MLA

^ What he said. Live transistor tab and risk of considerable voltage on heat sink, so for testing only and to be treated as an open lid amplifier. The original question for this testing was whether Joachim's latest current source can match the EL34 in performance sonically. After quite a few hours of listening, my conclusion would be yes; a Megatron modified with this solid state CCS is a really great amp (just as the regular Megatron is, btw). Designing a pcb for it where the CCSs can be mounted inside the chassi on proper heat sinks would be a great next step. Let's see if we can convince Joachim to take this on... .

These are really nice; drop in replacement for the EL34s in the current source positions. Just plug it into the tube socket and then spend some time comparing tube to solid state CCS sound. Joachim was kind enough to provide four of these for a Megatron we're experimenting with in Gothenburg. The current source itself is really nice and in comparison to the EL34's, might have an edge when it comes to transient reproduction (completely subjective opinion of course).

all good, but output capasitance 60pF vs 11pF Noted that and am wondering for this application; how much does it matter when used as a current source? I don't know enough of the theory to figure that one out. Help... ?

So, the golden rule for Kevin Gilmore boards seems to be to always follow the board. Schematics are sometimes earlier drafts rather than the finished design, which can be confusing. I have build both the 1.8 GRHVs and the .6 Carbon boards which you have with success. If you go for +/-400V rather than 450V, SimmConn has a nice post on how to push the pzta42 back into a more linear region. The BOM excels and links to Mouser carts posted by mwl168 in the first post of this thread both open and download without issues for me. Should be a good place to start.

GRHV does not fail gracefully... most or all of the active parts might need replacing. Sören B posted a good guide for trouble shooting somewhere.

Next step is usually swapping the tubes between boards to see if it's the boards or the tubes. In the process, cleaning the tube pins might be a good idea; if some have oxidized and have poor contact, hum may occur.

Indeed. Damn, I actually thought I had figured something out

Really nice! From picture, it looks like Ecc99s or similar replacing the ksa1156s?

Working on a BH at the moment, and given what you found regarding Cob for 2690; do you think it would be worthwhile to use TTC004b instead, or is the difference too small? I guess you may have checked it but discarded it as an option?

Very good suggestion; pentaconn is a good idea but the shorting risk sucks.

Not my field of expertise, but I would i imagine the start sequence is designed to start from full power, so if it was me I'd rather just flick the switch and see what happens as I would be worried that gradually ramping up voltage might leave parameters out of spec at the wrong time... But that's mostly a guess :).

That's the LT1085 regulator, right? Formula for setting voltage is on page 11 or 12 in the data sheet if I remember correctly.

Another option is https://www.mueller-rondo.com/eng/ in Germany. They build exactly to spec, sometimes cheaper than Toroidy, and turnaround has been about a month on previous orders.

For what it's worth, my CFA3 bords fell 2,5V short in terms of offset when I built them. In my case, swapping all the red LEDs to a version with much less forward current than the ones I installed first (I'm now using LTL-4221, which has 2mA I believe) put all numbers back in the right places.

For me they ship to Sweden without problem. Don't know what you need to do to make international delivery happen though...

Interesting. I believe I followed the board and came up with about 60mV offset w/o servo.

Toroidy will do whatever you need in my experience (including dual primary); anything T2 will be custom spec anyway. Mueller-Rondo lets you specify the exact diameter when you place an order, so that's quite helpful. The larger Mueller-Rondo (think 300VA) have a bit more mechnical hum than the corresponding Toroidy's (have built T2 psus with trafos from both). Not a dealbreaker if you're not sitting absolutely next to the psu box. Also, if you go with 3 trafos (I think that's what's in Kevin's original chassis?) I think if you spread the load none of them needs to be more than 150VA each, which should sort that problem.

Good riddance

10m90s in stock at Mouser; currently 2237 of them. Go get 'em while they last.

They are probably genuine if it was long ago, but may not necessarily have been new/unused. Anyway, replacing those and seeing if you get a clean signal could be an easy first check. Where do you connect to ground for the SE output? Needs to be from signal ground if I remember correctly; Kevin had some very clear instructions on that early in the thread.

Since you're using a GRLV, dirty power would be unlikely I think. I would replace those transistors for starters. Mpsa06/56 are in stock at least at Mouser.

True. Will keep supply voltage low and bias reasonable.

So, if all goes well, I'm picking up a pair of Quad 2805s next weekend, which means I have to build an appropriate amp for them The Uberamp I have boards for and that's probably what I'll build, but I'm wondering if a cfp3Large (with the Sanken output boards) would be a good fit also? Or is that a less good match?

If I read simmconn's post right, simmconn measured G to B- (over the 20K resistor) for those 20+ volts, not Vgs directly. So your 21.8V is that measurement and thus indicates all is well