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

[slidincharlie (Carlo P)]

Hello fellow ampagers!
I have had the power tranny of my mid-60s Silvertone 1482 amp rewound.
First thing: there is a new wire that wasn't there in the original tranny. The tech called it "grid". Any idea of what this wire is for? I left it unhooked for good.
Second thing: the heater wires have slightly different, and higher than stock, tensions: about 6.6 and 6.4 Vac, measured with no tubes in place.
Third - and most important - thing: I turned the amp on for a first check, with only the rectifier tube (an RCA 6X4) on. After less than one minute there was a weak crackling noise and the tube lit off. The heater and AC voltages are still there, but the tube shows no sign of life (no dc high tension). Also, the power light shut off shortly after the recto.

What can cause a rectifier tube and the power lamp to die when the ac voltages are fine?
I'd love to find the cause of failure before putting a new 6X4 in the amp.

[52 Bill]

Originally Posted by slidincharlie (Carlo P)

The tech called it "grid". Any idea of what this wire is for? I left it unhooked

Internal shield?

Originally Posted by slidincharlie (Carlo P)

the heater wires have slightly different, and higher than stock, tensions: about 6.6 and 6.4 Vac, measured with no tubes in place.

What are these voltages measured in reference to, ground? From wire to wire?

Originally Posted by slidincharlie (Carlo P)

The heater and AC voltages are still there, but the tube shows no sign of life (no dc high tension). Also, the power light shut off shortly after the recto.

Does the filament still light up? Read the ac voltage across the socket for the filament (pins 3 and 4), and across the pilot lamp socket.

[trevorus]

I would guess that your newly wound PT is over-volting the recto. it's only rated for 325v a side, by the datasheets. My 1483 came with a 5y3GT in the recto spot, but that is with a stock transformer. Measured potentials were in the 700 volt range peak to peak, no load. That is skirting the edge of the data sheet's ratings as it is. You might give a 5U4 a try (you'll have to change the pinouts when changing tube types), as it can handle 450 volts a side, but before anything else, I would measure voltage on you HV winding with no load. Be careful measuring it, though, that crap hurts, and can kill... Also, you may look into your filter capacitors to make sure they are rated for whatever voltage your PT is kicking out.

[Enzo]

Yes, what is the HV AC from the transformer?

Why does the 6X4 blow? HArd to say. What had the tube experienced from whatever caused the original transformer to need replacement? The tube could have been damaged, just waiting to die.

The 6X4 is a small 7-pin miniature, it would take some serious effort to stuff a 5U4 in there, aside from its needing a 5v heater winding. And I don;t know if there is room in the cab for a tall 5U4.

Rectifiers can arc over if the voltage is insane, but usually it is excess current draw through it that kills them. COUld be from leaky or shorted caps, bad tubes, shorts. Maybe whatever killed the first tranmsformer is still wrong in the amp.

[slidincharlie (Carlo P)]

Here's a combined reply (a schematic is attached).

The AC HV from the tranny is 350 V on each end (referred to ground), which is pretty normal since the schematic calls for 335 Vac (I've been told here that I can expect quite a drop in AC HV from a 6X4 when all tubes are in place). All filter caps and most other caps are fresh (yet I don't know if one has developed a short somewhere).
Also it is possible that the 6X4 was already damaged from the previous tranny failure, but I wanted to eliminate all possible causes of damage before using a new tube.
The heater voltage is 6.6 and 6.4 V referred to ground (plain 13 V across heater pins and across pilot lamp legs).
There IS heater voltage throughout the heater circuit now (no tubes in place for the moment).

This amp quit working all of a sudden several months ago after a couple years of normal operation. Only evident symptom of death was a shorted power tranny. That tranny had been rewound, too (by a different person). Since the first 'rewinder' had no experience with musical applications of a tranny, I thought that the tranny died due to poor craftsmanship. The second 'rewinder' does build trannies for a local boutique amp manufacturer.

[Robert M. Martinelli]

Hi Carlo,
the schematic you posted calls for 6,3 VAC PT sec. and heaters, this means that you should not have 6,3-6,4 VAC between each PT heater winding leg and GND, but rather 3,15 VAC.

With some 12,6/12,8 VAC across them, your heaters are bound for destruction, as, getting twice the voltage, they will have to dissipate four times their rated power.

Another point I can think of...An 6X4 can withstand 450 VDC between the heater and the cathode, and you don't have a separate winding for the rectifier's heater, this means you MUST NOT connect the rectifier's cathode together with one heater's leg, as by doing so you're creating a short to ground ( from the heater to the center tap of the PT heaters' winding ).

Hope this helps

Best regards

Bob

[Enzo]

You think he's going to exceed 450VDC on that cathode, Bob? That is 100v over the circuit spec.

[Enzo]

Hey Bob, you get all over this forum, so you;re the perfect guy to tell. I'm on vacation until tuesday, if anyone misses me, tell them. I'm too lazy to turn off my email link, I'll probably get a ton of that.

[Robert M. Martinelli]

Originally Posted by Enzo

You think he's going to exceed 450VDC on that cathode, Bob? That is 100v over the circuit spec.

Hi Enzo!
No, quite the contrary, I was rather concerned about a "short to ground" issue, probably I didn't manage to explain it very clearly, I cited the 450 V spec to highlight that the 6X4 rectifier tube doesn't need one heater's leg to be connected to the cathode as it can withstand very high cathode-to-heater voltages...Thanks for asking though, because it gave me the chance to re-read my post and to understand how badly I expressed my thought....Hope this time I' ve managed to be clearer....

Enjoy your vacation, everyone here will miss you, so be back to us ASAP ( or bring a laptop with you to keep in touch with us.....just kidding! Get some well deserved rest and forget about us ! )

Cheers

Bob

[slidincharlie (Carlo P)]

Jesus... something odd is going on here...
I can't believe the guy rewound the tranny the wrong way...
Back from work I'll check the amp and post again.

[52 Bill]

Originally Posted by Robert M. Martinelli

With some 12,6/12,8 VAC across them, your heaters are bound for destruction, as, getting twice the voltage, they will have to dissipate four times their rated power.

Yes, this is why I asked if the filament was still lighting up.

I could imagine that an old tube would die, but when he said that the tube and the pilot lamp died at the same time, I thought that there might be a problem with the filament voltage.

[slidincharlie (Carlo P)]

I have re-read the heater VAC with the secondaries and the heater CT on air:
there are about 6.4V between either heater secondary and its CT, and about 12.8V across the two secondaries: same as with the PT fully hooked.
Also, the heater CT is made of two wires inside the same tubing: is this normal? Actually it happens the same in my 1472 amp, which is quite similar to this 1482.
EDIT: I made a mistake in the starting post. The green (currently unused) wire is labelled "screen", not "grid" as I stated above.
Can this wire play a role in the "double heater voltage" issue?

Now, where is the problem? Did the tech rewound the heater coil the wrong way? Can this be corrected? May I use a resistor to get half the heater voltage?

[Robert M. Martinelli]

Hi again Carlo,
As I stated in my previous post and as Bill suspected, your heater's winding is wound wrong, as it gives you twice the voltage you'd need out of it.

Now, for the possible solution :

You stated that the heater's winding center tap has two wires tied together. This center tap consists of the "end" of the first 6.3 VAC winding tied together with the "start" of the second 6.3 VAC winding.

If you separate these two wires, you now have two separate 6,3 VAC windings ( a simple measurement will confirm this ), and you can try to use only one ( if its current rating is enough ) or, better yet, put the two windings in parallel ( the "start" of the first tied together with the "start" of the second - the "end" of the first together with the "end" of the second ) Should you connect them "wrong" ( out of phase ) you won't have any voltage out of them; should this be the case, simply swap one winding's connections and everything should now be ok.

This way you'll get a 6.3 VAC winding with twice the current capability.

You' ll no longer have a center tap, but this can be very easily remedied by adding a couple of 100 Ohm resistors ( or a hum balance pot ) between each 6.3 VAC leg and GND ( for hum-canceling purposes ).

Hope I've made things sufficiently clear, if not, don't hesitate to ask for further help!

Best regards

Bob

[Steve Conner]

Great idea

Just be careful, if you parallel the windings wrongly, you'll not just get no voltage, you'll short out the whole transformer and might burn it up. (Don't ask me how I know this...)

If you think about it, the two wires that were previously inside the same tube should NOT end up connected together again. Following this rule, there are fewer ways left to screw up.

Of course, if one half reads 6.6 and the other 6.4, there's always the possibility that our foolish transformer winding friend has used one extra turn in one of the two sections. (0.2v is about right for one turn.) Paralleling them will end in disaster. I recommend connecting one end, then rubbing the free ends together with the power on and looking for sparks (there should be none) Also ask the guy who wound it if it's OK to parallel the windings. Better still, send it back and get him to parallel them for you and test it

In fact, maybe when all's said and done, it's less hassle to separate the windings and run half of the tubes off each one.

PS, a wire marked "Screen" is a Faraday screen between primary and secondary, and should be connected to mains ground/chassis. It helps to stop noise from the mains getting into your amp.

[Robert M. Martinelli]

Hi Steve,
Thanks for appreciating my idea and for the additional "caveat".

Carlo,
I have attached a ( poorly drawn ) drawing to make things clearer.

Cheers

Bob

[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)]

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)]

Originally Posted by MWJB

(...) 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)]

Originally Posted by MWJB

(...) 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?

[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]

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]

Originally Posted by 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.

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?