The speed you have displayed in building this Amplifier is one of the best i have seen on the net,which makes one really understand the building of each circuit.💯💯
Mark, thanks for taking the time to put these step-by-step videos out there. You are giving so many a huge "leg up" into a wonderfully addictive hobby. This amp was my first DIY build back in 2013. Wish I had this video, then, to walk me through it. I just dove in knowing so little about "how" and "why".
I love watching this guys videos. He takes his time to make sure you get it. I can't take my meds that make me drowsy because his mild voice knocks me out to sleep & I have to go back.
I built one and it sounds excellent (gec kt88 and Philips ecc85) ,no tweaks necessary. Using 89db speakers but would like to try pi or econowave at 95 db for better dynamics. Absolutely no regrets as first amplifier I've ever built.........Many thanks Mark !!
This is a hero movie ! A lesson in life :) This is so good to watch. Once i'm through all the part i'm begining to build mine. Just a little bit afraid because i'm in europe so i have to pay attention because of the difference on the power side.
It’s unlikely in the extreme that flipping the switch during operation would damage the tube; the thermal mass of the plate (pin 3) would prevent the tube from getting red hot in the few milliseconds that the grid (pin 4) is floating, unless the tube was already poorly biased and red-plating anyway. Now, if this was a solid-state amp, then - yes - I would put a warning label on the switch to “not change during operation”, as semiconductor electronics are a lot less tolerant of B.S. than tubes are! 🤠 The real problem might be a loud >pop< in the speaker as the switch is thrown. Also, I don’t see in the video where you wire up the LED in the cathode circuit of the preamp tube. I’ve also never seen anyone design a tube amp with a diode in the cathode bias, though I have seen designs with a small battery (or a B- supply) to provide a negative bias.
I'm loving this project. No need to apologize for cutting away for the LED explanation. That's the kind of thing I want to see. I wonder why we use LEDs rather than a diode without the light emitting property. Maybe it's just because there are so many LEDs around??
Wow lots of details and explaining how and why, plus detail how to test and setup. Keep up the great videos. Like Mr Carlson Labs 👍 I think after this I’m ready to build my first high voltage tube amp. Up till now 48V has been the highest.
You could use a 1W White “lighting” LED in the cathode circuit of the KT88, these can operate easily at 350 mA or so, though their Vf is typically 3-5 VDC; you’d have to adjust the value of the 500 ohm cathode bias resistor you have there now. But it might be interesting to see if the LED brightness changes as the amplitude of the audio signal changes. 😁
Conceptually, your teachings are starting to make sense to me, learning with every video and elated when I know exactly what you're about to say! One question though, the audiophile being inside me keeps on asking one question during this build: wouldn't using one side of the small signal 6N1P tube for the right channel and the other for the left make crosstalk between the channels happen? It seems to me that the 2 different signals are super close to each other at the electron level. I wouldn't ask except for it seems odd to me that you've taken great pains to twist the heater wires, keep the power supply as far as possible from the amplifier section, crossed two wires at a 90 degree angle, etc.....
For those interested in more info on using LEDs for bias, there is lots of info in Morgan Jones book "Valve Amplifiers". Specifically he uses the either the HLMP6000 Red or what he calls the "Cheap Inefficient Red", for voltage drops of 1.63 or 1.7 at 10ma. He also discusses the other colors and their respective voltage drops.
the led's remind me of a luxman "brid' lv105 amplifier from the late 80's. it had led's in a tube front end with transistor back end. fet's fed the tubes that fed the mosfet outputs if i remember correctly. it's been a long time since ear food (muffin and jeff-winchester va)sold these things.
Hi Mark, love your build series here, great job on explaining, I just eat popcorn and enjoy watching someone else solder. This is a great help to those new to electronics. BTW I would like to help you with a small thing, as you say, if you see something, say something. So as it turns out I speak some Japanese and Nichicon is pronounced [nee-Chee-kon] no big, deal I just thought you might like to know. I buy lots of them from Digikey and they are great electrolytic capacitors.
Maybe as an option the switch at the back could be mounted internally with stand-offs.. Just to prevent any “accident” to the output tubes while in operation.
I have sometimes built radio equipment where I made every component parallel to an edge of the chassis. Nothing at 45 degrees, including wiring. This looks quite good, but RF engineers complain about it!
You should have made it easier on yourself and made an LM317 constant 5 ma. current source so you don't have to keep tweaking your power supply. Now that I think about it you could make yourself a permanent test jig with the LM317 IC and make a permanent tool for this application with little effort. Great vid. Thanx.
This is going show how little I know about this, but..... Why don't you use an adjustable bias setting like a pot? That way if you change tubes, you could easily change the bias. Told you it was going to show my ignorance!
what's the best component of a tube (pin) to lead an effect into -if one wanted to include an input jack to plug into the circuit? probably varies from app to app, may even feed it into the last part of the circuit instead? but, say I wanted to install a reverb effect into a F Vibrochamp?
What is the correct bias voltage for KT-88? I have a 1 year old chinese Audioromy 808. Class A/B push-pull. 2 KT-88's per side. After 580 hours one of my ElectroHarmonix KT-88's went blue and a pop and no sound on right channel. Asmall bank of 3 large resistors came unsoldered. I fixed it. Put back in Shuguang tubes, went fine for 900 hours and one Shuguang went blue and pop and the left channel was out. Same thing other side. A friend on facebook said the bias voltage could be way high, but my amp doesn't have a pot to change it. Maybe it's self biasing? What do you think? He told me to put a 1 ohm 5 watt resistor between one of the tube socket pins and ground and measure voltage across and use a formula to calculate. Problem is I don't know the formula and I also don't know what the bias should be for my particular amp. Does the amp matter or is it the tube bias? It also can uses 6550's if they have a higher bias setting.
I think you should be careful about your disclaimers that there is no ground current flowing through the chassis. Current takes proportional paths to ground, not the path of least resistance. In this case (since you're using a steel chassis and copper wire) much of the current is going through the wire (by an order of magnitude). However if you were using aluminum chassis about a 1/3 of the return current would be flowing through the chassis (between the second ground connection and the original star ground).
Another question: Why are you using an IR LED for 6N1P bias? Is it solely for the low voltage drop an IR LED offers? A red or green LED drops about 1.8V, not much different than an IR LED. A green LED could then double as a "Condition Green" indicator. No LED lighting up would indicate a blown heater in the 6N1P, or do I have this wrong?
No, you definitely don’t want a switch that has make-before-break action here; it would short the top half of the output transformer through the 1.2 kilohm resistor, which would cause the magnetic field to collapse and generate a nasty voltage spike. Also, it would put more current through the bottom half of the winding (from B+) to the center tap, and to the plate of the output tube and could result in shortening the life of that winding over time, not to mention throwing off the bias on the tube due to the increased B+ at the plate, although only momentarily.
I'm very much enjoying this series, thanks for the detailed explanations. I will add this amp to my list of projects for the coming year. Question: How tough would it be to add bias pots and a center-zero meter to this amp so tubes of questionable pedigree can be balanced more precisely? This is a feature on some high-end amps like the $3500 Cayin A100T you did a review on a year ago. Could you also do this to an existing amp thus turning a mediocre amp into a better one?
Hi Mark, Thank you for the very nice videos. Most of the tube amp builds I have seen are using 100k volume pot. Would appreciate it if you can explain why this resistance value is preferred compared to say 50K pot. How do we determine the appropriate value for a given tube amp project? Thanks again.
At the end of the day, its just a voltage divider. You can use whatever size you want as long as you scale the other components to match. 100k is just very common.
mfr58: I think it actually does. If you were to monitor the current through the LED it would vary based on the AC signal, but all the time keeping the DC bias point relatively constant.
The layout looks a bit busy. Did you consider the possibility of using turret board or vector board with vector clips to solder the components to? An awful lot of commercially made "pro" audio equipment used turret board construction. The point to point wiring just does not look nice. In the world of commercially built electronics point to point wiring was reserved for very simple circuits or to shave a few cents off of the build cost.
Compared to most tube amps, this wiring is pretty darn simple. Rarely do you see turret boards in audio gear due to the complexity and space requirements for 2 channels. I have seen some and some that are beautiful (Leaks, McIntosh) but I enjoy good old hand wiring myself.
A few observations: I must disagree with the wire bypassing the chassis. Electricity takes the lowest resistance path, not the shortest. (Actually, it's a ratio depending on impedance.) Chances are the chassis is a far lower impedance than that thin wire. If the coupling cap is big enough, and a quality film cap, differences shouldn't be audible. Why not tie a 1Meg resistor from screen to B+ to "stupid proof" the rear panel switch a bit? Why isn't the LED bypassed? Diodes tend to be noise sources. Zeners are far (far) worse in this regard, but a forward biased diode creates noise too. Instead of a LED, why not an adjustable VBE multiplier?
Keep in mind, I'm trying to build this as close to the Kegger design as possible. Did you watch video one where we discussed all that? Once built, I have many tweaks I plan to do to it and will do that as a "tweaks" video. On the grounding thing, I'll make some comments in my next video. I know many major amp designers / builders who use this technique, done so many times myself with deadly quite noise floor. I'll show some examples in the video.
Ground wire bypassing the chassis. I have been professionally designing and building industrial electronic control systems for over 40 years, and in our equipment those ground wires are almost always used. The chassis is nowhere as low an impedance as it may seem. The DC resistance may measure zero ohms, but introduce AC and the impedance can go through the roof. UL does not require these bonding wires "for nothing" - they have their reasons. My only suggestion - make that bonding wire a good heavy gauge, like maybe 14 AWG.
hlmp6000 i have 1600 of them a lot of the standard red led's are electrically identical this i know from building led display projects there is no magic in the hlmp6000 or any other standard red led
Keep in mind, this isn't my design. You should click the DIYAudio link above and post in that thread about your thoughts on the LED needing a cap. I've seen a lot of cathode biased LED setups and can't remember seeing any with caps across them.
Raw LED's are not noisy, especially when powered by DC. LED lamps can be noisy but that is due to the integral SMPS used to power them from the AC line.
Understood. But from what I have researched, any raw LED noise is in the microvolt range at worst. Yes, there is an optimum forward current for minimum noise (type dependent). From what I have found LED's are used a lot in carhode bias circiuts, with no noise complaints from builders.
