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Old 9th March 2004, 02:55 PM   #1
Fuling is offline Fuling  Sweden
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Default Whatīs wrong with Hammond 1628SE?

Hi.

A friend of mine is working on an SV811-3 amp using the mentioned 5kohm OPTīs. A couple of days ago he fired up the prototype and ran some tests which showed some serious shortcomings in the upper frequency range , -6dB at 20kHz together with a dip at 13kHz.

Further measurements told us that it is the transformer that is the problem, the signal is OK when measured on the plate of the output tube. At first he hardly believed what he saw (two sets of transformers were tested do eliminate the risk of a single manfacturing fault), but to confirm it he made a whole new prototype using entirely different tubes and PSU.
Same thing happened again.

BTW: He also checked the stuff he uses to do the measurements and that equipment is just fine.

What is happening here?
All other Hammond 162(X) SE transformers reach 20k without problems, so why not the 1628??
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Old 9th March 2004, 03:31 PM   #2
EC8010 is offline EC8010  United Kingdom
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Is the cathode adequately (measurably) decoupled at 20kHz? If it weren't, ra would rise, and that could cause the HF loss in the transformer.

Another thought: SE output transformers can change their HF response depending on which way round the primary is connected.

Finally, I'm assuming that the transformer was intended for this application. Transformers have a limited number of octaves of bandwidth, and matching them to the application is a case of sitting that bandwidth symmetrically (in octaves) about 1kHz. It might be worth checking that you don't have an excessively good LF response. If you have the ability to measure distortion, drive the amplifier to full power, lower frequency, and look for a rise in distortion. A well-matched SE transformer is usually good to 25Hz.
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Old 9th March 2004, 04:28 PM   #3
Fuling is offline Fuling  Sweden
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The 811-3 circuit employs fixed bias so there is nothing in the cathode to bypass (ok, 10 ohms or something for current measurement).
Besides, my friend is very competent when it comes to tube amp designs, he has designed and built lots of really good sounding amps. I honestly donīt think that he has made any simple mistakes here. Everything has been double-checked.

The Hammond datasheet specifies what wire that should connect to what and the transformer was wired that way.

SV811-3 is mentioned in Hammonds list of suitable triodes for the 1628SE, so I canīt see anything wrong here. I thought for a while that the transformers were mislabeled, but that doesnīt make sense since everything works fine up to 12-13kHz.
Bass response was 1db down at 20Hz if my memory serves.

The weird thing here is that, as I mentioned, the transformers were tested in a completely different circuit (EL34 SE) and showed exactly the same behaviour.
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Old 9th March 2004, 04:43 PM   #4
dhaen is offline dhaen  Europe
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Quote:
the signal is OK when measured on the plate of the output tube
That can only mean the supply impedance is rising at HF.
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Old 9th March 2004, 04:57 PM   #5
Fuling is offline Fuling  Sweden
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Quote:
That can only mean the supply impedance is rising at HF.
IF he used the same type of PSU filter caps for both prototypes, that figures. Iīll ask him!
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Old 9th March 2004, 05:11 PM   #6
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by dhaen
That can only mean the supply impedance is rising at HF.
Well spotted! I didn't think of that one.
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Old 9th March 2004, 05:17 PM   #7
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Quote:
That can only mean the supply impedance is rising at HF.
Hi John,

Could you explain that? In my language?

Cheers,
Bas

Edit: I mean why is it fine on the plate and messed up after the transformer? Does that mean that the psu does not have a low impedance?

PS. Olimex quoted me 35$ including shipping for 6 of those boards. I am going for 6 of those...to do some debugging..
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Old 9th March 2004, 05:18 PM   #8
Fuling is offline Fuling  Sweden
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Well, thinking of it...
If the signal is correct before the transformer but not after it, doesnīt that indicate that something is wrong with the transformer? But of course, the PSU is in series with the transformer and if the PSU impedance rises...

Still I donīt want to think that Hammond makes a transformer that falls at 13k and specifies it to 20k.
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Old 9th March 2004, 05:19 PM   #9
Fuling is offline Fuling  Sweden
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I forgot to mention something: My friend also tried using 1629SEīs (6,5kohm) instead, and they worked alot better. Still not perfect though, the dip at 13k was still there.
Some kind of resonance at 13k in the PSU, maybe?
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Old 9th March 2004, 06:49 PM   #10
Fuling is offline Fuling  Sweden
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Someone at audioasylum has measured this frequency response:
http://world.std.com/~doyle/845seBode.html

Seems like it is the transformers after all.
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