Restoring Amp tubes - from 7581 to original EL37

[Fast996]

I have two A-30 Crown mono blocks that have been modified from the original EL37's to ECG7581's. I have plenty of EL37's from my stash.

The transformer used is a Hammond 1650H which seems not to be a good match to the EL37. https://www.hammfg.com/files/parts/pdf/1650H.pdf

I'd like to find another output transformer that is a better match.

A 6.6k plate to plate i think is to high....maybe 4.0k
primary seems to be 400-0-400
the output impedance would be for 8 ohm speakers
I'm new to this,so please bare with me.
attached is a A-30 schematic
and the original output was 30 RMS and is now 40 watts
Can anybody help with the other necessary specs for the new OT's
I think there are better OT than the Hammond

EL37 @ The Valve Museum

The output transformer company sent me this.....

As for the output 4k plate to plate, (need UL taps?) if so what %?

80/80ma idle (little high there)

12 ohm secondary (8-16)

40 watts capable.

8db NFB,

 

[HollowState]

I see no real need to replace the output transformer. Hard load impedance values for output tube pairs isn't written in stone, but can vary somewhat depending on plate voltage and how much you want to push them. EL37s, 7581s and 6L6GCs are all similar in ratings. 4K ohms is more suited to 6550s and KT88s. Just know that using a primary load that's on the low side will provide a little more maximum power out along with higher distortion. While a higher impedance will produce a little less max power with accompanying lower distortion. And you're probably not going to be able to hear much difference between the two unless you're really picky.

The one modification I would make to this amplifier is to install a separate low value (5 or 10 ohm) resistor in each output tube's cathode so you can adjust each tubes grid bias voltage to balance the current between them for something around 50 milliampers each. You'll need to add a second test point as well.

 

[Fast996]

I see no real need to replace the output transformer. Hard load impedance values for output tube pairs isn't written in stone, but can vary somewhat depending on plate voltage and how much you want to push them. EL37s, 7581s and 6L6GCs are all similar in ratings. 4K ohms is more suited to 6550s and KT88s. Just know that using a primary load that's on the low side will provide a little more maximum power out along with higher distortion. While a higher impedance will produce a little less max power with accompanying lower distortion. And you're probably not going to be able to hear much difference between the two unless you're really picky.

The one modification I would make to this amplifier is to install a separate low value (5 or 10 ohm) resistor in each output tube's cathode so you can adjust each tubes grid bias voltage to balance the current between them for something around 50 milliampers each. You'll need to add a second test point as well.

Thanks....I am picky...not kidding either.

 

[mondogenerator]

Why switch back to EL37 at all?

7581(a) are expensive enough and very good valves, in my limited experience of them.

And the pair of EL37 I *had* sold instantly for £££s, even without emission figures