Help finding correct component with Fender Hot Rod Deluxe

[abbey road d enfer]

The thing is, I don't know what a "flyback" diode is (could someone please explain) and I assume that although marked C5 on the PCB, it should be CR5 on the schematic. I also assume it's a protection diode and after a quick search I think the correct value is 3000V/0.25A.

A flyback diode is used to damp overshoots that happen when an inductive circuit (such as a transformer) is not properly damped, which happens if the loudspeaker is disconnected. It protects the output transformer and the output tubes.

-Can this diode be replaced by another one like 4004 or 4001? I'm asking cos I got a bunch of those that I use as rectifiers for amps

You could temporarily use a 1N4007 that is rated at 1000V, but you really should use the real part or a correct substitute.

-Should I check other components that could have been damaged aswell (apart from the obvious resistor next to the diode of course) and if so, any suggestions?

I believe you already have the answer; one of the tubes and its associated screen-grid resistor are gone. I would recommend replacing both screen-grid resistors with wire-wound 5W+ type.

 

[sonolink]

Thanks Abbey!!

Btw, I'll post some porn soon on the other thread

Cheers
Sono

 

[Mylithra]

While I have seen tubes go into run away and for the short time they were working before they blew up, they were putting out more than enough power to blow a lower wattage speaker but that a little more uncommon. If you take a meter (must be an RMS type meter) to the speakers, it should give you roughly the speakers resistance. If its more than a few R too low or no resistenance then they could have been damaged (Short in the coil), or infinite resistance (Coil burned open), but if its within a couple R of what the speaker is rated for, its probably fine.

 

[sonolink]

I think it's fine!!  8)

https://imgur.com/a/pjQu5cq

 

[abbey road d enfer]

If you take a meter (must be an RMS type meter) to the speakers, it should give you roughly the speakers resistance.

For measuring resistance, a meter does not need to be RMS. Resistance measurement is DC. RMS really pertains to AC.

If its more than a few R too low or no resistenance then they could have been damaged (Short in the coil), or infinite resistance (Coil burned open), but if its within a couple R of what the speaker is rated for, its probably fine.

All true, whilst being slightly OT, though.

 

[moamps]

It's interesting that Fender do not use that flyback diodes in all Deluxe models.
Also you should adjust the bias of new output tubes to get 60mV at the cathode resistor.  That's the first thing you should do after replacing output tubes. I would check also each cathode current, they must be equal.

 

[Mylithra]

For measuring resistance, a meter does not need to be RMS. Resistance measurement is DC. RMS really pertains to AC.
All true, whilst being slightly OT, though.

He said he wasn't sure if the speaker was damaged. I do think its nice that they actually put a 1r resistor off the cathode for easy bias. In some of their amps, its a lot more of a pain.

 

[sonolink]

It's interesting that Fender do not use that flyback diodes in all Deluxe models.
Also you should adjust the bias of new output tubes to get 60mV at the cathode resistor.  That's the first thing you should do after replacing output tubes. I would check also each cathode current, they must be equal.

You're right and I am aware of that. I told him to change the tubes cos I imagined that the reason why the amp failed during his gig was a bad power tube. Once that ruled out, I was going to have him come over to bias it but unfortunately, the amp failure involved other components. As soon as I get them replaced and checked voltages, etc, I'll bias it. Thanks anyway for pointing it out

Cheers
Sono

 

[sonolink]

Hi again

I replaced the components and switched the amp on. With tubes I get a frying noise when flipping the stand by switch on...
https://instaud.io/3bkk
Please listen til the end. Thanks

Any ideas?

 

[abbey road d enfer]

Hi again

I replaced the components and switched the amp on. With tubes I get a frying noise when flipping the stand by switch on...
https://instaud.io/3bkk
Please listen til the end. Thanks

Any ideas?

There are many possibilities. Can you have oscilloscope pics of the signal on the plates of the power amp section? Not only the 6L6's but also the PI.
I could be one of the octal sockets is contaminated, or worse, an arcing transformer.
Can you reduce the mains voltage from 230 to about 180V, to see if it still happens?

 

[sonolink]

There are many possibilities. Can you have oscilloscope pics of the signal on the plates of the power amp section? Not only the 6L6's but also the PI.
I could be one of the octal sockets is contaminated, or worse, an arcing transformer.

I have an oscilloscope. A ****** Owon. Should I ground the black terminal and attach the other terminal to the plate pin on the tube?

Can you reduce the mains voltage from 230 to about 180V, to see if it still happens?

Mmmm not really....I think I'd need some gear I don't have for that...

Thanks for your help Abbey
Cheers
Sono

 

[Mylithra]

Make sure your probe and scope is rated for the voltage you're about to throw at it. I have a 100x probe I use for work on the HV side of an amp.

 

[abbey road d enfer]

I have an oscilloscope. A sh*tty Owon. Should I ground the black terminal and attach the other terminal to the plate pin on the tube?

Yes. As mentioned by Mylithra, there several hundred volts there, so the usual precautions are de rigueur.

Mmmm not really....I think I'd need some gear I don't have for that...

One thing you can do is use the old trick of connecting a light bulb in series. It limits the in-rush current and protects the amp in case it goes in runaway mode. You need a real incandescent bulb, not an LED nor a fluorescent; harder and harder to source theses days.

 

[sonolink]

Make sure your probe and scope is rated for the voltage you're about to throw at it. I have a 100x probe I use for work on the HV side of an amp.

Yes, I'll check that first. I think I have a multiplier too on the terminals. I haven't used the oscilloscope for a few years now and I originally bought it to calibrate a tape machine...

Yes. As mentioned by Mylithra, there several hundred volts there, so the usual precautions are de rigueur.

Indeed they are. Seriously, I'll be very careful. I always am because I'm **** scared every time I've got to chopstick or something like that...

One thing you can do is use the old trick of connecting a light bulb in series.

That's good news. I have a bulb limiter that I use to fire up for the first time the amps I build. I'll use that then. IIRC, it's got a 100w bulb I think.

I'll measure all that and report back.
Thanks to both for your guidance
Cheers
Sono

 

[abbey road d enfer]

I have a bulb limiter that I use to fire up for the first time the amps I build. I'll use that then. IIRC, it's got a 100w bulb I think.

You'll need less than that I think; start with 40-60W.

 

[sonolink]

start with 40-60W.

Ok. So I switch the amp on with the bulb limiter with a 40-60w bulb and read voltages between plates and gnd with the scope and report back. Correct?

Cheers
Sono

 

[abbey road d enfer]

Ok. So I switch the amp on with the bulb limiter with a 40-60w bulb and read voltages between plates and gnd with the scope and report back. Correct?

Cheers
Sono

Yes. You need to select the lightbulb for about 20% less than nominal at the amp, so the heaters work almost normally.

 

[sonolink]

Ok so I checked my gear and the bulb is a 60W and the scope probes are supposed to handle 300V on the 10x position. I guess that's enough since I'll be using the limiter and the schematic says that voltage should be around 60VDC right?

Cheers
Sono

 

[abbey road d enfer]

Ok so I checked my gear and the bulb is a 60W and the scope probes are supposed to handle 300V on the 10x position. I guess that's enough since I'll be using the limiter and the schematic says that voltage should be around 60VDC right?

Cheers
Sono

Nominal voltage on plates is about 430 V, peaking at 700+ when driven.

 

[moamps]

.....the schematic says that voltage should be around 60VDC right...

Values in oval frame are AC (audio, 1kHz test tone) and in rectangular are DC.