Another Scott 222C
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FalconEddy
Audio Mastering Engineer
Yes, DC balance pots. Sorry, it was a typo.
Correct, as I said, his amp has no bias adjustment; only the later versions of the 222C have them. Although they increase/decrease the negative voltage level feeding grid 1, the end result is an adjustment to the bias as measured at the cathode (or at the anode). Hence,(IMO), a bias adjustment pot.
As far as the grid resistor theory. . . there's nothing to argue about. I just wanted to know why you felt it wasn't a plausible theory. Now I know.
As always, thank you for your valued time and input.
. . Falcon
FalconEddy
Audio Mastering Engineer
NOSValves
Super Member
and a completely different phase inverter circuit which allows it to "Have the AC BALANCE POTS" 
One of the stripped down parts is the phase inverter circuit.... the 299B was the last integrated Scott amplifier with AC balance pots...
The Late model 222C you are referring to was really a 222D with a 222C face plate. The circuit is way different on that model of the 222C I call it a 222C/D its not all that common. That amp almost perfectly matched the 222D schematic... shares just about nothing with a real 222C circuit.
One of the stripped down parts is the phase inverter circuit.... the 299B was the last integrated Scott amplifier with AC balance pots...
The Late model 222C you are referring to was really a 222D with a 222C face plate. The circuit is way different on that model of the 222C I call it a 222C/D its not all that common. That amp almost perfectly matched the 222D schematic... shares just about nothing with a real 222C circuit.
Jim McShane
Well-Known Member
You mean my "dubious" opinion?
You stated that after Gordon called you on it. And honestly, it strikes me in the same vein as "have you stopped beating your wife yet?"
Then complete it for us, would you? If your honest opinion is that it's incomplete then please share with us what is missing.
Nothing "snide" about that remark, is there? And it absolutely does prevent the tubes from running away. Not all tubes will/do run away, but it's difficult if not impossible to tell which ones will act up. So a prudent course would be to take appropriate precautions.
Again, that remark isn't snide or nasty at all, is it? I'd take your criticisms more seriously if it wasn't a case of the pot calling the kettle black, as the old saying goes.
Hmm... why would you even try it if you "knew" it was a bad idea, and you'd never had any trouble with runaways? :scratch2: Inquiring minds want to know.
Also, a question and a request:
1. Why would I do it in one channel only?
2. Compute the gain difference for us. I want to know EXACTLY what the difference is.
Here's a hint - if you take a Fisher 800B and change the 330K grid resistor to 180K you change the effective plate load from 39.6K to 36K. If you change to 200K from 330K it's 36.7K. That assumes the balance pot is at about midpoint, 12K.
That's less than a 10% difference, which does not translate directly into a 10% reduction in gain; and since concertina phase inverters have a gain of just less than 1 already, it's not going to have much impact in any case. There will be even less impact on the closed loop gain once you add in the effects of the loop NFB.
Nobody said it was a perfect solution, I never did. I've posted many times about the small change in operating conditions it causes for the 12AX7.
You are wrong, that is not what I said at all. Read it again:
"And in your Scott 222C while indeed there are 330K resistors on either side of the DC balance pot, there is an additional 270K resistor from each of the 7189 tube grids to ground. This yields an effective grid resistance of roughly 150K. That works well with the stouter triode section of a 6GH8 or 6U8, which is much more muscular than the 12AX7 a la Fisher - and can handily drive the 150K load."
Where does it say they need to be changed? WHERE?? Nowhere, that's where. In fact it says the grid resistance is about 150K (okay, that's approximate, it's actually 148.5K), and that means that no change is needed. And it also says the Scott is set up to drive the 150K-ish load just fine.
Now, look again and tell me where I said Scotts need the resistors changed.
Okay, here comes the usual spiel - "if they'd just make them like they did in the old days...". Well they don't. And you aren't privy to my conversations with the people who make these tubes, so you have no way of knowing what I say to them. But REGARDLESS of all that - if there were no current production 7591s, most of the amps we're talking about couldn't be used at all. Or they could be converted to 6L6s or something I guess - I don't like to do that. So rather than sit around saying "woe is us", I chose to do something about it.
I don't want you to EVER complain about my "snide" remarks again. What was the remark above if not snide and condescending?:thumbsdn:
I never said that, show me where in this thread I said that. Don't put words in my mouth.
What is your PROBLEM? Underwear too tight? Why all this negativity? There are 100s of people ON THIS BOARD that successfully use modern production tubes and enjoy the sound they produce.:yes: Are you upset that I want to help them?
As did RCAs, GEs, Sylvanias, etc. They all followed a typical bell-curve distribution. I had nearly 800 GE 6550As pass through here - under a given set of conditions the cathode current varied by over 100%! I had a number of tubes in the low 50s (ma) area, and I had a number of others in the 110-115 ma range. You act as if production variation didn't exist before now. Well, it did.
Or you make a simple set of parts changes and you can bias them wherever you want, up to the tube limits. But I want to point out something else:
Again, you act as if this only occurs in modern tubes. Grid leakage has been an issue in power tubes for a long time - remember the 8417? So an excessively cold bias will fix the problem. Of course, if you never turn on the power switch they won't run away either.
I can't speak to what Paul thinks, I've had very little contact with him although he seems like a real good guy to me. I don't know Doc at all - except sadly what I've seen posted about his business issues. So I can't comment.
But in general you are correct on one thing. Bias a tube cold enough, and it'll behave itself under almost any circumstances. Not everyone is happy with that option though.
I would just add that I dispute your timeline. IIRC some early Fishers used the 12DW7, they went out of production, and the later stuff used 12AX7s.
In general, the most desirable tubes for use in concertina phase inverters have high Gm and low Rp. No one disputes that. So with that in mind, tell me how a 12AX7 could possibly be considered a low Rp or high Gm tube with an Rp of about 62K and a Gm of 1200-1600 - versus say the low mu section of a 12DW7, with a nominal Rp of about 7700 and Gm of 2200. So, you are saying Fisher made the change to 12AX7s because they were better for the intended use? At about 1/2 the Gm and 6-7 times higher plate resistance they were better for the application??
I wasn't in the room when they made the decision. But it would be difficult to not conclude it was a cost savings issue - one less tube to stock, and 12AX7/ECC83s were cheaper than 12DW7/ECC832s - especially when the 12AX7s were bought in even larger quantities from whoever was the supplier.
Honestly, I don't think you want an answer. I think you just want to punish the world for it not being 1965 anymore. RCA, Sylvania, Telefunken tubes - they're all gone, so we all have to deal with it. I also think you don't like anyone else "playing in your sandbox", so to speak. And I think you are a "true believer" - and your mind is closed to ideas that don't fit your idea of how things should be done. There's nothing wrong with that, nothing at all. Just recognize not everyone shares that mind set. And that there is more than one solution to most problems.
If you choose to reply, you can have the last word. I am finished with this discussion, I have to move on to other tasks. The floor is yours...
NOSValves
Super Member
The only thing I have to say is both times we have discussed this you have thrown the first stone insulting my integrity....simply because I do not except your theory.... You come at me with nasty snide remarks and YES you will get them back in kind. Come at me in a reasonable manor and I'll be reasonable....
I could careless if it was 1965 or 2065 rude behavoir is rude behavoir.
I could careless if it was 1965 or 2065 rude behavoir is rude behavoir.
kvflyer
Super Member
FWIW, I have a different approach. These Scott amplifiers worked well when they left Maynard, Mass. Components that were used were were usually better than the usual consumer standard. However, over time, some of those components have failed. Let's face it, they were not supposed to be still working 50-60 years later! Some capacitors were better than others. What comes to mind is Mallory FP electrolytics and Astron electrolytics. If the amp was used moderately and not allowed to get excessively hot, many of the Mallory are still working. I am not trying to say, "Don't replace them", rather they may still have a lot of life in them.
Astron capacitors seem to be more likely to fail. And let us not forget that the white ceramic shell American Radionic Ceracap. Yes, some of them even still work. But they are often leaky. And a leaky capacitor connected to the grid of an expensive Telefunken 7189 or Westinghouse 7591 is a recipe for disaster.
Siemens had a good selenium bridge rectifier. But over time, they too have reduced output.
All of that being said, my approach is simple.
Replace any Ceracap.
Replace the Siemens selenium bridge with a silicon bridge (available from Radio Shack)
Replace the first resistor after the bridge because a one junction silicon bridge has less voltage drop than the selenium. I usually use a 33Ω 5-watt wire wound resistor. Measure the voltage and if it is as per the schematic, it stays. If not, adjust its value up or down to get the correct negative voltage.
I use an ESR meter and check the electrolytic capacitors. If they fail the test, the can is usually re-stuffed to retain the original appearance. Takes about 45 minutes to an hour to do one. Nothing wrong with using new JJ 32-32µF electolytics (about $9 each)
And depending on the model, the bypass capacitor on the 6U8/7199 etc. gets replaced. It could be .25µF or 4µF etc.
Tubes tested. Only tubes that are bad are the outputs in most cases but even "weak" ones will often perform fine. The 12AX7/ECC83 made by Telefunken are always good (if someone didn't pilfer them first!).
Bring it up slowly on a variac.
Sit back and enjoy!
Notice the only "Modification" is the changing of the 18Ω 1-watt resistor to a 33Ω 5-watt resistor.
"Your results may vary" but it works for me. Is it a good thing to put a 10Ω resistor in series with the cathode of an output resistor to measure the current flow? Of course it is. But the only amp that I did that to was my Sansui 1000A. And I did that because it has a separate bias control for each 7591.
Years later, someone will own my stuff. They won't have much difficulty trying to figure out what I did. And, re-stuffed electrolytic capacitors may be noted somewhere so that they are not just replaced later because they "look original".
Just my 2¢. I have not intended to offend anyone. Some of us were regulars on the HH Scott Secure Forum and "Discussions" like this flared up from time to time there as well. To each his/her own opinion.
Still neat stuff isn't it?
Astron capacitors seem to be more likely to fail. And let us not forget that the white ceramic shell American Radionic Ceracap. Yes, some of them even still work. But they are often leaky. And a leaky capacitor connected to the grid of an expensive Telefunken 7189 or Westinghouse 7591 is a recipe for disaster.
Siemens had a good selenium bridge rectifier. But over time, they too have reduced output.
All of that being said, my approach is simple.
Replace any Ceracap.
Replace the Siemens selenium bridge with a silicon bridge (available from Radio Shack)
Replace the first resistor after the bridge because a one junction silicon bridge has less voltage drop than the selenium. I usually use a 33Ω 5-watt wire wound resistor. Measure the voltage and if it is as per the schematic, it stays. If not, adjust its value up or down to get the correct negative voltage.
I use an ESR meter and check the electrolytic capacitors. If they fail the test, the can is usually re-stuffed to retain the original appearance. Takes about 45 minutes to an hour to do one. Nothing wrong with using new JJ 32-32µF electolytics (about $9 each)
And depending on the model, the bypass capacitor on the 6U8/7199 etc. gets replaced. It could be .25µF or 4µF etc.
Tubes tested. Only tubes that are bad are the outputs in most cases but even "weak" ones will often perform fine. The 12AX7/ECC83 made by Telefunken are always good (if someone didn't pilfer them first!).
Bring it up slowly on a variac.
Sit back and enjoy!
Notice the only "Modification" is the changing of the 18Ω 1-watt resistor to a 33Ω 5-watt resistor.
"Your results may vary" but it works for me. Is it a good thing to put a 10Ω resistor in series with the cathode of an output resistor to measure the current flow? Of course it is. But the only amp that I did that to was my Sansui 1000A. And I did that because it has a separate bias control for each 7591.
Years later, someone will own my stuff. They won't have much difficulty trying to figure out what I did. And, re-stuffed electrolytic capacitors may be noted somewhere so that they are not just replaced later because they "look original".
Just my 2¢. I have not intended to offend anyone. Some of us were regulars on the HH Scott Secure Forum and "Discussions" like this flared up from time to time there as well. To each his/her own opinion.
Still neat stuff isn't it?
kvflyer
Super Member
I forgot to mention that I use either the "Orange Drop" or yellow "Lemon Drop" capacitors, nothing exotic. Just depends on what I have in the box or which way the wind is blowing.
And on the early LK-72 amplifiers, those two .05µF disc capacitors connected in parallel on each 7591 get replaced with a .1µF tubular capacitor. New caps are smaller and there is enough room for them...
And on the early LK-72 amplifiers, those two .05µF disc capacitors connected in parallel on each 7591 get replaced with a .1µF tubular capacitor. New caps are smaller and there is enough room for them...
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Jim McShane
Well-Known Member
I don't want to re-ignite this whole thing, but if you review the earlier posts in the thread, Craig described the resistor/cap changes as "dubious". His exact post was:
According to Merriam-Webster, dubious means:
My response was to describe the electrical mechanisms involved, including the impact on the prior 12AX7 stage. And I restated that it worked well - and others here can confirm that. He also stated that I left out information. So in my post I provided the information so he (and others) would be able to read it if he hadn't seen it before. I believe a reasonable person would agree his next post was inflammatory. But...
...all the "who shot who?" aside, I challenged Craig to provide an explanation as to why the fix is "dubious". I asked him to please post the "missing" information he keeps referring to - I've seen none of it. I asked him to calculate the gain change in the prior stage that the resistor change causes. He has not. I even calculated the new effective resistance, Craig has not posted any hard data to support his position that it's "dubious".
So while the quarreling may be over, I don't think any reader should be confused about the outcome here.
The resistor and cap change is not a "dubious" solution. It works, it works well, and it has sound physics backing it up. Every circuit design involves compromise - and while the resistor/cap solution isn't perfect, neither was Fisher's original design. The new circuit is an effective and inexpensive solution to a problem that does rear its head from time-to-time nowadays.
Craig's solution - a cool bias of the 7591s - can also work. It is a compromise, just like the resistor value changes. The physics dictate that it is a solution - a tube passing less current has less electron "build-up" on the grid to cause trouble regardless of how well or how poorly a tube is made. I just consider that his recommended compromise is not as well optimized as the resistor/cap change.
Which is the best solution is a matter of opinion - but it is not appropriate to say my resistor/cap fix is dubious - meaning "of doubtful outcome". It is not of doubtful outcome.
So while the "shouting" may have died down, the FACT remains that the resistor/cap fix is very effective, easy to implement, and inexpensive. No amount of histrionics can change that. So until Craig or someone else provides an explanation based on the physics involved that discredits my fix, people reading this thread should regard it as a viable and usable technique - regardless of who said/wrote it. It is NOT just a matter of differing opinions.
WopOnTour
Hoarder Extraordinaire
In my experience, this just isnt the case. Aged Mallory FP electrolytics are just as likely to fail as Astrons or any other brand. Their initial quality just isnt a factor. I find it has a lot more to do with regularity and duration of usage as well as the lengthy periods of unused storage. When an amp is "found" in a garage after sitting for even 10+ years, the capacitors may initially appear to be working and OK (and may even test OK) but are often doomed for failure witin the first few hundred hours.
Why would you risk the potential damage of a rare and expensive piece of vintage equipment for a $20-30 capacitor? (and a 1/2 hours work) My advice is always to replace em all (and stuff as required for asthetics if that matters to you) Otherwise it's just a ticking time-bomb IMHO.Sure, even new modern capacitors fail - but it's far less likely.
JMO
WopOnTour
NOSValves
Super Member
BINGO!!! That really was my intent..... I guess I could of chosen a better word. I just find it frustrating the way misinformation ends up getting spread by some of the DIY'ers...and incomplete information is spread by some professionals.
I also stated that Jim makes blanket statements about the need to change this resistor value on various brand amplifiers and that this change alone will make all things hunky dory... It will not and is not always required. I posted the other changes required. Which is what I mean by incomplete information.... Jim seems to think I'm referening his thoery of why to change this value as the incomplete information. I understand his theory. I think he is making a mountain out a mole hill but that is his right.
I still question any reason to change it at all. As Jim stated most of the new production tubes handle the stock circuit just fine and he has no way to pre-test and cull out the few that will have issues... Well I call the tubes that do have issues "Defective". EH should ask and pay the cost for its dealers to replace these defective tubes and then disect them and see what part of the production process/materials that is causing the issue and fix it. With cathode resistors installed on all output tubes it would be easy to find the bad ones a few hundred hours after being placed in service.... The bad tube will have drifted upward in bias compared to the other matched tubes.... another way to find the culprits is to test the control grid negative voltage of all four tubes on a fisher receiver they should all be very close like in the mV range of each other. Any tube with apreciably less negative voltage surely has grid leakage to confirm move the tube to another socket and see if the problem follows the tube.
His website states.....
"Important note! There have been some cases where the new production 7591A tubes (EH and JJ) have had trouble
in circuits using a very high value grid resistor - as vintage Scotts, Fishers, and others do. The best solution is to change
all the grid resistors to 180K-200K (from the old value over 300K - sometimes they even exceed the old RCA tube manual
recommendation!), and change the coupling caps to .1 uf or .22 uf. I stock all the parts you need, the parts cost is only
$10.00 to $15.00 per amp, depending on the coupling cap value and voltage."
My point is this change alone will not make things hunky dory... and is absolutely not required on almost all the Scott amplifier with DC balance controls.
I posted the addition changes required. This is the incomplete information I am referencing.
This above modification does effect the performance or the Fisher receivers. I suggested doing it to one channel and witness/test the results... I get back why would I do that? My question would be why the heck not? You do it to see the real world results of the change in circuit.
Jim seems to think I'm advocating not using these tubes when I have stated countless times I use these modern production tubes all the time and have for like 6 or 7 years with no major issues.. Yes I prefer the NOS versions but that is true of just about any tube type. Maybe Jim thinks I'm in the tube sales business...... I do not sell tubes. I try to avoid the tube selling business like a plague.
Craig
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Alright before this gets to out of hand,
I believe everything that "needed" to get out here has been, so I'm closing this thread as of now.
This has been hashed enough, any more of this will mean taking further actions for those involved.
-----------------------------------
If the OP needs more help/info please startup a new thread.
I believe everything that "needed" to get out here has been, so I'm closing this thread as of now.
This has been hashed enough, any more of this will mean taking further actions for those involved.
-----------------------------------
If the OP needs more help/info please startup a new thread.
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