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Post subject: Power & output transformers
Posted: Mon Feb 24, 2014 4:57 pm
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So I found a DuKane PA amp that had two 5881s, a 5U4 and a 12AT7 (minus the tubes) recently so I have two good transformers for a start on a new tube amp. My question is how do I know what amperage the PT will handle (i.e. what power tubes I can run)? I checked voltages and it measures 750v, 6.3v, and 5.0v on the pairs. The OT measures 252 ohms across the red & green leads and 127 & 125 ohms across the green & yellow and red & yellow leads respectively.

I read here somewhere there is a winding factor to deal with but I can't remember how to determine that. The OT connections were clearly marked 8 ohm, 4 ohm, 70v and 25v balance. It seems like I have some good parts to start with but I just wanted to make sure before I put the effort into building something like a 5F6.

Any thoughts?

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Post subject: Re: Power & output transformers
Posted: Mon Feb 24, 2014 6:13 pm
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With 750 volts available at the power tranny's secondary you could prolly run anything up to and including a pair of 6CA7/EL34's. Mebbe even 6550/KT88's. The 6.3 volt and 5.0 volt taps are heater supplies for the pre-amp/power-amp tubes and the rectifier, respectively. If 5881's were the original power-tube complement you can substitute any set of 6L6GC's you prefer.

Arjay

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Post subject: Re: Power & output transformers
Posted: Mon Feb 24, 2014 6:53 pm
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jimolson1963,

Was the 750VAC measured from CT to end? Or end-to-end? And was the original circuit's bias type on the 5881 cathode-biased or fixed bias?

I'm thinking it was cathode-biased, as you have no tap for a bias supply. Unless it was tapped off B+.

The output tranny's (OPT) impedance needs to be measured using a signal generator. Ratio of voltages between the primary and secondary will give you the winding ratio. From there you can find the impedances. But, my guess is the ratio is around 30:1, with primary impedance ~ 6500 ohms, for a pair of 5881 (6L6WGB). If the original unit had one speaker, prolly the secondary's impedance is 8-ohms. 4-ohms for multiple speakers.

To make life simple, stick with 6L6 types. The 5881 is a very good tube. Or even the coke bottle 6L6GA or -GB. These are great sounding tubes, that are cheaper than the Holy NOS 6L6GC. The PT and OPT wiring is most likely set for 5881 use and not hardcore 6L6GC use. Current-wise.

You may want to use a cathode-bias arrangement. This eliminates having to figure out a fixed-bias circuit. A 250-ohm, 10 watt resistor tied to the pair's cathode, bypassed with a 50mfd/100VDC is standard for a dual 5881 cathode-biased amp. The amp will be in Class "A" push-pull mode. The tone will be closer to original Bassman or Super Tweed tone.


http://www.webphix.com/schematic%20heav ... _schem.pdf



Makings of a very nice DIY amp! Keep us informed. :D :mrgreen: :D


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Post subject: Re: Power & output transformers
Posted: Mon Feb 24, 2014 10:16 pm
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I measured it end to end, I have no way to know what the center tap wire is if there is one, I can't find a diagram for this transformer anywhere. The numbers don't return any results at all no matter where I search.

I've got 6L6GCs, 5881s and EL84s so it's wide open as far as output tubes go.

The wires are:

Green (pair) 6.3v
Yellow (pair) 4.8v
Red/Yellow (pair) 750v
Yellow/Red & Green/Yellow (grounded together on a ground lug)
Blue (no connection)
White (to a funky 1/2" square multi-plate thing on a ground lug)
Black (neutral power connection)
Red/Black (hot power connection)

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Post subject: Re: Power & output transformers
Posted: Mon Feb 24, 2014 10:41 pm
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jimolson1963 wrote:
White (to a funky 1/2" square multi-plate thing on a ground lug)


That sounds suspiciously like a selenium rectifier.

Arjay

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Post subject: Re: Power & output transformers
Posted: Tue Feb 25, 2014 1:37 am
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jimolson1963 wrote:
I measured it end to end, I have no way to know what the center tap wire is if there is one, I can't find a diagram for this transformer anywhere. The numbers don't return any results at all no matter where I search.

I've got 6L6GCs, 5881s and EL84s so it's wide open as far as output tubes go.

The wires are:

Green (pair) 6.3v
Yellow (pair) 4.8v
Red/Yellow (pair) 750v
Yellow/Red & Green/Yellow (grounded together on a ground lug)
Blue (no connection)
White (to a funky 1/2" square multi-plate thing on a ground lug)
Black (neutral power connection)
Red/Black (hot power connection)



The white wire is the tap for a bias supply and 1/2-inch square thing is a selenium diode. The amp had a fixed bias style circuitry. The paired yellow wire = amp had a tube rectifier. Need to replace the selenium diode (toxic component) with a regular 1N4007 or UF4007 diode.

Yellow/Red prolly is the center-tap of the primary winding of the PT. Since you measured from end-to-end. The 750VAC = total AC voltage or (375-0-375 peak-to-peak).


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Post subject: Re: Power & output transformers
Posted: Tue Feb 25, 2014 9:27 am
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Awesome, thanks. I will check that when I get home.

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Post subject: Re: Power & output transformers
Posted: Tue Feb 25, 2014 3:14 pm
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Seems to be a neat unit. The grn/yel should be the 6.3V center tap. So this is just a power amp, and would need a pre amp. Cool building block!! Art

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Post subject: Re: Power & output transformers
Posted: Tue Feb 25, 2014 6:23 pm
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Maybe a DuKane 1U460A amplifier. There is a layout on the Net on a 1U460A-Bassman conversion. I can't find a schmatic on the Net of the 1U460A.

http://www.webphix.com/schematic%20heav ... n_5f6a.pdf


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Post subject: Re: Power & output transformers
Posted: Tue Feb 25, 2014 8:52 pm
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That's cool, but mine is a very narrow chassis so I'm using a Yamaha B100ii (solid state) head cabinet that looks like a Bassman head only not quite as wide and a few inches deeper. I already have the bottom of the chassis replaced and will be cutting out the tube holes tomorrow.

If the previous owner hadn't used an engraving pen to etch their church name in the faceplate I could use it but I'll have to make a new one and get the pattern silk-screened on it.

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Post subject: Re: Power & output transformers
Posted: Tue Mar 25, 2014 5:47 pm
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I just finished a build based on the 5F6 Fender Bassman schematic for this project and I have no sound. I do however have an extremely high surplus of voltage on pin 3 of the output tubes and I'm not sure why. According to the schematic I should be seeing around 440v there but it's almost three times that. It stabilzes around 500v across the first 80uf cap and it reads 1192v from ground to the positive side of the second cap. Way too high.

I've been through my wiring a few times looking for mistakes but I just don't see any. What in the world would cause that?

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Post subject: Re: Power & output transformers
Posted: Tue Mar 25, 2014 7:19 pm
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If you have no current through the output tubes, I would expect to see a higher voltage on the Plates (pin 3). Doubtful that the voltage that you measure was really 1192 VDC, maybe your reference is not really ground. Check that all grounds are connected to the chassis, including the negative of the power supply.

If there is no current through the tubes, check that you have voltage to the screen grid (pin 4).

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Post subject: Re: Power & output transformers
Posted: Tue Mar 25, 2014 7:27 pm
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It's reading 1169 on pins 3 & 4 on both tubes and unreadable on pins 6 (too high).

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Post subject: Re: Power & output transformers
Posted: Tue Mar 25, 2014 10:25 pm
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Still sounds like your reference is not actually ground. Does the AC supply from the transformer that feeds the high voltage rectifier, have a center tap that should be connected to ground?

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 12:00 am
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Jim,

Does this 5F6 build use a GZ34/5AR4 rectifier? There is no way that this rectifier can handle 1KV. Is this VDC or VAC read? Like Mike said, better check your center taps are grounded properly. Should have two grounded wires off the PT (usually green-yellow and red-yellow). Be sure your tap for the bias supply is not accidentally grounded (usually red-blue or red-green).

Good luck! Keep us informed. :D


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