Bottlehead Forum
Bottlehead Kits => Kaiju Stereo 300B amp => Topic started by: Sp33ls on April 15, 2018, 04:01:01 PM
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I made some progress this weekend. I've got the C4S boards built and ready to go in, so when I get the time, I'll get them in and wrap up the build.
But, I had a couple of questions. I made a really silly error and had one of the rectifiers leading from the transformer placed in the wrong direction in the initial power supply test step. When I plugged the cable in, I heard a buzz for about a second, then a faint pop from the fuse giving way. After remedying the situation, I replaced the fuse, but at first with a 1.6A fast blow which didn't stand a chance. 2A fast-blow fuse also blew instantly. But, placing a 3A fast-blow fuse in there held. Voltages checked out fine.
I then continued and measured with the 300B tubes in place. When I turned the amp on, I heard another very slight pop, but the fuse didn't give way. The tubes were glowing, and I heard a very slight hum sound coming from the amp itself. I thought it was odd, so I unplugged it, and check everything again. I couldn't find anything, and when I plugged it back in for the tests, I didn't hear the pop or buzz this time. The measurements were within spec. What are your thoughts on why that buzz showed up but disappeared after powering off -- and do you think the transformer will be okay?
And, will there be any noticeable effects of one side (10U vs 15U & 53U vs 65U) having a slightly higher VDC measurement? I believe 53U was about 75.5-76V and 65U was about 77V. I did notice that the 300B tubes had varying measurements: [74mA/6.0mA/V] vs [72mA/5.8mA/V]
Of course, the driver testing and actual listening will grant more insight, but I'm wondering if I should be paying extra close attention to something before arriving at that point.
Edit: @PB, would you recommend I disconnect the UF4007's from the transformer and perform the transformer test just to be certain? Or do you think that things should be okay given that I've passed high voltage power test?
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If your DC voltages are OK, the transformer is OK. A little noise from the transformer may just mean that you should tighten it down again.
The different cathode voltages on the 300Bs is indicative of slight differences from tube to tube and potentially resistor tolerance. You have one tube drawing 76mA and one tube drawing 77mA, there isn't anything wrong with this.
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Funny enough, I noticed after sending this post that the transformer was a tad loose. So, maybe that was the culprit after all.
Good to hear. I figured this might be the case, but I like to hear the reassurance. After completing the build, I'll post back the results.
Thanks, PB!
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So, I finished up the build, and it seemed like things turned out just fine. I began to slowly turn up the preamp and that's when I noticed that the left channel was significantly more quiet than the right channel. I remembered that the hum level was higher in the right channel (8.2VAC) vs the left channel (2.2VAC). Then, after a few minutes of moderate-high volume, I noticed that the amp began to distort more and more and the output began to lower gradually.
I am not testing with sensitive speakers at the moment (88 dB), so I don't expect much in terms of sonics and output until I get my full-range drivers hooked up (97 dB).
I performed another voltage check:
10U: 249V
14U (IA): 463V
15U: 225V
OA: 299.1V
57U -- 58U: 123.5VAC
The resistance check on the ground buss was all less than 1 Ohm. I did notice something interesting with the capacitor when measuring 18U. It started below 1 Ohm, then shot up to something like 400, and then came back down. I measured it a minute later and it remained at 0.6 Ohm.
The power transformer hum is back (& also present with no line or speaker connections). I noticed that it's more audible with the 300B tubes in place, but still slightly present with just the driver tubes.
I'll go through tomorrow and check for cold joints.
In the meantime, any other measurements you think I should be making?
The 300B's have a nice, blue glow to them.
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What DC voltage do you get at 18U? Is the jumper between 18U and 20U touching 19?
What voltage do you get on either side of the giant 1K resistor on the offending channel?
Also test the AC voltage between pins 1 and 4 on each 4 pin socket.
This sounds a lot like a solder joint that isn't all the way soldered or maybe not soldered at all.
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Did you mean the jumper from 19 to 23? It looks fine. Same with the lead from the cap from 15U to 17L.
18U: 0VDC, should it not be 0 since it's connected to ground? The lead from the prior cap at 15U is at 227 VDC.
The 1k resistor seems to be around 70VDC.
One of the 4 pin sockets is 5.22VAC, and the other is 5.33VAC.
Would testing the ground/chassis against the chassis of the pre-amp for AC voltage show anything? When I just did that it showed 87VAC.
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sorry, I meant testing the voltage at terminal 19 or 23, just to be sure it's not 0.
70V across the 1K resistor means the 300B is conducting.
The filament voltages you have look great.
What preamp are you using? Tell me about the age of the wiring in your house?
Seeing 87mV might not be abnormal, seeing 87V would be disturbing.
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Yeah, it's about 75VDC across the 1K on both sides.
0.019VDC at 19U & 23U. Does that seem concerning?
Right now I'm using a re-built Marantz 2220B as the pre-amp -- until I get my hands on a BeePree ;) I just hooked up one lead to the ground lug, and one end into the ground port of an outlet and got 0VAC. I think there might be a ground issue with the old Marantz since you have to wire the ground yourself on the back of the amp.
Edit: That being said... would measuring any difference between the ground lug and the ground port of the same outlet really tell us much? Is there a better way to test the ground?
I live in a high-rise that was built 3 years ago, so should be relatively new.
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I'm getting my terminals a little mixed up. 17 and 15 should have about half the B+. 19 and 23 should indeed be about 0V.
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Changing the tubes around does not seem to effect the left channel.
I torqued the transformer down. Still making the buzz :( My Crackatwoa has never had this problem so I can't imagine it's my home's wiring.
Any measurements I can make on the transformers?
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I'd figure out the left channel issue first before worrying too much about the transformer.
What I would do next is to use an 1/8 to RCA cable and just check the Kaiju with your phone as a source to be sure that it isn't source related.
Beyond that, you'll want to play a 60Hz tone into your Kaiju (you can download a tone to your phone or a signal generator app), then we can measure AC voltages along the signal path of the amp to see where things are dropping out.
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Okay, so using a 60Hz signal at Max volume from my phone, I see 1.88VAC at the RCA jacks and the trim pot leads.
Measuring the output terminals (6&3) of the chokes, I noticed that I had miswired the second choke... I guess I became a little too anxious at the end of the project lol.
So, now I see 11.94VAC on the left and 11.52VAC on the right.
Is this indicative of a solid output level and disparity between the two channels? I realize this is what the trim pots are for, just looking for advice on performance.
And about that damn transformer talk... 😂
I'm wondering if installing the DC filament might take care of it..?
Thanks a lot, PB!
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Go ahead and have a listen to the amp. Tell me about how the choke was miswired...
The transformer screws can be as tight as you can get them with a handheld screwdriver.
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OT, not choke.
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Sorry, yes, I meant to say OT miswired. Output is at 8 ohms. I had the output wired at terminal 6 and directly below it (terminal 1?) instead of terminal 3.
Just to ensure I'm using the correct terminology... Is the choke the inductors on top of the OT?
It sounds pretty good, definitely much louder than last night. I just wish I had access to more than my current pair of B&W bookshelf speakers to really give it a critical listen. But, from the 5 mins I had with it before having to run to an appointment it sounds great. I'll have to give an update on impressions in a bit.
Thanks again.
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The output transformers are on the bottom side of the chassis with solder terminals and the plate chokes are up top with the blue, red, and black wires coming out.
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The output transformer is the one with sticker that says "OT-5."
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So, I installed the DC filament kit. Voltages checked out okay afterwards. 0.0mVAC background from the posts. For some reason, I could only get the noise down to 8mVAC each channel prior to the kit.
I was wondering if experiencing "pops" and "crackles" from the speakers when turning the Kaiju on/off is normal?
I cranked the transformers down, but still getting the buzz noise from the amp :-\
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Yes, there's a little bit of noise when you turn the amp on and off.
Depending on your meter, it may not resolve low AC voltages all that well.
Another possibility for a buzzing transformer is DC pollution on the powerline. Do you have any big motors running in the house? Some dimmers can also pollute the line badly enough to disturb a power transformer.
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I use a Fluke 117, so it should be pretty solid.
I tested my the outlet next to where the Kaiju is plugged in... the DC voltage fluctates between about 0.014 to 0.021VDC. Would this be high for the transformer?
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The easiest way to know would be to just pop it into your car and plug it in at your workplace to listen to PT noise.
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Still noisy outside the apartment :/ But, I just thought of something...
The manual points out that you should scrape away paint around the screw holes of certain areas. I scraped away the paint from just about everything on the chassis. But, I did not do that for the end bell.
I figured the end bell was just a protective / decorative piece and didn't need to really be grounded at all.
Do you think this could be a problem?
Thanks again.
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It's a safety problem, at least. And a potential hum problem, though unlikely to be a cause of the mechanical hum you are observing.
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Okay. That being said, I did not paint the underside of the bell. When I get back from traveling for work next week, I can try scraping away the paint around the screws on the end bell and see if that makes a difference. Hopefully that'll do it.
How often do noisy transformers pop up for folks?