Odd Mod- Tweed Bassman question

vistacruiser · #1 12-08-2012, 10:35 AM

In an earlier post we discussed a '56 bassman 5e6-a amp I received. " '56 Bassman speaker load question " And I enjoyed a lot of helpful info from the TBer's that responded. This is regarding the same orphaned amp.
After re-capping the amp I discovered what appears to be a "mod", the purpose of which I don't understand.

The "normal" input jack is wired to both sides of the first pre-amp tube, in parallel. And from both coupling caps a wire returns to the "volume 1" pot.
The "bright" jack is wired directly to the "volume 2" pot, bypassing the first pre-amp tube. The bypass wire doesn't match the color of the rest of the wiring, yet it is an old cloth insulated wire, (yellow/brown stripe).
From my schematic it looks like the two inputs share the 1st pre tube, one going to each side, and then back through the caps to the vol pots.
The end result is that Input 1 is very loud and distorts at a level of about 1/4 turn, while input 2 (the bright channel) can be maxed and still be at "practice level".

I am interested in why this was done, and what use it would serve, before I attempt to change it back. I am doing this as a hobby interest, not a tech, and while I have made successful repairs to a handful of amps, this is the first "tweed" I have serviced.
Any thoughts appreciated, thanks.

Codger · #2 12-08-2012, 11:46 AM

Because each plate of the first stage is AC coupled to the same point it would look like two triodes paralleled with a common plate resistor of 1/2 the normal value. The signal gain should be the same as before but the noise gain should be 3dB lower.

So - lower noise.

vistacruiser · #3 12-10-2012, 12:28 AM

So the reason for the mod may have been to reduce the noise floor of channel 1, at the cost of a useable bright channel? Was that commonly done with these? I guess if the amp was so noisy, that would make sense. Think I'll change it back and see. Thanks Codger.
Also, I noticed that the bright cap is missing from the bright (channel 2) vol. pot.

1958Bassman · #4 12-10-2012, 05:24 AM

The amp shouldn't be noisy enough that this is necessary- I would change it back to its original configuration.

Codger · #5 12-10-2012, 05:29 AM

WRT the reason I suppose it is possible. It also may give a slight increase in gain for that channel. Looks to be less than 1dB though.

wcriley · #6 12-10-2012, 06:01 AM

It's also possible that a previous owner changed it just for the sake of "modding" it.
Kinda' like some people who "upgrade" their bass guitars by changing pups, bridges, pots, caps, etc.

vistacruiser · #7 12-10-2012, 09:18 PM

Quote:

Kinda' like some people who "upgrade" their bass guitars by changing pups, bridges, pots, caps, etc.

I was just thinking...you know...when I claimed above, that the amp distorted at 1/4 turn of channel 1 vol pot, I had my flame painted Squier P-bass plugged into it, with both the DiMarzio and Quarter pounder P pick ups in series. With the stacked and blend pots wide open and the orange drop completely rolled out. May be I could get better tone from it with a bone stock P-bass.

wcriley · #8 12-11-2012, 06:07 AM

Yep!

1958Bassman · #9 12-11-2012, 07:46 AM

Quote:

Originally Posted by vistacruiser

I was just thinking...you know...when I claimed above, that the amp distorted at 1/4 turn of channel 1 vol pot, I had my flame painted Squier P-bass plugged into it, with both the DiMarzio and Quarter pounder P pick ups in series. With the stacked and blend pots wide open and the orange drop completely rolled out. May be I could get better tone from it with a bone stock P-bass.

If it sounded a bit like the bass on Frank Zappa's song 'Apostrophe', I suspect your pickups are too hot for the input to stay clean. I have an SD Curlee bass and when the pickup, which is also a DiMarzio, is in series, it overdrives the input of every amp I have plugged into. The first time it happened with my '58 Bassman, I thought I blew it up. If I went back to the Parallel position or turned the volume control down, it cleaned up.

I knew someone who did a similar mod to his Deluxe Reverb and it had the effect of making a two channel amp into a one channel amp with one stage driving the other, which worked great. This amp isn't the same and it doesn't work the same way. He did something similar to mine and it added a bit of gain, but not enough to make it useful for guitar/overdrive. I changed it back because A) these weren't as valuable at that time and if it had damaged anything, it would have reduced its value and B) it didn't work. It works better for bass if you don't have any added gain at the input.

vistacruiser · #10 12-11-2012, 08:56 PM

Quote:

If it sounded a bit like the bass on Frank Zappa's song 'Apostrophe',

While I can't claim to sound anything like Jack Bruce, my amp can!
Thanks for pulling out the Zappa reference, that gives me something to shoot for, very nice!

A while back, when the model P was alone in an old fender bass, I had the s/p switch wired as on your SD Curlee. But this time the DM is wired series as is the 1/4 lb bridge pup, and the s/p switch is between the two P pup's. So yea, it'll drive an amp pretty good.

Got to think of it as if the input gain is on the bass, and use it to drive the amp into distortion as desired, or roll back to clean it up.