Starting to populate the chassis....

[BrianL]

Thanks,
No scope,
Plugged into the effects input and still no sound.
Yeah, I'm gonna do a continuity test on all the preamp brd wires...
BrianL

[BrianL]

How much voltage can I feed the tube heaters,specs says 5.7 to 6.9. I’ve seen videos of giving Marshall’s 12vdc. BrianL

[dbharris]

Hi Brian,

Your heaters are AC, not DC. Ideally you are within 10% +/- of 6.3Vac when testing with tubes. But some variance is normal based on line voltage fluctuations etc. What voltage are you measuring?

You can measure it on the preamp socket with one probe alligator clipped to ground and the other on pin 9 and then pins 4/5 (3.15 is what is expected for each). Or you can measure across pin 9 and pins 4/5 where 6.3 is expected. I prefer to measure each side of the heater string to ground, as it seems easier and safer to me.

-Dan

[BrianL]

I'm measuring 5.2VAC. across the pins.
I have a measured input of 117VAC house current.
Transformer is supposed to be putting out 6.3VAC 8amps.
But it's not...
When I test I get 2.6VAC on each pin.
I'm thinking of getting another transformer like I put in for the relay switches and
resistoring it down to 3.45VAC each lead or 6.9VAC across the pins.
BrianL

[GAStan]

Are you by any chance using a light bulb limiter?

You may have an out of spec transformer.

[dbharris]

What PT and OT are you using? I don't think you should need a separate filament transformer.

[BrianL]

Thanks,
I shouldn't need a separate transformer but just as a test to see if it helps.
I could bring down each 6.3VAC to 3.45VAC (high limit) and see if it makes a difference.
It's a lot of un-needed work but if it helps. why not try it.
The PT is out of warranty so I would have to buy a new one anyways.($100.00)

I was using a bulb, it would dim and come back to normal when turning the amp on,
then it would dim again and come back to normal when taking off standby.
The OT is from Tube Depot, a Hammond 1760W Twin Reverb type 100watts
Download PDF file to see the specs.
The power transformer is from Weber, Marshall style WPT100 100watts
see attached pic. I typed in the little numbers to track what I was getting. Stock specs are the bigger, darker numbers...

[dbharris]

Voltages will be lower than expected with the light bulb limiter in circuit. As long as you don't have a bright glow, indicating a problem with the circuit, then it is time to remove the limiter and test voltages without it. I bet your heaters will be right where they should be.

Typically you have the lightbulb limiter and variac (if you have one) in place for the initial power up. Once no major issues are detected, is best to remove those.

-Dan

[BrianL]

So I was a dummy, I thought a light bulb in my power strip that I plug my amp into would work as a test light.
I guess I was wrong, so I built the test light thing on this web site:
https://robrobinette.com/Tube_Amp_Start ... e_vignette
Pulled out the tubes, unplugged guitar and speaker, plugged amp into light checker,
plugged light checker into power strip,
Turned amp on, light goes brite, then dims, warm up tubes for a bit,
turn standby to operate, light goes brite, then dims.
So I guess so far I'm good...
I will run the startup tests from this website tonight and try to figure out why no sound.
Thank you for your help and patience...
BrianL

[martin manning]

I thought a light bulb in my power strip that I plug my amp into would work as a test light.
I guess I was wrong, so I built the test light thing on this web site:…

Yea, the bulb has to be in series with the load (amp).
There are several bulb limiter examples on TAG here: https://ampgarage.com/forum/viewtopic.p ... 06#p237806

…plugged amp into light checker,
plugged light checker into power strip,
Turned amp on, light goes brite, then dims, warm up tubes for a bit, turn standby to operate, light goes brite, then dims. So I guess so far I'm good...

Yes, sounds good. I would do a voltage survey, power tube plates, screens, and grids. Is your power tube bias somewhere close? You might hear popping as you probe, indicating that signal is getting through. Then check preamp tube plates, grids and cathodes. If the voltages look reasonable then suspect a wiring error, a break, or an unintentional grounding in the signal path.

[BrianL]

Thanks Martin,
I did a voltage survey/check a startup describes, I thought it came out good.
I have set -57 volts on PT Bias. Did hear a couple pops but then they stopped.
I will check continuity on wire connections and check for wiring errors.
Need to rewire my power switch too, I don't have power going thru the fuse and then to power switch.
I have neutral thru the fuse...

Did find something I thought was strange though:
While checking this part of step 6 of the above startup procedure

"If the light bulb dims with all the preamp tubes installed then measure the voltages at all the preamp pin tubes. Nine pin preamp tubes' pins 4 & 9 and 5 & 9 will have 5.9 to 7v AC heater voltage and the pins 1, 3, 6, 8 will have DC. Normal preamp gain stages should have voltage on their plates and cathodes but not on their grids. Phase inverters will have voltage on their grids. Cathode followers may have high voltage on their plates, grids and cathodes."

Preamp tubes are 12AX7's, tube one being the tube to the right of the footswitch.
I'm getting :
Tube 1 pins 4-5 to 9 3.5VAC pin1 122VDC pin3 0VDC pin6 124VDC pin8 0VDC This tube has resistors and capacitors with it
Tube 2 pins 4-5 to 9 3.5VAC pin1 126VDC pin3 0VDC pin6 128VDC pin8 0VDC This tube has resistors and capacitors with it
Tube 3 pins 4-5 to 9 3.5VAC pin1 195VDC pin3 38VDC pin6 203VDC pin8 38VD This tube doesn't have any resistors or capacitors associated with it.

Why would I be getting higher voltages on tube 3 ??? Is this normal?
BrianL

[dbharris]

Yes, that is correct the plate voltages will get higher from v1 to v3. Are these voltages still with the light bulb limiter? What wattage bulb are you using? Is it lighting up bright or just a flash with power and another with standby flipped?

I would address the AC in to be properly fused and make sure your standby switch operates as intended too.

Then remove the limiter assuming the bulb isn't glowing bright and record all voltages. Your heaters will not change from one tube to another, so just measure those once.

How are you setting power tube bias?

-Dan

[BrianL]

Hi,
V1-V3 voltages were with the light bulb limiter,
100 watt clear light bulb so I can see the filaments
turn on amp, goes brite then dims
turn standby to operate, goes brite then dims.
Bias
Without Power tubes in I turn on the amp, set to operate
I check the wiper on the 10K bias pot which is between tube 5 and 6
I set it to -57VDC.
Startup procedure says should be between -30VDC t0 -60VDC, -60VDC being better than -30VDC
6L6GC should be 54.55 at 60% according to a chart on Tube Depot
Thanks, BrianL

[erwin_ve]

Hi,
V1-V3 voltages were with the light bulb limiter,
100 watt clear light bulb so I can see the filaments
turn on amp, goes brite then dims
turn standby to operate, goes brite then dims.
Bias
Without Power tubes in I turn on the amp, set to operate
I check the wiper on the 10K bias pot which is between tube 5 and 6
I set it to -57VDC.
Startup procedure says should be between -30VDC t0 -60VDC, -60VDC being better than -30VDC
6L6GC should be 54.55 at 60% according to a chart on Tube Depot
Thanks, BrianL

Without power tubes it is useless to set a bias voltage. Also it isn't about the specific voltage, it is about the current flowing through the tube with the B+ voltage as a marker for you calculating the current. Ohms law and power calculating is mandatory.
Do you know howto?

[BrianL]

erwin,
I don't know how to specifically do it. I have watched and read several videos and descriptions, so I have an idea.
I guess it's not as easy as just turning the dial to -57VDC...
Still learning,
BrianL