Kegger/blueglow kt88 project
- Thread starter Tubeglowpio
- Start date
invaderzim
Super Member
When I added the grid stopper resistor and the grid leak resistor there wasn't a noticeable difference; maybe a bit cleaner sound. When I upped the capacitors from 10uf to 30uf it seemed to have a more defined sound with improved dynamics when sounds hit fast. When I added that resistor it just opened up and I can sit for hours and listen to it now.
I was building a chip amp at the same time and it was sounding pretty close to the tube amp until I added that resistor.
I'd make a lousy reviewer because I can't tell you what is good about the sound but no matter how much I thought I liked the sound of the amp before I'd turn it on to listen and then 15 minutes or a half hour later I'd turn it off and go find something to do. Now I can just listen until something comes up that I have to do. I've stayed up too late a few times with 'just one more song'
I was building a chip amp at the same time and it was sounding pretty close to the tube amp until I added that resistor.
I'd make a lousy reviewer because I can't tell you what is good about the sound but no matter how much I thought I liked the sound of the amp before I'd turn it on to listen and then 15 minutes or a half hour later I'd turn it off and go find something to do. Now I can just listen until something comes up that I have to do. I've stayed up too late a few times with 'just one more song'
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JohnT123
Super Member
What I'd like to know, is how to evaluate the amp without access to 30K in test gear. People ask about the specs all the time and all I can say is that it was designed to sound good, and it does. I'm going to scope mine and study the output waveform across the spectrum. It will show if there's any weak spots but nothing in terms of actual frequency response or distortion. If anyone know other testing methods I'd love to hear it.
invaderzim
Super Member
It would take something like the Analog Discovery2 with the software from TheStuffMade
Ruidoso Bill
Active Member
Well, I know I had something wrong, so with everything in place especially around the nine pin left me unable to isolate it. Hopefully this time going a lot slower and a lot more testing between. Will it cause harm to tubes to run the heaters up with nothing else? I have put in a new ul switch and color coded everything red for right, blue left, to help me make sure nothing is crossing over between channels. When it comes to the tubes home much needs to be wired so I don't damage anything?
primosounds
Powered with pure tube sounds.
I don't remember if your build started from scratch or was a rebuild. You always need to start work with the power trans and power supply and then the amp circuit. Also the terminology you use indicates that you are still unfamiliar with tube equipment. For example, not sure what this means?
"When it comes to the tubes home much needs to be wired so I don't damage anything?"
To answer the question about tube heaters. It probably wont cause any catastrophic harm to run the heaters without anything else, but it is way easier to just use your meter on the ACV setting and test the result at the end of the heater string to know whether you have proper voltage, assuming you have soldered the wires to the correct pins. It is very easy to incorrectly wire the tube sockets. I still do it occasionally after working on toob amps for many years. Also pay attention to your soldering technique with properly dressing your connections to avoid shorts from nubs of cut ends shorting on something. Avoid stressing the sockets pins by too much manipulation or too much heat. Too much time heating the solder joint will weaken the metal and cause it to break, sometimes within the socket. If the tube pin breaks within the socket you will have a very frustrating time diagnosing what went wrong as there will be no visual clue.
"When it comes to the tubes home much needs to be wired so I don't damage anything?"
To answer the question about tube heaters. It probably wont cause any catastrophic harm to run the heaters without anything else, but it is way easier to just use your meter on the ACV setting and test the result at the end of the heater string to know whether you have proper voltage, assuming you have soldered the wires to the correct pins. It is very easy to incorrectly wire the tube sockets. I still do it occasionally after working on toob amps for many years. Also pay attention to your soldering technique with properly dressing your connections to avoid shorts from nubs of cut ends shorting on something. Avoid stressing the sockets pins by too much manipulation or too much heat. Too much time heating the solder joint will weaken the metal and cause it to break, sometimes within the socket. If the tube pin breaks within the socket you will have a very frustrating time diagnosing what went wrong as there will be no visual clue.
JohnT123
Super Member
When you wire up the power supply and heaters there's no need to have the tubes installed. The voltages will be reading high due to a lack of loading. Early on you just want to see if there's the correct voltage on the correct pins.
Sorry, didn't see that @primosounds already provided an excellent answer.
Sorry, didn't see that @primosounds already provided an excellent answer.
Ruidoso Bill
Active Member
Build is from scratch. Thanks for the advice. My question was about the heaters. When Trouble shooting in the amp sections, if tubes are installed, must I have everything connected ie B+, Signal generator, dummy load etc connected or possibly harm the tubes. The reason I ask is I was planning to do things in steps since I had a shorting issue. Yes, I haven't done anything with tubes in 55 years since I got my first tube tester. Old dog still learning.
Ruidoso Bill
Active Member
Thanks, like I said below I was planning in doing it in steps and don't want to damage anything.
primosounds
Powered with pure tube sounds.
That plate to plate resistor is a feedback resistor which helps to control non linear behavior. For multi grid tubes the use of fb greatly improves the performance of the circuit, which results in better sound quality. The key is that it is NOT global negative fb which includes the output transformer. To me the use of GNFB adds an artifact to the sound. Maybe a very high quality OPT will not have that added component but i have not explored that. Most people when they get a very expensive OPT will also use an expensive high power triode.
primosounds
Powered with pure tube sounds.
Heaters are the simplest aspect. as long as they are wired in properly, either working or not, and separate from the high voltage circuit. If you are testing the B+ circuit, separately, for voltage that can be done by itself. Don't forget to check the cathode circuit. But once your entire amp is complete i prefer to use good sounding, "junk" speakers as a load since i want to hear the sound of the amp to find out if is good. I find it more informative and easier to know if there are issues with the quality of the sound than looking at some frequency on a scope. I use practically the same 4 or 5 cds for testing audio in my shop which quickly reveals to my ears if there are issues with the sound. With a variac on around 80% of wall voltage there will be enough output to produce sound from the speakers. It may not be good sound since the circuit is not fully energized but it will tell you that initially the circuit is working. At this point voltages should be fairly stable. If B+ starts dropping or a resistor starts to get hot or the smell of something overheating are warnings to shut things down and inspect. At the least using a variac will allow you to save components from the wheelie bin.
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Ruidoso Bill
Active Member
Thanks for the information, as always detailed and well thought. Prior to the fuse blowing at full wall voltage it was working at 80% on the variac, worked up to 110v, it's when it was up to wall volatage (120 vac) it started blowing the fuse.. Too bad I donated a bunch of speakers to the humane society thrift shop, I now only have good ones, I started with the dummy load, once nothing was smoking tried the speakers.
Thanks again, very much appreciated.
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Ruidoso Bill
Active Member
Made some good progress today with no load B+ is 585VDC, heaters with the just KT88's 6.5VAC, after bringing it up on the variac first went to Wall ac, no blown fuses or issues, so Guess that means the PS is in working properly. Still waiting on some parts but did a lot of rewiring in the amp section, think this time it will work (thinking Positive).
Ruidoso Bill
Active Member
That's good news. I'd keep it on the variac during the testing. You don't want to push the filter caps past their voltage ratings. In my brain the power supply and related wiring is like our circulatory system and it's really the most important part of any build.
Thanks John, as a retired senior radar rf type engineer, just trying to get back (stay in the game) the best I can, not totally familiar with audio tube terminology , just really enjoying all of this.
Thanks to all for info and support.
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Ruidoso Bill
Active Member
Just an update, still struggling here with this amp build, I have rewired it and still blowing fuses when hit it with full wall ac. Works fine with a 1khz tone if I bring it up on the variac, all voltages check ok. Last time it blew the fuse I heard what sounded like arcing on the power supply side. Pulled out the PS Transformer, removed the end bells, checked and put heat shrink on all the leads, if you remember I pinched one between the bottom of the transformer and the chassis, actually cut the insulation, I must have damage the transformer under the wrap/insulation, although resistance checks with an ohm meter seems fine.
Nothing else to do other than replace the PS transformer. Ordered one from Edcor yesterday so who knows when that will come in.
I am so far into this amp, I won't give up now.
Oh well, that's it.
Nothing else to do other than replace the PS transformer. Ordered one from Edcor yesterday so who knows when that will come in.
I am so far into this amp, I won't give up now.
Oh well, that's it.
JohnT123
Super Member
It sounds strange to me. If you can bring it up on the variac without problem but not if you simply turn on the power, I have to wonder about simple stuff. The GZ34 will bring the hi voltage up slowly unless you put too much capacitance in the first power supply node, and that could cause it to short. The fuse should be a time delay or slow blow to handle the initial inrush of current. It's a bit of a head scratcher.
Ruidoso Bill
Active Member
Tell me about it, I'm losing sleep over this thing, I just think it's possible I damaged the something in the winding insulation of the ps transformer, so when it sees the 119.6 coming it breaks down? Just a guess but I have built this thing the equivalent of about 3 times now. I have 3 GZ34's on my shelf, eliminated that as a cause several times back. I have replaced the big caps in the ps a couple of times, same with the other caps on the amp side. I am using slowblow 2 amp fuses, also have a thermistor in it. Like I said I could hear something like arcing, only thing close to where I heard it is the transformer. the tone sounds sweet when I bring it up slowly, all the way to max on the variac, if I turn off the power switch and turn it back on say after a minute or so, I actually watch the B+ drop down before hitting the switch again it blows the fuse.
The GZ34 does put out a little hum when up at full ac power, not through the speakers, I can hear it even on dummy load, I asked before if that's normal, hope so.
Ruidoso Bill
Active Member
BTW, Since I am following Mark Walker, Blueglow, please give him a shout, he had a heart attack and mentions it in his Soapbox Sunday March 8, 2019. One class and smart guy!
