Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on November 15, 2013, 02:43:00 pm
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Hi,
this is another question inspired by a 1960 Vibrasonic I have on the bench for a check-up.
I'm looking for a good source of transformer codes.
The reason I'm asking is that the amp (with a JA code on the tube chart) has what looks like a Fender replacement output transformer. It has a larger footprint than the chassis was originally drilled out for. Extra mounting holes have been added for it as it is wider than the original set of mounting holes. It has been in the amp for a long time judging by the condition of it, and has definitely been soldered in after the amp was built originally.
The code is 022897/606-2-36 and is stamped FENDER.
From what I can make out this is a Fender replacement for the Vibrasonic and Showman?
Possibly a Schumacher from the 36th week of 1962?
The main question is, if it is a replacement, was it a direct replacement? And if not would it have changed the sound at all? I guess there is always the option of installing a Mercury Magnetics replacement if so…..
Any help/comments/advice/opinions/theories/general banter would be great!
Cheers, VB
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I'm looking for a good source of transformer codes.
Better put on your Sherlock hat, as nobody ever has everything rolled up in one place.
The 6G13A Vibrasonic schematic (http://www.el34world.com/charts/Schematics/files/fender/vibrasonic_6g13a_schem.pdf) lists the PT as part number 67233 (almost certainly an old Triad part number, but not one you'll find in a Triad catalog).
In Fender Transformer Charts (example here (http://www.unclespot.com/FenderXFMRchart.html)), 45548 (or 45550) is listed as the output transformer for the 6G8 Twin. Checking the 6G8 Twin schematic (http://www.el34world.com/charts/Schematics/files/fender/TWIN_6G8.pdf), 45548 is indeed listed as the OT.
BTW: the first time I ever saw one of those charts was in Dave Funk's Tube Amp Workbook, but I don't know if someone published earlier than that. The chart I linked doesn't even have the Vibrasonic at all.
The code is 022897/606-2-36 and is stamped FENDER.
Possibly a Schumacher from the 36th week of 1962?
Schumacher, yes. But other 60's Fender amps seemed to transition to the "125P___" or "125A___" numbering system. Look for yourself at Fender schematics from the early-mid 60's.
The main question is, if it is a replacement, was it a direct replacement?
So early on, there seemed to be a link between the Vibrasonic and the Twin, even though the Vibrasonic only had a pair of 6L6 output tubes.
Maybe you never ran across the "Vibrosonic Reverb (http://www.thevintagesound.com/ffg/vibrosonic_reverb_sf.html)" which was around a decade later. This was basically a silverface Twin Reverb, but with a different nameplate and a 15" speaker. I briefly had one a long while back.
If you look at the schematic for that amp (http://www.thevintagesound.com/ffg/schem/twin_reverb_sf_100_schem.jpg) (silverface Twin Reverb/Vibrosonic Reverb), you see the part # 022897 cross-referenced with 125A30A and noted as for the Vibrosonic only.
Now all the "Twin Reverb" power transformers were called "125A29A" and referred to 4x 6L6 to 4Ω; the single Showman and Vibrasonic Reverb 125A30A would be 4x 6L6's to 8Ω. When the Vibrasonic was first introduced, it made sense to take the 45548 (4x 6L6:4Ω) and attached a single 8Ω speaker to reflect the appropriate load for 2x 6L6's.
The code is 022897/606-2-36 and is stamped FENDER.
...
Possibly a Schumacher from the 36th week of 1962?
Based on the evidence from the silverface Twin Reverb/Vibrasonic Reverb schematic and the date range given for the Vibrosonic Reverb on the Fender Amp Field Guide, I'd say the OT is 36th week of 1972, and was ordered as a replacement for a damaged Vibrasonic OT. Unfortunately, when that repair was made, the model of the similar name was really 4x 6L6's:8Ω. As you suspected, this had to be an old repair.
So your output tubes probably see half the load they were intended to see when the amp was originally designed. Unless your speaker is a 16Ω JBL D-130 (the Field Guide suggests the stock speaker was a D-130F, but the prototype D-130 was most-commonly seen as a 16Ω speaker).
Other than impedance mismatch, the core is bigger to handle more power than the original 2x 6L6 unit, so it no doubt gets good, wide clean frequency response.
Whether to change the OT for the smaller version, or swap speakers to correct impedance mismatch and enjoy the bigger OT... well that's another call only your customer can make. Maybe present pros/cons for both this and the speaker repair issue & let him decide.
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Hi HBP,
I really appreciate you taking the time to give such a comprehensive answer with so many references. Thats certainly a bit of sleuthing to work out the TF replacement as being one for a 70s VibrOsonic instead of a 60s VibrAsonic! :thumbsup:
One thing I omitted to state that it is a 5G13 tube chart with a date stamp of JA, so that makes its a Jan. 1960. It also has the "centre volume" pot. and the empty control position next to the speaker jack on the rear.
Its a JBL D130F speaker that measures a dc resistance of just over six ohms.
Cheers, VB
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One thing I omitted to state that it is a 5G13 tube chart with a date stamp of JA, so that makes its a Jan. 1960.
Gotcha. But the 5G13 schematic/layout didn't have any transformer info to go on. :icon_biggrin:
Its a JBL D130F speaker that measures a dc resistance of just over six ohms.
This is news to me, but apparently all the earlier D-130's were 8Ω, regardless of what the speaker said. If DCR was below 8Ω, then it was an 8Ω speaker. Later JBL's that were actually 16Ω speakers measured more like 11-12Ω DCR.
That's according to Harvey Gerst (http://www.gearslutz.com/board/so-many-guitars-so-little-time/417825-about-my-d130-jbl.html), who designed the D-130F and was around for production of the original D-130.
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Hi,
Just realised that I never updated this thread with the results of the OT replacement.
I ended up with a Mercury Magnetics Type FBFDS-O (a 125A29A type) which is very large lump. It sounds full and rich, with a deep bass that is not plummy or woolly with a smooth top end without a hint of harshness. It is NOT a small difference at all. I'm very impressed indeed.
:icon_biggrin:
It's been a very long haul with this amp, with many, many things wrong with it, and I'll do a post in a while with the outcome.
Thanks again for all your help. :worthy1:
Cheers
VB