Petite boutique HELP

[Tubelab_com]

It is common in tube amps to operate the heaters a few volts above ground to reduce hum and reduce the possibility of H-K shorts. I usually make a two resistor and cap voltage divider off of main B+, but many amps choose to steal from an existing low voltage source, the output tube cathode.

In an amp like this any of the 3 possible choices will work fine. Ground the wiper, connect it to the output tube cathode, or make a voltage divider to feed 20 volts or so to the wiper. A 4th choice exists, and is often used in guitar amps. Just tie one side of the heater winding directly to ground (or the output tube cathode) and ditch the pot altogether.

 

[audiopro]

Looks like a nice little build. One safety problem. Your fuse is on the wrong side of the switch.

 

[DrAwesome]

Thanks again mrtube- though I do t understand about half of what you said :/

Dan- wrong side? Power to switch then fuse is how uncle Doug runs things- the old champs seemed to fuse one leg and switch the other. I’m open to compelling input but it looks like it has been done several ways

 

[PRR]

... it looks like it has been done several ways

It has.

As long as you ALWAYS un-plug before you stick your hand in there, it does not matter.

If you are going to produce for sale, the safety inspection agencies like to see things a certain way. But you got the one intercom, and its wiring was probably accepted when new, and may be retained.

 

[DrAwesome]

Check this out- the published schematic seems wrong the plates should be 1 & 6
But this says pin 1&7. I pin 7 is the cathode- not pin 3. And the heater is pins 3&4 - this is not a good sign.

 

[DrAwesome]

Safety check, bulb current limiter, plug it in... nothing :-/
Filament power good 6v, glow, warm
B+ about 280
No sound whatsoever
I guess it’s troubesh time

 

[DrAwesome]

IT WORKS!!

A simple misplaced connection, as you may have suspected.

So it makes noise, not loud, sounds - meh, but it’s sound. Now I can play with components and see what happens. Thanks for the support

 

[DrAwesome]

A little help?

So after a break from this project- I thought I would try to tune it up a bit... short story, the output transformer only puts out ~50vac what the heck?!? I disconnected it from the circuit completely and measured the voltage, yup 54volts. How is this possible?

 

[PRR]

You had 280V DC before.

How can you have 50V AC now??

 

[DrAwesome]

That’s what I said!

I’m willing to consider that I was looking at 2.80 at pin 7 of the 6cu5 and mis read the meter.

Any ideas bout how to proceed?

At first I just wanted to see if I could make it function- now I really want see if I can get a decient sound.

 

[DrAwesome]

So sorry - the “that’s what I said!” May sound like a chastisement - it’s intended to be “I agree! How did that happen?!?”

 

[DrAwesome]

Here’s a picture of the offending transformer- completely detached and on the bench ~54vac on the secondary

Now what? Voltage doubling circuit? I have a 325-0-325 but I think that would fry the whole thing.

While I wait your input I shall continue reading the “hundred buck amp challenge” thred and fill my skull with information as I gawk in amazement