Crackling and popping from my Rev c

[Shannon Parks]

Since you've already tried removing all negative feedback, I'd start there again. Then lift one leg of both R38 & R39, thus disconnecting the feedback to V1 from V2. I can't quote chapter and verse from RDH4, but for those that don't know, that feedback to the cathode of V1 achieves similar results as a bypassed cathode would - namely increased gain and lower output impedance. BTW, Earle, these are also from the same spots you were probing.

If this doesn't help, I would then add a ~300pF, 1kV cap from each of the EL84 plates to ground. Though since you've already stated you've disconnected all your feedback, I doubt this will help in your case.

Finally, do you have all the grounding attached directly to the PCB? An external star ground could be causing some strange return issues. Disconnect any external chassis grounds in your tripler circuit and have them directly attached to the PCB.

Shannon

[Guest]

I've tried your suggestions Shannon, all to no avail. I had the earth to the PCB anyway, but did notice the earth to the chassis was relying on the PCB mounts only. I ran a wire to a solder tab; this "slightly" fixed the problem. Might be some mysterious earth bug somewhere hmmmmm. I'm about to head back to the shed and stare at it for a while, might even brake out the heavy artillery and fire up the oscilloscope (Tektronix dual trace valve type of course: wink: ).
If the answer doesn't jump up at me. I'll do some output trannie impedence testing at different frequencies? I think I read somewhere that you should rate the impedence at 20hz, I think I measured these at 1khz. Maybe the problem is there.
I'll keep you posted
Thanks.
Earle.

[erichayes]

Hi All,

Earle, when I read that you were using a voltage tripler, my neck hairs went perpendicular. A tripler supply has a very high internal impedance, which translates into instability; the supply is relying on filter capacitance, rather than power transformer capability, to develop the necessary high voltage and current the output circuit wants to see. What can happen--does happen--is, on current peaks, the filter caps get sucked dry momentarily. The silicon rectifiers respond almost instantaneously, but can't charge the caps up to their limits because there's not enough horsepower in the PT to get the job done. Once this instability is created, it morphs into a low frequency oscillation of the B+ voltage which causes motorboating.

I believe that you're duplicating the problem by touching the cathodes of the phase splitter with your DMM, which causes a similar momentary imbalance in output tube conduction.

I mentioned to Shannon in a recent email that I regard an amplifier as a modulated power supply. If the power supply is lacking in any regard, there is no amount of electronic voodoo that can be performed to make a good amplifier.

Scout around for another PT that will provide, at the least, 300V@120mA for FWB, or 600VCT for standard full wave rectification. The more current you can throw at the rectifiers, the better. And keep us posted!

[ehtcom.]

Hi Eric.
Thanks for that info. I'm pretty sure you're right on the mark. The sound it makes fits you're fault description. I've got another trannie I was saving for a 6v6 project I'll drop it in to test. It has a 300-0-300volt HT (also has a 5v tap. hmmm 5u4) :wink:
I'll let you know how it goes.
Thanks.
Earle.

[EWBrown]

I, too, would concur with the voltage tripler being the culprit. Full wave center tap, or full wave bridge are best, then voltage doubling is next best, anything else is asking for problems when there is some significant current consumption.

The rev C board is already set up for a center tapped HT transformer winding, 300-0-300 to 330-0-330 is the design range, I've used the Hammond 272FX and 272HX (300-0-300 at 150 mA and 200 mA respectively, and the original or replacement PA-774 (330-0-330 @ 180 mA) power transformers.
These are 115-120 VAC primary, so would not be suitable for operation in Australia or Europe, unless an external step down transformer is used.
Hammond does make "universal" power transformers (370 series) that will work on 220-240 as well as 120 VAC, but these are fairly expensive.

If you choose a transformer with a 300 to 330 VAC HT winding, then a full wave bridge of four rectifiers can be used, and the + should be connected to the location on the board where the two uF4007 cathodes are connected together, the FWB will have to be mounted off board in this case.

My next one, in process will be a rev B using a 325-0-325 power trannie,
a 5AR4/GZ34 rectifier and 5H choke, connections will be similar to those shown for using the Heath AA-151 transformers.

/ed Brown

[Thermion]

Earle,

I realize this is a little off the post subject train, but you hinted at it. I used a Hammond 272JX PT in my Rev C project, (300-0-300) and used tube rectification in a Dyna style pi configuration. I tried 5ar4s, 5U4s, and CV 378s. With this PT, the 5AR4 gives me a B+ of about 380 v. Using the CV 378 drops the B+ to about 345 v. And the 5U4 was about 330v. If you plan to use a 5u4, I would get a PT on the order of 330-0-330 to get the b+ up there.

JT

[Guest]

Hi JT.
Thanks for the info.
I've refitted the original PT and used a voltage doubler circuit. I found a secondary tap on the original of around 30Vac, and shunted this with the original 100 to give ~130Vac. This doubled, rectified and filtered gives me about 320 Vdc (~300Vdc at the plates). I have bypassed R33 to bump up the voltage for V1 and V2 and have set the el84's bias at 290mv . Initial testing is very good. Not scratching or popping, even when driven flat out.
I'll bench test it for a while to see how it goes.
Thanks.
Earle.