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Thread: How to test output transformer performance for overperforming amp

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    How to test output transformer performance for overperforming amp

    Hmmm, not sure what information can be gleaned from this scenario but I built this amp and it uses the Hammond 278CX PT which gives me about 590V B+ with four power tubes. My OT is Hammond 1650T which is listed as 120W output tranformer.

    My question is, this amp with 590V at the plates does about 150W at clipping. So by the numbers it is exceeding what the OT is made for, however it doesn't get hot or anything. It seems to withstand it without a problem. I don't think it will fail, but my question is, is there a way to determine how good its performance is other than just swapping OTs to see if there's a significant sound difference.

    I ask because I am considering buying a Heyboer 150W OT with the same 1900 ohm primary to see if the amp will sound better, fuller, better bass response, etc, when it is played at high volume.

    The amp really will ONLY be played at high volume because that's what it's made for. The amp seems to get maxed out at about 6 on master volume (like most amps usually seem to do). it gets kind of grainy and trebley sounding and adjusting tone controls starts to have less effect. Maybe the amp is must maxed in general, or maybe the speakers are maxed and since they're putting out full volume they can't really control the sound as much anymore. Maybe we are reaching power amp distortion and negative feedback is feeding a distorted signal back to the PI, unlinearizing the amp in general. I dont' know. But I wonder about the OT. Any thoughts other than just swap the OT and see?

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by nsubulysses View Post
    ...My question is, this amp with 590V at the plates does about 150W at clipping. So by the numbers it is exceeding what the OT is made for, however it doesn't get hot or anything. It seems to withstand it without a problem. I don't think it will fail, but my question is, is there a way to determine how good its performance is other than just swapping OTs to see if there's a significant sound difference...
    One test that you can do is to measure the full power bandwidth of the amp. Or just measure the maximum clean output at various frequencies. Your Hammond 1650T OT is rated at 120W, 30Hz to 30kHz. The 120W is not an absolute limit at all frequencies. If it is able to put out "150 W at clipping" as you stated, that is not necessarily exceeding its rating. We also don't know how much clipping you are allowing and that makes a significant difference in your measured result. I assume that your test frequency is the typical 500 Hz to 1 kHz that many people use. If you lower the test frequency I'll bet that you will find that the max clean power drops to 120W or less as you approach 30 Hz. You would need to make sure that the dive signal remains the same because, if there is somewhere else in the signal chain that attenuates the amplitude, that will obscure your test of the OT. From your description it seems that the amp is working pretty well and I would not put the OT at the top of the list of culprits that could be causing the sound characteristic that you don't like.

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    Last edited by Tom Phillips; 02-26-2017 at 02:38 AM.

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    What Tom said,
    The 1650T is regularly used by DIY guys for 150W Bass Amps and 200W Guitar Amps.
    When you are NOT trying to reproduce 30Hz then the power handling goes up. (The Power Handling is mostly in the Low Bass Frequencies)

    For guidance: On a HiFi Monoblock with 1650T and a quad of KT88 (B+ at 510V) I found that:
    at 18Hz core saturation commenced at 50 Watts
    at 30Hz it would core saturate at its rated 120 Watts
    To me that says that at 60Hz (and above) it could handle >200Watts with ease.

    Fundamental of low E on your guitar is 81Hz, on a 4 string bass then low E is 40.5Hz but Bass Amps often have significant cut at these low frequencies (-6dB at 40 Hz is very typical).

    Cheers,
    Ian

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    Last edited by Gingertube; 03-07-2017 at 03:25 AM.

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    Sorry I fell off from this thread. I had to do a bunch of repairs and of course could not work on/test my own amp

    Anyway, I did this test incorrectly and have not had time to do it again yet. I could get basically full power at all frequencies but I have to boost the input signal a lot. I guess the true way to do this test is to test all frequencies with the same standardized amplitude input signal, maybe like 100mV or something. And that I should input this signal at the PI rather than the input of the amp to test the frequency response of the OT/output section rather than the preamp and the rest of the amp. Is this correct?

    I tune my guitar to A standard so almost an octave down

    Also from Gingertube saying -6dB at 40 Hz is typical, it seems like getting a 150W spec'd OT would not help my low frequency response at high volume?

    I will do the test with standardized amplitude input signal at different frequencies later today and post my results. thanks !

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    If you can get full power at all frequencies, then you don't need to worry about the OT. You need to tailor the frequency response of the rest of the amp.
    Unless I am misunderstanding the issue?

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    I tested it two ways -- signal at input of preamp and signal at PI

    1V pk-pk signal at input with master volume on 10 and gain volume set at point of clipping (gain volume on about 2) with 1K sine wave as my bench point. Without adjusting any amp volume settings and making sure 1V pk-pk was accurate through the frequency settings I got

    4 ohm load

    1KHz -- 24.5V -- 150W
    900 -- 23V --- 132.5W
    800 -- 19.5V -- -95W
    700 -- 16.6V -- 68.9W
    600 -- 14.9V -- 55.5W
    500 -- 12.9V -- 41.6W
    400 -- 11.2V -- 31.4W
    300 -- 10.2V -- 26W
    200 -- 8V -- 16W
    100 -- 4.24V -- .25
    80 -- 3.18V
    60 -- 2.25V
    40 -- 1.26V
    20 -- .46V


    With signal input at PI I needed 8V pk-pk to bring the amp to clipping with 1K sine wave. 8V pk-pk pretty much did the same for all the other frequencies too

    it sums up as this

    1K - 200hz -- 24 - 24.5V -- 144 - 150W
    100hz -- 23.9V -- 142.8W
    80 --- 23.9V -- 142.8W
    60 --- 23.7V -- 140.4W
    40 ---- 22.5V -- 126W
    20 --- 19.3V -- 93W

    Hmmmmmmmmm

    If I add more bass in the preamp the sounds is not so in your face. Maybe it is what it is. and the speakers can't take that volume and still do the full frequency range

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    Last edited by nsubulysses; 03-08-2017 at 12:03 AM.

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    Something strange happens though. When the signal is applied at the input jack the signal at the input (purple) will start to distort when the amp reaches clipping at output (yellow). All the way up in volume until the output section begins to clip, the signal at the input jack is a perfect sine wave

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    Last edited by nsubulysses; 03-08-2017 at 05:23 AM.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by nsubulysses View Post
    ...I guess the true way to do this test is to test all frequencies with the same standardized amplitude input signal, maybe like 100mV or something. And that I should input this signal at the PI rather than the input of the amp to test the frequency response of the OT/output section rather than the preamp and the rest of the amp. Is this correct?...
    Yes. An also verify that your PI to PA coupling caps are large enough capacitance that they are not rolling off the frequency response. You could do that by monitoring the AC amplitude feeding the PA grids.

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    Quote Originally Posted by nsubulysses View Post
    ......Maybe it is what it is. and the speakers can't take that volume and still do the full frequency range
    Good point. What are you using for speakers and what's the intention here. For example and some extremes, an open back guitar cabinet isn't going to put out the low end that a ported sub cabinet is. Maybe (and speculating) you need to explore a speaker setup and/or cabinet that is more efficient in the low end.

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    It is a sealed 4x12 with internal baffle that divides it into two 2x12s. I have 2 weber grey wolves and two Celestion G12K 85s. The Celestions are definitely the more trebley, middy ones and are probably also a little less loud than the Webers.

    If I replace them with two Eminence EM12s (EVM-12L inspired I guess) I bet it would be much better. They are supposedly very clean and full and loud.

    I upped some of the capacitors that parallel the grid resistors in the preamp in the mean time and it's a bit better. like from 470pf to 1000pf or more. At least I know the OT is not the choke point

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    Supporting Member The Dude's Avatar
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    If you could find some EV Force 12's, those would be a good choice, also. And, about 1/2 the weight. Well, half the weight of the EVM-12L anyway. I guess only slightly lighter than the Eminence.

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    You're not going to find a speaker that will reproduce much below 50 Hz. Subwoofers tend to be high-passed around 40 Hz, even. An average ported bass cab is down 3dB by 50-60Hz. With a sealed bass cab that's closer to the 80-100Hz range.

    The bigger issue here is that your preamp is cutting a ton of bass. Your test where you injected signal right at the PI shows that the PI, power tubes, and OT are doing a fine job for a guitar or even bass amp. Heck, even a Hi-Fi amp... so the loss of clean bass - or early clipping of the bass - is happening before the PI.

    I looked in the other thread and I don't know which preamp schematic you went with, but you might want to either try increasing the bass early in the preamp or cutting more treble late in the preamp.

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    Quote Originally Posted by nsubulysses View Post
    I tested it two ways -- signal at input of preamp and signal at PI

    1V pk-pk signal at input with master volume on 10 and gain volume set at point of clipping (gain volume on about 2) with 1K sine wave as my bench point. Without adjusting any amp volume settings and making sure 1V pk-pk was accurate through the frequency settings I got

    4 ohm load

    1KHz -- 24.5V -- 150W
    900 -- 23V --- 132.5W
    800 -- 19.5V -- -95W
    700 -- 16.6V -- 68.9W
    600 -- 14.9V -- 55.5W
    500 -- 12.9V -- 41.6W
    400 -- 11.2V -- 31.4W
    300 -- 10.2V -- 26W
    200 -- 8V -- 16W
    100 -- 4.24V -- .25
    80 -- 3.18V
    60 -- 2.25V
    40 -- 1.26V
    20 -- .46V
    You are NOT testing the OT response but the full amp, which to boot being a guitar one has huge bass cut and top boost.

    With signal input at PI I needed 8V pk-pk to bring the amp to clipping with 1K sine wave. 8V pk-pk pretty much did the same for all the other frequencies too

    it sums up as this

    1K - 200hz -- 24 - 24.5V -- 144 - 150W
    100hz -- 23.9V -- 142.8W
    80 --- 23.9V -- 142.8W
    60 --- 23.7V -- 140.4W
    40 ---- 22.5V -- 126W
    20 --- 19.3V -- 93W

    Hmmmmmmmmm
    Now we are closer, but even here you have coupling cap (amp input and power tube grids) time constants, a fancy name for bass cut, that are in the path.

    If you want to measure OT response in a Lab, you´ll probably drive it with an SS amp, simply because these can easily supply a flat frequency response, are constant voltage output, very low distortion, etc.

    If you want a more realistic frequency response measurement, then drive it with tubes but use 10X the standard coupling cap values, simply to put them out of the picture; then you are certain that any variation comes from the transformer itself.
    Tubes, of course, are flat from DC to several MHz or at least, worst case, tens of kHz..

    If I add more bass in the preamp the sounds is not so in your face. Maybe it is what it is. and the speakers can't take that volume and still do the full frequency range

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    I'm still recuperating from some plumbing repairs so if I sound foggy, take it with a grain of salt and tell me I'm drooling.

    The limitation on power transfer ability in a transformer is ultimately either thermal (something melts in the fervent heat) or impedance (it can't let enough power into the M-field gyrations to get it into the secondary.)

    The melting we understand. The impedance issues are more subtle. At high frequencies, the amount of power the transformer will let in is limited by the series leakage inductances. At low frequencies. the primary inductance shunts incoming signal through thr primary without ever jiggling the M-field to get to the secondary. Between these two extremes, it's mostly thermal. So a transformer power rating must ***always*** include frequency limits to make any sense.

    The power rating of the transformer INCREASES from the power max at its lowest frequency. This is because the impedance of the primary doubles with each octave higher signal. A transformer that can do 100W at 30Hz can usually do 200W at 60Hz and 400W at 120Hz. That's the reason for those teeny, tiny transformers in power supplies. You don't need a big transformer at 100KHz to 1MHz.

    Because of this, I seen no conflict between what you saw and the transformer's specs.

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    Quote Originally Posted by nsubulysses View Post

    If I add more bass in the preamp the sounds is not so in your face. Maybe it is what it is. and the speakers can't take that volume and still do the full frequency range
    Don.t forget the feedback network

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    Yes the feedback network. I think I am also reaching power stage distortion and that also contributes to the grainy sound and lack of ability to control it with the tone controls. I recently increased my feedback resistor from 110k (16 ohm tap) to 150k and it tightened up the sound and removed s bit of that unpleasant distortion characteristic. It also allowed me to add bass in the preamp in ways I wasn't able to before because it made the sound too flubby.

    Going back to post 7, can anyone say why my input signal begins to distort when my power stage does ? This is really weird to me. I should check a bunch of other amps this same way to see if that happens

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    Quote Originally Posted by nsubulysses View Post
    Going back to post 7, can anyone say why my input signal begins to distort when my power stage does ?
    Check the grounding of your test set up. It could be that the load return current is running through the scope or sig gen ground.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nsubulysses View Post
    Hmmm, not sure what information can be gleaned from this scenario but I built this amp ...............................
    Sorry but that is a long thread , it shows 4 or 5 amp schematics and half a dozen circuit snippets, lots of changes along the way, can you please repost here the *one* schematic you actually built and are asking about?
    Thanks.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nsubulysses View Post
    Going back to post 7, can anyone say why my input signal begins to distort when my power stage does ? This is really weird to me. I should check a bunch of other amps this same way to see if that happens
    I have a couple ideas at the tip of my tongue, but can´t comment until I´m certain about the exact circuit of your amp and a detailed description at how and where you are injecting your test signal.

    And yes, I *have* seen amps where drive signal distorts at about the same time as power tubes clip, but there´s 3 or 4 possible scenarios, would love to focus on yours specifically.

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    Hi JM, I will draw the schematic. I am out of town for 2 days but I'll try to do it on Sunday and post it. I would love to hear your thoughts

    thanks

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    Last night I tried to draw the schematic. It took over 10 minutes to find a piece of paper. When I did, I realized all the pencils I had were so old that the eraser was so dried out it could no longer erase, merely just smear lead all over the paper.

    I'll have to try again today and find some better materials

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    Quote Originally Posted by nsubulysses View Post
    Last night I tried to draw the schematic. It took over 10 minutes to find a piece of paper. When I did, I realized all the pencils I had were so old that the eraser was so dried out it could no longer erase, merely just smear lead all over the paper.

    I'll have to try again today and find some better materials
    ...and write a blues song about your soul-crushing experience with the pencils and paper

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    Ok JM I just need you to fax over your signed NDA otherwise all rights and access to materials that involve helping me will be revoked

    Preamp
    Power/output

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    Just out of curiosity, how can the negative bias supply be derived from the B+ DC voltage? Wouldn't a reverse diode there just block the positive DC entirely? Wouldn't the bias supply have to tap from the AC portion BEFORE the rectifier?

    Justin

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    Thank you Justin. You are correct

    I drew it wrong. keep em coming. Although it should be mostly correct other than that I think. Please allow me to eat my words if anything else looks weird.

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