It worked!
But no guitar sound yet. Can't get more than 9 mA measuring from cathode to ground, over a 1ohm resistor.
Got to check the bias voltages again.
Too much hum too... got to do something about that.
Tommy
Fired it up finally!!
Moderators: pompeiisneaks, Colossal
Re: Fired it up finally!!
Changed the bias resistors, and it helped. Still couldn't get more than 15mA between cathode and ground. But I should get some sound from it still, shouldn't I
Did some quick measures with everything in place:
All plates on V1 and V2 were about 205-211 V, that's not too bad I think.
PI plates were right under 300V
Output tube plates were about 404V.
(25/400)*0.7 gives me roughly 43mA, so I still don't have half the current needed. But should I get some sound when I plug in the guitar, having only 15mA on the output tubes?
Way too much hum...I wonder how I'm going to deal with that..
Tommy
Did some quick measures with everything in place:
All plates on V1 and V2 were about 205-211 V, that's not too bad I think.
PI plates were right under 300V
Output tube plates were about 404V.
(25/400)*0.7 gives me roughly 43mA, so I still don't have half the current needed. But should I get some sound when I plug in the guitar, having only 15mA on the output tubes?
Way too much hum...I wonder how I'm going to deal with that..
Tommy
Re: Fired it up finally!!
When measuring across the 1 Ohm resistor, you should be reading in millivolts, not milliamps. Because of the 1R the conversion is 1:1. (Just checking to make sure you have your meter on the correct scale.)
I'm not sure but I think you have to double the value of the pots and tail resistors when you're running dual bias because, in a sense, you have parallel resistors in the circuit. IOW, you may need 20k pots with 54k tails to get the same bias control as a single 10k pot with a 27k tail. Hopefully a real guru will drop into this thread.
I'm not sure but I think you have to double the value of the pots and tail resistors when you're running dual bias because, in a sense, you have parallel resistors in the circuit. IOW, you may need 20k pots with 54k tails to get the same bias control as a single 10k pot with a 27k tail. Hopefully a real guru will drop into this thread.
Re: Fired it up finally!!
For the 'no sound' problem you can 'tap' the plates of the tubes with the (red) tip of your DMM probe whilst you have it out taking voltage readings.
You should be able to hear some sort of noise from that tube, if not that's where you problem is. If you're using newer ceramic tube bases make sure the pins of the tubes are making good 'contact' with the base. Some new tube pins are thinner now days. Just had problems myself with a new build, K pin on the 1st preamp tube not making good contact causeing 'no sound'.
You should be able to hear some sort of noise from that tube, if not that's where you problem is. If you're using newer ceramic tube bases make sure the pins of the tubes are making good 'contact' with the base. Some new tube pins are thinner now days. Just had problems myself with a new build, K pin on the 1st preamp tube not making good contact causeing 'no sound'.
Re: Fired it up finally!!
What kind of amp are we talking about here? Dual bias shouldn't matter in regards to the bias voltage available from the bias supply as far as resistor values go. It only lets you match the tubes a little better if the output tubes aren't matched. Do you have a 6v pilot lamp or a 110v pilot lamp? If your pilot lamp goes out with the tubes in and you get a LOUD hum then you may have a bad tube or there is something shorted in you heater circuit. A loud hum can also be created if you have too much current flowing through your output tubes which would point to a shorted output tube or a no bias condition on that tube.
Re: Fired it up finally!!
No change in values needed. You're just supplying a potential, no current.Normster wrote:I'm not sure but I think you have to double the value of the pots and tail resistors when you're running dual bias because, in a sense, you have parallel resistors in the circuit. IOW, you may need 20k pots with 54k tails to get the same bias control as a single 10k pot with a 27k tail. Hopefully a real guru will drop into this thread.
Re: Fired it up finally!!
The problem was the secondary on the OT. Switched them and it stopped screaming.. I had positive global feedback, not negative.
I ended up using 56K tail resistors on the bias pots to get a voltage about -50. Don't know if it's because I have two pots or not, but 15K didn't give me bias voltage close to what I need.
The pilot lamp etc. is not a problem anymore.
As for the bias measure, I did a mistake. It was late at night, and I put my meter on mA when I measured.. that's why I got only about 8mA... I realized this later, and measured using mV ang the lowest bias I could get was 50mV=50mA. This is way too high, and is caused by the tail resistors that I changed from 56K to 24K. I'll put the 56K back in, and measure again.
Besides from the no guitar sound, hum seems to be the biggest problem. Really bad hum, even on lov volues, and if I turn the drive/level pots, even worse.
Here is how I grounded:
Long bus running along the pots:
All pot grounds go here, all cathode resistors/bypass caps, ground on clean/OD switch, input jack. The end of this bar is grounded at the power transformer mounting screw.
Short bus running along the power supply:
All filter caps (except first pair) is grounded along this. The end of the bar is grounded at the same place as the one above.
The 6.3V center tap and the high voltage center tap, are both soldered to the negative side of the first filter cap, which is then connected to the same screw as the two bars above.
The speaker jack, the bias cap, and the 1ohm cathode resistors, have a wire of ther own, running to the same point as all the ones above.
I'm afraid that the two center taps solders aren't as good as they should be. But are there anything in this grouning scheme that is all wrong?
I'll post some pictures soon.
Tommy
I ended up using 56K tail resistors on the bias pots to get a voltage about -50. Don't know if it's because I have two pots or not, but 15K didn't give me bias voltage close to what I need.
The pilot lamp etc. is not a problem anymore.
As for the bias measure, I did a mistake. It was late at night, and I put my meter on mA when I measured.. that's why I got only about 8mA... I realized this later, and measured using mV ang the lowest bias I could get was 50mV=50mA. This is way too high, and is caused by the tail resistors that I changed from 56K to 24K. I'll put the 56K back in, and measure again.
Besides from the no guitar sound, hum seems to be the biggest problem. Really bad hum, even on lov volues, and if I turn the drive/level pots, even worse.
Here is how I grounded:
Long bus running along the pots:
All pot grounds go here, all cathode resistors/bypass caps, ground on clean/OD switch, input jack. The end of this bar is grounded at the power transformer mounting screw.
Short bus running along the power supply:
All filter caps (except first pair) is grounded along this. The end of the bar is grounded at the same place as the one above.
The 6.3V center tap and the high voltage center tap, are both soldered to the negative side of the first filter cap, which is then connected to the same screw as the two bars above.
The speaker jack, the bias cap, and the 1ohm cathode resistors, have a wire of ther own, running to the same point as all the ones above.
I'm afraid that the two center taps solders aren't as good as they should be. But are there anything in this grouning scheme that is all wrong?
I'll post some pictures soon.
Tommy
Re: Fired it up finally!!
Got the bias rigt, and adjusted it to 40mA. Still much hum, but not as bad as yesterday.
Around 230V on all preamp tubes (that's too high on V1..right?). About 1.7V on all cathodes on the preamp tubes.
I managed to get some guitar sound out of it. I turned it up quite high, and there was a faint, almost sitar sounding, guitarsound..
I think the glow from the preamp tubes and PI tube is very weak. Even in total darkness, it's almost hard to see the glow.. is that normal? Could there be a problem with the heaters?
I run the 6.3V wires from the PT to the first 6L6. Then I use 18gage wire from there on to the next 6L6, and further on to the 3 12AX7 tubes.. Could too thin wire give too low heat, or will it heat normally, but leave the wires hot, if there is too much current going through them?
Tommy
Around 230V on all preamp tubes (that's too high on V1..right?). About 1.7V on all cathodes on the preamp tubes.
I managed to get some guitar sound out of it. I turned it up quite high, and there was a faint, almost sitar sounding, guitarsound..
I think the glow from the preamp tubes and PI tube is very weak. Even in total darkness, it's almost hard to see the glow.. is that normal? Could there be a problem with the heaters?
I run the 6.3V wires from the PT to the first 6L6. Then I use 18gage wire from there on to the next 6L6, and further on to the 3 12AX7 tubes.. Could too thin wire give too low heat, or will it heat normally, but leave the wires hot, if there is too much current going through them?
Tommy
Re: Fired it up finally!!
18gage wire should be fine for heater string.
I'm not a big fan of star grounding, I would try grounding the 'Long bus running along the pots' near the input jack and see if that helps. Others will chime in here. I'd make sure those CT groundings are as good as possible.
Some 12ax7s glow brighter than others (or maybe just easier to see) depending on brand , Check with your DMM and make sure you have 6.3v on the heaters. Tubes can also cause hum and noise, try swapping.
Do you have the Jfet circuit connected? if not you'll need a 150k resister to ground on the end of the dropping resistor string to bring the voltages down another 20v.
I'm not a big fan of star grounding, I would try grounding the 'Long bus running along the pots' near the input jack and see if that helps. Others will chime in here. I'd make sure those CT groundings are as good as possible.
Some 12ax7s glow brighter than others (or maybe just easier to see) depending on brand , Check with your DMM and make sure you have 6.3v on the heaters. Tubes can also cause hum and noise, try swapping.
Do you have the Jfet circuit connected? if not you'll need a 150k resister to ground on the end of the dropping resistor string to bring the voltages down another 20v.
Re: Fired it up finally!!
will measure the heaters, but as long as the cathode voltage is ok, it shold be ok I'd think.
I'll also put in the 150K to drop the voltages a bit.
I'll also do something about the grounding...bug I'd love to get some sound first...
Tommy
I'll also put in the 150K to drop the voltages a bit.
I'll also do something about the grounding...bug I'd love to get some sound first...
Tommy
Re: Fired it up finally!!
Sound problem could be alot of things. You could be losing signal to ground somewhere. Try making up a test probe and trace the signal.
A wood chopstick, .047uf/630v cap, 2 x length of wire (4-5 foot long), 1/4" jack. Solder one end of cap to wire and insulate( use tape or heatshrink), solder other end of wire to the Hot of the 1/4" jack. Tape the cap to the chopstick so that the unsoldered end of the cap sticks out 1/2" or so. Solder one end of the other length of wire to the nuetral(cold) lug of the 1/4" jack and solder the bare end of that wire to a alligator clip.
Your test probe is now complete. The 1/4" jack gets plugged into another amp to amplifier the signal as you check.
you'll need a signal source (cd player, radio) with 1/4" jack to plug in the input of your 'D' clone.
Plug your signal source ( cd player) into the "D" clone, the other end of the test probe into your reference amp, connect the alligator clip to the chassis of the 'D' clone, now trace the signal with the probe starting at the input though the circuit and see if you can hear where the signal losses strength. Might help, might not.
A wood chopstick, .047uf/630v cap, 2 x length of wire (4-5 foot long), 1/4" jack. Solder one end of cap to wire and insulate( use tape or heatshrink), solder other end of wire to the Hot of the 1/4" jack. Tape the cap to the chopstick so that the unsoldered end of the cap sticks out 1/2" or so. Solder one end of the other length of wire to the nuetral(cold) lug of the 1/4" jack and solder the bare end of that wire to a alligator clip.
Your test probe is now complete. The 1/4" jack gets plugged into another amp to amplifier the signal as you check.
you'll need a signal source (cd player, radio) with 1/4" jack to plug in the input of your 'D' clone.
Plug your signal source ( cd player) into the "D" clone, the other end of the test probe into your reference amp, connect the alligator clip to the chassis of the 'D' clone, now trace the signal with the probe starting at the input though the circuit and see if you can hear where the signal losses strength. Might help, might not.
Re: Fired it up finally!!
Here is what I measured after I put in the 150K at the last filter.
V1:
Pin1 - 198V
Pin3 - 1.68V
Pin6 - 203V
Pin8 - 1.57V
V2:
Pin1 - 203V
Pin3 - 1.63V
Pin6 - 204V
Pin8 - 1.59V
PI:
Pin1 - 315V
pin6 - 308V
6L6 #1
Plate - 441V
Pin5 - -44V
6L6 #2
Plate - 440V
Pin5 - -44V
B+1 - 442V
B+2 - 440V
B+3 - 432V
B+4 - 315V
B+5 - 311V
Bias is set at 40mA
All heaters measured about 6.0V AC
These values look pretty good to me. Right around 200V, and cathodes are roughly where they should be..enough to make a signal anyway.
When I ground the grids on V2, the hum stops. In clean mode, nothing happens when I ground the grids....naturally.
But on V1, nothing happenes when I ground the grids..it just keeps humming. If I tap pin 2 and 7 on V1, nothing happenes, completely dead.
I have checked the wiring on V1..and it's ok. I checked the solderings, nothing touching each other, no bad solders.. Could it be the socket?
This is driving me mad!
Tommy
V1:
Pin1 - 198V
Pin3 - 1.68V
Pin6 - 203V
Pin8 - 1.57V
V2:
Pin1 - 203V
Pin3 - 1.63V
Pin6 - 204V
Pin8 - 1.59V
PI:
Pin1 - 315V
pin6 - 308V
6L6 #1
Plate - 441V
Pin5 - -44V
6L6 #2
Plate - 440V
Pin5 - -44V
B+1 - 442V
B+2 - 440V
B+3 - 432V
B+4 - 315V
B+5 - 311V
Bias is set at 40mA
All heaters measured about 6.0V AC
These values look pretty good to me. Right around 200V, and cathodes are roughly where they should be..enough to make a signal anyway.
When I ground the grids on V2, the hum stops. In clean mode, nothing happens when I ground the grids....naturally.
But on V1, nothing happenes when I ground the grids..it just keeps humming. If I tap pin 2 and 7 on V1, nothing happenes, completely dead.
I have checked the wiring on V1..and it's ok. I checked the solderings, nothing touching each other, no bad solders.. Could it be the socket?
This is driving me mad!
Tommy
Re: Fired it up finally!!
You've checked the relays are working and the wiring (to and from) is 100% correct??
At this stage it looks like V1 has a problem ( a test probe is always handy at this stage), could be faulty socket ( grid pin not connecting properly or shorting ect). have you checked for leaky caps (all) in the section from v1 to v2?
At this stage it looks like V1 has a problem ( a test probe is always handy at this stage), could be faulty socket ( grid pin not connecting properly or shorting ect). have you checked for leaky caps (all) in the section from v1 to v2?
Re: Fired it up finally!!
Take your time my friend, when you get in a hurry is when you'll get bit.
Check the voltages at the inputs of every tube (pins 2 and 7) starting at the PI and working upstream. Follow the circuit right to the previous tube's output. At each point, "scratch" your probe on the solder joint to get it to pop. When you find a section that won't pop, stop and fix what's wrong before moving on.
Build the "sound injection probe" per tonelabs post. That will certainly tell you where the problem is in a hurry
I've put 1M resistors where 1K should go, forgot a ground, you name it...
Check the voltages at the inputs of every tube (pins 2 and 7) starting at the PI and working upstream. Follow the circuit right to the previous tube's output. At each point, "scratch" your probe on the solder joint to get it to pop. When you find a section that won't pop, stop and fix what's wrong before moving on.
Build the "sound injection probe" per tonelabs post. That will certainly tell you where the problem is in a hurry
I've put 1M resistors where 1K should go, forgot a ground, you name it...
Re: Fired it up finally!!
I'll make the probe.
I have one amp with speakers, but the Dumble has no speaker...is that a problem?
I noticed something. When I turn up the master volume, the hum gets louder. But when I adjust the volume (next to the input jack) nothing happenes...even if I turn it all the way up, the hum stays 100% the same. Could it be the pot?
Tommy
I have one amp with speakers, but the Dumble has no speaker...is that a problem?
I noticed something. When I turn up the master volume, the hum gets louder. But when I adjust the volume (next to the input jack) nothing happenes...even if I turn it all the way up, the hum stays 100% the same. Could it be the pot?
Tommy