18W PT secondary voltage vs. DC B+

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jimoreilly
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18W PT secondary voltage vs. DC B+

Post by jimoreilly »

I’m hoping someone can clear up some confusion I have with the 18W PT output compared to the rectifier output.

Most schematics I’ve seen (Doyle 1993, p210 / model 1974, and many other schematics at this site, tubestore.com, mhuss.com and others) show the PT secondary output at 300 VAC and the DC output of the EZ81 FWR at 325V DC.

Unless I’m missing something, it seems to me that the DC output (even after accounting for circuit loading) should be higher. For instance, 300 VAC should yield a peak voltage of about 420V, and with about 35-40V dropped across the rectifier, I’d expect the DC voltage at the first filter to be somewhere closer to 375-380V, not 325V.

What am I missing?

Thanks,

Jim
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Re: 18W PT secondary voltage vs. DC B+

Post by JMPGuitars »

You're likely looking at schematics that used differently rated PTs. The standard that we used is actually 290-0-290, but that was rated for lower AC source voltage. So in reality, it actually is closer to 300-0-300, and does often give around 375VDC rectified voltage (depending on your rectifier type).

For the classic 18 Watt sound and behavior, we prefer that be closer to 345VDC post-rectifier (B+) voltage.

I don't recommend following a schematic calling for 325VDC B+. You should also keep in mind, some of the schematics you'll find (including some on this site) were created to work with specific transformers. If you're looking at a schematic that shows ~325VDC as the B+, see if you can find a newer version from that series, or ask about it specifically.

Thanks,
Josh
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jimoreilly
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Re: 18W PT secondary voltage vs. DC B+

Post by jimoreilly »

Thanks, Josh - good info - however, my question had less to do with the PT secondary output at 290V or 300V - and yes, that will vary with input for a specific primary-to-secondary winding ratio - than the fact that if one has 300V AC (RMS) at the PT secondary output, it seems highly unlikely to get a rectified DC B+ of only 325V - unless there was something else really loading the supply down, which I don't see.

The EZ81 rectifier used typically will have 35-40V drop from what I've seen, and thus the peak voltage of a rectified 300V AC should be closer to 420V, with perhaps 40V dropped across the rectifier, we'd have a B+ closer to 380V.

I wonder if this was just a mistake in labelling the voltages in those very old Marshall schematics . . .

For what it's worth, the Mojotone schematic does show ~380V at the rectifier output - which seems reasonable - and of course, if one wants to drop the EL84 plate voltage a bit, that can be done post-rectifier and 1st filter output.

Jim
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Re: 18W PT secondary voltage vs. DC B+

Post by JMPGuitars »

You're right, on those old docs it is weird. Maybe they were lying based on things they assumed from the schematics they stole the circuits from. 🤣🤣

Take a look at schematics for the Watkins Dominator. That has the same 325VDC B+, but they used a 275-0-275 secondary.

Don't trust Mojotone schematics, though that might be the only thing almost accurate on it.
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