5.2k and 6.6k Express OT

[xtian]

What?!? How could the number of turns in an OT vary with voltage?

[Mark]

I think the confusion is I'm referring to the DC rail voltage. I came across a record player valve amp. The rail voltage was 200VDC and the output transformer was 2.5K to 4ohms.

[Littlewyan]

Thats why some Marshalls were so hard on the valves. The impedance stayed the same, 3.4K, but the voltage varied between 380VDC and 500VDC! Then again with tolerance levels back then the impedance varied a bit as well

[guitarmike2107]

The output transformers input impedance is dependent on the rail voltage. The lower the rail the lower the primary impedance.

think that should say the selection of the output transformer primary impedance is dependent on operating voltage, once you have a 6.6k primary that what you have regardless of voltage.

you can easily try mismatch impedance's to see how much of a change it makes, plug a 16 ohm into an 8ohm output on a Marshall, tonally I find them very similar, but the power sections normally do distort easier which is what Ken was going for I believe and may make the amp more adjustable from the guitar volume knob

[pdf64]

As the primary impedance increases, eg above the 'max power optimal value', so will the stress on the screen grids.
May explain the 1k dropper for the screen grid HT node.

[andresound]

To repeat Mark's question

"What is the definitive rail voltage of the Rocket?"

Sorry I am a newbie and have just built a rocket. Great sounding amp.
What is the desired rail voltage for a rocket? Also, should we be shooting for 100% dissipation on the tubes. Obviously, if the mains voltages varies, that could push the tubes over this mark. In my country, we regularly see changes of at least 10%. Even while taking notes of voltages, by the time I get to the end of the exercise, the mains voltage has changed completely!! So bias calculations are not always accurate?

[BLT]

In the "Trainwreck Files" there is thread called "Rocket Documents," and in that thread
there's an Excel file named "Rocket Voltage Chart" that has expected voltages of both
rail and tubes.

[RJ Guitars]

To repeat Mark's question

"What is the definitive rail voltage of the Rocket?"...

I would suggest that we really don't know but if you end up with around 300 volts feeding the OT and 90 volts on V1 you'll be in the ballpark with at least one of the early originals. I put together pages of notes on the Rocket way back when we did the group build and we had a pretty good idea but never an exact answer. I found some info that I copied from a post by "2tone" here on the TAG forum (an original Rocket owner) that indicated that the earlier Rockets used a lower voltage PT than the later ones.... I can't find that post so maybe it wasn't here on TAG. I did print it out so I have the printed copy in my books here.

I've seen inside one of the later original Rockets and it used the Exact same PT as a Liverpool - A Pacific part# 11410 power tranny on the 250-0-250 secondary. It had an internally added 5V tranny for the rectifier since that tap is not there on the standard Pacific PT.

Read the threads listed below and you'll learn most of what has been published on the Rocket builds.

http://ampgarage.com/forum/viewtopic.php?t=5964

http://ampgarage.com/forum/viewtopic.php?t=24912

http://ampgarage.com/forum/viewtopic.php?t=8394

...So bias calculations are not always accurate?

I suppose if your math is good you could say your calculations are accurate but I think there are too many variables in each tube and amp circuit to really hit it right on the money. Bias calculations are useful... Bias measurements are real... my expectation is they rarely align perfectly.

[Scott_F]

Great info