early Ampeg B15-N -> no sound

[traynorbassmate]

Please see pictures here:

b15-n - a set on Flickr

I'm on the job to repair a friend's B15N.
Tried to dig up info about this model with little success. Dating is frustrating as I only have a serial number to go by - stamped inside the speaker cab (104118...1961?), the speaker is some old looking canadian "Marshland" brand, model Hawk 3-15. Inside the amp the pots are blank. It has a green jewel pilot lite instead of the lucite. The tolex is blue but with box designs, not diamonds.

Down to business - just cracked it open and first thing I see amongst the crusty crumbling wire is severed connections having to do with the speaker connection (8-pin amphenol socket). Of those, the only two remaining connections are (1) wired up to the standby switch, joining to the PT center tap, and (2) wired to the tip (+) connection of the ext cab jack. As far as I can tell, the OT secondary (YL/GN?) arrive at the ext cab jack as well. GN to gnd, YL to 'switched' terminal (NC).

this makes no sense - CT and one half of the OT? no wonder no sound from the speaker cable that way. Further puzzling is the OT connecting to the ext cab output - GND and Switch. meaning + gets nothing when a cable is inserted

Judging by the schematic located under the amp it seems the 8pin plug was designed to automatically standby the amp if it became unplugged - very classy. Maybe this just needs to be hooked up right? cross your fingers for the OT's life.

Thanks for reading!

[T-Bird]

Hi.

Didn't look the pics, I'm having a bit of bad luck with my browser crashing, but the schematics are widely available, and they do look pretty clear for me.

"Crossing fingers for the OT" never sounds promising, "do no harm" should apply to techs as well as medics.

Edit: Just to be clear, I'm not tech, just a hobbyist, and I haven't ever had the pleasure of having a B15 in front of me or even play one.

Looked at the pics, nothing too bad there, normal band-aid crap people do.

Your output-octal pinout however doesn't match the schematic I have in front of me, and quite frankly, Your explanation either means that the OT has been swapped for a single tap one, or You have no idea what You're doing.
I do sincerely hope it's the former.

BE SAFE.

Regards
Sam

[johnk_10]

no offense, but from your description, it sounds like you aren't very familiar with the B15N. IMO, the speaker isn't the original one and has probably been replaced years ago (old Traynors sometimes used Marsland speakers) and the pots and filter caps should have date codes on them (on the pots, sometimes it's written on the sides of them and not the back).
if you follow the schematic there shouldn't be any problems diagnosing it. it's a very simple amp.

[coreyfyfe]

On the dating front, definitely a 60-61 amp. They only used the jewel light like that on the first version, changing to the lucite logo after. The tolex should be what is known as navy random flair, and that speaker is definitely not original.

Electronics-wise I can't help you, but that's a very rare early example of a great amp.

[beans-on-toast]

Originally Posted by traynorbassmate

Down to business - just cracked it open and first thing I see amongst the crusty crumbling wire is severed connections having to do with the speaker connection (8-pin amphenol socket). Of those, the only two remaining connections are (1) wired up to the standby switch, joining to the PT center tap, and (2) wired to the tip (+) connection of the ext cab jack. As far as I can tell, the OT secondary (YL/GN?) arrive at the ext cab jack as well. GN to gnd, YL to 'switched' terminal (NC).

this makes no sense - CT and one half of the OT? no wonder no sound from the speaker cable that way. Further puzzling is the OT connecting to the ext cab output - GND and Switch. meaning + gets nothing when a cable is inserted

Based on the pics and your description, the speaker connection wiring is correct.

There are three wires from the output transformer, black (common return), yellow (8 ohm), and green (16 ohm). The ext speaker jack should be isolated from the chassis with a set of isolation washers that prevent a ground loop. Yellow goes to the jack switched terminal. Green goes to the jack sleeve, as does the 10K feedback resistor. Black goes to the ground bus, the location varies but it is best (quietest) near the ground point for the phase inverter (V3) components, the 25uF/25V cap in parallel with the 1K ohm resistor..

On the octal connector, pin-1 goes to the tip of the ext speaker jack. I assume that this the black wire in the pic. Pin-3 and pin-7 are shorted at the octal connector. Pin-5 goes to the standby switch. Pin-7 goes to ground bus at the power supply end as close as possible to the first capacitor (C1B on the schematic) common connection point. Don't ground it to the chassis.

In the speaker cabinet, at the male octal connector, pin-1 (green) goes to speaker (+), pin-3 (black) goes to speaker (-), pin-5 and pin-7 are shorted at the connector.

Make sure that the switch contact on the ext speaker jack is clean. The speaker out flows through this contact point.

The standby/speaker interlock scheme is not a safe thing to do. I disable it and not run the high voltage return through the speaker cable. I connect the standby under the chassis. If you want to protect the output transformer in case the amp is turned on without the speaker connected, add a 250 ohm/10W resistor from the 16 ohm tap to ground. They did this on the later models and it will provide short term protection.

If this doesn't resolve the problem, run an instrument cable from the ext amp jack (this is pre-amp out/power amp in) to the input of another amp. This will allow you to test the B-15 pre-amp.

[traynorbassmate]

Thank you everyone! This forum is the place to be.

John, I definitely know close to nothing about the B15. Its hard to search for it on the forums because the 3-character rule makes it very inaccessible. Google only brings up history lessons - which are really cool and informative but not completely satisfying my thirst for tech knowledge about these amps. I am however knowledgeable in push-pull and single ended tube amps, high voltage, etc. Thanks for the schematic - I was having trouble coming up with anything but the 4-input model.

Also one of the most confusing parts to do with this amp is getting a sense of which wires exit the transformers - the turret board gets right in the way and will not budge even when unscrewed. That, and someone has already been under the hood and messed around with things, so i'm trying to get a sense of what and why things were augmented - with the help of you good people.

B-O-T, thank you for confirming the wiring. Looks like I'm coming up close to your explanation:

The OT is original, and i've been able to trace the wires down - Primary; BL & BN connect to the 6L6 plates, RD connects to the rectifier and filter caps. On the secondary, BK goes to ground only (this is one part of the mod, it does not hit the speaker output), this connects to the same gnd bus as the V3, as you describe, but the cap value is 50uf/50v instead. YL connects to the EXT jack switch, GN to the EXT jack GND (jack is isolated from chassis).

So it seems that everything is wired okay except for the speaker wire which has been muddled up. I'll fix that up as per original. I repair Leslie speakers and am used to large DC returns sent along the speaker cables so i'll leave it alone for now. Will test the preamp tonight, thanks for that tip.

Much appreciated!

Adam

[johnk_10]

cool. i think that you'll find it to be a pretty simple amp to work on. good luck and let us know how it goes. you have a very nice amp there.

[beans-on-toast]

Originally Posted by traynorbassmate

On the secondary, BK goes to ground only (this is one part of the mod, it does not hit the speaker output), this connects to the same gnd bus as the V3, as you describe, but the cap value is 50uf/50v instead. YL connects to the EXT jack switch, GN to the EXT jack GND (jack is isolated from chassis).

Adam,

It sounds like the ground connection is at the power tube 50uF/50V cathode cap and resistor. This is fine. Unless you are saying that someone installed a 50uF instead of a 25uF at the phase inverter.

I work on Leslies and Hammonds as well. I wonder if they came up with the idea of sending the high voltage through the speaker cable and then justifies it by saying that Hammond does it.


David

[traynorbassmate]

Sorry - my mistake. The 50/50 is the 6L6 cathode cap. The ground bus is shared by everything (filters, cathode caps, etc...) so why does it matter where the OT gnd is connected?

A few questions:

When was the M15 in production? Because the cabinet label is M15 scratched out and B-15 hand written over it. Just curious.

The schematic under the amp is for both the B12 and B15, labeled March 1961. The V3 cathode cap is labeled 100/25. Here's what I find installed in its place:



Is that 25uf/6V?

In Joe Piazza's 62 schematic he adds a 20uf/500v cap at the 6L6 screens, but they are also wired up to the 20/500's at the filter section of the schematic. Was this Joe's mistake or did they add a 3rd 20uf capacitor to the later versions? An attempt at cleaning up some extra ripple?

[johnk_10]

the B15 and B12 are the same amps, they just have different sized speakers. i've seen some that match Joe's schemeatic exactly, but IMO, that 'extra' 20uf/500V cap is not a mod. here's one of the original ampeg B15N schematics that is a bit different than the one he drew up:
http://www.freeinfosociety.com/media/images/366.gif

IMO, someone replaced the 25uf25v cap on V3 with a 25/6.

[traynorbassmate]

Are you sure you posted the right link? That's for the 4-input model, totally different filter values all over the place.

[nobodysfool]

Wow! That's older than the 1964 model I used to have. I sold it to a guy out in Oregon on EBay. It was all original, and worked great. Had the Lucite light panel, and all the original paperwork, including a little envelope that you could send the Lucite panel away to get custom engraved, for 50 cents. The one end of the card folded over to hold the 50 cent piece....I hope the guy who bought it is still enjoying it.

[johnk_10]

Originally Posted by traynorbassmate

Are you sure you posted the right link? That's for the 4-input model, totally different filter values all over the place.

there's a lot of B15N models. your's should have schematic to it glue on the bootom of the tray.

[stiles72]

Cool amp - would love to see some full pics including the cab

[beans-on-toast]

Originally Posted by traynorbassmate

The ground bus is shared by everything (filters, cathode caps, etc...) so why does it matter where the OT gnd is connected?

A few questions:

When was the M15 in production? Because the cabinet label is M15 scratched out and B-15 hand written over it. Just curious.

The schematic under the amp is for both the B12 and B15, labeled March 1961. The V3 cathode cap is labeled 100/25.
Is that 25uf/6V?

In Joe Piazza's 62 schematic he adds a 20uf/500v cap at the 6L6 screens, but they are also wired up to the 20/500's at the filter section of the schematic. Was this Joe's mistake or did they add a 3rd 20uf capacitor to the later versions? An attempt at cleaning up some extra ripple?

The B15 is one of the quietest amps you will ever find. The system that they adopted works well. It is set up as a bus and connections are made from low to high current. There are no high currents running near low currents where interaction and noise can occur. This makes a difference. There is one ground point (except for the ext amp jack) and that is at the input jack. There isn't a ground at the power supply end, just a connection to the bus. The highest current loops are at this end.

Normally on these amps, the black OPT secondary is found at the phase inverter ground. If found that if the speaker return is also close to here, the amp is even quieter. Normally it is connected closer to the power supply ground points.

That capacitor in you pic is not original. According to the schematic, it should be a 25uF/25V. I don't know where the 100/25 comes from that you referenced. I assume that the 25/6 is 25uF/6V. The typical voltage read at the (+) end of the cap connected to the cathode is around 3.1V DC. Maybe that is why they thought a 6V cap would be ok to use when they replaced it. I can't comment on voltage swings such as when the amp is turned on.

The M15 was a guitar amp with tremolo. I believe that it was introduced in 1959. THe best way to estimate the manufacturing date of your amp is to look at the codes on the pots (137XXYY -> CTS year week), or caps. The lead time between component dates and shipment of the amp was short. Your serial number dates the amp to April 1961.

There are some minor differences in what the schematic says and what sometimes found in the amp. Ampeg was always changing things. Eventually the schematics would catch up with the revisions. This amp had the extra 20uF/500V on the 6L6GC screens. It was removed in all later revisions of the B15. They didn't need it.

Here is another copy of the schematic. Original but not as clear as the one John referenced.

[traynorbassmate]

Thank you so much for all this insider info - this is stuff I was after.
I love Ampeg - play a 74 SVT and have worked on it plus others like the V4B. I've only heard great things about the B15 though have never had the pleasure of meeting it! I'll definitely post some additional pictures soon.

I am beginning to wish this amp was mine

Ok - that schematic is nearly identical to whats glued under my amp chassis. But have a look at the 25M-25V script and tell me the 25 wasn't edited in at a later time. Mine clearly says 100M-25V. More ripple protection?

The volume pots are blank. I've read date codes in the past, I know what to look for - but these suckers are blank!! The only caps that have info are the 40/40 & 20/20 & 50 and my guess is 1959, 1960, 1960. The red Astron caps have no dates.

20/20 @ 450:

EM 00559
1081 6032 85C

40/40 @ 450

EM 00660
1081 6032 85C

50 @ 50

EM 00560
1081 6046 85C

(all 85C's are temperatures)

[johnk_10]

all of the caps with those numbers were made in 1960.
the 100uf cap on V3 (instead of the 25uf one in the schematic which, BTW is the stock value) is the cathode bypass cap (AFAIK ampegs didn't use german capacitors). it has nothing to do with hum or filtering. the cathode bypass cap connected in parallel with the cathode resistor. by altering the values of the cathode resistor and cathode bypass cap, it is possible to roll off various degrees of bass with this triode stage, and higher values allow more bass. the cathode resistor and plate resistor control the biasing of the tube. a cathode bypass cap generally gives the stage more gain.

[beans-on-toast]

Originally Posted by traynorbassmate

Ok - that schematic is nearly identical to whats glued under my amp chassis. But have a look at the 25M-25V script and tell me the 25 wasn't edited in at a later time. Mine clearly says 100M-25V. More ripple protection?

The volume pots are blank. I've read date codes in the past, I know what to look for - but these suckers are blank!! The only caps that have info are the 40/40 & 20/20 & 50 and my guess is 1959, 1960, 1960. The red Astron caps have no dates.

The phase inverter design that they used wasn't new, it was used in older Ampeg amps as well as radios at the time. Earlier models like the 625D did have a 100uF/12V cap in this position. That must be where the 100uF comes from. Maybe they started with 100uF and switched to a 25uF/25V cap. It isn't unusual for Ampeg to have errors in their schematics. All the subsequent B15 models, the N, NA, NB, NC, NF, and the Heritage have the 25uF/25V cap. I would go with that value. I assume that there was a reason that they went and stuck with the 25uF value. I suspect that the higher gain with the 100uF would lead to more distortion. Since you have it on the bench, it would be interesting to try the two values and see.

Since they aren't EIA codes on the caps, interpreting them is more difficult. Even with EIA codes they can be difficult to interpret. 59 and 60 is a good guess but your cab dates to 1961. You tend to find a few months between date codes within the amp and the cabinet date, not a spread of (1959-1961) years. Although with Ampeg, I wouldn't discount anything odd. There is another thing to check out. Sometime there is a piece of paper with a date and transformer model number on top of the potting material under the transformer. You would have to remove the transformer to look for it and most of the time it isn't there.

[traynorbassmate]

Cool stuff. All the electrolytics are being replaced as soon as I confirm the amp works. I checked continuity from the ext jack tip to switch and I believe this was the reason for no signal.

The speaker cable is toast. I'm going to omit the high voltage from the speaker cable and do an in-chassis stand-by, let me know the following is OK:

- Ext jack tip to Speaker +
- Ground bus to Speaker -
- PT center tap to stand-by switch, switched to ground bus.

Is that OK?

Pardon the noob question, but what does the CT have anything to do with the circuit working? Obviously it has everything to do with it...but humor me. I've never worked on an amp that bypassed this way. Some amps don't even have a CT. The entire circuit is grounded to chassis through the input jack. 1) Is this not a "Real" standby, whereas the tubes are hit with full voltages upon power-up? (thus not being a stand-by in the "tube-amp" sense of the word?). 2) The speaker is always "seeing" the 8ohm tap and chassis ground...how does the PT CT determine if the amp plays or not?

What should I do about the Ground switch? I typically see it being disconnected completely, with the amp always chassis grounded via a grounded AC plug. I don't like the idea of the amp not being earth grounded.

[johnk_10]

if it were me, i'd leave the amp completely stock and just get it running to spec. there are thousands and thousands of them running perfect completely stock, and IMO, there's no reason to change it (plus it'll retain more of its value).

the only thing that i would change is the AC cord to a grounded 3 prong one and you won't have any need for the ground switch and cap.

the standby switch on a typical fender amp just turns on/off the B+ supply, so if you're still going to change the amp to have one, i'd do it that way using the ground switch that you'll no longer need.