Jess Oliver lives on through The AMPEG PORTAFLEX CLUB - now up to lucky 7!

[stiles72]

Yeah those look original. As long as they work, I would be sure to hang onto them and pass them along to the next owner should you ever sell your amp. I have a full set of Ampeg branded tubes for my B15 that took me a while to collect, but I don't run them in it anymore since finding Ampeg branded tubes is getting harder and harder.

[dangorange]

Originally Posted by stiles72

Yeah those look original. As long as they work, I would be sure to hang onto them and pass them along to the next owner should you ever sell your amp. I have a full set of Ampeg branded tubes for my B15 that took me a while to collect, but I don't run them in it anymore since finding Ampeg branded tubes is getting harder and harder.

Yeah, this was kind of my thinking when I bought a new set. They sound great, but i know at some point I'll need to replace them, so might as well do it now while they still sound good.

[coreyfyfe]

If you're going to replace with new production tubes, I've used the JJ and Winged C 6L6GCs in the past and had good results with both. Going NOS will definitely be a bit costlier, and IMO if you have a good NOS set I would go try out a newer set and see how they sound in comparison. As far as pre-amp tubes, I've used the sovtek, tung-sol reissue, and several NOS varieties. They've all been pretty close IMO with the tung-sol RI and NOS being my preference. I had some odd-brand NOS tubes that I was given when I bought my first B15 but the ampeg labeled tubes sounded better in that amp. I'm currently using a 6188 (military spec 6SL7) in my V1 slot and like it a lot, and they can usually be had for less than a similar NOS 6SL7 would cost you.

[beans-on-toast]

Originally Posted by dangorange

Hi, sorry if this is a often asked question, I have a 1966 B15N with the 30w fixed bias circuit and want to replace the original tubes that came with the amp, partially to see how fresh tubes would sound. I bought a set from fliptops and am wondering if it's ok to simply pull the old ones and replace without checking voltages etc. I know it's best to have it checkout out when replacing tubes, especially the power tubes but I do not have the necessary equipment on hand and the set I bought has matched pair of 6L6GC.

I can wait if it's a bad idea, though....

If you plug the new tubes in and the amp is not distorted, and the plates look orange as opposed to red, you should be fine.

It is true that Ampeg designed these amps to be plug and play, even the fixed bias ones. Back then, the tubes were consistent and they could be installed without modification. Of course, the cathode biased amps are self biasing so they are plug and play. The way to tell if your amp is cathode or fixed biased is to check what is connected to pin-8 on the 6L6GC power tube sockets. If it is grounded, your amp is fixed bias. You will also see a single diode which is part of the bias circuit. If there is a 250 ohm 10W resistor and a parallel 50uF capacitor connected to pin-8, it is cathode biased.

There isn't a bias pot inside the amp so you would have to remove a resistor and solder in a new one to change the bias. Sometimes people mod the amp to add a bias pot to make life a little easier.

When the amp was built, Ampeg assumed that the wall voltage was 117 VAC and the optimal cathode current was attained with a bias voltage of -50 VDC. The bias voltage delivered inside your amp depends on your wall voltage. If your voltage is 120-125 VAC, as is typical today, the required bias voltage for optimal operation will change slightly. A higher wall voltage will also mean that the voltage on the power tube plates will be higher. This changes the optimal cathode current.

For your amp and a given plate voltage, which you measure, you can determine what the optimal cathode current should be using this calculator. At the very top of the page you select class AB, tube type 6L6GC, then enter the measured plate voltage measured with the tube plugged in. The calculator will give you the optimal cathode current. With a wall voltage of 117 VAC and a healthy set of power supply capacitors, the plate voltage should be around 450 VDC, the bias calculator tells you that the cathode current should be about 47mA. This cathode current is adjusted by changing the bias voltage. A more negative voltage, say -55 VDC will give you less cathode current than a more positive bias voltage, say -45 VDC. The higher the cathode current, the hotter the tube is running. Too hot, and the tube will burn up because it simply can't dissipate the heat. Too cold and the amp will sound lifeless. Just right and the amp sings. But just right is a range of cathode currents. This is why, to some extent, you can plug in a new set of tubes and the amp will work without the need to re-bias. Optimizing the bias is like setting the amp to work at the peak of a bell curve. Either side of the peak, the amp is still going to work within spec. Go too far on either side of the peak and operation is too hot or too cold. The tube specs and plate voltage dictate what is too hot or too cold.

It never hurts to have your friend check out the bias if you want it optimized. The bias probe that you mentioned is the best way to go because it measures (indirectly) the actual idle current flowing through the tube. This allows you to select the bias voltage that provides optimal current flow.

Setting the bias isn't difficult if take care. That's how you learn. Even though you've done it before on your own, take the time and have your friend show you how to check out the amp.

Also, those old tubes could still have a lot of life in them. Don't be too quick to replace them. If they can be biased to spec, they are still good.


David

[beans-on-toast]

When Ampeg built these amps they never would have guessed that these amps would still be in service 50 years later. It is pretty amazing and a testament to how well they were designed and built.

At the same time who would have imagined that 50 years later NOS tubes would still be available. We are really lucky, companies today don't stockpile inventory like they used to.

[christw]

Consumer electronics have continued in the direction of plug, play, and throw away for a while now. I think that's rather unfortunate but what can you do? I still like carburetors and tubes because I can service them myself to a certain extent. I guess I'm more "old school" than most in my generation though. SMPS what now?

dangorange, I was going to tell you to hang onto the originals but I was already beat to it.

[dangorange]

Thanks very much for the info David, I will be showing my guitarist your post and I'll try to hang out with him and work on the amp to learn a few new things. Very informative and great link too!

[MoosBros]

I have had some of those.. the one I miss the most was a B12XT 2x12 guitar combo. Sold it long before I realized how valuable it was...The vintage B15N was sold, but replaced with the B15R, and after years of searching, finally got the matching extension cab too...The VT-22s got to be too heavy to carry and the B-25B I shoulda kept too.. Ohh well..I always use the little SWR Baby Blues for gigs the last 15 years tho with a 2x10 or 1x15... sometimes with the SVT 808 cab if I gotta be loud......I'm old, and lazy...nice place here...

[P Town]

Originally Posted by MoosBros

I have had some of those.. the one I miss the most was a B12XT 2x12 guitar combo. Sold it long before I realized how valuable it was...The vintage B15N was sold, but replaced with the B15R, and after years of searching, finally got the matching extension cab too...The VT-22s got to be too heavy to carry and the B-25B I shoulda kept too.. Ohh well..I always use the little SWR Baby Blues for gigs the last 15 years tho with a 2x10 or 1x15... sometimes with the SVT 808 cab if I gotta be loud......I'm old, and lazy...nice place here...

Interesting that you mention the B12XT. I recently bought a B12XTC, (which I tried hard to talk myself out of). This is a guitar amp, with four twelve inch Jensen C12Ns. I will share some pictures after I have some time to see what I have here. At low volume, the guitar speakers are a little brighter sounding than I prefer, but they surprised me with some good tone, and volume without complaint. Once I connect a real bass speaker to it, I will find out if I made a usefull purchase.

[Vintage-Blue]

Originally Posted by John D

It looks like a good match to me, but we'll see how it turns out when Mark gets his order. The good thing is that it's reasonably priced for a one-off DIY job. Mark will probably want to find a bulk source, but that shouldn't prove too difficult, if this is the right product.

I just got my sample pieces today. I can't imagine finding anything else that is as close of a match as this imitation leather black levant. To my eye it's the same pattern. The thickness is right. It even tears the same. I'm curious to see how it does doing some tight bends and getting glued down. I'll be out of town for a couple of days but I'll try to do a quick test this weekend.

I'm glad you saw this stuff John - I completely missed it when I visited their site!

[beans-on-toast]

Sounds great Mark. I was wondering if the water based contact cement would be too thick to use with such a thin material with the tight bends and if some other glue, like 3M90 spray or Titebond II would be better.

Also it would be interesting to test a small strip for flamibility. It is nitrocellulose based.

[Bass 45]

That material is a good find. I will be getting some for a B18X restore that I am working on.

Also, it seems to be the same material on my Ampeg bass cases from the '60's.

I was thinking of using 3M also. Tolex glue would probably turn it to gum.

[John D]

Originally Posted by Vintage-Blue

I just got my sample pieces today. I can't imagine finding anything else that is as close of a match as this imitation leather black levant. To my eye it's the same pattern. The thickness is right. It even tears the same. I'm curious to see how it does doing some tight bends and getting glued down. I'll be out of town for a couple of days but I'll try to do a quick test this weekend.

I'm glad you saw this stuff John - I completely missed it when I visited their site!

That sounds good, Mark. I hope it works out for you guys. Just thinking, though. Wouldn't the same company have the proper glue? By the way, last night, I was looking at a Bible my daughter gave me, and I'm fairly sure it's covered with this material. If it is, you won't need to worry about making bends. The stuff on the Bible has three 180 deg. bends where they wrapped it around a thicker substrate with no problems. Will be looking forward to pictures of your tests.

John

[John D]

Just looked on Hollander's site again. It appears that they use PVA for all their adhesive applications.

[beans-on-toast]

The price of this product is very reasonable so that's a big plus.

They say that their PVA dries clear. I think that Titebond which is also PVA has a slight yellowish tinge. I sometimes use a white PVA product called Weldbond to glue felt to wood and metal. It dries clear and flexible. This stuff seems to glue anything. The only problem with this type of glue is that it needs to be held down till the glue dries. A 3M spray might be easier to work with.

[stiles72]

Originally Posted by Vintage-Blue

I'm glad you saw this stuff John - I completely missed it when I visited their site!

Now John, if you could just find some Navy Random Flair!

[FenderAmpeg]

Originally Posted by stiles72

Now John, if you could just find some Navy Random Flair!

Found some.....already in use

[John D]

Originally Posted by FenderAmpeg

Found some.....already in use

I just turned green.

[hennessybass]

Originally Posted by John D

I just turned green.

Ya. That's pretty bad a**.

[Ric Vice]

Originally Posted by FenderAmpeg

Found some.....already in use

Tom,
When you're using a extension cab with the B-15 and those older Octal connectors, did you make a special cord that goes from 1/4" phone plug to and Octal connector? I can't tell what's connecting them to the head as they are blocked by the cabinet. Just wondering.