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Official Ampeg Portaflex Club

#8,002
beans-on-toast said:
I'm always amazed how well small amps can drive big cabinets.

When you order the 4-pin to 1/4" adapter, verify that they short pin-2 to pin-3 on the connector. I assume that they do but it wouldn't be a bad idea to check.
Click to expand...

They do, I have one of the adapters. Now you've got me wondering how well my B15 would do with the 8x10, either alone or in combination.

David - could you speculate on how much (potential) stress this could put on the transformer? Would a 12/16 mismatch be better than a 4/8?
 
#8,003
12 is closer to 16 than 4 is to 8 so the former presents less stress. Measure your 810 with an ohm-meter and see what the speakers average. A 4 ohm speaker might measure 3.2 ohms. A 4 ohm cab could be a little different. If the ohms, what they call DC resistance, is the same, there is no difference in terms of stress if you connect a 4 ohm speaker vs a 4 ohm cabinet.

A lot of people run 4 ohm cabs with their B-15 without any problems. It won't hurt to try it to see how you like the sound.
 
#8,005
beans-on-toast said:
12 is closer to 16 than 4 is to 8 so the former presents less stress. Measure your 810 with an ohm-meter and see what the speakers average. A 4 ohm speaker might measure 3.2 ohms. A 4 ohm cab could be a little different. If the ohms, what they call DC resistance, is the same, there is no difference in terms of stress if you connect a 4 ohm speaker vs a 4 ohm cabinet.

A lot of people run 4 ohm cabs with their B-15 without any problems. It won't hurt to try it to see how you like the sound.
Click to expand...

The last time I measured the impedance on my 8x10 it gave me 5. My 4 ohm B25 cab gives a proper 3.2-4ish reading. Not sure what's up with the 8x10 though. I've done the 9 volt test so I know they're all connected, but haven't pulled them to see if they give me different readings individually.
 
#8,006
JimmyM said:
The one caveat I'd say to running a cab at a lower impedance is the amp does tighten up to a degree. This is why Jess liked it.
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It also changes the performance of the amp a bit. There's no free lunch. I suspect that people came to Jess asking if there was a way to make the amp a little louder. This was one thing that he could do. Of course, it makes the transformer and tubes work a bit harder which is why I don't recommend it. As I've said in the past, on some amps you can do this with no ill effects, on some amps you can't, but there is also a gray area where you can do it but are pushing the amp. I believe that the B-15 falls into this category.

I know that he envisioned the amp as a bit stiffer which is why he lobbied to have the choke added in the HB-15. What I've wondered about is why didn't he offer this as a B-15N mod. Jimmy, when you sent your amp in to him, did he mention adding a choke instead of re-coning the speaker to 4 ohms?
 
#8,008
coreyfyfe said:
The last time I measured the impedance on my 8x10 it gave me 5. My 4 ohm B25 cab gives a proper 3.2-4ish reading. Not sure what's up with the 8x10 though. I've done the 9 volt test so I know they're all connected, but haven't pulled them to see if they give me different readings individually.
Click to expand...

When making decisions like this, it is always good practice to measure the DCR, don't assume. It is so easy to do so why not.

It would be interesting to do a survey and have everyone with an 810 measure their cabs with a meter. Everyone using a different meter would skew the results a bit but it would still be interesting. For instance, is there a difference between a flat back and an 810e. As far as I know, DCR doesn't change with age, although the speakers impedance can change. I'm just wondering if the DCR of the old cabs is the same as the DCR of newer cabs.
 
#8,009
beans-on-toast said:
When making decisions like this, it is always good practice to measure the DCR, don't assume. It is so easy to do so why not.

It would be interesting to do a survey and have everyone with an 810 measure their cabs with a meter. Everyone using a different meter would skew the results a bit but it would still be interesting. For instance, is there a difference between a flat back and an 810e.
Click to expand...

It would be interesting. I've always chalked it up to the cab/speakers being 40 years old and having drifted in spec a bit, but honestly, I don't even know if that's a realistic scenario or if one or more of my speakers are defective in some way and doing damage to my amp. Given the scope of the undertaking I've been going with ignorance is bliss.
 
#8,010
More than likely Corey, your speakers are all wired in parallel. The DCR is probably just a little high. Better to live a blissful life than to put a screwdriver through a cone. :p Also, keep in mind that some ohm meters are not accurate when measuring low resistances. Your meter might say 5, but the actual value could be less. How does your meter do when you measure a one ohm resistor?

Let's consider a few scenarios:

If all the 32 ohm speakers were wired in parallel (32 || 32 || 32 || 32 || 32 || 32 || 32 || 32) it works out to 4 ohms, expect the DCR to be 85% less or around 3.4 ohms.

If you have one pair wired in series and the remaining six in parallel (32 + 32 || 32 || 32 || 32 || 32 || 32 || 32) the calculated impedance works out to 4.9 ohms. The DCR would be about 4.2 ohms.

If two pairs of paralleled speakers were wired in series then four speakers paralleled (32 + 32 || 32 + 32 || 32 || 32 || 32 || 32), the impedance works out to 6.4, the DCR would be around 5.4 ohms.
 
#8,011
beans-on-toast said:
I know that he envisioned the amp as a bit stiffer which is why he lobbied to have the choke added in the HB-15. What I've wondered about is why didn't he offer this as a B-15N mod. Jimmy, when you send your amp in to him, did he mention adding a choke instead of re-coning the speaker to 4 ohms?
Click to expand...
He did not. Don't know why but he never brought it or the 4 ohm speaker thing up.
 
#8,012
beans-on-toast said:
More than likely Corey, your speakers are all wired in parallel. The DCR is probably just a little high. Better to live a blissful life than to put a screwdriver through a cone. :p Also, keep in mind that some ohm meters are not accurate when measuring low resistances. Your meter might say 5, but the actual value could be less. How does your meter do when you measure a one ohm resistor?
Click to expand...

Spot on with a 1 ohm resistor. Other cabs and speakers I've measured all show the appropriate/expected dc resistance so maybe it is time to check. Gotta go dig out that clutch head driver...
 
#8,014
beans-on-toast said:
It also changes the performance of the amp a bit. There's no free lunch. I suspect that people came to Jess asking if there was a way to make the amp a little louder. This was one thing that he could do. Of course, it makes the transformer and tubes work a bit harder which is why I don't recommend it. As I've said in the past, on some amps you can do this with no ill effects, on some amps you can't, but there is also a gray area where you can do it but are pushing the amp. I believe that the B-15 falls into this category.

I know that he envisioned the amp as a bit stiffer which is why he lobbied to have the choke added in the HB-15. What I've wondered about is why didn't he offer this as a B-15N mod. Jimmy, when you sent your amp in to him, did he mention adding a choke instead of re-coning the speaker to 4 ohms?
Click to expand...

running the wrong impedance = less power out of the amp. I don't think a choke would help that much in an amp this size, a dropping resistor works just fine! If you wanna stiffen up the HT bigger caps and faster diodes always help.
 
#8,015
They did add the choke in the Heritage B-15. The designers wanted a stiffer power supply. It would be a simple mod to bypass it if someone who had that model didn't want this feature. They also supplemented this by boosting the capacitance a bit. I like the boosted capacitance. Not sure if I would want the choke.

I think that the design intent was to provide the vintage tone but to help the amps performance a bit by delaying the onset of distortion and adding a bit more headroom thus getting a little more out of the amp.
 
#8,017
I totally didn't need this cab, but it was just so damn clean!

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