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 🙂
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 🙂
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
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.
Anybody else out there interested in helping me with this?
I'll also be building a custom cabinet for the amp when its complete.
I'll also be building a custom cabinet for the amp when its complete.
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!
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!
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.
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.
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.
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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?
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?
If you need more power, run the bass output into an active or passive splitter, with one half going to the tube amp and the other going to a 1970s thrift store stereo (can I recommend a Kenwood? or perhaps a flame linear?) with two cabs. Or run a voltage divider across the speaker terminals sending 1v or so RMS to said SS power amp. Onstage, a balanced out into the PA would be nice. Otherwise, 50-60W is reasonable given the right amp Q, and some pro bass speakers (many are 96db+). Unless you play death metal, then go class D or use the 6L6 amp to modulate transmitting tubes 🙂.
Honestly with MI amps the same output power issues arise as with hifi, namely different powers suit different drivers & listening environments. Sometimes you need tuned bass bins and 5000w, sometimes 50w and the right drivers is too loud! Depends on the room damping, how insane the drummer is, etc. I can get the neighbors mad with 30w and the right stacks 🙂. Can barely hear it in a damped room.
Honestly with MI amps the same output power issues arise as with hifi, namely different powers suit different drivers & listening environments. Sometimes you need tuned bass bins and 5000w, sometimes 50w and the right drivers is too loud! Depends on the room damping, how insane the drummer is, etc. I can get the neighbors mad with 30w and the right stacks 🙂. Can barely hear it in a damped room.
hey-Hey!!!,
Your '35 looks very similar to my AU36( my very first tube amp). Mine had its OPT replaced with a Stancor 8053 and its performance was very good. I made some connector mods and turned it over to a guitar player loaded with 6AR6's instead of the 6L6's it came with. Its power TX stood the extra 1.2A of 6.3V load w/o issues. Last I heard from him it was going strong and played often.
cheers,
Douglas
Your '35 looks very similar to my AU36( my very first tube amp). Mine had its OPT replaced with a Stancor 8053 and its performance was very good. I made some connector mods and turned it over to a guitar player loaded with 6AR6's instead of the 6L6's it came with. Its power TX stood the extra 1.2A of 6.3V load w/o issues. Last I heard from him it was going strong and played often.
cheers,
Douglas
There are quite a few tubes in there, including four power tubes. It looks like the PT would be up to the task! The OPTs may or may not be sufficient. You may want to try cathode feedback like Leslie did or many on this site have explored, since that'll give you more bass response and better damping. The gNFB loop may be able to be tuned for bass depending on the transformer, gNFB is as close to voodoo as it gets without a scope though!
If you're feeling extra experimental, you can either parallel the OPTs (since they're similar) and use 4x power tubes or you can use the 70v taps on the first transformer to drive the grids of the second pair of tubes in class B. Paralleling seems to have a good effect on frequency response in my limited experience with it (dissimilar 6V6 transformers driven by 6AV5s). Good luck, that chassis is at least a great starting point, you can always replace the big stuff later.
If you're feeling extra experimental, you can either parallel the OPTs (since they're similar) and use 4x power tubes or you can use the 70v taps on the first transformer to drive the grids of the second pair of tubes in class B. Paralleling seems to have a good effect on frequency response in my limited experience with it (dissimilar 6V6 transformers driven by 6AV5s). Good luck, that chassis is at least a great starting point, you can always replace the big stuff later.
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