I'm working on a Baldwin 40 watt organ amp that was brought to me as non-functioning and humming badly. I'm running through a dim-bulb tester. I found a dead rectifier tube (5U4) of a pair, and one 6L6 with white getter. I checked the power transformer and it's making about 430VAC per side to ground, so that's good. I replaced the dead 5U4 and the dead 6L6 with a pair of NOS RCA 6L6GBs. The first cap is a 4uF oil cap and I'm getting only 260V for B+, which seems low to me. Schematic doesn't have any voltage readings. My Kill-A-Watt tells me that I'm only pulling 0.6 A from the wall, which also seems low, but the dim bulb stays pretty bright. It will pass a signal but the volume is very low, albeit pretty clean sounding. Any suggestions for where to look?
Have the ratings for the choke labeled with a 2 that goes between the transformer and ground?
It is looking like a choke input supply so the plate voltage is not going to be too high (maybe 450).
Also if the 4 microfarad capacitor after the rectifier (Labeled with an 8) is bad you would get hum and low voltage..
This amplifier is asking each pair of 6L6s to deliver 20 watts. It was built to be left on a long time without tube failure.
Also you will want to see how much voltage you have between the 6L6 cathodes and ground. If the bypass capacitor is leaking badly you will get too much plate current draw and lower power supply voltage.
It is looking like a choke input supply so the plate voltage is not going to be too high (maybe 450).
Also if the 4 microfarad capacitor after the rectifier (Labeled with an 8) is bad you would get hum and low voltage..
This amplifier is asking each pair of 6L6s to deliver 20 watts. It was built to be left on a long time without tube failure.
Also you will want to see how much voltage you have between the 6L6 cathodes and ground. If the bypass capacitor is leaking badly you will get too much plate current draw and lower power supply voltage.
I don't have any ratings for the choke, but I got this thing working a few years ago with new output tubes, new cathode bypass cap and new coupling capacitors, so I don't suspect the choke. As to the 4uF cap, I'm getting no hum in the speaker at all, just the low voltage and the too-bright dim bulb with low volume level.
There's only 1.7V across the 125ohm cathode R. That doesn't seem high enough.
The output transformer primary halves measure 78 ohms and 69.8 ohms. Voltage drop across the output transformer is about 4 times higher on the 78 ohm side (0.0086V) than the 69.8 ohm side (0.0023V).
There's only 1.7V across the 125ohm cathode R. That doesn't seem high enough.
The output transformer primary halves measure 78 ohms and 69.8 ohms. Voltage drop across the output transformer is about 4 times higher on the 78 ohm side (0.0086V) than the 69.8 ohm side (0.0023V).
329 sounds about right, something is dragging your voltage down. Does the transformer get really hot quickly?
Baldwin Schematic with voltages.
I have several of these amps. They are good bass and midrange amps but the OT performance drops off dramatically after about 8Khz.
The choke is 2.7H and 35 ohms. The power transformer is center tapped 350 - 350 volt 380ma secondary.
329 volts B+ without tubes is very low.
The power transformer also provides power to a banks of 6SN7s in tone cabinets. It does not drop voltage like a choke input supply because it is not choke input. The B+voltage was 450+ without the tone cabinet bank of 6SN7s. I had to change the dropping resistors to get the voltage down to the values shown on this schematic. In my final iteration I used Sovtek 6L6WXT+ tubes and let the voltage rise. But I still had to adjust the dropping resistor supplying voltage to the screens and paraphase inverter.
The choke and the power supply capacitors are the only parts connected to ground. If you pulled all of the tubes and there are no wiring errors than those would be the only parts that could pull down the power supply voltage. However, if that was happening I would expect the PT to get very hot since you have to dissipate a lot of power to cause a 200 volt drop.
Maybe the PT is bad?
Steve
I have several of these amps. They are good bass and midrange amps but the OT performance drops off dramatically after about 8Khz.
The choke is 2.7H and 35 ohms. The power transformer is center tapped 350 - 350 volt 380ma secondary.
329 volts B+ without tubes is very low.
The power transformer also provides power to a banks of 6SN7s in tone cabinets. It does not drop voltage like a choke input supply because it is not choke input. The B+voltage was 450+ without the tone cabinet bank of 6SN7s. I had to change the dropping resistors to get the voltage down to the values shown on this schematic. In my final iteration I used Sovtek 6L6WXT+ tubes and let the voltage rise. But I still had to adjust the dropping resistor supplying voltage to the screens and paraphase inverter.
The choke and the power supply capacitors are the only parts connected to ground. If you pulled all of the tubes and there are no wiring errors than those would be the only parts that could pull down the power supply voltage. However, if that was happening I would expect the PT to get very hot since you have to dissipate a lot of power to cause a 200 volt drop.
Maybe the PT is bad?
Steve
What wattage bulb are you using? It could be your using too low a wattage bulb and this is restricting the amp too much. Four 6L6's should be pulling around 200mA Ia. Check the AC ripple on the HT/B+, also check the resistance to ground with the amp off and caps fully discharged. Another wrinkle is to substitute an ammeter for the HT fuse or main fuse to monitor current draw.
Andy.
Andy.
In the schematic I posted the loaded voltage with the tube cabinet bank of 6SN7s is 395 volts. In my amp without the tube cabinet it is 450 volts. I would expect the power transformer to get hot if the voltage is being pulled down to 329 volts.
My suggestion is to test the power transformer and work forward from there until you can isolate the cause of the problem.
Remove all tubes, don't put a load on the secondary, and test the ac voltage of the secondary high voltage winding from center tap to the other 2 leads.
Steve
My suggestion is to test the power transformer and work forward from there until you can isolate the cause of the problem.
Remove all tubes, don't put a load on the secondary, and test the ac voltage of the secondary high voltage winding from center tap to the other 2 leads.
Steve
You CANNOT operate the amplifier normally with the bulb tester connected.
That is ONLY for initial power up, to avoid severe damage from shorts etc.
Remove the bulb tester. THAT is your problem.
The bulb tester is ONLY for initial power up to avoid heavy shorts and damage.
Then it MUST be removed before proceeding further.
That is ONLY for initial power up, to avoid severe damage from shorts etc.
Remove the bulb tester. THAT is your problem.
The bulb tester is ONLY for initial power up to avoid heavy shorts and damage.
Then it MUST be removed before proceeding further.
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Thanks, this is helpful information. The schematic I was using was missing a lot of detail.
100 watt incandescent bulb. I can try a 200 watt bulb to see what that gets me.
I’ve run my 200 watt monoblocks through three 100 watt incandescents and a variac to get the incoming voltage up. I don’t get the full 600V B+, but close enough where things run normally and I can tell whether there is a fault or not.
Some builders of large solid state amps around here have been known to use space heaters or toasters in place of the dim bulb. Same thing, just allows more power.
Some builders of large solid state amps around here have been known to use space heaters or toasters in place of the dim bulb. Same thing, just allows more power.
Look at Rayma's post #11.
Remove the bulb, all tubes and do the test I suggested in my post #10
Steve
Remove the bulb, all tubes and do the test I suggested in my post #10
Steve
Well, I took it off the dim bulb, put a quad of NOS RCA 6L6GC and things seem to be working correctly, but the B+ is about 30 V too high, at ~425 depending on line voltage, dropping to about 415 on the anodes of the 6L6GCs. The tubes are pulling about 93mA per pair. This all seems to be on the high side and is likely why it ate one of the Chinese tubes he had in there. I added a 300 ohm 25 watt resistor after the 2nd 4uF cap, and that tamed the voltage to about 370 on the plates and about 82mA per pair of plate current (measured voltage drop across each half of the OPT). I think I'm gonna let it go like this.
I wouldn't worry about 415 volts on the anodes.
This amplifier was originally designed for 6L6 tubes.
At 93ma per pair you are running the tubes at less than 19 watts plate dissipation. 6L6GC tubes are rated 500 volts on the plates 450 volts on the screen and 30 watts plate plus dissipation.
I use 6L6WXT+ (6L6GC equivalents) in my amps and hence am not concerned about the higher voltage and dissipation.
Steve
This amplifier was originally designed for 6L6 tubes.
At 93ma per pair you are running the tubes at less than 19 watts plate dissipation. 6L6GC tubes are rated 500 volts on the plates 450 volts on the screen and 30 watts plate plus dissipation.
I use 6L6WXT+ (6L6GC equivalents) in my amps and hence am not concerned about the higher voltage and dissipation.
Steve
https://tdsl.duncanamps.com/link.php?target=25C52D
This link shows the plate voltage to be 360 for PP AB1. 370 is well within tolerance. I don't think 415 will add to tube longevity.
This link shows the plate voltage to be 360 for PP AB1. 370 is well within tolerance. I don't think 415 will add to tube longevity.
Look at what it says for maximum values for AB1 operation. 500 volts plate and 450 volts screen.
The impedance of the output transformers in my amps are 3300 ohms. Double that value for a single tube and you get 6600 ohms.
The cathode bias example is using a 9000 ohm impedance.
It is not high voltage that shortens tube life it is the power dissipation.
The 6L6GCs are operating at approximately 50% Plate dissipation.
Did you measure the anode voltage from plate to ground or plate to cathode? In a cathode biased amp plate to cathode is the true anode voltage.
Steve
The impedance of the output transformers in my amps are 3300 ohms. Double that value for a single tube and you get 6600 ohms.
The cathode bias example is using a 9000 ohm impedance.
It is not high voltage that shortens tube life it is the power dissipation.
The 6L6GCs are operating at approximately 50% Plate dissipation.
Did you measure the anode voltage from plate to ground or plate to cathode? In a cathode biased amp plate to cathode is the true anode voltage.
Steve