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Excess Line Voltage vs Amplifier Sound

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Posted on January 8, 2017 at 17:57:25
sayntjack
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Recently I have been spending some time with a couple of my amplifiers which use power transformers built in the late 50s and mid 60s when line voltages were typically 110-115 Volts vs my house voltage which runs between 118 & 124. At times I have noticed that my 1957 Heathkit W5-M's sound a little edgy on the mids & highs. A check on voltage at these times typically finds 123 / 124. Reducing this voltage to 118 using a tapped autotransformer eliminates the problem. Last night after about 3 hours of listening to a 6B4G triode version of a Dynaco ST70 I began to notice a significant lacking in bass weight. On a hunch I checked the voltage and noted it to be 124. With a quick reconnection to an auto transformer, not only was the bass restored it seemed to have a little more weight than was usual at my lower line voltage of 118. The output voltage of the autotransformer - 110!

The bottom line here seems to be that if you want your vintage tube equipment to sound right (and likely last longer) then you should ensure that its supply voltage is kept around 115. Most commonly this can be achieved by either an external device such as a variac or an auto transformer or by a replacement power transformer with a primary winding rated for nominal 120 volts such as those offered for Dynaco's by Bob Latino.

 

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RE: Excess Line Voltage vs Amplifier Sound, posted on January 8, 2017 at 21:07:30
DAK
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Plugging your amp into a line conditioner that outputs 117v is an easy way around the problem. Just be sure you get a unit that has enough current capacity for your needs.
There are things you can do to the power supply during the rebuild to bring your amps' B+ down. Increasing the size of the resistors in the filter supply or adding a choke if space permits or another choke can reduce the B+ and improve the sound quality of your amp.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 8, 2017 at 21:39:56
sayntjack
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The issue with excessive line voltage I think is not limited to high B+. Rather, at 124V, saturation of the power transformer core and the accompanying increase in harmonics and waveform distortion are possible issues.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 8, 2017 at 23:35:17
91derlust
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A ~10% increase in line voltage is saturating the transformer core? I'd prefer a transformer with more headroom than that.

Filament/heater over-voltage could also be cause of the sonic issues you mention... and will reduce tube life.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

Brown box, posted on January 9, 2017 at 01:17:10
FenderLover
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I know some ppl that use these on their early 1950-60s era Fender amps. It makes the tweed era amp sound less strident or "pushed" --- according to them. Most noticeable in mid to higher volume ranges.

I have yet to try them. I usually play with rail resistors to get into original nominal voltage range, esp with phase inverter and gain stage-input tubes. These areas seem to suffer the most from excessive B+ plate voltages.

Note: All the ppl, which I know that use the Brown Box use it to control AC line voltage. Not specifically for tone or amp response control.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 08:16:17
Palustris
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speculation: the forming of a theory or conjecture without firm evidence

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 08:31:06
krankkall
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Bucking transformer.

I made one out of a TRIAD USA 6.3v/4A filament transformer, a Leviton hospital-grade outlet, small metal outlet box, and fused it too boot.

Steve

 

Oh man. Got to love the guitar nuts., posted on January 9, 2017 at 13:25:09
Chip647
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Or you could get a $100 Variac and be better off.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 13:40:11
xaudiomanx
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I can't comment on what you are experiencing sonically but I do know using the older transformers designed for 115VAC will give you a higher B+ if the B+ isn't dealt with by lowering it. That does shorten tube life but usually what I've always done with that is lower the negative voltage on the grid. As for sonics I do know if you overdrive a tube(which is what a lot of the so called "modders" do) it can tend to sound thinner with less attention to the bass. I don't think it's the bass that is compromised. I think it is more the upper ranges become more prominent. But I too am a novice with a small opinion but I do know what I hear.

Also, I was told by many in audio whom I respect that any type of device used to either condition the AC or lower it squashes the sound stage. Now! That I did experience. Not to a huge degree but in an A-B comparison I did hear it. I was always told the only units that should go through a line conditioner of any type should either be a CD/transport-Dac or a preamp. Anything needed to draw a lot of current should go directly to the wall outlet.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 16:08:06
sayntjack
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Hi Krankkall. Regarding your 6.3V/4A buck transformer approach, I assume that the transformer is a 120V pri, 6.3V sec and that you have connected the secondary in series with the primary in subtractive polarity to get ~114V. Given the secondary rating of 6.3V/4A indicates that the transformer is 25VA capacity (6.3V x 4A). If this is so, then the current rating of the 120V winding is only 0.21A (I=25VA/120V) which is inadequate for the typical 1.5 - 3 amps a typical 20-50W vintage amp is likely to draw.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 16:56:25
krankkall
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Largest tube amp I have is 8 wpc, works for me.

Steve

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 17:09:29
Steve O
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It's the secondary rating that counts in a bucking arrangement since its that winding that is in series with the mains. In the subject case, the rating would be 4A Max although I'd be inclined to derate that to ~ 3A to stay on the conservative side of things.

 

Is it wise to trust a guitar nut with a variac? (nt), posted on January 9, 2017 at 17:13:21
Steve O
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RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 18:40:13
sayntjack
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Hi Steve. I assume that the 8W that you refer to is the power output of the amp, which does not directly relate to the input power / current. With the output tube plate dissipation, filament power and power factor of the transformer are taken into consideration, it is likely that your amp is drawing at least 0.5 amps at 120V - twice the current rating of the buck transformer 120V winding if it is a 25VA transformer.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 18:53:59
sayntjack
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Hi Steve O, I disagree, the windings must be rated for the current that is passing thru them. In the case where someone is taking the 5V secondary winding of say a Dynaco ST70 amp and put it in series with the primary there would be no problem as the transformer primary winding current is already rated for the amp's load and the current rating of the 5V secondary is more than adequate. In the case of what I think Steve was doing using a stand alone transformer, the VA of the transformer must be high enough that the current rating of the 120V primary winding can carry the current being drawn by the load. Jack

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 9, 2017 at 19:36:25
91derlust
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Just to clarify your statement (it can be interpreted two different ways):

"That does shorten tube life but usually what I've always done with that is lower the negative voltage on the grid."

Do you mean lower as in going from a) -55V to -60V, or b) -55V to -50V? For a given plate voltage, example a. will reduce dissipation (less plate current) and example b. will increase dissipation (more plate current).

I only ask because when many people see "reduce" or "lower" they are thinking in absolute values. In the case of negative values, they might assume you mean the second example (-55V to -50V).

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

You sure about that?..., posted on January 9, 2017 at 20:28:44
Steve O
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...using a simple single primary/single secondary transformer in a buck (or boost) config, it is the secondary current rating that counts because the secondary must pass full current drawn by the device for which it's bucking voltage. Since we know the secondary volts and amps we also know its VA rating. Neglecting losses, in this case the VA rating of the primary = VA rating of the secondary. If the the VA rating of the secondary is not exceeded it will not be for the primary either. So effectively, secondary current rating becomes the limiting factor: the primary goes along for the ride with its current reduced by the factor Vsec/Vpri , again neglecting losses.

If I get time I'll wire up something, make some measurements and report back!

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 10, 2017 at 06:34:21
xaudiomanx
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Yes Sir,

Bad choice of words. I meant going from -55V to -60V but in essence that is lowering the voltage on the grid. But if you look at your response it is adding(raising) the negative voltage on the grid but it lessens the current draw of the tube and the total power supply.

I have modified many Dynaco ST-70 and Mark III amps and they sounded best running the tubes extremely conservative. I alos did very little to the power supply as far as adding Uf's to the caps and the such. It wounded better with almost the stock values. The biggest difference I found sonically was going from electrolytics to film caps in the power supply with oil caps as coupling. Many might not like the sound but I think it sounded softer and more natural.

Again, just my opionion!

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 10, 2017 at 11:51:38
91derlust
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Your choice of words was was technically correct - I was just clarifying for those that are less so.

Yes, although I run a different type of amp, I also run my tubes conservatively. For a desired power output, I'd prefer to have a higher dissipation tube run conservatively than a lower dissipation tube run near max dissipation.

I think we might have similar tastes in musical presentation.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: You sure about that?..., posted on January 10, 2017 at 12:57:56
sayntjack
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This is pretty basic stuff. To establish the rated full load current of the windings of a two winding single phase transformer, take the VA rating of the transformer and divide it by the rated winding voltage.
For example, for a 50VA transformer with a 120V primary and a 6.3 volt secondary, the rated full load currents are: Primary, 50/120 = 0.42A, Secondary, 50/6.3 = 7.9A.
So, if you wanted to make a buck transformer out of this by connecting the secondary in series with the primary in subtractive polarity (120-6.3 vs 120+6.3) and you needed it to be capable of supplying 3 amps to your load then the transformer would need to be rated 360VA minimum (120V x 3A) to have a primary winding capable of 3 amps.
To verify, I took a 135VA transformer, 120V pri, 6.3V sec. With my line voltage I got 128V additive polarity & 114V subtractive. Wired in subtractive polarity I connected a 60W light bulb and measured 474mA in the wire to the bulb. In the incoming wire to the transformer primary from the wall outlet I measured 480mA - a little higher as expected because of the transformer magnetizing current.

 

RE: Excess Line Voltage vs Amplifier Sound, posted on January 10, 2017 at 16:17:46
xaudiomanx
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Thank you for the kind words. Whenever I posted in the past I would get criticized in a big way so I just read and try to learn from the more informed.

 

Ah-Ha, posted on January 10, 2017 at 16:48:35
Steve O
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..."So, if you wanted to make a buck transformer out of this by connecting the secondary in series with the primary"

This isn't a conventional circuit for a stand alone bucking transformer. Conventional arrangement places primary directly across the mains and the secondary in series with the load with the combo across the mains.

I too set up a circuit using a tiny 115V - 6.3V/1.5A filament transformer in buck mode supplying 100V/1.5A to a resistive load (150VA or W). Measurements were just as I described previously and the 10VA transformer handled the 150VA load for 1hr w/o issue. i.e. no transformer buzzing and normal operating temp.

Will edit in circuit diagram and meas later.

Test circuit:


 

RE: Ah-Ha, posted on January 10, 2017 at 17:50:49
sayntjack
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Yes, that would be great if you could post the circuit diagram as our discussion is a little in the dark without it.

 

Diagram posted (nt), posted on January 10, 2017 at 18:16:08
Steve O
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RE: Diagram posted (nt), posted on January 11, 2017 at 10:03:03
sayntjack
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Hi Steve. I'm not seeing your posted diagram but no matter. I have set up my buck circuit again & retaken measurements. Results - I'm more than a little embarrassed. My statement regarding calculation of rated winding currents of a transformer is correct - the rest, regarding current thru the primary 120 Volt winding in the buck configuration is not correct. I apologize to all. With the 60W 120 volt bulb bucked with the 120 / 6.3 V transformer to operate at 114V from the 122V incoming, the lamp current thru the 6.3V winding was 474mA and the current going into the 120V winding was 58mA. I'm scratching my head a little on this but again I apologize to all. Jack

 

RE: Diagram posted (nt), posted on January 11, 2017 at 10:10:34
sayntjack
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I see your diagram - same circuit as mine. Thanks Steve.

 

Okey doke (nt), posted on January 11, 2017 at 15:20:53
Steve O
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