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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Mar 11, 2014 2:25 pm
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Just a fast tip on restoring the old Bassman heads safely, an 18/3 grounded plug is the perfect fit for the hole in the vintage Bassman heads, using a 16/3 guage cord is a huge hassle, it pushes the back panel out in the far right upper corner and its all you can do to use a stress relief retainer on them.
18/3 gauge cords are tough to find and the typical amp repair parts house wants big bucks for them !!! fortunately most Computer power cords are 18/3 gauge or at least enough of them that a trip to my local thrift store got me a nice 8' 18/3 for .79 cents plus tax.
I just finished the job on my latest & greatest 67 black line drip edge Bassman.

Of coarse while in there I noticed the Screen resistors and the Grid stoppers are still the old carbon comps and have strayed a bit past the 10% range so.......... 8)

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Mar 11, 2014 11:42 pm
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Very cool, Chris !!! 18 GA is really sufficient(IMO) for these amps !!! I usually go with the water-proof power tool cables. But that is over-kill. Rock that Bassman 'till you Roll !!! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed Mar 12, 2014 1:42 pm
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Hey art !

Ya I used a 16/3 in one of the rebuilds and it was darn near impossible to use a strain relief
If fact I believe what I did was just put two layers of shrink tubing on it to keep it safe.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Aug 03, 2014 6:37 am
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WOW almost 30K hits on this thread
Its been a while since I've had time to visit
I'm still on the hunt for more Bassman heads to play with !
I absolutely love taking worn out Bassman heads and making them sound new again and I learned most of it right here and from many of the dedicated fender junkies here.

PS I have a bassman 135 on the bench waiting for the OK to get it back into shape.
Talk about heavy !!!

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Sep 22, 2014 7:36 am
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The fella finally gave me the green light to repair this monster :roll:
only took about three months
A little off topic being a totally different animal than our beloved Bassman heads, the Bassman 135 has bad tubes, filter caps and a snapped off potentiometer.
fun stuff !
Image

Image

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Sep 22, 2014 8:07 am
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a fuzzy picture but you get the idea
The Bassman 135 is a very simple circuit

dig those 220uf first stage filter caps ! in picture #2

Image

Image

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Sep 22, 2014 8:21 am
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These UL Fender amps need the best main filter caps and toughest 6L6GC tubes. I recommend either the Tungsol 6L6GC-STR or their 7581. If you have access to "real" mil spec Philips-ECG or GE 7581A's --- go for it. They can handle a lot of plate voltage and cathode current. Last a long, long time. GE recommended max dissipation on the 7581A is 35 watts per tube. These are one of the output tubes made in the "Holy" Owensboro, KY plant. Along with the famous 6550A.

Good luck, on the R&R! :D


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Sep 22, 2014 9:22 am
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going with F&T for the caps

I have access to some decent sovtek 6L6's this guy is on a very tight budget so......
Trying to find a quad of super 6L6's is out of the question.

this is what was in there

Image
Image
Image

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Sep 23, 2014 6:42 am
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You may want to try to bias the 6L6GC a bit cooler than the 30-35 watts per tube. I believe that Fender Bassman 135 setup the amp to have this range of output. Most "regular" 6L6GC types made today may have issues with this range. Really, this is 100% max dissipation recommended. It's one of the reasons Fender went to "STR" tubes. Special Tube Requirement types.


I'd try 20-22 watts per tube, first. See what the tone is like. Unfortunately, the Bassman 135 does not have a bias pot. It's a balance pot. So, you either will need to modify the balance pot circuit to a bias pot circuit. Alternatively, you can play with the 1.2k-ohm and/or the 2.2k-ohm dropping resistors. Or the circuit to ground 33k-ohm resistor. All three within the bias power supply circuit.

Since the amp is not yours, you prolly should change the set resistors. Unless, the owner is willing to pay to convert the amp to bias pot adjustment.

If it were my amp, I'd go the balance -> to bias pot route.

http://ampwares.com/schematics/bassman_135.pdf


Here's a link to basic layout changes, in going from balance to bias pot. You need to alter the specific layout wrt the year/model Fender amp. Note, the changes to the grid-to-ground resistors on the 6L6GC's.


http://el34world.com/charts/bias_conversions.htm




Here's a link to adding a bias pot to a balance pot era Twin Reverb. The TR's balance pot circuit is close to the Bassman 135.


http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm


Note this statement in the link:

" Make sure you use a resistor between the pot and the connection to ground to prevent the power transformer from being damaged by preventing the negative bias voltage from being directly shorted to ground. Use at least a 3.3K resistor here, maybe even a 4.7 to 6.8K. "


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Sep 23, 2014 8:01 am
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Thanks Steve !
As always your a wealth of information.
They sure spent a lot of time and money on this circuit just for the sake of using unbalanced tubes ! a balance pot and a hum pot.

Again this guy has little $$$ to throw at this monster, I don't know if it is worth too much investment honestly, I see some bass players using them.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Sep 23, 2014 9:00 am
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Chris,

This amp, of any amp, needs a bias pot. Or you'll need to find very tough 6L6GC's. At least alter the dropping resistor(s) in the bias supply so that the tubes are running around 20-22 watts per tube. And not 30-35 watts.

Really at 30-35 watts, only NOS STR or 7581A will last. You can try the Tungsol 6L6GC-STR or 7581, but they are really 30 watt (max) tubes.

Keep us posted!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Sep 25, 2014 5:40 am
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will do steve !
for whatever reason I'm not getting e-mail showing response to any postings.
Parts should be here tomorrow, hopefully I'll get into it this weekend.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri Sep 26, 2014 2:29 pm
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Filter caps done, new tone pot done, about to fire it up and start checking voltage etc.....
and I notice the hum balance pot is cooked, like as in at some time smoke puffed from her.
Not exactly sure how I missed that ???
No harm, no foul ! I was just hoping to get some readings and check specs.

and guess what, no 100 ohm bias type pots anywhere to be found outside of mail order :roll:

Monday it is !

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri Sep 26, 2014 6:40 pm
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Chris,

I think the balance pot is same value of most Fender bias pot. 10K-ohm linear. But, have a fourth tab connection. Maybe, find one on EBay. You may be able to use the usual 10k-ohm pot.


http://bmamps.com/Schematics/fender/bas ... _schem.pdf

https://www.tubesandmore.com/products/R-V10KL-BP-FND


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri Sep 26, 2014 6:53 pm
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The one in it is 100 ohms
Clearly marked on the back of the pot

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