What amps use the HOT!! cement resistors and zeners?

[R.G.]

I know the Carvins and Hot Rods. Which other amps use the power-resistor-and-zener approach for making +/- 14V for opamps and such?

I'm writing up the more general case following the discussion on Carvins.

[Stevem]

RG, off the top of my head I say try taking a look at Musicman amps.

[maxkracht]

Pretty sure Peavey did this in the 80's on a handful of amps, but they were a bit more conservative with the starting voltage. Don't think I have ever seen one burn through the PCB like in fenders, but same basic design and they get fairly hot. Can't recall model numbers.

[dorrisant]

Blues Jr
Crate Blue Voodoo BV60 and BV100
Marshall DSL and TSL

Just a few I could think of right off the bat... We know many more exist.

I hate this method = too cheap of a build to get a PT winding for it... so, 2* resistors, zeners, filter caps = happier bean counters.

I call it brute force... copped that from somewhere else, probably Enzo, or RG...

[R.G.]

Many thanks for the pointers. The collective experience here is great!

[FUCHSAUDIO]

Honestly, it's not the hot resistors (assuming they are used within their ratings), it's how they are implemented. If they are on a crappy, single-sided circuit board, not elevated so they can breathe, they will be a problem.
On the Hot-Rods, I usually replace the resistors and the zeners, and raise off the board, using ceramic standoffs, OR I put aluminum housed resistors on the chassis under the board and run wires back to the old resistor locations.
Usually, the resistors are used within their ratings, but not adequately ventilated or poorly mounted.

[pdf64]

I’ve often wondered why amp designers use dropper resistor and zener, rather than standard 3 terminal regulators, eg 78xx and 79xx family?

[johnnyreece]

The old Crate Vintage Club series used something similar, although they did appear to have a dedicated winding. They just used much smaller value resistors, but if memory serves me correctly, they got HOT. Forgive me if this isn't what you were looking for exactly.

[Ten Over]

More Fenders
60 Series
Vaporizer
The Twin
Super Sonic 60
Super Champ XD
Super Champ X2
Pro-Tube Amps
'94 Twin
'93 Super/Concert

Crate
BV-50H
BV-60H
KBA-60
VC-5XXX
VC-3112
VTX200S

[R.G.]

@FUCHSAUDIO:
That approach works, all right. Using higher-rated resistors and/or moving the generated heat off to somewhere else can take care of it. This is a simple, expedient way to fix an amp with this problem.

My disagreement with the resistor-and-zener approach is that it constitutes penny-pinching by the manufacturers that wastes power and causes some fraction of the buyers to incur repair bills. Given my experiences designing equipment for manufacture, it's (IMHO) sloppy and inelegant design that moves costs to the user; worse, the costs could be avoided.

We were beaten over the head and shoulders that if fixing a problem in the design phase costs $1 per machine, fixing it in the manufacturing stages costs $10 per machine, and fixing it once it's in a customer's hands will cost at least $100 per and maybe much, much more. It's a formalization of the classical folk wisdom of accountants and MBAs leaving out $0.10 parts and causing a higher failure rate in customers' hands.

@pdf:

I’ve often wondered why amp designers use dropper resistor and zener, rather than standard 3 terminal regulators, eg 78xx and 79xx family?

Two reasons: Second, 7800s and 7900s can only withstand about 35Vdc as a maximum input voltage; and first, it's far cheaper to use a winding you already have (as in a bias winding) that makes too high a voltage (~70Vdc or so) and a resistor/zener wasting power than to design in something more elegant.

@johnnyreece and Ten Over: again, thank you. I suspect that many of these will be modifiable to a lower-power method.

Now that I think about the circuits a bit, they're in the same design family as capacitive-dropper power supplies. These supplies can produce a (f'rinstance...) 5V power supply directly from 120Vac by using the impedance of a capacitor in the range of 100nF to 470nF. The capacitor limits current, and a zener contains the resulting rectified voltage. Capacitive reactance does not produce heat like a resistor dropper would, so these things are good for quick and dirty logic supplies that don't need line isolation. Subbing in a capacitor with Xc = 200 at 60Hz (about 13.2uF if I got the decimal points right) would be a non-dissipating replacement for the 200 ohm resistors in these supplies. But of course, 13uF film caps are way more expensive than wirewound resistors, so again, the decision is for lower cost rather than lower power waste.

[martin manning]

We were beaten over the head and shoulders that if fixing a problem in the design phase costs $1 per machine, fixing it in the manufacturing stages costs $10 per machine, and fixing it once it's in a customer's hands will cost at least $100 per and maybe much, much more. It's a formalization of the classical folk wisdom of accountants and MBAs leaving out $0.10 parts and causing a higher failure rate in customers' hands.

...And irreparable damage to the reputation of the manufacturer, resulting in $$$$ in lost revenue.

[maxkracht]

I think Fender has proven that if you start with a good enough name, you will not lose much revenue from faulty products. Just has to look close enough to the classic thing on the outside and make it past the warranty period.

[FUCHSAUDIO]

@FUCHSAUDIO:
That approach works, all right. Using higher-rated resistors and/or moving the generated heat off to somewhere else can take care of it. This is a simple, expedient way to fix an amp with this problem.

My disagreement with the resistor-and-zener approach is that it constitutes penny-pinching by the manufacturers that wastes power and causes some fraction of the buyers to incur repair bills. Given my experiences designing equipment for manufacture, it's (IMHO) sloppy and inelegant design that moves costs to the user; worse, the costs could be avoided.

We were beaten over the head and shoulders that if fixing a problem in the design phase costs $1 per machine, fixing it in the manufacturing stages costs $10 per machine, and fixing it once it's in a customer's hands will cost at least $100 per and maybe much, much more. It's a formalization of the classical folk wisdom of accountants and MBAs leaving out $0.10 parts and causing a higher failure rate in customers' hands.

@pdf:

I’ve often wondered why amp designers use dropper resistor and zener, rather than standard 3 terminal regulators, eg 78xx and 79xx family?

Two reasons: Second, 7800s and 7900s can only withstand about 35Vdc as a maximum input voltage; and first, it's far cheaper to use a winding you already have (as in a bias winding) that makes too high a voltage (~70Vdc or so) and a resistor/zener wasting power than to design in something more elegant.

@johnnyreece and Ten Over: again, thank you. I suspect that many of these will be modifiable to a lower-power method.

Now that I think about the circuits a bit, they're in the same design family as capacitive-dropper power supplies. These supplies can produce a (f'rinstance...) 5V power supply directly from 120Vac by using the impedance of a capacitor in the range of 100nF to 470nF. The capacitor limits current, and a zener contains the resulting rectified voltage. Capacitive reactance does not produce heat like a resistor dropper would, so these things are good for quick and dirty logic supplies that don't need line isolation. Subbing in a capacitor with Xc = 200 at 60Hz (about 13.2uF if I got the decimal points right) would be a non-dissipating replacement for the 200 ohm resistors in these supplies. But of course, 13uF film caps are way more expensive than wirewound resistors, so again, the decision is for lower cost rather than lower power waste.

I don't disagree. I have a dedicated winding that makes 20 or so volts +/- and it runs relays, digital reverb, etc. It's not a substantial increase in the cost of the transformer to do it this way, but Fender penny pinches like crazy !
John Suhr mentioned that he suggested they move their jacks off the main board to avoid damage when someone tripped over a cable etc (something most repair shops do as routine), and it was something like a dollar or two per amp, and got shot down...

[xtian]

Why are we complaining? Those Hot Rods keep me in business!

[R.G.]

Why are we complaining? Those Hot Rods keep me in business!

Think of it the way the MBAs do - having a premium solution in your pocket means you can offer a simple repair at the existing price, or a premium repair for a premium price.