Tweed Deluxe OT in AB763 Deluxe?

[Thisismyname]

I'm rebuilding a DR into a single channel, no FX amp.
I have a Pacific Audio tweed deluxe OT on hand. I thought it was a brownface/blackface OT (I guess that OT is in an amp of mine 4 hours away as part of a friend's woodworking project)

So I have this tweed OT. I cannot access the spec sheet at Pacific Audio, so I'm not sure how capable it is. Can I use this OT safely? Will the higher voltages be a problem?

[Thisismyname]

Ok, I found the spec sheet a different way. Their site is weird.

The only difference is that the blackface/brownface OT they sell has an 8K primary and the tweed OT they sell has a 12K primary. I'm sure this is fine, although I expect there would be a tonal difference between the two.

What I might expect tone-wise from a 12K primary OT when the amp calls for an 8K?

[Willabe]

Tweed Deluxe rated at 15w, BF Deluxe reverb rated at 22w.

[Thisismyname]

Tweed Deluxe rated at 15w, BF Deluxe reverb rated at 22w.

What's your point?

Pacific's spec sheet for both transformers lists them as 20 watts transformers.

[mresistor]

Why would pacific rate a deluxe OT at 12K ohm center tapped knowing it will be most likely connected to 6V6 tubes?  Seems odd.

[Thisismyname]

disregard

Disregard what?

[mresistor]

Sorry there is not way to delete a post after you make one so I had deleted some text where I had my head stuck   and the info was wrong.  So I said disregard then I changed it above.


Are you going to run it in class A config?

[Thisismyname]

Sorry there is not way to delete a post after you make one so I had deleted some text where I had my head stuck   and the info was wrong.  So I said disregard then I changed it above.


Are you going to run it in class A config?

I'm just wiring the stock normal channel and stock power amp and power supply. The amp has a little less bass in it and a large value mid pot. But the amp is stock otherwise.

[mresistor]

If the OT primary is 12K center tapped wouldn't that be 6K per side?

[Willabe]

Tweed Deluxe rated at 15w, BF Deluxe reverb rated at 22w.

What's your point?

Pacific's spec sheet for both transformers lists them as 20 watts transformers.

Output transformers are a big part of an amps sound.

Like I said, normally a Tweed Deluxe is rated at 15w, BF Deluxe reverb is rated at 22w. Their rated differently and have different primary's, can't just swap them without a change in performance. They will sound different, distort differently, bass could be different. Put a lower power rated OT, but same primary/secondary, in an amp it will shave the shave the the high end and the low end frequencies off. The less it's rated for the more it will shave off the ends. 

Deluxe reverb preamp/PI will drive the 6V6's harder than the Tweed Deluxe preamp/PI. But the OT's primary is higher, it will sound, respond differently.

But this company says their both rated at 20w, which you didn't tell us. So it's just the primary that could change the amps sound.

The dcv difference will not be a problem.

[Willabe]

You might have to play with the -bias R values to get the -bias in the correct range because of the 12K primary.

[tubeswell]

Whether 12k plate-to-plate will be too much load resistance for a pair of 6V6s in push-pull depends on how you configure the output stage. 12k gives a shallower load line gradient, which would cross the Vg0 grid curve below the knee if you were running at normal deluxe reverb voltages. In this condition, screen current would rise rapidly when you drive a big signal through the amp. You can avoid this by either halving the speaker impedance (to get a 6k load, which is better for a DR running above 420V), or by lowering the B+ voltage (to something like 300V), or having the screen voltage reduced by about 60V or more below the plate voltage (which drops the grid curves down). You could also run a cold bias but that still might not be enough to keep screen current from spiking under big signal conditions if the B+ is too high.

[pdf64]

If the OT primary is 12K center tapped wouldn't that be 6K per side?

At lo signal levels in the class A area of operation, yes.
But at higher signal levels, the output valve conduction angle will falls below 360°.
In that condition, when current only flows in half the OT primary, the voltage ratio is halved so the impedance ratio is quartered.
So the class B loadline is 1/4 the full winding impedance, ie 3k.

[tubeswell]

If the OT primary is 12K center tapped wouldn't that be 6K per side?

12k plate-to-plate. The opposing signal phasing means that pair of tubes works as one 'joined-up' tube as seen by the primary, with the 2 x the current of a single tube during the Class A cycle. In this condition, each half of the primary 'sees' positive current (from one tube) going in a clockwise direction through the primary winding as well as negative current* (from the other tube) going in an anti-clockwise direction - i.e. 2 lots of current in each half of the primary compared to if only one tube was conducting.

In the B cycle, only one tube is conducting, and only in one half of the primary - resulting in 1/4 of the impedance compared to the A cycle.


*Negative current going in an an anticlockwise direction is the same thing as positive current going in a clockwise direction. But technically, it is 'negative current' in relation to the B+ voltage reference point.

[mresistor]

So  wouldn't it help to halve the speaker load? Like use a 4 ohm speaker on the 8 ohm tap? ( when using a 12K ohm primary/8 ohm secondary OT with 6V6s ) 


I do realize that it is permissible to use either a 4 ohm or a 16 ohm load on a Fender amp with 6V6s and an 6.6K or 8k OT primary and 8 ohm secondary - that the vacuum tube PS will tolerate the mismatch if used modestly.