Make sure the power xfmr you bought is the right one

[nevetslab]

Monday I found the power xfmr in one of our Fender 65 reissue Twin Reverbs had a shorted primary. Pulled it out of the chassis, set it aside, looked up the part at Antique Electronic Supply, found a Hammond P-T290FX that's used in Fender Twin Reverbs, Dual Showmans, and had that part ordered.

It arrived today, so I pulled the chassis back out and moved it to the service cradle, opened the box to find a much larger core than what failed (ok, more headroom), and mounted it. Seeing two of the four HT leads being Red with a GRN stripe, another Red with a Yellow stripe, and an orange wire coming out of the primary side, I stopped to look at the schematic drawing of this xfmr.

Sigh..........HT winding has a C/T and a 60V tap. Just what you need for the older Twin Reverbs and Dual Showmans. NOT FORM-FIT-FUNCTION drop in replacement for the 65 Reissue Twin Reverb. Grrrrrr..............

A further look on line, I didn't find one, outside of going directly to Fender and ordering the replacement, which I should have done in the first place. That would have taken longer, which was why I went this way, not giving it a second thought (trying to get enough billable hours to make it thru March financially, adding this amp repair to the hours).

So, I'll make it work. With more work than I had planned, and will no doubt have to tweak the Tremolo ckt, the bias ckt, move the S/B switch between the first filter stage and the input to the O/T Primary C/T and choke. Removed the 1A 800V rectifier diodes, and dropped in UF5408 3A Ultra Fast rectifiers in D2/D4 positions was as far as I got today before packing it in to catch my bus home.

Sometimes I just don't remember to look closely.

65_Twin_Reverb_SvcMan.pdf
Hammond P-T290FX.pdf

[J M Fahey]

Ouch!!!

With more work than I had planned, and will no doubt have to tweak the Tremolo ckt, the bias ckt, move the S/B switch between the first filter stage and the input to the O/T Primary C/T and choke. Removed the 1A 800V rectifier diodes, and dropped in UF5408 3A Ultra Fast rectifiers in D2/D4 positions

With due respect and given the extensive modifications, I would order the proper one.
Sorry about the financial considerations but I donīt think a few extra dollars (you canīt be charging *that* much ayway, itīs a plain vanilla Twin Reissue job) to justify, not only the rework, but something worse in my view: next time that Twin needs servicing, next Tech will swear and curse "the tyro who made a mess out of a simple repair".
Including possible danger because of the "miswired" standby switch which he wonīt be expecting, unexpected bias circuit, weird Tremolo, etc.
Sorry and said with utmost respect.

It would be different if it were, say, a custom build because then next Tech will be there with both eyes open, but not with a "standard" well known amp.
Maybe you should leave a note glued to amp chassis explaining it?

[drewl]

Agreed, I didn't want to point it out but JM is right.

I have bought the wrong parts before, only to find the correct part later.
Chances are you will be able to use the wrong trans for another repair later, I know I have.

[nevetslab]

Ouch!!!

With due respect and given the extensive modifications, I would order the proper one.
Sorry about the financial considerations but I donīt think a few extra dollars (you canīt be charging *that* much ayway, itīs a plain vanilla Twin Reissue job) to justify, not only the rework, but something worse in my view: next time that Twin needs servicing, next Tech will swear and curse "the tyro who made a mess out of a simple repair".
Including possible danger because of the "miswired" standby switch which he wonīt be expecting, unexpected bias circuit, weird Tremolo, etc.
Sorry and said with utmost respect.

It would be different if it were, say, a custom build because then next Tech will be there with both eyes open, but not with a "standard" well known amp.
Maybe you should leave a note glued to amp chassis explaining it?

Sure glad to have excellent advise! I wasn't exactly looking forward to spending the extra amount of time to 'make it work'. I took another shot at finding the replacement xfmr, and it turned out that search led me back to Antique Electronic Supply where we bought this xfmr I received but, they DID have the correct xfmr for '65 Twin Reverb Reissue, drop-in replacement, and $22 less. I contacted them to verify that, explain my hasty mistake, and got an RA number to pack up and send this one back, and get that one sent to me. I did have to do a dumpster-dive to retrieve the outer shipping box, which had all of the order info on the label, so all will work out, and NO oddball '65 Reissue Twin Reverb in the fleet to confuse someone later on. Outer box's inside packing material got wet from other trash it was bagged up with, but will recover ok.

I did find it odd NOT being able to find this transformer in their usual search tree. That only leads to the larger Hammond xfmr which retrofits the older Twins & Dual Showmans, having the C/T lead and the 60V tap for the bias circuit/Tremolo. So, they will get with their IT guy in charge of setting up their products for internet search and make it appear.

Many thanks for proper advise, Juan & drewl!!

[drewl]

There you go, a happy ending.

Took me three tries once to get the correct PT for a Peavey Classic.

Kept the wrong ones and so far have used one of the two.

[g1]

As you have found, a vintage Twin PT is not directly compatible with the re-ish Twin PT, regardless of what brand.
This is the case for several different re-issue models, seeing a bridge rectifier on the schematic is sometimes a tip-off.

[J M Fahey]

There you go, a happy ending.

Took me three tries once to get the correct PT for a Peavey Classic.

Kept the wrong ones and so far have used one of the two.

I always love Happy Endings.

[nevetslab]

After Antique managed to loose the 1st Pwr Xfmr I sent back in exchange for the correct #037610 65 Reissue Pwr Xfmr, I finally received it yesterday, and got it installed, first verifying the connections/windings, as well as the mains current/plate current/bias (matched quad set of J/J 6L6GC's), idle current/wattage being 1.32A/131W with 33mA plate current, 14W, 442VDC on the supplies. All seemed ok.

This morning, as I had moved the chassis back into the cabinet just before I closed down the shop yesterday, and briefly ran the amp under moderate burst pink noise, not even showing wattage change during the burst signal vs idle, I set it to run. After an hour and 15 min, I checked to see what the temperature on the core of the PT felt like, putting my hand on it and pulled it off immediately, as it was bloody hot! So was the chassis rear, by the power tubes.

I checked the surface temp with a Fluke 80PK Surface Probe, finding it was 71 deg C, 55 deg on the bottom end bell, and the chassis was 62 deg C! So, I'm pulling the chassis back out to have a look at the power supply filters. The Choke core temp was 47 deg C, and O/T core temp a mild 41 deg C. I know the power xfmr in Twins run hot anyway, but it usually takes a good deal of time to get them really hot. This doesn't seem right!

Before this amp failed, it was noted that the amp was running VERY HOT. That trend appears to have continued.

[nsubulysses]

Off topic but wondering why you don't order from CE dist? They are the same company as AES and the transformer is $90 instead of $102. I'm sure they would be happy to set up an account for you since you buy from them anyway.

of course amplifiedparts has free shipping over $89 order so if you need to order a speaker or a transformer, sometimes its a toss up if its cheaper to order from amplifiedparts/AES rather than CE anyway!

[Leo_Gnardo]

of course amplifiedparts has free shipping over $89 order so if you need to order a speaker or a transformer, sometimes its a toss up if its cheaper to order from amplifiedparts/AES rather than CE anyway!

Yes, ^^^ this drives me kookoo. I'm going to have to run parallel shopping lists from now on at CE and AES/AP, do some price comparing before finalizing the orders. Tired of shelling out $30-65 shipping UPS for a typical order. Seems the attitude is "we gonna get your money any way we can." Other than that, I'm a long time customer and I do recommend them.

[nevetslab]

Off topic but wondering why you don't order from CE dist? They are the same company as AES and the transformer is $90 instead of $102. I'm sure they would be happy to set up an account for you since you buy from them anyway.

of course amplifiedparts has free shipping over $89 order so if you need to order a speaker or a transformer, sometimes its a toss up if its cheaper to order from amplifiedparts/AES rather than CE anyway!

I had looked into that years ago, and as I recall, it was because I wasn't set up as a recognized business. CenterStaging does have an account with them, but, going thru that path creates long delays in waiting for someone to approve the purchase, so I've just been buying directly thru AES, pass along the cost + shpg to the customer. I do at least hold off as long as I can to get sufficient parts on my shopping list so I don't get hit continually with shpg costs.

[nevetslab]

After Antique managed to loose the 1st Pwr Xfmr I sent back in exchange for the correct #037610 65 Reissue Pwr Xfmr, I finally received it yesterday, and got it installed, first verifying the connections/windings, as well as the mains current/plate current/bias (matched quad set of J/J 6L6GC's), idle current/wattage being 1.32A/131W with 33mA plate current, 14W, 442VDC on the supplies. All seemed ok.

This morning, as I had moved the chassis back into the cabinet just before I closed down the shop yesterday, and briefly ran the amp under moderate burst pink noise, not even showing wattage change during the burst signal vs idle, I set it to run. After an hour and 15 min, I checked to see what the temperature on the core of the PT felt like, putting my hand on it and pulled it off immediately, as it was bloody hot! So was the chassis rear, by the power tubes.

I checked the surface temp with a Fluke 80PK Surface Probe, finding it was 71 deg C, 55 deg on the bottom end bell, and the chassis was 62 deg C! So, I'm pulling the chassis back out to have a look at the power supply filters. The Choke core temp was 47 deg C, and O/T core temp a mild 41 deg C. I know the power xfmr in Twins run hot anyway, but it usually takes a good deal of time to get them really hot. This doesn't seem right!

Before this amp failed, it was noted that the amp was running VERY HOT. That trend appears to have continued.

After the amp had fully cooled off, I removed the chassis, checked the DC Current into both the O/T Center Tap, seeing a steady 130mA @ idle, and about 20mA thru the Choke feeding the screens and the rest of the tube compliment. Then, turning the chassis over, I clamped the current probe onto the Red Lead feeding the 1st Stage Power Supply Filter bank C30/C31 (220uF/500V). Illinois Capacitors. Look at the discharge during the 'off' cycle thru the Bridge Diodes. I've never seen this before.



Amp is in idle state, same AC mains condition as stated above. This looks wrong.....looks like the 1st stage filter caps are fully discharging every 8.33mS. The AC ripple voltage across them reads about 2.7V RMS. Looking at the current thru the choke, I see maybe 3mA ripple in that 20mA DC current flow.

[Dave H]

That looks inverted. I'd expect the flat part to be at 0mA with positive charging current pulses (as below).

Edit: I think the flat part is above zero because it's AC coupled.

[nevetslab]

[ATTACH=CONFIG]52998[/ATTACH]

That looks inverted. I'd expect the flat part to be at 0mA with positive charging current pulses (as below).

Edit: I think the flat part is above zero because it's AC coupled.

[ATTACH=CONFIG]53002[/ATTACH]

Actually, it's not. From the 0mA baseline, center graticle on the scope, you'll see the flat part of the waveform is ~ 150mA, then we're seeing this negative-going charge. Initially, I had the current probe coupled in the opposite direction, but that put the flat part of the curve at -150mA. When I had the current probe on the output from the filter stack, feeding the primary C/T of the O/T, that was a straight line @ 130mA above 0mA. In this particular sensing of the positive end of the diode bridge feeding the stacked filter caps in the doghouse, I'm seeing what I was not expecting.

I'm about to remove the pair of 220uF/300V caps, and put a single 100uF/500V cap there and see what that yields. Current Probe amp IS DC Coupled, Scope is DC Couple. More to follow...........thanks

[nevetslab]

Charging Current into PS Cap C30 100uF/500V Fender '65 Reissue Twin Reverb

Actually, it's not. From the 0mA baseline, center graticle on the scope, you'll see the flat part of the waveform is ~ 150mA, then we're seeing this negative-going charge. Initially, I had the current probe coupled in the opposite direction, but that put the flat part of the curve at -150mA. When I had the current probe on the output from the filter stack, feeding the primary C/T of the O/T, that was a straight line @ 130mA above 0mA. In this particular sensing of the positive end of the diode bridge feeding the stacked filter caps in the doghouse, I'm seeing what I was not expecting.

I'm about to remove the pair of 220uF/300V caps, and put a single 100uF/500V cap there and see what that yields. Current Probe amp IS DC Coupled, Scope is DC Couple. More to follow...........thanks

I've replaced the pair of Illinois 220uF/300V caps C30/C31 with single 100uF/500V cap (C30), checked all the wire connections on the PS PCB, connections on the main PCB, and reran the measurements, with additional results.



The second scope image is with the current probe reversed, which offsets the flat portion of the curve into the negative side, while current into the cap is positive. I also added scope probe to show the AC ripple across C30. I then moved the current probe to measure current into Gnd from the (-) side of the diode bridge. That shows the current flow into C30 from ground, along with the same 10V P-P ripple. Note NO DC shift in the baseline as happens measuring the current into C30 from the (+) side of the bridge. Then I moved the current probe to show the current from C30 into the OT Primary C/T. I checked the inductor on my GR 1568 bridge, measuring 3.24H, Q of 12.5



The point behind these measurements was to see what is causing so much temperature rise in the new power xfmr installed. As I don't see any difference between using the original 220uF/300V caps and a 100uF/500V cap as a replacement, it doesn't seem like that is responsible. Is it just the xfmr design that's resulting in such high core temp, chassis temp to exist? I know these Twins run hot, though this one seems abnormally hot for just after a couple hours at idle. The previous PT failed, and was stated to be running REALLY hot before the primary shorted, taking it down.

What am I missing here?