5E5-A low output volume

[Corcia]

I've added the pot with a resistor in series and set the bias current to 52mA, coherently the voltage at the bias resistor is exactly -32V so i guess it should be fine. The amp seems louder but i've only made a test drive with vol @3 (it's 23:41!). But which is the relation among bias, power and output volume? Does Half bias current mean actually half power? By setting bias from 26 mA to 52mA the output volume increased but did not double for sure

[loudthud]

There is no direct correlation between the DC power with no signal and the output power. If the bias current is too low, the amp will seem cold and lifeless. The gain will be somewhat low, tone won't be as good as it could be. But without a good measurement of output power, any advise as to why the amp is not as loud as your Deluxe Reverb is just a guess. A wiring error, a bad tube or other component could be the cause. A quick look at the output with an oscilloscope will reveal if the amp is working as it should or has some kind of hidden fault.

A common mistake is installing the wrong value resistors, usually because the builder does not know the color code. A novice builder should measure the resistors with a DVM before installation, just to make sure. Once the resistrors are installed, they may not measure correctly. One end of the resistor will need to be unsoldered before measurement.

There should be one wire from the output transformer, the 4 ohm tap, that should not be connected to anything. Do not connect it to ground !!!

[Corcia]

There is no direct correlation between the DC power with no signal and the output power. If the bias current is too low, the amp will seem cold and lifeless. The gain will be somewhat low, tone won't be as good as it could be. But without a good measurement of output power, any advise as to why the amp is not as loud as your Deluxe Reverb is just a guess. A wiring error, a bad tube or other component could be the cause. A quick look at the output with an oscilloscope will reveal if the amp is working as it should or has some kind of hidden fault.

A common mistake is installing the wrong value resistors, usually because the builder does not know the color code. A novice builder should measure the resistors with a DVM before installation, just to make sure. Once the resistrors are installed, they may not measure correctly. One end of the resistor will need to be unsoldered before measurement.

There should be one wire from the output transformer, the 4 ohm tap, that should not be connected to anything. Do not connect it to ground !!!

Loudthud, that's exactly how i built the amp: i've tested each component (including caps) with a RCL digital meter before installation, and double (triple!) checked layout and wiring and everything seems to be fine. the 4 ohm tap on the OT is not grounded, i've isolated it and left apart.

regarding Bias, since i've increased the bias current it appeard a constant Hum, hum is present even with volume knobs to zero and no input jack connected; such hum is not so loud, it is enough to play even at minimum volume to forget about it but when not playing it is clearly audible...any guess?

I've found a lot of "oscilloscope software" do you think a software oscilloscope might work? in case which are the tests i should run?

thank you (all) so much for spport!

[loudthud]

I've found a lot of "oscilloscope software" do you think a software oscilloscope might work? in case which are the tests i should run?

Here is what needs to be done. A Sine wave is feed into the amplifier. The amplitude of the sine wave is not too important as long as it is not clipped by the input stage of the amp. 1 Volt RMS is about as big as the input stage can handle. Something between 0.1VRMS and 0.5VRMS would be safe. The amplitude can then be adjusted by the amp's Volume control. You can find software that will generate a sine wave using a PC's sound card. The frequency is not critical, anything between roughly 200Hz and 1000Hz will work. The Sound Card Oscilloscope will be used to monitor the output of the amp, it needs to be connected through an attenuator to the speaker output of the amp. It is best to use a "dummy" load, an 8 ohm resistor to connect to the amp instead of the speaker. This will prevent damage to the speaker and your hearing. The amplitude of the sine wave will be increased by adjusting the amp's Volume control until clipping is observed on the oscilloscope display, then just below that point, a DVM will be used to measure the amplitude at the output of the amp in Volts RMS. Power output is then computed by the formula P=E^2/R.

25W at 8 ohms would be 14V RMS or 20V peak. A PC sound card probably can't handle a signal that big so the signal will need to be attenuated. A simple voltage divider will do, or even just a pot between the amp's output and the sound card. Again you don't need anything exact, you just need the signal to be small enough that the sound card does not distort the signal.

What may be happening is that one side of the waveform is clipping prematurely limiting the volume. The oscilloscope display will reveal this.

[Corcia]

Here is what needs to be done. A Sine wave is feed into the amplifier. The amplitude of the sine wave is not too important as long as it is not clipped by the input stage of the amp. 1 Volt RMS is about as big as the input stage can handle. Something between 0.1VRMS and 0.5VRMS would be safe. The amplitude can then be adjusted by the amp's Volume control. You can find software that will generate a sine wave using a PC's sound card. The frequency is not critical, anything between roughly 200Hz and 1000Hz will work. The Sound Card Oscilloscope will be used to monitor the output of the amp, it needs to be connected through an attenuator to the speaker output of the amp. It is best to use a "dummy" load, an 8 ohm resistor to connect to the amp instead of the speaker. This will prevent damage to the speaker and your hearing. The amplitude of the sine wave will be increased by adjusting the amp's Volume control until clipping is observed on the oscilloscope display, then just below that point, a DVM will be used to measure the amplitude at the output of the amp in Volts RMS. Power output is then computed by the formula P=E^2/R.

25W at 8 ohms would be 14V RMS or 20V peak. A PC sound card probably can't handle a signal that big so the signal will need to be attenuated. A simple voltage divider will do, or even just a pot between the amp's output and the sound card. Again you don't need anything exact, you just need the signal to be small enough that the sound card does not distort the signal.

What may be happening is that one side of the waveform is clipping prematurely limiting the volume. The oscilloscope display will reveal this.

Thank you so much for your support, i've disappeard for a while because I decided to buy an oscilloscope and i'm waiting for it to be delivered (it should happen today).
BTW the scope i bought is this one: SainSmart DSO203 Portable ARM Digital Oscilloscope, 4 Channels, 72MHz Analog Bandwidth: Amazon.com: Industrial & Scientific hope it wil fit the purpouse!
once i'll have it in my hands i will start to check the amp.

[loudthud]

That looks ok on the surface, BUT I'd like to see more specifications. The most important is the input sensitivity range and input breakdown voltage. The voltages in a tube amp could kill the probes or the scope. It looks like probes are included, are they X1 or X10? They appear to have SMB type connectors which means the scope is not compatible with the vast majority of probes made over the last 50 years. That might be fixable with adapters. The right probe will protect the scope from sudden death. A scope like this is difficult to repair, more than likely it will just have to be replaced.

The other thing you need is some kind of signal source. A sine wave generator, a sound card with software, or a CD player with a disk of different waves. Lastly you need some kind of "Dummy Load", just a big 8 ohm resistor rated atleast what the power output of the amp is expected to be or higher.

[Corcia]

That looks ok on the surface, BUT I'd like to see more specifications. The most important is the input sensitivity range and input breakdown voltage. The voltages in a tube amp could kill the probes or the scope. It looks like probes are included, are they X1 or X10? They appear to have SMB type connectors which means the scope is not compatible with the vast majority of probes made over the last 50 years. That might be fixable with adapters. The right probe will protect the scope from sudden death. A scope like this is difficult to repair, more than likely it will just have to be replaced.

The other thing you need is some kind of signal source. A sine wave generator, a sound card with software, or a CD player with a disk of different waves. Lastly you need some kind of "Dummy Load", just a big 8 ohm resistor rated atleast what the power output of the amp is expected to be or higher.

i took a closer look at the scope, it has a built-in signal generator (10Hz to 8Mhz 2.8Vpp Square Wave, duty circle 10~90% adjustable / 10Hz to 20Khz 2.8Vpp Sin, Square, Saw, Triangle wave ); it came with two probes, each probe has a switch for X1-X10 mode; maximum allowable voltage is 80Vpp (X1) - 400Vpp (X10); resolution is 20mV-10V/div (1-2-5 step) on x1 probe / 200mV -100V/div (1-2-5 step) on probe X10.
i cuold not find a 8 ohm/40W resistor, but i was able to find two 18 ohm/17W resistors, i was thinking about the parallel of the two but it will be 9ohm instead of 8ohm, for sure it will not damage the amp (will it?) but i don't know if the 9 ohm could jeopardize the test.
anyway, which should be the correct testing procedure? i will substitute the speaker with the dummy load, feed the amp with a sine wave (amplitude? frequency?)... and than? where do i have to connect the probe? do i have to test each stage of the amplification in order to find out where is the problem (if any)?

[Dave H]

9 ohms will be fine. Just remember to use 9 for R in the V^2/R power calculation. I use a 100W, 10 ohm HSC100RJ from Farnell (they don’t stock 8 ohms). It has a 50ppm temperature coefficient which is good. The previous 8 ohm I had from eBay drifted as it got hot making my 30W amp look more like 34W.

To measure amplifier output power connect the probe to the dummy load then use Loudthud’s method in post #19 above except you won’t need a separate signal generator, voltmeter or attenuator as your scope has a sine wave generator and RMS voltmeter built in and you have x10 probes. If you set the probe to x10 remember to set this on the scope too or the measurements will be a factor of ten out.

[loudthud]

First issue to tackle is the Dummy Load. 9 ohms is pretty close and your amp will not be damaged. If you can find something close to 72 ohms (68 or 75 ohms at 3 to 5 Watts), that in parallel would bring 9 ohms down to 8 ohms. The dummy load needs to be connected to the amp instead of the speaker. A length of wire with a phone plug on one end will do the job. Additionally, a meter or scope needs to be connected across the dummy load to measure and observe the output voltage. You can do this at the end of the wire at the dummy load resistor or at the speaker jack inside the amp.

When making a Power calculation by looking at a Voltage waveform, you need to know the RMS value of an AC waveform for the calculation to be accurate. Most Digital Volt Meters assume that the waveform is a true Sine Wave with no distortion. The more distortion in the waveform, the less accurate the meter reading. More expensive meters will take the waveform into account, these are called "True RMS" meters. Because of this feature, the meter can sell more money and will usually say True RMS on the faceplate. I don't know what kind of DVM you have. The scope may have a function to compute the RMS value of a waveform. Again, I don't know enough about the scope from the Amazon webpage.

An X10 scope probe usually has a "Compensation" adjustment. This adjusts the probe so that the X10 attenuation is accurate at all frequencies and is not affected by the input capacitance of the scope or the probe cable. Your scope should come with instructions on how to do this. The usual procedure is to connect the probe to a source of square waves of about 1kHz and adjust a small adjustment screw at the probe tip or near the probe connector until the tops of the square waves are flat.

The last task is connecting the signal generator to the input of the amp. Ideally you would have a cable with the connector for the generator on one end and a phone plug on the other. If that is not available, a cable with the connector for the generator on one and some form of clip lead (alligator clips) on the other can work.

It is very important to make sure that the ground side of the generator and the ground side of the scope connection both go to ground at the input and the speaker connection. If this is not done correctly, a short will occur that could damage the scope. The easiest way to prevent damage is to connect the scope probe ground clip to the chassis of the amp and observe the signal (connect the scope prove tip) at the speaker jack.

[Corcia]

i've made the first tes with scope and here the results:
input: sine, 1 kHz, RMS 100mV, Vpp 264mV, Vmin -124mV,Vmax +124mv
as per the output the wave form seems to be preserved up to volume at 8 (a bit more than halfway), but it shows a sort of ripple which is eliminaed by setting he treble pot to zero.

with volume at 4 (9:30) the output is the following:
1001 Hz, RMS 7.2V, Vpp 18V, Vmin -8.4V,Vmax +8V
here the scope capture

with volume at 6.5 (12:00) the output is the following:
1002 Hz, RMS 12.8V, Vpp 35.2V, Vmin -17.2V,Vmax +16.4V
here he scope capture

with volume at 9 (15:00) the output is the following:
1002 Hz, RMS 17.6V, Vpp 41.2V, Vmin -20.V,Vmax +19.6V
here the scope capture

with volume at 12 (17:00) the output is the following:
1001 Hz, RMS 18V, Vpp 41.2V, Vmin -19.6V,Vmax +20V
here the scope capture

to explain the "ripple" due o high i mentioned above, her the scope capture with volume halfway but wih high control rolled all the way off

[loudthud]

The position of the Volume control does not mean much. What concerns us is the maximum size (Voltage and Power) and shape of the signal at the output. If the signal applied to the input is too big, the sine wave would be distorted no matter how far you turned the Volume control down. In that case the signal generator amplitude should be reduced. This is how a "Clean Boost" pedal creates distortion. It increases the amplitude of the guitar signal so that it is too big for the input stage of a guitar amp to amplify without distortion.

I'm not sure what all the numbers around the edges of the screen mean. Is there an operator's manual for the scope online anywhere? I see 100Hz on the screen. Is that the generator frequency? Try changing the frequency to see if the amplitude changes. The little ripples you see could be an oscillation or just noise. It's hard to tell with a digital scope. Does the amp make it's own signal when you disconnect the generator and turn the Volume and Tone controls all the way up?

Try the same test on the Deluxe Reverb. Which amp makes a bigger signal? Note that the tone controls of the Deluxe Reverb cause the signal to change amplitude as the generator frequency is changed. This is normal. Mid range frequencies around 500Hz do not pass very well.

Bad weather here, I may be offline for a day or two

[Jazz P Bass]

A little late to the party here.

My take is the amp looks fine.

If you look at the 'volume at 6.5' screen shot, you can see the signal is nice & clean.

!2.8V RMS is the max clean that the amp can provide.

The rest of the increased volume shots show the amp distorting exacty as I would expect it to look.

[frus]

A little late to the party here.
12.8V RMS is the max clean that the amp can provide.

If that's through a 9 ohm load, then max output would be around 20W
That IS a bit low, my 5F4 produces 32W into the specified (4 ohm resistor) load (and my B+ is around 420V)

[Corcia]

i was able o find a 36W 8 ohm resistor (8,08 Ohm), so measuremens have been done with such resistor
i made new tests at different sine wave frequency: 100Hz, 500 Hz and 1kHz;
first of all the hum: amp makes a hum even with volumes to zero and with no jack in, its rms vlue is 35mV (it is not luod) here the track: , this hum is not influenced by volumes, but it disappears if bass dial is set to zero and treble to max.
the amp has some volume dependent hum but it only shows if volumes are set at 11.
with an input signal of 100mV rms at 100HZ max clean is 14.8Vrms here the shot:
with an input signal of 100mV rms at 500HZ max clean is 14.2Vrms here the shot:
with an input signal of 100mV rms at 1kHZ max clean is 14.0Vrms here the shot:
i've also measured my Deluxe reverb, and feeding in a 100mVrms signal at 100Hz max non distorted signal is as high as 12.8Vrms, here a capture:

[Corcia]

I discovered another odd behaviour of the amp: when turning the bass control off the volume drops dramatically, is it normal for such amp/circuit or It might be the symptome of some problem?