Tell me this, how did I find more modern equivalents? The power supply also uses one A94 and one A44. How can I find modern equivalents of these two? I have often wondered how to (easily) find direct substitutions in the past. The online sources I have found so far are not helping much as the more modern equivalents I have found are also listed as obsolete and out of stock at Mouser and Digikey.
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Simonism, thanks for the YT link. I saw a YT years ago that was the poor man's version, it went like this:
1. Insulate ALL power transformer secondary wires with tape or heat shrink.
2. Insulate ALL primary wires of the OPT.
3. Connect a pair of inexpensive, high efficiency, low impedance headphones to the OPT secondary. (I have a pair of old $40 Sennheisers).
4. VERY CAREFULLY, apply wall voltage to the power trans primary.
5. Try the OPT in various orientations around the PT to determine the lowest hum level heard in the 'phones.
I've also done this live in an amp to find the best position for a PS choke. THIS MUST BE DONE WITH UTMOST CAUTION!
S.
1. Insulate ALL power transformer secondary wires with tape or heat shrink.
2. Insulate ALL primary wires of the OPT.
3. Connect a pair of inexpensive, high efficiency, low impedance headphones to the OPT secondary. (I have a pair of old $40 Sennheisers).
4. VERY CAREFULLY, apply wall voltage to the power trans primary.
5. Try the OPT in various orientations around the PT to determine the lowest hum level heard in the 'phones.
I've also done this live in an amp to find the best position for a PS choke. THIS MUST BE DONE WITH UTMOST CAUTION!
S.
Alltransistors.com has a substitution and cross-reference function. If that doesn't work, then you have to search thru Digikey/Mouser using the numbers from the data sheet.
jeff
Great. It took a little work to figure out how to use the site, but I used their tool to load the old transistors' specs into their search tool, cleared the package style from the search since it was wrong, and then it produced a list of possible substitutions.
I haven't ever read a spec sheet for a transistor before. Someone please check my work to see if I have these two substitutions correct. Did I check all the right things?
and
Generic MPSA44/94 were being made as KSA by Fairchild, and it doesn’t surprise me a bit if they’ve gone EOL/NLA. Zetex/Diodes, Inc has always made that special TO-92 which does a better job of cooling despite being physically smaller. It’s even flat on the same side as normal American TO-92’s. They are in general very good parts. A bit pricier than the generics, but if you need them you need them. They are the right ones. Low capacitance for when that is required, and will run at voltages that would be iffy for the more common MPSA42/92.
Thanks for the confirmation. This is my first time ever looking at transistor specs, so I wanted to double check. I have everything in my shopping carts except the various small resistors now. Maybe I can get all of those added this weekend and finally place my orders. I'm still working on the all black paint for the chassis. The weather has not been cooperating.
@Never Get Old
According to the thread title this is a kit build. Don’t they include the necessary parts?
I’ve never built a PS that uses transistors. What purpose do they serve and what’s the advantage, if any, over a more traditional power supply configuration (CRC, CLC, LC)?
According to the thread title this is a kit build. Don’t they include the necessary parts?
I’ve never built a PS that uses transistors. What purpose do they serve and what’s the advantage, if any, over a more traditional power supply configuration (CRC, CLC, LC)?
A cheap and relatively small (except the heatsinks) way to regulate B+ and screens.
jeff
@FlaCharlie
Well, that would be my fault for not being 100% clear in the title line. Yes, it is (was) a kit, and yes they do (did) include the necessary parts. Since this is my second one of these, and since many people online have wondered how this amplifier might be "upgraded" or "improved" I decided to build a second one using premium quality parts, including upgraded output and power transformers. There is another thread for the first one that I built, which did use all of the kit parts.
In this amplifier, the three transistors regulate the 300 volt screens (not B+). A full amplifier schematic is in post #1 (that schematic will be modified to include independent resistors and cathode bypass caps per tube and also the "Allen Wright" bias mod discussed previously) but the regulated power supply remains the same (schematic below). Since the amp is running in pentode mode, according to my research on the subject, the screen voltage regulation is necessary to prevent what so many people refer to as the "dreaded" "pentode sound." For some reason, pentode seems to be widely accepted in overseas markets like Japan and China but not anywhere else. I won't weigh in on that discussion. However, this design, whatever the reasons may be, has proven itself to me well enough to warrant a second experiment. In fact, the one that I built from a kit (in the first build thread) is among the best amplifiers I have ever used. All of the details are there and here, although it would be a whole lot of reading to go through it all. For anyone who is interested in all of the details of this little pentode amp, they are good reads though, and you can learn from my experiences and mistakes.
Build #1: https://www.diyaudio.com/community/...s-before-i-build-maybe-during-if-i-do.394962/
Power supply as it stands now:
Well, that would be my fault for not being 100% clear in the title line. Yes, it is (was) a kit, and yes they do (did) include the necessary parts. Since this is my second one of these, and since many people online have wondered how this amplifier might be "upgraded" or "improved" I decided to build a second one using premium quality parts, including upgraded output and power transformers. There is another thread for the first one that I built, which did use all of the kit parts.
In this amplifier, the three transistors regulate the 300 volt screens (not B+). A full amplifier schematic is in post #1 (that schematic will be modified to include independent resistors and cathode bypass caps per tube and also the "Allen Wright" bias mod discussed previously) but the regulated power supply remains the same (schematic below). Since the amp is running in pentode mode, according to my research on the subject, the screen voltage regulation is necessary to prevent what so many people refer to as the "dreaded" "pentode sound." For some reason, pentode seems to be widely accepted in overseas markets like Japan and China but not anywhere else. I won't weigh in on that discussion. However, this design, whatever the reasons may be, has proven itself to me well enough to warrant a second experiment. In fact, the one that I built from a kit (in the first build thread) is among the best amplifiers I have ever used. All of the details are there and here, although it would be a whole lot of reading to go through it all. For anyone who is interested in all of the details of this little pentode amp, they are good reads though, and you can learn from my experiences and mistakes.
Build #1: https://www.diyaudio.com/community/...s-before-i-build-maybe-during-if-i-do.394962/
Power supply as it stands now:
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@Never Get Old
Ahh, thanks for clearing that up.
While none of the power supplies I’ve built use transistors I have used a voltage regulator tube before, though it doesn’t supply pentode screens. It supplies plate voltage to a pair of 26s. I admit I used it mostly just to try it, I’m not convinced it has any significant benefit in that situation. It does look cool.
And more recently I’ve used a little SMPS whose 300v output voltage is regulated. It has transistors, of course, but I’m clueless as to how a SMPS works. It’s convenient and works well enough as a turn-key, “black box” solution.
While I’ve often read suggestions that regulated screen voltage is desirable I always thought the main objection to pentode was the need for NFB, especially when globally implemented. Vintage amps are mostly pentode and I’ve never seen one that has regulated screen voltages. Lots of them are highly regarded, especially ones that are PP EL84. I don’t know about preferences in China but I always thought the Japanese preferred SETs.
I’ll leave it at that. I don’t want to sidetrack your thread.
How difficult do you figure it will be to implement the mods you’re planning using the same circuit board as the kit? Did you also do those on your earlier build?
I would expect it to be easier building it point to point. Hacking up a circuit board to make changes would not appeal to me. I’m not a fan of circuit boards, so maybe that’s just my bias. Perhaps it’s not that involved.
Ahh, thanks for clearing that up.
While none of the power supplies I’ve built use transistors I have used a voltage regulator tube before, though it doesn’t supply pentode screens. It supplies plate voltage to a pair of 26s. I admit I used it mostly just to try it, I’m not convinced it has any significant benefit in that situation. It does look cool.
And more recently I’ve used a little SMPS whose 300v output voltage is regulated. It has transistors, of course, but I’m clueless as to how a SMPS works. It’s convenient and works well enough as a turn-key, “black box” solution.
While I’ve often read suggestions that regulated screen voltage is desirable I always thought the main objection to pentode was the need for NFB, especially when globally implemented. Vintage amps are mostly pentode and I’ve never seen one that has regulated screen voltages. Lots of them are highly regarded, especially ones that are PP EL84. I don’t know about preferences in China but I always thought the Japanese preferred SETs.
I’ll leave it at that. I don’t want to sidetrack your thread.
How difficult do you figure it will be to implement the mods you’re planning using the same circuit board as the kit? Did you also do those on your earlier build?
I would expect it to be easier building it point to point. Hacking up a circuit board to make changes would not appeal to me. I’m not a fan of circuit boards, so maybe that’s just my bias. Perhaps it’s not that involved.
Because this amplifier, and also another Chinese amp kit that interests me, both use pentode mode, I did a lot of reading in forums from around the world using online translation tools. China and Japan were the only places I found pentode still regarded as a viable hi-fi option today. Yes, some like triode purity, but pentode gives us power and not everyone wants to pay for a 300B SET amp. Single-ended pentode (SEP) apparently also is very highly regarded, not just pentode PP as in this amp, yet good luck finding many designs or even much information about SEP outside of China and Japan. Last time I tried, I hardly turned up anything.
Whether the screen voltage regulation is a significant part of the great sound of this little pentode PP amplifier, I guess we will never know unless someone wants to build one without the regulation for comparison, which isn't going to happen. I'm also not converting it to UL or triode. I'm sticking with the pentode design.
Click here for post showing bias mod
I can either mount the little tab strip onto the PCB in the unused area as shown there or mount it off to the side. Very easy. I only need to cut two traces on the PCB. Very easy. Mouser no longer stocks the tab strip, but they can be found on ebay. Of course that caused yet another shipping charge to be added to my total. Adding the extra 2 resistors and increasing the size of the caps for the Allen Wright mod gives this layout on the tab strip:
Although I did draw out a point-to-point wiring option, in this extremely compact chassis, the circuit boards are nice.
No.
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I forgot, I did one mod to the first kit I built, and that was increasing the cathode bypass caps:
In the bias module illustrated above, 1000uF are available, and they will fit physically, so I might as well use those. However, I just realized that I haven't shown any small-value film bypass caps for them. Given the small size of the mini tab strip, the only ones I can fit physically without too much trouble are 0.22uF. I have always heard that "ideal" is at least 1uF to 5uF, but they would be too large for the space.
In the bias module illustrated above, 1000uF are available, and they will fit physically, so I might as well use those. However, I just realized that I haven't shown any small-value film bypass caps for them. Given the small size of the mini tab strip, the only ones I can fit physically without too much trouble are 0.22uF. I have always heard that "ideal" is at least 1uF to 5uF, but they would be too large for the space.