Hey guys,
I am rebuilding an HF-87, the original power transformer was measured to be defective and a replacement was built by the original manufacturer. Except a mistake was made on my part on the HV coil of the transformer.
There is no center tap on the winding.
Is there a way to add a center tap for the build? From what I have seen online, there are ways to simulate a center tap but is there a way to build one without adding another tube?
Thanks
I am rebuilding an HF-87, the original power transformer was measured to be defective and a replacement was built by the original manufacturer. Except a mistake was made on my part on the HV coil of the transformer.
There is no center tap on the winding.
Is there a way to add a center tap for the build? From what I have seen online, there are ways to simulate a center tap but is there a way to build one without adding another tube?
Thanks
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Do you by chance have dual primaries? Then you can series them, instead of in parallel (for a 120V line).
Otherwise you'd need a 2:1 stepdown transformer before the existing one, to use a FWB circuit.
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I reached out to the manufacturer that built the transformer to see if a center tap can be added.
I am quite a bit new to building power supplies and still learning. I have seen virtual center taps for the filament secondary but I wasn't sure if one could be made for the HV secondary.
If there are any other options short of buying another transformer or sending this one back that would be great.
The specs of the transformer are:
0-120v black-black
Secondary 1 HV 0-350vac @ 400mA red-red
secondary 2 bias 0-50vac @750mA blue-blue
secondary 3 fil 0-12vac @ 750mA yel-yel
secondary 4 fil 0-6.3vac @ 8 Amps green-green
Sounds like you've gotten lucky - if it is a single 350Vac winding you can bridge rectify and end up with a more efficient power supply. Disclaimer: I am not familiar with the amp in question and this is assuming that the amp wants an HT rail in the 450-490 volt range.
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This is a copy of the power supply schematic:
The schematic does not show a center-tap but the service manual and original transformer has a lead on the secondary side that is grounded in white:
There are no direct specs for the white lead and my original transformer does not have any resistance from the white lead to any of the other leads. It may be defective as it was very old. That's why a replacement was made.
Thanks!
The white lead was probably an electrostatic shield layer between the primary and secondary. Your no CT HV winding should work fine with the schematic shown. (which is a voltage doubler supply) However, are you sure the original amplifier used 700V B+? (times 1.4 yet!)
You probably need to re-do the power supply to be just a bridge rectifier. Then 350V x 1.4.
Looking at the EICO HF-87 schematic, looks like the B+ was 478 Volts.
http://www.vintagevacuumaudio.com/schematics-manuals/eico/eico-HF-87-manual.pdf
So just convert to a bridge rectifier supply scheme using the 350 VAC winding. That will give the 478 Volts expected.
You probably need to re-do the power supply to be just a bridge rectifier. Then 350V x 1.4.
Looking at the EICO HF-87 schematic, looks like the B+ was 478 Volts.
http://www.vintagevacuumaudio.com/schematics-manuals/eico/eico-HF-87-manual.pdf
So just convert to a bridge rectifier supply scheme using the 350 VAC winding. That will give the 478 Volts expected.
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I think I got confused with the white wire. The B+ was certainly not 700V.
I am having an issue with my circuit though. The B+ was charging to 500V+ and my transformer was possibly arcing or shutting off.
I used a dim bulb tester and the bulb dimmed on startup with no tubes installed and when the B+ charged up it then began clicking.
I am rebuilding the supply but I am not sure what I can run unloaded safely.
UH-OH! If you put that 350 VAC HV winding into the existing power supply, it is going to voltage double that to 700 x 1.4 = 980 VDC!!! Something will FAIL.
Clicking not good. Arcing over somewhere is probably limiting the VDC to the 500 VDC you are seeing. Do not run it this way!!! You will need to determine what has been arcing over. More fixing will be coming up. The electrolytic caps are now suspect.
Clicking not good. Arcing over somewhere is probably limiting the VDC to the 500 VDC you are seeing. Do not run it this way!!! You will need to determine what has been arcing over. More fixing will be coming up. The electrolytic caps are now suspect.
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If the replacement power transformer was wound to original HF-87/HF-89 specs the B+ would be around 180 VAC under load, maybe 10-15% higher than that unloaded. That 180-ish VAC would give you proper DC voltage out of the voltage doubler.
If the transformer was wound to deliver 350 VAC then - as has been said - the voltages applied to the components of the power supply have been FAR above the voltage ratings of the capacitors and possibly even the diodes. The resistors may have survived but virtually everything else is suspect and should be replaced.
You NEED to contact the transformer supplier and find out what the voltages on all the secondary windings are supposed to be! Until you know that you won't know what is going on.
If the transformer was wound to deliver 350 VAC then - as has been said - the voltages applied to the components of the power supply have been FAR above the voltage ratings of the capacitors and possibly even the diodes. The resistors may have survived but virtually everything else is suspect and should be replaced.
You NEED to contact the transformer supplier and find out what the voltages on all the secondary windings are supposed to be! Until you know that you won't know what is going on.
The spec I listed earlier was the exact email sent back from the transformer manufacturer. With the red/red HV leads disconnected from the supply I can measure 350V AC lead to lead.
The 1.4 x factor is for unloaded peak voltage. (the electrolytic caps will charge to the sine wave peaks when no load is present, like before the tubes warm up) Loading will pull that down some. Usually just by 10% for most designs, but maybe more for the doubler design. C16 and C17 electrolytics in the power supply are rated at 300V each (page 5). So would handle up to 600V summed as they are.
The voltage doubler part is contained in the schematic. The two stacked capacitors give it away, along with the diode configuration.
The 478VDC figure came from page 6 (pdf page 23) of the EICO manual for pins 3 and 4 of the EL34 tubes. (plate and screen grid)
Your transformers should work fine for the amplifier IF the power supply is converted to a bridge rectifier instead of a voltage doubler configuration as shown on the schematic. Likely that C16 and C17 are damaged though, best to replace them when re-wiring the power supply to a bridge configuration.
http://www.vintagevacuumaudio.com/schematics-manuals/eico/eico-HF-87-manual.pdf
The voltage doubler part is contained in the schematic. The two stacked capacitors give it away, along with the diode configuration.
The 478VDC figure came from page 6 (pdf page 23) of the EICO manual for pins 3 and 4 of the EL34 tubes. (plate and screen grid)
Your transformers should work fine for the amplifier IF the power supply is converted to a bridge rectifier instead of a voltage doubler configuration as shown on the schematic. Likely that C16 and C17 are damaged though, best to replace them when re-wiring the power supply to a bridge configuration.
http://www.vintagevacuumaudio.com/schematics-manuals/eico/eico-HF-87-manual.pdf
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Smoking,
Thank you very much for your help. You are completely right. Its been a while since I worked with these circuits.
I pulled up a schematic capture for the circuit. 360VAC is too high for the voltage doubler.
The caps C16 and C17 that were purchased are rated at 400V. If they are measured with the fluke to be at the right capacitance within tolerance can they still be tested in the bridge circuit?
I will look into changing the configuration to a bridge rectifier as suggested.
Thank you very much for your help. You are completely right. Its been a while since I worked with these circuits.
I pulled up a schematic capture for the circuit. 360VAC is too high for the voltage doubler.
The caps C16 and C17 that were purchased are rated at 400V. If they are measured with the fluke to be at the right capacitance within tolerance can they still be tested in the bridge circuit?
I will look into changing the configuration to a bridge rectifier as suggested.
I have a new HF-89 transformer, wounded by Heyboer in front of me. It was for replacing one of 2 HF-87's that I had, but I sold one of the HF-87's later. Hence, I can measure how a new HF-87 should look like.
If the PT is at the right specs, you should see around 180 VAC across the 2 red wire. And yes, the white wire is just for shielding, it is isolated from the other wires. So, your assessment of the original transformer may or may not be valid using the white wire resistance. So, what was the other measurement from this old PT? Did you get any voltage from the 2 red wires? This is to rule in or rule out the PT is the cause of the "symptom".
One thing to keep in mind, you will need at least 500VDC rated cap if you redo the power supply with full wave bridge. If you didn't connect your new C16 and C17 to this new transformer to the doubler circuit, then you can stack these 2 caps as one smaller 800V cap. If you did, the caps could be damaged by the over voltage, hard to tell.
If the PT is at the right specs, you should see around 180 VAC across the 2 red wire. And yes, the white wire is just for shielding, it is isolated from the other wires. So, your assessment of the original transformer may or may not be valid using the white wire resistance. So, what was the other measurement from this old PT? Did you get any voltage from the 2 red wires? This is to rule in or rule out the PT is the cause of the "symptom".
One thing to keep in mind, you will need at least 500VDC rated cap if you redo the power supply with full wave bridge. If you didn't connect your new C16 and C17 to this new transformer to the doubler circuit, then you can stack these 2 caps as one smaller 800V cap. If you did, the caps could be damaged by the over voltage, hard to tell.
Fred,
Thank you for lending a hand. I want to go ahead and use the transformer that was wound by Heyboer. I measured 350VAC with my fluke across the 2 red wires. Not 180VAC as it is supposed to be.
Given that, would a circuit like this work?
The original two diodes are GI856's, the application did not have that same model available, but I can incorporate the matching diodes.
Thank you for lending a hand. I want to go ahead and use the transformer that was wound by Heyboer. I measured 350VAC with my fluke across the 2 red wires. Not 180VAC as it is supposed to be.
Given that, would a circuit like this work?
An externally hosted image should be here but it was not working when we last tested it.
The original two diodes are GI856's, the application did not have that same model available, but I can incorporate the matching diodes.
I de-soldered both C16 and C17 from the circuit. There is no bulging physically to the caps and both measure to ~510uF with the fluke. No short was measured either. I also measured both of the JJ caps and all the readings are <10% tolerance with also no bulging.
I may have been lucky that the caps aren't blown. Unless there may be another way of testing them.
All of the testing was done under dim bulb but I am not sure is that contributed to them not blowing.
I may have been lucky that the caps aren't blown. Unless there may be another way of testing them.
All of the testing was done under dim bulb but I am not sure is that contributed to them not blowing.
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