Dual Bias supplies on a ST70 ?

[EdStiles]

Given the problem of adjusting the bias on a ST-70 shared supply where you have to go back and forth between each side because every time you adjust one side, it affects the other:

How about building dual bias supplies, one for each channel, sharing the bias tap off the transformer into 2 rectifers, caps etc, doubling the resistor values?

Is there any technical reason this wouldn't work? It would seem to me that it would add another layer of isolation between the 2 channels.

Even more outrageous, a supply for each output tube to isolate interactions?

Thoughts???

Thanks,

Ed

[EWBrown]

I found that it's more of a factor of the B+ voltage being "pulled" down as the bias current is increased, and vice versa. I especially notice theis when initially setting up a "blue board" ST35, each bias adjustment throws the other three tubes a small amount, so it can be like trying to shorten the legs on a chair Yellow_Light_Colorz_PDT_09

If I were to do it all over again (I may well, a I have an ST70 in need of a DIY driver board and some other fixes), I'd go with making each EL34 / KT77 individually settable, this would require four 20 - 25K pots (this way the other range-setting resistors can remain at the original value) .

Replace the old selenium or silicon bias supply diode with a uF4007 (or better) and double the eiectrolytic cap value, and this should help straignten things out. I'd also use 10 ohm, 1% resistors 1 Watt for each cathode, this makes bias setting a cinch, f'rinstance 50 mA would read 500mV on your DVM, 60 mA would read 600mV, etc. No need to go for heavy duty 10 W resistors here, the resistor's dissipation at 50 mA is on the order of 25 milliwatts, 60 mA is all the way up to 36 milliwatts, so even a 1/4 or 1/2 W resistor will do the trick, and a smaller one can act as a safety fuse if a tube should go into "china syndrome" mode.


/ed B in NH

[Uncle Ned]

I found that it's more of a factor of the B+ voltage being "pulled" down as the bias current is increased, and vice versa. I especially notice theis when initially setting up a "blue board" ST35, each bias adjustment throws the other three tubes a small amount, so it can be like trying to shorten the legs on a chair
/ed B in NH

Not only that, because the bias comes off a tap on the B+ winding, more load on the power xfmr B+ is going to change the bias supply voltage slightly as well.

Regulating the bias won't fix the problem, either, because in a UL design, the screens aren't regulated. Also, the B+ isn't regulated, so the static bias current would vary with the wall voltage with regulated bias, as it is, the bias tends to track the B+ and be somewhat if not perfectly self correcting.

There's one guaranteed way to fix the side-to-side bias problem: Trade the ST70 for a pair of MKIV's ;-) Otherwise it's like cleaning up garage floor oil stains under a Triumph, it's just some extra work that you have to deal with if you own one.

[dhuebert]

it's like cleaning up garage floor oil stains under a Triumph

I just had to comment: I have a 1970 Triumph TR6

http://www.diytube.com/gallery/full_frontal.JPG

Uncle Ned, You made me laugh!

I set the bias three or four times early in the life of my ST-35 and forgot about it. Unfortunately, the TR6 is not so easy. I got my garage floor epoxied and the oil drips drive me crazy. I spent HOURS trying to get it dry. I know it's possible because I've seen others that don't leak...

If you think you're a good mechanic and are looking for a challenge, try a Triumph. Their secret factory slogan was " You will live to regret it" Nah, I love the stupid thing.

Don

[EWBrown]

As I remember, the "C Chong" cap boards and bias boards have a zener regulated bias supply (either 75V or 68V zener, depending on MKIII or MKIV / ST70 circuit) which would not allow the bias voltage to track the plate voltage, bur otherwise I have had good results with these. The new P-782s and PA_060s are so beefy, that they don't droop very much, anyway... Serious "overkill" can be a good thing, sometimes Yellow_Light_Colorz_PDT_01

/ed b in NH

[Uncle Ned]

I just had to comment: I have a 1970 Triumph TR6

Golly, that's like watching The Night Of The Living Dead again.

Unfortunately, the TR6 is not so easy. I got my garage floor epoxied and the oil drips drive me crazy. I spent HOURS trying to get it dry.

Put some cardboard under the car. I thought that was the first thing they teach British mechanics. (the second is "install an aftermarket ignition system", I think)

I know it's possible because I've seen others that don't leak...

Uh huh. I suck my gut in when my wife has her friends over. Doesn't mean I don't have a pot belly.

If you think you're a good mechanic and are looking for a challenge, try a Triumph. Don

If you think you're a tough mechanic, try tightening Citroen wheel bearings to the factory-spec one thousand foot pounds. I recall one of my high school friends getting some free replacement Craftsman breaker bars from Sears trying that.

Me, I stuck to Mopars (not any more, I'm a Toyota man now). I think I could still swap a starter on a Slant Six without getting under the car. In the dark.

[EWBrown]

Something I remember about Lucas Electric, AKA "The Prince of Darkness"

The question is: "why do they drink warm beer in England" ?

The answer is: Fridges made by Lucas Electric".

and then, back on this side of the pond...

MOPAR: don't that mean "My Old Pig Ain't Running" ? Yellow_Light_Colorz_PDT_07

/ed B in NH

[Shannon Parks]

Thinking out loud and I think Don asked this awhile back: has anyone built/owned a zener or otherwise regulated bias in a AC mains fluctuating zone (aka the US)? Curious if the volume drops at all when the mains drops (since the bias doesn't).

Mopar story:

My '74 Plymouth Scamp's alternator crapped out in the WV pan handle twenty years back on a Sunday afternoon while I was on a road trip. Under a tight schedule, I thought the trip was going to be delayed at least a day or so. This area was backwoods. Anyhow, within a few minutes of noticing my dying battery, I pulled into a junkyard/car shop and left the car running. Could they help? Why of course. Anyhow, they dropped what they were doing, grabbed an alternator from the back, went to work and let me drink all their coffee. Thirty minutes later they said they were finished and I asked the damage. "Ahh, 20 bucks should do it." That's all I had, other than credit (no good 'round there!), so I was ecstatic. Less than an hour delay, I was back on the highway, cranking up the stereo again.

One hour later the blasted Scamp's timing chain shattered at 80mph and bent up all the valves.

Valley. To mountain top. To valley again. Last Mopar I've owned.

Shannon

[dhuebert]

Two, two threads in one!

Listen: I had a 1968 Dodge Monaco. Seven friends and I could ride quite comfortable, I loved that car. If I remember correctly, gas was $0.35 a litre tho...

On the subject of fixed bias, it seems to me that if B+ drifts downward due to dropping mains, Vbias should drift upwards, compensating, yes, no?

EDIT: I guess Ned already said that

There should be some pithy quote here about the urge to tweak lifting us out of the neolithic...

Don

[Uncle Ned]

Thinking out loud and I think Don asked this awhile back: has anyone built/owned a zener or otherwise regulated bias in a AC mains fluctuating zone (aka the US)? Curious if the volume drops at all when the mains drops (since the bias doesn't).

I can't imagine that the volume would be affected much at all
since the bias only sets the static bias current, it doesn't
change the power output substantially unless it's way off.

The old Grommes 260 had regulated bias, but it had regulated
screens as well.

One hour later the blasted Scamp's timing chain shattered at 80mph and bent up all the valves.

I learned to replace the timing chain & sprockets as routine maintenence, the nylon sprocket always craps out about 60,000-90,000 miles. They could've made those old Mopars a lot more reliable with some simple fixes like cast iron sprockets, cast iron distributor housings and better ball joint seals.

I'd regale you with more car anecdotes, but I can use 'em to fill a lot of blog pages, when I get around to filling blog pages :)

[Pyre]

Hi there,

I have a modified ST-70-C3 that has 4 bias pots, one for each tube. If anyone would like a schematic I can email you a pdf. Im not sure if the circut is that similar to the original ST-70 but it could help.

[sorenj07]

just on the subject of fluctuating mains, I've seen anywhere from 119V to 128V on my house as I have a power conditioner with an LED readout for a home theater system (sand, not mine )

[jeffdavison]

Suppose you had the space for an additional transformer... could you simply duplicate everything to create a true dual mono power supply? Including the 5ar4, cap board etc? Only use one filament tap on each transformer and eliminating one bias adj. pot ? assuming keeping one pot for each channel.
The only thing shared would be the power chord.
True its a costly endeavor and room isn't really there on the standard chassis for two PA-060's.... but if there was room? Basically putting 2 MKIV's on a single chassis?

Now if I get creative, I think I will persue this, but instead of using two PA-060's I'll use two toroids and mount them on top of the chassis vertically between the two output transformer. The ratings for the toroid are near exact of Triodes "Dynaclone" PA-060. This frees up room under the chassis being able to place two chokes where the end cap of the old single PA-060 would have gone, as well as giving room for an additional C. Chong cap board. I have taken measurements and everything will fit! There will still be room on top of the chassis for a pair of Edcor 60w opt's and the original placement of the bias pots can remain. The toroids would have to be slightly sunk into the chassis if the cover is to be used for clearance, maybe a half inch or so, but nowhere near the under chassis real estate the PA-060 demands. Now a quad of KT88s, 6550's, kT90's KT100's etc can really sing! Driver board should not be an issue, just keep the left and right channels separate. This will eliminate most of the problems of interaction between left and right sharing a single rectifier tube let alone the additional strain on it and the extra demand from a single transformer.

Am I mad?... I just like the single chassis form factor and the size of the ST70. Must be mad....I'm a Fiat guy and the problems you've all had with the trumpets are minor in comparison....oh but when they're dialed in!

Jeff Davison

[Uncle Ned]

Suppose you had the space for an additional transformer... could you simply duplicate everything to create a true dual mono power supply? Including the 5ar4, cap board etc? Only use one filament tap on each transformer and eliminating one bias adj. pot ? assuming keeping one pot for each channel.
The only thing shared would be the power chord.
True its a costly endeavor and room isn't really there on the standard chassis for two PA-060's.... but if there was room? Basically putting 2 MKIV's on a single chassis?

Well, there's TWO 360V windings on the toroid, so if you don't insist on using a tube rectifier, you could just put a bridge rectifier on each winding.

Am I mad?... I just like the single chassis form factor and the size of the ST70. Must be mad....I'm a Fiat guy and the problems you've all had with the trumpets are minor in comparison....oh but when they're dialed in! Jeff Davison

Hmmm.. likes Fiats and wants to make as ST70 into a dual mono 60W/ch amp... I think you answered your own question. Yeah, I owned a Fiat, too. And a Studebaker. Great when they're dialed in , if you don't mind adjusting a a dial with a lot of slop in it. BTW, a Studebaker flathead 6 can do 100 mph... takes awhile to get there, though!

[EWBrown]

I have a big hefty power trannie that I salvaged out of a "hamfest special" all-tube laboratory pulse generator, that set me back all of $5, plus I got a batch of very nice tubes thrown in as part of the deal Yellow_Light_Colorz_PDT_01 . (I still thank Gary K for deciding not to buy it, and leaving it for me...)

The trannie has a 360VAC, 500mA winding, and a 6.3VAC, 14A filament winding. The primary side has two 120VAC windings, so it's good for 120 or 240 VAC. It definitely as plenty of tubular testosterone potential, I could probably get away running a PPP EL34 / KT77 stereo design, or quad channel PP with this beastie... Four 1200V 6A HEXFREDs and some hefty big-azzed electrolytics, and it'll make for a very "stiff" power source, indeed... Yuh, it doesn't have a bias voltage tap or winding, but that's what small 48VAC transformers were made for...

It's a bit too large (and way too heavy) to fit comfortably onto a standard ST70 chassis, as well as being vertical mount rather than the horizontal PA060 style.

Just remember, never turn down a good deal at a hamfest or swapmeet...

/ed B in NH