question about my large mystery power transformer

[Hebert]

I ran ac through it and tested the wires.
Blacks are primary
Reds are HV with no center tap
large green and large brown are 6.3 with a small green wire as the center tap
there is a small leftover black wire that has no connection with the other wires
and the two yellow wires that I thought would be 5 were in fact 29.1
the amp I want to build does not use the 5v so whatever (but still weird that it is 29.1) but I do need a center tap for the HV.
what do you think?

[silverfox]

Are there any numbers on the transformer?


The 29 volts is likely a bias tap. You do not need a center tap on the HV.


It would be nice to know how many amps the filament leads are, as well, the HV.


silverfox.

[Hebert]

Are there any numbers on the transformer?

The 29 volts is likely a bias tap. You do not need a center tap on the HV.

It would be nice to know how many amps the filament leads are, as well, the HV.

it is 39051 I can't find anything about it. The plan I am using does show a center tap for the HV; two HV wires to the rectifier tube and a third (the tap) to the negative end of a capacitor. can a tap not be used?

Edit; Untangled quote, Willabe.

[sluckey]

Reds are HV with no center tap

What voltage do you measure between the reds?

[Hebert]

Reds are HV with no center tap

What voltage do you measure between the reds?

they measure 373

[sluckey]

That will produce 527V unloaded B+.

[PRR]

> HV with no center tap

Use a Bridge Rectifier.

As Sluckey says, this will make more Volts than we usually want, and more than standard filter caps can stand.

The 29V may be bias as-said or may be for some transistor frill.

The unconnected black wire is probably an internal shield to divert crap from elevator motors or radio transmitters on the same power line.

[kagliostro]

If you don't adopt other sistems, like a Fixed Voltage Regoulator (a VVR with a trimmer to set voltage instead of a pot), one other way can be to use an hybrid FWB (Solid state + Vacuum Tube), this will result in a drop of the HV, only you haven't a 5V winding so you are limited on using a 6.3V heated Tube Rectifier (with the connected limitations)

Other way can be to use the 29V winding as to drop the voltage on the primary side of the PT

read here

http://el34world.com/Forum/index.php?topic=25306.0





So, assuming your primary is 120V you'll have 373VSC - 29VAC = 344VAC ~282VAC that rectified (using Solid State diodes) and leveled will give you a B+ of around 480V  ~294,8VDC

But this solution require that the 29V winding can afford at least the current that flow on the primary winding

--

If your 6.3V winding has enoug current available also this can be a way to tame B+

http://el34world.com/Forum/index.php?topic=25190.msg272815#msg272815



with a 6.3V heater winding to obtain the max voltage drop you must connect the two heaters in parallel (not in series as shown, that is for a 12.6V heater winding) and the two tubes in series as to maximize yhe voltage drop

If the 29V winding is enough current capable and the 6.3V winding can feed all the tube of the circuit plus a pair of Tube Rectifiers, you can adopt both solutions at the same time to maximize the voltage drop on B+




The red diodes are there only to increase the ability to afford high voltage of the Tube Rectifier, adopting also the 29V winding connected in counterphase HV AC will drop but also 6.3V will drop a bit, you must control that the Heater voltage don't drop too much

--

If you want to go the Fixed Voltage Regoulator way give a look to the following circuit

This is a standard VVR


--

this version (with two mosfet, nowaday you can find cheap mosfets with good spec for this use) is designed to avoid problems if the mosfet burns

(instead of 1M pot use 1M trimmer, you are interested in a fixed voltage, not a variable one)



Franco

[Hebert]

I was hoping to build this: https://robrobinette.com/Bassman_Micro.htm#Bassman_Micro_LTP

[Hebert]

If you don't adopt other sistems, like a Fixed Voltage Regoulator (a VVR with a trimmer to set voltage instead of a pot), one other way can be to use an hybrid FWB (Solid state + Vacuum Tube), this will result in a drop of the HV, only you haven't a 5V winding so you are limited on using a 6.3V heated Tube Rectifier (with the connected limitations)

Other way can be to use the 29V winding as to drop the voltage on the primary side of the PT

read here

http://el34world.com/Forum/index.php?topic=25306.0





So, assuming your primary is 120V you'll have 373VSC - 29VAC = 344VAC that rectified and leveled will give you a B+ of around 480V

But this solution require that the 29V winding can afford at least the current that flow on the primary winding

--

If your 6.3V winding has enoug current available also this can be a way to tame B+

http://el34world.com/Forum/index.php?topic=25190.msg272815#msg272815



with a 6.3V heater winding to obtain the max voltage drop you must connect the two heaters in parallel (not in series as shown, that is for a 12.6V heater winding) and the two tubes in series as to maximize yhe voltage drop

If the 29V winding is enough current capable and the 6.3V winding can feed all the tube of the circuit plus a pair of Tube Rectifiers, you can adopt both solutions at the same time to maximize the voltage drop on B+




The red diodes are there only to increase the ability to afford high voltage of the Tube Rectifier, adopting also the 29V winding connected in counterphase HV AC will drop but also 6.3V will drop a bit, you must control that the Heater voltage don't drop too much

--

If you want to go the Fixed Voltage Regoulator way give a look to the following circuit

This is a standard VVR


--

this version (with two mosfet, nowaday you can find cheap mosfets with good spec for this use) is designed to avoid problems if the mosfet burns

(instead of 1M pot use 1M trimmer, you are interested in a fixed voltage, not a variable one)



Franco

so basically this transformer is overpowered and I need to tame it?

[kagliostro]

I did a mistake

373VAC - 29VAC = 344VAC is not correct

if you subtract 29V from the primary supply your voltage on the secondary HV winding will be

~282VAC NOT 344VAC


----

so basically this transformer is overpowered and I need to tame it?

It depends in which are the available currents on the windings

High voltage is favorite for big tubes with high current consumption

---

I think you must put a load on the windings and increase it till you see a 5-10% drop in voltage, the current that the transformer is delivering at that moment is the current available, as to be sure, leave the load connected and control after 1/2 - 1 hour if the transformer temperature is accettable or is too high if it is too high decrease the load and mesure newly the curent

Franco

[sluckey]

I was hoping to build this: https://robrobinette.com/Bassman_Micro.htm#Bassman_Micro_LTP

I would look fo a more appropriate transformer.

The voltages from that PT could be used with EL34s or 6550 or KT88 ***IF*** the current is sufficient. Certainly not something for a 12AU7 power amp. How big is it? how much does it weigh?