6DN7 tube HiFi Amp MORE mods Part 3 // New transformers, resistors and Constant Current Device.

[@doubledigitdreams]

I’ve ordered 90% of the components needed to build this amp. The files you have shared and these videos have given me the confidence in undertaking this project. Thank you.

[@MikeFreda]

Thanks for commenting. I hope it goes together smoothly for you! Cheers.

[@paulbrodie]

Mike, good video! All this electronics stuff is Way over my head, but you did a great job explaining. I did notice the Aermacchi poster on your wall... Hope you are well, talk soon!

[@MikeFreda]

Thanks Paul! Things are going good. Busy as always. I’ll be in touch soon! Cheers.

[@DerekRonin]

Happy Holidays, Thank-you for being so helpful!

[@MikeFreda]

You’re welcome, thank you! Happy Holidays to you as well!

[@DerekRonin]

Thanks for another amazing build instructional, I'm still putting the pre amp together.

[@MikeFreda]

You’re welcome. Now is a good time of year to work on a few projects! Cheers.

[@rajoconnor]

Very cool! I’m definitely interested in building an Amp

[@MikeFreda]

It’s a really good sounding amplifier. Even the first/base version is pretty good! Cheers.

[@andydelle4509]

I would bet those old 1960s output transformers out perform both the new Hammond and the Edcor.

[@MikeFreda]

To my ears, I cannot hear an audible difference between the Edcor and these older OT's. I purchased and installed the Edcor OT's to show people what will work and what is readily available to purchase. I also wanted to see if there was any difference between the two... not really. The Hammonds are okay as well but they to cost way more than the Edcor's. I also feel that the extra tap's/windings may affect things as well. Cheers.

[@robertchauval]

Like your build.. very tidy.. and really great documentation. Im a big fan of the industrial look for amp cases.. I like how youre not buying into the "sand-snobbery" BS and focused on valid technical improvements over golden-ear ones :-). An anode CCS will very much improve input stage PSRR and tube linearity... well worth doing.. Bottlehead use exactly the same sand CCS (C4S) and they have a great rep.. then theres the gyrator version of the CCS ala Bartola etc?? Another thing to try would be replace the input stage cathode resistor with an active regulator like TL431 at 2mA and 2.6V it would slot in easily?? In HV ccts I try to default to series resistor strings as the volt drop across ea resistor is significantly reduced reducing aging and current leakage (dust) issues down stream etc. A minor issue with your calcs for the CCS that may confuse those less technically familiar (Im an EE). At 9.47 the arrow points to the base of the top BJT and you state 0.95V across it when you should be pointing at the emitter. Got me going for a while till I confirmed the volt drop across the LED=1.6V and of course the standard Vbe=0.65V thing leaving 0.95V across Rset. .. Not trying to be a knocker just a correction .. Im really keen to see where you take this lovely lill amp and see whats capable of

[@MikeFreda]

Thanks for commenting. This amp was a labour of love. I’m very happy the way it turned out and I use it almost every day. I’ll make the CCS diagram a bit more clear. Thanks for pointing it out, it seemed self explanatory to me when I did it. Cheers.

[@CraigHollabaugh]

Nice work, thanks for taking us along your build series. Where did you read about constant current source loads improves power supply rejection?

[@MikeFreda]

Thanks for commenting. I read this in John Broskie's Guide to Tube Circuit Analysis & Design (Tubecad Journal) It's also buried in the Electronics theory in the Morgan Jones book... not to mention a couple forums (like Audio Asylum and DIY Audio) There are some talented people in there that love this theory. Cheers

[@CraigHollabaugh]

@@MikeFreda thanks, I'll look through these references for commentary this.

[@marty1776]

Do you ever add an active tone stack to your amps? Thank you. Great videos

[@MikeFreda]

Thanks for commenting. On stereo HIFI amps that is just used for audio playback, I typically don’t add a tone stack. They can be tricky to employ, they can add some noise into the system and can rob some power. All this is not that good for lower power SET amps. For guitar amps, yes, I do. Cheers.

[@marty1776]

@@MikeFreda I had another question, when you connect your source audio to the preamp, do you worry about it not being a line level output? How do you go about that? Thanks

[@MikeFreda]

Yes, you have to make sure the level you are using is the right voltage level. Anything that is RCA is typically line level. Headphone jacks could be a “maybe”. You would want to measure that and/or keep the volume low. I do that with my iPhone. Speaker jacks would be a no go. At the end of the day, if you are not sure you could measure the voltage swing (AC) on using a multimeter. Cheers.

[@ibrahimkocaalioglu]

Single lm317 can be used as constant current source

[@MikeFreda]

Thanks for commenting. A LM317 is designated as a 4.5-40v voltage regulator. I've seen many use then on the cathode where there is lower voltages but never on a plate. I'm intrigued how you would accomplished this? Looking at the data sheet, It's not clear whether or not 300-400VDC should be applied to the LM317. Furthermore, what sort of ground loop or switching noise would it induce on the plate? I'm all ears and eyes! Cheers

[@johnstuchlik5828]

I'm curious about your work shop speakers.looks like 8 dual cone and tweet.grs pioneer? What crossover? Enclosure vol.iwould like to make something similar for low watt amp.

[@MikeFreda]

Thanks for commenting and noticing. They are a real sleeper set of speakers. Almost embarrassing how good they are for the cost. The drivers GRS 8FR-8 Full-Range 8" Replacement Speaker Pioneer Type B20FU20-51FW and the tweeters are Dayton Audio ND20FA-6 3/4" Soft Dome Neodymium Tweeter. They are crossed over high with just a 2.0uf cap... but you can play with that to it sounds good to your ears. The 8" driver is a full range and its start to roll off at 11k hz. Plug the values into a Xover calculator then order a bunch of different values. (2.5 to 1.5) The box size is 18" x 12" x 11" (outside dimension) and the port is 4" long x 2-1/4" dia. It's made out of 3/4" Baltic Birch ply and extra stiffening / diffusion strips are glued inside along all 6 sides. No nails or screw, all glue. They are worth making especially if you have a lower power SET amp. cheers

[@johnstuchlik5828]

Thanks for the details.ihave a pair of the grs as well as the pioneer bofu.started a t-line for pioneer but never finished.picked up the grs copy and thought about a simpler box.also have that Dayton tweet so I think build something similar.also have 1 in tympany tweet with small wave guide bc25 I will probably have to pad if combine with bofu.also have 6dn7 6j5 so maybe duplicate your setup.inclined to try6em7 first as the lower plate resistance of out triode might be an advantage.could always build 1 ch. of each.probally use 125 dse as I have a pair.cant duplicate your fine fab work,so use bake pan upside down with thin plywood for reinforcement.

[@MikeFreda]

Nice, I hope it works out for you! Just start building! Cheers

[@jukingeo]

@Mike. I like to keep things simple, and normally don't like putting sand in a tube circuit, but if adding the constant current source DRAMATICALLY increases the sound quality, then I would try it. So that is my question. Does it significantly affect the sound, or is it just marginally better?

[@MikeFreda]

Thanks for commenting. Oddly enough, when it come to guitar tube amps, I'm more of a purist and do not install "sand" in them. However, in HiFi amps like this one, there are benefits. I find adding a solid state CCS provides better ripple rejection (whatever is left over from the PSU filter circuit) which makes for a blacker/quieter background. This makes for lower distortion, tighter bass and better imaging due to better channel separation. Under critical listening (headphones or a controlled environment) it is noticeable. Most of my HiFi amps have some sort of current or voltage regulation. Cheers

[@jukingeo]

@@MikeFreda Ok, fair enough. I know what you mean about guitar amps. I used to fix them, and they are more critical of sound that even a tube rectifier has an effect on it due to the current sag at high volumes. Solid State rectifiers in a tube guitar amp gives a fuller lower end at high volumes, but distortion isn't as nice. At low volumes, I can't really tell the difference. But for Hi-Fi, you don't really need a tube rectifier. Yet, I still like to use them anyway.

[@chrisreich40]

Let me make sure I understand the wiring of that constant-current source board. Red goes to B-plus, and red goes to the tube plate, and red goes to ground. Got it! 😆

[@MikeFreda]

Yes, different color wire would make it easier to follow especially for debugging. It all made sense to me at the time plus I didn’t have another color of 600v Teflon solid core wire at the time. I’ll remember that for my next build. Cheers.

[@marty1776]

Do you have a hi-fi supply site you’d recommend in the US? For the caps and future builds. Thanks again

[@MikeFreda]

Thanks for commenting. I typically order from Mouser and Antique Electronic Supply. For filter caps, there is a large selection of them. The main criteria is Voltage, heat range and life span so you have to pick wisely. Cheers.

[@marty1776]

@@MikeFreda I didn't see a chassis layout file, like dimensions(like you did for the 12AU7 linestage). Did you make one? totally cool, if not, i'm about to buy metal and just thought i'd ask. Thank you

[@MikeFreda]

No, I did not do a chassis layout. You could probably screen shot the chassis footage on the first video and draw it out by hand. I suspect most people would just do a large top plate so my layout probably wouldn’t work. If you want something, I could do a quick sketch for you. Cheers.

[@mauricedemel6142]

Some times cc generators are good.Some times bad. Isolate ccg from signal path.Otherwise the sound is dull.(muddy mid)

[@doubledigitdreams]

Just curious, instead of using 3 50k resistors could you just use one 150k resistor? I’ve already added this in my amp and did notice an improvement, i used 2 resistors in series to get close to the 160k specified in the worksheet. I’d like to implement this circuit in my other builds as well.

[@MikeFreda]

Thanks for commenting. Yes, you could only use one if the power rating is adequate. I'd recommend least 2-3x the wattage for a safety margin. It's cheaper to install 2 or 3 one watt resistors than a larger wattage one. I typically have 1/2 watt and 1 watt in my parts bin. As for value, as long as you are +/- 10% of the calculated value you will be fine. I have found that It's really not that critical. Cheers.

[@doubledigitdreams]

@@MikeFreda thanks for confirming. Im actually putting together a second amp from spare parts. I’ve salvaged a box full of iron and luckily have everything I need. What I would really like to do is add a potentiometer in the cathode bias circuit of the output section of the tube to help adjust the bias. I have acquired a dozen or more 6dn7 tubes and notice when I’m swapping tubes the 16.5 volts on pin 3 can vary by a couple of volts. I’d like to be able to match the output current of each tube maybe even run them cooler or hotter. Still haven’t figured out how to make that happen though.

[@MikeFreda]

You're welcome. I'd probably just run the tubes as is and not worry about having an adjustable bias. Set the bias at ~7-8 watt and leave it. Having a potentiometer on the cathode "may" induce some noise especially when it gets older and starts to oxidise. I can't really hear a difference between 7-10 watts other than a bit of headroom. If you are determined to make that happen I'd probably use a large 5 watt resistor that is almost the proper value and install a very small value pot in series to allow you to add just a bit more resistance. The higher the resistance, the lower the bias. cheers.

[@doubledigitdreams]

@@MikeFreda i figured after adding your subsequent mods another can’t hurt😁. If it’s too much trouble I’ll ditch the idea. A few days ago I put together the entire circuit on a breadboard with success, i also went ahead with adding a 100ohm pot and ended up using an 820ohm resistor to give me about 2v of adjustability, i was a bit confused though. On the test point of the pot and resistor I would adjust to 16.5 and on pin 3 there would still be a slightly higher voltage (17.8). I’m guessing the voltage on the pot and resistor would be the right one? What do you think. Sorry for the bombardment of questions,

[@davidnguyen5028]

I’m thinking about switching out coupling capacitors from .02uf 500v to .01uf 800V, would this change the amp drastically? These are on a dual EL84 push pull design. TIA.

[@MikeFreda]

You may hear a difference depending on the age or brand (i.e. leaking, high quality vs low quantity) If it's a good quality Cap, it should sound a bit better after it breaks in. The value between the two is negligible. You can input the values into a Bandpass frequency calculator and see what the roll off is. Never hurts to try. Cheers

[@davidnguyen5028]

@@MikeFreda These caps are Orange Drops, I’ve heard they’re decent also I have them on hand. Didn’t want to place an order with Mauser

[@MikeFreda]

Yes, they are good. A step up would be something like Solen, Clarity Caps, Auricap… or some Oil in film caps. Then you you can definitely hear a difference. Cheers.

[@davidnguyen5028]

@@MikeFreda Thanks!

[@ChickenPermissionOG]

carbon film resistors are terrible.

[@MikeFreda]

How so? I use them for their low noise characteristics. They do have their place in the circuit. Why do you feel their are terrible?

[@ChickenPermissionOG]

@@MikeFreda metal film are better they don’t go bad as quickly as carbon resistors. They’re not as noisy. Carbon resistors are just less costly.