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Old 21st September 2011, 06:06 AM   #61
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Location: Sydney
Hey everyone,
Pete this tube amp looks great and by all reports sounds great too. I just purchased the red board from Pete a few days ago and while I await it's arrival in Australia I have been doing some research etc.
I have a question regarding the OPT, Pete mentioned that if money was not an object he would have went with the ones from Sowter, I am new to tube amps so this may be an easily answered question. What Sowter transformers would you go with, I have built many amps in the past but no tube amps so these are a little new to me ie the output winding values etc. I thought initially the Sowter 8968 This however could be overkill not really sure.
Will a gain a huge benefit from using the higher quality OPT.
Maybe I should just build the tube amp with the Edcor transformers and maybe switch them out later, that's just doubling the cost though?

Any light on this matter would be great, or perhaps recommendations for other higher quality OPT that will gain performance.

I read that another member had issues with the audible hum in the amp, as I am in Australia and our supply line is different will I experience any greater sources of line hum?

Probably all dumb questions for you tube heads but hey I am only just crossing over to the dark side.

I have found these on eBay in Australia and thought they might do the trick

1 x 5K UL PP 4/8/16 ohm 25 W Output Transformer for Valve tube Amp NEW


Thanks guys and gals

Archie

Last edited by Archie1979; 21st September 2011 at 06:33 AM.
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Old 25th September 2011, 08:28 PM   #62
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I finished mine today and connected JP1 on PCB because of Pete's and highflyin's comments. Even with the jumper I have hum with volume all the way down and hum doesn't change with higher/lower volume. All parts within whole amp are the same as required on the BOM with Hammond aluminum chassis.
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Old 28th September 2011, 04:22 PM   #63
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I still get a very light hum in my unit even with the JP1 connected that's unnoticeable when music is playing. It's because the transformers are magnetically coupled, which is surprising given their distance on this design. The only way to get rid of it is to move them even further away on a wider chassis, which would have been too much work in my case. Since you're the third person experiencing the issue while using the Hammond chassis, it may be a good idea for Pete to take down that recommendation on his page.
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Old 28th September 2011, 05:21 PM   #64
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The hum I get is loud enough that I can still hear it in background when music is playing at lower volumes. I'm using 64 ohm HD280's. With my 88db speakers, hum can be heard 1 feet away from woofer(at quiet times). Hum can be a big problem with >95db speakers.

I read somewhere Pete was saying that even he has hum issues. He said even with his high impedance cans, he can detect a tiny bit of hum which he reports is louder with 32 ohm Grado's.

I wonder if there is something out there; some sort of fix/dampening product to dampen the magnetic coupling. I like this amp, it plays about as detailed as my bigger amps.
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Old 28th September 2011, 07:43 PM   #65
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I think I will try DC on the filaments on mine. When I get it back from my step-son.
Al
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Old 30th September 2011, 04:52 AM   #66
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Vey unfortunate day today..got one bad 13EM7; saw sparks inside tube and some smoke out from tube only.

I noticed it quickly and turned off the amp within 20 seconds..do I need to change any resistors and capacitors now? Got new tubes and amp sounds fine but hum has gotten a bit louder from that channel. R10 looks bit weird. It has kinda grey shade allover the green body now. C7's shiny metal part now has a bump and is not flat like it used to be like C8. I don't know if they were like that since assembly. Should I also change other resistors and capacitors as well??
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Old 30th September 2011, 04:03 PM   #67
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Quote:
Originally Posted by imnewbie View Post
Vey unfortunate day today..got one bad 13EM7; saw sparks inside tube and some smoke out from tube only.

I noticed it quickly and turned off the amp within 20 seconds..do I need to change any resistors and capacitors now? Got new tubes and amp sounds fine but hum has gotten a bit louder from that channel. R10 looks bit weird. It has kinda grey shade allover the green body now. C7's shiny metal part now has a bump and is not flat like it used to be like C8. I don't know if they were like that since assembly. Should I also change other resistors and capacitors as well??
Sounds like a plate-to-cathode short in VT2A...

The resistor is probably OK - you can verify with an ohmeter that the resistance has not changed significantly (make sure you wait until it's been powered off for a while). If the cap has a bulge, you should replace it... it may fail (sometimes spectacularly) at any time.

If the hum got louder, that may mean that the cap is leaking a lot of current, and if so it'll get hot and fail sooner or later.

Other parts are probably OK. If there were damage to something else (like the OPT) you'd probably hear it, and if there were damage in the power supply I think you'd be blowing fuses.

Pete
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Old 30th September 2011, 04:14 PM   #68
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I'm not sure what to tell you guys with hum issues. I may not be as sensitive to the hum as others, but on reasonably efficient (~90-92dB) desktop speakers about 2 feet from my head, I hear no hum. Also not noticeable in AKG 701's or HD600s. With Grados I have to be in a quiet room to hear any hum.

I did some experiments when prototyping that showed a lot of magnetic coupling between the power transformer and OPTs. That's why I had Edcor put a flux band on the power transformer. Basically, I moved the transformers apart until I found the hum level acceptable, and that's how wide I made the chassis.

If the hum is constant, with the inputs shorted, and is also present with the volume control all the way down... the issue is most likely magnetic coupling. There is also a little bit of hum from B+ ripple, but it is not as significant (at least in my case).

Assuming that the issue is magnetic coupling (flux leakage from the power transformer), there are a couple of things that might make things worse than what I experience. If the line voltage is high (mine is 120V), the transformer will leak more flux. Same likely is true at 50Hz. The transformer looks a little small for the power output, so I do wonder if Edcor cut it a little tight, and the transformer is close to saturation. That makes it leak a lot of flux.

If any of you have access to a variac, try dropping the line voltage 20% or so, and see if the hum decreases.

Another thing to try is to rig in more B+ filter capacitance (parallel in more capacitance across C9/C10) to see if that makes a difference.

Pete
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Old 30th September 2011, 04:50 PM   #69
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I have a choke in my power supply, will try more capacitance and dropping the voltage when I get it back. Have not tried it on speakers, but will. I am using grados as well.
Al
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Old 12th October 2011, 11:53 PM   #70
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220uf 450V cap Panasonic EET-UQ2W221DA under Mfg PN will not fit FPE Case. It's 30mm.
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