Pots Pots and more Pots do I need a scope.

[bobva]

Sorry guys this my IM lost babble.

Im NOT any way shape or form of a electronics guy. Although I can rewire a boat and older General Motors cars and trucks simple stuff. Opps forgot I can recap speakers.

​

All of my amps have these pots Im not talking about the face pots.

The bias pots are actually the most strait forward pots
I got a rebuilt ST-70, Fishers all in need of rebuild X-202-B, KX-200 also has a balance pot piece of cake,,,The 500b does not have bias pots but they can be installed. Plus other that are siting in a what to do pile.

The pots Im talking about are the
Phase Inverter pots 500b
Phase inverter AC balance, Output tubes DC balance X-202-B
Plus others.

How does the average Joe adjust these?
You can buy a recap kit from ebay and reputable dealers and recap your unit. Don't these pots need to be tuned. Every time you change a tube don't you got to retune these pots?

Here's why this Lost and confused babble comes from. My ST-70 had a Issue took it to the amp guy down the the road. Got the unit back he set bias he said they were cooking at the 1.38vdc I was running them at. I asked what did he set the bias at. He said he use his scope to set the bias. Got home put her on line she sounded much better. He set the bias Right side @.90vdc Left side @.95vdc St-70 calls for 1.56vdc IIRC.

 

[bobva]

Oh BTW there was nothing wrong with the ST-70. Installer error Me

 

[gadget73]

This should get you where you need to be on the phase inverter pots

http://audiokarma.org/forums/index.php?threads/quick-and-easy-400-phase-inverter-adjust.559415/

Its written for the 400, but its the same idea.

Not being familiar with the X200B's setup I can't say for sure how DC balance was intended to be adjusted in stock form. if it has cathode resistors you can measure across, either stock or ones you added, adjusting the pot for the same current through both tubes is probably correct. If it has none of that, it likely was supposed to be adjusted for minimum hum at the speaker terminals. Instructions should be in the service manual if someone doesn't chime in with particulars.


Stock bias setting on an ST-70 is 1.56 volts for two tubes across the stock 15.6 ohm resistor. If that resistor value is different, or if you have one per tube, you'd need to set it otherwise. It needs to be 50ma per tube, a little math magic with that plus the resistor value will tell you the correct voltage. In stock form, the tubes are not being abused. Setting to a lower current only increases distortion.

 

[bobva]

Thank you gadget.. For reading my babble.
I try to not comment in the tube or for the most part anywhere on this forum. Truth be told 98% of everything I read is way above my pay grade.

I like my old junk most of it sounds pretty good to these old ears. After my St-70 insedent And Im fully retired now It's time I learn more about the toys on my shelf. So I started reading owners manuals and service manuals.

Opened this http://www.fisherconsoles.com/non console manuals/fisher x202b 40001 49999 sm.pdf on the last page it states set bias at 0.8 then for the AC % DC balances ETC it states use IM Distortion Analyzer and set to X. First thing that when threw my head what the heck is that?

In the mean time I've been kicking around the Idea of getting a scope. This guy is down the road a little way for me. https://www.ebay.com/sch/shorewa8/m.html?_nkw=&_armrs=1&_ipg=&_from= But I need to relearn how to us it.

Anybody know of a Good electronics 101 book for dummies. So I can maybe get a better understanding on what Im reading in this forum.

Thank you
Bob

 

[gadget73]

AC balance is the phase inverter. You can use the resistor measurement method in the sticky. If you've got an intermodulation distortion analyzer it may give slightly better results but setting it with the ohm meter should get you plenty close enough.

DC I'd add 10 ohm cathode resistors to and just set to the same voltage across, or set for min hum. Both come out about the same, but DC volts across a resistor is easier to measure than AC hum unless you've got a sensitive voltmeter or a scope.

 

[primosounds]

Oh BTW there was nothing wrong with the ST-70. Installer error Me

You need to check the bias setting resistor on the cathodes. I would think that if he set the bias voltage to what you measured that would mean he changed out the resistor to a 10 ohm one. Otherwise your bias current would be way off and the amp would not sound very good at all.

 

[primosounds]

Anybody know of a Good electronics 101 book for dummies. So I can maybe get a better understanding on what Im reading in this forum.

Thank you
Bob

Try to get a copy of the ARRL handbook. Used ones are pretty cheap on epay. It was made to help lay people get their ham radio license. But also it has basic electronics and a lot about tube technology.

 

[bobva]

You need to check the bias setting resistor on the cathodes. I would think that if he set the bias voltage to what you measured that would mean he changed out the resistor to a 10 ohm one. Otherwise your bias current would be way off and the amp would not sound very good at all.

Thank you that makes a lot of sence to me. He didn't change no resistor in it. The ST is not stock looking maybe the builder did.

Try to get a copy of the ARRL handbook. Used ones are pretty cheap on epay. It was made to help lay people get their ham radio license. But also it has basic electronics and a lot about tube technology.

Yes found Lots of them them on ebay Thank you.

 

[bobva]

Ordered a book its as old as me 1961.

 

[triode17]

Bob - The photo shows the correct bias-set resistor of 15.6 Ohms. So if : "He set the bias Right side @.90vdc Left side @.95vdc St-70 calls for 1.56vdc IIRC." Then it's set way low! Equivalent to 46% and 49% of maximum Plate dissipation. Dyna had the amps. biased to 80% of Max Pd, which is a bit high and makes the amp. sound too thick on the low end to me. I set mine to 75% and it sounds better.

 

[bobva]

Bob. I set mine to 75% and it sounds better.

Quiky math 1.2-5 vdc. paper math 75% of 1.56 = 1.17 is that about what yours are set at?

 

[triode17]

Quiky math 1.2-5 vdc. paper math 75% of 1.56 = 1.17 is that about what yours are set at?

No, you are missing how this works. At 1.56v, THAT's the 80% Power dissipation level that Dynaco spec'd. To calculate the power at idle, first you take the Biaset voltage reading and divide it by 15.6 Ohms. So 1.56v divided by 15.6 Ohms, gives 0.1A or 100 ma per pair. Each tube draws 50 ma. times the plate voltage of about 425v. Multiplied together gives you 21.25W. But remember, you are measuring the total power diss. including the screen power. What I do is subtract 5% for the screen power ( I rate this conservatively) or multiply by 0.95 which = 21.25 x 0.95 = 20.18W. Divide that # into the max. rated power of 25w and you get 0.8075 or 80%. If you set it to 1.45v /channel, the same math will give you 75%.

 

[bobva]

WoW I real got to take a math class.

1.45vdc I guess the amp guy doesn't know stereo.

Funny thing about this ST I found it local on CL here in seattle. I snooped around on ebay and found a ST that was a close cousin Im thinking same builder. I go threw his past sales more cousins. Sent him a pm. His reply (Bias should be about 1.4V Happy New Year!) I P&C his reply.

Thank you triode17 You rock

 

[gadget73]

Honestly the stock dissipation setting is not excessive. Turning it down really does little but increase distortion.

Some reading justifying the stock setting from someone who is a bunch smarter than me

Dynaco ST-70: Base Line Testing

 

[triode17]

Honestly the stock dissipation setting is not excessive. Turning it down really does little but increase distortion.

Some reading justifying the stock setting from someone who is a bunch smarter than me

Dynaco ST-70: Base Line Testing

I had read this too but I don't hear any increase in distortion. Plus at 80% Pd, it sounds too thick, fat, call it what you will. To each his own.

 

[gadget73]

Indeed. I go by measurements because its far easier to discuss a hard objective measurement vs personal perception. You're certainly not going to hurt anything by turning it down, so if you like it better that way who am I to say you're wrong?

 

[truep]

IMHO, the best way to adjust the AC balance is with a dual channel scope. Set the channels to AC,you put a probe on each output tube grid, invert one channel,and set the scope to add. input a sine wave around 1khz.
Adjust the output (Terminated to an 8 ohm 20 w. resistor) to about 3V and adjust for a null. I have found this gives the best sound on my VTL amps. Note, if you increase or decreases the output the null point changes.

 

[Dandy]

Bob, if you decide to get a scope Tektronix is a good make. And get one that works. It's hopeless trying to learn if the scope is faulty.

 

[bobva]

IMHO, the best way to adjust the AC balance is with a dual channel scope. Set the channels to AC,you put a probe on each output tube grid, invert one channel,and set the scope to add. input a sine wave around 1khz.
Adjust the output (Terminated to an 8 ohm 20 w. resistor) to about 3V and adjust for a null. I have found this gives the best sound on my VTL amps. Note, if you increase or decreases the output the null point changes.

Cool now I just got to figure just how to do that.

BTW I did get a tec-465 real good condition. came with inst. manual, probe parts, bag and what looks like never bin used shade hood,, whats real cool is the cover still has the factory molded styrofoam in it.
P6105 probe
10X
100MHz
10.5 5pF
10 M ohm
1meter -- 81 inch meter

I'll have fun figuring this out.

I do got a question Ive watched a lot of you tube vids and I can't see how you plug a signal gen in to a scope and then use the scope probe on the thing your working on.

Ive seen how to plug the two together BMC gen out to channel 1 or ch 2 on the scope.
This is the stuff I find.

Thank you

 

[Dandy]

That sounds like a great scope. Well done. I'm also still learning the basics.