Choosing between 10K or 8K primary will depends on your speakers. The original 10K design was meant to keep things happy no matter if your speaker spend more time near 8 ohms or 4 ohms. If your speaker don't go down to 4 ohms much, choosing 8K in this case will give you more power. In short (with some hand waving) , you get about 10% more at 8K if you speaker is 8 ohm. But if you guess it wrong, at 8 K and 4 ohms, you end up with 10% less than 10K and 4 ohms. All these are explained in his website, section 4B.
10K primary is a good choice. If you do choose 8K, since changing OPT is a drag, you might want to add a bias adjustment circuit before hand to adjust (increase) Ip, if you end up not liking the 8k result.
10K primary is a good choice. If you do choose 8K, since changing OPT is a drag, you might want to add a bias adjustment circuit before hand to adjust (increase) Ip, if you end up not liking the 8k result.
I do believe a company in your area of the world called altronics.com has the m1115 line transformer which may be a VERY cheap substitute for an output transformer. http://www.altronics.com.au/p/m1115-8-ohm-to-1.25w-15w-pa-speaker-transformer/
Some 70 volt 10 watt transformers may work although not gapped. they are cheap so don't expect great sound but you may be surprised
use the .675 watt and common taps
Some 70 volt 10 watt transformers may work although not gapped. they are cheap so don't expect great sound but you may be surprised
use the .675 watt and common taps
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Thanks mike 567,
Over the years I've dabbled in electronics off and on, and Altronics and Jaycar are the go-to's in my part of the world, both keep some really good stuff.
Unlike the US with over 300 million people and the largest economy and distribution network on the planet, we've got bugger all here, just a big flat country with nobody in it...
I did look at the M1115 in my travels but dismissed it immediately as being too lightweight, but thanks for taking the time to go look.
I'll be sticking with the 125CSE or the 125DSE, although I've found a guy Graham, in Oz who will wind up a couple of ROLA OPT's with 10k primary and excellent frequency range for around USD75 each. (A 125CSE will set me back around USD45). I had a bit of a chat with him and his trannies might be just the ticket...
Over the years I've dabbled in electronics off and on, and Altronics and Jaycar are the go-to's in my part of the world, both keep some really good stuff.
Unlike the US with over 300 million people and the largest economy and distribution network on the planet, we've got bugger all here, just a big flat country with nobody in it...
I did look at the M1115 in my travels but dismissed it immediately as being too lightweight, but thanks for taking the time to go look.
I'll be sticking with the 125CSE or the 125DSE, although I've found a guy Graham, in Oz who will wind up a couple of ROLA OPT's with 10k primary and excellent frequency range for around USD75 each. (A 125CSE will set me back around USD45). I had a bit of a chat with him and his trannies might be just the ticket...
Hi Guys,
Now I'm getting down to it, on Fred's schematic for the MiniBlok SET there is C5 for which the + goes to ground. Why would this be so..?
I don't know enough to know the answer, but I'm sure all you folks out there do. It's the only polarised cap on the schema which is oriented this way so there must be a reason..
Please elucidate..
If I knew how to post the schematic I would, but I'm still working on how to post a picture...
Cheers, Martin
Now I'm getting down to it, on Fred's schematic for the MiniBlok SET there is C5 for which the + goes to ground. Why would this be so..?
I don't know enough to know the answer, but I'm sure all you folks out there do. It's the only polarised cap on the schema which is oriented this way so there must be a reason..
Please elucidate..
If I knew how to post the schematic I would, but I'm still working on how to post a picture...
Cheers, Martin
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It's part of a voltage doubler that uses the heater supply to create a NEGATIVE bias of 35V for the grid of the output section. See how it's similar to the voltage doubler of the B+ circuit? This is actually quite a clever design using existing heater supply to form the negative grid bias. One of the reasons I LOVE what Fred did way back when...I built this as my first tube amp and it was fabulous, but also a terrific learning opportunity because of how well he explained things. Do read carefully his description here: The MiniBlok SET Amp: How It Works.
And while you're at it, thank his daughter, Rhiannon, for keeping Fred's website alive. Buy Violins from a Symphony Violinist: Fiddleheads.ca
And while you're at it, thank his daughter, Rhiannon, for keeping Fred's website alive. Buy Violins from a Symphony Violinist: Fiddleheads.ca
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Thanks Carlp, I have indeed sent Fred's daughter Rhiannon an email with my personal thanks for keeping Fred's legacy alive..
Much thanks for your response. I've read the entire Miniblok site several times in an effort to understand the hows and whys of Fred's design, and I'm slowly picking up bits as I go.
Very little of it is intuitive because I have so little knowledge, but as the saying goes about the longest journey starting with the first step, I've taken a few small steps. I'm confident that this build won't present too many issues, since it's clearly aimed at the new chum and within my skill-set.
With regard to the voltage doubler for the B+, I can see what you mean now it's been pointed out. I'm using Fred's addendum for 240vac supply here in Oz, with its slightly different capacitor and bridge set-up, and have cut/pasted the schematic accordingly. I've also dumped the various LED's from the schematics. I don't want or need lights of any kind except the warm glow of the 13EM7's, and Fred made the LED's optional.
I've replaced R1 with a small choke (3Hx100mA) at mike567's suggestion, but I can keep that option open to see any difference. It will be an easy swap...
Regards, Martin
Much thanks for your response. I've read the entire Miniblok site several times in an effort to understand the hows and whys of Fred's design, and I'm slowly picking up bits as I go.
Very little of it is intuitive because I have so little knowledge, but as the saying goes about the longest journey starting with the first step, I've taken a few small steps. I'm confident that this build won't present too many issues, since it's clearly aimed at the new chum and within my skill-set.
With regard to the voltage doubler for the B+, I can see what you mean now it's been pointed out. I'm using Fred's addendum for 240vac supply here in Oz, with its slightly different capacitor and bridge set-up, and have cut/pasted the schematic accordingly. I've also dumped the various LED's from the schematics. I don't want or need lights of any kind except the warm glow of the 13EM7's, and Fred made the LED's optional.
I've replaced R1 with a small choke (3Hx100mA) at mike567's suggestion, but I can keep that option open to see any difference. It will be an easy swap...
Regards, Martin
I tried to pack mine into a tiny enclosure (roughly 3 x 5 or 3 x 6 and 2.5" deep) and had problems with hum. But the hum was small all things considered. Don't try forcing it into such a small container. And report back if you compare the resistor vs choke. I'd be interested to know how they compare. 3H seems small for that position but I haven't thought much about it.
Good luck with the build. If it weren't for Fred's explanations I'd say this isn't a simple first amp (mostly for the interesting PS and negative bias). I liked the LEDs as they helped me visualize the charging of the B+ caps.
Good luck with the build. If it weren't for Fred's explanations I'd say this isn't a simple first amp (mostly for the interesting PS and negative bias). I liked the LEDs as they helped me visualize the charging of the B+ caps.
Thanks Carlp,
I'll be mounting the works in some boxes I ordered from Aliexpress. 7 inches wide x 2inches high x 11 1/2 inches long. I've worked out these dimensions to allow for the three transformers to be encapsulated in aluminum covers and plenty of room inside for the bits. I've read that hum can be an issue so they will require a bit of experimentation to get the layout right.
Fred's explanation is brilliant even for someone like me to get a grip on things. The old hands would take it in their stride but rarely does any technical explanation cater for dummies; it assumes some prior knowledge. Fred's does to some extent but then an explanation to cater for someone who has never before even seen a vacuum tube for example would be bigger than War and Peace.
I've looked for a long time for a simple schematic for a tube amp and my search ended when I came across the MiniBlok. Here was something I could build that didn't scare the hell out of me, and every detail was well covered. The fact that this little amp is universally acclaimed sealed the deal. A SET 1 watter, what could be better?
Over the years I've built a number of amps, all SS, so I know enough to not be afraid of high voltage, but this will be my first tube. I've got a number of amps i play with at home, my favourite being a triode strapped SE El34 jobby which I've modified a little. All hand wired, a thing of beauty internally, good enough for me to entirely trace the schematic that came with it, boosting my confidence enough to attempt to build a simple one myself. I don't need to know every electrical parameter, though I'm gradually learning, and I know how to sling solder, so I have no misgivings about giving it a go.
I'll keep folks informed as to my progress...
Cheers, Martin
I'll be mounting the works in some boxes I ordered from Aliexpress. 7 inches wide x 2inches high x 11 1/2 inches long. I've worked out these dimensions to allow for the three transformers to be encapsulated in aluminum covers and plenty of room inside for the bits. I've read that hum can be an issue so they will require a bit of experimentation to get the layout right.
Fred's explanation is brilliant even for someone like me to get a grip on things. The old hands would take it in their stride but rarely does any technical explanation cater for dummies; it assumes some prior knowledge. Fred's does to some extent but then an explanation to cater for someone who has never before even seen a vacuum tube for example would be bigger than War and Peace.
I've looked for a long time for a simple schematic for a tube amp and my search ended when I came across the MiniBlok. Here was something I could build that didn't scare the hell out of me, and every detail was well covered. The fact that this little amp is universally acclaimed sealed the deal. A SET 1 watter, what could be better?
Over the years I've built a number of amps, all SS, so I know enough to not be afraid of high voltage, but this will be my first tube. I've got a number of amps i play with at home, my favourite being a triode strapped SE El34 jobby which I've modified a little. All hand wired, a thing of beauty internally, good enough for me to entirely trace the schematic that came with it, boosting my confidence enough to attempt to build a simple one myself. I don't need to know every electrical parameter, though I'm gradually learning, and I know how to sling solder, so I have no misgivings about giving it a go.
I'll keep folks informed as to my progress...
Cheers, Martin
Yes, Fred was very clear about bleeding the caps and being on guard all the time.
When I was around 4 years old I put a live electrical cable into my mouth; an extension cable my grandmother had left lying around switched on after finishing some ironing. The belt I got off that nearly launched me into space. It was the hard way to learn about the stuff...
When I was around 4 years old I put a live electrical cable into my mouth; an extension cable my grandmother had left lying around switched on after finishing some ironing. The belt I got off that nearly launched me into space. It was the hard way to learn about the stuff...
I've built one of a pair of monoblock to Fred's schematic, and made a start on the other.
No-one was more surprised than I was to hear the thing firing up the first time. That it actually worked at all is something of a miracle for a noob like me.
Anyway I'll have some pictures to follow.
I didn't deviate from Fred's schematic much, after all what do I know...?
I substituted a choke for R1 and that was it. I used a Sprague Orange Drop for C7, figuring that this coupling cap is important. I've ordered some Russian pio's as trial substitutes for no other reason than the idea of these caps, designed only for a 'cold-war' military purpose, being used in audio long after the collapse of the USSR, gives me a kind of perverse pleasure, their sonic benefits notwithstanding...
Oh, and I left out C6, only because I realised I'd forgotten it. Then I figured that my Marantz CD player might have some DC blocking on its output, so I took the risk...
I got some nice ally transformer covers and potted the transformers. I laid the power transformers on their backs in the covers before potting, not so much to avoid any coupling issues but so they'd fit better.
The powers are Hammond and the OPT's I had custom-wound by a chap in Oz for a delivered cost of AUD240.00 the pair, and they have a declared response of 20-20k.
I used boxes from Aliexpress, and made sure all components were fixed to the top half of the box, so that the bottom identical half could be removed to expose the guts. Having three transformer cubes sat on the top of the box meant that I could lay the amp upside down as a steady platform to work inside, and I made every effort within my scant knowledge to carefully lay out the internal wiring.
When I fired the thing up for the first time there was a minor hum from the speaker, but after a few minutes this disappeared to almost the point of non-existence. I might fish around inside and wiggle wires around a bit whilst connected up to check any change in the hum, but really for what my expectations are, this amp is virtually silent!
I followed Fred's check list and when I post some pictures I can include a check list of expected and measured results. Generally all measurements are within cooey of Fred's so that was a plus. The B+ is a bit high at 370v, so perhaps someone can chime in there.
What else?
I can't wait to complete the other amp so I can get some indication of the soundstage etc, but even with one amp it's enjoyable to listen to. Constructor bias? Sure, must be some of that in the mix, but listening to well-known tracks reveals that this amp is at least as good as others I have, hopefully better, but it's early days yet...
There is something kinda nice about the idea of one tube doing it all, and let's face it, I can't think of a simpler schematic than this MiniBlok. All credit to Fred for this beautiful and simple design, genius if you ask me. The implementation is down to the builder and I've done, I think, a reasonable job, logically following the layout with no deviation that I'm conscious of. The guru's will likely put me straight on a few points, and I welcome any comments...
Pictures to come...
Cheers, Martin
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Is this 370V measured with all the tubes installed and running? If it is, it is "problematic" high not "bit" high. This circuit is designed for 210V B+. If it is an unloaded measurement, what is B+ when the Amp is running?
Thx for the report, Martin. Looking good. I seem to recall that the B+ voltage was a bit higher than 210, but I don't remember if that was at the plate or somewhere in the PS. It's been a while. Martin, if you used a choke in place of R1, what was it's DCR? Can you check the voltage at the output of the PS transformers (the second that steps the voltage back up to 120v) and at the doubler caps?
Allensoncanon. I misread my own results....
With the tube removed I got 338v B+. It should be 300-320v.
With the tube installed I got 265v on the plate. It should be 200-220v.
Remember here in Oz, we're on 240v and Fred kindly made the changes on the schematic. I used a bridge, and there is one less PS capacitor. Whether these changes for 240v will make much difference at B+ I don't have the knowledge. I know mains voltage can fluctuate so maybe that's a factor.
According to the 13EM7 specs, max DC (?) plate voltage is 330v, so at 265v mine is well below max it seems to me, or am I missing something.
All the other measurements are good. I'll post these results later...
I'm using an el cheapo Fleabay meter which may or may not be accurate.
Calp asked about the choke DCR. Hammond 156M with a dc resistance of 86 ohms. (which means absolutely bugger all to me)
As for the other measurements I'll do those next time I've got the bottom cover off and let you know Carlp...
Cheers, Martin
With the tube removed I got 338v B+. It should be 300-320v.
With the tube installed I got 265v on the plate. It should be 200-220v.
Remember here in Oz, we're on 240v and Fred kindly made the changes on the schematic. I used a bridge, and there is one less PS capacitor. Whether these changes for 240v will make much difference at B+ I don't have the knowledge. I know mains voltage can fluctuate so maybe that's a factor.
According to the 13EM7 specs, max DC (?) plate voltage is 330v, so at 265v mine is well below max it seems to me, or am I missing something.
All the other measurements are good. I'll post these results later...
I'm using an el cheapo Fleabay meter which may or may not be accurate.
Calp asked about the choke DCR. Hammond 156M with a dc resistance of 86 ohms. (which means absolutely bugger all to me)
As for the other measurements I'll do those next time I've got the bottom cover off and let you know Carlp...
Cheers, Martin
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Thanks Carlp...
My enclosures are 10" x 7" x 2" high, so plenty of room. I didn't want to be a slave to the idea of forcing the bits into a tiny enclosure as Fred did, figuring I'd have some room to move wiring around or re-route if I wasn't so constrained.
Cheers, Martin
Ah, I get it now. With the 156M choke DCR being 86ohms, is it a case that the choke value should be close to R1? In this case 100ohms. (although other electrical parameters may need taking into account?)
If this is so, I should have used a choke more closely matched to R1.
Starting to make sense. I chose that particular choke because of a suggestion that it should be around 3H. (Although someone mentioned that 3H seemed a bit small)
No matter, I can fiddle around a bit more as I learn a bit more..
Cheers, Martin
Following from the above conversation, to bring the resistance R1 up to 100ohms with an 86ohm choke in place, do I place a 14ohm (or close) resister in series with the choke?
So, disconnect one wire of the choke, add in the resistor and reconnect?
This is assuming that the DCR of the choke is too small at 86ohms, when it should be 100ohms.
Cheers, Martin
So, disconnect one wire of the choke, add in the resistor and reconnect?
This is assuming that the DCR of the choke is too small at 86ohms, when it should be 100ohms.
Cheers, Martin
Again, following from above conversations, I noted in Fred's Part 4 (Construction and testing), that the second mains transformer need only be 1A, but he said that using a 2A is entirely ok and will give a slightly higher B+ voltage.
This is what I did to avoid any confusion, so it's possible that my higher-than-normal readings are due to this....?
Cheers, Martin
This is what I did to avoid any confusion, so it's possible that my higher-than-normal readings are due to this....?
Cheers, Martin
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