Webcor tube amp, how can I turn this into a bass/guitar amp?

[SGS]

I got my hands on this old Webcor amplifier and from what limited information I can find online it appears that these were used as the speaker and amp section for reel-to-reel players and record players. It has an RCA type input on the side right next to the power cord plug and that's it, no volume knob or EQ or anything else.

I only pulled the woofer (think it's an 8") off the grill to check its condition and it is perfect. The surround and the paper cone show no signs of wear or rot. I tested it out with a guitar, J bass, and 5 string and everything sounds fine. The speakers even handled the low B without getting muddy.


The tubes are one 12AU7, two 12AB5, and one 5Y3. The AU7 and AB5s are all original Webcor tubes. The 5Y3 I wasn't able to pull out easily so I just left it in there. The chassis is stamped with 5Y3 right next to it so that's how I figured out what that one is. I can't make out the writing on it with the way its facing so I'm not sure if its a Webcor tube or not.


How much work and money would I be looking at to turn this into a little practice amp? I'm talking about taking the amp out of that case and building another for it, adding a gain control so I don't overload it with too hot of a signal, a 3 band EQ, and a pre-amp output so I can run it into a larger amp when I want more volume but still have a tube preamp. I would leave it as is but its pretty quiet with a guitar plugged in and nothing else. I know I could "boost" the signal going in by throwing a pedal in front of the input and turning up the level but I'd have no control over how much is too much.

I like DIY stuff and if its possible for me to do this without electrocuting myself then why not do something unique? Sure, it might only have a 10w output so we're definitely talking about a practice amp here, but if its not going to cost me an arm and a leg and it will sound good when done then it could be a fun project.


I took pics so you guys can see what I'm working with.

[SGS]

More pics.

[SGS]

Close-up pics of the amp section.

[T-Bird]

Hi.

That looks very cool and clean.

So cool and clean in fact, that I'd leave it alone and would build all the pre-amp parts into a separate enclosure, perhaps flip-top style into the front panel since it's easily switchable/reversible.

Congrats, a cool find indeed.

Regards
Sam

[SGS]

I was thinking about doing that, however, I'm not sure how well the speakers will hold up to bass when I'm getting full power out of it. It was designed for full range music, thus the 3 speakers, and I'd rather have it focusing its energy into a single 10" bass guitar speaker.

I'd also rather have the guts transplanted into another box with everything in it at once, keep it very simple by having it altogether.


I have no clue how to do the pre-amp parts though, nor do I have an idea how much work and $$ it will be. Any idea?

[Rick Auricchio]

A pair of 12AB5s (6v6 equivalent) gets you about 10w power output with a good output transformer. More likely you're getting five watts.

You may be better off flipping the unmodified unit to a collector. You'd end up rebuilding everything.

[T-Bird]

Hi.

Originally Posted by SGS

I was thinking about doing that, however, I'm not sure how well the speakers will hold up to bass when I'm getting full power out of it. It was designed for full range music, thus the 3 speakers, and I'd rather have it focusing its energy into a single 10" bass guitar speaker.

Since the baffle is easily replaceable, there's no reason why You wouldn't be able to swap the speakers as well.

Originally Posted by SGS

I have no clue how to do the pre-amp parts though, nor do I have an idea how much work and $$ it will be. Any idea?

Well, since the power transformer is most likely too small for any additional draw, You'll need a PT that shouldn't cost more than $20 or so. Two PT system is prone to hum, but additional filtering will take care of that.

As for the pre-amp itself, on one end of the spectrum You have the Bassman (RCA) with two jacks, one tube, 4 resistors, 4 capacitors and two potentiometers IIRC. ~$20 if You shop wisely and don't look at the top of the line parts.

On the other end, there's Ampeg V9 for example with two somewhat linked preamp channels, over 100 parts in total. IIRC again.

In between there's every imaginable variation, obviously.

Unless there's a special attachment to that particular amp, I'd build a Pro Champ (IIRC) clone if the 6V6 tone is to Your liking. I don't know what the value of Your amp is, but I'd imagine that any collector who's willing to buy it, can give two working amp sections in trade.

I do recognise the temptation of modding, especially an amp that looks as cool as Yours does, but I also do recognise the potential value reduction.

Regards
Sam

[SGS]

Rick - I've always wanted to play with stuff like this, see what I could build on my own and what I could modify from old parts. With the way I want to do this I'd basically be ripping the amp section out and it would be easy to put back in, would have to disconnect the pre-amp I want to add and change the input back to the RCA type instead of a 3/8" instrument jack.

TBird - as far as hum, this thing is very quiet on its own. I can barely hear a hum when its plugged in on its own. However, I would agree on probably replacing the transformer. I don't want to use the 12AU7, I want something "grittier" in the preamp. The 12AU7 is meant for very clean sound reproduction and I do want to get some tube crunch off this. I was talking to someone else who said that the transformer in there may not have the juice to handle a 12AX7, so replacing it is probably a good idea.

As far as what I want to build preamp wise, I have no clue. I'd like to stay on the cheaper end because this is my first time doing it, so dropping $100+ to do something like the Ampeg V9 would be a bad idea. I'd like to stay under $50 for transformer and preamp parts, but I do want to do low/mid/high on the EQ, so that'd be 4 pots total.

If this goes well I'd be tempted to try again with something bigger/better.

But...I have no clue what I'm doing or where to start. Fortunately I know how to solder and I'm capable of reading schematics. I can't find a schematic for this amp as of yet to see where I'd be able to cut in to add the controls. Hell, I don't even know if they go before or after the preamp tube.

Where do I start?

[christw]

Dude, forget hot rodding this amp. Keep it as it is. It's super cool. If I had one one hand, I'd certainly give you an amp build kit for that thing! Then you could build your own hot rodded amp (thinking Weber or Ceriatone or something) and get exactly what you want the first time around without tearing up what is already a very cool amp!