Converting a Stromberg Carlson AU-35 into a Bass guitar amp?

[Pckid212]

Is this possible? Ive always wanted a tube amp for my bass and I can pickup this Stromberg Carlson AU-35 for $25

I'm looking to get a really deep growling bass amp, if that helps with designing this.

Here are some pics:




I would rate my skill level with electronics about a 7 on a scale from 1 to 10 and I'll also have help with this (my dad is an Electronics Engineer).

Here is a schematic that I have found for this:


I'm looking to get 45-50 watts in PP or 90-100 watts in dual PP. I have no problem buying different transformers, caps, resistors, etc.

The tubes used are (3)6SJ7, (3) 6SC7, (2) 6N7, (4) 6l6G, and (2) 5U4G

Thanks

[Tom Bavis]

25 and 50 watts are more like it - should be plenty with efficient speakers. A little more power would be available by dropping 5AR4s or a solid-state sub into the rectifier sockets, and 6L6GCs into the outputs. But maybe not enough increase to HEAR...

I measured a similar Stromberg output transformer and it went down to 55 hz at 25 W - these look to be a little bit bigger. Bigger than a Bassman transformer, I would say...

The Sams folder is on my Stromberg page: Made In Rochester

[Pckid212]

Originally Posted by Tom Bavis

25 and 50 watts are more like it - should be plenty with efficient speakers. A little more power would be available by dropping 5AR4s or a solid-state sub into the rectifier sockets, and 6L6GCs into the outputs. But maybe not enough increase to HEAR...

I measured a similar Stromberg output transformer and it went down to 55 hz at 25 W - these look to be a little bit bigger. Bigger than a Bassman transformer, I would say...

The Sams folder is on my Stromberg page: Made In Rochester

50 watts would be alright for gigging I guess. This amp will drive a 100 watt 4 ohm 15" Jensen MOD

I wouldn't mind buying a new/used Marshall or Fender OT to gain more wattage if possible.

[Pckid212]

Anybody else out there interested in helping me with this?

I'll also be building a custom cabinet for the amp when its complete.

[m6tt]

Personally I'd start a bass amp project by clearing out just about everything except magnetics and tube sockets! I'd aim for SS power rectification. The power transformer and output transformer are really the limits on maximum power output. SS rectification will help, freeing at least some current in the PT and removing some impedance from the B+. I'd check out something like schematic heaven or elsewhere and examine a bass amp with similar tubes for the preamp. You want high B+, proper DC bias (low distortion, more headroom) in the preamp. Expect to pay out the nose for coupling caps around 1uF 630v for good bass response, unless you DC couple or use feedback loops in the preamp. The power stage you should do the loadlines on to figure out where to bias it etc. If you need wires, grab an old computer PSU, plenty of colors, enough to follow the old RETMA style, looks nice and easier to fix later.

Ultimately, most of these PAs were designed to do a lot of stuff a bass amp doesn't, like amplify bass frequencies . It'd probably work as is, but you can do better with a redesign. Simple is better, keep B+ high and make sure to use a frequency response starting at the lowest note the bass can produce. You will want gNFB around the output stage or cathode feedback to get more lows through the OPT.

Cheers & good luck, nothing like rocking out on something you made!

[Eli Duttman]

This amp will drive a 100 watt 4 ohm 15" Jensen MOD

If, as I suspect, that speaker driver has a single voice coil, you are asking for trouble. Paralleling sections into a single, shared, voice coil frequently results in lots of heat and poor sound. You can't bridge the sections, because the O/P trafos lack the necessary (sic) 2 Ohm taps. Perhaps a dual voice coil driver that's usually found in a subwoofer is the answer to the problem. Energize only 1 or both voice coils as circumstances require.

[Pckid212]

Originally Posted by Eli Duttman

If, as I suspect, that speaker driver has a single voice coil, you are asking for trouble. Paralleling sections into a single, shared, voice coil frequently results in lots of heat and poor sound. You can't bridge the sections, because the O/P trafos lack the necessary (sic) 2 Ohm taps. Perhaps a dual voice coil driver that's usually found in a subwoofer is the answer to the problem. Energize only 1 or both voice coils as circumstances require.

It is a single voice coil. I haven't hooked it up yet, I was meaning it will be driving a Jensen MOD (after its complete), because thats what I have in my SS cabinet.

[whitelabrat]

Originally Posted by m6tt

Personally I'd start a bass amp project by clearing out just about everything except magnetics and tube sockets!

I'd take it even further than that. Guitar amp. Yes. Bass Guitar amp. No way. Mabey for low volume practice at home, but forget it for gigging.

From my experience 100w is a minimum to keep up with a drum kit and even then isn't enough. You'll need big mama output transformers for that job since you'll need serious low frequencies. Don't forget the low E has a 41hz fundamental.

You can do it though, but you probably won't get more than a good clean 20-25w per channel out of it. You can have each channel drive a separate speaker, perhaps two 15 inchers for best efficiency? You'll want to beef up the electrolytics or you'll not have much attack on a note.

[jjman]

My friend plays bass thru a 4x6l6/EL34 head. 725v on plates and 325 on screens. Just about class B. It's a Music Man HD130. Plenty of volume for gigs. I'd go for it. SS rectification and new beefy OP tranny. Probably a new power tranny too if it's not up to the task. I think it could look great with a new paint job. Or go crazy and get it chromed or powder coated.

[Pckid212]

725v on the plates is a bit high imo. I plan to run 450-500 volts on my 6l6 plates. And yes, it will need a new power tranny, the current power tranny only puts out 390v or so.

I'm not really into restoring the old enclosure. I'm going to build it in a new cabinet and cover that in Tolex that matches my SS cabinet.

What would you guys recommend for a Power Transformer and an Output Transformer?