Another 5B6

[Phil_S]

Many thanks to ToneMerc and everyone else who gave me a helping hand on this amp.

I have not yet played it...maybe time will permit that later today but I'm not optimistic. I have a number of things to do and with the nice weather today, I'd rather fit in some wood work on the headbox, which I need to do outside and I'll be lucky to get to that by 4:00.

TM's keen eye spotted the wiring error in the PI. Now that it is corrected voltage readings are:
B+1 362
B+2 (screen supply) 306
B+3 (PI and pre) 289

6L6's:
Plates 352 and 354
Screens 306
Cathode 23.64

6SL7 2nd triode:
Plate 165.2
Cathode 2.114

6SL7 1st triode:
Plate 166.9
Cathode 2.092

6SC7:
Plates 121.0
Cathode 1.497

Grid voltage is near 0 everywhere.

I am figuring static plate dissipation at about 15.6W per tube and total current draw for the pair at about 95mA (includes cathode current). I'm thinking this is reasonable, but would welcome comment on whether I got the calculations done correctly.

I also grounded pin 1 of the 6SC7 (thanks again). I hope I'm not premature in thinking this amp is finished, or nearly so aside from any tweaking.

[ToneMerc]

Many thanks to ToneMerc and everyone else who gave me a helping hand on this amp.

I have not yet played it...maybe time will permit that later today but I'm not optimistic. I have a number of things to do and with the nice weather today, I'd rather fit in some wood work on the headbox, which I need to do outside and I'll be lucky to get to that by 4:00.

TM's keen eye spotted the wiring error in the PI. Now that it is corrected voltage readings are:
B+1 362
B+2 (screen supply) 306
B+3 (PI and pre) 289

6L6's:
Plates 352 and 354
Screens 306
Cathode 23.64

6SL7 2nd triode:
Plate 165.2
Cathode 2.114

6SL7 1st triode:
Plate 166.9
Cathode 2.092

6SC7:
Plates 121.0
Cathode 1.497

Grid voltage is near 0 everywhere.

I am figuring static plate dissipation at about 15.6W per tube and total current draw for the pair at about 95mA (includes cathode current). I'm thinking this is reasonable, but would welcome comment on whether I got the calculations done correctly.

I also grounded pin 1 of the 6SC7 (thanks again). I hope I'm not premature in thinking this amp is finished, or nearly so aside from any tweaking.

Your welcome Phil, glad we got it sorted out. Looks like just some additional adjustment of the power rail and you will be squared away.

Mike

[tsutt]

my ignorance showing again. TM adjusting the rail what would you adjust and why? how would yo accomplish it.

thanks

[ToneMerc]

my ignorance showing again. TM adjusting the rail what would you adjust and why? how would yo accomplish it.

thanks

I would increase the PI plates 15-18V and bring the 6SC7 plate voltage up 35-40V to increase the headroom. I found a 30V delta among the half a dozen 6SL7 PI tubes I tried and I also decreased the 250K plate resistor value to increase the preamp voltage up into the 160V range as seen in the original circuit. In Phil's case his B+ is about 30-35V lower than what's typically found, so I would adust the second 10K resistor in the rail. For a harp application though, as it stands this might be just the ticket.

TM

[tsutt]

Thanks i will study your comments. tryin to learn.

[ToneMerc]

...... tryin to learn.

Your welcome, me too!

TM

[Phil_S]

I'm too preoccupied at the moment to calculate this. I wonder how much difference it makes to use 270K vs 250K on the plate load R's. It seems common values these days are 220K and 270K. Of course, this can be fine tuned with multiple R's if so inclined.

I did not have 10K R's for the power rail. It is done with 2 pairs of 5K 3W in series. A bit of jumper wire over one of the second dropping resistor pair will probably do the trick if I want to raise the voltage. I am working at home on Tuesday so I will have a lunch break or after breakfast break when I can turn it up and see what I've really got before I modify it.

Since I'm using Russian 6P3S (not the heavier duty 6P3S-e not real 6L6, I'm actually very happy the voltages worked out the way they did in the power section. I have read these can actually be quite toneful if the plate and screen voltage are not pushed so high.

In a brief test last night (less than 5 minutes) I had quite a bit of 60Hz hum, but there was no bottom on the chassis and it was on the floor in front of the speaker. There was also a florescent on nearby, so we'll see. My limited test suggested it is a good sounding amp and it's got more clean headroom than I expected.

[Phil_S]

I was suspecting an inconsistency between the HT VAC supply and the rectified voltage. Indeed, there was a problem, covered here in a separate thread, and new voltages posted, for those who are curious.
https://tubeamparchive.com/viewtopic.ph ... 803#243803

I think this one is going to be too loud to keep, as I don't play out. I will finish the head cab and then think about selling it. It is likely to take a few weeks until the cab is done.

[Phil_S]

Did a brief test drive for lunch today. This is a very satisfying amp, but I will never be able to dime it in the house. That's bittersweet. IMO, there is plenty of clean headroom below full volume or by rolling off the guitar.

I managed to find a 35V 22u radial cap on a circuit board and salvaged it for the power tube cathode It took some lead extensions...

The remaining problem is 120Hz hum. Plugging in a cable makes it worse. Pulling V1 pretty well cures it. I think it is the tube and not lead dress, which is very clean and there is plenty of "air" to go around. Chopsticking disclosed nothing, though tapping on the grid input wires was audible.

I am wondering about the wisdom or lack thereof of keeping the 6SC7. I have 3 metal ones and they all do the same thing. Yes, I grounded Pin 1 to the chassis. I could easily change it to a 6SL7 of which I have a few glass ones. More difficult, could change to a 5751.

Or is this as simple as using shielded from the jack to the tube? I guess it makes sense to try the easy stuff first.

[Phil_S]

This is an update, in the remote chance anyone is interested.

I've nearly completed a cab. I butchered the corners. Nevertheless, many thanks are due to Taylor Cox for his excellent video. I used 3M Super 77 and bought vinyl upholstery from JoAnn Fabric with the deep discount coupon from the Sunday newspaper. From afar, it looks OK. I have to figure out how to mend the corners because I am not getting into a do over. I also get a headslap award for writing TOP on the fabric and drawing a placement box, but putting the bottom where it says top. The seam is on the top but I did a decent job with that.

Here is a picture that shows the seam, but it doesn't look that bad in real life and I think I can mitigate it some, still.

Today I had time for a lunch break while working from home so I moved a 2x12 into the living room and gave it a test drive. The room is 14' x 33' and has wall to wall carpet. This soaked up the sound and allowed me to open it up quite a lot. I found out a few things, but mostly that it is a very pleasing amp to play. It has a range of clean and dirty. With volume at 5 it is still frikin' loud. I was able to dime it and that makes lots of good dirt! Even while dimed, I can dial back the guitar to clean up. I've got a Korean PRS with P90s and a Guild Starfire IV (think poor man's ES-335) both of which sounded great. I was wishing for a Tele to mate with this amp. I can imagine that a pedal steel player might really like one of these.

I managed to lick the hum. There is a small bit of noise, but fully acceptable as far as I'm concerned. Really, it is a very quiet amp.

I apologize for not posting clips. I am not a great guitar player and this is made worse by not having had either guitar out of the case for more than 5 minutes at a time in the last 9-12 months. I will have to build up some chops for a sound clip.

The preamp tubes are a metal RCA 6SC7, a GE 6SL7, a pair of Russian cheap 6p3s (a 6L6 compatible), and RCA 5U4GB.

I am waiting for a friend who has a fully equipped wood shop (table saw, planer, band saw, etc.) to cut and mill front and back panels that will fit properly. I think he's got some Cherry scraps for me.

The only problem I think I've got is the pilot doesn't light up. I didn't have any #47 bulbs while the build was going on. I will have to pull the chassis one more time to look at this as it is strange. I'm sure I can fix this even if I have to replace the lamp fixture.

[M Fowler]

Looking good and I think the 5b6 is one of the sweetest tweed tone circuits around.

Mark

[Blackburn]

Hey Phil, how do you like the metal 6SC7? I have two of the same RCAs in my 5C3 PTP build and I dig their tone. Very nice cleans and if I fix up the amp a bit and switch to a regular cathode biased V1, the grind should be a good deal better.

[Phil_S]

This is my first experience with a 6SC7. I bought 3 of them from the bargain bin at this place for about $10 http://www.fourwater.com/master/masterlink.htm

It is not clear to me how it affects the overall tone of the amp. I can only judge in context of the whole. I like this amp a lot. From a single good test drive, I think it has an impressive range of possibilities. I did not expect this from a tweed circuit. I was imagining more of a one trick pony like the 5E3.

[telentubes]

Transformers are finally all here for the 5b6.
Edcor PT XPWR179-120.
Here's a link
http://www.edcorusa.com/p/820/xpwr179_120

Allen OT, TO35MT,
Allen link...
http://www.allenamps.com/parts.php#transformers

Chassis is 2 1/2" X 8 1/2" X 15"
I have Tino's book, but I'm going to do it with a circuit board, and not point to point.

I'll start a new thread when I get started. This will take a while, as with all my projects, but it will get done.

[Blackburn]

Just be extra careful with lead dress and component layout...

Twas the death of mine...