PRS HDRX amp

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Roe
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Re: PRS HDRX amp

Post by Roe » Fri Oct 21, 2022 7:13 am

masonbeckham wrote:
Thu Oct 20, 2022 5:49 pm
Going off of memory and recordings of my JH100, it feels like a Tube Rectifier amp vs a Solid State Rectifier compared to the WC amp. There was a lot of “sag” in the JH100. It was quite trebly too, and the bass would get woofy when you added any in my case.
I had one on my bench. It has 600v across the standby switch that dropped to ca 545v at idle, depending on bias. It did sag a little, but not much more than normal for a plexi. Its one of the better new marshalls, but it did tend to eat new kt66s
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mark
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Re: PRS HDRX amp

Post by mark » Fri Oct 21, 2022 8:21 am

Thanks for your feedback Roe.

What was different circuit wise?

I would have thought there would have been measures to save the output valves.

Regards

Mark

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Re: PRS HDRX amp

Post by Roe » Fri Oct 21, 2022 10:48 am

mark wrote:
Fri Oct 21, 2022 8:21 am
What was different circuit wise? I would have thought there would have been measures to save the output valves.
A jtm45/100 with the socalled dickinson mod (33k/500pf. etc), except for the 1k screen resistors.

the 1k was added and the voltages lowered slightly to save tubes. the chinese tubes handle these voltages better than the russian ones (even if the gold lion RIs sound better in almost all cases). I tried many tubes that redplated at 545v, even with a coldish bias
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daveweyer
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Re: PRS HDRX amp

Post by daveweyer » Fri Oct 21, 2022 9:31 pm

The crossover distortion is not a problem until the tubes reach a 50% duty cycle, meaning one pair is cut off for exactly half of the waveform. If the tubes are run so that they operate at slightly more than half of each cycle, they will overlap slightly at the zero crossing.
As the plate voltage drops during high current situations, the virtual bias of the tubes becomes less unless the bias supply also drops by the same percentage. On the SL type amps, with a separate bias winding, the supply usually does not drop as much as the plate winding supply, meaning that the tubes can be cut off for over half of the wave cycle, also meaning you will have crossover distortion as well as the harmonic and intermodulation distortion.

If this doesn't make sense, think of the current in a 6550 at a plate voltage of 560 volts with 63 volts of negative bias on the grids, about 15ma. Then consider the same tube with 460 volts on the plate with the same bias on the grid--the current in the tube drops substantially, and if the screen voltage drops a lot, the current can go to zero at the zero crossing. This is one reason that classical Class B amps normally require regulated supplies; there is a huge difference between idle current and full output current. That's why, if you want the most power out of the tubes, and you want them to hold up the best in long term square wave duty, you need to run them in Class B. As we have said many times here, Class B is defined as the tubes being in cutoff for a SUBSTANTIAL part of the wave cycle.
Jimi's first request was for the output tubes to hold up better, so his amps wouldn't blow up on him all the time.

There are cheap and dirty work arounds, and you can make it work. In the rock music world, the crossover distortion ends up being at full volume, so what's not to like? At 15ma current in each output tube at idle you will not hear crossover distortion and your string sustains will be clean all the way to noise.

mark
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Re: PRS HDRX amp

Post by mark » Fri Oct 21, 2022 10:24 pm

Roe wrote:
Fri Oct 21, 2022 10:48 am
mark wrote:
Fri Oct 21, 2022 8:21 am
What was different circuit wise? I would have thought there would have been measures to save the output valves.
A jtm45/100 with the socalled dickinson mod (33k/500pf. etc), except for the 1k screen resistors.

the 1k was added and the voltages lowered slightly to save tubes. the chinese tubes handle these voltages better than the russian ones (even if the gold lion RIs sound better in almost all cases). I tried many tubes that redplated at 545v, even with a coldish bias
Thanks for sharing your findings Roe. I appreciate it. So basically it’s much like the Ceriatone version of Hendrix’s amp.

Did you consider doing the zener diode trick to reduce the voltage on the plates of the output valves?

Many thanks Roe.

Regards

Mark

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Re: PRS HDRX amp

Post by daveweyer » Sat Oct 22, 2022 12:34 pm

A few months back I read this sweet little history of Marshall amps, including diagrams of various power supply arrangements, transformers, and related detail. At the margins I thought I saw that Roe and Shakti were authors.
Roe is this true or did I get this mixed up with something else?

mark
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Re: PRS HDRX amp

Post by mark » Sat Oct 22, 2022 5:19 pm

Thanks for your reply Dave. Did you try the zener mod to get rid of crossover distortion as you say it’s almost unavoidable in guitar amps.

https://www.aikenamps.com/index.php/wh ... distortion

https://music-electronics-forum.com/fo ... -zener-mod

Regards

Mark

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Re: PRS HDRX amp

Post by daveweyer » Sat Oct 22, 2022 8:05 pm

Mark I used the zener to lower the screen voltage at idle, and to keep the screen dissipation below the peak value given for the tube at max power.
The screen supply ideally should be about 300 volts with a beam power output tube running 500 to 600 volts on the plate, but if you can lower the screen voltage by 70 to 100 volts below the plate voltage you can get away with this and still have high sensitivity in the output tube. Plus you will not need such a high negative bias voltage. A zener can hold the screens below the plate by a fixed amount, but it has to have enough power dissipation to drop that much voltage and still supply the screen current; in this respect it is much better than using larger value screen resistors.
You may need a couple of 10 watt zeners to make this system reliable

I read the posting about crossover and clamping distortion; I might just add that I use .015 coupling capacitors for the output tubes for the reasons outlined in the article, and other reasons not mentioned in the article. I have also found that when the output grids start to draw current, there is a very nice non-linear function working on the driving wave creating a very desirable type of distortion. Generally speaking you design the driver stage to run out of headroom at about the voltage it takes to drive the output tube grids into positive current territory so the output tubes don't go into Class C operation, which is really what the obviously intelligent person who wrote the piece on blocking distortion was talking about. Class B operation does not have crossover distortion unless the tubes have a non linear response. Since most pentode output tubes are not linear as they reach their "remote cutoff" zone, there is a wiggle at the zero crossing, unless other compensation is made to linearize their response. And, as the author states, if the driver stage has a fixed upper limit to its plate swing, but can continue to expand the swing to the negative, there can be a time during the cycle zero crossing when neither output tube is conducting--that's called Class C.
But in reasonably designed Class B circuits, and since this non-linear response at the zero crossing happens at maximum volume, the crossover distortion is the smallest part of the distortion created by the stage.
Guitar amps running in Class B are generally not run to maximum efficiency because there are too many variables to guarantee the tubes will exchange their current drawing part of the cycle at exactly half, so you sacrifice some efficiency and let them overlap at the zero crossing. I try to design so that at maximum power the tubes are getting close to a 50% duty cycle but can't get pushed beyond under any known condition.

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Re: PRS HDRX amp

Post by yladrd61 » Sun Oct 23, 2022 3:23 am

Here is a video from PRS with more on their version of the "Hendrix" Amp. I have seen the Amp they claim to have used as the basis for this new Amp, I was able to see that it had the West Coast sticker. Of course anything that uses Jimi's name or likeness has to be approved by Janie to be legally sold as such.

https://youtu.be/6Pe1udi0i_4

mark
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Re: PRS HDRX amp

Post by mark » Sun Oct 23, 2022 3:55 am

Dave thanks for giving me your time explaining the theory behind your amp mods. I will have to reread what you said and think about it. The FET driver is suppose to be a good thing. I haven’t experimented with it yet but it gets a decent rap.

http://www.geofex.com/Article_Folders/m ... tfolly.htm

I agree Randall Aiken is a very smart guy.

YLADRD61 thanks for video clip, I had seen that clip previously. It seems like PRS even copied the excessive B+ voltage the older amps had.

Regards

Mark

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Re: PRS HDRX amp

Post by daveweyer » Sun Oct 23, 2022 1:23 pm

There is one other entity legally entitled to use Jimi's name and likeness, and that is the Jimi Hendrix Foundation Of America. Janie fought this in court, but when the judge advised she was likely heading for losing control of Jimi's estate altogether, her lawyers persuaded her to immediately drop the suit. The ruling is quite clear, that the Foundation Al Hendrix set up to realize Jimi's dream of using his power and legacy to help disadvantaged children, is legal, and is also authorized to use Jimi's name and likeness to achieve its goals. By the time the lawsuit was over, the Foundation was pretty well depleted of funds, energy, and vitality, and apparently Leon's chicanery while on the board did not help. Maybe Janie accomplished her goal anyway, which was to destroy any competition in the Jimi Hendrix market. I guess only time will tell on this account.

Some speculate that Janie found out the Foundation was exploring releasing an actual Jimi amp as designed by West Coast Organ and Amp, and engaged PRS with the only thing she had which still had a remnant of a West Coast mod in it. I am not privy to the inner details of course, but I continue to wonder why they did not call me for getting a reference, at the very least.

In the mean time I built Mason's Marshall amp to the West Coast specs which were being implemented at the time of the last upgrades on Jimi's equipment. From there on out it was experimental stuff not yet used by Jimi on the road, but possibly in the studio; I rely on Jimi "golden ears" and Jimi gear afficionados to help me identify which gear this might be. You all have been very helpful in this regard.

mark
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Re: PRS HDRX amp

Post by mark » Sat Nov 05, 2022 3:02 am

Soon Julijan Eric will have one, and Xplorer too, because I sent them kits with all the right parts.
I look forward to hearing Mason’s amp as well as the amps from the guys you sent kits to.

Is this something you do commercially or something for mates?

The other question is what’s in the kits? 🤭

Thanks for your time.

Regards

Mark

daveweyer
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Re: PRS HDRX amp

Post by daveweyer » Sat Nov 05, 2022 5:29 pm

The kits I provided were basically promotional, but they were part of some larger thinking on the particular subject of whether or not there is a genuine interest in the guitar world of today toward Jimi's actual historical sound and relationship with West Coast Organ and Amp. Would there be enough interest to make it worthwhile to provide these kits to the general public? Would these kits compete with George and his Marshall reproductions or could they work together? Lots of questions to answer before making more than exploratory moves.

The kits had the transformers, the electrolytic capacitors, some resistors and special signal caps, and the zener regulator for the screen grids. All these were the components for West Coast upgrades back in the day (the zener was special). The first step was to get these kits or amps in the hands of devoted Jimi players so they could judge for themselves whether they were, or were not, getting the sound and feel they heard in their heads when they listened to Jimi play live.

So far so good it appears.

mark
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Re: PRS HDRX amp

Post by mark » Sat Nov 05, 2022 7:27 pm

Good to hear that results have been good so far, that’s always encouraging. Looking forward to hearing the amps.

The only question that I haven’t asked us did Hendrix have speaker preference?

That’s a big part of it too and there’s no one following the post with a vested interest so far.

daveweyer
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Re: PRS HDRX amp

Post by daveweyer » Sun Nov 06, 2022 9:56 pm

There have been a million discussions about Jimi's speakers, and lots of trials by Jimi fans of various combinations in their own cabinets.
I have repeated this story ad-nauseum, but when Jimi had his gear prepared for the spring tour of 1969, Bob Hovland and I got 24 Celestion clones from Thomas organ company, and the West Coast Organ and Amp crew replaced the units in six of the Marshall cabinets that had been just shipped over from Britain. Jimi loved the sound of those speakers, They were alnico with small voice coils, and they compressed nicely at a bit lower power while having a more bell-like high frequency response.
Otherwise Jimi was known to like the Celestions with the heavier cones.
I have tried to clone one of the original Thomas speakers, but with not much luck so far; all the American manufacturers from the day have morphed into other companies, and everyone has had to get various parts from China, The last couple of years have been tough--supply chains and all that.

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