B15- change or leave parts

[pbman1953]

I just picked up a mid 70's B15N. Original owner with no work done. Out the gate its really quiet and punchy. Because of the age I decide to test the parts.


The tubes tested well


Parts tested-


The original paper can cap is a 40/40/40/500v-


Measurements-


40 >  55uf


40>   62


40>   92




30/600v-  92


2- 10Uf/100v-   17 & 19 uf


There's 1 - 25uf /25v, that shows "OL" on the meter.


Carbon comp resistors-


2- 22k  > 21k


1- 100k >  105k




Bias-


Tube A- 508 volts / Cathode Current @ 61.7 =  31.4 watts


Tube B- 476 volts/ Cathode Current @57.8 =  27.6 watts


From reading this is common for a B15, but seems high for a 6LC




Questions on replacements-


Can cap- change due to being out spec?


Same  for the 30/600v cap & 2- 10uf caps?


For the carbon comps- they're in spec but old- change anyway, since I have it open?


For the 25/25v- why would it be un-read- bad part?




Thanks

[sluckey]

For the 25/25v- why would it be un-read- bad part?

yes

[pbman1953]

For the 25/25v- why would it be un-read- bad part?

yes

Thanks, what about the bias? Should I get a different value bias cap to bring it down some?

[sluckey]

Changing a bias cap won't bring anything down.

Based on your statement "Out the gate its really quiet and punchy.", I would not change anything except that 25µF/25V cap that tested open. BTW, you cannot test that cap in circuit because of the 1K resistor that's parallel to the cap.

I would just turn the amp on, play it occasionally, and closely monitor for signs of smoke, bad noises, hum, etc. Mainly keep an eye on the filter caps and touch them to see if any are getting warm or hot. The filter caps should operate at room temperature. Check the B+ voltages periodically to see if they are staying about the same.

[sluckey]

Oh yeah, the bias is too high. I'd feel better if they were idling at about 20W. What voltage do you have on each bias cap and also pin 5 of each 6L6? Expecting you to post four numbers.

[pbman1953]

Oh yeah, the bias is too high. I'd feel better if they were idling at about 20W. What voltage do you have on each bias cap and also pin 5 of each 6L6? Expecting you to post four numbers.

Pin 5's- 49.4 each


caps-  8.1 each- negative leg

[sluckey]

It's impossible to have -49v at pin 5 but only -8v on the caps!

However, you need to increase the voltage on pin 5 to run the tubes cooler. Probably -52v to -55v would be good. I'll tell you how to do that when you tell me what the real voltage is on each of those bias caps.   

[pbman1953]

It's impossible to have -49v at pin 5 but only -8v on the caps!

However, you need to increase the voltage on pin 5 to run the tubes cooler. Probably -52v to -55v would be good. I'll tell you how to do that when you tell me what the real voltage is on each of those bias caps.   

Maybe it's the meter I'm using. I went back to my old school meter. I bought a new Klein, but not sure if I'm keeping it.


I'm getting -046 in all 4 spots

[sluckey]

Which Klein? I have a MM700 and love it.

[pbman1953]

Which Klein? I have a MM700 and love it.

MM450

[sluckey]

The MM450 is a respectable meter that should have no problems reading -50VDC. Surely you read page 10 of the instruction manual???

[pbman1953]

The MM450 is a respectable meter that should have no problems reading -50VDC. Surely you read page 10 of the instruction manual???

I didn't but that's exactly how I have the leads and use the Sel button. when needed. I tried again with the 450 and got - 46 on all 4 point's


Thanks!

[pbman1953]

Is the voltage correct?

[PRR]

Measure at C19 to avoid voltage drop due to meter loading.

[sluckey]

Is the voltage correct?

No. It's too low.

[pbman1953]

Measure at C19 to avoid voltage drop due to meter loading.

c19 measures -50

[pbman1953]

I suppose I could install a bias pot on the back side. Would I need a staked pot, to replace the 2- 10uf's?

[sluckey]

Here's my suggestion for a bias mod. Go ahead and replace C15 and C19 just so you will have good fresh caps. Then replace R34 (47K) with a 39K and 50K pot as shown in the attachment. This bias schematic came from your B15N schematic. Everything has been removed except the bias circuit for clarity.

[pbman1953]

Very cool, thanks!


Do you know who has a locking 50k pot?

[sluckey]

Google and ebay have them. Search for 50K locking potentiometer. Available in RV4 style full size pots, although I prefer the RV6 style mini pots as seen on my Sunn Sceptre.

Here's one...

https://www.ebay.com/itm/202145285373?hash=item2f10cc44fd:g:CJkAAOSw3ZtaL1CM&amdata=enc%3AAQAIAAAA4MzyF8SFwIPDBRtTzv177aHXQDuWYc86GG%2FBYVX7YH3p0Lax9YBhTPk3oA6%2FJaKEcOALJkNHqI2lu%2BcBRwHuK8FAZheesQGX0zVBchEhWNAXj9lxu%2BC7795RXHsYMt1et%2F5AGTh%2FsJM0YFuqbtogXODO%2FxH550Ai5LfY9hOngSCgSSEJioxFEC9Ils%2Fpp3QDEKxuxKCrQ%2FXAETWHEqIT1X3dzlwGhuvOEVPQdtIpAw5I1C4YzpOqlu1Yk9tY6UPeDyExXYAoF4aPn58HEgze1J23Mfmd9wVo%2FRo2%2FFBlwfbO%7Ctkp%3ABk9SR4zW0MKIYg

[pbman1953]

Google and ebay have them. Search for 50K locking potentiometer. Available in RV4 style full size pots, although I prefer the RV6 style mini pots as seen on my Sunn Sceptre.

Here's one...

https://www.ebay.com/itm/202145285373?hash=item2f10cc44fd:g:CJkAAOSw3ZtaL1CM&amdata=enc%3AAQAIAAAA4MzyF8SFwIPDBRtTzv177aHXQDuWYc86GG%2FBYVX7YH3p0Lax9YBhTPk3oA6%2FJaKEcOALJkNHqI2lu%2BcBRwHuK8FAZheesQGX0zVBchEhWNAXj9lxu%2BC7795RXHsYMt1et%2F5AGTh%2FsJM0YFuqbtogXODO%2FxH550Ai5LfY9hOngSCgSSEJioxFEC9Ils%2Fpp3QDEKxuxKCrQ%2FXAETWHEqIT1X3dzlwGhuvOEVPQdtIpAw5I1C4YzpOqlu1Yk9tY6UPeDyExXYAoF4aPn58HEgze1J23Mfmd9wVo%2FRo2%2FFBlwfbO%7Ctkp%3ABk9SR4zW0MKIYg

Bought it!


As far as the pot location, on the back, there is some open space. It would be closest, on the board side I need,  if I put it next to the AC cord . I could place it an inch away from the cord, is ok as far as noise interference?

[SoundmasterG]

Keep in mind with Ampeg B15's that these do tend to have more of a problem than some other designs with the power transformer eventually failing. They are somewhat under-designed for the circuit demands, and in the case of electrolytic caps in the power supply getting marginal and failing the power transformer can fail instead of just the fuse blowing. This is true of any amp of course, but these B15's seems to have more power transformer failures than many other designs.


If it was my amp I would replace all of the electrolytic caps in the amp just to be on the safe side. New power transformers are available, but they aren't cheap. I fixed a B15N from 1967 for a guy a few years back that made funny noises on stage until the fuse blew, and someone had replaced some of the electrolytics in the power supply but not all. The power transformer and output transformer both failed and the power transformer leaked some goo all over the inside of the amp chassis. I had to pull everything out to clean it all off, then replace both transformers. Unfortunately I didn't realize the output transformer had failed also at that stage of the repair and had to take the board out along with all the connections  again to replace the output transformer once I discovered that tidbit. Best to have peace of mind and not have to worry those caps will fail down the road.

Greg

[pbman1953]

Hey Greg,


I've planned all along to replace the old stuff. Just looking for confirmation since it was so quiet from pickup day. I expected it to be hummy but in turn it was whisper quiet. That's why I asked the questions but I figured to bring it up to date while it was part. Plus Sluckey came up with a nice upgrade to bias circuit.


Thanks!

[mresistor]

Save the paper/cardboard shell on the cap can it will fit nicely over a new CE Manufacturing cap can.

[pbman1953]

Save the paper/cardboard shell on the cap can it will fit nicely over a new CE Manufacturing cap can.

Like that

[pbman1953]

I was able to remove the paper cover off the old can cap. Is the black goo silicone? It's more tar like now

[PRR]

Silicone was hardly known when B-15 was king.

It is tar. Chisel a chunk out of OLD(*) pavement, a chunk of this cap-junk, set on fire, compare and contrast.

((*) post gas-crunch "tar" is not as juicy as the good old stuff.)

[pbman1953]

Silicone was hardly known when B-15 was king.

It is tar. Chisel a chunk out of OLD(*) pavement, a chunk of this cap-junk, set on fire, compare and contrast.

((*) post gas-crunch "tar" is not as juicy as the good old stuff.)

Worked like a charm!


Question since the vendor forgot to send me a a couple parts - I'm missing the 2-22k 2 watt resistors. Without them, but leaving the old ones in, can I bias the amp? Do they have an effect on the bias?

[sluckey]

yes. no.

[pbman1953]

After turning it on, one tube is 21 watts and the other is 19


The Hum balance doesn't seem to do anything


Anything to test for the hum?

[sluckey]

Sometimes it's difficult to hear a change in hum when the pot is turned. Connect your meter leads across the speaker. Set the meter to read small ac volts. Slowly, slowly turn the pot from end to end and notice where the voltage is minimum. Then set the pot for that minimum reading. This is easy and fast with an analog meter or a digital that has a bar graph. Looks like that MM450 does not have a bar graph. That's why you must turn the pot slowly.

If you still can't get the hum minimized, just set the pot to mid range and move on.

[pbman1953]

Sometimes it's difficult to hear a change in hum when the pot is turned. Connect your meter leads across the speaker. Set the meter to read small ac volts. Slowly, slowly turn the pot from end to end and notice where the voltage is minimum. Then set the pot for that minimum reading. This is easy and fast with an analog meter or a digital that has a bar graph. Looks like that MM450 does not have a bar graph. That's why you must turn the pot slowly.

If you still can't get the hum minimized, just set the pot the o mid range and move on.

Thanks!


I do notice since the new parts is a volume reduction. At full rotation it's not distorting
. All I replaced were the 7 box cap, but sounds good. s, 30/600v cap, can cap 40-40-40, 2 bias caps, 2- 22k 2 watts. Can any of these have effect on volume?

[sluckey]

Everything has an effect on volume. Some more than others. Changing the bias caps may very well have increased the negative voltage on the grids of the output tubes. This will definitely lower the volume. Did you check the grid voltage before you began "fixing" the amp? Did you do the bias mod? How did you adjust the bias? What did you set if for?

[pbman1953]

Everything has an effect on volume. Some more than others. Changing the bias caps may very well have increased the negative voltage on the grids of the output tubes. This will definitely lower the volume. Did you check the grid voltage before you began "fixing" the amp? Did you do the bias mod? How did you adjust the bias? What did you set if for?

. I didn't check the voltage before any changes. Please send me the instruction to measure the grid voltage. I installed the bias mid and it worked fine. I set the bias with my Bias Rite and one is at 21 watts and the is 19. Voltage was 509 and cathode current was around 38 to 41 mv.
Im going away for a few days so I'll have to finish when I come back

[pbman1953]

Has anyone a chance to read reply #33?

[sluckey]

Has anyone a chance to read reply #33?

yes

[pbman1953]

Has anyone a chance to read reply #33?

yes

And what would like to see first? How to test?

[sluckey]

Have you forgotten that you measured the grid voltage just last week? I swear you must do a brain dump every day. After hanging out on this forum for over 14 years and twiddling with some very sophisticated amps, you must have picked up something as simple as how to measure grid voltage.

And what would like to see first? How to test?

I'd like to see you pick up the spoon and feed yourself for once. I'll still be around to help you cut the tough meat, but no more spoon feeding! So, pick up that spoon. If you can play a P bass, you can do this too.

[pbman1953]

Ouch


I'll have a measurements back by Monday

[pbman1953]

I'm back-


Pin 5- 59 volts on both tubes






Bias cap-


c19@ -1.7

[sluckey]

Pin 5- 59 volts on both tubes

OK

Bias cap-
c19@ -1.7

IMPOSSIBLE!

[pbman1953]

Pin 5- 59 volts on both tubes

OK

Bias cap-
c19@ -1.7

IMPOSSIBLE!

I'll double check.


Does the pin 5 seem right?


I'm measuring the c19 on the positive leg. Is that right or should I go by the negative leg?

[sluckey]

Sigh!   

[pbman1953]

I remeasured again -




Pin 5 was -58 volt on both tubes


c19 on the negative side was - 59

[pbman1953]

Am I being boycotted? 

[sluckey]

Am I being boycotted? 

No. But you are now on my ignore list so I will no longer see your posts. It's just too painful and frustrating to try and help you.

[pbman1953]

Am I being boycotted? 

No. But you are now on my ignore list so I will no longer see your posts. It's just too painful and frustrating to try and help you.

Well. I sincerely apologize and appreciate what you've helped me with. How can I repair the relationship and get back on track?

[sluckey]

There is no relationship. There is no "on track".

LET IT BE! I'M DONE.

[pbman1953]

HI All fellow techs out here,


I've come back with some good news. I solved my issue and it was starring me in the face. Seems like this amp likes the power tubes to run on warmer side. If you reduce the bias the a 60-70% range you lose gain. It will never distort but it won't have any real gut when you push it. I have it about 22-23 watts (6L6) and its fine.


There were some other "friends" out there who had a pinch more patience and were willing to help me. Don't take this as a knock because most of you have patience and some don't. I come here to continue the fun and I'd say that I could help sometimes and be happy to.
 

Thanks
Rich