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Post subject: Improving sustain with upgraded decoupling circuits
Posted: Sun Jul 09, 2017 2:29 pm
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OK, so after hearing the great sustain produced by the Trainwreck Climax, I realize that the large number (5 ) of preamp filter caps is not so much for hum reduction as decoupling each stage from the power supply.
So I did more digging into decoupling techniques and may have a way to improve sustain by improving decoupling.
Decoupling is the method of isolating the ac signal of one amp stage from getting into the dc power supply and talking to other stages. It reduces low level sensitivity.
The inductor in the power supply is a decoupling device to separate the power stage from the rest of the amp stages. It removes the high frequency current ripple from the power supply and sends smooth dc to the preamps.
The remainder of the filter caps are used as decoupling devices for subsequent preamp stages. There is little 60 cycle hum to filter out, so their primary function is to filter out the ripple caused by the ac signal going through the tube.
In an amp like the HRDX, there are four plates connected to one 20uf filter cap. On some BF/SF amps there were 5 plates to one cap. After hearing the Climax, and seeing what's in it, I can't help but think that there is room for improvement. Leo may not have had the bucks to put them in, but when tone trumps cost, it may be worth it.

The two things you need for good decoupling is a low impedance to ground at the relevant frequencies and close proximity to the device to be isolated.
While the big filter caps are great for 60 or 120 hz rectifier circuits, they aren't so great at filtering higher frequencies. At high signal levels, the interconnected plates start to interact in two ways.
Out-of-phase ripple from a high gain circuit can affect the performance of a low gain partner, reducing it's sensitivity .
In-phase ripple reduces the sensitivity of both signals by reducing the available current to the plate.
The value of the cap to ground determines its decoupling frequency. Using two caps that are half the value of the original raises its decoupling frequency. Using a large cap and a small cap in parallel sums their decoupling effect.

So it would seem that using overvalue caps in the preamp stages would be counterproductive. There is little hum and poorer high frequency decoupling. I guess that's why Leo used the smaller values in the preamps.

The next virtue of good decoupling is proximity to the device. The Fender blast can is obviously not in close proximity to the plate resistors. The reason for the design is safety. When capacitors were made with paper, the possibility of a fire was very real. I first realized this when I was repairing an old Film-O-Sound amp. The electrolytics were in a metal box, soldered shut. If there's anything worse than shouting , Fire, in a theater, it would be the projector starting one. So in order to get the commercial safety certification, caps had to be in flameproof cans or covered. This is the Fender blast shield.
But in the 1970s, the paper was mostly replaced by plastic. The UL810 standards specified flame retardant materials. Capacitors no longer caught fire. The Fender blast shield is obsolete. When I do my next cap jobs, I'm going to try mounting them internally with a .1/600v cap in parallel.
I think I will also try separating some of these interconnected plates with 10K resistors and smaller value caps for each.
While the old fenders share a lot of plates,, they are between channels and not usually interfering , but it will be a simple job to hook the ground of the caps to the brass strip and the positives to the plates.
Now it would be simpler to connect to the paired 100k resistors, but separating them would be better. 4 filter caps with resistors would be ideal, but in practical use both channels are seldom used together. So a cap for V1 and V2 pin 1 plates and a cap for V1 &V2 pin 6 plates would get you the same bang for your buck and still isolate the first 2 stages.
The HRDX and that series all use 4 plates when the drive channel is used. It is an amp known for little natural sustain and could probably benefit the most from upgrading the decoupling circuits. 10k and 10uf X 4 should do it. I think pulling the filter cap to the far right and replacing it with a pack of smaller caps taped together would connect into one pulled leg of the plate resistors .

I'm kinda at a disadvantage here, because while I can figure this theory out, I haven't figured out how to measure it. Plainly this needs a musical ear to appreciate relative sustain. Not being a musician, it wouldn't be me.
So I'm gonna invite those that won't shock yerselves tryin , go for it. Let us know how it works out.


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Post subject: Re: Improving sustain with upgraded decoupling circuits
Posted: Mon Jul 10, 2017 3:17 am
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Very interesting !


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Post subject: Re: Improving sustain with upgraded decoupling circuits
Posted: Mon Jul 10, 2017 8:05 am
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The Valve Wizard has a nice write-up on PSU designs, on his website.

http://www.valvewizard.co.uk/smoothing.html

Kevin O'Connor has a nice discussion on PSU de-coupling, in his The Ultimate Tone Vol 1 book. Chapter 2. Esp, motorboat de-coupling.

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Post subject: Re: Improving sustain with upgraded decoupling circuits
Posted: Mon Jul 10, 2017 10:11 pm
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Yes, I looked at some of the Valve Wizard content and found it useful.
I got started on this because of a HRDV. I put power tubes in it 18 months ago. When I saw it again last week, I found 3 blown 22/500 electrolytics. Now usually, I would call it cheap caps and just replace them without a second thought, but a bell ringing in my head reminded me that there is a 100% chance of any one part failing, but the odds of simultaneous, identical failures goes up astronomically. There was something that killed these caps. The amp isn't old, but the customer plays loud and often. Since there was little hum in the circuits of the failed caps, I looked elsewhere and started researching this decoupling concept. The caps don't have a lot of 60 cycle to deal with, so the stress must be coming from the higher frequency ac signal. So I put .1/600 poly caps in parallel with the new electrolytics . The customer thought it had more sustain, but there was no baseline to compare to.
The research into UL and the flammability problem was the surprise to me. Fender could modernize the reissues by loosing the blast shield and achieve better sustain by mounting the caps locally. Or would that be heresy?


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