WT Grant Bradford 1-tube Amp Questions

[electra225]

The reason I mentioned cutting out the selenium completely is this. I had a nice Emerson 659 AM-FM radio that I replaced the selenium with a 1N4007. I used one lug of the selenium as a tie point and cut out the other leg so I would isolate the selenium from the circuit. The radio worked okay for awhile until, one day, the volume dropped appreciably. I removed the chassis and found B+ really low. What had happened, apparently, is that the selenium shorted internally and was leaking to the chassis, even though it was technically isolated from the circuit. I removed the selenium completely, replaced the 1N4007 with another one and used a terminal strip for connections. My eyesight isn't good enough anymore to make sure dangling connections are not touching something else. I have to use a terminal strip for peace of mind, but that's just me. From that time on, I totally remove the selenium rectifier and toss it so I know it will cause no further issues.

[Hi-Fi-Mogul]

The reason I mentioned cutting out the selenium completely is this.
I had a nice Emerson 659 AM-FM radio that I replaced the selenium with a 1N4007.

I used one lug of the selenium as a tie point and cut out the other leg so I would isolate the selenium from the circuit.
The radio worked okay for awhile until, one day, the volume dropped appreciably. I removed the chassis and found B+ really low.
What had happened, apparently, is that the selenium shorted internally and was leaking to the chassis, even though it was technically isolated from the circuit.
I removed the selenium completely, replaced the 1N4007 with another one and used a terminal strip for connections.
From that time on, I totally remove the selenium rectifier and toss it so I know it will cause no further issues.

Very good advice. I actually was going to take the anode end of the 1N4007
off of the selenium tab, just to see if it made a difference.

This selenium rectifier is a bit odd. It has a plastic sleeve that insulates
the selenium plates from its metal mounting body.

For drilling out the selenium mount rivet, I wish there was more room in this
tiny chassis !!! That's my biggest complaint here.

[electra225]

Some of the little record player chassis were not designed to be worked on, I really believe that. They were entry-level many times and repairing them, even back then, was not a consideration when they were designed. They had no idea somebody, 60 years hence, would be going to any trouble to restore them. Some of the chassis were really tight to work on. Even in the good old tube days, some things were designed to be "use and toss" like most of what is made today is. The more things change, the more they stay the same.

Can you get to the rivet from the top and grind it off with a Dremel instead of drilling?

Did removing the selenium make a difference in voltage?

I am very interested in following this project, Mr. Mogul. If I am a tad tardy in reply, it's "Western Red" day and I am keeping one eye on your posting and one ear on Western Red......

[Hi-Fi-Mogul]

GRRRRRRR.

New good 1N4007.

Wiring completely separate from the body of
the selenium rectifier.

No change at all in any of the voltages.

Rectifier cathode still 114 V
Plate 86 V
Grid 89 V

The voltages start out OK at about 105 VDC on the plate
and a bit higher on the grid.

Over 10 minutes, they drop down to original numbers.

Would a wonky tube cause this ?

I've seen problem tubes, even if they test "good",

[electra225]

Do you have a spare tube you can sub? I still don't understand why B+ is so low at the cathode of the rectifier. It sounds like the load is changing, getting greater, as it warms up. If you had grid emissions in the tube that would change the plate current. If you pull the tube, that will open the filament circuit and probably kill the phono motor, but it would also remove part of the load on the B+ line. Since there is a separate AC line for the motor and filament of the tube, and a separate AC line for the rectifier, I think you could do that. Pull the tube, then see what B+ does just for grins. Let us know what you find.

[Hi-Fi-Mogul]

It's time for a break. I'll resume tests Sunday.

I don't have a 25EH5, but I do have a 25C5.

Would that do just for testing purposes ?

I know it's not an exact match, but I'm not worried
about sound quality at this point !

[electra225]

The 25C5 should work. Did you remove the tube to see what B+ does?

[Conelrad]

With AC power in a rectifier situation, here is what happens:

Line voltage rectified into DC with a cap-input filter will be 1.4114 (peak to peak waveform) times the line E. Current available will be limited as you approach the point of the filters ability to provide the smoothing effect. Circuit voltage will vary widely under load.

Line voltage rectified into DC with a choke-input filter will be .707 (RMS waveform) times the line E. Current and voltage available will hold up under variable load until total overload.

I hope this clears up the jargon a bit.

D

[hermitcrab]

Solid State rectifiers are more efficient so the output will be a tad higher than the original selenium... I did the same to a Transoceanic H-500 ... had to insert a 100 ohm 1/2 watt in line ... I didn't need to stress out those crazy expensive 1 volt tubes

[Hi-Fi-Mogul]

The 25C5 should work. Did you remove the tube to see what B+ does?

The voltage to plate & grid with tube REMOVED is 162 VDC.

I subbed in the 25C5, and the grid voltage stayed at 95 VDC.

Plate at 90 VDC.

These are 5 volts higher than with the 25EH5 tube.

[Conelrad]

IS that grid voltage positive?

If so, that is way off.

It should be a negative bias voltage, usually no more than 20-30 Volts.

Those conditions will eat tubes.

[Hi-Fi-Mogul]

That is G2, screen grid voltage.

I had posted that earlier, and should have been
consistent on the last post.

[Conelrad]

Ah, OK. If that was control grid E, one could understand my concern...

[electra225]

I think we can all agree that 162 VDC is more what one would expect on the plate of the OP tube with no tube installed. It drops 67 volts when the tube is plugged in. I haven't done the math on paper, but in my head, that is over a third of total B+ loss when you plug in the tube. I'm sorry, and I may be wrong, but that seems like a lot of load for one tube to me. One would think that if the tube was conducting that hard, it might be red-plating. I reckon my next question would be what is the cathode voltage? How is the output tube cathode biased? Is something driving the tube too hard, pulling down B+. If you can get B+ on the tube plate at about 150 volts or so, that should be a hot little record player. Volume-wise, I mean.

[Hi-Fi-Mogul]

The cathode resistor is 470 ohms.

The tube has no red-plating.

I'll check the cathode voltage tomorrow.

A normal SE plate voltage value for a 25EH5 tube is 110 volts.

I'm also going to dig in and replace the old crystal cart
with the good one I found cheap.

Maybe this will turn out to be a tempest in a teapot !

[Hi-Fi-Mogul]

1. Cathode voltage is 3.6 Volts.

2. The replacement cart's mount shaft is too short !

I mounted a P-228 cart to test the sound,
but the amp definitely needs a 2-3 volt cartridge.

Sound output is clear.

3. I have several Electro-Voice powerpoint mounts and
carts.
They put out over 1 volt, but the problem is how to
mount them.

The BSR Varco cart was mounted by one screw through the
top of the tonearm head.

There are no 1/2 inch mount posts inside the tonearm.

[TC Chris]

There are no 1/2 inch mount posts inside the tonearm.

It ought to be easy to make a little sheet-metal bracket to convert from center-hole to1/2" holes.. Or get some aluminum stock from the hardware store and tap some threads in. Improvise!

Chris Campbell

[Hi-Fi-Mogul]

It ought to be easy to make a little sheet-metal bracket to convert from center-hole to1/2" holes.. Or get some aluminum stock from the hardware store and tap some threads in. Improvise!

Chris Campbell

Chris,
I decided earlier to use a cut to fit little wooden block.

I'll use a small wood screw through the top of tonearm,
and glue the block onto inside tonearm.

I will attach the power point bracket mount with small
wood screws into the block.

The power point bracket is made so the cartridge is
angled when mounted on regular 1/2 inch wide holes.

I'll have to drill those holes at a 45 degree angle or so.

[electra225]

I don't know what the cathode voltage should be. All the Magnavox record players I have, both great and small, specify 11 volts on the cathode. The idea is to maintain the cathode more negative than the the plate to limit the current thru the tube. Too low of a cathode voltage will cause the plate current to increase, since the cathode is less positive in relation to the plate. Raising the cathode voltage will have the opposite effect. When the plate current in the output tube raises too high, it is hard on the output transformer and, in extreme cases, can red plate the output tube. Since Magnavox uses about 325 VDC on the plate and 11 volts on the cathode, your little record player uses about 1/3 the plate voltage of the Magnavox and 1/3 the cathode voltage. Using that rather unscientific equation in the absence of a proper schematic, you can try what you have and see how it works. I hope you don't have a tempest in a teapot. We appreciate you sharing your project with the forum and for the opportunity to offer our two cents.

You bring up another interesting point. You are going to try a 1-volt cartridge on a one-tube player with no pre-amp tube. If you have satisfactory results, I may try using a changer with a ceramic cartridge on my little GE stereo that uses a crystal cartridge. I'm interested to see how your experiment goes.

[Hi-Fi-Mogul]

Greg,
I've learned a lot during the conversations about this player.
I'll certainly report back on the E-V 89 power point (Astatic 911).

One thing I noticed viewing the 25EH5 tube data for "typical
operating characteristics" :

For the cathode resistor, a value of only 62 Ohms resistance
is listed.

This amp uses a 470 Ohm cathode resistor (reads 470 Ohms).

I don't know what effect the value of a cathode resistor has
on the rest of the inner workings of a tube.
I think they are for biasing the base current draw ?

I could not find a clear answer doing some research on that.