4-4-0 [2 NEW BUILDS WITH PICS; THE 2 WATT WRECK]

[pompeiisneaks]

Okay. So i will put my red and blk probe and trim it untill
It reads 6.5 ma on the meter and repeat the same for the other trim pot

Am i right?

Thanks

Well technically you measure the Volts DC on the drop across the ground resistor, and get mV at about 6.5, Assuming the 1 ohm resistor with ohms law means 6.5mV = 6.5mA yes, do not put your meter in amps measurement, you need to use that a completely different way and it's more dangerous. Just make sure you're measuring the voltage drop not amps directly.

The idea you've stated is right, get each to about the exact same current.

~Phil

[mehfuzhoss]

Thanks.

[mehfuzhoss]

i have reading 19ma per side. which is a little higher than 6.5ma. amp sounds good to my ear. is 19ma too high for the tube? the trim pot wipe runs out and i can't lower it any further. please advise.

[pompeiisneaks]

i have reading 19ma per side. which is a little higher than 6.5ma. amp sounds good to my ear. is 19ma too high for the tube? the trim pot wipe runs out and i can't lower it any further. please advise.

The datasheet maximum was 5 watts, 6.5ma is 75% of that or

X * .75 = 6.5

that means that max current should never exceed 100% or

6.5/.75 = 8.667 mA is 100% dissipation,

to figure out your diss, you have

8.667mA * Y = 19mA

Y = 19/8.667 = 2.19 or 219% dissipation, seems a bit hot to me, yes. Are the tubes redplating yet?

To get it lower, you need to add more resistance to the bias circuit so that you can slow current down more, you'd need to increase the resistance to lower the voltage, I 'think' that would be the 15k 1W to lower it down. Also check the bias voltage coming out of the wiper on the pot, it shows in the schematic that it should be around -12.2 to -12.3VDC if it's not, then you need to drop it down to that range.

~Phil

[mehfuzhoss]

i am using a 6sn7gtb --> 12AY7-->12AX7-->12AX7

all components are as per the ax84 layout. bias reading is reflected on two pics for both sides. the amp is very very loud and it doesn't distort untill 80% volume. i would like it to break up early lets say at volume 3/4. is this do able? if so, please guide me on that.

as for bias current what can I do, and redplating is not happening as of yet. i have played amp for 2/3 hours after built.

[mehfuzhoss]

to check the plate volatge of the 6sn. i set my meter to read DC high volatage set to 500, power up amp with spkr connected, sit for few minutes, then black probe to ground and red probe to pin 3 of that tube. i have no reading and showing all 000. does it mean i need a new DMM?

[mehfuzhoss]

on pin 2 i get 277 VDC the blue wire from PT. pin 3 is connected to the bias test probe red.

[pompeiisneaks]

It is possible your multimeter is malfunctioning, or your leads themselves aren't making good seat in the meter. 000 means that either the voltage reading is not in the range of that part of the meter (per that picture somewhere between 200-500VDC which seems impossible) or your amp wouldn't be working. The negative bias readings of -20.1 and -19.4 means that you're definitely not in the range you'd want.

I think, as I recall, a higher negative bias voltage, the less current it allows to flow, so this means you've set the bias a touch colder than the expected values. BUT since you're reading completely differnet values on the 1 ohm cathode resistors, it begs the question again if the DMM isn't bad?

It depends on how much you can spend on a DMM, but the Aneng an8008 gets rave reviews and costs about 25$ mostly around here. Shoot it's even lower now on banggood:

https://www.banggood.com/ANENG-AN8008-T ... rehouse=CN

If you can afford it, I recommend stepping up to the Fluke 117 as i own it and it's been outstanding for me. It's available on amazon, and most other usual places but does cost about 180 bucks.

~Phil

[sluckey]

to check the plate volatge of the 6sn. i set my meter to read DC high volatage set to 500, power up amp with spkr connected, sit for few minutes, then black probe to ground and red probe to pin 3 of that tube. i have no reading and showing all 000. does it mean i need a new DMM?

Not at all. Pin 3 is the cathode and will only have a few millivolts, same as your bias test points. But you have your meter set to read BIG volts. A reading of 000 is no surprise.

[pompeiisneaks]

Oh yeah I should have thought to check that, the pinout can be found on a datasheet, the plates are on pins 2 and 5, see here:

http://www.radiostation.ru/tubes/6SN7.pdf

I forgot it was dual triodes and thought your 377VDC was the screens, but they don't have them on this amp

~Phil

[mehfuzhoss]

back to Phill plz. I connected 12K ( since i don't have 15K ) in series for both the trim pot. so all together I now i have 68K + 12K on both pots. readings are now as follows: -

VR1 : -

lug 1 -10.56
lug 2 -5.90
lug 3 -5.89

VR2: -

lug 1 -10.56
lug 2 -5.88
lug 3 -5.87

bias reading has not changed and still sitting at 19 for both sides. pic attached for resistor added. the red arrow showing where i added the 12K resistor in series. plz advise.

[pompeiisneaks]

That type of resistor in series won't impact the bias at all, it makes the pot work differently, you need to swap out the resistor I mentioned that's in circuit so it adjusts the negative bias. does the bias pot take it farther down to negative numbers like -24 or something? It's the dropper resistor that's needing adjustment so it makes the voltage change up or down, the pot just allows the up/down to be adjustable


~Phil

[mehfuzhoss]

i will use a 50k pot for both trim pots. how about that? will it work then!

[mehfuzhoss]

before the trim pots were 25KB each

[pompeiisneaks]

In theory that should work, if the amp needs more range and not less (i.e. less voltage drop) I can't remember off the top of my head which it is. You usually want the bulk of the voltage drop happening in the fixed resistors and have a bit of variability in the bias pot so people cant as easily overshoot massively to lead to runaway, but it's your call.

~Phil