Classic 30 replacement transformer (Hammond)

[Steve Conner]

All good points Steve - but the point that I have been trying to make with the MM V Jr mod, is that the video shows a considerably reworked amp with an additional power tube & ALL new transformers - differences are typical of a new OT alone, in ANY amp.

Just didn't want this detail to go overlooked.

Maybe MM were worried that nobody would notice the "huge" difference from their wonderful aftermarket OT? :-)

[abram]

on the topic of transformers in the pc30, a hammond 290bx should be an adequate replacement PT for the classic 30 right? the schematic reads 332vdc to plate after the bridge rectifier. how much of a difference would a two volt difference make?

[Enzo]

The stock transformer will cause B+ of 332vDC? Yes, but only if the mains are at 120VAC. For every volt up or down from that the mains travels, the B+ will travel 2.77v. SO it is not likely the B+ sits there at a steady 332v anyway. I often see the mains at 123vAC here, and that 3v mains difference translates into a change in the B+ of about 8v.

2v on a 332v rail is a .6 of 1% difference. The variation in all the parts in the amp is way larger than that. No one would ever hear a 2v difference on the B+. And I doubt anyone will notice that 8v change either.

Unfortunately you have made a faulty assumption. The PV transformer causes 332vDC in the amp. That is DC. It takes about 235vAC to make that. The Hammond you propose has a secondary voltage of 330vAC, that's AC volts. rectify and filter 330vAC, and you get B+ of 467vDC. That is an increase of 135v, and I guarantee there would be a noticable change in the amp.

[abram]

beautiful. that's the exact answer i was hoping to see. thanks for the push in the right direction.

[Enzo]

You wanted to see that 2v didn't matter? Or you wanted to find out that that transformer was not appropriate?

[abram]

i was hoping to understand how big the difference was between rectified AC to DC voltage. a lot apparently. that opens a door when tracing out schematics that don't list the AC voltage off the power tranny out right. if you don't mind me asking, what's the formula you used to calculate the post-rectified DC voltage?

[Enzo]

Ac voltage is measured in RMS, which, without getting into it, means 0.707 of the peak value. And going the other way, the peak value of the AC is 1.414 times the RMS value. You got one, you got the other.

When you rectify AND FILTER AC voltage, the caps in the supply will charge up to the peak voltage of the AC. There will be some losses in the rectifier. If you have solid state rectifiers - plain old diodes - that loss is somewhere up to maybe one volt. On top of 300 and some volts we can ignore that.

Tube rectifiers have a much greater voltage drop, as much as 50v. But that is a lesson for some other amp that actually HAS a tube rectifier.


SO to get the DC result of your AC rectified, we take the AC and multiply by 1.414.

Hence 330VAC will make 466VDC. More or less.

You want to get 332VDC? Peak of the AC is 332v, then times .707, and we get 235VAC. Verify: 235 x 1.414 = 332.

And if it makes you more comfortble, multiplying by 0.707 is the same as dividing by 1.414. Multiplying by 1.414 is the same as dividing by 0.707.

So remember that 0.707/1.414 number, and understand what happens to AC when rectified to DC and filtered.


And if you rectify but do not filter, you wind up with pulsing DC, you DC voltmeter will average out the reading, and it will be closer to the AC RMS voltage than the peak voltage we are expecting.

[abram]

ahh. power transformers are rated RMS and not peak-to-peak. that was the missing piece of the puzzle and the tid bit i needed. you're a wealth of knowledge, fine sir.