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Post subject: Bassman 135 help OT
Posted: Sat Aug 12, 2017 4:47 am
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Hello to all my bassman 135 keep blowing OT, I replaced the Ot once, and the new hammond transformer for 135 is already broken. All the tensions from power transformer seems to be good... I don't know what could it be.
Thank you


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Post subject: Re: Bassman 135 help OT
Posted: Sat Aug 12, 2017 7:50 pm
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It is unusual to blow output transformers. Let alone 2.
Usually the main fuse will protect it in the event of a tube failure.
Check for proper fuse value before replacing the OT again.
Do you know what the Hammond model number is?
You might consider changing the 470 ohm screen grid resistors to 1K to limit current.
You might also connect a 1 Amp fuse inline with the red wire.This will give you a convenient location to measure current to see if it exceeds the rating of the transformer.


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Post subject: Re: Bassman 135 help OT
Posted: Thu Aug 24, 2017 8:50 am
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I'm remembering another ultralinear OT that went down because of tube failure. It was in a high end tube stereo from Japan.
The screen grid resistors connected to the ultralinear taps were 5 watt rated. This may seem like a good idea to a boutique builder. But in failure mode, such as a screen grid short, the winding of the transformer shorted out because the resistor was too big to fail. A 1 watt resistor would have saved the OT by opening up and disconnecting the screen grid.
Only the ultralinear output transformers are susceptible to this issue. Still, I don't upgrade wattage ratings on SG resistors. They're supposed to fail to protect the amp.


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Post subject: Re: Bassman 135 help OT
Posted: Thu Aug 24, 2017 1:00 pm
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Tim, I think just the opposite.

You want the screen resistor not to pop and act like a “fuse.” AFAIK, the screen grid provides a constant, positive voltage to strongly attract electrons from the cathode. It also isolates the control grid from the plate which reduces parasitic capacitance (Miller Effect) between them, thus increasing the tube's gain and stability. This is especially nice when the amp is pushed to max volumes or runs into momentary instabilities. You want this screen grid and its resistor intact, at all times. As a popped screen resistor will actually make the tube less stable.

I've been using 3-watt metal oxide and 5-watt wire wound screen resistors. Even going to 1k-ohm, versus 470-ohm OEM resistors in Fender amps with a history of instability issues. Like frequently blown fuses and tubes --- and even a few with blown output transformers.

Now, for the grid stoppers, you want smaller 1/2-watt resistors. This is to prevent high voltage (and current) passing onto the signal grid --- from a failed coupling cap. You want them to pop, if they see too much wattage passing though.

I've seen caps between the phase inverter and output tube pass 5 or more VDC. This in effect, is changing the idle bias from a negative voltage (from the bias supply) to a positive one. Instant fried tube and output tranz (OPT), unless you have a real fuse between the output tubes and the OPT, to save the iron. This is why I never use boutique PIO coupling caps on the output tube signal grids. They are just not as stable as a good Mylar cap. Though, some ppl's (including mine) experience with Russian mil spec, 600VDC and 1KV rated K40Y-9 metal clad PIO have been very good.

:)


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Post subject: Re: Bassman 135 help OT
Posted: Fri Aug 25, 2017 3:40 pm
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Yea, I've gone back and forth on this one.
I haven't seen a screen grid resistor go bad by itself. The failure occurs when they get taken down by a shorted tube.
So a larger rated resistor will overload the power supply until the tube melts down or something else breaks. In the case of an ultralinear OT, it can take down the secondary winding. Its the only scenario I can think of that would take down 2 transformers like the OP described
If I thought the 1 watt would wimp out under normal operation, I would upgrade it, but as a safety device during failure, I'm sticking with em. There are many fusible circuits in modern designs. This looks like one of Leo's.


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Post subject: Re: Bassman 135 help OT
Posted: Sat Aug 26, 2017 8:37 pm
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It is a common misconception that SGRs are supposed to act like a fuse for some sort of protection in a tube amp. Fuses are used for protection, not so with most resistors. Resistors that are meant to fail for circuit protection are called "fusible resistors", and these type of resistors are not used for SGRs.

You want your resistors to hold, and fuses to blow, therefore the proper size fuse should be used to protect the circuit, including protection of the SGRs.

Increasing the ohmmage and the wattage of SGRs is a good idea for many amps.

TimsAudio wrote:
...So a larger rated resistor will overload the power supply until the tube melts down or something else breaks.....


Yes, the "something else" should be a fuse..... that's what fuses are for. The SGR failure is usually due to a bad tube anyway, so why would you want to protect a bad tube? :lol: :wink:

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Post subject: Re: Bassman 135 help OT
Posted: Mon Aug 28, 2017 9:37 am
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I'm not looking to protect the tube. It's already history by the time the resistor disconnects.
Most power supplies can handle the overload from a shorted tube. Usually the main fuse blows and its over.
In the case the OP described, the SGRs are powered through the output transformer. If the power tube shorts, the OT windings will overheat and destroy the OT without blowing the fuse.
A blown sgr is an indicator of tube failure. Now the OP didn't indicate that there were blown tubes connected with the OT failure, but I wouldn't be surprised.
A fuse on the OT would be ideal, but doesn't come stock.


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