5F6-A 2204 Build

[thetragichero]

what's the other red wire read?
5f6a schematic i pulled up had 600v caps so maybe it's totem pole time for at least the first node? choke should help resist inrush current for the screens

 

[Chris-in-LA]

what's the other red wire read?
5f6a schematic i pulled up had 600v caps so maybe it's totem pole time for at least the first node? choke should help resist inrush current for the screens

The amp has been running and sounding great for hours, since 4/27. When I turned it on yesterday evening, I heard a clicking noise like a circuit breaker was letting go when I took it off standby. The filaments looked a little brighter than usual, but I wasn’t sure. That’s when the 500v cap popped and smoked. The amp was working but went dead within 1 minute of powering up.

I can power it up without the rectifier and check the voltage to the rectifier socket and heaters in the morning. But it did work initially after I replaced the blown cap with a 450v spare I had and it was plugged into the light bulb limiter. It also worked fine on my variac at 110vac. It blew the cap again when the voltage hit 120vac.

 

[neikeel]

Chris
Are we replying here or in the technical section? ;-)

 

[Chris-in-LA]

Chris
Are we replying here or in the technical section? ;-)

In the workshop section.

 

[Chris-in-LA]

In the interest of finishing up this thread, the PT had some internal shorts and Mojotone is sending a new one. In the meantime I opened up the old PT, found 2 shorts in the leads due to crushed wires in the end cap. The leads were repaired and the amp runs great. It sounds a lot like a regular 2204. It has chime and thump. Substituting .022 caps for the .1 caps as in a regular 2204 keeps the bottom end tight. The EQ section responds very much like a 2204 in that it can get very bright unless you keep the mids and treble down. It is heavy, but not as heavy as a jcm800 combo. I ended up putting Fender-style removable casters on the bottom, that seems to make it much more manageable.

 

[Chris-in-LA]

I was referencing a 5F6-A schematic so I spoke from that. It is also why I asked @Chris-in-LA which or what schematic he was using.


A fuse in the utility to primary of transformer works whether located on hot or neutral line.
It forms and is part of a circuit through the transformer primary.


The Mojotone schematic is pretty much the original Bassman schematic.
See the in line fuse and power switch below.

@Chris-in-LA, are you using a different schematic?

View attachment 88500

Oh, that’s your question. I’m using the Mojotone schematic.

 

[mickeydg5]

If you have other bad components in the amplifier, those can also blow the next transformer.

Hint(s).

I had to mention that.

 

[Chris-in-LA]

If you have other bad components in the amplifier, those can also blow the next transformer.

Hint(s).

I had to mention that.

It wasn’t bad components that blew the last one, it was a short in the 100v tap that caused an extra 100 volts to hit the rectifier. All that extra voltage took out a filter cap. Everything else looks pretty good and I’ve been running the amp for a number of hours since the repair of the old PT. I don’t plan to use the new PT for now.

 

[mickeydg5]

It wasn’t bad components that blew the last one, it was a short in the 100v tap that caused an extra 100 volts to hit the rectifier. All that extra voltage took out a filter cap. Everything else looks pretty good and I’ve been running the amp for a number of hours since the repair of the old PT. I don’t plan to use the new PT for now.

So you are not using the amplifier for a while?

It is strange that a bad transformer from the manufacturer could cause a rectifier tube to spark but I guess anything is possible.

 

[Chris-in-LA]

So you are not using the amplifier for a while?

It is strange that a bad transformer from the manufacturer could cause a rectifier tube to spark but I guess anything is possible.

You may be right because the replacement rectifier never arced even when there was so much voltage filter caps were blowing.

 

[mickeydg5]

You may be right because the replacement rectifier never arced even when there was so much voltage filter caps were blowing.

You did use the 22uF power capacitors at the rectifier VDC out?
Two 22 = 44 total. The 5AR4/GZ34 can only handle so much.

 

[Chris-in-LA]

You did use the 22uF power capacitors at the rectifier VDC out?
Two 22 = 44 total. The 5AR4/GZ34 can only handle so much.

Yeah. I guess that putting 120vac into the 100v tap was too much.

 

[mickeydg5]

Yeah. I guess that putting 120vac into the 100v tap was too much.

Shit man. I thought for some reason you did that. Maybe you had mentioned it earlier.
If you did then you placed approximately 540-550VDC on the plates and almost 8VDC on the filaments/heaters. That would fry a lot. Do not arbitrarily do anything like that (again).
That would and should void any warranty.

 

[Chris-in-LA]

Shit man. I thought for some reason you did that. Maybe you had mentioned it earlier.
If you did then you placed approximately 540-550VDC on the plates and almost 8VDC on the filaments/heaters. That would fry a lot. Do not arbitrarily do anything like that (again).
That would and should void any warranty.

Dude, I didn’t do it on purpose, the insulation on the 100v tap was damaged and shorted into the cap and caused the problem. Yes, voltages were crazy until I put the amp on a variac and turned down the 120v to 100v. Once I found the shorted tap, voltages returned to normal. Mojotone sent me a new PT.

 

[mickeydg5]

Dude, I didn’t do it on purpose, the insulation on the 100v tap was damaged and shorted into the cap and caused the problem. Yes, voltages were crazy until I put the amp on a variac and turned down the 120v to 100v. Once I found the shorted tap, voltages returned to normal. Mojotone sent me a new PT.

I got you. I had read few things and saw the crunched wires and whatnot.
Did you do that to the wiring or did it come that way? I would have sent that transformer back if it showed up like that.

That did compromise other components so check every thing including fuses. Swapping out fuses and rectifier tube may be necessary.

 

[Chris-in-LA]

I got you. I had read few things and saw the crunched wires and whatnot.
Did you do that to the wiring or did it come that way? I would have sent that transformer back if it showed up like that.

That did compromise other components so check every thing including fuses. Swapping out fuses and rectifier tube may be necessary.

It came that way. I told Mojotone that my PT wasn’t working and they sent me a new one. I took the broken one apart because they didn’t ask for me to return it. I wanted to see why it didn’t work. I actually fixed it before the new one got here. The new one doesn’t have the extra taps, has more room inside and looks much better inside. It fried one fuse, two filter caps, and two rectifiers. Everything else is fine.

New one.


Old one.