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[Ampeg B15R] Following Jess Oliver's advice...

#81
JimmyM said:
I'm not a huge high end fan but I like more than 3k. 4k at the least.
Click to expand...
Feel the same way.... don't really care for treble-y bass tones (Jamerson/Dunn/McCartney/etc. are my templates). But a 3K roll-off is too low. You lose the sound of the pick articulating the note.

JimmyM said:
So will that be it for mods or are you going full-on Fliptops?
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Well, we subscribe to the theory of continuous improvement around the ol' studio here :)! But seriously, I've got a lot of gear to play nurse maid to. I can't play favorites... just finished doing a big 'round of improvements to the drum set (cymbals/snares mostly). And then there's the studio piano, which I am always upgrading when funds allow (it's an 1890s upright, so it'll be a money pit, er... work-in-progress for as long as I own it).

As far as the B15r goes - I'm going to do the speaker next week (Beta 15a), then swap the tubes (JJ E34L's, and maybe try something different in the 12AX7 spot). I'm fairly certain I'll eventually buy Bruce's ported grill. It's just too tempting. But I'm going to wait & let someone else be the first guinea pig on that one. :p
 
#83
Wait a sec...doesn't the crossover still shut the woofer down at 3k when the tweet's turned off? Has on every other Ampeg cab I've used with one, although most shut it down at 4k.
 
#84
Triboost said:
As far as the B15r goes - I'm going to do the speaker next week (Beta 15a), then swap the tubes (JJ E34L's, and maybe try something different in the 12AX7 spot). I'm fairly certain I'll eventually buy Bruce's ported grill. It's just too tempting. But I'm going to wait & let someone else be the first guinea pig on that one. :p
Click to expand...

What's the word, Triboost? Did you try the new speaker yet?
 
#85
JimmyM said:
Wait a sec...doesn't the crossover still shut the woofer down at 3k when the tweet's turned off? Has on every other Ampeg cab I've used with one, although most shut it down at 4k.
Click to expand...

I missed this one Jimmy.

The crossover, which is acting as a high pass filter, is only wired into the horn. In the off position, it opens the circuit and takes the high end driver out. You just have the woofer.

The woofer is connected directly to the amps output and makes a circuit to ground. It is wired separately from the other driver and the crossover isn't in that circuit. So the high end rolloff of the woofer is coming from the design of the driver.


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#87
Beans, thanks tons for the info!

But, now, I don't know what to believe! How do I reconcile the schematic you posted against what Triboost reported?

Is the schematic you posted one that you put together by inspecting the crossover? If you got the schematic from somewhere, is there any chance that it's wrong?

Triboost said:
Thanks!

I just finished testing the cab using the stock speaker. Tried out a '72 Fender Tele Bass and an early '70s Gibson SG bass. Big improvement! Now I can hear the mids! That ultra-low problem is tamed too, which was the big issue I've been trying to solve.

I can even dial in a bunch of treble (via the B15r head) if I so choose, and then I'm actually getting more "picky" Rickenbacker-like tones. This has never happened before!

I think I know what happened with my particular rig. I purchased it used, and the original seller told he had disconnected the tweeter. I was aware, even back then, that this was a common mod to this amp. However, I did not know it was done incorrectly and the crossover circuitry was still in place.

So now, for the first time, I'm hearing the stock speaker without that big 3000hz low-pass roll-off (it is 3000hz, isn't it?). I recommend trying this, if you haven't already. Sounds great... can't wait to see where a tube swap & new speaker gets me next.
Click to expand...
 
#88
Stu, that's the crossover schematic from SLM that is used in the B15R.

All I can say is that when a used amp or cab is purchased, there is always the possibility that it has been modified. You can't be sure what, if anything, was done without examining it. Maybe there is more to the story concerning the mod to Triboost's cab. As I mentioned, the off position removes the tweeter so I'm wondering what mod was done unless it did the same thing that the switch does.


David
 
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#89
Did not know this, David. Always thought that once the tweet kicked in, it would cut everything else off. Interesting...I now have a little science experiment on my gig today, as I'll probably get a 410he supplied instead of the 810e I keep asking for on this gig ;)
 
#90
It is doing what you are hearing, just not in the way that you think it is.

The woofer does diminish because it can't reproduce high end so above its spec it rolls off. So with the high pass filter on the horn letting it kick in for higher frequencies coupled with the woofer rolling off you have a balance. At the high end it will sound like the woofer isn't there. Play low notes and you hear it through the woofer, play high notes and the horn predominates.

High frequencies carry so it doesn't take much to make it sound like they predominate. If you unhooked the woofer, the sound would get thin. Although the woofer rolls off it is still there. Like the megaphone vocal in Big Sugar's Diggin' a Hole ([Invalid or Expired Link Removed]).
 
#91
Guys, I'm not sure why my 15r sounds better - but I swear it does! Not a subtle change. Definitely better. If that schematic beans posted applies to my cab, maybe my crossover was just broken? I doubt it was pre-modded in any way. But if I recall correctly, my crossover's rotary switch never seemed to do anything to the sound. So, dunno. It could be that my rotary was broken in such a way that the crossover was permanently turned on.

Stu, I have since installed the Beta 15A, and it does sound good - but I can't say it sounds better/worse than before with the stock speaker. Just good, as always with this amp. Next up is the tube swap... will post back when that's done.
 
#92
I believe you! Sometimes happy accidents occur and the results are quite surprising.

Your switch should have had a noticeable effect on the sound as you rotated it so something is up for sure. A tech could examine the circuit. There are a number of possibilities. If the light bulb is burned out, the crossover will not work. There could be something wired wrong. Maybe a component is out of spec. As you said, the switch may not be working properly. If you ever have a tech look at it, here is the layout of the B-15R crossover circuit that I posted earlier.

 
#93
I hear you Beans! And thanks for posting all the schematics.

Like I said, this thread is becoming a great resource for these 15R's... I'm printing these and storing them with the crossover in case I ever try to use it again (not likely, but you never know). I'm guessing a component is blown and/or the switch is malfunctioning.
 
#94
beans-on-toast said:
I believe you! Sometimes happy accidents occur and the results are quite surprising.

Your switch should have had a noticeable effect on the sound as you rotated it so something is up for sure. A tech could examine the circuit. There are a number of possibilities. If the light bulb is burned out, the crossover will not work. There could be something wired wrong. Maybe a component is out of spec. As you said, the switch may not be working properly. If you ever have a tech look at it, here is the layout of the B-15R crossover circuit that I posted earlier.

Click to expand...

But, Beans if I understood your earlier post, and the schematic, correctly, isn't it the case that the woofer gets a full range signal no matter what the position of the horn knob is? So, the only way the crossover could be broken that would affect the sound coming from the woofer would be for the woofer to get no signal at all?

Since Triboost reported more audio content coming out after removing the crossover (and horn), I'm still confused about how that could be explained by a broken crossover.

I'm in Rawlins, Wyoming, tonight in the middle of moving across the country, so it will still be a while yet before I can experiment with my own 15R.
 
#95
You're right if it is wired as per the schematic, the woofer doest have the crossover in the circuit. Since Triboost's circuit wasn't working the way it should have, there is always the possibility that someone rewired it, different from the schematic, in a way that put the woofer in the crossover circuit. That would be an explanation as to why it sounds better now.
 
#96
Stupid Question: I have been researching and wanted to verify the EL34/6L6 switch and the 4ohm/8ohm switch. There's the block and the gap. I assumed the "block" portion was the indicator but in the manual it looks like (if the block is the indicator) the it is set to EL34 and 4ohm - neither which are stock settings. Most pictures I wave seen shows the block side indicating the EL34 bias and it seems unlikely that the majority of people switched out the stock tubes.... anyway.
 
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#97
punchclock said:
Stupid Question: I have been researching and wanted to verify the EL34/6L6 switch and the 4ohm/8ohm switch. There's the block and the gap. I assumed the "block" portion was the indicator but in the manual it looks like (if the block is the indicator) the it is set to EL34 and 4ohm - neither which are stock settings. Most pictures I wave seen shows the block side indicating the EL34 bias and it seems unlikely that the majority of people switched out the stock tubes.... anyway.
Click to expand...

I just looked at my B15R -- the void should appear near the text indicating your intended selection - NOT the block portion of the switch.

Here is mine pictured - set for 6L6 tubes and 8 ohm speaker:

IMG_3103.JPG
 
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#99
This type of switch is meant to be actuated by a flat blade screwdriver. Insert the blade in the gap, and move it to your desired position.

And since this thread has been ressurected:
Could someone give me the dimensions of the transformer cans? I have an empty B-15R chassis that I'd like to built up.
The transformers/cans are unobtainable, so I'll have to make them myself.

punchclock said:
Stupid Question: I have been researching and wanted to verify the EL34/6L6 switch and the 4ohm/8ohm switch. There's the block and the gap. I assumed the "block" portion was the indicator but in the manual it looks like (if the block is the indicator) the it is set to EL34 and 4ohm - neither which are stock settings. Most pictures I wave seen shows the block side indicating the EL34 bias and it seems unlikely that the majority of people switched out the stock tubes.... anyway.
Click to expand...
 
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#100
Are you sure about the switch settings Rich?

Convention is, the block indicates the setting. In this case, the switches slid to right would be 6L6 and 8 ohms. The problem with these sitches is that the fasteners can be removed and the switch turned around. That would reverse the chassis labeling.

The proper setting can be confirmed with a volt meter. The DC bias voltage can be measured between R55 and R56 relative to the chassis. Set the meter for VDC, red lead at the junction of the resistors, black lead on the chassis. In the 6L6 setting the bias voltage should be around -50 volts, in the EL34 position the bias voltage is around -40 volts. The amp should be in playing mode, standby off. If you want to take the readings without the speaker cabinet connected, remove the power tubes first. Before removing them, label them to keep track of their socket location.

The schematic is in the TB Portaflex wiki, schematic section: http://www.talkbass.com/attachments/b15r-schematic-26701h4-pdf.767853/

Here is the manual: http://www.ampeg.com/pdf/B-15R.PDF.


Cc: @Bass 45
 
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