Fender Bandmaster (clone) 5E7 problem

[jezzbo]

last weekend I disconected the negative feedback. No difference, the bulb still doesn't go dim.

I also de-soldered the powertube sockets. I'm gonna resolder them again this week.

One thing I noticed. By looking at the schematic of a Super Reverb AB763, I noticed a switching jack on the speakers outputs.
Does it realy have to be a switching jack? I used regular monojacks.
Maybe this is my problem?

Thanks

[Mark]

The switching jack is there in case you forget to plug you speakers in, I'm led to believe it prevents a very large voltage being developed across the secondary of the output transformer. Not as good as a large 100+ ohm 10 watt resistor.

[Structo]

Just so we are all on the same page, when you built your current limiter, you did put the light bulb in series with the hot side of the circuit, right?

Like this drawing shows?

[img:800:618]https://lh3.googleusercontent.com/-ufJU ... imiter.jpg[/img]

[jezzbo]

Indeed Tom ,I build it like that.
The bulb current limiter does work with another amp (my Express clone)

Since the switch in the switching jack only works when there is no speaker connected then it seems to me that this is not the problem. I always connect the speaker or in this case a 4Ohm 100W load.

I still have to resolder the powertube sockets.
I'll keep you posted.

Thanks guys.

[Mark]

What stage are you up to with this amp?

[jezzbo]

I'm sorry Mark but I'm afraid that I don't understand your question.

[Mark]

What do you know that you didn't know when you started this thread. What faults have you fixed, what problems still remain with this amp?

[jezzbo]

What do you know that you didn't know when you started this thread. What faults have you fixed, what problems still remain with this amp?

My problem hasn't been fixed yet.
short circuit somewhere in the amp, only when I plug in the powertubes. If I can believe my bulb current limiter.
(when switching standby on, the bulb goes a littlebit dim, but not completely. When I do the same thing with my other amp, the bulb goes almost completely dim.

Preamptubes only and the amp is fine. (completely dim bulb)

- it ain't the powertubes (checked with another set)
- PT voltage is rather low when powertubes are plugged in. Otherwise it's fine.
- completely remeassured my turretboard. (and looked for shorts)
No faults so far. I didn't take it out of the amp though. Just meassured every component and meassured the elko's. Maybe I should meassure the normal caps for leakage? I haven't done that yet.

- Now I need to rewire the powertubesockets.

Could it be that the OT has a short?
wires where connected correctly, but I mean inside the OT?
(I doubt it though, it's a new Heyboer)


So basically the bulbcurrent limiter still shows that there is a short.
I'm affraight to power the amp up without the bulb limiter.
(don't want to fry the transformers)


Thanks for helping.

[Mark]

short circuit somewhere in the amp

So if you use the bulb current limiter and plug your guitar in to the jack can you get a sound out of the speakers?

Have you tried a different power transformer?

That is to say use a wire with alligator clips to go from the rectifier diodes of one amp to the filter caps of the other amp.

The preamp tubes don't present much of a load for the power transformer, the output tubes do. The output tubes heater current alone is 1.8A or 11.34 watts. The preamp tubes use 1.8 watts in total.

If you think the problem is in the output stage try disconnecting the primary leads to the power tubes, tape the output transformer primary leads up so they are well insulated.

See what happens next.

[jezzbo]

short circuit somewhere in the amp

So if you use the bulb current limiter and plug your guitar in to the jack can you get a sound out of the speakers?

Have you tried a different power transformer?

That is to say use a wire with alligator clips to go from the rectifier diodes of one amp to the filter caps of the other amp.

The preamp tubes don't present much of a load for the power transformer, the output tubes do. The output tubes heater current alone is 1.8A or 11.34 watts. The preamp tubes use 1.8 watts in total.

If you think the problem is in the output stage try disconnecting the primary leads to the power tubes, tape the output transformer primary leads up so they are well insulated.

See what happens next.

Thanks for helping Mark.

Good idea about pluggin in a guitar and hooking up some speakers.
I will try that as soon as the amp is wired up again.

About "disconnecting the primary leads to the power tubes" I will try too.

I have not yet tried a different PT.
This one is a brand new one (from tubeampdoctor.com)

Probably somewhere this weekend. (busy week at work)

Thanks again, I'll keep you posted.

Jesse

[Structo]

Just an FYI because I didn't see it mentioned when I quickly scanned the thread, anytime you have power tubes in an amp you must present a speaker load on the secondary.

Be it a dummy load or a speaker.

If you don't do that you stand a good chance of damaging or ruining the output transformer.

If you know that and are doing that, disregard and carry on.


Also, if you are beating yourself to death over this problem, walk away for a day.
I know you will be thinking about it but you will be able to come back to it with a fresh mind and maybe some new ideas.

[brentm]

I can't help but follow this thread a little... I have an original tweed bandmaster....

so this is a side question. however, is it a good (best) practice to test each change along the way?

1. new PT (tubeampdoctor) - TEST
2. new OT, Heyboer OT for bassman. secondary = 2Ohm, 4Ohm, 8Ohm. (8Ohm is not connected) - TEST
3. Made a new turretboard with (mainly) Allen Bradley 1W resistors and sozo blue caps. - TEST
4. Changed the biasing to blackface specs. (following the Ceriatone layout) - TEST
5. Installed a switch to select 2Ohm and 3 speakers or 4Ohm 2 speakers. - TEST

In my troubleshooting world (computers/networking), there's nothing worse than a hot mess of a several changes in succesion and not knowing what and where the problem began. Typically, I see this in more Junior level techs. Those who have pulled enough all-nighters as a result of their own heavy hand seem to take a more methodical and calculated approach.

[Reeltarded]

Go to Hell Brent. Stop calling me old.

[vibratoking]

Reel - I'm pretty sure he called you stupid too. I am not sure which one you should take greater offense to?

[Reeltarded]

Old

haha!