Jess Oliver's legacy - AMPEG PORTAFLEX CLUB part 9

[coreyfyfe]

Originally Posted by JvN

Hmmm... Not a single sound. Amp's External speaker out is 8 ohms, cab is 4. Should I use the fixed speaker out; is that 8 ohms as well? Mine doesn't have an XLR plug, so I'll have to find a adapter for the 1/4" jack first.

The main output has to be plugged in first. That 4 pin jack is wired to the standby circuit so it will not come out of standby without the cab plugged in. The extension speaker connection is the second tap of the output transformer, and is rated for an 8 ohm connection in series with the first (ie a total load of 16 ohms). Fliptops sells a ready made 4-pin to 1/4" adapter if you want to buy one, though I don't know their rules about international shipping. You could likely source the parts and build your own just as easily.

[beans-on-toast]

I would carefully check the speaker out wiring to make sure everything is wired correctly. Normally, the 4-pin connector goes into the B-15 cab. The speaker out is intended for an 8 ohm extension cab. You can't be sure how they wired everything or what mods related to the speaker out they might have made so this needs to be checked out.

As Jimmy said, these amps will work with a 4 ohm cab so that isn't your problem.

Test the amp and B-15 cab on its own. If that works, plug the 810 into the ext speaker jack and see if that works. Both cabs should be working. You normally need to short pin-2 and pin-3 together on the speaker cable that comes out of the amp end for the amp to work. This happens when you plug the cable from the amp into the 4-pin connector on the cab. A mod can be done to undo this and is necessary when the 4-pin connectors are replaced with 1/4" jacks.

[tonynoriega]

Originally Posted by beans-on-toast

Good to hear that it sounds great. Hope that you enjoy it!

There are two cement resistors on the right hand side marked 250 ohm 10W and 1W ohm 10W. Both these resistors can get hot, especially the 250 ohm one. Try not to let the blue 20uF capacitor come in contact with the resistors. It might just be the camera angle, but I would raise the blue cap up a little if it is.

Those original red Astron capacitors are worth their weight in gold. People pay a lot of money for them if they test good. It is always a good idea to get back your old parts.

Thanks for your advice Beans-on-Toast. I had them save all the parts they replaced, including the red capacitors.

Today I'll check on the resistors to make sure they're separated sufficiently from the 20uF capacitor, as you suggested.

[JvN]

Thanks Coreyfyfe and David. If I connect the full B-15 plus the 810, I get awesome results. No distortion way past 1300. Fantastic! Great for small to medium gigs. Can't post the pic, somehow I exceeded my quota. So I definitely need the 4-pin adapter if I wanna run the 810 alone. Fliptops ships abroad, don't worry. I'll order one.

[beans-on-toast]

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.

[coreyfyfe]

Originally Posted by beans-on-toast

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.

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?

[beans-on-toast]

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.

[JimmyM]

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.

[coreyfyfe]

Originally Posted by beans-on-toast

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.

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.

[beans-on-toast]

Originally Posted by JimmyM

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.

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?

[JvN]

Will do David - thanks.

[beans-on-toast]

Originally Posted by coreyfyfe

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.

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.

[coreyfyfe]

Originally Posted by beans-on-toast

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.

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.

[beans-on-toast]

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. 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.

[JimmyM]

Originally Posted by beans-on-toast

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?

He did not. Don't know why but he never brought it or the 4 ohm speaker thing up.

[coreyfyfe]

Originally Posted by beans-on-toast

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. 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?

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...

[beans-on-toast]

Being a klutz, over the years I've managed to poke holes in a couple of cones. Just sayin...

[MAMMOTHvolume]

Originally Posted by beans-on-toast

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?

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.

[beans-on-toast]

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.

[christw]

Originally Posted by beans-on-toast

Being a klutz, over the years I've managed to poke holes in a couple of cones. Just sayin...

Last year I picked up a '72 810 with original speakers and knocked the grill frame through the top left speaker while moving it as soon as I got home.