5F6A low volume

[vinceg]

Hey, we are only 4 hours away!!!
Italy +1 GMT ; Argentina -3 GMT

Back to the transformer, it seems to be perfect


So now weŽll reconnect it step by step.

Start by reconnecting filaments .... *ONLY*
Do not "save time" connecting anything else.

1) Test first with bulb limiter that we have filament voltage across proper tube pins at every tube. No tubes plugged in. 6.20 V

2) Now that each tube pin has about half that relative to ground.3.10 V

3) Now repeat tests with tubes in their sockets, *still* through builb limiter.
I donŽt care you will now have way less than 6.3V at each filament, we are just testing winding does not collapse/short under real load.

Post results.

Now I accompany my daughter to school. Later, within an hour, I will do the test with the tubes in

[vinceg]

Hey, we are only 4 hours away!!!
Italy +1 GMT ; Argentina -3 GMT

Back to the transformer, it seems to be perfect


So now weŽll reconnect it step by step.

Start by reconnecting filaments .... *ONLY*
Do not "save time" connecting anything else.

1) Test first with bulb limiter that we have filament voltage across proper tube pins at every tube. No tubes plugged in.6.20

2) Now that each tube pin has about half that relative to ground.3.10

3) Now repeat tests with tubes in their sockets, *still* through builb limiter.~5V between filaments wires, ~2.5V between each filament wire and ground. On V1 the voltage between each filament wire and ground is ~2.2V and ~2.5V
I donŽt care you will now have way less than 6.3V at each filament, we are just testing winding does not collapse/short under real load.

Post results.

I presume that above values was less than 6.2V because of the lamp. By the way, this time the lamp lit up normally and immediately the brightness went down, you could see the orange tungsten filament, like tubes filaments.

Here I am

[J M Fahey]

Ok, now connect bias winding *only* and check that you have bias voltage and that it reaches power tube grids.

You will have less than expected values, say, -20/25V instead of around -50V, thatŽs because of the bulb limiter, no big deal.

I want to be certain that you have negative bias before we apply high voltage to plates and screens ... which would be deadly if unbiased.

Like the other thread says: letŽs check the obvious.
We canŽt assume "everything is fine" without proof.

[eschertron]

Ok, now connect bias winding *only* and check that you have bias voltage and that it reaches power tube grids.

You will have less than expected values, say, -20/25V instead of around -50V, thatŽs because of the bulb limiter, no big deal.

I want to be certain that you have negative bias before we apply high voltage to plates and screens ... which would be deadly if unbiased.

Like the other thread says: letŽs check the obvious.
We canŽt assume "everything is fine" without proof.

I'm assuming 'bias winding' indicates 'bias tap' and 'center tap'. Let's be sure Vince understands before we spin out of control

[vinceg]

Ok, now connect bias winding *only* and check that you have bias voltage and that it reaches power tube grids.

You will have less than expected values, say, -20/25V instead of around -50V, thatŽs because of the bulb limiter, no big deal.

I want to be certain that you have negative bias before we apply high voltage to plates and screens ... which would be deadly if unbiased.

Like the other thread says: letŽs check the obvious.
We canŽt assume "everything is fine" without proof.

I'm assuming 'bias winding' indicates 'bias tap' and 'center tap'. Let's be sure Vince understands before we spin out of control

In fact I'm not sure I understood. Juan talks about negative bias and this suggests that I need to connect the Red / Green wire (50V output) to the diode of the baseboard.

Also, he says he want to check if I have bias voltage at power tubes grid.

I have already measured the voltage between "bias tap" (Red/Green) and "center tap" (red/yellow).

Anyway, I want to be sure I'm doing the right thing. Also, now the sockets are disconnect from all wires, then I have connect the 220K grid resistors to power tubes for I be able to read negative bias voltage.

[eschertron]

Let's see if I understand. Do you have any connections other than the wires from the power transformer disconnected? If so, I don't think that was necessary. At this point, the red high voltage wires should be disconnected and taped off to make them safe. Other than that? Nothing that I can think of.

With the amp (except the red wires) wired up properly, the bias circuit will take that 50vac from the red/green and red/yellow pair and convert that to a negative DC bias voltage that can be read between ground and the grid pin connections on the output tubes sockets. If you don't get the negative bias at the power tube pins, then any high voltage will kill the tubes PRESTO!

[J M Fahey]

To make it clear, we are going step by step.

I never said "disconnect everything and restart from 0", whatŽs implied is than any new suggestion *adds* to what was already there. Unless specifically told otherwise.

So Red/Yel which is the high voltage winding is already connected to supply/chassis ground and stays there .

Filament windings also stay there.

Only "news" is that NOW we connect the bias tap where it belongs, the small bias supply, so to anything it includes: its own diode, resistors, capacitors, trimmer, etc. , I shouldnŽt need to name all besides saying "connect bias tap where it belongs".

connect bias winding *only*

means "add that only and nothing else", since vinceg is very impatient, and when told to go one step forward, he walks 10

Juan talks about negative bias and this suggests that I need to connect the Red / Green wire (50V output) to the diode of the baseboard.

Yes.

Also, he says he want to check if I have bias voltage at power tubes grid.

I have already measured the voltage between "bias tap" (Red/Green) and "center tap" (red/yellow).

You measured AC voltage at a winding, now I want you to measure Bias Voltage which is DC.

Anyway, I want to be sure I'm doing the right thing. Also, now the sockets are disconnect from all wires, then I have connect the 220K grid resistors to power tubes for I be able to read negative bias voltage.

Did you disconnect everything from power tube sockets?
Why?
I only asked you to disconnect *transformer* wires fron anything so they can be measured without interference, but nothing else.

[vinceg]

I have all wires disconnected except those that go to pots. The red high voltage wires are disconnected and taped.

Red/yelllow is connected to ground. If I connect Red/Green to diode and the grid resistors to power tubes I'll be able to read negative bias at pin socket. Right?

[vinceg]

I had disconnected everything for safety before you gave the instructions step by step.

Anyway I understood correctly and I assure you that I do not want to run.

Ok, tomorrow morning, at 8.00, when you are steel asleep, I will proceed with this step and update you.
It is inplied bulb current limiter use.

[Helmholtz]

I have not measured between Bias and center tap because last night while I was measuring the 500mA fusein bias wire is burned, then I'm scared. I suspect the problem is at 50 VAC out winding.

From this I assume that there is a 500mA HT fuse wired between red/yel and ground, like in a JTM45 circuit. It is not shown in the layout.

[Reader]

Hi Vincenzo,
I think it would be good if you will post an updated photo of the amp. The photos in your posts #2 and #3 are very good quality but each photo shows only a portion of the circuitry. Just one new photo is requested. A photo that shows the entire view of the inside of the chassis. This will help the members see first hand the current state of the amp.
Sincerely,
Bill

[DrGonz78]

I think it would be good if you will post an updated photo of the amp.

Just looking over the original pictures it appears that turret board is not mounted to the chassis. My thought here is that the underside of the board might have the ability to short something out to the chassis. This might not be the case but since it does not appear to be mounted I felt it worth mentioning.

[J M Fahey]

Red/yelllow is connected to ground. If I connect Red/Green to diode and the grid resistors to power tubes I'll be able to read negative bias at pin socket. Right?

Yes ...if wires leading to the socket are also connected

[vinceg]

Goodmorning people!

I connected the bias winding and check for negative bias voltage at power tubes grid. The voltage is -44VDC each power tubes.

Before proceeding with the next step I want to point out that I replaced the rectifier valve with diode like picture below, in case this could be a problem for our work

[J M Fahey]

No problem.
Now you connect Red_T and Red_B to the rectifier, and turn amp pn.
Still with lamp limiter and remember to connect some speaker to the output.
All controls set to 0.
Do you get high voltage?
Check main filter caps, plates, screens, and follow the line which feeds the preamp tubes.

You will have less than normal, probably around 300V DC.

Post results.