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

[glebert]

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

I always get frustrated trying to figure out reasonable bias voltages because everyone says "you don't set a voltage, you set the bias based on current" and won't even give a range or a starting point. I think if you start with a bias of something like -60V you will be safe for either type of tube, but perhaps very cold, but I am happy to be corrected.

[Enzo]

Pins 4 of the power tubes are SCREEN grids. And I kinda think those resistors are not going to ground, but to B+ instead.

Pin 5 is the CONTROL grid.

[Enzo]

I always get frustrated trying to figure out reasonable bias voltages because everyone says "you don't set a voltage, you set the bias based on current" and won't even give a range or a starting point. I think if you start with a bias of something like -60V you will be safe for either type of tube, but perhaps very cold, but I am happy to be corrected.

Oh in general, as we use them, 6L6 use something in the -55v area, while EL34 use more like -40-45v.


We don't specify certain voltage because all tubes are different. But unless they are odd, they will tend to fall into a range. Likewise, I can't specify a particular current as we don't know what the plate voltages will be .


Some OEMs will specify a bias VOLTAGE, because they know that most tubes will operate at reasonable range at that voltage.

[TelRay]

Pins 4 of the power tubes are SCREEN grids. And I kinda think those resistors are not going to ground, but to B+ instead.

Pin 5 is the CONTROL grid.

you're totally right! my mistake, those (470 Ohm in theory and 1 K Ohm in this case) resistors are placed between PIN 4 and point C (+478 VDC)

[TelRay]

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

yepp... will BIAS it taking into consideration that the MAX PLATE DISSIPATION of EL34s is 25W and be conservatively below that

[g1]

I always get frustrated trying to figure out reasonable bias voltages because everyone says "you don't set a voltage, you set the bias based on current" and won't even give a range or a starting point. I think if you start with a bias of something like -60V you will be safe for either type of tube, but perhaps very cold, but I am happy to be corrected.

In this case he has cathode resistors, which is why I asked for the voltage at the power tube cathodes. Then idle dissipation can be quickly calculated, and we can either say 'bias voltage needs to go more negative' etc., without specifying numbers.

I do understand what you are asking though, and I usually look at equivalent established circuits for ballpark numbers.
Your suggestion of -60V for a safe start point also seems reasonable (for typical grid drive class AB).

[TelRay]

In this case he has cathode resistors, which is why I asked for the voltage at the power tube cathodes. Then idle dissipation can be quickly calculated, and we can either say 'bias voltage needs to go more negative' etc.

sorry i missed this bit
if i understand correctly what you are suggesting is that I measure VOLTAGE from PIN 1 (or 8 as rhey are bridged) to GROUND and having a 1 Ohm Resistor my X mV readings will be equivalent to X mA
then adjust the BIAS knob until I am around 35 mA in each tube
is that right?
any further calculations necessary (using plate voltage, etc)?

[Helmholtz]

then adjust the BIAS knob until I am around 35 mA in each tube

THe schematic shows 1.2R (not 1.0R) cathode resistors. So 35mV corresponds to only 29mA cathode current.

[TelRay]

THe schematic shows 1.2R (not 1.0R) cathode resistors. So 35mV corresponds to only 29mA cathode current.

you are (as always) totally right
I forgot to mention that the actual resistors in the amp are 1 Ohm (BROWN, BLACK, GOLDEN stripes)
the coldest BIAS voltage is -48 VDC. I got 35 mV DC at both tubes with -37 V.
Plate Voltage was 482 VDC
is that OK?

[Helmholtz]

you are (as always) totally right
I forgot to mention that the actual resistors in the amp are 1 Ohm (BROWN, BLACK, GOLDEN stripes)
the coldest BIAS voltage is -48 VDC. I got 35 mV DC at both tubes with -37 V.
Plate Voltage was 482 VDC
is that OK?

Means around 15W or 60% plate dissipation. Should be fine.

[TelRay]

Means around 15W or 60% plate dissipation. Should be fine.

thx!

[Enzo]

Why on earth would someone use 1.2 ohm resistors in that application? It isn't difficult to punch it into a calculator, but harder to do in your head.

[TelRay]

Why on earth would someone use 1.2 ohm resistors in that application? It isn't difficult to punch it into a calculator, but harder to do in your head.

hahahaha... totally true!
it might be a 20% safety margin for dummies

[TelRay]

maybe the last (hopefully the last!) thing to fix on this amp is the complaint from my friend that the VOLUME control is always too loud and does not go down fully even with the VOLUME KNOB at ZERO.

so, I went a bit deeper into the SYMPTOM:

- there is SOUND with the VOLUME KNOB at ZERO
- this SOUND is not affected by BASS or TREBLE
- going UP in VOLUME to something close to the "ONE" mark the SOUND starts to be affected by the TREBLE and BASS controls

what I've checked:

- completely removed the VOLUME POT from the circuit and measured 20 Ohm to 1,016 kOhm from the wiper to both HOT and COLD lugs (it is a 1M pot). So I think the range the pot is controlling is OK
- all connections between VOLUME, TREBLE, BASS are according to the schematics (though there's always the chance I've missed something)
- VOLUME pot correctly GROUNDED through the INPUT JACK

the only thing that seemed odd to me is that both the VOLUME and TREBLE pot housings showed continuity to GROUND but not the BASS pot. However I connected it to GROUND and there was still SOUND at 0 VOLUME



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

thanks for the help in advance!

[Enzo]

Probably crosstalk.