Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frankenxtein on November 26, 2016, 01:25:39 pm
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Not sure this is
Two tape recorders into one amp:
Parts from V-M 730
PT (12691-13 549-6537) sec (300-0-300) Don't have good specs on it , it ran 5 tubes not counting the eye tube has 6.3v tap just for it's 6CA4 (EZ81) that is the only 6.3v I was planning on using on this PT. The other 6.3 winding appears to be separate.
Parts from Wollensak t1500
PT Just for filament heater supply , the 12.6v (.4A) for the 2- 12AB5's and using it's 6.3v (.75A) for a single 12AX7
OT (pri 8.8k sec 130 ohm (7 ohm) that is currently hooked up to those 12AB5's. I haven't settled on a design yet. Nothing fancy .... anyone see a problem? Other than doing something with the 6.3v on the small PT not drawing down the voltage. I'm anticipating that but other than adding a lamp or 2. Was planning on using the same Deluxe Micro preamp circuit.
I've only built a couple SE amps a Silvertone 1481 , and a Kalamazoo Model 1 that uses a Micro Deluxe preamp. From the power tube on back it's a Kalamazoo , filter caps and such.
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I didn't understand well the spec of your transformers, can you post the schematics where they were used ?
Franco
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I didn't understand well the spec of your transformers, can you post the schematics where they were used ?
Franco
The PT providing the HT is from a Voice of Music Reel to Reel I've got a lame schematic of that ;)
What I'll call "the other" is here https://www.dropbox.com/s/5iipqogkdy99b5w/OTFT.pdf?dl=0 (https://www.dropbox.com/s/5iipqogkdy99b5w/OTFT.pdf?dl=0)
Can't get a snapshot of it
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Well so much for this idea ..... sorry ..... I'm not not sure what to make of the PP OT primary one side reads 230 ohm the other 180 ohm? On top of that the 12.6v transformer is really in bad shape. The leads are pretty crumbly .... can't , wont use it :BangHead: It all worked before harvesting the two transformers from the Wollensak :laugh: Oh well might just put the Voice of Music PT on the shelf for a bit.
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Here is the other schematic
(http://i.imgur.com/IDD44mp.jpg)
OK, so
the T-1500 PT has
a 200v winding that supplies a 12AX7 and a 12AT7 tube (really few mA)
a 6.3v winding that supplies the same tubes = 600mA
a 12.6v winding that supplies the two 12AB5 tubes = 400mA
B+ for power tubes is from the line using a doubler, but this insn't of our interest
the Voice of Music PT has
a 6.3v winding that feeds 2 x 6EU7, a 6BQ5, an EM84, and a 12AU7 = total 1820mA
a 6.3v winding that feeds the 6CA4 = 1000mA
a High Voltage winding that before after rectify give a B+ of 340v
At the end you have two TA with
One is
200v few mA (just feed a pair of preamp tubes)
6.3v @ 600mA
12.6v @ 400mA
The other is
HV for 340v B+ (if rectified with the 6CA4 tube)
6.3v @ 1820mA
6.3v @ 1000mA
so this are the values to consider in your new construction
Consider that you must not control an OT using the winding resistance, because of the way of construction often one branch is longer (and so has more resistance) than the other, to control an OT you must control the turn ratio
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I've just seen this
http://stonemarmot.com/rantrave/ak9.html (http://stonemarmot.com/rantrave/ak9.html)
(http://stonemarmot.com/images/AK9a.jpg)
and this
http://www.tdpri.com/threads/inexpensive-lightweight-can-of-whoopass.478718/ (http://www.tdpri.com/threads/inexpensive-lightweight-can-of-whoopass.478718/)
(http://i406.photobucket.com/albums/pp142/printer2_photo/Guitar%20Amp%2002/6AK5Bassman_zps86cef4ab.jpg)
Franco
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Thank you Kagliostro , that's some much needed info on the V-M 730. Trying to scrounge up a 12.6v Transformer here has been fruitless. Other than a wall wart. The Wollensak (1958) that I thought would be a great source is really in poor shape.
I'm afraid to send more money down that hole. Hoping to learn a bit more about PP amps kinda took a hit on this one. I have much more to learn. Put the 12AB5's on the shelf. There will be another day for those. Again thank you.
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> B+ for power tubes is from the line using a doubler
DIRECTLY from wall-plug!! This is NOT SAFE for guitar amplifier.
Actually, if you did everything exactly right, including the unusual driver transformer, it "could be safe". But I would NOT go there. Proper isolating power transformers are not hard to find. Even new ones (which are not rotten). Certainly cheaper than a good shock.
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Hope it's okay to change the topic a bit :dontknow:
Daring as I am ... it was a drag when I noticed the T1500 was a widow maker (see) post #3 in this thread. Definitely not much used except for a 1/4" jack , pilot lamp 12AT7 tube from the T1500 , the rest VM730. PT , OT , 12AU7 , 6CA4.
Had built one before with a 250-0-250 with diode rectifier , changed it 6X4 and EL84 , stolen from VM730.
Used the VM730's higher voltage transformer in the
SE Micro Deluxe II
All tubes heaters off the green 6.3vac set of leads
(@6.9vac still ? with 3 tubes and lamp)
B+1 359V
B+2 283V
12AT7 CBS
V1A Plate (pin 1) 150V
V1A Cathode (pin 3) 2.02V
V1B Plate (pin 6) 147V
V1B Cathode (pin 8) 2.05V
12AU7 V-M Westinghouse
V2 Plates (pins 1 and 6) 334V
V2 Cathodes (pins 3 and 8) 13.26V (470ohm resistor)
6CA4 V-M
V3 470v @ rectifier W/O tubes
(470ohm dropping resistor)
373vdc @ rectifier W/tubes
One end effects the other it seems.
Change the Cathode resistor you have change the rectifier voltage dropping resistor. The voltages at the Preamp change too.
The numbers at the Preamp stage are the closest I've gotten to the original design?
Not really sure where V2 is biased , it sounds pretty good but it probably wouldn't make it on the Space Shuttle.
13.26 divided by 468 = .0283
.0283 X 334 = 9.45
It's a Direct TV box upside down everything to spec except 12AT7 pretty much
A link to Rob's Deluxe Micro https://robrobinette.com/Deluxe_Micro.htm