Help with Ampeg B15n diagnostics

[Tonegeek]

THis is the fixed biased one. It has very low power and sounds really muddy. I suspect the OT is going bad but not totally convinced. I pulled the 6L6s and when I put 4v AC on the 8 ohm speaker output I only read 29V on the primary . I would expect to get more than that. When I tried this with a DR tranny (8 ohm tap), I got 67v on the primary. Tubes test good, and plate voltages are in spec. I get signal to both power tubes. bias is 22ma and 30ma respectively on the output tubes (not matched obviously but should be good enough to get more than 2v output, which is about all this thing can manage. The unknowns are the tranny and the signal voltage I should be seeing at the input to the power tubes. I do have equal signal at each tube, just don't know what voltage is needed to drive the tubes to full power. I did notice the B+ drops about 60 volts when I crank the amp full tilt. Seems reasonable given the rectifier tube and demand. Can cap is original so I clipped a new one across the Output tube section but it made no difference. There are other filter caps inside that have been replaced, or clipped in parallel with the main cap. The can should be replaced but even at lower volume with good voltages, the amp sounds bad.

[dorrisant]

I could be totally wrong but I think the above numbers would equate to a 7.5 to 1 turns ratio and an impedance ratio of 56 to 1. With an 8 ohm load the plate to plate load would be 448 ohms... that is if the OT would even function at all. Way too small of a load and it might explain the 60v loss at full bore.

Have you measured the resistance of the primary leads to the center tap? See if the values look close? If you have an LCR Meter you can check AC resistance and the impedance of the two halves of the primary and compare. I think these measurements should be relatively similar... if not, then the OT is toast.

Can you inject the 4vac into the primary and report what the voltage is at the secondary?

Tony

[Stevem]

If the PI is pumping out enough drive signal you should then see that 29ma of idle current go up to at least 70ma and if that's the case and the amp still sounds wimppy with early distortion then Ya the OT is shorted !

[Leo_Gnardo]

It's lookin' bad for the good guys... not often you get an "in between" transformer failure but it is possible. If you have another OT - brand doesn't matter - what matters more that it can handle 30-50 watts - remove the B15's OT primary leads, substitute your known good OT, run the secondary straight to the speaker, see what kind of tone & power you get. If it's obviously better with the substitute OT, the B15's OT is toast. Then it's a matter of do you buy another ($$$$$), plunk a raw-looking OT onto the B15 chassis, or my favorite, do the mad scientist re-potting routine.

Of course replace filter caps that are failing or you will be doing the same with the PT.

[dorrisant]

Steve & Leo

I'm trying to make sure I have my head wrapped around this... am I way off with the numbers above or the advice?

Tony

[Stevem]

His B+ dropping by 60 volts is prof plenty for me that the outputs are trying to there best to drive the OT but can not due to the short it has!

[Leo_Gnardo]

Steve & Leo

I'm trying to make sure I have my head wrapped around this... am I way off with the numbers above or the advice?

Tony

I'm sure we all have a similar take on transformer failures: try to prove it's anything but the transformer. Your numbers & advice perfectly good Tony; I respect you and your work. Personally I skip to "sub the transformer" rather than do the math, just lazy I guess. Some of my more mathematically inclined friends rib me for it "you're such an empiricist!" while I claim I'm just saving time. And calculator batteries.

[Tonegeek]

Based on the excellent suggestions, it is looking grim for this OT. The bias is going way up on the tubes under load and the B+ is dropping. I will take measurements tonight on each leg to center tap and see what I get. I also read an interesting article by RG Keen on using a battery and neon bulb to test for a shorted winding. He sounded very confident in the test. The article is here:
http://www.premierguitar.com/articles/t ... r-tester-1

I shall return...

[Tonegeek]

Bad OT confirmed. One leg to CT is 138k . Looks like its $250 for a replacement that retains the original vibe. Any recommendations on a cheaper sub? MC has a Bassman with multiple taps I could use. I hate to drill holes in this classic amp though. I just suspect this particular customer is not going to want to spend the dough to do it right.

[Leo_Gnardo]

Bad OT confirmed. One leg to CT is 138k . Looks like its $250 for a replacement that retains the original vibe. Any recommendations on a cheaper sub? MC has a Bassman with multiple taps I could use. I hate to drill holes in this classic amp though. I just suspect this particular customer is not going to want to spend the dough to do it right.

If you don't mind spending some time, charcoal, wax, sand, wrinkle-finish spray paint you can do it for less. I'm sure MC's Bassman OT will work just fine.

I wait for a calm windless day to roast a transformer on the hibachi. Raining lightly, so much the better for the sake of fire safety. All this so you won't get neighbors alarmed at the tar smoke which will inevitably happen. Wear some old jeans & long sleeve shirt you won't mind if they get tar on 'em. Safety goggles if you're so inclined. Get some briquets started 10 or 12 should do, meanwhile take 2 cardboard boxes, fit one inside the other and wet 'em down. Have a pair of heavy duty leather gardening/workshop gloves on, start cooking your transformer. Have a scrap of wet cardboard handy to wrap around the transformer for a cooler grip. When the potting tar melts, dump upside-down into the wet cardboard, shake out everything you can. Including the deflicted OT. You can douse the charcoal now & discard the mess in the wet box. Leather gloves off now.

Wirebrush on a drill or dremel - wizz off the remaining paint. Now you need a nice dry warm day to spray a new coat of wrinkle paint on the transformer can. Clean any crud from the mounting studs so you can spin on nuts when the time comes.

Remove bells from your replacement transformer & reassemble.

Double boiler - carefully melt some wax and have dry sand ready. I got fancy and bought pure white modeling sand at the hobby shop. You could pick any color you like, you're not going to see it again. Pour a little wax into the tranny can, then place your new OT in there, wires sticking out. Scoop wax then sand then wax then sand into the transformer can until you have it nearly full. Push the wires into position so they'll go back thru the chassis holes then finish off with a final layer of wax. Don't overdo it, you don't need a mound of wax in the way when you bolt the transformer in place. FWIW I use beeswax, it smells nice though a bit expensive.

Under normal operation your OT will never melt the wax. Melt temp is 145F. I've done this several times with PT's & OT's, it's a lot of work and hassle but feels great when it's done. A warmed-up B15 with a little beeswax odor is a wonderful thing. You just have to be careful playing it outdoors in warm weather, local bees may show up to check it out. Properly biased even the PT doesn't melt the wax. If you ever need to unass the tranny from the can, double boiler it, no hazardous trial by fire will be necessary

Or spend $250 plus shipping. What kind of fun is that?

[Stevem]

Leo ,I too hate when I have to do the math and most times with a issue like this you can even just look into the plate structure of the tubes in a dark room and see them flash up brighter as they try to drive the shorted OT and get a good feel that they are being drivin pretty well!
I do understand the feeling though of wanting to fully confirm that a 250 buck OT is needed!

I have also found that the neon bulb test works out better on A PT then some OTs, that's why checking for the current rise in the cathode of the tube is the fastest and definitive test you can make in my mind!?
Tonegeek, with whatever OT you place in the amp for its owner install a inline fuse holder stuffed with 200 ma fast blow fuse in the OTs center tap, and then also place a 5 to 10 watt 250 ohm resistor across the speaker output.

Also if someone has installed the so called protection diodes from the outputs plate to ground REMoVE them, as all they surve to do is protect the tubes at the expense of the OT from what I have seen!

[Tonegeek]

Leo, Thanks for the tutorial on reusing the can. I may present that to the customer, although he already seems ok with buying a replacement from Fliptops. However I may revisit that if it saves him some money AND makes me a little extra AND I get some satisfaction from recycling! BTW I thought paraffin might be a good wax to use but it seems the melting point may be lower than that of beeswax. Anyhow you sold me on beeswax when you said it smells good. Maybe it will compensate for the smokey bar smell most of this old stuff puts out!

[pops]

Bees wax is a great wax, but it holds moisture if i remember correctly. The moisture i holds can rust from the inside unless you live in a dry environment, i would use the paraffin.

[Leo_Gnardo]

Thanks for considering the idea. We need more mad scientists. I started down this trail by being a cheap so-and-so, not gonna pay $150+ extra for a tin can. My scottish genes at work.

AFAIK standard white paraffin wax also has a 145F melt point. I suspect candle makers dope it up with stearic acid & whatever else possibly to increase the melt temp or keep the wax stiffer at temp & avoid guttering candles. I'd just as soon stick with plain bees and/or paraffin wax with sand filler for transformers, leave out the sand for pickups.

Stevem, I only apply flyback quench rectifiers when I see the need for it: spikes flying way past the margins when driving an amp hard square wave territory into a speaker. Some amps do it, some don't. And you won't see it while driving a resistor load. 3 x 1N4007 in series for most amps as Ken F recommended, 4 for higher voltage amps like V4, never had a failure. But I have seen some noobie techs put 'em in backwards, then wonder why they're poppin' fuses and OT's.

[JMFahey]

LOVED the transformer (can) recycling, very detailed and accurate.

That said, I also trust wax or paraffin potting ... in pickups which live mounted on a guitar surface, but I wold be very nervous using such low meltingn pointb stuff, also so easy to run anywhere when melted, inside a tube amp, in a closed case and surrounding a heat generator ... sorry, transformer.

There is a reason oldtimers used tar for such purpose.

Personally, I'd dry the new transformer at least overnight in a low set oven to be certain of driving all moisture away, let it cool and poy it inside the can with porring polyester rosin (no need for expensive epoxy)

If you wish, you can add the suggested amount of ground quartz (a.k.a. fine sand) to make it thicker, save some $$$ and improve thermal characteristics.

FWIW Fender has filled its transformers with such a mix for ages. (think 80's TWINs)

Not quite recyclable the net time, but since it will easily last over 50 years, why worry?