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Old 18th April 2018, 10:43 AM   #11
zxx222 is offline zxx222  Europe
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Are you sure you used right HT secondaries of your power transformer?

With something like 200-0-200 (234V unloaded) your B+ is way to low and tubes are now working as they should due to different bias point.

You should use PT with 370-0-370 HT secondaries to have proper B+.
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Old 18th April 2018, 02:16 PM   #12
Tubelab_com is online now Tubelab_com  United States
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Quote:
His meter was showing around 440 volts, I am only getting 234 volts?.....I did get a chance to check B+ voltage today, it is 452 vdc
Your readings are normal. Ty's checkout was done on an older vintage board that did not have D3 and D4. The rectifier tube is connected directly to the transformer, so he gets the same voltage readings. Your board has two additional diodes to help prevent damage to some of today's less than stellar quality rectifier tubes. This will confuse the voltmeter since it's reading half wave AC. Your 452 volt B+ indicates that the rectifier and power transformer are working fine.

Verify that the 5 volt windings go to the rectifier tube (side of board), and the 6.3 volt windings go to the output tubes (back of board). Swapping these will cause distortion.

It's difficult to determine the source of your distortion without some more information. You chose 4 ohm OPT's, are your speakers 4 ohm? Are they wired correctly red to B+, Blue to tube plate? Measure the voltage at the output tube's cathode (connect the meter across the large white resistor next to each tube) That is the best indicator of proper operation in the output tubes.

Post some pictures of your board and the wiring to it so we can look for any obvious errors. DC voltage readings on the 12AT7 pins would help too.

It is possible that your amp is working properly. Try CFB, UL mode, and both. Some speakers need the lower output impedance afforded by CFB or the extra headroom of UL.


Quote:
I did not install D1 or D2 at all, was this a mistake? I also have installed a choke, Edcor CXC100-5H-200mA, could this cause a problem? I did not install R1
No these are correct. I assume that you are using an Edcor power transformer. What transformer are you using.

You have three different threads asking the same questions with bits of information in each one. Please keep them all in this one so I don't have to read 3 different ones to piece together all the clues as to what might be wrong.
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Last edited by Tubelab_com; 18th April 2018 at 02:21 PM.
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Old 18th April 2018, 03:25 PM   #13
mudiwater is offline mudiwater  United States
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I am using the Allied 6K7VG power transformer. I am sure that the 5 volt tap and the 6.3 tap are connected correctly. I have taken the board back out to double / triple check my work, all resistors, capacitors and semiconductors are, I believe installed correctly. I've looked at all solder joints for proper "wetting", cold joints etc.... all looks good to me.

I will get it put back together, take the readings you suggested, take a couple pictures and post my findings.
I do not feel as positive about the OPT hook up, I will pay extra attention to that this time.
I bought 4 0hm OPT's because I am in the process of putting together a 4 ohm MTM speaker, currently running 8 ohm speakers.

George, sorry about the scattered threads, I appreciate everyone's help very much.
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Old 23rd April 2018, 08:58 PM   #14
mudiwater is offline mudiwater  United States
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Ok, I have some more measurements, next I hope to get a few pictures. I will first say that I installed R17 & R27 (560 ohms) thinking of using EL34's, but the amp actually sounds much better with KT88's in place, although the power transformer seems to get a little warm.

12AT7
Pin 1: 222 v
Pin 3: 2.07 v
Pin 4: 6.4 v
Pin 5: 6.4 v
Pin 6: 216 v
Pin 8: 1.98 v
Pins 2,7 and 9 was not sure how or what to check? ( i'm very much a novice)

Measurement across R17: 47.0 v
R27: 44.9 v, I could easily have calculated this wrong, but with a 560 ohm cathode resistor, the above readings are 84 mA and 80 mA of current, correct?

B+ is 446

I have used the Allied 6K7VG transformer, I looked at the tubes and application chart, it appears the 560 ohm cathode resistor could be used on both EL34 and KT88 tubes with B+ at 446 volts, but something tells me this is not optimal for KT88?

I mentioned above that with EL34's my amp does not sound good, worse with the Svetlana some better with JJ's, but the sound really improves when I install the EH KT88's.

I have a few 12AT7 tubes, a couple older RCA and a new JJ, the JJ has a little hum, does't sound great, the RCA seem fine. I'm not sure what the measurements of the 12AT7 should be?

I am currently wired in UL with feedback. Balance of the amp seems to be just a little heavy on the left output.

I appreciate any help or advice very much,
Thanks
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Old 23rd April 2018, 10:45 PM   #15
PDL is online now PDL  United States
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I think the problem lies with the eight ohm speakers on a four ohm tap.

Could you borrow some four ohm speakers to validate this?
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Old 23rd April 2018, 11:10 PM   #16
mudiwater is offline mudiwater  United States
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Thanks, I do have an old set of Boston Acoustic towers that are 4 ohm, they aren't great speakers, but they have the same bass distortion. Like I said, much much better with KT88's, so I think that I'm close, hopefully just overlooking something.

It may just have been a mistake to buy 4 ohm OPT's? I have some Scan Speak drivers and tweeters, my plan was to build MTM speakers with them, 4 ohm
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Old 24th April 2018, 12:52 AM   #17
PDL is online now PDL  United States
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What's the sensitivity of the Boston's?

What's the volume at when the bass breaks up?

What kind of music are you listening to?
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Old 24th April 2018, 03:10 PM   #18
mudiwater is offline mudiwater  United States
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91db, the are a TMM tower


I would say moderate volume, not trying to make my ears bleed or anything


Music, a lot of different taste, Pink Floyd to The Clash, The Band, Bowie, Al Dimeola, Mozart.............I listen to a pretty wide variety, not to much hard rock or country


I will get some pictures posted in the next day or two, maybe someone will see an obvious mistake that I have made?


Thanks to everyone for your help
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Old 24th April 2018, 10:27 PM   #19
cnau is offline cnau  Canada
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Beginner here but I think the KT88 works better than an EL34 into lowish OT primary resistances which suggests maybe you have the screen (UL) rather than plate hooked up for Triode mode?

Using an 8 ohm speaker on a 4 ohm tap would give you lower distortion and less power (higher OT resistance; hope I got that right).
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Old 25th April 2018, 09:25 AM   #20
mudiwater is offline mudiwater  United States
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Thanks, it is currently in UL mode with feedback. I believe it is wired correctly
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