Tubes up #102 build

[andresound]

Finally decided to do tackle a scratch ODS 100w build. Picked up a board set and chassis from RJ a while ago. Eventually tracked down a loaner 1972 BASSMAN 100 for iron duties. Problem is, I had forgotten the chassis I received was a “upside down, tubes up, inside out” version. Talk about a challenge!!. I have tried searching for info and layouts of the tubes up type...zippo!!! If anyone can steer me in the right direction, i would appreciate it.

I have only built one other ODS (documented here) and many thanks to all on this forum, which was #40 with built in dumblelator and into a 68 Bassman chassis. Still have this amp and love it.

Here is the progress so far.

[andresound]

Going to go with the #102, 4th gen classic with skyline update. Because chassis only has one input hole, I would like to still do the FET channel on a relay. Advice and suggestions are most welcome

[martin manning]

Schematic layout for switchable FET is here: https://tubeamparchive.com/viewtopic.php?f=5&t=31084 With only one input jack hole you'll need to find another place for an externally adjustable FET gain pot if you want that.

[andresound]

Thanks Martin. I saw that sticky . I do have a hole on the rear for FET trim on the rear.

[andresound]

A little progress. Waiting on my 5uf Sprauge cathode caps to travel 6000km!! Very frustrating living this side of the world.

[andresound]

Is it ok to use high quality film caps on the cathodes? What difference would it be sonically, as apposed to lytics? I do have some 10uf Sprauge’s, but that would be TOO much?. Maybe just for CL1?

[martin manning]

I'm sure they would work just fine, and have infinite life as compared to electrolytics. Pricy, though, aren't they?

[andresound]

Martin, films are also rather large. I think I will be patient and wait for 5uf sprauge. The sprauge also turn out to be pricey, unless I put through a bigger order with other items. The freight kills us in this part of the world. .

Martin, are you saying I could use 10uf for CL1 and CL2? I do have these values in sprauge lytics. I also have my last 2x 5uf Sprauge lytics I could use for the OD1 and OD2.

[andresound]

Some progress. Finally finished wiring up. Still need to don a final check. Found some mistakes “right off the bat”.

[martin manning]

Martin, are you saying I could use 10uf for CL1 and CL2? I do have these values in sprauge lytics. I also have my last 2x 5uf Sprauge lytics I could use for the OD1 and OD2.

Sorry, I guess I missed this! You could, that is the low-plate scheme as we know it from #124. The difference will be a little more bottom end.

[stelligan]

Looks great!
Everyone else here is so tidy. My 102 looks like a rat's nest....
Good luck on the power up. I really have been loving mine

[andresound]

Martin, are you saying I could use 10uf for CL1 and CL2? I do have these values in sprauge lytics. I also have my last 2x 5uf Sprauge lytics I could use for the OD1 and OD2.

Sorry, I guess I missed this! You could, that is the low-plate scheme as we know it from #124. The difference will be a little more bottom end.

Thanks Martin, but this is a high plate version. I have already gone with the 4.7 sprauges, which eventually arrived. I am trying to stick to the 102 schematic. Must admit, the tubes up thing was a challenge and don’t recommend it unless you are an experienced builder (which I am NOT). I realize the reason for the normal tubes down version (combo’s), but thought... tubes up means less heating of components (longevity etc..) and this was going to be a head. If I do another, it will definitely be the regular tubes down version.

Will report back when she is up and running.

[norburybrook]

Looks great!
Everyone else here is so tidy. My 102 looks like a rat's nest....
Good luck on the power up. I really have been loving mine

I'm not in the tidy camp my amps sound great though
#102 is a great all round amp.


MC

[andresound]

I now have a problem on powering up. With the light-bulb limiter.. No tubes installed.

220uf in series/stanby/choke and OT center tap/22uf/2.7k/22uf/22k/40uf/2.2k/40uf/150k to ground (no FET). Have checked and re-checked values of string and all OK. All caps read OK (a little high while in circuit-measured with FLUKE DM).

1. HT behaves correctly. Bright then dies down to dim glow. So first first pair of 220uf in series seem OK.
2. Stanby swith activation (next part do PS string) does a strange thing. Bulb briefly brightens.. then dims..THEN...after maybe 2 seconds, suddenly pulses brighter (every second or so)!! Almost as if a cap is discharging, then recharging etc.

I have not left the amp on in this state for more than a few seconds, for fear of destroying something (OT). Should I fuse the OT center tap just in case?

Please help guys.

[martin manning]

I think maybe a a failing electrolytic.