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

[MWJB]

Red Bank NJ Danelectros (Silvertone/Wards Airline) used some wierd filament circuits...I would rather see shots of the tube wiring, than trust the 6.3VAC on the non-original schem.

[slidincharlie (Carlo P)]

Thank you guys.
Prior to applying the "parallel windings mod" suggested by Bob, I confirm that the two windings have slightly different readings referred to ground: 6.4 and 6.6 VAC respectively. May I parallel them?

Furthermore here's another clue:
after splitting the two-wire CT, while I get a 6,4 (or 6.6) V reading between one CT wire and its own end, I get about 25-30V between one CT wire and the end of the other winding. Is this what you expect?
That is:
CT(1) to end-of-wire(1): ca. 6.5VAC
CT(1) to end-of-wire(2): ca. 25VAC

[Robert M. Martinelli]

Hi again Carlo,
as Steve said, if the windings are "unbalanced" then it's not safe to parallel them; at this point you have two choices: take back the PT to your friend and have him rewinding it properly ( one 6,3 VAC winding with twice the current capability ), or use the two 6,3 VAC windings you now have to separately supply the preamp's and the power amp's heaters ( e.g. if you're concerned about hum you could supply DC to the preamp and AC to the push-pull, which is hum-canceling by design ). I'd personally take the first "road", easier and more "true to the original", but the choice is of course yours.

As to the "weird" readings you're getting, I'd say they could be "ghost" voltages, especially if you're using a DMM, which has a very high input impedance/resistance that renders it more accurate than a "needle" MM, but also rather prone to pick up "fake" readings. I'd bet they will disappear once the circuit will be connected to a load.

This is why I also have,( and still use a lot ) a very good needle MM, an ICE680R; maybe not as accurate as a modern DMM, but very good when working where stray/induced voltages can be present, like in a tube amp.

Hope this helps

Best regards

Bob

[Steve Conner]

Hi all,

I wouldn't worry about the 25V AC reading. It's almost certainly a "ghost voltage" as Bob said: voltage picked up from other windings through the inter-winding capacitance. It doesn't have enough current to activate a needle meter. (and yes, I agree that an old-fashioned analog meter is good to have around for this reason... Avo Model 8 anyone?)

However I do think the 0.2V difference is worrying. When I said to try the spark test, I meant that you should wire the windings as in Bob's "After" diagram, but only make one of the two connections that he shows. Then carefully make the other connection with the power on. If there is a voltage imbalance between the windings, then a fairly heavy current will flow when you touch the wires together, and you'll see sparks.

Instead of just touching the wires together, you could make the connection through your DMM's 10 amp range (AC of course) or use a clamp-on ammeter if you have one lying around. If the windings were perfectly balanced, the current would be zero.

[slidincharlie (Carlo P)]

Low heater voltage (5.8V)

Back I am.
A brief summary: The rewound power tranny in my 1482 Silvertone amp has got incorrectly wound heater coils: they give 13 Vac across and 6.6-6.4 Vac between each end and the central tap.
Update: taking it back to the guy who rewound it is not a feasible option. Thus I have used one coil to supply the tubes (including the 6X4 rectifier) and left the other coil unhooked.
My problem: although I have about 6.5 Vac across the two wires unhooked, I get only 5.9 Vac when everything is conneed and the amp is on.
The DC voltages in every single point of the circuit are ok and close to specs, and the amp sounds fine (maybe a little bit crucnhier than before, but this IS a crunchy sounding amp).
I told the guy to allow for 3 A rating in the heater coils.
Is less than 6 Vac a major problem or may I live with it?

[MWJB]

If your ears suggest that you can live with it then live with it. Personally I couldn't, 5.9VAC will normally just make an amp sound wheezy, splatty, with poor fidelity...to me it sounds like a malfunction, rather than character. I'd want over 6VAC, 6.2VAC min?

What happens when you try a SS rectifier/Copper cap?

Older Dano/Silvertone/Wards amps often used the series heaters of the 12AX7 tubes to provide cathode resistance for the power tubes. Also frees up 0.6A of heater current. You might need an additional cathode resistor (100ohms?) in series if trying this, this approach usualy makes for a noisier amp too. If a 12AX7 heater fails it'll take your power tubes with it....maybe you could work a compromise that gave survivable dissipation with just the cathode resistor but still decent dc voltage on the 12AX heaters? See Wards GDR 8517A for an example.

[slidincharlie (Carlo P)]

hmmm... Too difficult for me. I am not able to follow your instructions.
Is there a specific technical reason that I have such low heater voltage?

If your ears suggest that you can live with it then live with it. Personally I couldn't, 5.9VAC will normally just make an amp sound wheezy, splatty, with poor fidelity...to me it sounds like a malfunction, rather than character.

Actually the amp sounds maybe a little fuzzier than before, maybe also with some buzz (like a ground issue) that it did not have.

[MWJB]

I can find 2 schems for the Silvertone 1482, neither appear to be original factory schems, one shows a centre tapped heater winding, the other does not...maybe there were 2 versions, maybe both are incomplete? Perhaps there is an error in what was conveyed to the tranny winder regarding heater secondary (0-6VAC, 3.15-0-3.15VAC, 6.3-0-6.3VAC etc..)?

Either way, I'd add a virtual centre tap from either leg that you are currently using (2x 68ohms to ground as on the 1481 schem) assuming that you are using a twisted pair, rather than a single wire daisy chain. This might help with your hum?

Pull the 6X rectifier and use diodes, see how much heater voltage this frees up?

[MWJB]

2x 12AX7, plus 6X5, plus 2x6V6 adds up to more than the 3A you requested (3.3A by my reckoning) so it looks like you don't have enough current capacity in the heater winding. Losing the 6X5 should free up 0.6A...might be enough? Pull the 6AU6 unless you need the vibrato.

[slidincharlie (Carlo P)]

I need the 6AU6 'cause I use tremolo...
How should I arrange diodes for a ss arrangement? I should use 1N4007's I suppose? May I copy from another amp circuit?
There is a Weber 6X4 copper cap replacement at US$ 22. The spec sheet says Imax=70mA, voltage drop @ Imax=22V.
Since it's a direct drop-in replacement it would be easier to use than a 1N4007 rectifier stage.

[Robert M. Martinelli]

Hi Carlo,
being 5,9 VAC a little low for proper operation IMHO ( even if some datasheets specify that the heater voltage can go as low as 5.7 VAC ), I would do as MWJB said, throwing ( not literally! ) the 6X4 away ,and throwing in a couple of 1N4007 in its place. You will have to add a series resistor though, which will serve two purposes: lower the HT ( 1N4007s have a very low voltage drop due to their very low differential resistance so without the additional series resistor your HT would be way too high ) and add some "rectifier sag" to the design ( because 1N4007s wouldn't "sag" under dynamic conditions.
A 470 Ohm 7 W resistor should be fine ( but you might want to use a 10 W one for peace of mind ), but, should your HT be a little low, you can move to a 390 Ohm 5 W one ( use a 7 W one for additional peace of mind ).

I have prepared a small schematic ( which includes the corresponding 6X4 pins ) to help you in this task - As to the diodes' pinout, the white stripe on the 1N4007 indicates the cathode. You can safely leave the heater supply wires ( pins 3 and 4 ) connected to the 6X4 socket, they won't do no harm.

Hope this helps

Best regards

Bob

[slidincharlie (Carlo P)]

Great! Thank you guys. Bob's drawing is very helpful. I have learned many things from this forum through the years, but I am still a morone when it come to electronics...
I'll post the results.

[slidincharlie (Carlo P)]

(...) Either way, I'd add a virtual centre tap from either leg that you are currently using (2x 68ohms to ground as on the 1481 schem) assuming that you are using a twisted pair, rather than a single wire daisy chain. This might help with your hum?

I had already put two 100 ohm resistors there from each of the power lamp's legs to ground. Any reason for using 68ohms? The 1472, which is essentially a 1482 running at lower overall voltages, uses two 68ohms, too.
Currently this amp (like many other old amps) has got the heater wires running as single wires adherent to the chassis. Arranging them in Fender-like twisted pairs running above tube sockets might help with hum, as you say? Isn't there any interference/oscillation issue if I modify the current heaters arrangement?

[MWJB]

100ohms should work just as well....but now I'm confused, you say the amp has "single wires adherant to the chassis"? A single wire daisy chain will have one leg of the heaters grounded, the other leg running to one side of the tube heaters (say pin 2 6v6, 4&5 12AX) with the remaining connection grounded - see an original Fender Champ, 5E3 Deluxe etc.

The twisted pair runs both legs of the heaters, one to either side of the tubes and is ground referenced via a centre tap or the 2 virtual CT resistors. Which do you have?

If you have a twisted pair simply running along the chassis floor, there is no reason to switch to suspending them above the tube sockets.

[slidincharlie (Carlo P)]

(...) The twisted pair runs both legs of the heaters, one to either side of the tubes and is ground referenced via a centre tap or the 2 virtual CT resistors. Which do you have?
If you have a twisted pair simply running along the chassis floor, there is no reason to switch to suspending them above the tube sockets.

Sorry for the confusion. I have a pair that runs both legs of the heaters, one to either side of the tubes and is ground referenced via the 2 virtual CT resistors. I meant that the two wires run parallel each other close to the chassis, i.e. they are not twisted and suspended like in bf or sf Fender amps. I take it that I may leave'em alone, right?