@6A3sUMMER and @Chris Hornbeck those are excellent suggestions. I am writing all of these notes to a file for future reference.
I wondered about that too, and I will be changing driver tubes this week. I have both 6P14P and 6P14P-ER output tubes.
I do think I'll add the 1k resistors because it's so easy and it just seems prudent to do so, even though I don't really think there is a "problem" here after further research. A lot of people would have just set the tubes to the setting where they like the sound (which is one reason to use the add-on bias adjustment kit) and never thought twice about it. The directions even say that the bias can be increased.
The current setting of each EL84 output tube should now be 35 mA. This can be adjusted to your preference via the on-board bias potentiometers, providing a useable range of 320 to 450 mV (32 to 45 mA).
I happen like 37mA and 40mA with these particular tubes. So, it seems that this option was contemplated as part of the design.
Right now there are just jumper wires from the PCBs to the #2 pins on the tube sockets that are about the same length as a resistor. "All" I have to do is remove the four jumper wires and bend the 1k resistors to reach from the pins to the PCB eyelets. The chassis is so small that I hate working inside it, but I can do it and have it back in service in a few hours. Even if not strictly necessary, it seems prudent. Both of my other tube amps have these in place.
I don't want to make multiple changes at once because that doesn't identify which change makes what difference. I will try the GE driver tubes first since they are arriving first. Next, I will try the 1k resistors, since they have to be ordered. Maybe next weekend. At some point in the future, I could move the NFB to the 4 ohm tap, but I probably won't ever go that far. I'll bet that after any or all of these changes I still prefer a higher bias setting, but I reserve the right to prove myself wrong. 😉
I wondered about that too, and I will be changing driver tubes this week. I have both 6P14P and 6P14P-ER output tubes.
I do think I'll add the 1k resistors because it's so easy and it just seems prudent to do so, even though I don't really think there is a "problem" here after further research. A lot of people would have just set the tubes to the setting where they like the sound (which is one reason to use the add-on bias adjustment kit) and never thought twice about it. The directions even say that the bias can be increased.
The current setting of each EL84 output tube should now be 35 mA. This can be adjusted to your preference via the on-board bias potentiometers, providing a useable range of 320 to 450 mV (32 to 45 mA).
I happen like 37mA and 40mA with these particular tubes. So, it seems that this option was contemplated as part of the design.
Right now there are just jumper wires from the PCBs to the #2 pins on the tube sockets that are about the same length as a resistor. "All" I have to do is remove the four jumper wires and bend the 1k resistors to reach from the pins to the PCB eyelets. The chassis is so small that I hate working inside it, but I can do it and have it back in service in a few hours. Even if not strictly necessary, it seems prudent. Both of my other tube amps have these in place.
I don't want to make multiple changes at once because that doesn't identify which change makes what difference. I will try the GE driver tubes first since they are arriving first. Next, I will try the 1k resistors, since they have to be ordered. Maybe next weekend. At some point in the future, I could move the NFB to the 4 ohm tap, but I probably won't ever go that far. I'll bet that after any or all of these changes I still prefer a higher bias setting, but I reserve the right to prove myself wrong. 😉
I still wonder about the fact that the original design used traditional ceramic disc caps in the feedback circuit instead of the mica caps that come with the kit. I wish I had been able to get a stable test reading on the tiny 18pF mica caps, but my meter wouldn't do it. I wonder what audible difference ceramic caps actually make in these 2 NFB positions, either the old style disc or MLCC? Like these:
https://www.mouser.com/ProductDetail/KEMET/C317C390JCG5HA?qs=sGAEpiMZZMsh%2B1woXyUXj5qSAxJpwJH10O/li4XMyOE=
and
https://www.mouser.com/ProductDetail/KEMET/C330C180JHG5TA?qs=sGAEpiMZZMsh%2B1woXyUXjyU7r8F9vEKY03fOpaHwq60=
Information sheet: https://www.mouser.com/datasheet/2/212/KEM_C1054_HIVOLT_GOLDMAX_300_C0G-1140500.pdf
General information on MLCC: https://www.digikey.com/en/maker/bl...ic-capacitors-and-practical-application-hints
"Good" old ceramic disc NFB capacitors in the ST-35, and nobody complained! 😀
https://www.mouser.com/ProductDetail/KEMET/C317C390JCG5HA?qs=sGAEpiMZZMsh%2B1woXyUXj5qSAxJpwJH10O/li4XMyOE=
and
https://www.mouser.com/ProductDetail/KEMET/C330C180JHG5TA?qs=sGAEpiMZZMsh%2B1woXyUXjyU7r8F9vEKY03fOpaHwq60=
Information sheet: https://www.mouser.com/datasheet/2/212/KEM_C1054_HIVOLT_GOLDMAX_300_C0G-1140500.pdf
General information on MLCC: https://www.digikey.com/en/maker/bl...ic-capacitors-and-practical-application-hints
"Good" old ceramic disc NFB capacitors in the ST-35, and nobody complained! 😀
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The ceramic caps of yore were well known to be of very broad tolerance specification, somewhat like electrolytics in that sense.
They were widely used in kits, especially the IF strip as 10,7 MHz bypass of FM receivers such as Eico & Heathkit.
My recollection is the 0.01 microF is series resonant when the leads are cut to about 1/2 inch.
C6 is used to fix problems sometimes encountered in FB amplifiers at HF due to stray capacities & inductances in OPTs.
I did run into such a problem when running a Hammond 1618 in a PP circuit many years ago. A small cap such as C6 was the remedy.
The primary in many low cost OPTs is wound serially, I found the capacity of the primary to the core was sometimes 200 pF greater
from one end than the other. 🙂
Given that the Kemet (or cheaper Murata) MLCC caps are 5% C0G caps, I wonder if there is any difference at all between those and the mica caps that came with the kit. I found this, but I don't understand technical details in the thread:
https://www.diyaudio.com/community/threads/using-mlccs-for-critical-audio-applications.279768/page-2
Wima makes a 39pF film polyproplylene cap but an 18pF isn't available. https://www.digikey.com/en/products/detail/taw-electronics-inc/FKP2J000391D00HSSD/16689451?0=/film-capacitors/wima&s=N4IgjCBcoEwAwBYCcVQGMoDMCGAbAzgKYA0IA9lANogDMSABAA4BiIAuqYwC5QgCqAOwCWXAPKYAsoWz4ArgCdCIAL6kAtBGggMkHARIgAbgKhd5sgxUjUwSGAA52nHpBABlM0IEBzFepio2qbmllQgAKzsytFAA
Sometimes I wonder if all of the modern components that we use to "clean up" the sound and "perform better" actually go too far and introduce sound changes that, had those "better" parts been available at the time, would have resulted in modification of the circuit or rejection of those parts. DynakitParts offers a carbon resistor option instead of metal film. If I had a billion dollars, I'd build 5 of these amplifiers (and then fly to somewhere for someone with knowledge and experience to put them all on a 'scope). Maybe we have gone too far and need to "dirty up" the ST-35 sound a little with some older-type parts. 😀
Seriously, I do wonder about the NFB caps because they are important.
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I find that using a pair of test leads on my DMM causes problems with capacitance readings of very small capacitances (like 18pF).
Trying to measure 18pF, and expecting a steady readings, and repeatable readings could be a problem.
1. Just move your arms closer to, or further away from the test leads and meter.
2. Try and connect once, and make a reading. Then move one of the leads around, reconnect, and then take another reading.
3. Just measure the leads by themselves, move the leads, etc.
You will see what I mean.
Trying to measure 18pF, and expecting a steady readings, and repeatable readings could be a problem.
1. Just move your arms closer to, or further away from the test leads and meter.
2. Try and connect once, and make a reading. Then move one of the leads around, reconnect, and then take another reading.
3. Just measure the leads by themselves, move the leads, etc.
You will see what I mean.
COG/NPO caps are perfectly fine for the job, with very low DA and minimal parasitic inductance. Mica is a "found material" and measured performance near the audio range varies with different tests and samples. It's great for RF, very stable with time and temperature (if selected for that), plenty of voltage rating available and possibly fine near audio frequencies. The little see-through polystyrenes aren't made anymore AFAIK (nobody -read Dupont- wants to make the sheet PS) so are probably only interesting to folks with lots of them laying around. Great, but hardly necessary.
The little WIMA FKP-2s are beautiful, and everyone should buy a whole collection while they're still available (wire lead parts are dinosaurs today, gone tomorrow) but 630 Volts rating is the bare minimum for the smaller one. Unless you're a grandparent, buy some nice resistors too. Tick, tick.
All good fortune,
Chris
The little WIMA FKP-2s are beautiful, and everyone should buy a whole collection while they're still available (wire lead parts are dinosaurs today, gone tomorrow) but 630 Volts rating is the bare minimum for the smaller one. Unless you're a grandparent, buy some nice resistors too. Tick, tick.
All good fortune,
Chris
Without an oscilloscope or audio board & software; and a 10kHz square wave generator or audio board & software . . .
You can not "See" what is happening before or after when you change any of the negative feedback parts.
The blind pianist, Erroll Garner wrote a tune called "Feeling is Believing". Great Musician! RIP
Do not try poking your fingers around on a vacuum tube amplifier, or it will make a believer out of you.
Just my opinions
You can not "See" what is happening before or after when you change any of the negative feedback parts.
The blind pianist, Erroll Garner wrote a tune called "Feeling is Believing". Great Musician! RIP
Do not try poking your fingers around on a vacuum tube amplifier, or it will make a believer out of you.
Just my opinions
Often small C caps can be successfully read by using a simple difference method.
Connect something like a known 50 pF cap to the cap meter leads first. Keep hands & other objects away, take a reading & record it
Then connect the unknown small cap to the same point on the leads, Now the difference can be seen, the actual unknown capacitance.
The measurement setup needs to be fixed in place during the test, otherwise errors sneak in.
How do I know all this Sh*t? I spent more then 10 yrs in a lab before being unleashed on the World. 😱
Connect something like a known 50 pF cap to the cap meter leads first. Keep hands & other objects away, take a reading & record it
Then connect the unknown small cap to the same point on the leads, Now the difference can be seen, the actual unknown capacitance.
The measurement setup needs to be fixed in place during the test, otherwise errors sneak in.
How do I know all this Sh*t? I spent more then 10 yrs in a lab before being unleashed on the World. 😱
Perfect. Good to know. Having my hands on the leads probably caused a lot of the issue. I was able to test the 39pF though. They came in at 34pF instead of 39pF, but that could be just my meter, my hands, leads, etc. Just for peace of mind, I looked at the NFB caps again, and just as I remembered they are clearly marked with the correct values. I mounted all of the components on the boards so that their values can be read easily.
Yes, I know having built two and rebuilt one. 380 volts in this case.
I know. However I am one of those "nut jobs" who believes that not everything can be measured. I might stick the Wima cap in the 39pF position or a Kemet MLCC just to see if I notice any difference. It's not changing the circuit values any. The orignal circuit used ceramic disc so if we want to be authentic we could use a couple of those ... 😉
I used a Wima cap on the input because that's what most people seem to do. Makes me wonder just how "bad" the old ceramic disc caps used in the input and NFB really were. They couldn't have been too bad. People still like the ST-35 as it was.
Isn't there a software oscilloscope that runs on a PC? I wonder if it works. I hate to go down this road because in a few weeks I quite literally will never use it again. Maybe there is a ham radio group or something like that near me where I can find someone with a scope. We have one stereo shop left, but I doubt they do this kind of testing. I think they just pack things up and ship them back to the manufacturer. When I owned the Threshold amplifier, it had to be rebiased every 6-12 months. It was very annoying. Fortunately, the shop wasn't too far away and they had a fully equipped lab with a tech who came twice per week, and they did it for free.
I don't think so given what I am seeing come out of China. You just have to put up with quality control issues and fakes. As long as there are audiophiles there will be audiophile components. I read today that if you break open a small MLCC cap today you might find a SMD inside with leads attached to it.
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Which really is my point. The "audiophile" market is flooded with Chinese fakes, and likely will remain so. I've personally seen everything from faked Telefunken ECC83s with a diamond on the base (concave rather than convex, but they'll soon remedy that, no doubt) to faked UF4007s. Why would anybody bother to fake UF4007s? They cost like 7 cents. Hard for me to not wonder about Stuff and broken cultures.
It's already too late to buy Toshiba FETs - assume they're all fakes without serious proof. Capacitors and semicons of all kinds are routinely fakes. Vacuum valves have been faked since before we were born, but it used to be just rebranding that everyone accepted - now it's very substandard junk at inflated prices for the "audiophile" and guitar amp markets.
Don't put up with junk from China. For small parts Mouser and DigiKey are still reliable, although both their websites are a trial. And if you're a serious builder, and not old like me, stock up. Even I still try to get enough depth and breadth of small parts to keep my generation younger building partner busy after I'm gone.
All good fortune,
Chris
Some of the Chinese kits and finished gear are ok if you get what someone else has tried and evaluated. I love my two Chinese DACs, and one only cost $59 and the other only cost $110. Both well worth it, and I have enjoyed fiddling with different op amps and small mods. This DynakitParts ST-35 is going to have a very hard time beating the "Huaji Audio" Chinese amplifier kit that only cost me $400 including shipping and tubes. The problem with both the Chinese kits and the finished gear is that it's all built for 110V so when plugged into my 121V there can be a problem when the extra 11V gets stepped up in the transformer, even if they claim it's ok on 115V or 120V. It cost me very little little to modify the amplifier kit, but it required consideration, patience, and in my case help from people here at DIYaudio to make sure I got it right.
Those are my sources for genuine parts like Wima caps, Dale resistors, etc. I don't know what's inside the "Wima" caps and "film" resistors that came with the Chinese amp kit, but they almost all tested very close to correct values and they sound great. Who knows what's inside if you break them open.
I'm not, and if anyone here ever sees me trying to build another amplifier, please stop me! 😱 The ONLY exception is: I may in fact build a second "Huaji Audio" kit if I decide that I like it better than the ST-35. I would build the second one with name-brand genuine components and try to find better output transformers for it. I now have three tube amps, and I have no place to store the other two while I listen to one. I can't bring myself to get rid of the Stereo-70 version ii for some reason though. I'm too sentimentally attached to it. It runs as hot as a fireplace, and I no longer have an open table/shelf on which it can sit. Sigh. Thus my adventures with EL84/6P14P amps.
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If you're having fun, yer doin' it right.
"Ain't nobody dirty business, how my baby treat me,
Nobody business but mine"
All the best,
Chris
"Ain't nobody dirty business, how my baby treat me,
Nobody business but mine"
All the best,
Chris
Agree,
As recipient of 2 US patents...
You are correct that a patent is only worth how much $ you can pay to defend it..
After 7 years- US patent runs out- then the lawyers say you NEED to extend protection on your intellectual property @ $200/ hr....
So, in my opinion, patent lawyers are just high priced salesmen...
Jim
1k Ohm grid stopper resistors arrive tomorrow. Maybe I can get them in place this weekend. It will be very interesting to see if they actually make a difference.
United States Postal Service LOST the General Electric 12DW7 tubes I ordered. Says they delivered them and they never did. 👎👎👎👎👎
The value of a patent has to be several hundred thousand dollars in revenue or licensing fees to be worth it. That's why I didn't do it. Today they just get copied overseas anyway where it's pretty much unenforceable.
United States Postal Service LOST the General Electric 12DW7 tubes I ordered. Says they delivered them and they never did. 👎👎👎👎👎
The value of a patent has to be several hundred thousand dollars in revenue or licensing fees to be worth it. That's why I didn't do it. Today they just get copied overseas anyway where it's pretty much unenforceable.
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Well, well. What a difference 4 resistors can make. After a few hours of fiddling around inside the tiny chassis, I got the four Dale 1k Ohm grid stoppers installed. I can find no reason why these should be omitted. Does anyone have a legitmate reason to leave them out, other than to save a few dollars? The original circuit does not use them. They replace four short jumper wires from the PCBs to the tube sockets, and they fit perfectly.
So far, I haven't had a chance to do any real listening. However, when I held my breath and turned on the amplifier after the installation, the bias pots needed to be adjusted a whole lot. I had to turn, turn, turn the four pots down, down, down. They were up at about 45 mA when I turned it on. The highest I had ever gotten before, even with the pots all the way up, was 40 mA. They were at 37mA before I installed the resistors. After a whole lot of pot turning, I finally got them down to the 35mA spec. So, I let it play in the background for about 2 hours while testing the bias once in a while. The bias setting seems to be more stable now, changing much less as the amplifier warms up.
As I said, I can't judge sonic changes yet. Maybe tonight or tomorrow. Given the above notes, what changed when I installed these four resistors? Did they likely stop some sort of oscillation that was happening? How did that affect the bias setting so much?
So far, I haven't had a chance to do any real listening. However, when I held my breath and turned on the amplifier after the installation, the bias pots needed to be adjusted a whole lot. I had to turn, turn, turn the four pots down, down, down. They were up at about 45 mA when I turned it on. The highest I had ever gotten before, even with the pots all the way up, was 40 mA. They were at 37mA before I installed the resistors. After a whole lot of pot turning, I finally got them down to the 35mA spec. So, I let it play in the background for about 2 hours while testing the bias once in a while. The bias setting seems to be more stable now, changing much less as the amplifier warms up.
As I said, I can't judge sonic changes yet. Maybe tonight or tomorrow. Given the above notes, what changed when I installed these four resistors? Did they likely stop some sort of oscillation that was happening? How did that affect the bias setting so much?
The ST-35 is stable with real EL84 tubes. But small changes can make it oscillate, even installing EFB
might cause real EL84 oscillate ( which is documented in the EFB documentation). As you use
cheap EL84 lookalike it's maybe an explanation. While you are "at it" you could install 100ohm screen stoppers too ..
might cause real EL84 oscillate ( which is documented in the EFB documentation). As you use
cheap EL84 lookalike it's maybe an explanation. While you are "at it" you could install 100ohm screen stoppers too ..
They are actually more expensive than some EL84, and they have better life than EL84, and the same specifications as EL84, and they are proven to work in this amplifier by many users. I have two types on hand. Been through that. Not an issue as I have stated repeatedly.
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@Never Get Old
Area circled in blue above appears to be a little concerning, i.e. insulation is burned off. Either replace or make sure wire is conducting.
Best,
Anand.
Area circled in blue above appears to be a little concerning, i.e. insulation is burned off. Either replace or make sure wire is conducting.
Best,
Anand.
You are correct and have excellent eyes. I did hit that wire briefly with the side of my soldering iron. It did not burn through the thick insulation. No conductor was exposed. It's one of the heater wires to the 12DW7.
The bias setting is much more stable than it was before, changing very little as the amplifier warms up. Still evaluating the sound.