JCM800 Pre-amp current?

[Jaicen_solo]

I'm planning to build this:

http://www.pisotones.com/GORG/Pablocarrillo/pre_JCM800_por_gak_2.pdf

I went through a few options like cloning a TSL preamp, till I found this project that seems to be all there and easily built.
I was wondering though how much current the the transformers will need to be capable of producing??

If anybody has built this, please do let me know what you think.

[Seljer]

Typically, around 1mA or 2mA per individual triode is needed for the HV section, so not a lot

The rated heater current is 150mA @ 12V for a single 12AX7

[Bunyaman]

I had build this preamp. It`s buzzing and I still can`t  liquidate it. For all I know, it`s a problem of this PCB. I`m not the only! But I still hope that I will liquidate this buzzing in the bearest future.

[defaced]

The first transformer is going to need to be able to supply the heaters and the current needed to drive the HV transformer.  Figuring 150ma the heater in each tube gets you to 300ma out of the 12v side transformer.  The current needed to drive the HV transformer is kinda sticky because you have to take what the HV circuit requires then back calc the LV side requirements. 

Figure each tube needs 2 ma (a little high, but conservative is better than marginal here), and that the transformer is 80% efficient. 

0.008 A * 220v = 12v * x * 0.8

X = 0.18 amps, or about 180 ma is needed on the 12v side of the HV transformer to get 8ma out of the high voltage side.  So the first transformer needs to be good for ~500 ma bare minimum.  I'd probably be looking for a 12v transformer that is good to 750 ma to 1 amp for first one, a slightly smaller transformer good to about 300 ma for the second one. 

[electrosonic]

The layout looks tidy on the PCB, but having all the filter caps grounded in between the input ground and output ground probably causes some hum.

It would be safer and simpler to buy a 12VAC wall wart and then have a single 12AC transformer in the unit to make the plate voltage.

Andrew.

[Jaicen_solo]

That's a really good idea, I think I'll do that. I'm sure I've got a 12vAC trannie lying around but shouldn't be a problem to get one.

With regards to buzz, I'm sure I'll be able to track it down if it is a problem.

Thanks for the input guys!

[electrosonic]

The is a diagram at the geofx site showing the proper grounding layout for a tube amp. The grounding scheme could easily be applied to this  Kind of overkill for just a preamp ( ie no power tubes) , but it couldn't hurt.  I did a quick search of the geofx site but I couldnt find the page. I will look again when I get a moment.

Andrew

[electrosonic]

The page is titled "Star grounding in tube amplifiers" at geofx.


Andrew

[PRR]

The heater draw is 0.3A _DC_. The AC-DC conversion has losses, 1.7 or so in books, 2 in practice. So T1 is at least 0.6A rating.

The plate load is 300V (raw) at 3mA. (Worked backward from tube and resistor values.) Again we double the DC to find AC amps, 6mA on the 220V side. Mirrored back to the 12V side, 110mA or 0.11A.

Total load appears to need 0.71A of AC current.

However T2 is a second conversion, small PTs are lossy. I'd aim 1.5X high for less sag.

T1 - line:12V 1A
T2 - 220V:12V 0.2A (used backward)

T1 = 12VA
T2 = 2.4VA

Agree that this PCB plan's grounding mixes heavy rectifier buzz-current with audio. If space were available, I'd throw ALL the power supply stuff on a separate board. (Not hard to do it on perf and just bend leads together.)

Alternatively: do NOT use the top-left green pad for any audio. Go over to Bass, go down to the medium green trace, cut it! Do not jump Volume to green as shown, jump the Vol pot green to Mid green to Gain green. Use the far-right green for ALL audio grounds. If you use insulated jacks, then bond the Case to the big bottom green trace. That still leaves V1 heater return contaminating audio ground. Being regulated, it "may" not matter. But you probably should cut the green trace into V1, jump from that socket solder-blob back to somewhere on the green near the 4,700uFd.

An additional thing to check: the 12V regulator must have a solid 15V or it will ripple-dip and trash-up ground any anything near it. Measure across the 4,700u cap. If that's not well over 15V (even with your big amp, washer, dryer, PA system loading the line), then T1 needs to be bigger, Either another Amp of current or a couple Volts more (but 17VAC is hard to find and 18V will need a large heatsink on the regulator).

[Jaicen_solo]

Actually, I was planning to lift all the AC off the board and perf it up in a seperately shielded box in a box affair. The reason is that at some point, it will be transplanted into my Valve Jnr copy, with a couple of DPDT relays for channel switching, at which point I will swap the power supply over to the internal supply in the Valve Jnr.

If I rectify the 12v AC output that should give me ~16v DC to feed to the rectifier?

I can easily find 1A and 500mA 12v transformers so that shouldn't be a problem.