Rocket Power Transformer Story - Revisited

[martin manning]

And switching in the cathode R bypass cap on the previous stage will boost its gain by about 6dB, so it all evens out.

[Blackburn]

Just tried this out in my most recent Rocket build and it's fantastic. 2tone is right that the stock Rocket is more open sounding. Cleans are better stock, but modded, the dirt is so classic AC30 like this, and IMO, preferred. The Z Wreck is setup like this, but with higher plate voltages in the pre and the comfort/speed switch. I'd been meaning to apply the 220k/22uf sometime and discovered that's what was being discussed here. A couple of alligator clips later...

[RJ Guitars]

I just wanted to say thanks to 2-Tone for giving us some real insight into the smaller details of the Rocket amp as well as all you guys for sharing and exploring this topic with everyone. It's been a good read. I've had a few issues to tend to lately so I've just been a spectator on this thread, but I am greatly pleased to learn a bit more about the Rocket amps.

One thing I have seen is that KF doesn't seem to change his dropping string resistors from the 18.2K - 9.1K - 9.1K standard. I haven't seen every Wreck but it's true in what I have seen even when the transformer voltages are changing a bit... not sure what it means but it's a curiosity??

After a bit of study and contemplation (while avoiding any actual written math calculations), I asked EDCOR to engineer a new power tranny at 252-0-252, 250 mA etc.. My attempt to hit the sweet spot using modern wall socket voltages without messing with anything else. It also fills in a hole in my current transformer stock.

Let me tip my hand to one other detail from the Rocket I saw... the power supply was not the same as the typical Wreck, although the dropping string and transformer were. It used a 20uF cap for the screen voltages, right after the standby switch and the 80uF stack. Has anyone seen this before?

[Zippy]

One thing I have seen is that KF doesn't seem to change his dropping string resistors from the 18.2K - 9.1K - 9.1K standard. I haven't seen every Wreck but it's true in what I have seen even when the transformer voltages are changing a bit... not sure what it means but it's a curiosity??

I guess it means either that resistor color matters more than voltage, or that we care a LOT more about minutiae than Ken did. Maybe the preamp voltages aren't as critical as we believe. (Don't tell the Dumble crowd that there are no magic preamp voltage values - they'll get very grumpy.)

[Blackburn]

Tubes and speakers are, in my mind, the most critical... And the circuit of course.

[redshark]

I'm sure there's a lot more than we know about the rocket OT.

I think we won't know the real truth unless we look at Ken's book.

[Mark]

I wonder how much the rail voltage has to increase or decrease before a difference in tone is noticed.

I think the big deal with the Rocket and voltages it the fact the output tubes are on the verge blowing their brains out with the stock AC-30 voltages. Yet if you drop the voltages you don't notice a difference in tone and you get a bit more reliability out of the output tubes.

Just my take on it all.

[RJ Guitars]

I wonder how much the rail voltage has to increase or decrease before a difference in tone is noticed.

I think the big deal with the Rocket and voltages it the fact the output tubes are on the verge blowing there brains out with the stock AC-30 voltages. Yet if you drop the voltages you don't notice a difference in tone and you get a bit more reliability out of the output tubes.

Just my take on it all.

That's kinda my take on it all as well. With today's line voltages creeping up here in the US, we could push the old AC-30 way past the sanity limit. I'm skeptical you can hear a lot of sonic difference as you approach the the edge of disaster... I think of it more as a somewhat broad sweet spot that occurs before you hit that extreme. This thought has to be valid if we believe that VVR technology works at all.

Here is a great read that will have you rethinking what your B+ is going to be at any specific AC voltage, given your local power quality. http://www.merrenaudio.com/power_quality

[rooster]

Wow, that Merrin read was nice. I can't believe he contacted the electric company, ha! He's lucky they didn't add some 7th order harmonics to the pie.

One story that I read that I still think about is the one about Laurens Hammond, the creator of the Hammond B3 and many other things. Including a gyroscope that was used in US rockets. Amazing guy. Anyway, back in the day, he was concerned about the power arriving at people's homes, and keeping his tone wheel equipped organs operating correctly. It seems that the power would vary anywhere form 50 to 65 cycles depending on where you lived and what power company was involved. Which meant his organs could potentially sound like ass. So he built large high quality clocks that operated at 60 cycles - if you fed them 60 cycle power, they would keep accurate time. And then he went around the country giving them to various power companies who would then hang them on the power companies' wall. Since an 8 hour shift was involved and nobody wanted an argument over what time it was, the various power companies started keeping their generators at 60 cycles. Hammond's tone wheel problems were solved.

Now, today, there's a whole different set of problems mostly centering on dirty power. ....Anybody want to start delivering audiophile grade hi fi systems to power plants? No?

[2tone]

I wonder how much the rail voltage has to increase or decrease before a difference in tone is noticed.

I think the big deal with the Rocket and voltages it the fact the output tubes are on the verge blowing there brains out with the stock AC-30 voltages. Yet if you drop the voltages you don't notice a difference in tone and you get a bit more reliability out of the output tubes.

Just my take on it all.

That's kinda my take on it all as well. With today's line voltages creeping up here in the US, we could push the old AC-30 way past the sanity limit. I'm skeptical you can hear a lot of sonic difference as you approach the the edge of disaster... I think of it more as a somewhat broad sweet spot that occurs before you hit that extreme. This thought has to be valid if we believe that VVR technology works at all.

Here is a great read that will have you rethinking what your B+ is going to be at any specific AC voltage, given your local power quality. http://www.merrenaudio.com/power_quality

I have heard day to day differences in my amps, everything set the same, dials untouched etc. Two things seem to be in play..both the line voltage swings ( minor effect if less than 3 volts difference) and the dirty power factor, which I believe is from heavy industrial inductive loads and capacitor charging motors etc. (big pump motors, machine shops etc.) It would have been interesting to see wave forms ala Merren's article in real time. I'm not sure if power conditioners could eliminate this stuff..

[Plexified]

What a great read guys . I remeber Ken writing in Guitar Player in the 80's about plate voltages for the EL84s . He honed in on 315 volts which had more grind and warmth and 370 volts for headroom and dynamics . He was also the worlds leading Vox expert at that time and he built the Rocket for himself. Remember also he was VERY well versed in magnetics and design . So when Pacific says they designed and built him only one set, I take that at face value because Ken would design his own spec sheets and was very specific about wire gauge, symmetry and winding techniques as well as the geometry involved including laminate composition like grain orientation and such. So he primarily went to a manufacture with a blueprint and said build this.

In the Vox lineage there were at least three different transformer manufacturers like Albion, Woden, and others . So Ken chose each part for a reason . He specifically learned that technique from repairing thousands of amps . Many while people waited he was THAT good .

So that 5v transformer was probably chosen to reduce noice and HEAT . That was one of the biggest problems on the Voxes as well as the Beewax potting that absorbed moisture and literally the transformers would rust from the inside out producing some very interesting variations as they aged . Ken was wise to all this . Chris Merren is like the ultimate transformer designer, because he is basically a NASA scientist, a great player, and spent years conversing with Ken . And I am really serious about the depth of Kens Transformer knowledge its staggering . So to me he taught me the most amazing things about the effect on the gauge of wire in the transformers and their impact on tone .

I have to find the article, but Roe did a masterpiece of writing the lineage of old Marshall Plexis from start to finish . In that body of work you can literally have an AHA moment realizing the effect of the wire gauge in transformers and it teaches you how important power transformer selection really is and its impact on tone . You can literally get from a nice soft ready to implode on the edge of insanity tone up to a super hard fast solid state like response here alone. Some food for thought. David

[Paultergeist]

Just tried this out in my most recent Rocket build and it's fantastic. 2tone is right that the stock Rocket is more open sounding. Cleans are better stock, but modded, the dirt is so classic AC30 like this, and IMO, preferred. The Z Wreck is setup like this, but with higher plate voltages in the pre and the comfort/speed switch. I'd been meaning to apply the 220k/22uf sometime and discovered that's what was being discussed here. A couple of alligator clips later...

I am having trouble understanding this switchable 220k/22uF mod.....but my Rocket clone is still not inside a cabinet.....it is just sitting there on the bench cradle, winking at me.....

.....any help?

[gktamps]

Just to add my findings so far to the mix, though not of historical nature:

I've now built Rocket amps with the following OTs:
Heyboers (3), a Pacific from RJ, original Dynaco A-470S (2), an Edcor from RJ, and a Pacific from Allyn Meyer.

The Dynaco are pretty tame - good clarity with the guitar turned down a bit, and nice crunch when turned up, but not wild, and not my favorite tone. The Heyboers have been somewhat variable in volume and distortion quality, but pretty good sounding overall, the Edcor really too raspy for my tastes. The Pacific from Allyn is incredibly well balanced between clarity and distortion with varying guitar volumes, but not as loud as the Edcor, two of the Heyboers, or the RJ Pacific. By far, the most impressive tone has been my most recent with Allyn's PT and OT from Pacific.

I'm currently trying to capture the tone of that build in one using RJs Pacific OT and a Heyboer PT. I've adjusted the voltage dropping resistors to get voltages almost identical between the two amps, and all other circuit values started out standard, but the tonal differences are significant. I'm not adjusting each preamp section to see what effects that will have. Planned to finally learn how to use my old Tektronix Oscilloscope, but it gave up the ghost, so I'm going only by ear now.

Cheers,

Greg

[Zippy]

Just to add my findings so far to the mix, though not of historical nature:

I've now built Rocket amps with the following OTs:
Heyboers (3), a Pacific from RJ, original Dynaco A-470s (2), an Edcor from RJ, and a Pacific from Allyn Meyer.

The Dynaco are pretty tame - good clarity with the guitar turned down a bit, and nice crunch when turned up, but not wild, and not my favorite tone. The Heyboers have been somewhat variable in volume and distortion quality, but pretty good sounding overall, the Edcor really too raspy for my tastes. The Pacific from Allyn is incredibly well balanced between clarity and distortion with varying guitar volumes, but not as loud as the Edcor, two of the Heyboers, or the RJ Pacific. By far, the most impressive tone has been my most recent with Allyn's PT and OT from Pacific.

I'm currently trying to capture the tone of that build in one using RJs Pacific OT and a Heyboer PT. I've adjusted the voltage dropping resistors to get voltages almost identical between the two amps, and all other circuit values started out standard, but the tonal differences are significant. I'm not adjusting each preamp section to see what effects that will have. Planned to finally learn how to use my old Tektronix Oscilloscope, but it gave up the ghost, so I'm going only by ear now.

Have you considered just swapping OT's in a single amp to hear the differences due only to the transformer?

[gktamps]

Absolutely, Zippy. That was next on my list, but I wanted to try reducing other variables in the amp first. I've already swapped other transformers in that same build, and the differences were notable, so I understand it is the most logical comparison, but I'm also trying to improve my voicing chops. Thanks!

Greg