The Bassman LIVES!!!

[Prages]

I never got around to calling my amp guy, but I went ahead and bought a pair of EH6L6s and a full compliment of preamp tubes, and the mighty Bassman once again breathes fire!

I guess I probably should still either call him so he can bias it or get a Bias Rite or something, but SHE LIVES!!!!

[Mark Wein]

:rawk:

[telecaster911]

If you wait till Pragestock, I'll show you how to bias it. Or maybe I can write something up. It's dirt easy. No Bias Rite necessary.

[Mark Wein]

OK...you inspired me to fix my Bassman cab tonight since I finally got a new input jack for it...

[Mark Wein]

OK. I just fired it up and it sounds killer with the greenbacks. Just have an issue where the jack for the speaker cab is grounding against the metal surround....

[Prages]

Yeah, Tele, if you can write it up, I'd greatly appreciate it.

I've read some stuff about sticking a resistor on pin 8 and measuring the voltage from that, but I wasn't really sure if I wanted to poke around with a soldering iron inside the amp. :o

[telecaster911]

Yeah, Tele, if you can write it up, I'd greatly appreciate it.

I've read some stuff about sticking a resistor on pin 8 and measuring the voltage from that, but I wasn't really sure if I wanted to poke around with a soldering iron inside the amp. :o

That's where I'm going...it's easy though, really easy. I did it to my bandmaster in about 15 minutes. Let me try to find a gut shot of a bassman to annotate. I don't know offhand if it's pin 8, but it is the leg with the ground strap that then goes to the chassis. Unsolder the ground strap from the chassis, then put the 1 ohm between the end of the ground strap and the chassis. Probably a nice 2 or 3 watter. then multimeter the dc voltage on each leg of the resistor. Should read about .035 volts which in turn is 35mA draw, right where I like to run 6L6's. Drain the filter caps and put a clip across to keep the voltage from blossoming back up before poking around in there. I'll write up something in the morning with pics.

[Prages]

Thanks, Tele.

Also, please explain how to drain the caps and such. Everything I've read just says "Dangerous voltages exist in tube amps. If you don't know what you're doing, take it to a tech."

[Prages]

Oh, and if you need a gut shot of the Bassman, I'll take one for you. Gimme a minute.

[Prages]

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts1.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts2.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts3.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts4.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts5.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts6.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts7.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts8.jpg

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/Guts9.jpg

[telecaster911]

Those are great. Can you take a pic of the filter caps under the can on the other side? I'm guessing you see the ground strap from the power tubes to the chassis and the bias pot, but I'll label them and post tonight along with draining the filter caps.

[Prages]

Filter caps:

http://www.thewindtunnel.com/GuitarPics/Bassman/Guts/FilterCaps.jpg

[Denverdave]

I love the wiring on older amps!

And congrats on reviving the Bassman!

[Prages]

I love the wiring on older amps!

And congrats on reviving the Bassman!


Yeah, and what is nice is that I've been reading a book about amps for the last couple of weeks, and I can actually almost follow the signal path all the way through now and can tell, for example, where the phase inverter circuitry is (and that it's a long-tailed pair), where the rectifier is, and kind of sort of what does what in the circuit.

It's still intimidating to look at though. :o

But it is a lot less confusing than looking at a PCB on a more modern amp.

[telecaster911]

Just about done...but the bass player is here to record now. Up later tonight :(

[Prages]

No big rush, Mark. I'm headed out of town right after work tomorrow and won't have the resistor until Monday at the earliest.

[telecaster911]

Buy 2 of 1ohm, 2 watt is fine (1% tolerance preferred). And 1 of 100K, 2 watt. Plus a pack of alligator clips, maybe 10 total for making some jumpers. :thu:

[telecaster911]

OK, here is what I came up with. Give it a read through, ask questions, etc. If anything is unclear, don't start into the project.

Disclaimer, this document is for Prages and Prages' Bassman amp only.

http://hc.bloodyvelvet.com/files/129/Biasing%20a%20Bassman.pdf

[Mark Wein]

OK, here is what I came up with. Give it a read through, ask questions, etc. If anything is unclear, don't start into the project.

Disclaimer, this document is for Prages and Prages' Bassman amp only.

http://hc.bloodyvelvet.com/files/129/Biasing%20a%20Bassman.pdf

I'm not doing this anytime soon but I wasn't clear on one thing:


These are the filter caps. Check each cap with your multi meter to verify the voltage that is stored.
Notice there are 2 banks of caps, 4 on the left, and 2 on the right. If the voltage is higher than 10VDC
in each cap, build yourself one of these.


This is a lead with an alligator clip on each end and a 100K, 2 watt resistor in line. This will safely
reduce the voltage below 10VDC. Do each cap individually. Measure the voltage again. If the voltage
is below 10 VDC, clip on a lead with 2 alligator clips but this time no resistor in line. Leave this in
place. One alligator clip lead will take care of the left bank of 4 caps. Do the same with the bank of 2
caps. The 2 caps will each need a separate alligator clip lead left in place. Usually I tie all of the leads
to chassis ground for a little extra safety, i.e. ground the left bank of caps to the chassis via a lead with
alligator clips. The right bank can be tied to the chassis on the side of the cap that has the legs tied
together.
Verify that there is no voltage

Where do you put the alligator clips?

[telecaster911]

I'll draw in the connections for the alligator clips in tonight on the filter cap picture.

[Prages]

Wow, tele! That's great. I'll pick up the alligator clips and resistors today (if Radio Shack actually has the resistors). I know Radio Shack has the alligator clips, but I don't know if they have anything over 1/2 watt resistors.

[Prages]

One other thing...

Once it's done, take those 1 ohm resistors back out, or just leave them in?

[telecaster911]

.Actually 1/2 watt will work. Let me look around to see what I have closer to 2 watt and I'll drop them in an envelope to you. I'm sure I have some. Yes, just leave them in place. I wish I would have taken a couple pics of the bandmaster when I did it. It looks almost the same inside from what I remember. Did you read the last paragraph? You may just want to hit .036 and call it a day.

[Prages]

Yeah, I read the whole article and printed it off to have in front of me when I try it.

Before you put anything in the mail, let me walk over to Radio Shack and see what I can find. Gotta go over there anyway to get the alligator clips.

[Prages]

Radio Shack sucks. :mad:

They had 1/2 watt 100k resistors, but only as good as 5% tolerance, and the only 1 ohm resistor they had was a 10% 10 watt unit that looked like a brick of C4. :o

I also didn't get the alligator clips because there was a huge line, but I'll get them sometime before next week.

There's one other electronic components distributor I know of in the area. I'll check them out before I go home today and see if they've got the resistors.

[Mark Wein]

I'll draw in the connections for the alligator clips in tonight on the filter cap picture.

Thanks!

[Mark Wein]

Radio Shack sucks. :mad:

They had 1/2 watt 100k resistors, but only as good as 5% tolerance, and the only 1 ohm resistor they had was a 10% 10 watt unit that looked like a brick of C4. :o

I also didn't get the alligator clips because there was a huge line, but I'll get them sometime before next week.

There's one other electronic components distributor I know of in the area. I'll check them out before I go home today and see if they've got the resistors.

The Radio Shacks here are now basically cell phone stores with a little drawer ful of components in the back. I bougth a set of flush cutters from them that broke with the very first cut a few weeks ago.

[Prages]

The Radio Shacks here are now basically cell phone stores with a little drawer ful of components in the back. I bougth a set of flush cutters from them that broke with the very first cut a few weeks ago.

That's pretty much what the setup is here too. Pretty useless for actual electronics components, though they do have a bunch of 5% 1/4 and 1/2 watt resistors in a bunch of different values.

The other place is about 10 miles from work, but they are an actual Electronics Specialty Company (that's actually the name of the place) and from what I can remember from the last time I was in there, it's basically a place like NAPA where you go up to the counter and tell them what you need, then they disappear into the warehouse in the back which is full of components.

I'm leaving work early to run some errands anyway, so I'll make them a stop on my tour. :o

[Mark Wein]

That's pretty much what the setup is here too. Pretty useless for actual electronics components, though they do have a bunch of 5% 1/4 and 1/2 watt resistors in a bunch of different values.

The other place is about 10 miles from work, but they are an actual Electronics Specialty Company (that's actually the name of the place) and from what I can remember from the last time I was in there, it's basically a place like NAPA where you go up to the counter and tell them what you need, then they disappear into the warehouse in the back which is full of components.

I'm leaving work early to run some errands anyway, so I'll make them a stop on my tour. :o

Most of our electronics stores have closed around here. I tried Frys last week and couldn't get most of what I needed so I had to place an order through Small Bear Electronics (which should be here today, btw). We also have Orvac electronics, which I haven't been to in years but I might make a trip there to see what they carry now.

[telecaster911]

I put together a nice pair of 1 ohm, 2 watters. Hand selected to match. I'll drop them in the mail.

[Prages]

I put together a nice pair of 1 ohm, 2 watters. Hand selected to match. I'll drop them in the mail.

Thanks, Tele. I guess you still have my address?

I'll buy you a beer at Pragestock. :o

For the 100k resistor, does that need to be a 2 watter, or can I get away with the 1/2 watt, 5% resistor from Radio Shack?

Regardless, I'm probably still going to make the trip to the electronics store to see if they have a better selection of stuff than Radio Shack. It'd be nice to be able to get some of these things locally. If they have the resistors, I'll probably go ahead and buy them so I can work on the amp this weekend if it's raining.

[sunvalleylaw]

Congrats Prages! I love Bassman amps! Mark, looking forward to your progress with yours too. :rainfro:

[telecaster911]

1/2 should be fine on the 100k. 100k is not critical at all. It's just so there is not a dead short initially...and subsequently a big ass spark:D

[telecaster911]

New Link with updated info

http://hc.bloodyvelvet.com/files/129/Biasing%20a%20Bassman.pdf

[ellengtrgrl]

Good Luck guys! As Mark said, biasing is pretty straightforward. As he also mentioned DON"T USE 2 HANDS inside the amp, when it's powered up. If I'm working with tube gear, I do like the oldtimers used to do - keep one hand in my pocket at all times. BTW, clipping one lead to the chassis works fine as a ground.

I know the first year Ampeg SVT used 6146Bs for power tubes. I wonder if anybody has used them for a guitar amp. A lot of 60s & 70s ham radios that used tubes (like my old Kenwood TS-820), used 6146Bs. With 50 watts plate dissipation, getting 100 watts out of 2, 6146Bs is pretty easy. On a guitar amp - think VERY LOUD!

[telecaster911]

Thanks for checking it out Ellen, I was hoping you would take a peek and double check me :-)

>> BTW, clipping one lead to the chassis works fine as a ground.
I figured if I'm telling someone how to do this, I wanted to make sure those caps were grounded (double redundant)...LOL. Last thing I want is someone to get hurt.

Later Ellen :wave:

[Prages]

I picked up the alligator clips and the 100k resistor on Friday, but no luck finding anything else. I went to the Electronics Specialty Company, but they do nothing but install alarm systems now. Then I went to State Electric, which was actually the one I remember going to before. They are mostly a data-com company now. I asked about resistors and they told me that 10 years ago they'd have anything I needed. :mad:

[Prages]

Tele,

Just curious here.

For the bank on the left, you've got the chassis ground clip on the positive side of the cap.

For the bank on the right, it looks like the positive side is up on the first cap, and the positive side is down on the second (the far right in my pics). Should the chassis ground be on the positive side of each cap?

[Prages]

I just made up the alligator clip with the inline resistor.

First, I fired up the amp, then turned it off, unplugged it, and connected the multimeter, just to see what was happening. It started out at around 12v and was slowly dropping.

Then I put on the resistored alligator clips, and it started dropping dramatically. Within a minute or so, it was down well below 1 volt. Took the resistor off, and the voltage started raising again. As soon as I stuck the alligator clip to the chassis, it went immediately to 0.

Pretty interesting stuff.

[telecaster911]

Put the jumpers on the plus side. I would not have though they would be flopped like that. I sent resistors to you. I matched up 2 pairs of 1 ohms and put a couple of 100k in there. They should be there Monday.

[Prages]

Put the jumpers on the plus side. I would not have though they would be flopped like that. I sent resistors to you. I matched up 2 pairs of 1 ohms and put a couple of 100k in there. They should be there Monday.

Thanks again.

When I was playing around a while ago, I put the chassis grounds on the + side of everything.

I have to admit, the one cap being reversed kind of surprised me too.

[Prages]

But, I see that apparently that is the norm for an AB165 circuit.

http://bwilliamson.home.mchsi.com/filtercp.gif

[telecaster911]

That's why these instructions are for you only. If something doesn't seem right, you'll catch it :thu:

[Prages]

I really can't tell you how much I appreciate all the help you've given me the last few months. You've single-handedly gotten me into a new hobby, and given me a whole new area of GAS. :D

[telecaster911]

Hehe. :D

[ellengtrgrl]

Restoring and working on old amps (and radio gear for that matter) is fun!!! :D It's a cool feeling when you get an old amp working great! I'd LOVE to get my hands on an old fixer upper Magnatone for cheap!

[Prages]

Bassman is biased (I think).

Got everything done today. I couldn't get both tubes to exactly .035v, but I set the hottest one to .035. The other is around .033. Could be the tolerance of the resistor, could be the cheap multimeter, could be not a perfectly matched set of tubes.

Regardless, I played it for about 1/2 hour or so at stage volume, and it sounds great.

Thanks again, Tele. Got a gig on Friday. I'm taking the Bassman, but I'll take my Classic 30 too, just in case.

[telecaster911]

There's always a small mA difference between the tubes, and they will drift apart over time. I usually adjust to the hotter tube, just like you did. The resistors were almost a dead match for each other. If you set it at .035 you should be good to go. If it sounds a little stale when you gig with it, go a little hotter. Look at the plates in the dark for good measure, but I'm sure it's fine.

[Prages]

There's always a small mA difference between the tubes, and they will drift apart over time. I usually adjust to the hotter tube, just like you did. The resistors were almost a dead match for each other. If you set it at .035 you should be good to go. If it sounds a little stale when you gig with it, go a little hotter. Look at the plates in the dark for good measure, but I'm sure it's fine.

It was actually running a little cold at first, around .033 on the hottest tube. I upped it to .035. Sounds good to me though.

And that old solder to the chassis was a bitch and a half to melt. I ended up breaking out the 100w soldering gun to get that stuff to flow. :o