Blackface Champ OT question

[jvm]

I have just replaced a OT in a blackface Champ. Previously, someone has installed a diode from 6V6 cathode to speaker jack positive terminal. Also, they added a 10 ohm resistor across the speaker terminals of the 8 ohm speaker that was installed. Should I remove these mods or are they ok? I suspect the speaker mismatch may be what killed the original OT.

[nickb]

I have just replaced a OT in a blackface Champ. Previously, someone has installed a diode from 6V6 cathode to speaker jack positive terminal. Also, they added a 10 ohm resistor across the speaker terminals of the 8 ohm speaker that was installed. Should I remove these mods or are they ok? I suspect the speaker mismatch may be what killed the original OT.

IIRC the speaker should be 4 ohm so the 10 ohm in parallel with the speaker is an attempt to correct that. No harm really but it's halving your output power. The diode mod just sounds weird. Are you sure about that? You might want to double check. Maybe others have some ideas.

Nic

[jvm]

Hey, nickb. I understand that they were trying to compensate for the load mismatch. Just wondering if I should recommend that they install a proper 4 ohm speaker. The diode thing is really strange but I am looking right at it. It is from the 6V6 cathode to speaker jack positive.

[nickb]

Hey, nickb. I understand that they were trying to compensate for the load mismatch. Just wondering if I should recommend that they install a proper 4 ohm speaker. The diode thing is really strange but I am looking right at it. It is from the 6V6 cathode to speaker jack positive.

I would put in the correct speaker.

I'm guessing the cathode (+ve) end of the diode is going to the 6V6 cathode. The peak output voltage across the speaker is about 6 volts and the 6V6 cathode is about 13V so i don't think the diode is doing anything. If the speaker went open then the voltage could be enough to raise the 6v6 cathode and start to turn it off i.e. an attempt at protection?

On the other hand if the diode cathode is to speaker than that will short out the cathode resistor, mess up the bias and cook the 6V6.

My motto, if in doubt take it out...

[jvm]

I agree, nickb. I think I will yank the diode and recommend replacing the speaker. Thanks

[lowell]

Mod sounds kinda cool to me. Question is, does it work fast enough to protect the OT?

[nickb]

Mod sounds kinda cool to me. Question is, does it work fast enough to protect the OT?

Hi lowell,

Yes it is kinda cool. I did a little playing around and it does work. The key thing is that the output tube's nominal cathode voltage be just a little higher than the maximum peak voltage at the speaker. I don't believe that is the the case here.

Since it's not really going to work here, I worried what happens if the diode goes short. In that case we cook the output tube and transformer, hopefully the fuse will then blow and save it.


Nic

[Amp Kat]

Would definitely change that back to original BF specs. Not worth an OT for what little difference the mod added.

[lowell]

Yah I can see how that diode shorting would cause the 6V6 to see the speaker load (4/8/16ohms) as a Rk. Would the OT go bye bye due to the redplating of the 6V6? Would this cause the OT primary to see a grossly low z? If that's the case I wouldn't see how a exaggerated low/high ratio would hurt the OT. Just thinking aloud here.