K-MOD101 Kit DYI Amp -40% Voltage at V1 plate (12AX7 pin 6)

[TelRay]

hi guys,
a friend built this amp and asked me to check it out after he finished it.
I've found most voltages to be OK with the exception of the second stage of V1, which has a -40% lower plate voltage and (maybe a related issue) 1.9 VDC going into the grid
Reference voltages are in light RED and what I measured or checked either have a RED (if value is out of spec) or GREEN background (if OK).
I instinctively checked both 100K resistors and they are OK. Any ideas?



LINK TO FULL SCHEMATIC

thx!!!

[glebert]

How can pin 3 and 8 have different voltages if they are directly connected? As far as the 2V grid voltage, maybe a leaky cap? Did you measure voltages with the tube out?

[Enzo]

If pin 7 has zero volts with no tube and +2v with a tube, then that sounds like a bad tube to me.

And as glebert said, a leaky cap in the tone stack could put +2v there.

[TelRay]

hi guys and thank you for the replies,

not sure what happened with the readings on PINs 3 and 8. These are my notes from a couple of weeks ago. Maybe misread, typo, or taken in different times with different wall voltage (don’t have my variable transformer available these days)

will definitely check the caps and readings with no tube in the socket for the +2VDC

what do you think about that low plate voltage (-40%)?

[glebert]

what do you think about that low plate voltage (-40%)?

With 2V on the grid I think you are close to saturation for the tube (which would happen at 0V diff between grid and cathode) so it will be drawing high current, so it makes sense that there is much more drop across the plate resistor.

One thing that confuses me though is that the voltage at pin 1 is also low, even though the node voltage at E is actually higher than spec. The excess current going though the second triode should be elevating the cathode voltage for both triodes, which seems like it should reduce the current through the first stage, not raise it. Maybe I am thinking about it wrong, tube theory is not my strong suit.

[Enzo]

A positive on the grid will cause the tube to conduct hard which will drag the plate voltage down. Fix the +2 on the grid and your plate will likely rebound just fine.

[TelRay]

A positive on the grid will cause the tube to conduct hard which will drag the plate voltage down. Fix the +2 on the grid and your plate will likely rebound just fine.

loud and clear guys, will look into it this week
thx!

[TelRay]

well... did some more research from the owner / builder and he told me the VOLUME knob was not very responsive so I started doubting wiring in the TONE STACK / VOLUME section, which is something I had not still checked (started from the POWER supply)

and... there was a mistake. the wire coming from the first half of V1's plate and going into the second half's GRID (through the TONE STACK and VOLUME) was completely bypassing all the caps... hence no surprise there was +1.9VDC at V1 pin 7 (GRID)



as predicted by the guys in the posts above, now that this has been corrected I have 0 V at the GRID and the PLATE voltage at pin 6 has bumped to 320 V (spot on)

further than the wiring error, I learned about the big influence of having a couple of volts at the grid can have dragging as much of 40% of the plate voltage down. very interesting!

so... that topic solved (thank you guys!). we can jump to the next.

my friend has sent me this amp fitted with 2x EL34s as POWER TUBES, is that right? the schematic shows 6L6s and I do not see any modification in the power amp section that would justify using any other type of tubes. my understanding is that those two type of tubes are not interchangeable. any thoughts?

thx!

[glebert]

and... there was a mistake. the wire coming from the first half of V1's plate and going into the second half's GRID (through the TONE STACK and VOLUME) was completely bypassing all the caps... hence no surprise there was +1.9VDC at V1 pin 7 (GRID)

That also explains the low voltage at pin 1.

my friend has sent me this amp fitted with 2x EL34s as POWER TUBES, is that right? the schematic shows 6L6s and I do not see any modification in the power amp section that would justify using any other type of tubes. my understanding is that those two type of tubes are not interchangeable. any thoughts?

thx!

If he designed it to have EL-34 it should be OK, the interchangeability issue AFAIK has to do with pin 1, which is N/C on 6L6 (but some people use as an extra connection point for other things) but is a suppressor grid on EL-34. Also the biasing might be a bit different (although a good bit of overlap), and the output impedance I think. I have amps that are designed to use either.

[TelRay]

thank you!

i have my doubts because in the schematic I only see 6L6s specified and I've seen no modifications in the amp that would justify the introduction of EL34s

https://www.modkitsdiy.com/sites/mod..._schematic.pdf

[g1]

What is pin1 of the power tube sockets connected to?
How does the voltage shown at power tube cathodes compare?

[TelRay]

PIN 1 on both POWER TUBES is bridged to PIN 8 and go to ground through a 1 Ohm resistor

additionally, I've found that each GRID (PIN 4) goes to ground through a huge 1K Ohm 5W resistor instead of the (1/2 or 1W) 470 Ohm resistor shown in the schematic (my friend is trying to recall why he used the 1K ones)

[The Dude]

1K screen resistors are more commonly used on EL34's and 470 ohm on 6L6's. Since you have EL34's in your amp......

[TelRay]

1K screen resistors are more commonly used on EL34's and 470 ohm on 6L6's. Since you have EL34's in your amp......

i suspected the change of 470 Ohm for 1K resistors could have been my friend's attempt to adapt the original design (for 6L6s) to be able to use the EL34s (I asked by he did not remember)

So, I can plug this thing into a speaker and a guitar safely now to give it a try? heheheheh...

thx!

[The Dude]

You'll want to double check the bias since the amp was designed for 6L6's and you have EL34's in there.