WT Grant Bradford 1-tube Amp Questions

[Hi-Fi-Mogul]

Upon opening up the cabinet, I was surprised to see it had a
one tube 25EH5 amp, with selenium rectifier.

I've got it all re-capped, but some questions remain.

I had to draw out the schematic, since there is none.

1. According to 25EH5 tube data, the plate and Grid2 voltages are pretty low.

Both 80 VDC.

2. Rectified DC voltage is 104 VDC, which also seems low.

2.a. The 220 Ohm 2watt resistor is VERY hot, could not touch for
more than 1 second.

It tests at 240 Ohms.

3. Tube tested very good.

4. Resistors are good except for R4 1.5 MEG, which is 0.95 MEG.

5. Original capacitor can used 50 and 30 mFd 150 V electrolytics,
replaced with 47 and 33 mF 160 V caps.

[electra225]

1. You're right, the plate and screen voltages are low. The RCA manual says 150 VDC on the plate, 130 VDC on the screen grid. Those are maximum voltages. But 80 volts is a tad low.

2. Rectified DC, measured at the cathode of the rectifier, should be something closer to line voltage. What was your line voltage during these tests? There is a 220 ohm, 2 watt resistor and a 1000 watt resistor in B+ ahead of the plate of the output tube, so there will be some voltage drop. What is the voltage at the cathode of the rectifier? At the first filter cap positive? At the output filter cap positive? If line voltage is 117 VAC or close, you may have a weak rectifier. You also don't show the heater circuit for the 25EH5. Some of those cheaper players used a 90-volt motor, in series with the 25EH5. The 220 ohm and the 1000 ohm power resistors will get too hot to touch in normal operation.

A suggestion I might offer would be to verify B+ at the cathode of the rectifier. If it is low at 117VAC line voltage, I would disconnect the rectifier from B+ then see if the voltage at the cathode rises. If it does, appreciably, you may have a too-heavy load in the B+ line. Discovering the defective component by isolation would be the next step.

Does the player work? Sound from speaker?

Edit: I see you list voltage at the line at 120 VAC. You are not going to get 150 VDC on the plate of the rectifier in any case. I'd say something like 115 VDC on the plate and maybe 110-112 VDC on the screen grid would be more in line. That said, and considering the 220 ohm and the 1000 ohm power resistors in the B+ line, your voltages are not all that far off. I'd like to see something closer to line voltage at the cathode of the rectifier, given that it is a solid state device. A few volts low on the plate of that tube may make more of a difference than some other tubes.

2nd edit: Since we don't know for sure, but assuming there is a 90-VAC motor in series with the 25EH5, that would account for full line voltage. Some of those SS rectifiers are in series with the motor and OP tube, since there is no filament. Some wire the rectifier on their own leg of AC with the motor and OP tube in the other leg. It might be well, then, to measure AC at the anode end of the rectifier. If it only 104 VAC there, you need to see if the motor is in series with the rectifier. If the motor bearings are stiff, causing the motor to draw more power, that might be lowering AC to the anode of the rectifier.

[Hi-Fi-Mogul]

Whoops. Forgot something.

Yes, there is a 90 VAC motor in series with the 25 volt tube.

I get sound from the speaker.

Motor cleaned, lubed, spins like a top.

I'm fishing around to make sure the amp is at
optimum operation, before I zip it up and
put in the new cartridge.

I know it won't be real loud, but it should be
loud enough.
That's what I want as the end game.

And, can I replace that 220 ohm 2 watt with a 5 watter ?
I've got one here.

[electra225]

I'm not sure I'd replace it at all. That is just 10% out of tolerance. How does it sound?

[TC Chris]

2nd edit: ... Some of those SS rectifiers are in series with the motor and OP tube, since there is no filament.

Typo here? You can't have an AC motor in series with a rectifier because then it would be seeing DC and wouldn't go.

Also, maybe a faulty filter cap, leaking? Even new ones may have QC issues.

Odd how the simplest devices can confound us.

Chris Campbell

[Hi-Fi-Mogul]

Greg,
I have not played any records, because the original
3 V crystal cart is defunct.

I need to replace the motor in the changer, because this
unit, like another, has all wiring soldered in, and I
had to remove the motor, to service the changer.

I found a good deal on a 2-3 V stereo cart, that I will
replace the old one with.
They are not easy to replace, so I had wanted to make
sure the amp was good prior to all of the above.

What I might try is to connect a free-standing BSR
with a ceramic cart, to this amp.

Then I'd at least get some idea
of the sound output.

Chris,
I posted in error when I said the motor was in series.
The wiring is the common 2 leg AC wires, one to
motor, and one to the amp.

I'm posting a 1 tube RCA schematic whose wiring is
very similar to this Bradford.
I used it to help guide me in diagramming the circuit.

And, how would I test for a bad filter cap ?
Piggyback another onto what is there already ?

Thanks, guys.

[Conelrad]

Ditch the Selenium and replace it with a 5408 diode or such.

They get leaky as they age, in use or not. RMS vs. P to P will get you close to that 150VDC then with that cap input filter.

DG

[Hi-Fi-Mogul]

Ditch the Selenium and replace it with a 5408 diode or such.

They get leaky as they age, in use or not. RMS vs. P to P will get you close to that 150VDC then with that cap input filter.

DG

I've got plenty of 1N4007s, as a selenium replacement.

I don't know what RMS vs. P to P means, but I'd like to ! [ : )

Thanks.

p.s.
Greg,
I connected a BSR with a 0.5 V cartridge to the Bradford.
The sound was very low, but it was clear and not distorted.

[electra225]

Chris, no typo, just advancing age. How about "parallel" instead?

[Hi-Fi-Mogul]

OK, I put in the 1N4007 rectifier and put in a 5 watt 220 Ohm
resistor after it.

Voltages are better, but still off.

Plate = 86 VDC

Grid 2 = 89 VDC

Post Diode rectifier = 114 VDC

1. What else could cause reduced plate voltage ?

2. Is there a way to calculate what the voltages
after each e-lytic cap should or would be ?

Thanks !

[electra225]

RMS=Root Mean Square. P to P= peak to peak.

https://en.wikipedia.org/wiki/Root_mean_square

http://www.learningaboutelectronics.com ... oltage.php

114 VDC is lower than what I would expect using a 1N4007 diode. I missed the part where you said you were working with a selenium rectifier. Maybe this will help figuring out voltage drop by a resistor:

https://sciencing.com/calculate-voltage ... 28036.html











You only have 28 volts less on the plates of the OP tube than at the cathode of the rectifier. Probably pretty close relative to rectifier cathode. I would try to find out why rectifier cathode voltage is so low. Lift the cathode of the rectifier then measure B+. Is it higher than when it is connected?

[Hi-Fi-Mogul]

Greg,
By lifting the cathode of the rectifier, do you mean
unsolder it from the 220 Ohm resistor ?

Thank you.

[Hi-Fi-Mogul]

Voltage directly off of the 1N4007 is 56 VDC.

So about 1/2 less than when connected to circuit.

[electra225]

That doesn't sound normal to me. Lifting the cathode should remove the load. Perhaps make some checks around the rectifier. Is it good? Correct polarity? The B+ voltage DC should be greater with a 1N4007 than with the selenium rectifier.

[Hi-Fi-Mogul]

I put red lead on 1N4007 cathode and black lead onto B minus wiring
to obtain voltage reading.

The 1N4007 checked out good when I
measured it on meter.

Forward 0.5 Volts, reverse no voltage.

Polarity placement correct.

I don't know if it matters, but the motor and tube filaments
are running when I check the B+ off of 1N4007.

[electra225]

What is the voltage on the anode of the 1N4007, the AC side? If it is on a separate leg of the AC from the phono motor, the motor running or not shouldn't affect voltage to the rectifier. The band on the rectifier is the cathode side. I have a little diagram pasted in a spot where I can't miss it with this information because I can never remember it......

Try measuring cathode voltage with the positive/red lead on the cathode of the rectifier and the black lead on common negative. This measurement, of course, will be DC. Common negative, not the chassis unless it is common negative. Best to put the negative/common/black lead of your meter where the negatives of the filter caps are connected.

[Hi-Fi-Mogul]

Anode voltage 122 VAC, house line voltage.

You can see the 1N4007 in the photo fairly clearly.
The red wire on its left is incoming AC 122 VAC.

So, if the 1N4007 is bad, is there any other tests to do
for that ?

Or just replace it with another ?

[Hi-Fi-Mogul]

And the meter leads were placed as you stated, when I
took readings.

[electra225]

If you have 122 volts on the anode and only 56 on the unloaded cathode, that doesn't sound right. It should be close to 150 volts if the rectifier is good. Many of the radio guys, when they replace a selenium with a 1N4007, put a resistor in B+ so B+ won't be too high. You have the opposite problem. I think you did right replacing the selenium as they are usually found bad, or will soon fail if they test good while you are working with them. 1N4007's are pretty cheap, so I would test and try another one. Let us know how it goes.....

Did you bridge the selenium rectifier with the 1N4007? I can't see well enough, but it looks like the selenium is still there, used as a terminal strip, maybe? You should have the selenium completely out of the circuit if it isn't already. Replace it with a terminal strip to make the connections. If the selenium is bridged, it may be dragging down B+. Pardon my poor eyesight if I'm seeing wrong....

[Hi-Fi-Mogul]

Yes, I cut off the cathode terminal tab of the selenium,
and used its anode tab for mounting 1N4007 to resistor.

I'll try another 1N4007, and get back !