Just to clarify: carbon comp are OK to use in non-critical places if you have them and don't want to throw them away - but don't buy them specially. Elsewhere use carbon film or (better) metal film. In most places it doesn't matter which, but metal film may have lower noise and lower distortion. You probably won't be able to tell the difference, so don't get worked up about trivia. Getting the right circuit and doing grounding right (etc.) is far more important than component details. Some people worry about the latter because they don't understand the former - it becomes a displacement activity.
DF96 makes an excellent point - some of the biggest problems in building amplifiers are ground-related hum.
Be sure to carefully plan your grounding scheme as you design the amplifier layout. The key here is controlling the ground currents to make sure they don't get into your amplifier by NOT depending on the chassis to be "ground".
One common method of avoiding ground-loop problems is to use a "star" grounding system that ties the grounds back to the power-supply and then to chassis.
There are good tutorials on this site (and on the internet, of course), but basically it boils down to some general principles:
1) Use isolated RCA jacks for the input connectors. I use jacks made by Rean (available from Mouser and other suppliers). These jacks come with little plastic shoulder washers that insulate the RCA jack from the metal chassis. If you use a non-conductive panel to mount your RCA jacks, you can use "regular" RCA jacks. Connect the RCA jack grounds into your system ground using good-sized wire (22-20ga is fine).
2) Connect your power ground back to the SECOND filter cap (C3 in the Nachbaur schematic). This ground is "quieter" than the C2 cap ground, which will be seeing nasty charging-spikes from the rectifiers.
3) Ground your system wiring to chassis at a single-point only, right where you bolt the green safety ground to the chassis.
I used a piece of 14ga solid copper wire as a bus for connecting grounds, soldering parts to the wire as it routes through the amplifier and connecting only ONE end of the bus-wire to the power supply ground point.
There are several excellent writeups on grounding to be found online, such as this one from Randall Aiken of Aiken Amps: Star Grounding
Ground-loops and hum are a big thing, easy to avoid with a bit of careful planning up-front and devilish to find later in the game.
One other major source of potential mischief is hum induced from the 6.3VAC heater supply. Be sure to tightly twist the heater wires (5-6 twists per inch), use 20-22ga wire, and keep them away from the input connetors. There is an excellent tutorial in the sticky-section of this forum on heater wiring with good photos. Since you have only one heater in this SPUD, you don't have to worry about how to interconnect the tube sockets.
What else can happen? Well, lots I suppose but these are probably the biggest problems in building audio amplifiers.
However, they are solve-able by doing a bit of research into the basics and trying to physically build things the way other people have successfully done. There are many fine examples of the work people have done on this forum and elsewhere on the 'net, you will quickly start seeing "good" layouts that will give you an idea of how to proceed with your design.
Good luck with the project!
Be sure to carefully plan your grounding scheme as you design the amplifier layout. The key here is controlling the ground currents to make sure they don't get into your amplifier by NOT depending on the chassis to be "ground".
One common method of avoiding ground-loop problems is to use a "star" grounding system that ties the grounds back to the power-supply and then to chassis.
There are good tutorials on this site (and on the internet, of course), but basically it boils down to some general principles:
1) Use isolated RCA jacks for the input connectors. I use jacks made by Rean (available from Mouser and other suppliers). These jacks come with little plastic shoulder washers that insulate the RCA jack from the metal chassis. If you use a non-conductive panel to mount your RCA jacks, you can use "regular" RCA jacks. Connect the RCA jack grounds into your system ground using good-sized wire (22-20ga is fine).
2) Connect your power ground back to the SECOND filter cap (C3 in the Nachbaur schematic). This ground is "quieter" than the C2 cap ground, which will be seeing nasty charging-spikes from the rectifiers.
3) Ground your system wiring to chassis at a single-point only, right where you bolt the green safety ground to the chassis.
I used a piece of 14ga solid copper wire as a bus for connecting grounds, soldering parts to the wire as it routes through the amplifier and connecting only ONE end of the bus-wire to the power supply ground point.
There are several excellent writeups on grounding to be found online, such as this one from Randall Aiken of Aiken Amps: Star Grounding
Ground-loops and hum are a big thing, easy to avoid with a bit of careful planning up-front and devilish to find later in the game.
One other major source of potential mischief is hum induced from the 6.3VAC heater supply. Be sure to tightly twist the heater wires (5-6 twists per inch), use 20-22ga wire, and keep them away from the input connetors. There is an excellent tutorial in the sticky-section of this forum on heater wiring with good photos. Since you have only one heater in this SPUD, you don't have to worry about how to interconnect the tube sockets.
What else can happen? Well, lots I suppose but these are probably the biggest problems in building audio amplifiers.
However, they are solve-able by doing a bit of research into the basics and trying to physically build things the way other people have successfully done. There are many fine examples of the work people have done on this forum and elsewhere on the 'net, you will quickly start seeing "good" layouts that will give you an idea of how to proceed with your design.
Good luck with the project!
One more suggestion: add a 150k (or so), 1W bleeder resistor across C3. This resistor will discharge the HV caps after you turn power off, but won't appreciably load the power supply.
It's a safety feature, not part of the signal path, and you can use some combination of your CC resistors for this (in case you've already placed your parts order).
It's a safety feature, not part of the signal path, and you can use some combination of your CC resistors for this (in case you've already placed your parts order).
Welcome to the club! Proper grounding can seem to be like black-magic, but there is some science behind it.
There's lots of variables involved, but the basic rule on grounding is to be aware of where signal and noise current is flowing and try to control it using wires. Also, avoid creating ground-loops, most commonly formed when grounds are attached to the chassis at multiple points.
View attachment Power Amplifier Grounding Scheme.pdf
This is a sketch of the grounding scheme I used on my Tubelab SE build. There are many ways to do this, the approach I took worked like a charm without any fussing. The amplifier noise floor is such that I cannot hear any hum or noise coming from my speakers - which is good enough for me.
The other aspect is that grounding is very layout dependent, there isn't any "cookbook" approach to grounding but if you study a few examples you will know more than many about how to best layout and ground your design.
One more note... Keep your power transformer oriented at right-angle to the audio output transformer(s). That is to say, orient the power transformer such that the laminations "point" at the audio output transformer windings. This will minimize coupling between the power tranny and the output transformers. Again, there's lots of examples on the forum and internet.
Let us know how the project turns out, too!
EDIT - The power tranny orientation might not be a problem with your parts, I just remembered you are using power transformers with horizontally-oriented frames. My comment above was more appropriate for when both audio and power transformers have vertically-oriented frames.
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Sam, thank you very much for your help. I will share the project building details. This is what I want to do. I am not electronically proficient and I do owe this forum a lot. The only way how I can contribute is to share my experience. It might take a while, ,but I will start soon to work on this project.
An extremely good primer on grounding can be found in David Davenport's article on this site
Audio Component Grounding and Interconnection - diyAudio
whilst Rod Elliott's safety ground article is also worth a read
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
Audio Component Grounding and Interconnection - diyAudio
whilst Rod Elliott's safety ground article is also worth a read
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
Choke for the Miniblok power supply
JPS wrote...
***I also used a choke instead of R1***.
Pleased I found your entry.
I was going to ask about using a choke instead of resistor R1, does something good to the system. Don't know the terminology, smooths out ripples or somesuch. Dunno, just a beginner...
May I ask what size choke you recommend?
On another matter, Fred N used the 125CSE OT, presumably to rein in the cost. Hammond say this OT is 'general purpose' and not HiFi. Is that because it fizzles out below 100hz? If it did say go down to say 50hz or less, would it then be considered suitable for HiFi use?
It wouldn't blow the budget much more by going to one of Hammond's larger OT's to get 20 to 20k. Not in the spirit of Fred's creation sure but why not spend a couple of bob more...
Just thoughts. Collecting the various parts at the moment. Being in Oz I need 220-240v primary. Not easy to find here with 12.6vac 2A secondary, but I'm probably not asking the right questions or looking in the right places.
Jaycar have something similar, but 12v 2A secondary not 12.6v. Any good?
I'm sorry if either of my queries have been dealt with elsewhere..
I don't know what I don't know, hence the Noob questions...
Cheers, Martin
JPS wrote...
***I also used a choke instead of R1***.
Pleased I found your entry.
I was going to ask about using a choke instead of resistor R1, does something good to the system. Don't know the terminology, smooths out ripples or somesuch. Dunno, just a beginner...
May I ask what size choke you recommend?
On another matter, Fred N used the 125CSE OT, presumably to rein in the cost. Hammond say this OT is 'general purpose' and not HiFi. Is that because it fizzles out below 100hz? If it did say go down to say 50hz or less, would it then be considered suitable for HiFi use?
It wouldn't blow the budget much more by going to one of Hammond's larger OT's to get 20 to 20k. Not in the spirit of Fred's creation sure but why not spend a couple of bob more...
Just thoughts. Collecting the various parts at the moment. Being in Oz I need 220-240v primary. Not easy to find here with 12.6vac 2A secondary, but I'm probably not asking the right questions or looking in the right places.
Jaycar have something similar, but 12v 2A secondary not 12.6v. Any good?
I'm sorry if either of my queries have been dealt with elsewhere..
I don't know what I don't know, hence the Noob questions...
Cheers, Martin
Hi Folks,
I've started collecting the bits for Fred's 13em7 Miniblok. Ordered 4 Hammond power transformers and soon to be ordering the 125CSE OT's per schematic and Fred's description.
If this amp works ok would it be worth swapping out, down the track, the 125CSE for the much better and much more expensive Hammond 1638SEA. I understand that there is no substitute for good iron on the OT's but not sure if a "budget" amp would be worth another AUD500 per pair of the 1638SEA (which incidentally is also suitable for 211 & 845-different league altogether)
I'm like Sgt Schultz, I know nothing, or very little anyway..
Cheers for any input guys, Martin
I've started collecting the bits for Fred's 13em7 Miniblok. Ordered 4 Hammond power transformers and soon to be ordering the 125CSE OT's per schematic and Fred's description.
If this amp works ok would it be worth swapping out, down the track, the 125CSE for the much better and much more expensive Hammond 1638SEA. I understand that there is no substitute for good iron on the OT's but not sure if a "budget" amp would be worth another AUD500 per pair of the 1638SEA (which incidentally is also suitable for 211 & 845-different league altogether)
I'm like Sgt Schultz, I know nothing, or very little anyway..
Cheers for any input guys, Martin
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Eli D argues that it's worth using CC resistors as grid stoppers because they are not spiral cut and therefore don't add inductance at crucial locations. Yes they add noise, but that noise is small compared to the benefits the resistors can provide in the right applications.
BTW, I built Fred's amp back in 2006 for my first project and it worked beautifully built stock. I would challenge anyone (DF96 in particular) to notice the noise difference, particularly in a small chassis as Fred built his. Hum and other noises are WELL above what the CC resistors would add (DF96 alluded to this and the value of getting grounding right, and s/he is right about that). And FWIW the hum and other noises are not loud nor do they interfere with the sonic qualities of his amp. It's a fabulous design. Build it stock, then modify and take notes. Maybe you'll hear something better using other resistors but maybe you won't...
Quite so mike567, the 125CSE would be the logical choice, but I couldn't help wondering if Fred's design, being intended as a budget amp, was the parts cost the primary consideration? Which then begs the question. Can it be improved with 'better' iron in the OT?
$AUD500 a pair for the 1638SEA would be considerable overkill but it's the only one I found in that range which offers 10k ohms primary, as specified with the 125CSE.
So all I did, not knowing much, was make the leap, looking for 20Hz to 20kHz frequency range.
I'm going with the 125CSE anyway for cost, and apparently it still has bass extension below its stated 100Hz cutoff.
Much appreciate the feedback, it's how I learn...
Cheers, Martin
That's smart. Build it as designed and see if you need more. Or try Edcor GXSE transformers (e.g. XSE at $20 for 70-18k or GXSE at $30 for 40-18k) - less expensive and typically better than the Hammonds CSEs. Of course, if you are in Oz I don't know about shipping or local sources...
I mostly use the GSXEs. Only weaknesses from my perspective are that they take time (generally built to order - they don't keep stock on hand) and they only have single output impedances (may not be a weakness if you choose well - I often go with 6ohm). I've used them quite a bit and am always pleased with them. You might then get away with a different rolloff in the input but I don't know what other design issues need to be considered.
Thanks Carlp
I did check out the GSXe's and they certainly look the business in Gentian Blue, good price too. Always liked Edcor, but in Oz, it's not quite so easy with an especially long lead time. We're a bit of a backwater here with sketchy and pricey access to what the rest of the world has to offer. I'm even struggling with obtaining the 13EM7's. I've got 4 Sylvania's coming from HK, but the shipper has gone quiet on me, so it may be a dud deal. These TV tubes should be everywhere, they must have made millions of the buggers even here in Oz (20 years behind the times) but they seem to be available anywhere but here...
I took advantage of the free shipping and ready supply from Mouser here in Oz for my power tranx, the 186D12 of which I ordered 4. I was going to go with the second tranx being a 1A per Fred, but for the price difference for all-186D12, they will do the job just as well. They're made in China sadly, but not altogether surprising. At least the Edcor should be made of good 'ol USA steel and copper, but these days, who knows?
Anyway MouserOz has what I need, including the 125CSE's, and the free ship makes all the difference for an old fly-blown pensioner like me...
Cheers, Martin
I did check out the GSXe's and they certainly look the business in Gentian Blue, good price too. Always liked Edcor, but in Oz, it's not quite so easy with an especially long lead time. We're a bit of a backwater here with sketchy and pricey access to what the rest of the world has to offer. I'm even struggling with obtaining the 13EM7's. I've got 4 Sylvania's coming from HK, but the shipper has gone quiet on me, so it may be a dud deal. These TV tubes should be everywhere, they must have made millions of the buggers even here in Oz (20 years behind the times) but they seem to be available anywhere but here...
I took advantage of the free shipping and ready supply from Mouser here in Oz for my power tranx, the 186D12 of which I ordered 4. I was going to go with the second tranx being a 1A per Fred, but for the price difference for all-186D12, they will do the job just as well. They're made in China sadly, but not altogether surprising. At least the Edcor should be made of good 'ol USA steel and copper, but these days, who knows?
Anyway MouserOz has what I need, including the 125CSE's, and the free ship makes all the difference for an old fly-blown pensioner like me...
Cheers, Martin
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Thanks mike567,
I suspect that it will be just as hard to source a 13FD7 here in Oz. Besides which I'm committed to Fred's thing. Most of the parts are in-hand or on way.
Something else too...
JPS mentioned replacing R1 100ohm 1w resistor with a choke. I've asked him for more info but not heard back yet.
Any idea what value choke would be needed, and would there be a tangible benefit?
Cheers, Martin
I suspect that it will be just as hard to source a 13FD7 here in Oz. Besides which I'm committed to Fred's thing. Most of the parts are in-hand or on way.
Something else too...
JPS mentioned replacing R1 100ohm 1w resistor with a choke. I've asked him for more info but not heard back yet.
Any idea what value choke would be needed, and would there be a tangible benefit?
Cheers, Martin
Alternative OPT's
Thanks for that mike567...
I ordered a couple of suitable chokes to give them a try. They were cheap enough so little lost if they don't do the business.
Another query if I may, either to yourself, or any of the folks out there...
Fred specified the 125CSE OPT as we know for the MiniBlok, with 10k primary. What would be the result if I went to say 8k primary?
Just curious, because it's all Klingon to me...
Cheers, Martin
Thanks for that mike567...
I ordered a couple of suitable chokes to give them a try. They were cheap enough so little lost if they don't do the business.
Another query if I may, either to yourself, or any of the folks out there...
Fred specified the 125CSE OPT as we know for the MiniBlok, with 10k primary. What would be the result if I went to say 8k primary?
Just curious, because it's all Klingon to me...
Cheers, Martin
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