First chassis - a photo journal

[dhuebert]

I'm glad to hear it worked! Your lead dress actually looks not too bad. I think I see bare wires though.. But a fine job on the metalwork.

What tubes did you use? Any trouble biasing them?

The 2x4s remind me of a certain Hammertone Viper.

Don

[Ty_Bower]

Thanks, Don. I guess the wiring turned out OK. It doesn't hum (at least not through the speakers) and that's good enough for me. I've tucked and taped the bare wires you see since that photo was taken. Maybe I'll try to get a "finished" bottom shot.

The tubes are some inexpensive Russian 6P14P tubes I picked up off eBay several years ago. I think I got ten of them for $20. What I assume to be date codes suggest the tubes were manufactured around 1981~1982. They are not in any way shape or form matched. I've had problems getting them to work in my other two EL84 amps because any given pair rarely pulls equal bias current, and my other amps cannot individually bias the outputs. I've made pairs where one tube pulls 60ma and the other only 30ma.

The 'X7 has a Realistic label on it. I'm not sure who made it. The 'U7 is a JJ. They both should be practically brand new.

I must confess I have not actually checked the bias. At $2 a tube, I won't cry if I cook them. I haven't seen any visible signs of distress (cherry red plates, etc.) so I'm not too worried. I turned all the bias pots to 430 ohms before I installed the tubes, thinking they would be "in the ballpark".

I'm glad you like my 2x4 box. :) Seeing as how the project has turned out to be a success, I might be motivated to build something a little prettier. Heck, I might even stain and poly the wood. The one-plate design makes it easy to replace the box without touching any of the wiring.

Thanks for the complement on my metalwork. I was really starting to get into it near the end. Maybe I'll have to build another one soon... :)

[mesherm]

Just a thought to keep in mind, one of the main reasons to bias the tubes on a push pull amp is that since the current through each tube in the pair flows in opposite directions through the output transformer, the magnetic fields in the tranny core cancel each other out. Using your example, if one tube ran at 30 ma and the other at 60 ma then the core would magnitized in the polarity of the tube running at 60 ma. Then during normal operation the core could magnetically saturate on music peaks on the 60 ma side. This would be like a soft clipping effect. Unlike single ended output transformers, push pull trannies do not have a gap to prevent magnetic saturation.
Some PP tranny manufacturers will actually specify how much imbalance the trannies can take before this happens.
I have worked on industrial oven control systems where they actually take advantage of the magnetic saturation effect by controlling the heater current with a large "saturable reactor" which is nothing more that a big transformer with an adjustable air gap in the "I" part of the EI tranny core. A fluid filled bulb inside the oven expands and contracts with temperature pushing a small piston connected to the air gapped part. As the temp increases the fluid expands and closes the air gap which causes the transformer to saturate. Once saturation occurs the AC current through the secondary can't increase and the heaters start to cool.

[Ty_Bower]

I must confess I have not actually checked the bias.

OK, I've checked the bias. B+ is running right around 390 volts.
TP1 = 314 mV
TP2 = 310 mV
TP3 = 356 mV
TP4 = 341 mV

TP1 and TP2 look perfect to me (the right side outputs), considering the B+ is just a bit high. I'd like TP3 and TP4 a little lower. If I leave it where it is now, that one tube at TP4 is barely under 14 watts.

Now to find my plastic tweaker...

edit: I've got TP1 & TP2 both set at ~311 mV. I can't seem to get TP3 any lower than ~341 mV, so I set it there and left TP4 where it was. I guess I either need to swap in a different pair of tubes for that side, or think about changing out the fixed resistors on the board.

[mesherm]

As long as the push pull pair is matched you should be ok. A difference between channels of 3 ma probably isn't worth changing resistors on the board. I have had some of those 6P14Ps accidentaly go dull red for a few minutes with no apparent damage so I doubt 35ma will hurt them.

[EWBrown]

Those radio shack / realistic tubes (like the AX7) can be from almost any country, including the land of the telefunken, west germany... Look on the bottom to see if there is a "diamond" shape embossed in the glass, with a number inside. If that is present, it's a tele!.

In the 50s and 60s, when the US economy was strong, the tubes from Europe (Telefunken and Mullard, for example) were often used because they were considered as being the low cost alternatives to relatively expensive domestic-made RCAs and GEs. The Asian and Soviet tube manufacturers weren't a factor back then.

I wonder if RS still honors their "lifetime" guarantee...

/ed B in NH

[Ty_Bower]

Here's some more pics of the amp in operation. I've got it on its side so I can check the bias voltages. I've gone through my (small) pile of 6pi14pi tubes, trying to find pairs that are close in current draw (at the same cathode resistance). I've also noticed that my B+ voltages are well over 400 volts. I'm not exactly happy about this, and not quite sure what to do about it. I think I may have accidently stumbled across a transformer that may have been better suited for a ST-70 than a ST-35.

[Shannon Parks]

Your higher B+ actually gives you a little more performance. Your caps are rated well enough, so go ahead and enjoy it.

[Ty_Bower]

Those radio shack / realistic tubes (like the AX7) can be from almost any country, including the land of the telefunken, west germany... Look on the bottom to see if there is a "diamond" shape embossed in the glass...

Nope, no diamond. There is a faint grey "5751" printed on the side, just barely visible beneath the "Realistic" emblem.

I've also got a set of Radio Shack 6BQ5 tubes which were very plainly made in the Reflector plant. RS even printed a "USA" on the side in white letters. Hah!

[Ty_Bower]

I just received a set of four 12BH7 tubes, and one 12BH7A. Judging by the construction, I think they're Ei tubes. I put one of them in place of the JJ ECC82 that I've been using for the last week or so.

I think the 12BH7 sounds a little better, although the difference is subtle. It certainly sounded good before, but now it's ever-so-slightly more natural and open. Or maybe I'm just imagining it. :)

In any event, I like the way the taller 12BH7 looks in the amp. I'm going to leave it there a while...

[Ty_Bower]

Sorry, but I just like this little amp too much. Haven't posted a pic in a while - need to post a new one. Here's a shot of her right side. I don't think there's been too many from this angle yet.

[Ty_Bower]

Time for another update... It's been 9 months since I finished this amp, and it's still my primary listen. Maybe if it didn't sound so nice, I'd be more motivated to finish the Poseidons. I guess it says something about the amp if it stands the test of time. It seems like a lot of the "regulars" around here crank out a new amp every week. What is your everyday "go-to" amp?

[crispycircuit]

I just reread your entire post. Great info, excellent pics, nice lay out, very inspiring. It was also interesting to hear you still love the amp and feel it's one the best sounding amps you've owned. I'm always curious to see how these new builds stack up after 6 months of testing/listening?? To me if it's still the best of the bunch, then sell the others and keep the spare amp collection down to 4 or so amps.... I agree, what is your favorite amp you own??? and pics would be interesting......

[sorenj07]

If your B+ is too high, you could always smuggle a decently sized choke (or two, maybe, if you can't spare the size) and run it closer to choke-input.

[20to20]

Ty,

Ditto on the compliments for your journaling on this build. It may be a few years old but it still has much inspirational value.

I'm nearing this point for my first scratch build. Pulling parts together and tools. I have almost the exact same tool set that you had then. Bench top hand drill drill-press stand, mitre saw, etc...

Question on the drill-press and using the stepper bit. Would you rather have a single 3/4" or 1-1/8" bit to use for holes? I'm working on an idea for a drill press that uses a large hand drill, "holstered" to a longer arm that drops down, hinged to the back of my bench. This would give a much larger area to work with larger sheets. So my choice now is whether or not to spring for a stepper bit set or buy the single large bits and a HD large chuck hand drill.