help understanding basics of output transformer

[antiunderscores]

i've been mucking around with a old general purpose amp i got a while back. i took a look at the output transformer and cant quite work it out.

on one side it has 4 wires coming out, 2 are linked to the output jack (one for active and one to ground) then another two are off to the side and soldered together, is this a tap for a different impedance or what?
Then on the other side of the output transformer it has 5 wires, two wires linking straight to pin 7 of each tube and another 2 linking 2 pin 9 through a 41 ohm resistor and the fifth heads back towards the big cap but i cant work out from there where it goes at 11 at night.

i get 2 of the wires are the 'output' from the power tubes but what are the 2 feeding back into them? and whats the fifth? a ground wire?

i just havent really seen that on any schematics so im wondering whats it's there for. cheers.

[Merlinb]

Sounds like an ultra-linear transformer. The two ends of the primary go to the anodes, two more are 'tappings' on the primary and go to the screen grids via the 41 ohm resistors, and the 5th wire is the centre tap (where you feed the HT to!)

The secondary sounds like a generic dual winding. I suspect that each of the two windings is 4 ohms, and you would wire them in parallel if you wanted to use the OT that way. As you have them, they are wired in series for 16 ohms (4 squared).

[MWJB]

I'm assuming that this is an EL84 amp.

"on one side it has 4 wires coming out, 2 are linked to the output jack (one for active and one to ground..." These are the secondaries & determine the output speaker load selected.

"then another two are off to the side and soldered together, is this a tap for a different impedance or what?" Looks like 2 unused taps? Are you sure that they are soldered together (possible), or do they terminate to a terminal strip (more typical)?

"Then on the other side of the output transformer it has 5 wires, two wires linking straight to pin 7 of each tube" These are the primaries, feeding the plates of each power tube.

"and another 2 linking 2 pin 9 through a 41 ohm resistor..." This is the power tube "screen grid" supply, your OT is designed for ultralinear use (as opposed to most guitar amps that simply tap the screen grid supply directly from the B+ rail).

"and the fifth heads back towards the big cap but i cant work out from there where it goes at 11 at night." The fifth wire is the connection to the main B+ supply (hi voltage dc supply) and probably connects to the output of the rectifier and the first (usually largest value) filter cap.

[antiunderscores]

I'm assuming that this is an EL84 amp.

yes, its got 2 el84, a 12ax7 which im pretty sure is the phase inverter and a mullard dual triode preamp which is similar to 12ax7 but has lower gain and a different pin pattern, but its name eludes me for the time being.

"then another two are off to the side and soldered together, is this a tap for a different impedance or what?" Looks like 2 unused taps? Are you sure that they are soldered together (possible), or do they terminate to a terminal strip (more typical)?

im certain they are soldered together and do not go to ground (if thats what terminate means)

"and another 2 linking 2 pin 9 through a 41 ohm resistor..." This is the power tube "screen grid" supply, your OT is designed for ultralinear use (as opposed to most guitar amps that simply tap the screen grid supply directly from the B+ rail).

so could i safetly remove the 41 ohm resistors? could i experiment by leaving one and removing the other? this amps actually intended for bass but i find a lot of bass amps pretty sterile so im trying to get a sound that is half way between a bass amp and a guitar amp if that makes a difference an impact on what i should and shouldnt be doing.


just a question or 2 about the first reply. are the taps the ones that arent normally in a guitar amp? and whats HT mean?


thanks for the help so far.

[Merlinb]

so could i safetly remove the 41 ohm resistors? could i experiment by leaving one and removing the other? this amps actually intended for bass but i find a lot of bass amps pretty sterile so im trying to get a sound that is half way between a bass amp and a guitar amp if that makes a difference an impact on what i should and shouldnt be doing.


just a question or 2 about the first reply. are the taps the ones that arent normally in a guitar amp? and whats HT mean?


thanks for the help so far.

Yes, the taps (called 'UL taps') aren't common in most guitar amps, but they are used in some guitar amps.
You could remove the 41 ohm resistors, but you'd need to connect the screen grids to the power supply by some other means instead (DO NOT JUST LEAVE THEM FLOATING!!!). It's probably worth playing with other parts of the amp before you mess with them.

HT means High Tension, which basically means High Voltage. In the US the term "B+" is often used. Eaither way it means the power supply.

[antiunderscores]

thanks, could i just short around the resistors or would that not impact anything in a sound sense?

and im still mucking around with the rest of the amp, i was just curious about this. speaking of which i have a volume pot and a master pot. the volume is a 500k audio pot and that is after the tone controls between the 1st and 2nd gain stage and then a 1Meg audio after 2nd just before the PI (the master control) and both of them make some loud noises when turned and also the master stops all volume until about 1/3 up, is there any way i can fix that problem or is it just the wrong value pots?

cheers

[Merlinb]

thanks, could i just short around the resistors or would that not impact anything in a sound sense?

It's very unlikely to make any difference at all- those resistors are to protect against HF instability, they don't do anything audible. (Actually, you could make a bigger difference by making those resistors bigger! Like 10k 2W or something.)

If your pots are making scratchy sounds then you may have some leaking coupling capacitors which need replacing. Ideally there should be no DC on a pot, only AC. Otherwise they may just be dirty or worn out. You could try cleaning them with some Servisol switch cleaner, or just try replacing them with new ones.

[tubeswell]

Another thing that can make a pot scratchy is loose socket pins (esp pre-amp plate pins) resulting from worn-out sockets making intermittent/light contact. With the amp off and drained, try wiggling the pre-amp socket pins with your finger. If you get any ones that feel loose, then you can try re-rensioning them from the outside with a little pin or wire to try and force the clamp part of the pin closer together (otherwise you would need to replace the socket)