Series/Parallel Heater wiring

[renshen1957]

Question:

I have a transformer with a 3.15v heater tap, is there an effective way to use this so I get 6vAC on both heater lugs of my preamp/power tubes? apparently the 3v isnt enough to light up my indicator light either

Hi

Why not a 6ax7 tube for the preamp? We use 12AX7 at the same way wired for 6.3 V, why not use parallel heaters for 6ax7. Might also work for the 6N2 Chinese tubes and the Rusky equivalent.

Just a thought

Best regards,

Steve

[renshen1957]

duplicate

[NickC]

Nick,
What's up with 7.3? That's not a typo, seven point three...you typed it more than once.
Phil

It's not a typo. It is the actual voltage on the power and rectifier tubes heaters from both the original and the Mercury PT replacement. I'm a novice amp-affianado admittedly, but I've not seen that on any other tube amp circuits. Weird, eh?

[renshen1957]

Nick,
What's up with 7.3? That's not a typo, seven point three...you typed it more than once.
Phil

It's not a typo. It is the actual voltage on the power and rectifier tubes heaters from both the original and the Mercury PT replacement. I'm a novice amp-affianado admittedly, but I've not seen that on any other tube amp circuits. Weird, eh?

Hi,

Did you measure the voltage on the secondaries without a load? You will see about 10% or so more voltage if measured in that manner.

Best regards,

Steve

[NickC]

Nick,
What's up with 7.3? That's not a typo, seven point three...you typed it more than once.
Phil

It's not a typo. It is the actual voltage on the power and rectifier tubes heaters from both the original and the Mercury PT replacement. I'm a novice amp-affianado admittedly, but I've not seen that on any other tube amp circuits. Weird, eh?

Hi,

Did you measure the voltage on the secondaries without a load? You will see about 10% or so more voltage if measured in that manner.

Best regards,

Steve

Yes, the Sovtek MIG50 power transformer is actually spec'ed with a 7.3V secondary winding for the PI and PA tube heaters. I've never seen anything like it before.

[beasleybodyshop]

For what it's worth, both the portnoy and bruce collins MIG50 schems did nothing for me.

The original factory schem lists component numbers (R1, C5, etc) that do not match up with anything on the board. As in, the C5 on the board is NOT the C5 in the schematic.

The normal/bright channel in my amp share a cathode cap/resistor, the schem shows different bypass cap/resistor combinations. Also the mixer resistors are different values, NFB is different value....

Im beginning to get a little wary of those schems. Ive taken to tracing out the circuit in long hand to wrap my head around it.

Im beginning to see why doing this over with a new board is appealing. oh well. The amp lives fine with the 3.15V heaters. Can't figure out why its like that. I found my center tap wire (white) for the heaters. it was soldered to some lug after the bias pot....I moved it to ground and all it did was blow the Mains fuse. lol. The more I mess with this the worse it gets!

[NickC]

Now that I think about it, I believe the heaters are elevated (the white CT wire that is part of 6.3V heater secondary with two green wires, gets elevated). Don't have the schematic handy at the moment, but I'll check later. I recall there is an error on the Collins schematic having to do with a big resistor either in the bias supply or power supply string, but need to check my notes .... can't remember exactly.

[Firestorm]

Now that I think about it, I believe the heaters are elevated (the white CT wire that is part of 6.3V heater secondary with two green wires, gets elevated). Don't have the schematic handy at the moment, but I'll check later. I recall there is an error on the Collins schematic having to do with a big resistor either in the bias supply or power supply string, but need to check my notes .... can't remember exactly.

V1 and V2 have elevated heaters. The Collins schematic doesn't show the series resistor for the voltage divider. I have read that it should be 1M, which would give 20ish volts elevation depending on the screen voltage.

[potatofarmer]

Sorry, i meant to say that both leads are 3.15v, with no centertap.

How are you measuring this? It sounds like you're measuring each lead to ground. For AC you should measure across both leads, e.g. AC voltage from green to green or brown to brown. If each lead has 3.15V to ground, then you have 6.3V across them.

FWIW, the Bruce Collins schematic shows the bias tap coming from the same secondary winding as the heaters for power tubes and PI, which seems moderately clever.

So, uh, what's going on here? Is the original PT blown and you're trying to replace it? Have you tried hooking up the new PT exactly as the original PT was connected?

A little more background could help us figure out why your lamp isn't lighting and why (I assume?) the new PT isn't working correctly.

[beasleybodyshop]

Sorry, i meant to say that both leads are 3.15v, with no centertap.

How are you measuring this? It sounds like you're measuring each lead to ground. For AC you should measure across both leads, e.g. AC voltage from green to green or brown to brown. If each lead has 3.15V to ground, then you have 6.3V across them.

FWIW, the Bruce Collins schematic shows the bias tap coming from the same secondary winding as the heaters for power tubes and PI, which seems moderately clever.

So, uh, what's going on here? Is the original PT blown and you're trying to replace it? Have you tried hooking up the new PT exactly as the original PT was connected?

A little more background could help us figure out why your lamp isn't lighting and why (I assume?) the new PT isn't working correctly.

Ah! this makes sense. I checked it again measuring across the heaters and i am getting 6.3vAC. The original PT blew a long time ago, and someone installed a new MM xformer into this thing. When I bought it the previous owner said he didnt know why the lamp wasnt lighting up. Turns out it had the wrong lamp in it - screw in type when it needed to be a bayonet lamp - now thats working! I just need to run out and grab more of those tiny newer marshall style fuses that this thing uses and try again.

[tubeswell]

Sorry, i meant to say that both leads are 3.15v, with no centertap.

So it has two leads and each one measures 3.15VAC when you hook up a meter between each lead and which other wire?