1966 Fender Bassman 100% Original

[jamaio]

A few weeks ago a good friend asked me if I wanted an old guitar amp that had been in his basement for 30+ years. I said yes thinking it would be an old hunk of junk, boy was I wrong. I couldn't contain my excitement when I saw the amp. Upon initial inspection I found the fuse blown. When I opened it up I found one of the heater wires lose, it had contacted a high voltage pin on one of the output tubes. I have not tried to fix the issue or power the amp up because all the caps are original and the power caps are leaking. I have ordered a complete cap set, 2 new output tubes, one original is cracked, and some other parts to convert the circuit from AB165 to AA864. I will also voice the Bass channel with a "tweed" mod to give it more gain.

Should be a great sounding amp when I am done.

[sluckey]

Wish I could feel the excitement but those tiny thumbnail pics don't reveal much.

[jamaio]

Wish I could feel the excitement but those tiny thumbnail pics don't reveal much.

Sorry, I posted smaller pictures to stay within forum rules.

[PRR]

> I have ordered a complete cap set, 2 new output tubes, one original is cracked

Yes.

> parts to convert the circuit from AB165 to AA864. I will also voice the Bass channel with a "tweed" mod...

No, not yet. Fix what is broke and play a while. There are no BAD Fenders. And modding an amp with unknown problems is a path to madness (or mild poverty). Maybe a 864 is 'better' than 165, but thousands of 165 served well.

> smaller pictures to stay within forum rules.

"maximum individual size 2048KB"

Your biggest pic is 39k, far less than allowed.

[dwinstonwood]

jamaio, I totally understand the temptation to dive straight in and start with the mods. But, I really think PRR makes a very important point. So, to stress what he said, get it working stock first, as best you can.

And, I would also like to see some larger photos. What an amazing find! I dream of stumbling across something like this. This stuff is out there, packed away in closets and attics and all but forgotten about.

[jamaio]

Yes.

No, not yet. Fix what is broke and play a while. There are no BAD Fenders. And modding an amp with unknown problems is a path to madness (or mild poverty). Maybe a 864 is 'better' than 165, but thousands of 165 served well.

jamaio, I totally understand the temptation to dive straight in and start with the mods.

Both of you speak with more reason than you understand without the rest of the story.

I have been going back and forth about keeping this amp stock or modding it. I have a 1967 Bassman that was partially modded when I got it. It had a good sound but I never heard the original sound. I completed the mod to AA864 with the tweed mod in the bass channel and it sounds awesome now. So for the ‘66 I can keep it original because it is original or I can make it sound awesome like the ‘67, but maybe it sounds awesome stock, if I mod it I will never know the “original” sound.

So for now I will keep it original. I have replaced the 2 prong power cord, I bypassed the ground switch, the death cap and the power outlet. I’m in the process of replacing the caps. I will leave the rest of the circuit original including the bias balance and see how it sounds. Maybe original is better than modded.

The heater wire at V6 had a cold solder joint, I could see the hole the wire came out of. The lamp was also blown.

I will let y’all know how it sounds once I’m done with maintenance.

Thanks for the encouragement to keep it original.

[jamaio]

I made the image smaller.

[jamaio]

All original

[jamaio]

New caps

[jamaio]

My ‘67 Bassman with 2x12 cabinet, the grill cloth has been replaced but I have the original. This amp looked liked it was locked in a closet for 50 years, it is very clean.

[valvetones]

Groovy!!
Excellent find.

[jamaio]

Replaced all the caps, replaced the 2 prong power cord, cleaned all the pots, jacks and switches, replaced the fuse and lamp. Powered up the amp with a light bulb limiter, everything looks good, no smoke!! 

Plugged the amp into wall power, attached test speaker and guitar, the amp sounds awesome!! Checked all inputs, normal and bass channel, volume treble and bass, bright and deep switches, everything works as expected.

Power amp off and connected bias meter at output tubes. Check bias at idle with all pots set to zero. The bias is balanced at 55.8 ma. Voltage on pin3 of both power tubes is 450 v which gives me a 25.11 dissipation. I have never set bias with a balance pot so I have no idea if this is good. I have attached the spec sheet for the tubes I am using for reference.

Any and all help on setting the bias properly will be appreciated.

Thanks!!

[sluckey]

Power amp off and connected bias meter at output tubes. Check bias at idle with all pots set to zero. The bias is balanced at 55.8 ma. Voltage on pin3 of both power tubes is 450 v which gives me a 25.11 dissipation. I have never set bias with a balance pot so I have no idea if this is good. I have attached the spec sheet for the tubes I am using for reference.

25.11W is pretty hot. About 83.7%. Most people like to bias between 60% and 70%, or 18W to 21W. As is, you have no means to adjust the amount of bias.

Here's a simple mod to allow for adjusting the amount of bias in addition to the balance adjust. I suggest adding a 1Ω resistor between pin 8 of each 6L6 and ground. You can easily do this right on the sockets. Measure the mV between pin 8 and chassis and set the BIAS ADJUST for the desired mV (same as mA). Now connect the red meter probe to pin 8 of one tube and connect the black probe to pin 8 of the other tube. Adjust the BALANCE pot for zero mV. May have to repeat these steps until the desired bias current and balance are achieved.

[jamaio]

25.11W is pretty hot. About 83.7%. Most people like to bias between 60% and 70%, or 18W to 21W. As is, you have no means to adjust the amount of bias.

Thanks Sluckey!!

Decisions, decisions!!

On my ‘67 Bassman I converted the bias balance to bias adjust, see attached picture. I could do the same on this amp and save the old parts to maintain originality.

[jamaio]

So I took the least invasive route, I swapped the 15k resistor on the bias balance pot with a 22k. I think I’m good now. 454 v on pin 3 of both output tubes, bias balanced at 42.3 mA, that gives me 19.20 dissipation.

Now I’ll put the chassis back in the cab and see how it sounds through my 2x12s.

Thanks again for all the help.

[sluckey]

So I took the least invasive route, I swapped the 15k resistor on the bias balance pot with a 22k.

You may need to do that again when you replace tubes. Here's a non-invasive version of the same mod using a Marshall style trim pot soldered to the back of the balance pot.

[jamaio]

You may need to do that again when you replace tubes. Here's a non-invasive version of the same mod using a Marshall style trim pot soldered to the back of the balance pot.

Thanks Sluckey!!!

I had the 22k resistor so I took a chance that it would work. I thought about smaller pots, we have some at work but today is a holiday because I live in New Orleans! I think they celebrate Mardi Gras in your area also.   

I’ll let you know what I find at the office tomorrow!

[sluckey]

Fat Tuesday! All day and late into the night. The city is shut down except for the downtown bars and restaurants.

And police and tow trucks.   

[jamaio]

I asked one of the techs at work for a 10k pot, he gave me a few to choose from. They are not as small as the Marshall type but will work. I think I will use the metal one and solder it to the chassis so I don’t have to drill a hole.

[sluckey]

Gonna take a lot of heat to solder that pot to the chassis. Might even damage the pot. Here's a different approach that will look good and not require any drilling.

Look at the simple "Z" bracket that I made for the bias pot in this pic....  http://sluckeyamps.com/phoenix/p11_big.jpg

Now imagine that the part of the bracket that contacts the chassis is about 1.5" to 2" long. Drill a 3/8" hole near the end. Remove the nut for your balance pot and put the bal. pot bushing through the hole in the Z bracket. Now reattach the bal. pot and Z bracket to the chassis with the nut. Probably should attach your new pot to the bracket before attaching to the chassis. Does this make sense to you?

[jamaio]

Does this make sense to you?

Yes, thanks!! I can attach a bracket to the transformer screw. I may need to make an “L” bracket because the pot post is long.

[jamaio]

I made a quick and dirty bracket, I will see how it fits after work.

[jamaio]

Well, the bracket I made was way too big so I mounted the small pot on the balance pot. The amp will never leave my house so it shouldn’t be an issue.

Thanks again for your help Sluckey!!

[sluckey]

Do you now have full control of the bias? Bias level and balance?

[jamaio]

Do you now have full control of the bias? Bias level and balance?

Yes, I have a good range also, works great!! I also added the 1Ω resistors as you suggested, I have done that on all the amps I have overhauled.

Thanks!!

[jamaio]

I have been running the amp on the bench the last couple of days, I read somewhere that new tubes need at least 5 hours run time for the bias to settle. I was getting different readings from day to day. I always let the tubes heat up for at least 5 minutes, the bias would climb slowly over the 5 minutes. The balance would drift up to 5 mA. So I decided to clean the tube sockets and bias balance pot. Wow!! That made a huge difference, the balance is now settled and has been even for the last 5 minutes.

450 v pin 3
42.4 bias
19.08 w dissipation

[sluckey]

That Klein meter should be set to measure mV, not mA!

[jamaio]

That Klein meter should be set to measure mV, not mA!

Oops!! Time to retest!!

I played through both amps at the same settings, the 66 sounds amazing but the 67 has more volume and has a fuller thicker tone with the mods. I have creambacks in the cab.

[jamaio]

That Klein meter should be set to measure mV, not mA!

Not sure why I set it that way today, I promise it was correct yesterday when I tested!! 

Still dead on!! I have a TAD Bias Master but I prefer setting bias with 2 (or 3) meters, for some reason my results vary with the Bias Master.

Thanks again for your help!!