express transformer question

[phillyhudson]

hi all , I've just made an express with an AC30 mains transformer and a tubetown express output transformer - sounds fantastic but not for me , I'd like to try a 2 x 6L6 blackface type circuit next , but is it worth converting what I already have or just keeping the express as an express and starting over with new trannies ?

ta

[Roe]

the voltages would normally be too low. 320ish volts is not enough for a blackface

[wpaulvogel]

the voltages would normally be too low. 320ish volts is not enough for a blackface

That’s probably what’s wrong with it now. The Express runs about 400 volts with the pair of EL34’s. He’s running EL34’s with EL84 voltage.

[RJ Guitars]

I didn't see the actual voltages posted... did I miss them?

Looking at the replacement transformers for an AC-30, I see a 300-0-300 secondary on the Hammond unit. My experience with Hammond has been that they always run a bit high on the voltages so if anything I'd expect this tranny to be too hot for an Express. I've seen other AC-30 PT's that were 290-0-290 and those might be getting closer to the original Wreck voltage specs. The AC-30 did run EL84's but it ran them pretty hot... usually near the edge of disaster but they sounded really good. The other issue with this PT is 120mA current output... this is quite a bit less than the traditional Wreck PT.

I'd be curious to see the actual voltage measurements on the B+

[Roe]

Original ac30s had 280-90VAC into a gz34, yielding 320VDC at idle

[wpaulvogel]

^^^^This^^^^
If by chance he’s getting the 400 volts idle voltage and it’s able to keep up. An Express has a stout power transformer without really high voltage but if it’s running about 330-350 volts DC, there could be a problem with the sound. Not familiar with the performance of the AC30 transformer but because it’s EL84 I’d expect lower voltage than optimal for the Express. A GZ34 really doesn’t drop more than about 10-17 volts depending on current draw and I don’t think solid state rectification is going to bring up the voltage enough.

[pdf64]

I didn't see the actual voltages posted... did I miss them?

Looking at the replacement transformers for an AC-30, I see a 300-0-300 secondary on the Hammond unit…

The 290CZ you attached is listed as a AC15 replacement, rather than AC30
The AC30 4xEL84 cathode current is 200mA at idle, so AC current would be higher, and a 120mA winding wouldn’t cope.

[RJ Guitars]

I didn't see the actual voltages posted... did I miss them?

Looking at the replacement transformers for an AC-30, I see a 300-0-300 secondary on the Hammond unit…

The 290CZ you attached is listed as a AC15 replacement, rather than AC30
The AC30 4xEL84 cathode current is 200mA at idle, so AC current would be higher, and a 120mA winding wouldn’t cope.

Aw yes it's easy to see the problem now... thank you for clarifying. Makes you really wonder about that 100mA spec on the new Pacific Wreck series trannies? Also starts a curiosity about what John Mark is using in the new half power Wrecks? Maybe that is what Pacific is offering over there as Wreck replacements?

The one I'm playing with on the bench is the Hammond 270HX with a 230mA winding. It has an option of 115 or 125 VAC mains. Turns out neither hit the magic 300 volts but I'll see if I can coax it there if I work at it. I've got ~122 volts on the AC here where I am at.

I'll have to find a good AC-30 candidate and measure voltages on it but as suggested I'm starting to think it will end up a tad low.

[wpaulvogel]

You can wire the 5 volt winding in series with the 115 volt winding and make it a 120 volt winding. That’s probably going to get you really close to the 300-0-300.

[joeboo88]

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