Power Transformer specs - help me understand what I can build

[Fraggy0117]

Hey All,

I bought an old Voice of Music record player for cheap, hoping to eventually use the Power transformer in a future build. However, my research is making me more confused about if the specs are up to snuff for what I was thinking about building. I've now built 2 amps, both with salvaged PT but following a known schematic (Matchless Spitfire).

I've attached the scan of the schematic on the inside of the player, which notes the power transformer is 320-0-320 VAC @70 MA. It's the 70 mA that has me suddenly worried, and I have come to a point where I feel like I've found conflicting information on what this PT is good for. I'm wanting to build a PP 6v6 with 3 12AX7 preamp/PI tubes, and the phono uses 1 6u6, 1 12AX7 and 2 EL84 in PP.

While searching for information I came upon multiple places that said to use this current draw calculator https://thesubjectmatter.com/calcptcurrent.html. Using that, It would seem that the power transformer doesn't put out enough juice (but perhaps I'm not putting in the correct info). That lead me to a search that found me a thread (https://el34world.com/Forum/index.php?topic=16664.msg164971#msg164971) that mentioned using a full wave bridge to get a total 140 mA - which seems more in line with what the first calculator says I need.

HOWEVER, I watch Uncle Doug often and in his scratchbuild series, he shows another way to pick a PT which is to look at an amp with a similar tube compliment to what you're building. Specifically, he looks at a Princeton reverb amp (https://youtu.be/JhbJ3d6Ph2M?t=507) which has essentially what I'm wanting to build (3 12AX7 + 6v6 PP) but with another tube for the reverb. The video shows that the Classictone PT for that amp is nearly exactly the specs of the phono PT I have: 650V @70 mA.

So where am I going wrong? Can I use this PT for what I want just as is? Do I need to use a specific solid-state rectification to get the amps up high enough?

[sluckey]

Using a full wave bridge will not double the current capacity of that transformer, so forget that idea. That PT worked well with PP EL84s and it will work well with PP 6V6s.

[PRR]

If you keep the OT and PT together as factory designed you are unlikely to get in trouble.

And: 6V6 uses significantly less heater power {edit: than EL84}, so the PT can maybe supply a little more B+ power.

[Fraggy0117]

Thank you Sluckey and PRR. I figured I was somehow overcomplicating the issue, and the full wave bridge doubling current capacity didn't make sense to me either.

The OT is physically very small from the photo - maybe the size of choke. If I was to instead use an OT from a Hammond amp rated for EL84/6V6 Push-pull, do I need to worry about it overtaxing the PT somehow?

[sluckey]

No.

[HotBluePlates]

... I bought an old Voice of Music record player ... Uncle Doug ... looks at a Princeton reverb amp ...

If you omit the stuff between the Input Jack & V1, get rid of the feedback provided by C6 and R1, and possibly adjust V2 so you can insert a Volume Control between V1 and V2, you basically have an AC10.

Single channel, no tremolo, no Cut control (yet) but it's all there.  You could convert the 6AU6 socket to EF86 (or not).

I think you'll experience the most success letting this amp be what it is, minus the EQ bits ahead of the preamp (which isn't a good plan for guitar).  At least one of those old controls can be your new Tone (Cut) control.