Building my Crack - Finally

[davew]

Here's a update on my progress. I made some changes to the PSU mods shown in my original post. I decided not to use the 470uF electrolytic cap in the PSU. I used all Mundorf MTubeCaps. A 47uF is in position 1 followed by a 30uF and then a 20uF. The 47uF is larger than the 20 or 30 but everything still fits! The Hammond 158M 10H chokes are still there.

I used the Mundorf M-Cap MKP 100uF 250V film caps to replace the output electrolytics. These took a couple of weeks since Parts Connexion had to order from their supplier.

I've completed assembly up to the output stage cathode resistors. I've attached 2 new pics. The first shows the chokes and PSU caps "spread eagle" at the moment to give me room to work. The second has the chokes secured and the caps sitting in place. I haven't secured the caps yet since I'll need to spread things out again for the final resistance and voltage checks. I am saving the Speedball for the future.

You'll see in the pics that I've used my own cloth covered wire throughout. I wasn't comfortable using just red, black and white everywhere. I tried to use a somewhat standard color scheme.

[Paul Birkeland]

Looks nice!  I tend to use that wire in certain projects when I want the vintage look.

I'd suggest installing 47K resistors at the input jack between the solder cup and the hot for each channel that will help the balance of the PEC pot at lower levels.

-PB

[davew]

I finished the build today. All I can say is OMG! Simply amazing. Absolutely no hum. There was a very noticeable background hiss though. That hiss turned out to be due to the stock Russian 12AU7A. When I pop in any other input tube the background hiss disappears and the amp is DEAD SILENT! So the stock 12AU7A goes to the junk pile/trash.

Now the real fun begins. Listening to all my music again.

I have several input tube choices to audition:
Sylvania JAN 6189
RCA Clear top 12AU7A
Amperex Bugle Boy ECC82
JJ ECC802S
and several 6SN7GTA and GTB as well. I will also soon have a RCA red base 5692 I just scored from eBay.

For the output tube I have several pieces of pristine 1955 RCA coke bottle JAN 6AS7G.

I just listened to a bit of Diana Krall - Live in Paris, JS Bach - Suite for Solo Cello #1 - Fournier and Gregorio Allegri - Miserere Mei. Nothing is broken in yet and it all sounds great.

I added a few new pics. First shows the stuffed box. Everything did fit. The second shows it running with the stock tubes.

@PB - Thanks for the 47k resistor suggestion. I did notice an imbalance at the very low end of the pot. It's only the first 10 degrees or so. If I find myself listening at those low levels I will then add the 47K resistors.

[davew]

And here's a pic running with the RCA 6AS7G and Sylvania 6SN7GTB.

Now to just find time to finish the base.

[Doc B.]

I think you may be giving up on that 12AU7 too soon. New tubes often need some run time, apparently to get the cathode emission up to the designed level. Try running that tube for 20 hours and see if the hiss is reduced.

[davew]

Thanks. I'll burn-in the amp with the stock tubes and see how it improves.

[George Napalm]

I had a similar problem with the driver tube (some 12AU7 I couldn't identify). I heard audible hiss and extensive burn-in didn't help. Since installing other tubes I'm amazed how dead quiet this amp is.

I'd suggest installing 47K resistors at the input jack between the solder cup and the hot for each channel that will help the balance of the PEC pot at lower levels.

-PB

Interesting! I have a slight imbalance at lower level too. Is this resistor supposed to change the taper of the pot somehow?

[Doc B.]

Interesting! I have a slight imbalance at lower level too. Is this resistor supposed to change the taper of the pot somehow?

No, it cuts the signal down a bit so you can put the volume control at a higher setting where the two channel levels track better. Pots tend to get farther apart balance-wise at the low end of the scale.

[Doc B.]

If you use a decent metal film resistor it should be fine, since that should be of a bit higher quality than the pot itself.

[George Napalm]

Ok! So to make sure I understand everything: "connecting solder cup and the hot" means connecting + and - of the input jack? I haven't seen the term "hot" before...

[davew]

Hot is an old school term for the positive side of a pair of terminals.  "... installing 47K resistors at the input jack between the solder cup and the hot for each channel ..." means remove the positive wire from the RCA jack center terminal (solder cup) and install the 47k resistor inline between the wire and the now vacant RCA jack center terminal.

------------wire-------->] RCA jack center terminal

becomes
                          47k resistor
------------wire----/\/\/\/\---->] RCA jack center terminal

I think there may be a picture of this buried in the gallery somewhere. If I find it I'll attach it here.

[George Napalm]

Thank you for the explanations. Now it's clear. No need to look for picture :-)

[Doc B.]

BTW - any relation to Wallace and Wilder?

[George Napalm]

BTW - any relation to Wallace and Wilder?

No, I was a big fan of death metal music at some point, especially the British band Napalm Death. I completely switched to classical music since then, but the nickname stuck. Moreover, George Rachmaninoff would sound too pretentious