Working on a Silvertone Twin Twelve 1484 amp. Uses a voltage doubler for the HV. Output should be 480 but seeing only 375. I'll assume these caps being 45 years old are weak resulting in the 100 volt loss.
Funny thing printed on the capacitors "Guaranteed for One Year" LOL
What is the sag in the rectifier tube/s?
No tubes in the supply.
This application seems hard on caps. Had four identical tuners where a cap performing this function failed on all at nearly the same time.
I don't understand why a conventional full way bridge was not used. This amp uses two 6L6GC power tubes, three 12AX7 and two 6EU7 tubes much like say a Fender Bandmaster (minus the 6EU7 reverb amp and phase inverter). But the Bandmaster uses the traditional supply. Doublers aren't known for their ability to supply high current so even in new condition this supply is going to sag more than traditional. I guess the next step is to procure some replacement caps and hope they don't exceed the performance of the original design necessitating replacing the diodes also. Last conventional supply in a Twin Reverb recapped and replaced the diodes in jumped the B+ from 475 to 540 requiring a change in the existing droppers and adding a dropper to the plate supply.
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=A0 London SW
Power surge??
** Lower voltage electros are cheap. ** No such difference exists between a full wave doubler and a bridge circuit.
The doubler simply use two caps of half the voltage and double the uF compared to a bridge.
Music Man amps all use doubler supplies for the HT - which has the additional benefit of giving a half voltage supply for the output tube screens.
.... Phil
A 100uf @ 450 vdc can't be much more expensive than a 250 vdc of the same value. And a doubler has twice the caps as a bridge correct?
I know the MM 112 has a 700v HT didn't know it was derived from a doubler. I was taught that doublers are less able to supply a constant current than a bridge. That was back in the 70's.
l.
ELectricity still works the same way as far as I can tell.
On 2/11/2010 7:43 AM Meat Plow spake thus:
I don't know about *constant* current compared to a conventional supply--that depends on regulation (if any) and overall power supply sizing relative to its load--but obviously doubling the voltage cuts the current in half. That's kinda an immutable law of physics.
There is no regulation. Voltage tolerances are 10% so that's a lot of leeway for the 475 volt plates, 460 volt screens.
I re-capped and chaged diodes today in this amp. B+ went from 365 to
470. The brand name on the caps is Planet. Never heard of them. Also saw a few smaller caps made by Sangamo.** There is more than one kind of " voltage doubler " circuit.
The two diode, two cap " full wave" doubler has no current limitations.
Clint Eastwood's famous remark applies.
.... Phil
This circuit uses 4 diodes and 4 caps. All were replaced yesterday and the HT is back up to 480. Actually it starts at 500 and drops when the tubes warm.
I don't know the cost of those caps back then but it seems that 4
350wvdc 100uf caps would cost as much as two 600wvdc 100uf caps.Now that the supply is repaired I'm off to the a buggered reverb circuit. If you tap on the tank you can hear the spring. Apparently no drive from the 6FQ7.
The amp has been repaired, your idiotic arguement is moot, move along.
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