Built another one -- 5E3

[Phil_S]

Had some fun using up stuff on the bone pile. This one is a mix of the old and the new. The derelict chassis is from an Arkay FL-30, whose iron now lives in a JTM45. This is my first PCB build, but I think it's my 12th amp. The PCB is here: http://www.tubedepot.com/pcb-5e3-err.html

I haven't got it off the bench, but it passed a good signal tonight to the bench speaker and passed the thumb test with an instrument cable.

For those who want to look: http://home.comcast.net/~psymonds/5e3err.html

Regards,
Phil

[Andy Le Blanc]

That's a cool way to do it, what does the chassis fit into?

I've been thinking off a suitcased sized thing with two simply bent
bits of sheet metal, it has to safely and reliably hold the components and dress.
but the layout will be more radio based than anything else.

That stacking allow a compact exterior, it looks like the inside of my old HP signal generator.

[Phil_S]

It came with an ordinary hi fi type metal cabinet. I'm afraid to use it because of the way I have the PCB in there and because it is metal. Also, it hardly seems worth the effort to modify the front panel on it -- extra holes and relocations, etc. I am going to build a pine box with dovetails for it. I'll put a metal or grill cloth screen on the rear. I'll probably get some very thin Balsa for a front panel. It will look nicer than it sounds.

I think I mis-spec'd the PT for this one, though it is hard to say that with a straight face when you consider the PT came off the bone pile. Voltage all around is on the low side. I replaced the dropping resistor between B+2 and B+3 with a 5K. Plate voltage on the finals is ~336, screens ~295. Plate voltage on V1 is 136 and 139. The gain stage on V2 is at 199. The concertina is 227 on the plate and 49 on the cathode. All around OK, but low headroom in the pre-amp.

I gave it a test drive early to day and it is not anywhere near 22W loud. I wonder what's holding it back? Static plate dissipation calculates to about 75%, which is way cold. I'll have to think on this. I've got some 6Π3C that I might put in there as soon as I verify the pin out is the same.

[Andy Le Blanc]

It could be the PT, play it a while and then check for heat.
It might not be rated for enough ma. for the tube type.
I've had that one happen, no fun..... check the V for other tube types,
with similar loading, you might be able to run other 0ctal based pentodes
that won't demand the same current, 6v6, 6f6, etc..., if that's the issue
it be better than scraping or investing in another PT

[Phil_S]

The Russian 6P3S (5881/6L6 type) draws 900mA heater each. I put them in. The PT didn't even break a sweat -- cold to the touch after about 5 minues. Two of those and two 12A?7 tubes are still only 2.4A. This PT is pretty beefy looking. The 6P3S, did cause a significant drop in plate voltage all the way down the line, so I'm thinking they draw lots of plate current. I'm thinking volts are down, amps are up, watts are about the same. I'm not going to leave them in.

In either case, this amp seems like a 1W loud amp, not a 20W loud amp. Tone's "not bad" but volume isn't appropriate. I've been over this and verified the build several times. No saying if I missed an error, but I tried.

Any ideas what might be bogging down this amp?

[jhaas]

Hey Phil, is that one of those "PCB's with Errors" that tube depot is selling? I'm curious what the "Errors" are. Obviously it doesn't appear to be too much to work around.

[FYL]

Any ideas what might be bogging down this amp?

Voltages are on the low side but acceptable.

You're using a bugged circuit board with some silly stuff added, such as the 1M/1K NFB section. I see that you've fitted R14 (which should be R24) but not VR4 - you've used an external pot. You could have an NFB loop with only R14 - a 1K resistor ! - thus mucho feedback and very little gain.

Check your wiring...

[FYL]

I'm curious what the "Errors" are. Obviously it doesn't appear to be too much to work around.

http://site.tubedepot.com/pdf/pcb-5e3-err_v2.pdf

[Phil_S]

FYL:
Good call on the NFB. I maxed the pot (meters at 1.2M) and now, at least, there is a little bark in the amp. I don't see a reason to keep it. I pulled the Fender schematic and it has none. Clip clip, it's out of there.

Naturally, I' a little disappointed about the PT. Unloaded it is 327-0-327. I suppose I was hoping against the odds. I should know better. Actually, if this is a 320-0-320, as I suspect, then rectified voltage on a 5Y3GT is right about where it should be.

I have two choices in stock. I can try a 5U4GB and see if the 5V supply balks at giving 3A. I can pull the socket (why waste a good socket) hang a terminal strip, and build a FWB or simple FW with diodes. The diodes will put the voltage up there where this amp wants to see it. Then, I'll probably have to return the last dropping resistor to 22K (presently 5K) to keep the lid on the preamp tubes.

I have no real way of knowing if the 5V winding will support 3A. I suppose I can give it a watchful try, but I'm the cautious type when it comes to this sort of thing. I'm rather ambivalent for all the right reasons. What would you do?

Phil

[Phil_S]

Heck, not much to lose here. The 5U4 is too tall to fit. So I put in the diode FW. Hopefully, tomorrow I can check the results. I'm semi convinced that low voltage is part of the problem and those diodes should jack up voltage quite a bit, maybe more than I'd like. Thanks.
Phil

[Phil_S]

Got it running much better now. I put in a solid state rectifier. B+ nodes at 405-355-327. Bias calcs to 15.5W static plate - hot hot hot. Plate voltage in the preamps, V1 165/162, V2 235. V2 concertina 269(a)/59(k). Volume seems to be about where it should be. I think this one is about as done as it will get except for the hum.

Hum is not bad. Filaments have a CT that is grounded. Not sure if there is anything more I can do here. I might move the 220K grid leaks for the power tubes off the first ground plane, which seems all wrong to me. I'll have to float one end, insulate, and run a wire.

Thanks for everyone's help.

Phil

[Andy Le Blanc]

If its cathode bias put the fil. ct. to the cathode, if it still hums lift it entirely
and reference the cathode thru a hum pot..