Wow, this is all really inspiring! Trying to get up the nerve to do a build now that I’ve brought a couple of Scotts back to life, through much help here.
Kegger/blueglow kt88 project
- Thread starter Tubeglowpio
- Start date
Thanks Mark! The mechnical hum for the PT really is not all that bothersome, but I would definitly be interested to hear your thoughts on possible ways to minimize it. Thanks again for putting this project together. I'm partically sad its done as it was so much fun to build.Very well done, looks better than mine I think
Stay tuned, I'll make a post on how to tame that Power Transformer hum...
Thanks! The powder coating is definitly a nice upgrade from what comes stock from hammond. Seems like others have experienced similar mechanical hum. Its a killer sounding amp. If taming it slightly is an easy fix I'll likely do it as some point, but it is so slight I'm not too worried about it either way.
kentuckycat
Active Member
I really love this amp. I have a bunch of other tube amps and this one certainly is my favorite. I've had a couple of small problems with it in the past, though. I'm happy to hear that Mark is going to come up with a fix for the minor PT hum. My biggest problem has been rectifiers arcing. I've gone through a few of them. The line voltage at my house is too high between 122vac and 126vac and I suspect that might be the problem but I don't know for sure. I've received help from several kind people on this thread and I've implemented a couple of their ideas to preserve these expensive tubes. I've added a dropping resistor and an inrush current limiter, which I believe helped. My last Gold Lion GZ34 lasted about 2 years and I'm happy for that. But a couple of weeks ago I switched the Kegger/Blueglow on and Dzzzztt! A little lightning show and poof went seventy bucks after shipping. Yesterday, I decided to add the "yellow sheet" mod which is a couple of diodes added to the rectifier to prevent reverse voltage. We'll see if I can get more than 2 years out of this one.
A small nuisance manifested when I switched the amp on after the rectifier modification; a series of about 5 or 6 speaker pops consistently occurs at startup and again at shutdown. I'm not sure if that it's related to the yellow sheet mod or not. I suspect that the two are related because it happened immediately afterward. My first thought was it's a capacitor discharging but why the pops at the startup? With other speakers the pops wouldn't be too big of a deal, but with 101 db efficient speakers, it's pretty loud. Any ideas? Thanks Jeff
A small nuisance manifested when I switched the amp on after the rectifier modification; a series of about 5 or 6 speaker pops consistently occurs at startup and again at shutdown. I'm not sure if that it's related to the yellow sheet mod or not. I suspect that the two are related because it happened immediately afterward. My first thought was it's a capacitor discharging but why the pops at the startup? With other speakers the pops wouldn't be too big of a deal, but with 101 db efficient speakers, it's pretty loud. Any ideas? Thanks Jeff
You need to replace that 1st solen cap if you built it with one. Even it's not a solen cap, replace the power supply caps. I had an amp using one of them that I very slightly physically damaged by reclamping it too tight when I was working on something else with the amp, and it fried the rectifier tube. Highly likely what you have going on is one of the caps in the power supply is shorting to ground on power up when it has initial voltage on it.
The solution for the buzzing Edcor power transformer is to replace it with a Hammond one
kentuckycat
Active Member
Yep. 1st cap is Solen. I'll replace it. Are Solen problematic where I should replace it with another brand? You think I need to replace the 100u/100u, too?
JohnT123
Super Member
That's a tough call. Both of those caps are about as good as it gets. It is possible that the rectifier blow outs have damaged that first cap, but I'd want to look into the entire amp. Mine has a pair of 5 amp diodes for the rectifiers because of the issues with 5AR4/GZ34 tubes. No soft start like the GZ34 but I do have a CL-80 thermistor installed on the supply line.
I'm not disagreeing with Steph but I think getting your wall voltage sorted is really important. I would imagine you've been seeing higher than normal voltages throughout the amp due to the high supply voltage.
Please keep us in the loop and best of luck getting it sorted. Also, try a plain old Sovtek GZ34. They're quite reliable and come off the same line as all the Russian rectifiers. You'll save some money.
I'm not disagreeing with Steph but I think getting your wall voltage sorted is really important. I would imagine you've been seeing higher than normal voltages throughout the amp due to the high supply voltage.
Please keep us in the loop and best of luck getting it sorted. Also, try a plain old Sovtek GZ34. They're quite reliable and come off the same line as all the Russian rectifiers. You'll save some money.
I have 124V and used a Hammond 200 series with the option to use 115 or 125 (my amp voltages are still on the high side). The Hammond PT started buzzing a little after several months. My new production Tung Sol has been working flawlessly for 2 years, Jim McShane recommended it over the Sovtek I was going to get. No thermistor or any other devices. I have also used some older coke bottle 5U4Gs without problems.
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I'd just try replacing that first cap, I used a 22uf in mine FWIW. Also, a bucking transformer does make sense and might help reduce the buzzing! I just wanted to share what happened to me. A defective cap can randomly take out rectifier tubes.
kentuckycat
Active Member
I did build a buck transformer with a Hammond 116N12B for my old Fisher receiver. It it takes my crazy line voltage down to around 116-117vac. I'll start using that for my KT88 SE, too. But not before I swap out that first cap. As always, I appreciate your advice.
I’ve gone through this thread a couple of times and thinking of going with Hammond over Edcor for the transformers and choke, despite the additional cost. Also would prefer something other than powder coat blue (know I can paint the Edcors but might be more trouble than it’s worth).
For those who used Hammond for everything, any issues and regrets instead of using Edcor? Thanks.
No experience with Edcor, but didn't want the (ugly) blue. Already had black OPTs, so I just used Hammond for the PT and choke, no regrets.
Very happy with the amplifier in my main system.
@reverend13
Forgot about the Edcor lead time, I was able to buy Hammond from Hawk and receive the in stock parts in 2 days.
@nerdorama
Re: zinc plated - that is something I just found out about a few days ago when searching Magnavox console pull rebuilds here on AK.
Forgot about the Edcor lead time, I was able to buy Hammond from Hawk and receive the in stock parts in 2 days.
@nerdorama
Re: zinc plated - that is something I just found out about a few days ago when searching Magnavox console pull rebuilds here on AK.
Thanks guys. I saw the zinc plated bells sold separately on their site so will send an inquiry over to them. I recall someone earlier in the thread may have asked them previously and was told they would only include them on an order of 10 or more? Maybe I misread that but I'll check with Edcor.
They got back to me surprisingly quick. They can send the zinc-plated bells separately but all transformers come with blue bells attached. I asked if it's just a matter of removing the blue ones and replacing them with the zinc-plated ones after they arrive, and they said yes. I thought there was more to it than that but I guess not.
John Ess
Well-Known Member
I know this will probably raise a few hackles around here, but I finished a build of this amp a couple months ago and after reading all the bad experiences with Edcor power trans and new production 5AR4's, I decided to take the less traveled path. I wanted to try and have the fewest issues to deal with and that's exactly the result. I went solid state with a 360 volt toroidal trans and Vishay HexFREDS. A 7 amp variac stays in line, allowing me to ramp up the voltage more gently than any tube rectifier. Power turns on and the initial current surge all takes place at about 150 volts. Several seconds later it goes up to maybe 300-350 and the filaments just start to glow. Keep it there for maybe 15-20 seconds and then slowly turn it up to full B+ of 460 volts. No sparks..no drama...and no buzzing transformers. It sucks when you have to cringe while turning any piece of gear on. And those new 5AR4's...they're barely bigger than a 5Y3GT. So last but certainly not least...sound quality. I'd say no one could tell if they didn't know the rectification was SS. And after pairing with the just finished preamp, it's just plain amazing...best audio I've ever had in about 30 years of this stuff.
I read people being concerned about painting over powder coat, don't be. Scuff them off with some 180-220 grit sandpaper and spray them whatever color spray bomb you want. There is no reason to buy new end bells.