Golden Falcon 400 tube amp

[Toltec]

Hey guys,

I'm looking for some information on this Golden Falcon 400 tube amp. It has a total of 6 tubes, 2 drivers, 4 output. 6LQ6s. It is an AC powered amplifier. Power supply electrolytics have been removed and am trying to figure out what values to use as replacements. I can't seem to find any information on this amplifier. It's a big one.

Schematic seems to not be available. Anyone have any suggestions of similar tube amps that I could use as reference for the value of these electrolytics? Seems a lot of these amps are just copies of one another. Hopefully we can figure out an amp that is pretty much built the same as this one. Here is a video showing an amplifier just like it. Here you can make out one of the caps is 1000uf 25V.



http://www.radiomuseum.org/r/goldenfalc_linear_amplifier_400_4_bands_ac_only.html

 

[BlowinSmoke]

Tube amplifiers require between 21-32 uf total capacitance. If that amp does not have a step start, which it probably doesn't then I would stick to about 21 to 25uf total. So how many caps are in there? Two? Just multiply the amount of caps TIMES the desired total capacitance. So if you have TWO caps in your amp and you want 25 total UF, then you need 50uf caps each. If you have 4 caps then you want 200uf caps. This is not too critical. Stay within 21-25 total and you will be fine.

 

[wavrider]

Usually 100MF 450VDC is what was in those old sweep tube amps.
Very few had equalizing resistors or bleeders so if you going recap it install them for safety.

 

[Toltec]

Aren't these capacitors usually wired in parallel BlowingSmoke? In that case wouldn't the values add up?

There is two big caps missing, there are a couple of other smaller value ones left. The ones that are missing used to be in two big holders.

 

[wavrider]

in series,,divide by amount of caps

http://www.cbtricks.com/Amp/maco/maco_300b/graphics/maco_300b_redraw_sch.pdf

you can look at the HV PS section of this schezo to get an idea of how the caps would be installed.

If you know how to install them,,if you do not then be careful lethal voltages in that PS

 

[nomadradio]

No info on that one here, or anywhere else I have ever seen.

Don't know what the high voltage was supposed to be. Don't know whether it used a full-wave voltage doubler or a full-wave bridge rectifier.

The only way to be sure what voltage rating your filter caps must have is to find out the AC voltage of the transformer's HV winding. This can be hazardous to measure directly, since many low-price meters will be damaged by AC voltage over 600 or 700 Volts.

Safest way is to set up a 12-Volt AC power source. A step-down transformer is the safest way. Plug the power cord into the 12-Volt AC source. Measure the AC voltage on the transformer's HV winding. The normal voltage it delivers will be that reading multiplied by ten. And your meter won't be exposed to anything over 100 Volts max.

A reading of 300 Volts or less indicates that the rectifiers are configured as a doubler circuit. A reading of 600 or less means that it uses a full-wave bridge circuit.

Some amplifiers like the Palomars use only 800 Volts DC, with a transformer voltage of about 250 Volts AC. Or 500 Volts, depending on the rectifier setup. This allows using two 100uf 450-Volt capacitors in series. Never put more than 90% of the caps' total voltage rating on them in normal use.

Others like Maco and D&A use 900 or 1000 Volts DC. Transformer voltage will be either 350 or 700, more or less. The D&A amplifiers can use two 500-Volt caps in series. The 900-Volt setup still keeps us below the 90-percent rule.

Maco would use 1000 Volts, which requires three 450-Volt caps in series. They add up to 1350 Volts, which gives you plenty of margin above the 1000 Volts placed onto them.

Here's what we use as a replacement for the Maco amplifiers.

White wire is hot, black wire is ground. Has bleeder resistors on it.

38 bucks on Fleabay.

But I see two options. Either an "overkill" solution like the one we sell, or make a measurement of the transformer's AC voltage. That will tell you what you can get away with, as far as a lower total voltage rating for the filter caps.

73

 

[BlowinSmoke]

Aren't these capacitors usually wired in parallel BlowingSmoke? In that case wouldn't the values add up?

There is two big caps missing, there are a couple of other smaller value ones left. The ones that are missing used to be in two big holders.

No, in a multi capacitor string of electrolytic caps in a power supply, the caps are usually in series. Just about ANY sweep tube amplifier will no have more than 900 volts on the plates. It is always a good idea to have some head room so use enough caps to keep you well above the operating voltage. If your amp had only 2 caps then most likely you would replace the missing ones with 50uf at 450v units.

 

[Toltec]

I ended up going with 100mfd @ 450. Someone with a similarly configured amplifier said that's what he had in his. I figure the extra headroom wont hurt, at least for the time being. The initial surge might be an issue though. Indeed both are in series. There is a 470K bleed resistor in parallel with these two.

The other two caps were 1000mfd @ 25V. Even though in this case those were still in place, they were replacements. Blown ones at that. There was a small puncture on the rubber side of each of them.

This thing actually had a lot more damage I had not noticed until I removed the board on which the caps are mounted. One issue is the blown caps i mentioned, which I've gone ahead and replaced. Two is the missing main caps, which I assume got taken out cuz they too got blown up, this too has been repaired. Three, there are several blown traces one of which is the ground that fed one of the blown 1000mfd caps, this trace has been bridged. Another blown trace was already repaired with a 0.5 Ohm 5 watt resistor. What the heck is up with that? None of the remaining caps nor the diodes were shorted when I pulled them. I'm kind of scratching my head as to what could have caused this much damage. Power surge? Had to have been one hell of a surge! Nothing else in the amp looks blown up. A quick continuity test from the plate caps of the tubes to ground reveals no obvious shorts. Im a bit baffled by how in parts this thing looks like it's had a go at being repaired and blown up again. What else should I check? Main fuse was also blown I can't even tell what the current rating is as there is a hole blown right where it used to be. All i see is a 2. Looks like a 20 amp by the size of the wire inside.

 

[Toltec]

Alright another update. Turns out our plate voltage is 800 volts. This puts 400 volts on each cap, they are in series. Caps are rated at 450 volts. That is slightly below 90% of 450 which is 405 volts.

Powering it up reveales the B+ comes up as soon as the power switch goes up. Three of the four relays in the amp slap on. This is without the amp connected to a radio or anything. Transistor that drives the switching circuit is not shorted.

Another issue I ran into is one of the 1000uf @ 25 caps wanted to blow up upon first power up. The voltage across it is negative even though the one I replaced it with was oriented in this same manner. I confirmed the correct polarity for both of these 1000uf capacitors by observing the polarity of the ones I replaced and by looking closely at the youtube video posted above so I don't know why on mine one of them wants to blow up.The other 1000uf cap in this amp is part of a negative supply and is meant to be positive side towards ground as confirmed by the one that was here originally and by the one that can be seen in the youtube video above. No problems here.

The positive side of the failing cap in question is hooked up to the anode side of a diode which makes me question if perhaps it is the diode that was put in backwards. It does look like it has been messed with and it is not of the same type as the others. Both cathodes connect to a lead that goes to the transformer. This same node connects to the grids of a couple of the tubes. Pin 11 to be precise. So perhaps it IS supposed to be a negative supply? What do you guys think? I understand there is usually a negative grid bias supply but I think there is too much evidence that points to this being the correct polarity for this capacitor. If you look closely in that youtube video you can see the top most blue cap is a 1000uf @ 25V capacitor and it's positive side points towards the right, this is the same orientation that I used.

Another thing however is that having the capacitor with the supposedly correct polarity made all the relays humm and once I flipped it "backwards" this issue went away.

EDIT: I just discovered that when I disconnect the lead that goes to the node in question, same one that goes to pin 11 on a couple of the tubes, the relays stop activating on power on. Further convincing me that this supply line is not supposed to be negative.

 

[nomadradio]

Pin 11? !!!

The 6LQ6 is a 9-pin tube.

What's up with that?

73

 

[wavrider]

Chinese knock off 6LQ6?? 12pin

the tubes laying on the work bench look like 12 pin, maybe you have some 6LF6 tubes?

1000mf at 25vdc sounds like keying circuit caps?? And or SSB delay.
It may also be the negative bias circuit, thus the + side of cap would go to Ground.
Just some thoughts thrown out there as there are several ways to bias the sweep tubes.

 

[Paul-W5lz]

For what it's worth to you... Having a capacitor rated for at -least- twice the voltage applied to it is a very smart idea (also a 'safety standard' in the commercial industry). Being rated at more voltage than required isn't a draw back in any way. There are always going to be 'surges' for various reasons, why not 'allow' for them?

 

[nomadradio]

So, on a 6LF6 12-pin tube, pin 11 (and 3) connect to the tube's screen grid. Many amplifiers simply ground this pin. Maco, Palomar and others would put a positive voltage on this pin, seldom more than 12 to 20 Volts. About 8 Volts in a Palomar 300A goes onto this pin. It's a "nitrous" strategy that will boost the tube's peak current a bit, but requires a higher negative bias voltage on the control grid at pins 5 and 9 to prevent the tube from overheating. Simply connecting the filter cap for this voltage to the tube socket's screen grid pin works okay until the tube arcs over or "flashes" inside. This will put a pulse of high voltage onto the low-voltage filter cap and pop it.

A series diode between the filter cap and pin 11+pin 3 will protect the filter but you should add a resistor from the screen grid to ground if you do. 10k is not a bad choice.

73

 

[Toltec]

Tubes are 6LF6. There is already one negative bias circuit, that's where the other 1000uf cap is and that one is hooked with it's positive side to ground and is happy that way. Has about -17 volts across it. This goes through a 1.5K resistor and feeds the tubes on pins 5 and 9, first grid.

The one that is faulty has about -11V across it. Definitely is related to the relay circuits because when I disconnect the lead that comes out of this node, the relays stop clicking on when I power the amp. So if this is indeed supposed to actually be a positive supply, the reason the relays come on with no signal is because with the polarity reversed, they're taking this line as their ground and the regular ground as their positive voltage rail.

What do you guys think? Say screw it and flip that diode the other way around? Someone must have made a mistake. As I mentioned it's definitely been messed with and this particular diode is of a different style than the ones used on the rest of the board. With how much damage and screwing around this thing had when it came in I wouldn't be surprised the person working on it would have been capable of getting a polarity wrong. Add that with the comments from nomad radio of that particular grid sometimes using a positive voltage. Also here is a screen shot of that same cap from that youtube video above. You can see the positive side of the cap facing towards the camera. Same way I connected mine. Too bad you can't see the diode.

I'll upgrade the voltage on the main caps once the amp is working. The 600V ones were a bit more pricey and I figured I should start with the 450V once in case something went wrong.

 

[Toltec]

Alright I went ahead and flipped the diode and now the voltage is about 17 volts positive. The relays have also stopped activating without the radio being in transmit mode. The relay for the SSB pre amp has also started to work while before it was dead.

I hooked the sucker up and sure enough it started to work. About 250 watts on low and 400 watts on high. The problem is at times the power became intermitant and I could hear a slight buzzing/humming sound coming from somewhere in the amp. The person listening to me on the other end also reported a 60hz hum and some noise that wasn't there initially. After powering the thing down and then back up after a few minutes, it would no longer put out power or it would put out very little. After a closer look I noticed the inside of one of the tubes was arching every time I tried to transmit. The arch is inside the plate and I can see it through the little holes in the plate.

After powering down and letting it cool once again the problem went away and the amp started to work again, this time without the arching or hum on the transmit audio. Will probably start failing again soon though.