Hot Rod Delux repair/mods

[Leo_Gnardo]

I would point you towards some reliable parts suppliers like Antique or Mojo for 500V filter caps. There's been a rash of "matryushka" caps - smaller caps packed inside larger cylinders & relabeled - for the past couple years. Something you definitely don't want to use in your repair. Read here:

Matryoshka Capacitors !!! The Heights of Lowness ;-) - coolhand - Tube DIY Asylum

and here:

The Amp Garage :: View topic - Inexpensive Electrolytic Capacitors

Also FWIW I've used 3 x 1N4007 since the mid 80's as flyback quench diodes. Ken Fisher suggested that in his "Trainwreck Pages" (look it up & have a read - it's a good practical education in tube amps.) If it was good enough for Mr. Fisher, it's good enough for me. For amps with extra B+ I'll use 4 in a row. If you have 3 kV's on the way, no harm.

The real reason to have those flybacks is - your speaker voice-coil/cone assemblies take some time to follow the power delivered by the amp because they have some mass & can't instantly move. THEN when they move back to and thru their rest position, driven by the spring system that is the cone-surround and spider, the speakers act as generators, not motors, and the power returned to the output transformer's secondary winding is multiplied by the transformer and shows up as voltage spikes on the primary - up to 3000 volts - and that primary is what, attached to the power tubes' plates. Tubes are pretty tough and although rated for 500-600V generally, can put up with this for years BUT if you limit the spikes to B+, which is what those flyback diodes do, you've taken away the stress of having to deal with those spikes, and hopefully will get increased lifetime for the couple-of-dimes cost of the rectifiers. You can watch the output of an amp that doesn't have flyback diodes on a scope and see those spikes when you're clipping the amp into a speaker. You won't see 'em when driving a load resistor because that physical motor/generator system doesn't exist. Some amps are worse than others at creating spikes. Those that don't - I generally leave the flybacks off.

Sorry to see your encounter with our pet SGM. Hope you've had your rabies shot. He means well. Just likes to bark a lot and occasionally bite. Feel free to bark back.

In my case I fix the tube amps, and let my brother handle the aircraft engines. He's been at Teledyne Continental lo these many years, and took his Jet Shop courses besides, so he can get a job in case T-C's new Chinese owners move the kit & kaboodle to Asia.

Together, we have it ALL covered!

[beezerboy]

see.... now that's some good stuff. I never thought of a speaker as a linear motor but of course it is. I'm very familiar with counter emf & it's effects in motors and generators & that makes perfect sense. perhaps.... and going out on a limb here... the PIV of the flyback 3kv diodes is too low. hard to imagine 3kv burning the insulation on the x-former's primary.... or is it?

the question I have is.... why did this amp implode? it was my brother's and I know for a fact anything I loan him will come back in a bag, in pieces. (picture Baby Huey with red hair & 6' 5) ... unfortunately, I do not know the circumstances, but he says it was only up to about 3, with a Vox Saturator

tell me the things that blow out the primary side of the output transformer

as for the guru.... I could care less about him, but if he has something constructive to say, I'll listen. I'll also assign it an appropriate value. go ahead.... redeem yourself.

[beezerboy]

oh, I got the 22Uf 500V caps from Newoldsound... they are IC brand, same as installed.... good enough right? the rest of the stuff is Panasonic, except the Vishey diodes... all through Mouser... odd that Mouser didn't have a 500v rated cap for me... only 450. I've used them for years & thought they had everything. same thing though.... $10 for parts, $10 for shipping

[oc disorder]

"why did this amp implode?"
Usually the o/p tubes mechanically fail and often arc from plate to heater. This can be due to old age , poor manufacture or a bias that is way out.
EDIT : *** and more often than I care to remember sometime in the past the mains fuse has been replaced with a larger incorrect value instead of the fast blow 3Amp for 120v units or 1.6Amp for the 230 volt units.
Enzo pointed out that the Hot Rod Deluxe III now has a heater fuse , perhaps Fender may fit HT fuses in the future ....?

http://music-electronics-forum.com/t33660/

[Leo_Gnardo]

the question I have is.... why did this amp implode? it was my brother's and I know for a fact anything I loan him will come back in a bag, in pieces. (picture Baby Huey with red hair & 6' 5) ... unfortunately, I do not know the circumstances, but he says it was only up to about 3, with a Vox Saturator

I know the type . . . add to the list my most recent tour manager. Baby Huey, that's just about right. A King Midas in reverse.

tell me the things that blow out the primary side of the output transformer

500V caps - Mouser used to carry the Sprague series that was claimed by many to have the "secret mojo", also physically much larger than your IC's or other brands. In recent times even Sprague planted a dinky IC-size cap in a larger sleeve, covered that with their blue plastic wrap, and sold for @ 3x what you'd pay for anyone else's cap. If Mouser doesn't have 'em any more, just as well.

OC mentioned internal tube arc/short, yes that'll get you an open primary winding in your OT.

Arcing will do it too. Breakdown voltage of air is @ 8000V DC per inch, dry air, sea level pressure. When you're looking at electrodes that are only @ 1/8 inch apart - that's 1000V on a good day. And 500V for 1/16 inch. Derate of course for damp air and less pressure - at higher altitude maybe like in Denver or uphill from there. A popular arc locatiion is between pins 3 (plate) and 2 (filament) on output tubes & sockets. Not much distance between the tube pins, and similar on the socket. Right there is a good reason for those flyback diodes.

- - - - - - - -

I'd be a gooroo too but the FAA put the kibosh on my flying carpet.

[soundguruman]

The way I am looking at it:

Fender has built the most reliable amp for decades. And there's a reason for that. I doubt that any other amps have proved as reliable.

Recently I serviced a 1956 Champ. It had all the original caps, and tubes. And it all still worked, just fine. There is no more reliable design, I doubt it.

Fender designed it with a 3KV diode. That's what I will use.
I am not pretending that I am a better designer than Fender, that would be a bit foolish.
I'm a pretty good designer, but not as good as Fender.

And as I said, it's pretty rare for these diodes to go bad. That's because they used a diode that "would not" fail. That's deliberate.
If there was a part that proved as reliable, and was cheaper, they probably "would have" used it.

And so, I will continue to honor that tradition.
If Fender says it belongs there, it belongs there.
I have repaired amps for 38 years...BUT...I will not pretend that my experience exceeds Fenders. Cause it don't.

[Enzo]

SO when Fender puts a 55v bias supply in a schematic, and calls for - AND INSTALLS - 50v caps, it is because of the flawless design philosophy?

When Fender installs 5w resistors that unsolder themselves from Hot Rod series amps, that is because of their flawless engineering?

When 82k phase inverter plate resistors go open way more than in other amps, it is because of being the most reliable amp in decades?

When we have to resolder the octal sockets on just about every Hot Rod amp that comes through the shop, I suppose that os beause they are so much better at engineering than we are as technicians?


Note those last three ongoing repair scenarios are from the very same amp you use as example of the diode choice that is beyond question.

[Leo_Gnardo]

SO when Fender puts a 55v bias supply in a schematic, and calls for - AND INSTALLS - 50v caps, it is because of the flawless design philosophy?

Hmmm, well there is that, plus the other items you mentioned Enzo. Heck in some tweed Fenders I've even found 25V caps in the bias filter position. AND some of those replaced with 25V caps by none other than the chief authority on everything Cesar Diaz. Which I replace with properly rated caps without hesitation.