Rate the Express output transformers.

[jeffkt]

Has anyone tried one
https://edcorusa.com/products/cxpp30-6- ... ransformer

[Roe]

I can confirm that in original 1980s train wreck express output transformer measured at 6.6 K

was that a Pacific or a Stancor?

[jeffkt]

Pacific

[RJ Guitars]

I can confirm that in original 1980s train wreck express output transformer measured at 6.6 K

Thanks for confirming that. I know the original guys on this forum were meticulous in their clone work and it seems unlikely that everybody would have missed this.

[gktamps]

Yes, thank you, Jeff. And thank you for the photos.
Whatever one thinks about the tone of a 7k5 OT (and a sample size of one sound file is hardly enough to judge either way) the confusing information from Pacific bothers me the most, since as you say, RJ, early builders/researchers here were pretty darned meticulous.
Jeff - I look forward to hearing your next recording through a cabinet.

[jeffkt]

Also I don’t understand why the power trans is the wrong voltage and does not have a 5volt tap for a rectifier tube that is used on the rocket
The HV taps should be 300-0-300
Not 280-0-280
Very strange to me

[RJ Guitars]

Also I don’t understand why the power trans is the wrong voltage and does not have a 5volt tap for a rectifier tube that is used on the rocket
The HV taps should be 300-0-300
Not 280-0-280
Very strange to me

That is something that I have studied and believe I understand. If you are familiar with "Loaded" vs "Unloaded" ratings on power transformers then those two labels could possibly mean the same thing. Heyboer always provided me transformer data with an unloaded rating. For a Wreck PT this was 300-0-300. However Pacific always provided transformer data using a "Loaded" rating and labeled them as 280-0-280.

On the bench with no load you'll measure ~300-0-300 for the secondaries on both of these units. I've never seen either of them loaded enough in a circuit to measure 280-0-280 but they both produce very similar voltages in an Express build.

The documentation on the later Rockets using Pacific power transformers show that a 5V power transformer was added inside the chassis to power the rectifier tube. That was not a very elegant way to do things but that was the way they got there.

[jeffkt]

My original express was 400 v dc b+
With the new pacific power I am only getting 385 v dc B+
Bias set at 46 ma

[RockinRocket]

My original express was 400 v dc b+
With the new pacific power I am only getting 385 v dc B+
Bias set at 46 ma

What's your wall voltage?

I have 2 Expresses built with Rocking Class Original spec Transformers and I get 400 ish biased arround 34ma. Wall is 121 122v

[gktamps]

I have 2 Expresses built with Rocking Class Original spec Transformers and I get 400 ish biased arround 34ma. Wall is 121 122v

Sounds like you are running your tubes around 55%, fairly cool. Your B+ would drop with hotter bias - Jeff's is about 70% of plate dissipation.

[RockinRocket]

Sounds like you are running your tubes around 55%, fairly cool. Your B+ would drop with hotter bias - Jeff's is about 70% of plate dissipation.

Yep, I think this usually sounds best with most amps but I wouldn't describe it as a cool bias. Depending on wall voltage and bias I wouldn't be surprised to see 20 dv difference though its just a guess.

[RJ Guitars]

Does anyone know what voltage a person might have expected from the wall 20, 30, 40, or 50 years ago? My understanding is that it has been creeping up over the years which begs the question about what the ideal rail voltage might be... or maybe does it matter?

I posted this before and it's a good read on AC power and how it can vary from place to place. Due to the "quality" of the power you might get a different DC voltage out while measuring the same AC voltage in when comparing two different locations. http://www.merrenaudio.com/power_quality

[gktamps]

Yep, I think this usually sounds best with most amps but I wouldn't describe it as a cool bias. Depending on wall voltage and bias I wouldn't be surprised to see 20 dv difference though its just a guess.

Yes, our wall voltage is often at 122, which can pose problems. And yes, cathode current will be different at different input voltage.

My "relatively cool" characterization is based on Class AB calculations for an EL34 at 400V B+, with 50% considered cool at 31.3 DC mA, 60% Average at 37.5 mA, and 70% maximum safe dissipation at 43.8 mA. https://elecurls.tripod.com/biascalc2.htm

If one agrees with Merlin that up to 85% is safe for Class AB, then cool and average readings would be higher.

I'm running my Express amps at about 34-35 as well.

[jeffkt]

Actually another good way to bias an amp is to get in a dark room have someone drive the amp hard watch the plates glow red and then back it down just a hair

[Littlewyan]

That seems a bit on the edge. What if your wall voltage is low that day, your amp could red plate when it goes back up or if you run it somewhere where the wall voltage is a bit higher.