a terminal strip when you change from the selinium rectifier to diode? Can someone point me to a thread or let me know what value etc...can't recall on what thread I saw it but seems you add a couple of resistors to compensate for the voltage change with the diode....anyone? Also, would it be wise to change out the bias pots if there are stability problems? Thanks...IA

Hey.
Bob has written in various threads of this forum on this issue and the problem (occasional) that can generate in the adjustment of the exchange pathways of selenium rectifier with a diode rectifier.
Bob brings the solution is, put a 10K resistor in parallel on between terminals 1 and 2 of the strip of seven shots that would cause an effective resistance of 5K. Another 20K resistor should be placed in the same way on the resistor located between the terminals 3 and 4 of the strip, resulting in an effective resistance of 6.66 K.
Anyway, our friend Bob may certify the validity of my comments.
A greeting.
Pepe.

Wildiowa, did you replace the selenium rectifier in the bias circuit with a modern silicon diode? If not, do so. That old selenium rectifier is well past its prime and could be drifting off into its usual failure mode, which is high resistance. When this happens there isn't enough negative bias voltage on the grids of the output tubes and they pass excess current (the voltage at the BIASET measurement point is too high), get hot, glow red, and fail prematurely.

Guys-

Yes the diode was my first fix as it seemed to be a critical issue and an easy place to start in rebuilding these things. As most will know you get kind of hooked on the process and I now have about $200 and several hours into it....you get obsessed with getting it "right" and keep going....I think I will add the resistors across terminal 1-2 and 3-4 as noted in severel posts here and see if that brings it into range. I also have not heard whether it would be wise to replace those bias pots, when biasing they seem to go smoothly into range and then seem to suddenly "jump" around to a higher or lower value, just wondering....have tried to spray cleaner into them and still have some issues unless it is the resistance issue with the new diode...appreciate all the help this forum is great...IA

How much is your bias drifting? I assume that you have no input signal during the testing. If so, bias drift can be a bad tube, dirty tube sockets, a bad bias pot or bad bias supply. If it is safe and convenient you could check the voltages at the bias cap and the bias wiper to see if they are drifting.

Well, I must admit perhaps my 4 year-old grandson was chatting into a mic I had hooked up to a mixer running through the amp when I checked. Should I have the inputs connected but no signal when I bias, or nothing at all into the input jacks? Seems before I would get one channel at about 1.5v then the opposite channel would be low; adjusted that channel to about 1.5v and the other channel was low...back and forth could not get them in balance. Then when I checked yesterday there was a big jump to 2 volts or better but that was when the kid was using the mic....so could explain that. Also could not find 10k or 15k 1watt resistors at Radio Shack...grrrr.

The bias adjustments on the two channels of a Stereo 70 WILL affect each other as they are adjusted. You either have to go back and forth several times, or intentionally set the bias on one channel so that the other will "bring it back to normal" when it is adjusted.

When adjusting bias, inputs connected with no signal is correct.

The inputs should be shorted. The amp should be on long enough to warm up. You should excercise the pots with the amp off.

After several emphatic but terse instructions, perhaps a bit of explanation would help.

The key when adjusting the bias is to have the output tubes in a quiescent (idle) condition, so that you are measuring the tube current in the absence of drive. Some amplifiers pick up noise if the input cables are disconnected, so I don't recommend disconnecting the input cables. Shorting the inputs doesn't hurt, but would only make a difference if the amp has a lot of noise pickup with the cables connected. If there is a drive signal at the tube grid, this will affect the current through the tube and the readings at the cathode resistor.

It is certainly a good idea to warm up the amp for 15 minutes or so before a final bias check, but if the bias is possibly incorrect, an initial check after 1 min is highly advisable.

Exercising the bias controls with the amp off is risky in my view, because the control may be left in a position allowing excessive tube current. If the control is dirty, it should be replaced since wiggling the control is not a permanent cleaning procedure.

wildiowa wrote:I also have not heard whether it would be wise to replace those bias pots, when biasing they seem to go smoothly into range and then seem to suddenly "jump" around to a higher or lower value, just wondering....have tried to spray cleaner into them and still have some issues unless it is the resistance issue with the new diode...appreciate all the help this forum is great...IA

Pots are not like switches, depending on what "cleaner" you use you can actually make it worse.

I recommend changing out your bias pots with "sealed" pots such as these:
http://www.ebay.com/itm/Potentiometers-5K-10K-50K-2W-5K-1K-5W-Linear-1PCs-/300450719052?pt=LH_DefaultDomain_0&var=&hash=item8bb3aef5c6

wildiowa wrote:Got the hum issue resolved on my SY70 with new caps, two new tubes, bias caps and quad cap....could bias it pretty well but seemed to drift, also changing one side altered the other, back and forth could never get it quite right but finally got it fairly balanced. Went in yesterday to fire it up and bias was way way too hot, could not bring it down into range. Did I see a suggestion to parallel a couple of resistors on a terminal strip when you change from the selinium rectifier to diode? Can someone point me to a thread or let me know what value etc...can't recall on what thread I saw it but seems you add a couple of resistors to compensate for the voltage change with the diode....anyone? Also, would it be wise to change out the bias pots if there are stability problems? Thanks...IA

I had this exact same issue on my ST-70 in February this year. The resistance of both cathode resistors (15.6 ohms) for the EL34s was drifting about significantly. This is easy to check. With the amplifier turned off and unplugged you should measure 15.6 ohms from pin 8 of either power take off socket to ground. When my amp had this problem the resistor values where off by several ohms and adding a little heat from an ordinary blow dryer drove their ohmage up into the 20 and even 30 ohm region. I got my replacements from dynakitparts.com. If they have run out of them, no worries. Replace them with 10 ohm 1/2 watt 1% resistors and bias for 1 volt. I realize the original post was many months ago but I hope this helps some one.

as soon as I saw your post I saw the problem, Hawaii-Ken has already hit it. 50 year old bias controls are flaky!!
Replace them!!

tubes4hifi wrote:as soon as I saw your post I saw the problem, Hawaii-Ken has already hit it. 50 year old bias controls are flaky!!
Replace them!!

You are most correct! Just weeks after i got my 50+ year old ST-70 i found out my Biaset resistors were drifting and replaced them, just weeks after that another bias issue arose.

One my bias pots let go. The right bias pot developed a near short from wiper to chassis (no kidding). It measured just about 80 ohms wiper to ground. I subsequently replaced both pots with new mil-spec ones.

But the "fun" was not over yet. A few weeks after the pot failure a bare spot i had not noticed on a bias circuit wire made contact with ground. Needless to say I replaced the wire.

After these experiences i closely examined all the wiring, checked every resistor visually and ohmically, leak checked the caps, and gave the solder joints a magnified inspection. I found things here and there to correct and replaced several caps as a precaution. Any way that was all done during Feb and Mar this year and my 50+ year old ST-70 has run very well ever since.

In my "unbiased" opinion these old ST-70's are great amps. Keep 'em glowing!