advice on OT

[loudthud]

Could the 6V6 tube socket be bad? When you have the wires from the OT disconnected, measure the resistance from pin 3 of the 6V6 to ground. It should be infinite, but it still could be a problem if the insulation of the socket has been weakened.

[d. spree]

Could the 6V6 tube socket be bad? When you have the wires from the OT disconnected, measure the resistance from pin 3 of the 6V6 to ground. It should be infinite, but it still could be a problem if the insulation of the socket has been weakened.

Thanks loudthud. I removed the socket and replaced it. Have not wired it up yet as I will need a new OT.

[d. spree]

So I'm going to order a new OT. I could return this one to Mojo and ask them to replace it - or I could buy one with a secondary common wired so it can be connected to ground at the speaker jack and that then grounded to the chassis. I am tempted just to order a different one, just spend the $30. Looking at one from Watts Tube Audio (I have liked their products) - this is the schematic. What do you all think - I presume the 8 Ohm output gets taped off as I am running a 4 Ohm speaker. I presume the red wire is the B+, which of the blue or brown gets wired to the output tube at pin 3? Here is the diagram: any thoughts are appreciated.



I also wonder if this damned package got dropped when being shipped - 2 tubes not good and I wonder whether the OT was a problem from the get go.

[J M Fahey]

That Classictone looks good.
But Mojo should stand for their product.
If you don't trust it, they should be able to offer a refund or at least credit towards other stuff you may need.

[eschertron]

What do you all think - I presume the 8 Ohm output gets taped off as I am running a 4 Ohm speaker. I presume the red wire is the B+, which of the blue or brown gets wired to the output tube at pin 3? Here is the diagram: any thoughts are appreciated.

You are correct, tape off the 8R lead if you are using only the 4R tap (cut it short and tape it if you KNOW you won't want to try the 8R tap, otherwise, leave it long but keep it out of the way). BTW, I've run across discussion that suggests tape has a chance to fall off - shrink tube and nylon tie wraps may be more secure.

Do some searching for the answer to which primary tap you should use. Not only the type of tube (6V6 vs 6-something else) but the B+ and the screen voltage, and the nominal impedance of the speaker you use (4R vs 3.2R for example) can inform just where you want the load line. The load line you choose will have an effect on the damping of the speaker and the peak-peak voltages seen on the tube plate. Or you can just go with one .. or the other.


edit: I wonder how many users test their products (have the capacity to test?) upon receipt. I don't. I guess I've just gotten lucky so far.

[d. spree]

That Classictone looks good.
But Mojo should stand for their product.
If you don't trust it, they should be able to offer a refund or at least credit towards other stuff you may need.

Yes, thanks will ask for refund

[d. spree]

You are correct, tape off the 8R lead if you are using only the 4R tap (cut it short and tape it if you KNOW you won't want to try the 8R tap, otherwise, leave it long but keep it out of the way). BTW, I've run across discussion that suggests tape has a chance to fall off - shrink tube and nylon tie wraps may be more secure.

Do some searching for the answer to which primary tap you should use. Not only the type of tube (6V6 vs 6-something else) but the B+ and the screen voltage, and the nominal impedance of the speaker you use (4R vs 3.2R for example) can inform just where you want the load line. The load line you choose will have an effect on the damping of the speaker and the peak-peak voltages seen on the tube plate. Or you can just go with one .. or the other.


edit: I wonder how many users test their products (have the capacity to test?) upon receipt. I don't. I guess I've just gotten lucky so far.

in the past I ordered from Bruce at Mission - always great products, no need to test, and the builds went fine. If he sold Champ kits that would have been the choice of vendor. never had a rpoblem with TubeDepot either - just don't like printed circuit boards and am about to gut a build I did from them and replace with traditional type board and circuits. Not sure how to check nominal impedance of speaker - is that simply a matter of testing resistance across the two speaker lugs?

[d. spree]

Thanks everyone who pitched in - this should have been a simple build but two of the tubes (JJs) were bad. The bad output tube took out the 470 Ohm bias resistor. After checking and rechecking the OT was fine - no continuity from either red or blue wire to the case and continuity on the yellow. However the wiring diagram and OT schematic was wrong - currently Mojo says blue wire to teh B+ and red to pin 3 - which is the reverse of what most OTs show. In fact this was incorrect and I wired red to B+ after getting a loud whine. Email from mojo says "oh yes, this is sometimes the case check before you cut the wires short!!! Well indeed it all seems fine. the voltages are higher than the Mojo drawing shows especially for pins 6 and 1 of the 12AX7 - about 190V rather than 165 on drawing. But she sounds superb.

I added a toggle switch to be able to use or bypass a 22uF Cap along with the resistor to ground at the 12AX7 cathode. Using the cap adds a really fatter tome. Love both settings. I would love to hang on to this little amp as it has a great sound but it will go to my friend's son.

[Bruce / Mission Amps]

By the way... the classic 50's way those Fender transformers were made "IS" with the ground lead soldered right to the OT's channel mount band.

[d. spree]

By the way... the classic 50's way those Fender transformers were made "IS" with the ground lead soldered right to the OT's channel mount band.

Yes. I used it and it has worked well. I will be returning the one I ordered from triode.

[Bruce / Mission Amps]

Well, I'd keep the Triode OT anyhow... they are very well made and you can use it for another build later.... looks like you are into it anyhow so why not just start stocking some cool stuff on your parts shelf?
I sand off the bottom of those vintage style OTs with the grounded magnet wire and use a star washer under them to "dig into" the chassis metal.
You can always solder a jumper lead onto it and shove it through the speaker wire grommet and solder that to the input jack... but if the OT is grounded as above, I think about 99% unnecessary.

[mcgruff]

I added a toggle switch to be able to use or bypass a 22uF Cap along with the resistor to ground at the 12AX7 cathode. Using the cap adds a really fatter tome. Love both settings. I would love to hang on to this little amp as it has a great sound but it will go to my friend's son.

I think this is a good mod for a 5F1. In my own champ I've got switchable options for 22uf & 1uf cathode resistor bypass caps on V1a, and 680nF & 470nF on V1b - this lets you shape the sound of the extra gain. I also added a (switchable) 5F2A tone knob, and yet another switch to break the NFB loop. They all provide something really useful without losing any of the amp's original character.