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Official Ampeg Portaflex Club

#1,321
Just scored an SB-12....

and a Motion Sound Pro 3T....

and a sweet Tokai Tele... (and four cowbells)

for $450!

I'm thinking of eBaying the motion sound (even though I've got keyboards that would love the thing) and basically getting the guitar and ampeg for free!

I played the amp for a few hours, and noticed that the speaker will probably need replacing. There's that telltale buzzzzzz in the note as it sustains-- happened only after it warmed up. Coils are rubbing I guess. I did push it a little to see what it would do, but nothing crazy. Who knows, maybe I did it :(.

Replacement speaker ideas? Or can the original be serviced? Thanks.

also, is this head cool w/ 4ohms?
 
#1,323
This is tough one...reconing speakers got a lot more expensive than it used to be, almost to the point where it's not worth it. If it's a JBL, Altec, EV, or CTS alnico, I would seriously consider it, otherwise I'd probably just get an Eminence Beta 12 like Bruce at Fliptops recommends.

Portaflexes are fine to run at 4 ohms, but from what I was told, you can only tap the output transformer properly for 4 ohm operation if you have two 8 ohm speakers plugged into each speaker out. Still, Jess Oliver says that it's fine to run a 4 ohm speaker in it and will sometimes recone people's 8 ohm speaker with a 4 ohm. But it does sound tighter and less tubey. I've done it with an 810 and it kills, but it does sound tighter.
 
#1,324
I was more wondering if it would be safe to hook my genz benz and GK 4 ohm cabs up to it.

Sounds like it needs to be specifically wired for such operation from the transformer...

I do think that emminence 12 makes sense. I don't need the extra authentic jensen. I'd rather have something I can crank without fear and have most of the vintage sound intact.
 
#1,325
You definitely can't crank the Jensen. It'll be safe to use the 4 ohm cabs for a little while. It's not something that takes out your whole amp at once. If anything, it's very gradual and seems to affect tubes more than the tranny. However, I wouldn't run it balls out at 4 ohms.
 
#1,326
Thanks everyone for the tube info. I've only tried new tubes, as I've said, but mostly because the cost of building this thing has shifted my money focus into getting it running and having the circuit sound it's best.

marsbel- I have only tried two cabs at this point, unfortunately both guitar cabs (ampeg jet 1x12 cab and fender closed back 2x12)... not exactly the best set-up for the b-15. I'm really looking forward to hearing it out of true double-baffle. I think I'm gonna harass the guy at my local music shop to let me plug her into a 70's b-15 cab he has so I can finally hear what you've all been talking about.

vintageblue- thanks for the support and the awesome work you do for everyone... i hope my cab will live at least sound as good as yours if it doesn't look it.

jnewmark- i have worked out the preamp schematic and parts list... even with a solid-state rectifier to keep down cost it would be around $150 in parts alone (excellent parts, but who would want less than excellent parts??) and that would be without a chassis box to put it in (which would add another $15-$50). I'm interested in building one personally, so when i finally do, I can send it to you to see what you think and what it might be worth.

-Jeff
 
#1,327
A little housekeeping:

#107 jeefberky
#108 bassgasser
#109 Washishimi
#110 Standalone

Jeef, despite it not being a "real" portaflex, I think what you're doing definitely needs to be recognized formally...

:bassist:
 
#1,328
jeefberky said:
Since there aren't any composite tone units around anymore, I had to build my own from individual components. I found that the Piazza schematics list 47pf and 470pf capacitors for the treble side of the stack on some models, but the real components are .00047 uF (500 pf) and .0047 uF (5000 pf), which you'll notice on the real schematic floating around of the B-15NB. If you use the 47 pf value, the treble knob is essentially useless. It will only cut and boost frequencies above guitar range (a great hiss reducer, but not a great treble knob!) However, this simple error did give some of the deepest bass I've ever heard , so it may be a potential mod if you dare sacrificing one of your channels and your speakers can handle it.
Click to expand...

There are a number of interesting things that you can try with the tone circuit. You can add a DPDT switch to bypass the tone circuit. By removing the tone circuit, there is no signal loss and no coloration of the tone. As you found, you can change the values of the components in the tone circuit. For example, if you want a mid boost you can add a switch in series with a capacitor and place this in parallel to the .00047 cap. Add a switch and series resistor in parallel to the 120K resistor in the tone circuit and this will shift the centre frequency. Select values that sound best to your ears.

You can try a tone stack simulator for the James circuit to get a rough idea of how changing component values will affect the circuit (http://www.duncanamps.com/tsc/index.html).

The dropping resistors on the inputs seem to have little impact on the overall sound. I tried a 100K, a 120K, a 47K, and no resistor, with little noticeable difference except in the case of no resistor, which was really present sounding, if that makes sense, but also picked up radio signals.
Click to expand...

Changing the value of these resistors will attenuate the input signal by a small amount. In theory, IIRC on the early B15 models, the first input on channel 1 attenuates about 11%, the second input about 2%. Hard to notice a difference. On the third input jack with no resistor, I find that a 47K with short leads connected at the tube socket, pin-1 grid input of the 6SL7, does a good job of attenuating radio signals.

For fun, I tried removing the 10K feedback resistor coming off of the 16 Ohm output transformer secondary to see if I could get a little more volume out of it. I did, but the trade-off was more distortion, less presence, and less clarity. It went back in.
Click to expand...

You could try a pot instead of the 10K feedback resistor to find a value that sounds best to you. Try a slightly smaller resistor like 8.2K.

What's your favorite 6L6 for a B-15?
Click to expand...

For a B15, I prefer NOS.

David
 
#1,329
Hey all, I posted a thread tonight about comparing tone settings with my 69 SVT and 64 B-15N to an amp with near flat response when the controls are at noon (Markbass Little Mark II). So just to get it on the record in here, I thought I would post the results here as well. I plugged both heads into my Vintage Blue double baffle cab and tried to find the near flat setting for both channels of the B-15N. The results kind of blew my mind and proved to me that my hearing isn't quite as calibrated to different settings as I thought it was.

To match the tone of the LMII set flat as close as I could, here are the knob settings for each B-15N channel:

Ch. 1: Treble 11:30, bass 11:00

Ch. 2: Treble 9:30, bass 2:00

At least that's how my slightly burned out ears perceived it. So to the best of my knowledge, and mind you this is far from scientific, channel one has a slight scoop to it. I knew it had a midbass bump but I didn't know the treble was a little brighter (I prefer the sound of it that way, though), and channel 2 is a bright channel, which blew my mind because it always sounded to me like channel 2 was a near flat channel. I never perceived it to be so different when I listened to each head on separate occasions. I was, however, able to match the near flat sound in both channels pretty easily.

So I guess that means I'm not superhuman and I have no idea what I'm hearing ;) Kind of sucks to have it slap you in the face, though. Oh well, I'll get over it.
 
#1,330
JimmyM said:
Still, Jess Oliver says that it's fine to run a 4 ohm speaker in it and will sometimes recone people's 8 ohm speaker with a 4 ohm. But it does sound tighter and less tubey. I've done it with an 810 and it kills, but it does sound tighter.
Click to expand...

An output transformer serves to match the impedance of the output tubes to the impedance of the speaker. It acts as an impedance multiplier. When you connect a 4-ohm speaker to on 8-ohm secondary tap, you decrease the input by a proportional amount. I believe that the impedance spec on the later model B15's (OT-214) was primary 6.5K, secondary 8 and 16 ohms. So this means that if you connect a 4-ohm speaker to the 8-ohm tap, the tubes are seeing a 3.25K:4 transformer instead of a 6.5K:8. Not an optimal impedance match. It will be hard on the tubes, and it would add some distortion.

Some amps can take this, others can't. Parts can blow. It depends on the specs of the transformers and the tubes that you are using. If Jess says it's OK, I believe him.


David
 
#1,331
JimmyM said:
Hey all, I posted a thread tonight about comparing tone settings with my 69 SVT and 64 B-15N to an amp with near flat response when the controls are at noon (Markbass Little Mark II).

To match the tone of the LMII set flat as close as I could, here are the knob settings for each B-15N channel:

Ch. 1: Treble 11:30, bass 11:00

Ch. 2: Treble 9:30, bass 2:00
Click to expand...

Very interesting results. Two things would be fun to try.

1) Do a blind test. Don't look at the amp and have someone else adjust the tone controls and see if you can repeatedly pick the same settings for a flat response. Your ears can play tricks on your auditory tonal acuity if your eyes are involved.

2) Include a newer head like an SVT-VR in the experiment as well. The components within the old encapsulated PEC tone circuit modules sometimes drift with time. Not to mention variations in actual component values. As a result, there will be tone differences from one amp to the next for the same settings. Repeating the experiment with a newer amp would tell you if there are any differences in the settings. Knowing the amount of variation in settings and tonal range would be interesting.


David
 
#1,332
1. Good idea. Our guitarist is real cued into tone, way more than me, so maybe I'll get him to do it sometime in the NF.

2. This year's SVT budget got eaten up by a grandson and two kids in AC school, so I'm afraid the VR test will have to wait ;) However, I've played a couple VR's, and despite it being a near-total redesign, sort of, I'm hard pressed to tell the difference on a practical level between a VR and my Blue Line. So I would have to think the results I got with it would be similar. I'm sure it wouldn't be exact, but definitely in the ballpark.

Then again, I botched the freq response of the second channels of these amps so badly that how the hell would I know? :hmm:
 
#1,334
beans-on-toast said:
Congratulations Grandad! Maybe Dino can get Ampeg to send you one for evaluation. Sounds reasonable to me :).
Click to expand...
I think it's even more reasonable if they build a new B-15NC just like mine and send it to me for evaluation along with the VR. And then maybe Markbass will send me a Classic 300 to make it totally fair.

I agree...sounds reasonable to me too, David ;)
 
#1,335
i get to request a number to the coolest club on talkbass!!! interesting story about this one. its a 66 b-15n if i was told correctly. bought it from a store online for $1000. had to have it shipped from connecticut to texas. i was so nervous about it being shipped but there was no other option.

so at about 11:00 today (i called in sick!) there's someone playing drums on my front door. i look out the window to see the fedex truck outside, so i went to the door and this really thuggish delivery guy was standing there with no package. i asked him if he needed any help with it, and he said no. but then he started talking about how "bad ass" my halloween welcome mat was (it has a skull on it and says "stay out") then he was saying he was going to come back later and steal it! :scowl: to which i laughed. he said he really likes skulls and stuff and if he didnt have to grow his hair out to go to court hed show me his spider tattoo on the back of his head. then he goes and gets the amp out of the truck CARRYING IT OVERHEAD! he comes up to my porch, lowers it to his knees, AND DROPS IT THE REST OF THE WAY!!! :mad:

i get it inside and open it. it was packed really well, but the head had separated from the body a little (one latch is missing). so i set it up and plug it in and its DOA! :bawl: so i unplug it and look at the back and see that the fuse holder is jacked up. the plastic near the cap was broken off (i found it in the box later), the fuse was broken, and the whole assembly was loose from the chassis. i opened the head to see that one of the wire had also been pulled loose from the soldering on the fuse holder. so i ordered a replacement from fliptops for $4. :hmm:

29ntn3a.jpg


23tgwpd.jpg


and a pic of the destroyed fuse holder:

29xt89z.jpg


all in all, it could have been much worse, but its still a huge bummer. do you guys think i should try to solder it back myself or take it to a tech when i get the replacement?
 
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#1,338
Standalone said:
by the way, no fair taking it away from the great state of Connecticut!
Click to expand...

ha, sorry. if i had the choice, i would have moved there instead. i love that area. theres a great shop in new haven i believe called gguitars.

JimmyM said:
Sorry bout your mishap. Not a biggie. Jess Oliver says it happens to all of them so he installs a flush mount fuse holder in the ones he restores.
Click to expand...

yeah, now that im over the initial shock, im just glad its a simple fix. i should have her up and running in no time! :bassist:
 

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