| Michael Chaffee |
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Reply with quote | #1 |
Hi -
I just finished building a stereo T16 several days ago. It's been giving me a little bit of grief and I'd be grateful for any help Sorry this message is long, but I don't know what I'm doing and want to include any details that might be relevant...
So initially I built the amp and everything seemed decent - great kit and excellent directions, by the way! Anyway, fired it up and although it sounded pretty fantastic, the output tubes were flashing at about 2Hz, out of phase with one another. I probably put four hours on the thing like that, and finally looked on this board and found someone else had had the same problem, and had fixed it by correcting a bad ground. Turns out I missed the ground wire on the "A" output board, so I corrected that.
Now, I can't see any more flashing in the tubes. But here's the problem: It generally sounds fantastic, but certain passages (generally human voices) cause the sound to crackle somewhat. The crackle is not unlike what I've experienced with bad tubes in a preamp. I'm reluctant to blame tubes, though, because it happens on both channels and is unchanged with different 12AX7's (only driver tube type I have spares of). The bias is correct and stable, and the bias fuses are all intact. The crackle is absent or too quiet to hear shortly after firing it up, and gets worse over maybe a half-hour or so. The crackle started before I corrected the ground wire.
Possibly related, transformer T1 physically vibrates somewhat. You can't hear it but you can feel it when putting a hand on the tranny. Additionally, T1 gets pretty hot, and T2 gets VERY hot - almost too hot to touch. I've checked inside the thing, and all the voltages marked on the board (-170, 630V, etc) are all about 1-4% high, except for the filament lines, which are all showing about 12.9VAC.
Anyone have any thoughts on possible culprits, or what to check next? For the record, it behaves the same with a GG, an old Dynaco preamp, or fed straight from either of two sources. It's driving QUAD ESL-57's.
Thanks in advance for any assistance.
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| Bob |
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Reply with quote | #2 |
The T 16 circuit boards are very confusing in that a lot of the pads are mislabeled and/or not used. For example, my first T 16 failed because I wired the B+ wire to a pad labeled "B+" which was incorrect.
Your transformers should not be extremely hot. Go through the instructions step by step and check all your work.
We all make mistakes from time to time. |
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| Michael Chaffee |
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Reply with quote | #3 |
Thanks for the info. I'm glad you mentioned that the transformers aren't supposed to get super-hot. Now I know for sure I'm not imagining things... But what's next? All the voltage-tagged pads on the boards (+170, etc) are showing the proper voltages. I've re-read the directions and re-traced every single wire in there, and they're all connected as instructed. I've pulled the boards and inspected them for scorch marks, toasted components, bad solder joints, etc. top and bottom - they look great. I've double-checked the installed polarity of all the diodes, electrolytics, etc.
What comes next, by way of diagnostics? To recap, transformer T1 vibrates a good amount at 60Hz, T1 is very hot and T2 is too hot to touch (T3 is pleasantly warm but not hot), and there's a crackle in both channels that only happens at certain frequencies in the human-voice range. Sound is otherwise magnificent. Bias adjusts to zero easily and stays there.
Again thanks in advance for any input.
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| Bruce Rozenblit |
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Reply with quote | #4 |
There is no problem with the transformers. The mounting plate for the filament transformer get too hot to touch.
The crackle bothers me though. Maybe the amp is clipping because your speakers present a very low impedance at certain frequencies.
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| Michael Chaffee |
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Reply with quote | #5 |
That impedance hypothesis is looking more and more likely.
Did a few more tests... When I threw a tone generator onto the amp, at higher volumes the intensity of the sound would pulsate pretty significantly at maybe 2-4Hz, independent of frequency. I assume this was due to some instability.
So I borrowed a pair of regular speakers - Dynaco A25 - and the amp behaved perfectly, playing both music and pure tones without any difficulty.
Then, for a laugh, I threw 10-ohm resistors in series with the QUADs. This completely ruined the frequency response, of course, but it got rid of both the pulsation from pure tones and the crackle in music listening. So I can't help suspecting that the QUADs' impedance is indeed too low for the T16 to drive stably.
I don't suppose there's some quick fix or small tweak to address this...?
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| Michael Chaffee |
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Reply with quote | #6 |
I should add, by the way, that I could not replicate the crackle using a pure tone. Just the pulsating; otherwise it seemed fine.
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| Michael Chaffee |
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Reply with quote | #7 |
I want to post a quick update, just so this thread is more-or-less complete. Yesterday I had the opportunity to take my T16 to the home of another QUAD owner, who wanted to audition the amp before possibly purchasing. His QUADs had recently been refurbished by an expert.
To my surprise, all the crackle and clip problems I had experienced with my system were not there at all! The amp sounded positively magnificent on his speakers - far better, to my ear, than the QUAD II amplifiers he had been using. It was good enough that he's seriously considering a T16 for his own system now. And of course now I get to start planning a refurb of my own speakers, ugh.
So to sum up, the stereo T16 is a really fantastic match for the QUAD ESL-57 - IF your QUADs are in good shape. Thanks Bruce!
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| Nikhil |
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Reply with quote | #8 |
I thought I should drop a quick note. Gosh, its been a while since Michael came over, but I would like thank Michael for taking the time to bring his amp. What stood out with the OTL is the bass that I had not experienced with other "OT" based tube amps with the Quad 57s. Hat off to Bruce's design ! I finally placed my order for the T16 stereo kit just yesterday. Can't wait until I get my hands on the kit ! |
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| Ed |
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Reply with quote | #9 |
Hi Michael,
I'm getting ready to build a T16 (I think) and saw this thread.
I'm glad you got to the end of it......sure did sound to me like you had a speaker problem.....I went crazy once in a similar situation. Turned out to be the drivers were slightly damaged...not enough to crackle all the time but just enough to make me think my amp was having problems.
Glad to hear you were able to hook it to another pair of speakers like you own.
How old are yours and are you going to have them rebuilt?
Is the amp still working OK?
Ed
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| Alex |
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Reply with quote | #10 |
I had the valves flashing when using my Quad ELS-57s, I still do if I do not turn the pre-amp on 20 seconds before the T16. I always thought it was my output valves so maybe I need to get a second T16 and enjoy the delights of 40 full blooded watts RMS to get rid of the problem, I am sure that is what Bruce would recommend, or he might try and persuade me to get a pair of beasts, shame the pound sterling has now gone down the pan thank to Gordon Brown.
I sure if it had not I would have talked myself into buying some more of Bruce's Kit including the phono amp.
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| Gary Kemp |
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Reply with quote | #11 |
Ah, but the pound is now $1.63; true, that is after a high six years ago of $1.90 (which made american products absurdly cheap), but also after a low of $1.20 in around 1989 (the obvious political point will be unmentioned). The pound vs the dollar is now at a normal, natural level, even though the surrounding conditions are anything but.
I don't know why I am commenting on this; I should get to work!
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