Problem: Weber 5F6A with very low output!

[tubeswell]

Well okay that's got me stumped. I better go measure my 5G9 again.

[erndawg]

Okay so I did this with my 5G9 earlier to check this again.

With -23V on the grid, the tube current was 31mA and the plates were at 371VDC, with -27V on the grid, the tube current was 27mA (more negative bias) the plates were at 364VDC.

Either your measurements are wrong or your amp has a problem. Since this amp has a bias vary tremolo, there is a bias/tremolo interaction taking place. Measure your bias voltage using a scope, and observe the AC modulation, or disconnect the tremolo from the bias so you can see what is truly happening when you bias a tube.

[erndawg]

This surely must be because the increased permanent negative grid voltage is lowering the overall positive plate voltage, which in turn would reduce the current flowing from the cathode to the plate. (less voltage = less current for the same resistance)

Making the grid voltage more negative does not "lower the overall plate voltage"; it causes the plate voltage to *increase* because it reduces the current draw through the tube.

[erndawg]

Remember we are not talking about a pre-amp stage here. There is no large resistor loading the plate in an output stage. The output tube has a reactive load through the OT, which has very little DC resistance, so the quiescent plate voltage of an output tube is virtually at the same voltage as the B+. So to my way of thinking, the only other thing that can change the permanent idle voltage at the plate is another permanent negative voltage applied between the cathode and the plate. (And the bias voltage is drawn from the PT in the same way that the B+ voltage is - so they are both contraposed to one another so to speak) Anyway that's my take on it.

Preamp stage or power amp stage - they are fundamentally the same. You *do* have a large plate load AC resistance (aka reactance) of many k ohms on the plate of each power tube.

Consider this: If you biased a preamp tube - either grid biased or cathode biased - towards its cutoff region, you would see virtually no voltage drop across the 100k (or whatever value) plate resistor.
And again, your assumption that you can just add/subtract voltages as you have been stating is just simply not how tubes work.

[tubeswell]

My apologies erndawg. I was wrong and you and JoeM were right.

I must've recorded the voltages the wrong way. I checked it again scrupulously this afternoon and got the result you both predicted.

So convinced was I in my erroneous observations, that I convinced myself with a load of bullshit

More fool me I suppose.

When I zapped myself last year, it must've done more subtle damage than I realised. I'll have to concentrate more before I put my mouth into gear.

[jibe]

PT specs can be found here: http://hem.bredband.net/b493830/022798eusch.jpg

I'll do new measurements during the week, didn't get a chans yesterday.
Before we fixed the short, I had 750AC (loaded, standby on) between
pins 4 and 6 on the rectifier. That'll make it 375 from ground and that's
in the range of what I measured the other day, think I remember it as
360AC (also loaded) from ground, didn't take notice more than leaving it
when I saw what I expected to see.

I have to install some kind of light in the cabin and take new measurements
during this week (only worked during the day before, we don't have light inside).
I'll get back with readings of the PT and before the bias circuit, in loaded and
unloaded state.




Thanks for bearing with me! This is well on the way to being a marathon thread.



//Jibe

[erndawg]

No problem tubeswell; just want to make sure we're all on the same page regarding advice on fixing this amp.

[jibe]

New update:


In standby from ground:
Before the diod on the bias circuit, -42VAC, drops to -40 when i flick the standby.
After, -57.4VDC.
-44.1 After the 15k Resistor.
Pin5 output -43.5

338VAC on pin 4 & 6 of recifier, drops to 330 when I flick the standby.
I have 228VAC in on the fuse (before and after) and I'm using the gray
240V on the PT.

B+ (A on the layout) is at 423V
B = 422V
C = 384V

I confirmed that the 15k in the bias circuit is in fact a 15k.
Also the two 220k before pin5 are correct values.

What do we make of this?
Seems to drop a few volts when hitting the standby switch.
ACs are on the low side? Should be around 360 on pin 4 & 6 on the rectifier
and before the diod in the bias circuit I should have 45AC, all according to specs.

I tried to remove one side on my 220k bleeders, cap + side, nothing changed,
but I did notice that my power caps drains without bleeders in just about the same time??. Is that supposed to happen?

Bias pot:
Bias - 1 ohm cathode - B+
-40V, 43mV, 419V MIN
-47V, 26.2mV, 434V MAX
-42V, 35mV, 423V BIAS





//Jibe

[MWJB]

Connect your meter set to mA <200mA accross the standby switch terminals, plug in the amp & throw the power switch. Now ALL current in the B+ supply is going through the meter, what do you get?

Should be plate current (x2), some screen current (<10mA x2), and then 1-1.5mA per 12AX7 triode (x3)...

So if you had 35mA per tube at idle, add another 6 or 7mA per power tube of screen current, plus 6-9mA for the preamp tubes, you might expect to see 90-100mA-ish.

This will hopefully tell us if something is loading down the B+.

Don't waste your time measuring "before & after standby" voltages, I'd expect to see a drop after the switch was thown.

It's a long thread, so forgive me if we've asked this before, what are your AC heater voltages?

[jibe]

Should the screen current be around 6-7 mA? According to my readings last time I had
1.4 mA. See my bias calculations on page 5.

I did check the heaters before we did the bypass of my RCA jack, everthing was fine then. We had 6.5VAC and 5.2VAC on the heaters, today I checked the heaters on the rectifier and it was 5.2VAC. I can't find anything on the output and preamp heaters in my notes but I remember it as no change from before.
I'll double check tomorrow when I try the current measurements.




//Jibe

[Diablo]

338VAC on pin 4 & 6 of recifier, drops to 330 when I flick the standby.
I have 228VAC in on the fuse (before and after) and I'm using the gray
240V on the PT.

//Jibe

Hmm. Perhaps you need to use the blue (220V) primary wire for your PT? It seems like you have low voltage everywhere on the secondary side including the bias. Before doing this, I'd wait for MWJB to weigh in on. I trust his advice more than my own.

[MWJB]

With 6.5 VAC on the heaters I wouldn't try a lower AC primary tap, it'll probably shoot up over 7VAC.

Jibe, you screens shouldn't be more that 6-7mA tops, say you had a 2v drop accross the 470ohm screen grid resistor that would be 4mA (might even be less). We're trying to establish whether you have a significant draw on the power supply, so were looking for large currentrs that we can't explain by the standard current draw.

The thread is too long & sprawling now for me to be checking back to old posts, please quote pertinent info in current posts.

...so, what current did you get accross the standby?

[jibe]

New measurements:

35.1v over Cathode resistor nr 1.
36.6 over nr 2.

0.670v over 470 screen resistor nr 1,
0.689 over nr 2 gives 1.43 mA on nr1 screen, 1.47mA on nr 2.

35.1 - 1.43 = 33.67
36.6 - 1.47 = 35.13

Plate voltage today: 432v gives Plate dissipation on nr 1 14.55 and on nr 2 15.18

bias voltage = -39.
b+: 432v.
Turning my 50k bias pot to the max doesn't give me more than 442v on the plate.

Above is the bias readings I did the other day.
My voltage drop over the 470ohm grid resistor is way less than 2V.
I don't know if that is significant to this problem, but there it is.

I haven't been able to do the current readings yet, I'll try to get that done during the day.



//Jibe

[jibe]

Connect your meter set to mA <200mA accross the standby switch terminals, plug in the amp & throw the power switch. Now ALL current in the B+ supply is going through the meter, what do you get?

Should be plate current (x2), some screen current (<10mA x2), and then 1-1.5mA per 12AX7 triode (x3)...

So if you had 35mA per tube at idle, add another 6 or 7mA per power tube of screen current, plus 6-9mA for the preamp tubes, you might expect to see 90-100mA-ish.

I have 4 places for the probes to be connected on my meter.
10A, mA, Ohm and of course a Common.

When probe is in 10A, power on, standby off, 200mA set on the meter I get 00.8 across the standby switch.
When probe is in mA and same settings I get nothing.

I don't know if my readings are correct.

I also double checked the heaters, it shows 2.95VAC from ground.




//Jibe

[MWJB]

Check the mA fuse in your meter and try again, use the mA scale.

Measure the heaters from one leg to the other, e.g. pin 2 of a 6L6 to pin 7.