Building my 2-EL84 Rocket Reverb tribute, and want to use VVR on the power tubes and the phase inverter only if possible, but may have a problem.
The reverb driver is supplied at (B+)2b in the attached schematic, so applying voltage control to the power tubes and PI will also affect this circuit. My instinct is to leave this node out of the VVR circuit so the voltages stay where they need to be to operate properly, but I'm not sure how I would do this and still control voltages at (B+)3b (PI). Is that even possible?
Should I include the reverb driver in the VVR circuit and let it be scaled? What would be the impact on the reverb?
Or, am I better off scaling the entire amp?
Thank you,
Greg T.
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Hi Jelle,
I have read the manual and have an understanding of the concepts and typical applications, but the manual and posts on this forum I've been able to dig up don't cover the effect of scaling the reverb circuit, particularly when it is supplied so early in the power rail. If it were further down where the preamp tubes are supplied, it would be a no-brainer for me, but right now, with this circuit, the reverb circuit is being supplied between the power tubes and the PI.
In an 18 watt clone I had with London Power Scaling, bringing down the voltage starved the trem circuit until it didn't work; I don't want to do the same thing with my reverb circuit.
Cheers,
Greg T.
Well, supply the reverb with the highest available non scaled source. Maybe do a parallel node to the one for the cathode follower stage. Have fun. Jelle
Yeah, tricky. But if you're scaling just the PI/PA you'll most likely need an extra stage of filtering at the start of the preamp supply anyway. I'd integrate the reverb supply into the non-VVR'd chain and adjust the power chain resistors to get the right voltage. You'd normally have to squeeze in another filter cap for PI/PA VVR anyway, but here you might need two to get the reverb supply voltage up where it should be but also adequately filtered so you don't get a heap of hum.
Interesting, Katopan. I've seen back and forth discussion about the need for added filtering, but my impression was the consensus was that it would not be needed. What values would you suggest?
I understand the power chain resistors will need to be adjusted, at least at R29, and possibly at R31, correct?
I've attached two schematic sections showing 1: VVR on screens and PI, and 2: VVR on PA and PI. I suppose I could also scale the entire amp...
There are a number of pros and cons to each approach, and I'm like a deer in the headlights now - don't know which way to go, because I'm not sure of the implications of each.
Thank you!
Greg T.
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If you scale the power screens without the plate, all the pentode curves squash down compared to your load line in a way that completely moves your operating point. So definitely do plate and screens, which keeps close to the same operating point as you dial it down.
When we were first trying out PI/PA VVR at 18Watt, with the preamp supply filter connected to the first filter cap (before VVR) it meant for those amps that one filter stage was being bypassed. Tried it and the extra hum was quite noticable. So in went the additional filter stage and it was back to normal very low levels.
Also, yes you will need to increase the power chain resistance to drop the reverb driver and preamp voltages back down to where they should be. In fact your PI power resistor also needs to be increased a little to compensate for the reverb driver and preamp current no longer going through it. You need to work out the current draw from each power node and recalculate resistor values needed to put the voltages back where they belong. Also to minimise extra filtering stages you could always have the extra stage, then the reverb recovery filter stage, then feed the preamp stage off that because it's a lower voltage.
The difference in VVR set up is only a few wires so I encourage you to experiment for yourself. Make the mod without the extra filtering stages and see how much hum it adds. Then you'll know for yourself that you need the extra stage.
I know everybody must think I nuts,...but!!! I got a VERY GOOD result using a voltage control to the output tube heaters. Absolutely nothing changes from the input thru the output,..only less electrons are boiled off in the output tubes giving less volume as you decrease the pot. Like the normal use of the VVR, it takes a few seconds to fully change levels.
And, dead simple to implement. Supply a 6VDC voltage to one side of the heaters, the other sides to ground.
I'm a a newbie that just finished my first Rocket and installed a VVR. I started out wanting to just control the plate and screen voltages and installed a PPIMV to control the input to the power section. It didn't sound very good. As soon as the output section starts to suck less power the preamp voltages soar like 40v. The preamp just sounded unpleasant no matter how it was adjusted. Every tweak to the VVR required a full adjustment of about everything.
BTW, I put 47uf on the output of the VVR, quiet and tight sounding.
I switched over to VVR-ing the whole amp and I believe it sounds much more usable although not perfect. You have to deal with or live with the scratchy volume thing and it thins out the sound some. You can reasonably compensate with judicious control tweaking. I kept the PPIMV (at least so far) and I can get some interesting sounds mixing the two...but it's still not perfect. I think I'll try controlling the heater voltage as suggested by ampdoc1, what an interesting approach.
While you can get some nice overdriven and over the top distortion sounds with the VVR and the volume cranked you can never get back to the pristine cleans like when the volume is in 3:00 to 11:00 position. I've bought a boatload of pedals over the years and they just don't sound as good as a cranked amp. I think the best solution is going to be two amps and one of those Weber amp switching thingees...time to build an Express.
Funny, you mention using a large rheostat. That's exactly what I what I used (3ohm, 25W) in 2005-06, with a Trainwredk Express, when I first used this idea. It was kludgy as hell, but it sonically, it worked perfectly.
The VVR is a much better solution. Doesn't need a complicated PS. I use the PS section from a Peavey Classic 20, although I use a beefier FWB rectifier because I also use the 6VDC (not thru the VVR) for all my other heaters. Caps and resistor values remain per the Classis.
If you do try this, please report your experience. With the first amp I built, I player the guitar , then turned it down, etc, etc, measuring the output voltage of the amp, until the speaker didn't respond well. My measurements at that point were well less than 1V AC signal output into the speaker.
