Looking for an Ampeg M15 PT replacement

[nateflanigan]

I need to replace the power transformer on an Ampeg M15, none of the available schematics (original or Joe Piazza) have a part number or voltages.  The power section is a 5u4 and two 6L6's, cathode biased.  Fliptops has one for $125 but that seems a bit steep, I'm thinking a tweed super or something similar will be close enough.  Second opinions?


Thanks!

[Jack_Hester]

Take a look at the M12.  It might very well be the same configuration.  I only made a quick comparison, but it shows a 700vCT - 90mA PT.  5Y3 rectifier, though, with 6V6's in the power amp.  May still be what you're looking for. 

Jack

[nateflanigan]

Thanks Jack, that's very helpful.  Since posting  i found the part number on the bad transformer itself (of all places) PT 113.  Which as far as I can tell is what was used in the m12's.  Thanks for posting the voltages.

[DummyLoad]

not a common amp so expect to pay more for specialty parts. worthy of resurrection though.


--pete

[nateflanigan]

I found this edcor that should work well (voltage wise) but I need to double check the mounting holes.
https://www.edcorusa.com/xpwr155

[DummyLoad]

B+ will be much higher with the 200mA edcor. ampeg is loading a 90mA 700VCT PT to get 380V B+. OK for a 15W amp using high plate impedance rectifier. 


i found this photo of an M-15 - they claim it to be a 35W amp with 1 x 15" speaker. IMO, the PT-113 you have is under-rated for that application. here this model that shows B+ and that you need a 680VCT PT that has a 3A 5V winding. the PT113 is 90mA secondary and likely has a 2A 5V winding for use with a 5Y3. looking at the pic, i see a similar published RCA conditions for a 25W amp 360V B+ with a 250R cathode resistor. ampeg pushed that plan, probably a lower Z OT + higher G2 voltage to get closer to 35W.


IMO the edcor XPWR041 PT would be a better fit. it may also be a drop-in or close replacement size-wise.



--pete

[DummyLoad]

link to amp for sale..


http://www.gbase.com/gear/ampeg-m-15-1962


--pete

[nateflanigan]

DL, that's great, you really hooked it up!  The edcor you mentioned would be a great fit. 

[PRR]

> M-15 - they claim it to be a 35W amp

How old is that? By the phone-number, pretty old.

I bet "6L6" *means* 6L6, not 6L6GC.

It is worked at 22 Watts per plate. This is very hot for any 6L6 before the GC, but they will play that way for weeks (long enough to pay-off the amp).

At that input, and cathode bias, it is hard to justify a 25 Watts "clean" rating. 35 Watts would be at many % distortion and a fairly "sour" sound.

PT needs to be 140mA DC. Final DC Voltage has to be pretty close to 380V. 400V DC will put the "6L6"es at 25 Watts, which is technically legal for any 6L6GC (therefore nearly all new-made 6L6-ish bottles, "6L6B", "KT66", etc), but IMHO is flirting with trouble, including tube run-away melting the precious OT.

[DummyLoad]

> M-15 - they claim it to be a 35W amp

How old is that? By the phone-number, pretty old.

I bet "6L6" *means* 6L6, not 6L6GC.

It is worked at 22 Watts per plate. This is very hot for any 6L6 before the GC, but they will play that way for weeks (long enough to pay-off the amp).

At that input, and cathode bias, it is hard to justify a 25 Watts "clean" rating. 35 Watts would be at many % distortion and a fairly "sour" sound.

PT needs to be 140mA DC. Final DC Voltage has to be pretty close to 380V. 400V DC will put the "6L6"es at 25 Watts, which is technically legal for any 6L6GC (therefore nearly all new-made 6L6-ish bottles, "6L6B", "KT66", etc), but IMHO is flirting with trouble, including tube run-away melting the precious OT.

the edcor i posted will work and probably fit without too much modification, if any. the 700V 90ma part that it had in it is very likely why it's fried. edcor are conservatively rated. i'd use 5581 or GC anyway.


there is a published condition in RCA datasheets that's close to the ampeg stats, save for g2 and another 20 B+ volts. datasheet conditions wants 9K load; ampeg is probably a more common 6600 ohm load? datasheet condition (pg4) states 25W w/ 360V & 9K load, 250R comm. K. wih about 29V Vk. based on the photo schematic i posted, indicated is 34V across Rk; which is 250 is around 136mA total. so about 23.5W / bottle is what i crunch. 5881 is 23W tube, so agreed, it would be pushed hard.


http://tubedata.milbert.com/sheets/049/6/6L6GB.pdf


nate, before you buy, you should at least make sure the OT is not fried. 


--pete

[DummyLoad]

the edcor is about 18V too hot with US made baldwin branded 5U4GB. i have an edcor XPWR041: i paid the $20.00 design fee in 2008 to have two of them built. like all edcor, they are overbuilt. i'm running russian "6L6GC" tubes that are marked "west germany", they came out of a tusc amp i scrapped for the fane speakers. they look like the russian 6p3s tubes. see photo's in link. this is running on my breadboard. i suspect that with a 5U4G and not 5U4GB that B+ would be closer to 380V target.

why? i was curious to see what the XPWR041 would do with modern replacement tubes, suspecting that it would be somewhat hotter than sim showed; sim plotted 385V: we see different...about 4% more. the tubes are in near new condition. the last photo (dark shot looking for plate glow) was taken about 30 minutes after warm-up: as you can see, all clear. bear in mind that i'm running with 1K screen resistors as well.

http://s2.photobucket.com/user/pmitchel/slideshow/Ampeg-M15-PS-Experiment


with 28V/250R bias is @ ~112mA... i'll give this a spin tomorrow with a TW 35W 6600R OT courtesy of doug & mojotone. curious to see what the results will be...


this is probably the part you want: https://www.edcorusa.com/xpwr245 if you want to play it safe...or if you simply just don't trust DL and his rickety breadboard and his beat fluke DMM and his crappy "mad in west germany" tubes...


primary - 120V AC measured.
HT secondary - loaded 342V-0V-343V AC
fil secondary - loaded 3.1V-0V-3.1V AC
node A 397V
node B 387V
node C 365V
Vrk = 28V

see attached schematic for rest of the telemetry. this circuit sounds pretty damned good. it's a spin on the casino el camino - i didn't want to tear-down the whole breadboard so i kept the preamp and grafted the ampeg output and PS. i may keep it. i lost perspective on how smooth the 6L6 sounds after fiddling with EL34 for the last few months.

--pete


EDIT: in attached schematic, R6 & R7 are 15K 1W MF not 47K. R6 node is 335V and R7 node is 380V

[nateflanigan]

Cheers guys, stellar info as always.  The OT passed RG keen's ohm meter test.  Any OT tests you like?

[nateflanigan]

DL, Your bread board looks great.  Any layout drawings or pics?

[DummyLoad]

Cheers guys, stellar info as always.  The OT passed RG keen's ohm meter test.  Any OT tests you like?

thanks, i do have a layout in the works. i'll post it when i'm satisfied all the pieces will fit together properly. the original project used EL34s and a hotter B+.

a turns ratio measurement of the M-15 OT would be nice. with all the tubes removed, if you could you connect a 120V isolation transformer to the primary leads  (connect 120VAC to output tube sockets pin3 to pin3) of the OT, then measure the ACV of the primary and secondary and post the numbers, that would be great.

--pete

[nateflanigan]

I went with the XPWR245, perfect fit.  The amp sounds great, thanks so much for the help DL.

[DummyLoad]

I went with the XPWR245, perfect fit.  The amp sounds great, thanks so much for the help DL.

YW and glad it's back up making music. 


--pete