Grandma's old Capitol Console

[mashaffer]

I finally got over to my brother's house to have a look at grandma's old console. It is a Capital unit with turntable and AM/FM radio. I couldn't find a model number but it does say FDS in script on the tuner and turntable. It has seperate speaker 3 way speaker cabinets with 8" woofers.

I couldn't get a good look at the tuner section but the power amp has 3 12AX7s and 4 25L6GTs so I am assuming a PP with concertina splitter. I couldn't see a rectifier tube but it has a medium sized power transformer and two smaller pieces of iron which are probably output transformers (seems more likely than chokes). These smaller transformers have the numbers 120034 308 932 on them.

One interesting thing about it which should help date it is that it has stereo radio in which one channel is tuned in on AM and the other on FM rather than a single multiplexed signal.

The unit is firmly under his control right now (uses it to hold a bird cage!!!) but if I ever get my hands on it I think I will fix it up for a nice mains amplifier (above 50Hz) in PP or possibly 4 channel SE.

I will post pictures when I get the film developed.

mike

[Tom Bavis]

With 25L6s, it's probably non-isolated from the AC line - heaters run in series, B+ is rectified line voltage. Safe enough when everything is inside the cabinet, but a shock hazard when connecting anything external - an isolation transformer is required to make this safe.

Simulcast stereo would date it to about '59-62. If you do come up with a model number, it's probably in Sams Photofacts...

[mashaffer]

Thanks for the info Tom. I thought about the non isolated issue but there is a tranny in there that sure looks like a power tranny. Any possibility that they used non-isolated for the heaters only with a tranny for the B+? I didn't have a lot of time to look so I am not sure if there was a seperate PS on the tuner chassis.

When I mentioned that I might steal the tubes out of it If they were not careful Cindy said that I could have the tubes. Not wanting to damage the value of a potential collector's item and lacking any iron to use them with I left them but since this is in the family and they are using it only for furniture I am tempted to see if they would let me take the PA chassis out and restore/mod it for use as a main PA in my stereo system. I think that I could do it in a way that would allow it to be put back at a later date if desired.

I would recap of course and replace any other suspect passive componants. I would review the PS for necassary safety modifications and proceed accordingly.

[Frank Berry]

Of course it's impossible to know without opening up the amplifier and tuner but I suspect that the larger of the three transformers is providing B+ voltage and filament voltage for the tuner and the 12AX7 tubes in the amplifier. It's likely that the 25L6GT filaments are in series and deriving their filament voltage from the mains.
If the rest of the amplifier/tuner is isolated from the mains by way of a power transformer, the chassis shouldn't be a shock hazard.

One easy way to tell for sure is to pull one of the 25L6 tubes and see if the other three 25L6 tubes lose their filament voltage.

It's a strange design but I've seen stranger arrangements.

[Eli Duttman]

Mike,

The 25L6 data sheet shows a 300 mA. heater draw. 4X 25L6s and 3X 12AX7s set up for 6.3 V. add up to a 118.9 V./300 mA. total. Current average line voltages are somewhat higher than those found in 1960. So, you might add a 3.9 Ohm resistor at each end of the series heater string, as compensation. 1 W. rated Carbon film parts will do nicely.

Do you plan on retaining the OEM signal topology? 3X 12AX7 sections/channel provide lots of gain. If a CDP is used as a/the source, excessive gain rates to be present.

[planet10]

I disassembled a Fleetwood console with the same configuration (but it had a separate stereo FM decoder)... the OPTs were very well made but tiny,

dave

[Frank Berry]

Originally posted by Eli Duttman
Mike,

The 25L6 data sheet shows a 300 mA. heater draw. 4X 25L6s and 3X 12AX7s set up for 6.3 V. add up to a 118.9 V./300 mA. total. Current average line voltages are somewhat higher than those found in 1960. So, you might add a 3.9 Ohm resistor at each end of the series heater string, as compensation. 1 W. rated Carbon film parts will do nicely.

Do you plan on retaining the OEM signal topology? 3X 12AX7 sections/channel provide lots of gain. If a CDP is used as a/the source, excessive gain rates to be present.

Eli,

Good math. Perhaps the power transformer is for the tuner?
The only way to tell is to look at the wiring in the chassis.

[Eli Duttman]

Guys,

It's reasonable for the power trafo to be involved in the B+ for both tuner and amp. The console has a TT that end users come into direct contact with. It will not do for an end user to get shocked. Obviously, we'll know, rather than guess, after Mike finishes tracing things out.

If my hunch about the B+ situation is correct, current is available for all sorts of interesting possibilities. A 6922 voltage amplifier DC coupled to a MOSFET "concertina" phase splitter is but 1 option. The small drive requirements of 25L6s offer considerable flexibility.

[mashaffer]

Thanks for all of the input guys. Eli, That was my initial guess on the heater wiring. Off hand I didn't know what the current requirements for AX7s was but I figured that those three paralleled in series with the power tubes would be just about the right voltage.

I think that I even have a 24V CT tranny of a couple of amps capacity laying around here that could possibly be used to provide isolated heater power to the 25V tubes.

I was scratching my head over the three AX7s also. Two tride VA stages and a phase shifter per channel seemed excessive off the top of my head since it was design (I think) for ceramic cartridges.

Is it theoretically correct to say that the higher voltage/lower current heaters are advantageous WRT induced heater PS hum?

The OPTs are indeed rather small (about 1.25x1x1 inches) but my intended use would have a powered sub for the really low stuff so they may be perfectly adequate as is or replaced by small Edcore as a worst case.

I will almost cetratinly be approaching him with the idea of getting that chassis out of there.

Of course since he is using the console and one speaker only for furniture I might be able to find replacement furniture for him and talk him out of the whole unit.

mike

P.S. Based on a quick lok at the data sheet it looks like 5-8 Watts in PP would be about what this unit would provide depending on whether in triode of tetrode mode and B+. Does that sound right?

[mashaffer]

The system is in three separate peices that can be set next to each other or separated.

The main console