[Dirk]

Hi,

All components do have a tolerance. Capacitors have 10-20%, potentiometers 20% or even more and voltages of course too.
The heater of the tubes is designed for 6,3 V. Let's reduce the 10% tolerance from the 6,3 V and you will get 5,67 V. So your heater voltage is still acceptable.
I also recommend to check the voltage with an other DMM with full bateries. Cheap DMMs and/or  empty batteries are a big source for wrong readings.
If the voltage is still to low than use an other primary to "raise" the voltages a little bit.

Best regards, Dirk

[Pringles1]

Thanks Dirk and all for yours replies.

I have reassessed all measurements with a professional multimeter, and are the same. The heaters measure 6.06 V (226 V at home). I have done the fifth check of the circuit, the resistances, the capacitors, etc. and again I have not discovered any error (and surely there must be some). I have checked the transformers, the voltages of the PT and the resistances of the OT and they are all correct. I have reviewed all the GND and they have continuity with the chassis. Regardless, I've plugged my bass in the amp, and the volume is still pretty low (I have to turn on full the bass volume and amp volume to have a comfortable volume at home. It still has quite a bit hummm too which decreases a lot when I touch any pot on the amp... Any idea where is the problem? Any help?

Best regards,

Jaime

[chaccmgr]

when the hum level changes when you touch a pot I would assume that you have a shielding/Ground issue. Maybe a ground loop in a shielded cable (both ends of the shield connected to ground where only one end connected is the right way)

You say, you have continuity from all GND-ponts to the chassis. Do you have continuity from GND to the PE-connection of the amp? Does your house installation have an Earth wire at all? I live in an old house and have these issues in some rooms.

Did you check the amplification in each stage to identify where you lose signal?
Did you measure the output of your bass? Having this number you could calculate the theoretical amplification factor in each stage depending on the other components and voltages and cross check with your measurements.

[Pringles1]

Thanks Robert,

I check the  shielded cables continuity, and the brigth jack (upper at picture) signal doesn't continuity with GND and the shield yes. But the brown jack (lower at picture) yes... When I put the plug,  the brown jack signal doesn't continuity with GND and shield yes.
Is it possible to check the amplification at each stage to identify where I lose the signal without oscilloscope?

[chaccmgr]

yes
you need a known input signal, say 50mV AC and a multimeter
measure it at the input of the tube (grid1) and note it
measure the signal at the anode and note it

do so at each tube, input signal vs. output signal at each tube

then we can get a feeling, where there might be a problem

[chaccmgr]

regarding the shields:
the shield is grounded, ok
the question is, if it is connected to GND more than once (on both ends). If yes, you have a ground loop. It must be connected to GND only on one end,

[Pringles1]

Yes Robert, it's connect only the end of the jack. The other end is not connected.

I make the measurements and tell you about!

Thanks,

Jaime

[chaccmgr]

Hi Jaime
Curious what your measurements will say

Regarding the hum:
Hum can have many causes, in most cases it is a grounding or wiring issue
I noticed that your heater wiring is kind of „spacious“. The wires are well twisted but run in very large loops. You could make them much shorter.
On the first tube the heater wires are very close to other pins, one wire even touches the anode cable. The other wire comes to the soldering pin from the inside. Solder it from the outside to get more spacing to Avoid hum coupling

The center tap of the heater secondary is connected to ground?

[Pringles1]

Thanks Robert,

Yes, the center tap of the heater secondary is connected to ground. And you are right. I have change to another socket that I know for sure is grounded and now the hummm is down. I'm surprised,  because I had tried it before and the humm persisted.

As for the output of the bass is 60mV (AC). When I play with a lot of punch, it's increase around 200-150mV. I have tested bass on another 3 Watt guitar tube amp and it has a volumen sounds similar to the Bflex, so I think it sounds comparatively low (with the same 8 Ohm cone) since BFlex has 18 Watts, right? I have also feel that when I connect the bass to the Brown jack of Bflex the sound is little louder, especially the bass.

Robert, in order to not make mistakes, the signal source has 50mV (AC) but the measurements are as follows? (Excuse my clumsiness but I want to make sure I make some good measurements for you can help me):
preapms: V1 pin 7 to 8 and the pin 2 to 1? Boths in DC or AC? From V2 the same?.
power section: V3 and V4 pin 4 or 5 to pin 3? (AC or DC). And in all the measurements do I put the ground on the chassis?
thanks for your great patience.

Best regards,

Jaime

[chaccmgr]

hi Jaime
on triodes the input pins are pins 2 and 7
output pins (anodes) are pins 1 and 6
pins 3 and 8 are the cathodes that will not help us

all measurements to ground

so you measure the Input to V1a on pin 2 and the output on pin 1. In this particular circuit the output is supposed to be roughly 35 times the input

same procedure on V1b, but the amplification factor here is supposed to be around 50 to 60 times

V2 is the phase inverter, here it is a paraphase topology. I'm not an expert for paraphase amplification factor but just do the same at this tube and post your results.

then we have a clearer picture and can move on

[chaccmgr]

Hi,
the brown input is supposed to be louder: the voltage divider between R2 and R30 leaves more signal to V1a. The bright input accentuates the highs, so it is according to the design that the Brown input is more bassy.

an 18W Amp is capable of delivering deafening sound levels in a small room. I can't crank my 15W Madam Guitar Amp with my bass to full volume without getting aching ears. So the expectation for the BFlex is to be loud.

Measurements and calculations:
I double ckecked and found an error re the amplification factors in my last post (calculators can be found e.g. on www.ampbooks.com):
Please set all controls to 12 o clock. All measurements are AC, reference to ground/chassis (you checked continuity from GND to Chassis?)
To measure the amplification factors you need a steady signal. Playing bass and measure will not suffice.
When measuring with a steady signal applied and your load and the speaker output is connected to an actual speaker, this can get quite loud and nerve wrecking. Get a dummy load (high power resistor) of 4/8/16R and connect it to the appropriate jack.
As Dirk mentioned, all components have tolerances so don't expect the theoretical values to be measured. But the ballpark figures must be ok of you have an error in component value or other.

V1a is supposed to amplify 30,6 times (in theory). If you insert a 50mVAC Signal to Pin 2 you should measure 1,53VAC on pin 1. Measure the signal at pin 2 and not at the input of the amp. Measure both and post.

Your Gain Control (Blast) is supposed to be logarithmic. Set to 12 o clock, the output on the middle lug is supposed to be around 10% of the input. Measure both and post.

V1b is supposed to amplify 43,59 times (in theory). Depending on your input signal you can do the math yourself. Measure both and post.

The tone-stack is supposed to attenuate the signal by -21dB (in theory) which equals to 0,089 times the input. Do the math based on your actual signal entering the tone stack, measure both and post.

there is no calculator on ampbooks.com to match the exact topology of this paraphase splitter. Especially R23 is quite a high value with 15k. No clue why that value was chosen. -->Dirk?<--
 I suppose, it was chosen to attenuate the signal to prevent the phase-inverter or the power tubes to clip
 --> if yes, this might be our point of signal loss
Measure both input and output for each side of the PI and post the values.

Operating Points of the power tubes:
Did you measure the quescent current of your power tubes? This is a crucial information for judging, if your power amp can deliver it's maximum power.
Double check the values of R28/29 if they are really 1k. If they are higher, this will reduce the volume.

[geowicht]

Dear all,and in particular to Robert.
I am deeply impressed how you guide through the different steps of checking a valve amp and doing the appropriate Math. This step by step guidance contributes so much to beginners and helps them (and me) to do it by there own. So pls keep going and many thanks for your time and efforts!

Cheers Gerhard

[chaccmgr]

 

[Pringles1]

Thank you so much Robert!
Of course without your help, wisdom and patience and the other wise men in the forum, it would be impossible for newcomers or novices like me!

I'm not quite sure if I did the measurements right. In V1 I have applied a 84 mV (AC) signal on Pin2 and in P1 the measurement has been 0.000 V (AC) and 192 (DC). I have also applied the signal on P7 and the reading on P6 has been 0.000.

I made a mistake and applied a 2.15V (AC) signal to Pin 2 and the reading to P1 46.4V and when apply to Pin 7, the reading to Pin 6 43.5V The measurement in the middle lug of Blast pot was 1.65V.

Then I applied a 440mV signal on P2 and the reading on P1 was 11.7V. and when applied at P7 (390mV) the reading at P6 was 9.98 V. The Blast pot 0.31.

I have also adjusted the Bias with the new multimeter and when I played the amp to the bass it sounds louder than before, but at full volumen of the bass and amp, the  windows don't move ...  .

[chaccmgr]

Hi Jaime,
Hard to say from a distance why your measurements are odd and varying.
Feeding 400+ mV will not help as any stage will go into clipping and you read a compressed signal instead of a correct one.
What is the frequency you are feeding? Most multimeters have difficulties reading frequencies below 1kHz