New power tranny: rectifier and power lamp blow (Silvertone 1482)

[MWJB]

Sure, leave as they are.

[slidincharlie (Carlo P)]

I have installed the solid state rig with 2 1N4007 diodes and a 390ohm 11W resistor in series, per Bob's suggestion. The voltage drop is a bit high: I have 310V B+ instead of 350V called by the (non-original) schematic. I think that I will put a smaller resistor if this will let me get
The 390ohm resistor gets very hot after a short while, but I guess this is normal, no?
The heater voltage is still low though: it's 6V when I turn the amp on, but it goes down to about 5.8 a bit later, when the amp is hot.
I am totally satisfied with the amp's tone. I am only concerned about the low heater voltage.

[65Fender]

Little series trick

Add 6.3V 250mA Incandescent Falshlight Bulb (2-Pack)
$1.79
Model: 44
Catalog #: 272-1108

From Radio Shack in series , put both heater winding back in. Then you can fine tune the heater volatge you want by changing bayonet type bulbs, cheaply. There are mny volatges to chose from and you have your heater current back.

[Robert M. Martinelli]

I have installed the solid state rig with 2 1N4007 diodes and a 390ohm 11W resistor in series, per Bob's suggestion. The voltage drop is a bit high: I have 310V B+ instead of 350V called by the (non-original) schematic. I think that I will put a smaller resistor if this will let me get
The 390ohm resistor gets very hot after a short while, but I guess this is normal, no?
The heater voltage is still low though: it's 6V when I turn the amp on, but it goes down to about 5.8 a bit later, when the amp is hot.
I am totally satisfied with the amp's tone. I am only concerned about the low heater voltage.

Hi Carlo,
Sorry about my HT figures being a tad on the low side, maybe your amp draws a little more current than I expected, BTW moving to a lower resistor will reduce the resistor's power dissipation, so I'd advise you to move to a 330 Ohm or even a 270 Ohm one ( 10 W will be safe ). Though the heater voltage is a tad low, some texts/datasheets state the heater voltage can be as low as 5.7 VAC, so, if you' re satisfied with the tone, I wouldn't worry too much about it.

Cheers

Bob

[melvin]

This may be silly to say at this point, but why not hook up that extra heater winding to just the old rectifier tube?

[MWJB]

Carlo,

What heater voltage do you get with no load on the circuit (no tubes)?

If your 6.3VAC winding is dropping to 5.8VAC then that is a cause for concern...if it's not 6.3VAC in the first place, then maybe it's not such a problem?

In the interests of research, check the heater voltage with the 6AU6 pulled but all other tubes installed, what do you get?

[Robert M. Martinelli]

Carlo,

What heater voltage do you get with no load on the circuit (no tubes)?

If your 6.3VAC winding is dropping to 5.8VAC then that is a cause for concern...if it's not 6.3VAC in the first place, then maybe it's not such a problem?

In the interests of research, check the heater voltage with the 6AU6 pulled but all other tubes installed, what do you get?

Hi MWJB,
the person who rewound the transformer rewound it ( wrong ) so that the no-load voltage now is 6.3V, so under full load conditions, a 9% drop, even if a tad too pronounced, is still acceptable IMHO. AFAIK the no-load voltage for a 6.3 VAC heater winding should rather be in the 6,6-6,9 VAC range, dropping to the "on-spec" value with the tubes back in.
JM2CW
Cheers
Bob

[MWJB]

"so under full load conditions, a 9% drop, even if a tad too pronounced, is still acceptable IMHO." From a tonal point of view, I can't agree, I'd want to see 6.1VAC upwards, but if Carlo says he likes the sound at 5.8, then it's not a concern for him.

[Robert M. Martinelli]

Hi again MWJB,
it seemed to me you were concerned about the drop being a sign of an excessive current, with the risk of burning the PT, so my post was related to that concern only, and not to the "sonic implications" a lower heater voltage can bring, because Carlo stated he's happy with the way the amp sounds.

In the old days manufacturers specs were rather loose, it's not unusual to find schematics ( e.g. Fender ) calling for a + or - 20% on the specified voltages.

Anyway,I totally agree with you about the need for the heater voltage to be as close as possible to the valves' specs.

Cheers

Bob

[MWJB]

Hi Bob,

Indeed, you're right. I agree if Carlo's happy with the tone, the only concern would be stress on the PT. Really, I just wanted to clarify for anyone else reading this, that might want to apply the info to another amp, that 5.8VAC is typically too low.

I think that 20% tolerance on the Fender schems relates more to component tolerances rather than operating voltages. Vintage Fenders tend to run quite consistently in that respect, with a reasonable current draw, within +/-3% of each other when considering like models? 20% voltage tolerance on an amp running 460vdc would be 368-552v...subsequently impossible to get consistant bias/heater voltages etc.

Cheers, Mark.

[slidincharlie (Carlo P)]

Hello guys,
I did some testing to give you some more clues.
- heater voltage with the heater wires unhooked or connected to the ss rectifier: 6.4-6.5 Vac.
- heater voltage with all tubes in except the 6AU6: 6.0 Vac.
- heater voltage with all tubes in: 5.9 Vac.
From then on, the heater voltage decreases very slowly. Now I'm testing: after 20 minutes it is still 5.9 Vac.
Last night I forgot the amp on for the whole night . The morning after it was at about 5.7 Vac.

[Don Moose]

Theoretially, tubes are OK with 6V3 +/- 20%, so 5.04 would be enough. Thing is, if you then have any issues with it, the heater voltage becomes the first suspect.

Personally, I would separate the two 6V3 windings and use one set for the outputs and the other for the preamp. A little ugly, but it keeps everything where it's supposed to be.

I recently had an Orange AD30 on the bench that lost the heater connection to 2 preamp and 2 output tubes. The 2 preamp tubes died of - cathode stripping, I think they call it - having the high voltage but no heaters.

If you have a recent Orange amp with a PCB and strings of twisted pair hittting a connector under every tube, expect to have this problem eventually - rip those connectors out and hard-wire the heaters.

Sorry for the threadjack

[slidincharlie (Carlo P)]

Personally, I would separate the two 6V3 windings and use one set for the outputs and the other for the preamp. A little ugly, but it keeps everything where it's supposed to be.

Don, do you mean rectifier and preamp tubes separately from the power tubes? I suppose that the circa 0.2 Vac difference between the two windings is not an issue here.
IIRC using each heater winding to supply two separate sections was suggested by someone above, then for some reason the ss rectifier setup seemed the preferred solution.
If I use both heater windings as suggested I should expect less heater voltage drop, correct?
Furthermore I might gain maybe a tiny fraction of Vac keeping the ss rectifier.
Any further advice before I proceed? Mark, Bob?

[Robert M. Martinelli]

Hi Carlo,
Don's advice is a good one, and, yes, using both the 6.3VAC windings will result in a lesser load connected to each one of them, so the heater voltage will surely be closer to the specs ( I'd say some 6.0/6.1 VAC ).

Being the output stage a P-P one, it tends to be hum-canceling, so I guess you can leave the heater wires the way they are ( not twisted ). The preamp's a different story though, and my advice would be to twist the preamp heaters' wires as tightly as possible ( a drill at low speed will do the trick nicely ).

As to the SS rectifier, the diodes/resistor arrangement makes for a good "tube rectifier" emulation, furthermore, it doesn't waste any heater current, so I'd leave it alone.

Hope this helps

Best regards

Bob

[slidincharlie (Carlo P)]

Need one more advice

I need one more clarification here...
I suppose that each separate heater winding, being without a central tap, will need to be referenced to ground, right?
While one winding is already ground-referenced via the two 100 ohm resistors at the pilot lamp, where should I put the other 100 ohm couple for the other winding?
EDIT: if I supply the 6AU6 and the power tubes (which are ground-referenced at the pilot lamp legs) with one heater winding, and the two preamp tubes with the other winding, may I simply attach the two new 100 ohm resistors to pins 4-5 and 9 respectively of one of the preamp tubes?