Tweed Tremolux build 5G9, need some help !

[Daver]

Check the voltages on all the pins of every tube. Do they seem correct? Are there any missing or really high readings? Do you get any noise coming through the amp when you probe for voltages? Also make sure all your grounds are connected.
Dave

[tubeswell]

(Carefully) take each one of the B+ filter cap/supply node voltages and take the plate, screen, grid and cathode voltages (all VDC) at idle and report back. You should have several hundy VDC on each B+ node and on the 6V6 plates and screens and the LTP plates, over 100VDc on the V1 plates, 1VDC or so on the V1 preamp cathode, and about 25VDC on the LTP tail and a couple of VDC more on the LTP cathode, and about -25VDC or so on the 6V6 grids.

Also (carefully) take the VAC voltage between both sides of the heater winding - the heater voltage should be measured between pins 2 and 7 of the 6V6s and pins 9 and 4/5 of the 12AX7s. In either case you're looking for just over ~6 VAC

Then, if the voltages look okay, check for DC continuity with your R-meter (and the amp switched off), between each part of the circuit you'd expect DC continuity (input jack tip to grid of V1, plate of V1 to coupling cap, coupling cap to volt pot input, pot wipe to grid of v2, and so on.

Lastly, if all of that checks out, (carefully) test the coupling caps for DC leakage - there shouldn't be any.

[Enzo]

And just for science, "SEL RECT" means SELENIUM rectifier. If you ever see one, it will be a small square thing looking like a tiny radiator. A number of square metal plates in a stack with a bit of space between each one. Before nice silicon rectifiers came along, seleniums were the choice for not having a tube rectifier. And OOOHHHH did they stink when they burnt out. A sort of burning rotted cabbage smell.

[tweedmarmot]

Hey guys,

Probing voltages here and there, nothing looks too off except the voltage on pin 3-8 of the second preamp tube. I read a big 49 v !?

what could this be ?

tubes are (from left to right) 5U4GB 6V6GT 6V6GT 12AX7 12AX7(this tube) 12AY7

Again, thanks everyone.

Stephane

[tubeswell]

Hey guys,

Probing voltages here and there, nothing looks too off except the voltage on pin 3-8 of the second preamp tube. I read a big 49 v !?

what could this be ?

That's the LTP cathode, and it should be around 27-28V. Check the tail resistor and bias resistor values again. (Should be 10k and 470R respectively). I take it the plate resistors are the correct values there?

[tweedmarmot]

hey hi guys, Thanks Tubeswell for all that help.

I am gonna have to double check the value of the 470R on the 6V6 tubes .. I am pretty that they are ok but still, I will double check.

Here's a "picture" of the voltage I have on the tubes pins ( from rectifier to v1 )

rectifier- 4. 330ac 6. 330ac 8. 444v
6v6- 3. 442 4. 440 5. -24,5 6. 440
6v6- 3. 442 4. 440 5. -24,5 6.442
12ax7- 1.293 2. 5,3 3. 8 6.440 7.290 8.300
12ax7- 1.246 2. 24 3. 37 6. 241 7. 25 8. 37
12ay7- 1. 187 2. 0 3. 2,9 6. 183 7. 0 8. 2,9

Now the 6v6 cathode is connect with pin 1 and 2 as on the layout.
everything is as the layout except the silicon diode connect to the 50k pot ( replacing the 56k resistor)

any ideas on what is wrong ?

Again, thanks to all and I am gonna double check those 470R ...

[Daver]

tubeswell is talking about the 470R for the phase inverter. Do you have your heaters wired so one green wire is going to ground like the original Fender schematic or do both green wires from the power transformer got to all the tube sockets? Do the tubes light up and get hot?
Dave

[tweedmarmot]

Both of them are connect but you're right, on the schematic, I should have one grounded. But all tube filaments are lighten up and warm.

So, any of the two can be ground ?

Stephane

[Daver]

It's OK if they are connected to the tube sockets but then you must disconnect pin 2 of the power tubes from ground. One of your heater wires is connected to pin 2, correct? Does your power transformer have a center tap wire for the heaters? Usually this would be a green wire with a yellow stripe. If so, this should be connected to ground. Look at a Fender 5F6A Bassman schematic. If you don't have the center tap you need to create one using two 100 ohm resistors. Each resistor goes from one line of the heaters to ground. Look at a Blackface Twin schematic. Is pin 8 of the power tubes connected to ground? That's the cathode.
Dave

[tweedmarmot]

Dave, I dont have a Filament center tap, so as you said, I will disconnect pin 2 from the 6v6. But I dont get the part of creating a center tap with 2 x 100ohms resistors !?

[tubeswell]

rectifier- 4. 330ac 6. 330ac 8. 444v
6v6- 3. 442 4. 440 5. -24,5 6. 440
6v6- 3. 442 4. 440 5. -24,5 6.442

Firstly these voltages are a bit too high for 6V6s. The 5G9 runs the 6V6 Plates at about 370 or 380, with -27V bias. I suspect your 6V6s are running at over maximum dissipation. This won't be why your amp isn't working yet, but you still need to address it. Have you measured the plate current? (plate current x plate voltage = plate dissipation. You want the tubes to be idling at between about 60-80% of maximum dissipation for a fixed bias amp - in this amp the cooler the better the effect for the tremolo)

12ax7- 1.293 2. 5,3 3. 8 6.440 7.290 8.300

More of the same preaching as above - the HT of 440 for the trem tube is getting up there. And I suspect that the cathode voltage of 300 on the CF stage will kill that stage before too long - the h-k insulation won't take it. So try and get the B+ down to 380 or so would be my first bit of advice.

12ax7- 1.246 2. 24 3. 37 6. 241 7. 25 8. 37

This is the phase inverter, with the grids at about 25V the cathode (Pin 8) ought to be about 27V. The fact that it is 37V (i.e. around 10V of bias) indicates that it is biased too cold. A 12AX7 inverting stage can cope with around -4V or so, but that is heading toward being cut-off for some of the -ve part of the grid swing. Check that you have a 470R cathode resistor there (and not some other value). This is the 1st thing I'd check.

12ay7- 1. 187 2. 0 3. 2,9 6. 183 7. 0 8. 2,9

These look okay (if anything - a little on the high side for a nice tweed sound from V1)

[Daver]

Pin 2 needs to be connected to one of the green wires, pin 7 to the other green wire. Pin 8 needs to go to ground. On the preamp tubes you should have a green wire going to pin 9 and the other to pins 4+5 together. To make an artificial center tap for the heaters one 100 ohm resistor needs to be attached between one green wire and ground and another 100 ohm resister goes from the other green wire to ground. Often this is done at the pilot light socket because it's a handy place to make the connections.

[tweedmarmot]

Ok Now I have redone my filaments to what is on the schematic ( one green to all filaments and one green to ground)

Firstly these voltages are a bit too high for 6V6s. The 5G9 runs the 6V6 Plates at about 370 or 380, with -27V bias. I suspect your 6V6s are running at over maximum dissipation. This won't be why your amp isn't working yet, but you still need to address it. Have you measured the plate current? (plate current x plate voltage = plate dissipation. You want the tubes to be idling at between about 60-80% of maximum dissipation for a fixed bias amp - in this amp the cooler the better the effect for the tremolo)

Great ! Now my B+ measure 370 ! ... !? -27V bias, is the limit of my potentiometer, is it ok ?

This is the phase inverter, with the grids at about 25V the cathode (Pin 8) ought to be about 27V. The fact that it is 37V (i.e. around 10V of bias) indicates that it is biased too cold. A 12AX7 inverting stage can cope with around -4V or so, but that is heading toward being cut-off for some of the -ve part of the grid swing. Check that you have a 470R cathode resistor there (and not some other value). This is the 1st thing I'd check.

Ok I measure that resistor and I can read 470,8 ohms.


Now I have sound when I turn on the standby switch, it's all crackling sounds without a cable plug in the inputs ..
Can that be because it bias too cold ?

Again, Thank you so much.

Stephane

[tubeswell]

Cracking sound is more like a bad solder joint - or it could be a bad tube. Try a tube swap first. If that doesn't work just try pulling out the pre-amp tubes and putting them back in one at a time to try and figure out which stage is the noisey one. Then report back

[tweedmarmot]

Ok, I swapped the preamp tubes and it doesn't follow any tube. So I pull out one tube at a time and it looks like it come from second stage. I have redone most of the solder joints in that area and still have those cracking sounds. I plugged my old telecaster in it and it sounds good. It gonna be real good when those garbage sounds will be fix...

Any other ideas about those cracking sounds ?