Output and Heater Questions

[nflrage]

Building a MoJo Britt 800 and consider myself above average with electronics having some formal training but this is my first build. Their schematic and layout are child proof. Build is done and prior to first start up went back to TRIPLE check all my work and everything is solid! First powered up on a variac with out any issues, then full on power without any pops, hisses, hums or smoking components. Main concerns are heater voltage is only 5.7 VAC...strange...also with a 16 ohm dummy load out of standby the load was not even getting warm so I hooked up to a cab and zero sound. Other voltages appear normal. Kind of open ended, but any ideas? I could use some help.

[bepone]

do you have any lab.equipment, sine generator or scope? heater voltage is not ok. it is on the lowest values.. check incoming wall voltage . remove light bulb if you power amp over it.
are you shure that voltages are correct? fastest method is to check all cathode voltages and you can write them here.

if without equipment..you need to start from 0, remove output tubes, leave only preamp tubes in. start the amp..hit the guitar and check output from the phase inverter tube PI with AC multimeter=?

[Phil_S]

5.7V is just a bit over 90% of 6.3V. As noted, this is at the very low end of acceptable. What is your wall voltage? What transformer are you using and do you have a wiring diagram for it? If you do, please post it.

I'm thinking there may be multiple primary taps, i.e., 110/120 or 230/240. (You don't say where you are located.) If you selected the 120 or 240 primary and your wall voltage is low, that could account for low heater voltage. Just guessing here.

[Ten Over]

Schematic

[nflrage]

check output from the phase inverter tube PI with AC multimeter=?

Don't have a phase inverter tube...No sine wave generator...just a VOM. I will supply actual readings shortly.

[nflrage]

Schematic

Thanks...already have this. Also the wiring diagram.

[nflrage]

5.7V is just a bit over 90% of 6.3V. As noted, this is at the very low end of acceptable. What is your wall voltage? What transformer are you using and do you have a wiring diagram for it? If you do, please post it.

I'm thinking there may be multiple primary taps, i.e., 110/120 or 230/240. (You don't say where you are located.) If you selected the 120 or 240 primary and your wall voltage is low, that could account for low heater voltage. Just guessing here.

Home voltage is 117 VAC.

[bepone]

check output from the phase inverter tube PI with AC multimeter=?

Don't have a phase inverter tube...No sine wave generator...just a VOM. I will supply actual readings shortly.

phase inverter tube is your tube V3,

pls remove all the tubes and measure heater voltage again, is your instrument ok=?

[pdf64]

…
Home voltage is 117 VAC.

Is that an assumption, or a measurement you’ve just taken?

[nflrage]

117 is a taken measurement. All voltage measurements are in line to what they should be now. Also, I was able to clear up the no output issue but not in a good way. When I take the amp out of stand by...I get an extremely loud and deafening hi-pitch tone (squeal) and that is with all the knobs at 0.

[wpaulvogel]

That’s a Heyboer power transformer and they are never low on heater voltage. You should have about 6.6 volts without any tubes installed. You can variac up to 120 volts but it’s not going to make up .6 volt. Check voltage from your earth ground to your neutral to confirm no voltage drop.

[nflrage]

check output from the phase inverter tube PI with AC multimeter=?

Don't have a phase inverter tube...No sine wave generator...just a VOM. I will supply actual readings shortly.

phase inverter tube is your tube V3,

Crap...my bad, I was thinking rectifier tube...DUH. Thanks!

[nflrage]

Check voltage from your earth ground to your neutral to confirm no voltage drop.

What???? From where to where?

[bepone]

Also, I was able to clear up the no output issue but not in a good way. When I take the amp out of stand by...I get an extremely loud and deafening hi-pitch tone (squeal) and that is with all the knobs at 0.

this is a good sign! amp is alive! you need to swap anode wires on output tubes, pin 3 on the sockets EL34! take wire from pin 3 from one EL34 and solder to the other EL34 pin 3 socket. and vice-versa

but for the heater problem, do one thing, remove ALL tubes, start the amp (power switch only, no need standby) and measure heater voltages again , what's the voltage= ?

[nflrage]

Also, I was able to clear up the no output issue but not in a good way. When I take the amp out of stand by...I get an extremely loud and deafening hi-pitch tone (squeal) and that is with all the knobs at 0.

you need to swap anode wires on output tubes, pin 3
for the heater problem, remove ALL tubes, start the amp (power switch only, no need standby) and measure heater voltages again , what's the voltage= ?

Thanks! I will be in the workshop shortly with a yummy cup of Joe. Will be posting measurements momentarily. I really do appreciate all the input from the community.