Vox Cambridge reverb quandry

[phydauex]

Going through and repairing a formerly "Dead" Cambridge Reverb (V3). Poer Transformer was shorted out, along with the reverb driver shorted. Replaced the power transformer with a Hammond unit that had roughly the same voltages, and higher heater current rating. Alls good, amp works fine now, except......

The indicator lamps don't/won't function. Let me back up a moment. The original power trans had a center tap foon the heater winding, the replacement I got didn't, so I've tied two 220r reistors to ground. The indicator bulbs were run in series off one leg of the heater windings and then grounded, either at the junction (Standby) or at the tail of the two bulbs (On). When I put the bulbs in, the heater voltage goes basically to less than a volt. With bulbs out the heaters are fine (3.15/3.15). I think what's going on is the two resistors tied to ground on the heaters are interferring with the bulbs or shorting them as it were. Not sure quite how to describe it. Suggestions on leaving the heater arrangement alone and getting functioning lights or changing the ground reference for the heater taps.

One other little issue, is when I dissconnect the fake ground for the heaters, the bulbs still don't work. I've replaced the bulbs with known working ones, and measured the old ones and they are not "open". The only unknown variable, is that since the amp came in dead, with no tubes etc., I don't know if the indicator bulbs were working before or not. This was an internet purchase, and former condition was unknown.

[soundguruman]

That's kind of silly.
Because anybody who can replace a PT can re-wire an amp to make light bulbs work.

[Jazz P Bass]

If there is not a center tap on the transformer for the heater circuit, you do not need to form one.
Run the heater windings straight to the tubes & the lamps.
No resistors.

[phydauex]

There was a center tap on the original (two green and a green/yellow - that measured equal ohms between them) and the schematic shows a center tap on it.

Thanks for the helpful response, I know it's silly, but I can't find a reason why they don't work and was looking for suggestions. It uses two different kinds of bulbs and 1/2 the power switch to connect one or both lamps to ground. Yet nothing but high current draw (heater voltage drop) and no light. They are the correct bulbs, as I looked at another one that does work.

[olddawg]

Does the new PT have any extra secondary taps like a 5v? Seems like to me the simplest way now would be to wire your filaments with no ground as mentioned before and use a different supply to run the lights. Or... You could remove the ground from the switch to the lamps and wire them across the 6v tap.

[Jazz P Bass]

There was a center tap on the original (two green and a green/yellow - that measured equal ohms between them) and the schematic shows a center tap on it.

Thanks for the helpful response, I know it's silly, but I can't find a reason why they don't work and was looking for suggestions. It uses two different kinds of bulbs and 1/2 the power switch to connect one or both lamps to ground. Yet nothing but high current draw (heater voltage drop) and no light. They are the correct bulbs, as I looked at another one that does work.

You cannot run the lamps to ground.
They can only go 'across' the heater pins.

[phydauex]

Yeah Old Dawg, There's an unused 5v tap I could use to power the lamp. It would be one lamp to show juice to the PT, but that would probably be just fine. Thanks for the suggestion! I just wanted it to function as originally, even though It's kind of a silly way to do it (stock that is).

[olddawg]

Yeah Old Dawg, There's an unused 5v tap I could use to power the lamp. It would be one lamp to show juice to the PT, but that would probably be just fine. Thanks for the suggestion! I just wanted it to function as originally, even though It's kind of a silly way to do it (stock that is).

Well you can. As both Jazz and I said. You can run the lamps across the filament string. You would need to disconnect the ground point on the switch and run a wire to the other side of the 6v tap from the switch. No ground. Also have to use 6v lamps. But using the 5 v supply would work too and you could ground the switch. There is more than one way to Egypt. Whatever you deem appropriate.

[R.G.]

I believe the original designers were playing games with the bulbs. The standby and on indicator lamps are driven from one side of the heaters to ground. In the "on" position, they run in series. When switched to the "standby" position, the second part of the power switch opens the high voltage CT and shorts the junction of the two bulbs to ground, which forces half the heater voltage across just the standby bulb.

This would work OK if the standby bulb was rated for much more current, it would be very dim when the power and standby bulbs were in series. It would get bright when the power bulb was shorted.

I don't know, but it appears that the bulbs must have had different current ratings, and that the hard center tap on the heater winding is necessary for this to work right.

[52 Bill]

R.G. is right as usual, the bulbs were two different numbers, and had different current ratings. I have the numbers written down somewhere, if I find it I will post the numbers here.

[R.G.]

If they're different numbers, chances are that's how it worked.

That doesn't help the OP much, though, as he's still stuck without a hard center tap on the heater winding, and the original wiring won't work without the bulbs being strung between half the heater and ground at high(ish) and variable current for the different bulbs.

I'll see if I can think up something simple to make the thing work without a heater center tap.

[R.G.]

I think I know how to do it. I believe that the same two-bulb string can be put across the whole heater winding, but the "ON" lamp shorted by a $0.80 triac-output photocoupler, helped by two diodes and a resistor.

This lets the standby switch to ground switch the 10ma or so in the photocoupler LED to the resistor CT, and probably won't cause hum imbalance because it's so small. The LED turns on the triac output to "short" the power lamp and let the standby shine.

I think it will be under $1.50, although it will require different lamp voltages. I believe 5V rated lamps will work, at two different currents ratings.

[phydauex]

Thanks everbody for the thoughts.
As usual, RG has the design know how to figure out a work around.

I figured the hard center tap was the crux of the issue, and yes the two bulbs drew different current (#45 & 1490 I think). Doesn't help that the schematic cuts off the end of the string, but from other discussions, it was supposed to go to ground. That's how the other one I have was wired. But w/o the CT, it wasn't going to work as before.

I used the 5v tap and wired it to either end of the sockets and used two #45's. That's a 3v lamp, so both lamps come on in either position of switch. Which is ok. That takes all the lamps off the heater string altogether, which should quiet the amp down a bit also.
Not elegant, but at least there's some indicator working.

Thanks again for the informatioin. I enjoy getting other takes on the" why's". I often know in fuzzy terms whats actually happening, but getting confirmation is the learning. Thanks.

[52 Bill]

You could ground one side of the 5 volt winding and still turn off one of the two bulbs in standby position. If you could figure out what two 5 or 6 volt lamps would work like the original circuit did with the 3 volt lamps, you would have it.

And yes, the correct original bulbs were a #45 and a #1490.

What transformer did you use as the replacement, a Hammond 270 something? How well did it fit, etc?

[phydauex]

Hey Bill,

I used a 292DX, which was abit higher on the B+, but had a higher heater rating (4A). It comes out to 345v on B+. The stock one I have sitting here runs at 317v, so not much higher. The fit was ok, it's physically thicker, but the hole spread was the same, side to side. About 1/4" longer front/back. Also, had wires coming out of both bell ends, so had to put some risers under it. Used thick nylon washers, worked real nice. This one shouldn't burn out like the first one did. It was shorted between the heater and the secondary windings. I think the 270 version had only a 3A heater, and I wanted to go with some extra capacity. Was looking at the ClassicTone 18 watt replacement and their AC15 model, but both had a different mounting dimension too, and 3A heaters, plus they were more $$.

I think it worked out fine, could have gone either way. None of the replacements I looked at had the same hole spacing.