Kegger/blueglow kt88 project

[JohnT123]

I like the look of those and they do impress folks that stop by for a visit. I have a dedicated Techics meter bridge and it can be switched to monitor a number things. My built in meters will be only measuring average power output. What are you setting these up to monitor?

 

[Ruidoso Bill]

View attachment 1457315
These are my VU meters that I'll try to use. Please ignore my ugly buffer boards, I managed to etch them mirrored so had to put all components in reverse and the OpAmp on the backside
At least they works, so that's fine for now.
The meters themselves fit in a 34mm round hole, they are pretty tiny. Also, they are back-lit,so I think they look nice.

Nice, those are less than 1 1/2 inches, my ECC85 socket is about 2 inches from the front of the chassis, Black on a black chassis would look impressive!

 

[Tubeglowpio]

View attachment 1457315
These are my VU meters that I'll try to use. Please ignore my ugly buffer boards, I managed to etch them mirrored so had to put all components in reverse and the OpAmp on the backside
At least they works, so that's fine for now.
The meters themselves fit in a 34mm round hole, they are pretty tiny. Also, they are back-lit,so I think they look nice.

I have some of those same meters, I've used them on a project in the past and thought they looked pretty nice going to the music. Do you have a schematic of the driver board you used. I used one of those china boards with the ic but would like to build one without the uncommon ic.

 

[Tubeglowpio]

Update that after my last post awhile ago on this thread, I'm holding off on buying the parts for the dual monoblock kt-120 version of this amp. My dad passed away last month so I had a lot going on and figured out it's not the best time to drop the money on the parts so I checked around to see what parts I had in stock and decided to build a 2A3 amp for now as I only need to order one more transformer for that project and a couple of parts and tubes. I hope everyones kt-88 amps are sounding good. I'm hoping to build one maybe around Christmas.

 

[tubetime]

Ran some tests on my amp. Hopefully pics are big enough to get info out of.

 

[Ruidoso Bill]

Update that after my last post awhile ago on this thread, I'm holding off on buying the parts for the dual monoblock kt-120 version of this amp. My dad passed away last month so I had a lot going on and figured out it's not the best time to drop the money on the parts so I checked around to see what parts I had in stock and decided to build a 2A3 amp for now as I only need to order one more transformer for that project and a couple of parts and tubes. I hope everyones kt-88 amps are sounding good. I'm hoping to build one maybe around Christmas.

Sorry about your loss, that can be tough. I think amp building can be a great thing to help you deal with things, maybe a good distraction. BTW, found those meters on Amazon, they actually have several, I am looking at honrizontal versus round.

 

[Heffa]

I have some of those same meters, I've used them on a project in the past and thought they looked pretty nice going to the music. Do you have a schematic of the driver board you used. I used one of those china boards with the ic but would like to build one without the uncommon ic.

I used this one, found in the instructions of a VU kit from diy-tubes.com.
The schematic itself is a variant of several others I found when searching online, so not sure where it originates from.
Anyway, this one is available freely in the instructions to diy-tubes.com's kit, so I hope it's ok for me to upload it here.

 

[Tubeglowpio]

View attachment 1459825
I used this one, found in the instructions of a VU kit from diy-tubes.com.
The schematic itself is a variant of several others I found when searching online, so not sure where it originates from.
Anyway, this one is available freely in the instructions to diy-tubes.com's kit, so I hope it's ok for me to upload it here.

nice, thank you. I was thinking of building one using a couple of opamps so I'll have to give this one a try.

 

[Tubeglowpio]

Sorry about your loss, that can be tough. I think amp building can be a great thing to help you deal with things, maybe a good distraction. BTW, found those meters on Amazon, they actually have several, I am looking at honrizontal versus round.

Thank you, yeah I was used to giving him a call everyday so not being able to talk to him has been hard and just weird, I am just really glad I got to see him the day before he passed. I'll never forget how hard it was to say bye while walking out of the hospice room slowly.

 

[justmikey]

Update that after my last post awhile ago on this thread, I'm holding off on buying the parts for the dual monoblock kt-120 version of this amp. My dad passed away last month so I had a lot going on and figured out it's not the best time to drop the money on the parts so I checked around to see what parts I had in stock and decided to build a 2A3 amp for now as I only need to order one more transformer for that project and a couple of parts and tubes. I hope everyones kt-88 amps are sounding good. I'm hoping to build one maybe around Christmas.

Sorry about your loss. Been there, it's hard but time will help.

I too was considering building a KT-120 version of this amp. I built the KT-88 version about 7 or 8 years ago, to what was essentially the original schematic before all the tweaks found on Audiokarma. I modded the power supply somewhat to use motor run caps instead of electrolytics, and a Triode Dynaco replacement PA060 transformer intended for ST-70 upgrades. Played it for a while then put it in my collection and move on to the next project.

So after seeing the Blueglow videos and the updated schematics, my interest is renewed. But since the original build, there's been lots of talk about KT-120's and I'm wanting to try them. If I upgrade my existing amp to the latest Blueglow/Kegger schematic, I may just include whatever mods are needed for KT-120's. I'm thinking this means increasing the cathode bias current (lower R) to use the increased power capability of the 120 vs the 88. I am also not opposed to raising B+ but that can only go so far unless I change from tube to ss diode rectification.

So what do my fellow tube geeks think is reasonable to revise this schematic to a KT-120? Has anyone tried it yet? I know people have simply plugged KT-120's into amps designed for KT-88's, but I'm looking to actually plan on it being a KT-120 amp.

 

[Tubeglowpio]

Sorry about your loss. Been there, it's hard but time will help.

I too was considering building a KT-120 version of this amp. I built the KT-88 version about 7 or 8 years ago, to what was essentially the original schematic before all the tweaks found on Audiokarma. I modded the power supply somewhat to use motor run caps instead of electrolytics, and a Triode Dynaco replacement PA060 transformer intended for ST-70 upgrades. Played it for a while then put it in my collection and move on to the next project.

So after seeing the Blueglow videos and the updated schematics, my interest is renewed. But since the original build, there's been lots of talk about KT-120's and I'm wanting to try them. If I upgrade my existing amp to the latest Blueglow/Kegger schematic, I may just include whatever mods are needed for KT-120's. I'm thinking this means increasing the cathode bias current (lower R) to use the increased power capability of the 120 vs the 88. I am also not opposed to raising B+ but that can only go so far unless I change from tube to ss diode rectification.

So what do my fellow tube geeks think is reasonable to revise this schematic to a KT-120? Has anyone tried it yet? I know people have simply plugged KT-120's into amps designed for KT-88's, but I'm looking to actually plan on it being a KT-120 amp.

Well I was planning on mono blocks so that way I could switch between different rectifiers to drop or raise voltage without having to worry about going over the current ratings of the rectifier. I was also planning on having a switch for different cathode resistor choices because I wanted to still be able to run el 34s as well. Since they were going to be monoblocks even with the kt-120 the current draw was going to be a bit less from each power transformer than 2 kt-88s so the voltages were also going to be slightly higher. I think you would just have to tweak the driver tubes plate resistor to get that back in a healthy range. I want to go all out on this one so I wanted the room for basically all motor run caps. I'm basically going to do the something similar with the 2A3 amp I'm about to start working on once schools out.

 

[Ruidoso Bill]

Finally got my amp on dummy load, looked at my ST120 everything looked fine, clean single waveform both channels, same exact setup, the KT88 amp looks like this



Any suggestions? That's a 1 khz input.

 

[Tubeglowpio]

Finally got my amp on dummy load, looked at my ST120 everything looked fine, clean single waveform both channels, same exact setup, the KT88 amp looks like this

View attachment 1461221

Any suggestions? That's a 1 khz input.

Looks like some sort of out of phase feedback. Did you follow Blueglow's schematic?

 

[Tubeglowpio]

How does it sound? if you have howling try to reverse output transformer leads.

 

[Ruidoso Bill]

Looks like some sort of out of phase feedback. Did you follow Blueglow's schematic?

Yep, Blueglow schematic, amp sounds very good, clean, quiet, was a little surprised to see this. No howling. Same echo with square wave.

 

[Tubeglowpio]

Finally got my amp on dummy load, looked at my ST120 everything looked fine, clean single waveform both channels, same exact setup, the KT88 amp looks like this

View attachment 1461221

Any suggestions? That's a 1 khz input.

Yep, Blueglow schematic, amp sounds very good, clean, quiet, was a little surprised to see this. No howling.

Yeah I just looked at the schematic and don't think it would be that as feedback isn't taken from secondary of output, still think something might be up with the feedback but if it sounds good and you are getting the correct power out then not sure why you have a second sine wave on the same channel that's out of phase but the same amplitude.

 

[Ruidoso Bill]

Yeah I just looked at the schematic and don't think it would be that as feedback isn't taken from secondary of output, still think something might be up with the feedback but if it sounds good and you are getting the correct power out then not sure why you have a second sine wave on the same channel that's out of phase but the same amplitude.

Thanks for taking a look, I'll look more at it tomorrow, just put me at a loss.

 

[JohnT123]

IS there a way to determine if the strange display is an issue with the amp or if it's something with your test equipment?

 

[Tubeglowpio]

IS there a way to determine if the strange display is an issue with the amp or if it's something with your test equipment?

I was wondering the same but he mentioned his other amp looked normal on the scope. Maybe try running the signal generator straight into the scope to rule that out at least.

 

[Ruidoso Bill]

Clear eyes this morning, checked my setup and all is fine, don't know what I was doing last night, signal looks very clean and starts clipping at about 28v Peak to Peak, square wave looks good also. I do notice the output on the left channel is slightly lower on the scope, didn't notice that during listening.




5v per division