5F6A problem?

[vinceg]

Hi guy, my name is Vincenzo and I'm new in this forum
I've built a clone of Bassman 5F6A from Ceriatone Layout. I think my amp sound great but I'm not sure.
Please, watch this video and tell me if a breath when I turn up the vol pot is normal or not.
Do not consider the part when I plug the cord because I've resolved it (a simple ground loop)
This noise seems to be a breath and it is only the Normal vol pot is up to position 7.
With the Bright vol pot the breath is less loud than Normal vol pot
Also, this noise increases when turn up the presence pot.

5F6A Hum - YouTube

p.s. I have to say that this noise do not disturb when I play, but if I not play it is annoying.

[frus]

what's the other end of your cable plugged into?
if it's just floating around, then it's expected behaviour
if it's plugged into the guitar, and its volume pot is on zero, check if all 4 amp inputs are shorting correctly when nothing is plugged in
if its plugged into the guitar, and its volume is up, then the problem is probably the guitar rather than amp

[Dave H]

It just sounds like the normal hiss you get from the pre-amp to me. Did you use any carbon composition resistors?

[vinceg]

It just sounds like the normal hiss you get from the pre-amp to me. Did you use any carbon composition resistors?

I used metal film resistor and carbon composition.

Since I'm not a expert on tube amp, you could tell me what I can do to make sure that the amp work fine? Voltage values at pins ecc, for example.
What do you think if I replace the presence pot with one 25k and add a 4.7k resistor like "59 reissue?
I noticed that at loud volume the power tubes 6L6GC emits small blue flashes when I play the string, but in one of these the flashes are more brilliant.

[Chuck H]

You should be able to find the voltages for that amp on line. Then you could check it yourself. If anything seems too far off you can post your findings here. Keep in mind that if your rectified HV is higher or lower than any printed example then the other circuit voltages will be too. So if, for example, your actual rectified HV is 15V higher than what you see on any schematic it wouldn't be odd or wrong for the plate voltages to be a little higher than the schematic also.

Blue coloration in operating tubes is normal. Since one seems more prone that could be an indication of one tube carrying more of the load. Swap the tubes and see if the brighter blue flashing stays with the tube or stays with the socket. If it stays with the tube it's nothing to worry about. If it stays with the socket there may be a problem.

I don't think changing the presence circuit from the one you have to the one you propose makes any significant difference. But that's JMHO.

I don't have any speakers hooked up to my pooter right now but compare your hiss level to any other amp of similar power set to the same gain level. with all else being equal (same guitar, cabinet, etc.) If your hiss level is grossly bad you'll hear it. Many new preamp tubes nowadays seem to be noisy right out of the box. You could try swapping a known quiet tube for each of your preamp tubes and see if it helps.

[frus]

what's the other end of your cable plugged into?
if it's just floating around, then it's expected behaviour
if it's plugged into the guitar, and its volume pot is on zero, check if all 4 amp inputs are shorting correctly when nothing is plugged in
if its plugged into the guitar, and its volume is up, then the problem is probably the guitar rather than amp

btw, I was referring to when you plug the cable in. hiss seems normal to me

[vinceg]

btw, I was referring to when you plug the cable in. hiss seems normal to me

Yes, I understand. The problem when I plug the cable in was resolved. It was only a grond loop caused by the input jack nut. Thank's!!

[vinceg]

Hi, I've measured the voltage value at tube pins.

V1 12AY7
1 173V
2 nothing
3 2.25V
4 short to 5
5 heater
6 148V
7 nothing
8 2.27V
9 heater

V2 12AX7
1 165V
2 nothing
3 1.1V
4 short to 5
5 heater
6 300V
7 167V
8 166V
9 heater

V3 12AX7
1 230V
2 24V
3 38V
4 short to 5
5 heater
6 225
7 25.5V
8 38V
9 heater

V4 6L6GC
1 short to 8
2 heater
3 400V
4 376V
5 -47.5V
6 400V
7 heater
8 24mV

V5 6L6GC
1 short to 8
2 heater
3 400V
4 400V
5 -47.5V
6 398V
7 heater
8 24mV

Heater voltage is about 5.9V instead 6.3V

Do yu think?

[vinceg]

I have a question on the Bias.
I read that the voltage at pin 5 of the 6L6 power tube must be as low as possible, like -50V, in order to prolong the tube. In my case, through the bias pot, I have -47.5V. But the cathode current drop to 24mA. I'm confused. If I wanted to set the cathode current to 70% of max I should turn the bias pot until get the value of 52mA but the voltage at pin 5 increased to -30V.
What should I do?

[Diablo]

I have a question on the Bias.
I read that the voltage at pin 5 of the 6L6 power tube must be as low as possible, like -50V, in order to prolong the tube. In my case, through the bias pot, I have -47.5V. But the cathode current drop to 24mA. I'm confused. If I wanted to set the cathode current to 70% of max I should turn the bias pot until get the value of 52mA but the voltage at pin 5 increased to -30V.
What should I do?

That's normal for setting bias. The amount of negative voltage required on pin 5 is a function of the tubes used. Not all 6L6 tubes bias at the same voltage. In the 1950s-1960s, manufacturers produced very consistent and high quality tubes. So, you could just stick in a tube and be confident the amp would work properly without a bias adjustment. With modern production tubes, you need to adjust the bias voltage with each set of tubes, in order to get the bias current (52mA in your case) you desire. However, there is no magic number for idle dissipation. 70% is a widely accepted safe limit for push-pull AB1 power tube bias. You may however find that you like your amp biased to some lower value - down to 50%. The lower value will give longer tube life. If you go too low with the idle dissipation though, eventually you reach a point where you get a cross over notch, and this produces an ugly sounding distortion.

[Chuck H]

Listen to Diablo WRT bias. Your voltages look hinky on V5 screen (pin 4). There should be a drop in voltage. Lower than the plate. Recheck that. Both screens should be close to the same voltage. If you still don't show a voltage drop on that screen try swapping the tubes to see if the lack of voltage drop follows the tube or stays with the socket.

[vinceg]

Listen to Diablo WRT bias. Your voltages look hinky on V5 screen (pin 4). There should be a drop in voltage. Lower than the plate. Recheck that. Both screens should be close to the same voltage. If you still don't show a voltage drop on that screen try swapping the tubes to see if the lack of voltage drop follows the tube or stays with the socket.

Are you referring a drop in voltage between pin 6 and pin 4? I run the measurament again and I read approximately 388V both pin 4 and pin 6. Both V4 and V5.
The measurament across the two screens resistor is 0.6V.
There is a problem?

[Chuck H]

The schematic shows a higher voltage drop for the screens. And in your above measurements you had only one of the power tubes showing that drop. Now your measuring the same voltage for both power tube screens and this seems fine.

[vinceg]

In fact! Maybe, before I was wrong to read the value. Anyway, the amp seems sound great. In a previous post I said about the light blue when play the string at high volume and one tube has more blue flash than other. As suggested I swap the power tube and the same tube have identical behavior, then there isn't problem.
I set the cathode current as suggested before.

[vinceg]

About the hiss, I tried the following:

1) remove one at a time V1, V2 and V3. In all three cases the hiss disappeared.
2) Swap V1 with the other preamp tubes. Swapping V1 with V2 the hiss disappeared. I the other combination the hiss is appears.

What do you think?