MoJo Brit 800 Build: At My Wits End!!!!

[nflrage]

I have been working on this "project" for a couple months now...from study and prep to build. Last week the PT kept blowing fuses and I was sent a new one. Prior to the PT dumping, I was getting some really low voltage readings across the whole thig. Except for the local power being supplied...that was ok. If someone has time, I have attached an image of the MoJo British 800 with the stock voltages they post and, in the yellow boxes in red text are the voltages I am getting. I cannot count how many times I have been over and over everything and as in the title, I AM AT MY WITS END! Any input would be appreciated.

[bepone]

i can help. what is the last status? pt replaced for the new one?

[_Steve]

Where are you based?

My guess is something is drawing current and dragging the voltage down. Maybe bad filter caps?

[bepone]

only online.. i can advise and you are making actions

[pdf64]

Take out the output valves and recheck.
Don’t refit them until you can monitor their anode or cathode current.

[statorvane]

Wow, your voltages are 70 volts low right out of the gate. Do you have spare rectifier diodes? Are they oriented correctly? The sound would be pretty bad if so, so may not be the issue, but right off the chocks your voltage is low.

[passfan]

I second pdf64 advice , remove the output valves and recheck voltages. Even with the low voltages the phase inverter looks like something is going on there. It’s voltages are way off and crossed up , even considering the whole amp being low. Double check your resistor values in the phase inverter area. Feedback is also looking funny , suspect a wiring error or wrong part in there somewhere. Pictures of the actual wiring would be useful at this point.

[Stevem]

I concur!
If with your output tubes pulled out voltages should be a tad above or at least equal what the layout calls for, and if this takes place then it’s my guess that if the secondary AC voltages on the PT test good then your amp is oscillating up at a frequency you can’t hear, and it’s driving the output stage full out .
This is the only thing that to me will pull down your V+ voltage by 70 volts unless you got yet another bad Pt!

Stranger things have happened though!

You can hook up your voltmeter set for AC across the speaker outputs to prove this out.

Also with the outputs in and out report back on that negative 60 volts you should have?

[nflrage]

FYI...pic shows what the PT should be putting out according to MoJo's documentation...I will remeasure with no power tubes...all components and wiring are correct. Will test diodes as well.

[Stevem]

Also is your 6.3 ac filament voltage correct?

[nflrage]

OK so I followed the advice given...here are my findings and build details:
1. First off I am running KT77s (not EL34s)
2. In the attached image of voltage readings the new readings (without power tubes is in a BLACK BOX with WHITE LETTERING.
3. Diodes are in the correct direction.
4. All components are testing correctly.
4. Heater voltages with power tubes in is 5.6...with tubes out, 6.3.
5. In my build picture, the mods (heaters and resonance) are installed for future use but NOT hooked up.

[pdf64]

How much HT current are the output valves drawing? I suspect it’s far too high, thereby dragging down the HT voltage.
Why is there VDC at the speaker circuit? Ahh, there’s possibly no actual feedback connection.
The 100k series feedback resistor looks to actually be 100ohms.
The lead forming / resistor positioning of the screen grid and control grid stoppers looks to be a ticking time bomb.

[sluckey]

The biggest thing I see with your new voltage readings is the low negative bias voltage. Get that up to -60V and the other voltages may improve. -38V is not enough to bias KT tubes. Lower the value of that 220K resistor that's connected to the bias diode until you have -60V at the output of the diode. I'd start with a 100K and then adjust up/down until I had -60V.

[nflrage]

We need to look at this from the start...not bounce around.
First of all, why does my rectifier voltage drop so much when I install the power tubes...since this is the first thing the power goes through, voltage should not be affected.
Why would the bias voltage also drop?
I will look at the 100k feedback resistor and correct if wrong...
There is NOT any voltage at the speaker outs (I don't know what you are looking at). And there is a NFB off the 4ohm tap.
I didn't measure HT voltage...where are those measurements taken?

[pompeiisneaks]

I wouldn't be assuming as a new amp builder how to tell the experts what they should 'bounce around to' as they intimately know what's needing review and what isn't.

Voltage drop with tubes in is normal. Once they start conducting, they create a voltage drop due to adding load to the circuit. How much they drop is a different story, and dropping too much may mean you're burning up tubes because something's wired wrong and allowing way too much current to flow. This negative bias being off is one of the primary ways you can do this. This sets the operating point of the power tubes and if they're pulling too much they'll redplate and die really fast.

The bias voltage is dropping because of the tubes starting to conduct.

The question about the speaker voltages were to determine if there's something flowing to the speaker through the OT or not.

The HT voltage can be measured where it first exits the diodes providing rectification. (Edit: I see you did measure HT voltage in both cases, so not sure who missed those)

~Phil