Low output on tmb ef86

[Bugman3183]

Hello all, I just recently built a tmb ef86 following a layout I found on the net and upon completion plugged into to tmb jack and got very low output. Next I used my dmm to take readings and noticed that I am get.ting nearly the full b+ of dc on the 6v6 grid.  upon  further investigation I also noticed that I don't get a reading at all at the bias node.  I then checked the ground side of the capacitor for continuity with my dmm and got a beep, then checked resistance from the ground side to the chassis which read .5 ohms.  Next I reflowed solder to the positive side and still reading.  I'm not really sure if the two are related but it seems to me that capacitor must be bad and wanted to get you guys opinion before I run off and order more parts.  Thanks.

[Bugman3183]

Oh yeah forgot to mention
 I'm using 6v6's instead of el84's

[tubeswell]

... very low output. ... I am get.ting nearly the full b+ of dc on the 6v6 grid.  upon  further investigation I also noticed that I don't get a reading at all at the bias node. 

Sounds like you've got the 6V6 wired up funny. Plate (connect to OT primary) = Pin 3, Screen = Pin 4, Grid = Pin 5, Cathode = Pin 8 (heaters = Pins 2 and 7)

I then checked the ground side of the capacitor for continuity with my dmm and got a beep,

Which capacitor are you talking about? The Cathode bypass cap going to the 6V6 cathode, or the coupling cap going to the 6V6 grid, or a filter cap (maybe) going to the 6V6 Screen? Need more information before you can be advised what the correct testing procedure is. Have you got a schematic you are working to? Can you post it here??

[Bugman3183]

Thanks for the reply, I was referring to the cathode bypass cap for the 6v6's.  As far as the layout I'm not sure how to post it but I'll take a look in the forum " how to" section to figure it out but its the ceriatone layout that I followed. As far the wiring on the 6v6's I downloaded the pinouts for the el84 and 6v6 and made changes accordingly per the layout. After re-checking connections it all line up with the pinout you just listed.  Thanks.

[tubeswell]

For a cathode bypass cap - with the cap in the circuit parallel to the resistor, you wont get an accurate DC resistance reading for the cathode resistor on your R-meter. It doesn't mean that anything is wrong with the cathode resistor (necessarily). If you measure about 19-20VDC or so at the cathode when the amp is on, then the cathode resistor is doing its job as intended. If you aren't measuring this ballpark voltage, then the cathode resistor may still be okay, it may just be that the tube is conducting way more current that it should be - you said you had full B+ on the 6V6 grid - that is wrong. The 6V6 grid (Pin 5) should be at 0VDC in a cathode-biased 6V6.

Or the cathode resistor may not be okay - but the way to measure it with your R-meter is to unsolder it from the cathode bypass cap first. You only need to unsolder the bypass cap at one end (either end will do) to check the cathode resistor's DC resistance properly.

[phsyconoodler]

If you don't get a voltage reading at pin 5 of the 6V6's then the cathode resistor/cap aren't grounded.
  DC volts here.
What resistor did you use?It should be about 250 ohms.

So you should get this type of voltage scenario
pin 2-3.2vAC
pin 3-near full DCV
Pin 4-near full DCV
Pin 5-about 16-20vDC
Pin 7-3.2c AC
 
Note: you should be using at least a 470 ohm screen resistor,not the 100 ohm that that 18 watters use.But thats not really an issue with low voltage 6V6's.They often run even higher screen voltage than the plates.

[Bugman3183]

Unfortunately I was unable to post the layout it saved to my phone as a jpeg file. You are correct, no voltage on pin 5.  The first thing I checked was the ground side of that capacitor for continuity and it seemed ok, but I guys I'll resolder and check again.  I used a 270 ohm 5 watt resistor for the 6v6 cathode but I did use 100 ohm for the screen.  Thanks.

[phsyconoodler]

ook at the attached layout.See where I circled in red?Make sure that point goes to ground.And make sure you actually used a 270 ohm and not a 270k cathode resistor.

[Bugman3183]

Thanks for the reply.  I just checked and I do have both of those areas connected to ground and appear to have continuity referenced to the chassis.  One other question that I have is that I recall reading somewhere that you can check ann electrolytic capacitor in circuit with a dmm by checking the resistance which should rise continually until it goes to infinity.  While trying this method inn the highest setting on my dmm, it never reached infinity so lowered the range and the resistance stopped at 274 ohms.  Does this mann that the 6v6 cathode bypass capacitor is bad, our is this not an accurate test?

[Bugman3183]

Sorry for the typos but I'm using a cell phone and this damn auto spell is killing me.  Lol.

[tubeswell]

So you should get this type of voltage scenario
pin 2-3.2vAC
pin 3-near full DCV
Pin 4-near full DCV
Pin 5-about 16-20vDC
Pin 7-3.2c AC
 

Of course you mean Pin 8 should be 16-20V
Pin 5 should be 0VDC in a cathode biased 6V6

[Bugman3183]

Hey guys, thanks for all the replies.  After reading all the responses it seems the best course if action would be to list all of my voltage readings so that you guys out there who know what they should be can point me in the right direction as far as what the problem could be. It's way past my bedtime and I'll post them tomorrow evening after work. I would just like to thank everyone for their
Input and advice. 

[phsyconoodler]

Yes,pin 8 should be 16-20v.Pin 5 is the control grid.Sorry!

[Bugman3183]

 orry for the delayed response, had a couple if really long work days.  finally took measurement s all with just the tube rectifier installed.


          V0         v1        v2        v3        v4+5
Pin:
    1)    247      462    450     450

     2)     0            0        0           0

     3)     0            0        0           0          499

     4)                                                         498

     5)                                                            0

     6)  405      462     450     493   

      7)    0           0          0        443

     8)    0            0         0          0               0


 Now after doing all this I'm wondering if these would've been more helpful if the tubes were installed.  Also I'd like to mention that I removed the cap and resistor from the standby switch for one reason or another but assumed they were there to resist popping noise when switching standby on, but figured I'd mention it just in case.  Any advice would greatly appreciated.  Thanks.

[tubeswell]

Appears from the number of pins on each socket that you don't have any noval sockets?

What type of tubes are in each socketZ?

And what voltages are the B+ reservoir cap node and each supply rail node?

[Bugman3183]

Thanks for the reply, I didn't post pin 9 but I should have specified that the heaters are within spec.  v0 is ef86 v1, 2& 3 are 12ax7 and 4+ 5 are 6v6's.  I'm currently not at home but when I get back I'll post the b+ and other voltages, but as I specified earlier in the thread I have 0 volts at the 6v6 cathode and bypass cap node.  Thanks.

[tubeswell]

You need to put the tubes in to take the voltages

[Bugman3183]

0k, here's the readings with tubes installed.


          V0       v1       v2       v3       v4+ 5

Pin
   1)   80       143    206    160

   2)   0          0         39       0

   3)   2          1         89       1       375

   4)                                                 367

   5)                                                 0.14

   6)  88        143    205    284

   7)  0           0         42      160

   8)  2            1         89     169      0

I also noticed that without a load all the heaters were right at 6.3vac for each tube(3.1+ 3.2) but loaded I got 49.3vac for each string on all the tubes.  Now I'm assuming that is because I created an artificial ct with two 100 ohm resistors and connected the junction to the positive side of the 6v6 cathode and bypass cap to elevate the heaters but I'm not certain that is the proper procedure.  I've got a b+ of 385, I'm also getting 385v at the first 32uf cap, 367v at the second one, 23v at the 6v6 cathode and bypass cap, 295v at the 16uf and 280v at the 22uf.  So that is all the info I have right now, if any more if needed I'll be than happy to oblige.  Thanks .

[tubeswell]

Can you please post the schematic you are working to as well?  That way we can see if the voltages make sense in terms of what to expect. TIA

[Bugman3183]

Unfortunately I didn't have a schematic I just used the layout that is posted earlier in thread.  After studying my previous post it seems that I may have a cold solder joint on pin 8 of v 5 since I have 23v on the 6v6 cathode node but nothing on pin 8 of the 6v6
So I'll check that out after work today.  Thanks.