[runeight]

Possibly, but not necessarily. Can you please report the other voltages at the test points. Thanks.

 

[wiatrob]

What Runeight said. Please post your measurements. It's possible that some of your tunbes mught be bad, but unlikely ALL are...
I had a similar problem on one of my builds and it turned out to be a cold solder joint in one of the Resistors near the tube.

Tripping e12 is normal when measuring some of the test points.

 

[lacrossebowe8]

Here are my test voltages:

TL+ 14.9V
TL- -14.95V
TH -12.56
TB+ 107.5 (with tubes in place)

I have 216 mV through both R10 L and R

TB+L 81.4V
TB+R 81.4 V

Both R6L and R6R are at 400 mV

For the next part I will discuss measurements with specific tubes.

Beckman 5963
TPL 76v
TPR 47.3v

JAN 12AT7
TPL 48.6V
TPR 81.7

If I switch the tubes
TPL 16.5
TPR 47.5

JAN 6189
TPL 1
TPR 1

If I switch
TPL 3.3
TPR 3.6

I will let it warm up for a while with these tubes to see what happens.

 

[runeight]

lacrossebow, my first thought is that Q1P,Q2P need to be replaced. I hate to ask you to do this, but I don't think we can be sure about anything else until we are sure about these devices.

Then, if you can, try to find tubes that you know are working and insert those. We can work from there.

 

[holland]

Runeight, do you mean Q1L, Q2L, Q1R and Q2R instead of Q1P and Q2P?

 

[runeight]

Not yet. It looks like the B+ for each side is pulling down. It's hard to say exactly what the cause is, but we first need to know that the final cap multipliers are ok. And if not, then we can move on to the other devices.

However, I'm not married to this approach. What are your thoughts?

 

[holland]

I see. I understood. B+ to be steady at 107.5V, and TB+L and TB+R to be steady at ~81.5V but TPL and TPR is having a larger drop than expected. My first thoughts lead to the current mirror as TB+L and TB+R are after the multiplier.

Some other thoughts. If TB+L and TB+R were purposefully set to 80V and not the max voltage, that's OK. If it's not then there may be some drop due to current flow through the multiplier (more current = more drop). That may also explain the large drop through (TB+L)-TPR and (TB+R)-TPR. Easy way to tell is to measure TB+L and then measure the output of the TL783 to get the drop across the cap multiplier. It should be a few volts at most. If it's not, then it's Q1P and Q2P.

After verifying that, measure the tail current to make sure it's steady, if so, it's definitely the transistors in the mirror or the tube(s). It's unlikely it's all the tubes that were tried, so I would tend to think the transistors.

If it's not steady, that may explain the variances through the HV chain due to current draw.

 

[runeight]

OK. I see what you mean.

The next step then would be to measure the CE drop across Q1P and Q2P to see if these transistors are good.

Lacrossbow, try this measurement next before replacing these two devices. Let's see what condition they are in.

Thanks Holland.

 

[lacrossebowe8]

I have set the TB+L and TB+R to around 80V. I am on vacation now so I will have to do this on friday when I get back. I will get my hands on some new tubes also.

Thanks for the help.

 

[lacrossebowe8]

How would I measure the CE drop across Q1P and Q2P?

 

[runeight]

Measure between the two outside pins.

 

[holland]

^what runeight said. If you can't reach you can measure the output of the TL783 (middle pin) and the test points such as TB+L and TB+R. If I recall the circuit properly, that is effectively the same.

 

[lacrossebowe8]

I got the tubes I have tested and they tested ok. I measured the voltage drop across Q1P and Q2P and found it to be:

Q1P about 1V
Q2P also 1V

I measured using both methods, using HV- and across Q1P and Q2P

What is the next step? Thanks.

 

[runeight]

Assuming that the tail current is set correctly, the next step is to replace Q1 and Q2 on both channels. And verify that R4 is 18k.

 

[lacrossebowe8]

I will need to order the parts to replace them. What else can I do/ what else should I order to ensure that I can fix the amp? I don't want to have to order 5 $3 orders and would rather order too many parts than too few.