Kegger/blueglow kt88 project

[JohnB1950]

kward, I've now changed my mind. I just ordered the epoxy from Amazon. I don't want to pull the transformer back out once I get mine built and tested. The only drawback is that I will never know if mine would have been noisy without potting. I don't expect at this point that Edcor has corrected whatever manufacturing flaw they have that's causing the issue.

 

[JohnB1950]

I have a transformer grounding question. I know that the bolts attaching the end bells are isolated by nylon shoulder washers because of induced currents in them. Do you grind off the powder coating on the bottom of the mounting tabs so that the end bells are grounded to the chassis for safety? I am almost ready to start mounting and wiring!

 

[opnly bafld]

The end bells are painted, that means no electrical flow right?
I mounted mine with rubber washers.

 

[opnly bafld]

Dave,

Your voltages are almost identical to my amp which used the Hammond 374BX. I have wondered if connecting the mains to the 110VAC input on the PT instead of the 120VAC would raise the B+ and related voltages by about 9% which gets the plate and cathode voltages into the Edcor range; Not sure how safe that would be, perhaps others can advise.

It would be more than 9%.
What are your heater voltages? That is what I would use to determine if I needed more or less voltage.
I used the 274BX because it has 115v or 125v and my house voltage consistently runs @ 124, I probably would have been fine with the 374BX, but it helps me sleep at night.

 

[philiprst]

It would be more than 9%.
What are your heater voltages? That is what I would use to determine if I needed more or less voltage.
I used the 274BX because it has 115v or 125v and my house voltage consistently runs @ 124, I probably would have been fine with the 374BX, but it helps me sleep at night.

My heater voltages are almost exactly 6.3VAC and the 374BX has a separate primary for the heaters.

Related to safety: the schematic does not have a bleeder resistor across the large capacitor and I would recommend that you consider adding one.

 

[philiprst]

Just a quick update based on living with the amp for a couple of weeks. Overall, I have been delighted with its performance and sound. It works perfectly from the standard output of my CD player and streamer and drives my fairly inefficient speakers at reasonable volumes in my small room. If I had a larger listening room then I would probably need to use more efficient speakers. Mine was built with Hammond transformers and chokes which seem to work fine for this application.

I am now halfway through a build of a tube preamp based upon the Aikido preamp. My main goal is to build a phono preamp based on the same topology but I am starting with a line preamp to see how that works out before adding the passive RIAA network and additional gain stage.

 

[kward]

kward, I've now changed my mind. I just ordered the epoxy from Amazon. I don't want to pull the transformer back out once I get mine built and tested. The only drawback is that I will never know if mine would have been noisy without potting. I don't expect at this point that Edcor has corrected whatever manufacturing flaw they have that's causing the issue.

Good luck! I doubt Edcor even views it as a manufacturing flaw. I think from their perspective they're just doing what they've always done. Why some (most?) Edcor PTs hum and a few don't is beyond me. I don't want to be bothered any more with trying to figure out whether or not a particular sample is going to hum so I always pot and mount on neoprene washers (...if it's an Edcor PT).

have a transformer grounding question. I know that the bolts attaching the end bells are isolated by nylon shoulder washers because of induced currents in them. Do you grind off the powder coating on the bottom of the mounting tabs so that the end bells are grounded to the chassis for safety? I am almost ready to start mounting and wiring!

I've tried this both ways--sanding off the paint from the tops of the mount flanges down to bare metal vs. not doing this. I have found no appreciable difference in the amp's performance either way.

 

[philiprst]

A few construction details of my build that might be of interest.

I used the 17x10" Hammond chassis that Mark used. It works fine but if I were to do it again I would probably use a slightly bigger chassis with a bit more depth (i.e. 12" vs 10") I put the mains toggle switch on the back rather than the front to keep all the PT transformer wiring in the corner of the chassis. I used good quality regular speaker binding posts but not the expensive ones that Mark used; same for the RCA jacks. I used good quality Vishay/Dale resistors throughout since I have a large stock of these and axial Spague capacitors for the cathode bypass. I used 500V rather than 450V rated radial electrolytics for the PS filter of the driver tube. Rather than mounting the 100+100uF can capacitor on a tag strip I drilled the chassis and mounted the capacitor on top of the chassis with a clamp (this was one reason I would have gone with a slightly bigger chassis). I put a thermistor in the power supply for a somewhat softer start and added protection diodes to the rectifier tube since I managed to destroy one (unprotected) and expensive rectifier tube during testing. I would probably have been better off using a cheap rectifier tube until I got the PS sorted out. I experimented with elevating the heater supply but found the amp to be dead quiet without it. If I was building another one (and I am considering monoblocs) I would probably consider violating the all-tube philosophy and try a solid state rectifier since I find those easier to deal with, more robust, and much less expensive. I used solid wire for most of the internal wiring since I like the way it can be bent and retains its shape.

My least favorite part of the buid was, as usual, the drilling of the chassis. I also found those small LEDs for the driver cathodes a bit difficult to deal with. The only issue I had was my careless mistake in the PS wiring which cost me a rectifier tube. Fortunately I tested the PS section before wiring up the rest of the amp and avoided any other collateral damage.

I think you could build this amp just using a multimeter for testing and verification but I have a low-end oscilloscope and signal generator that I found to be useful in being able to verify clipping levels and get a feel for the harmonic distortion using the FFT function.

I had a lot of fun and minimal frustration with building this amp. There are so many resources available to help, including on here and in Mark's videos. It was also a great learning experience.

 

[StepheK]

A few construction details of my build that might be of interest.

I used the 17x10" Hammond chassis that Mark used. It works fine but if I were to do it again I would probably use a slightly bigger chassis with a bit more depth

I agree with all your suggestions, but especially the chassis. A 17X12X2 is perfect. I used a 100UF 500V after the big hammond choke and then added a smaller second 1.5H choke inside followed by a 250uf, 500V JJ cap rather than that 100+100 and it worked well. I know that is likely overfill but that is how I like doing things

 

[JohnB1950]

I used the 17x10" Hammond chassis that Mark used.

I am also building one but just getting started. I have all parts I hope except knob and feet. My chassis top plate is 1/8" aluminum plate 18 X 11.5 with 3/4" walnut base. I think I ended up with 4-3/8 height. I think I'll have plenty of room. I just finished the chassis last night and have added the back panel parts. I am anxious to start wiring and soldering! However, I'll be visiting my daughter at the beach for a week or so and won't be able to make any substantial progress for a couple of weeks. Also, I went ahead with ordering the thermal potting epoxy and am going to do the potting before mounting the PT.

 

[cirrusdave]

A few construction details of my build that might be of interest.

I used the 17x10" Hammond chassis that Mark used. It works fine but if I were to do it again I would probably use a slightly bigger chassis with a bit more depth (i.e. 12" vs 10") I put the mains toggle switch on the back rather than the front to keep all the PT transformer wiring in the corner of the chassis. I used good quality regular speaker binding posts but not the expensive ones that Mark used; same for the RCA jacks. I used good quality Vishay/Dale resistors throughout since I have a large stock of these and axial Spague capacitors for the cathode bypass. I used 500V rather than 450V rated radial electrolytics for the PS filter of the driver tube. Rather than mounting the 100+100uF can capacitor on a tag strip I drilled the chassis and mounted the capacitor on top of the chassis with a clamp (this was one reason I would have gone with a slightly bigger chassis). I put a thermistor in the power supply for a somewhat softer start and added protection diodes to the rectifier tube since I managed to destroy one (unprotected) and expensive rectifier tube during testing. I would probably have been better off using a cheap rectifier tube until I got the PS sorted out. I experimented with elevating the heater supply but found the amp to be dead quiet without it. If I was building another one (and I am considering monoblocs) I would probably consider violating the all-tube philosophy and try a solid state rectifier since I find those easier to deal with, more robust, and much less expensive. I used solid wire for most of the internal wiring since I like the way it can be bent and retains its shape.

My least favorite part of the buid was, as usual, the drilling of the chassis. I also found those small LEDs for the driver cathodes a bit difficult to deal with. The only issue I had was my careless mistake in the PS wiring which cost me a rectifier tube. Fortunately I tested the PS section before wiring up the rest of the amp and avoided any other collateral damage.

I think you could build this amp just using a multimeter for testing and verification but I have a low-end oscilloscope and signal generator that I found to be useful in being able to verify clipping levels and get a feel for the harmonic distortion using the FFT function.

I had a lot of fun and minimal frustration with building this amp. There are so many resources available to help, including on here and in Mark's videos. It was also a great learning experience.

Some great comments Philip...I agree with many of the things that you say.

In case somebody comes to this thread before they begin their build I would like to offer my thoughts after completing my amplifier:

Summary:
I would do it again in a heartbeat ...I learned so much, and feel a huge sense of achievement having built the amplifier that I now listen to as my "daily".

FYI I followed Mark's series (almost) religously ...Hammond chassis; component placement; wiring schema; etc.

Things that I would definitely do the same again:
Watch Mark's videos thoroughly before starting, and then refer to them frequently during the build
Watch Stephe's (Skunkie's) videos
Take my time with the construction
Buy a stepper drill bit (great invention)
Buy hole punches (although I didn't splurge on expensive ones so they may not last me for many more builds)
Buy Edcor OPTs and choke
Install a Thermistor
Use a Blue LED / mains switch
"Float" the 6.3v heater wiring with a virtual center tap ground
Buy/use an isolation transformer / Variac (I did buy a Variac that is also an isolation transformer although I know that most of them are not)
Buy a big box of different sized heat shrink
Build Rob Robinette's bucking transformer: https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer ...although I am finding that the lower voltage from the Hammond PT (compared with the Edcor PT) is negating its use/need

Things that I would do differently:
Use solid wire for the wiring inside the amp (in a newbie's hands, stranded does not create a neat look)
Buy a Hammond PT from the onset ...the buzz I got from the original Edcor PT drove me cuckoo
Find a better way to measure /drill the tube sockets and their mounting screws ...I always found myself having to make "adjustments" with my Dremel. There must be a better technique ..???
Develop the technique for soldering coax to high-quality RCAs ...that was more difficult than most of the other soldering
Place the cathode resistors on the outside (top) of the chassis (a la Skunkie) ...keeps the heat out and gives easy access for measuring voltage (and subsequently cathode current)
Buy a deburring tool (sooner)

I hope that this is helpful for anybody thinking of building this amp.

In the words of the great guru, Nike ... Just Do It!!

Dave

 

[dyehardfan]

Watch Stephe's (Skunkie's) videos
Dave

Would you please point me to those videos? I just finished Mark's series and have read this entire thread in the last couple of days. I am trying to psych myself up to tackle my first point to point build and want to learn as much as I can before hopping into it.

Thank you.

 

[JohnB1950]

Would you please point me to those videos? I just finished Mark's series and have read this entire thread in the last couple of days. I am trying to psych myself up to tackle my first point to point build and want to learn as much as I can before hopping into it.

The first video of the four part series is

You can go to her youtube channel at https://www.youtube.com/channel/UC3tTF80zeW_HDmfsLt-u-8Q and search for KT88.

 

[philiprst]

Some great comments Philip...I agree with many of the things that you say.

In case somebody comes to this thread before they begin their build I would like to offer my thoughts after completing my amplifier:

Summary:
I would do it again in a heartbeat ...I learned so much, and feel a huge sense of achievement having built the amplifier that I now listen to as my "daily".

FYI I followed Mark's series (almost) religously ...Hammond chassis; component placement; wiring schema; etc.

Things that I would definitely do the same again:
Watch Mark's videos thoroughly before starting, and then refer to them frequently during the build
Watch Stephe's (Skunkie's) videos
Take my time with the construction
Buy a stepper drill bit (great invention)
Buy hole punches (although I didn't splurge on expensive ones so they may not last me for many more builds)
Buy Edcor OPTs and choke
Install a Thermistor
Use a Blue LED / mains switch
"Float" the 6.3v heater wiring with a virtual center tap ground
Buy/use an isolation transformer / Variac (I did buy a Variac that is also an isolation transformer although I know that most of them are not)
Buy a big box of different sized heat shrink
Build Rob Robinette's bucking transformer: https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer ...although I am finding that the lower voltage from the Hammond PT (compared with the Edcor PT) is negating its use/need

Things that I would do differently:
Use solid wire for the wiring inside the amp (in a newbie's hands, stranded does not create a neat look)
Buy a Hammond PT from the onset ...the buzz I got from the original Edcor PT drove me cuckoo
Find a better way to measure /drill the tube sockets and their mounting screws ...I always found myself having to make "adjustments" with my Dremel. There must be a better technique ..???
Develop the technique for soldering coax to high-quality RCAs ...that was more difficult than most of the other soldering
Place the cathode resistors on the outside (top) of the chassis (a la Skunkie) ...keeps the heat out and gives easy access for measuring voltage (and subsequently cathode current)
Buy a deburring tool (sooner)

I hope that this is helpful for anybody thinking of building this amp.

In the words of the great guru, Nike ... Just Do It!!

Dave

Thanks so much Dave. I had completely forgotten about the Variac, which I have, and is probably the most useful thing I had to help me with the testing.

A trick I used for testing the cathode bias for each tube is to (temporarily) put a 1 ohm resistor in series with the cathode resistors and measure the voltage across that which is equal to the bias current. You can also permanently wire those 1 ohm resistors into the circuit and connect them to jacks so you can read the bias when the amp chassis is closed up but that's only really worth it is you add a trim pot so you can adjust the bias on the fly.

For the tube socket holes in the chassis I have a pair of chassis punches although, as you note, decent ones are very expensive. For aluminum, I find that a carbide hole saw works just as well. The holes for the mounting screws never come out exactly right so I used a drill that was oversized by about 0.5mm which provides enough wiggle room to compensate for any small misalignment in the hole. I am sure there are better techniques that others use.

 

[cirrusdave]

The first video of the four part series is

You can go to her youtube channel at https://www.youtube.com/channel/UC3tTF80zeW_HDmfsLt-u-8Q and search for KT88.

JohnB

Thanks for doing this for "dyehardfan"!! They are the key ones that Skunkie did that I found helpful ...although I find some of her others, not focused on the KT88 SE, also VERY interesting.

D

 

[JohnB1950]

small misalignment in the hole. I am sure there are better techniques that others use

I have my chassis completed and am waiting for the potting epoxy to harden in the Edcor PT. I have all the parts(I think) and will start wiring tomorrow if all goes well. For the tube socket holes, I used a carbide hole saw to cut the main hole for the socket, then held the socket in place and used a self-centering pilot drill for the mounting screws similar to what you do for a door hinge. My chassis main plate is 1/8 aluminum from the local salvage yard.

 

[JohnB1950]

In aluminum, a step drill also works very well if you already have one.

 

[philiprst]

John: that's great suggestion to use a self centering drill bit for the tube socket mounting holes.

I have actually used chassis punches, step drills and carbide hole saws to create the tube socket holes. They all work. The advantage of the chassis punch is that the hole is very clean with virtually no burrs. Step drills create more burrs which have to be dealt with. Carbide hole saws create even more burrs, especially in steel, but you can minimize this by drilling halfway through on either side. The cheap chassis punches (the ones in the $10-15 range) do work but they blunt fairly rapidly, in my case after a couple of projects. The Greenlee punches last significantly longer but are extraordinarily expensive. I assume that in the olden days people knew how to sharpen these punches.

 

[philiprst]

Just another update from my project. The amp has been in my system now for several weeks and is working well without any issues. I am very happy with it.

Inspired by the success of that project I decided to attempt a build of a preamplifier based on the so-called Aikido circuit. I have completed the PTP build of the line preamp version with a robust power supply and a regulated DC heater supply and it is working well with no noise/hum issues so far. Of course, if one is using a CD or other source with the standard output then a line preamp is not really required with the Kegger power amp unless you wanted to add tone controls or switching. But my plan is to now add the RIAA equalization and another gain stage to make it into a MM phono preamp at which point issues of noise and hum become more critical. One nice thing about the Aikido topology, apart from it's theoretical noise rejection, is the ability to swap tubes in and out with minimal changes (if any) in the circuit component values. This allows one to easily experiment and tune with the gain of each stage. If anyone is interested I could post more details of the build here or, perhaps more appropriately, start a new thread on this subject.

 

[JohnB1950]

I decided to attempt a build of a preamplifier based on the so-called Aikido circuit

Do you have a link to this circuit or your build details? I've been looking at the 4S preamp design as maybe a next tube project. I plan on using my Blueglow amp in my garage/shop, so it will mostly be driven by a Raspberry Pi/Boss 2 DAC running Volumio. The DAC will output 2 Volts, so I'm not sure I will really need a preamp.