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Hoffman Amps Forum image Poll

Are you interested in purchasing junk tube gear on Ebay?

No, not at all.
7 (11.1%)
slightly, if the price + shipping is cheap enough
31 (49.2%)
moderately, but I don't have the time to search for deals
10 (15.9%)
usually, especially if someone posts pre-screened items of interest
3 (4.8%)
Absolutely, it's my main source of cheap parts.
12 (19%)

Total Members Voted: 63

Hoffman Amps Forum image Author Topic: favorite old gear to buy for parts  (Read 214129 times)

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Offline EKDENTON

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Re: favorite old gear to buy for parts
« Reply #350 on: October 06, 2016, 05:39:42 pm »
This one looks much like it and has 9 tubes. different cabinet without the turntable and all but it may be the same schematics. 1949.


http://www.radiomuseum.org/r/geloso_super_g901st_g901st_g901.html


Did someone add the radio to the record player cabinet or was it al built by Geloso?


Guess its the same as kaliegsto posted. :icon_biggrin:

You only fail ... if you quit trying.

Offline Paul1453

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Re: favorite old gear to buy for parts
« Reply #351 on: October 06, 2016, 07:11:01 pm »
I did build a modified Ampeg R-12 with the 6SL7 PI driving a PP 6V6 output section.

I really like the tone of that output section!   :icon_biggrin:

Offline uki

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Re: favorite old gear to buy for parts
« Reply #352 on: October 06, 2016, 07:16:32 pm »
Thanks for the inputs guys !!

Yeah the plan is to use the power section and add a preamp to it, conversion like, the schematic would help but I still can draw it by looking at the circuit although some component values will be missing but that can be worked around, I'll make a thread soon for this specific topic, it is going to be an amp definitely !

@EKDENTON I did saw that one too before, been searching for about 3 days now.
Looks like the whole thing was mounted here, the pickup was a Garrard(England), the speaker is a Hamilton(US), and the machine is from Geloso but the dial plate is for the stations for the epoch back then and in portuguese.
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Offline PRR

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Re: favorite old gear to buy for parts
« Reply #353 on: October 06, 2016, 08:09:36 pm »
Whoa! Blast from the past!

Offline uki

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Score !
« Reply #354 on: October 22, 2016, 07:03:17 pm »
Score !!

Check out guys !! 1961-1962 Tape recorder Geloso G268 !

2x 12ax7 Phillips Miniwatt
1x 6AQ5

3x Iron and components !!

http://www.radiomuseum.org/r/geloso_registratore_per_alta_fe.html
http://www.arimi.it/wp-content/Geloso/Bo81.pdf


« Last Edit: October 22, 2016, 07:29:37 pm by uki »
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Offline Willabe

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Re: favorite old gear to buy for parts
« Reply #355 on: October 22, 2016, 08:24:53 pm »
Nice!   :icon_biggrin:

Offline kagliostro

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« Last Edit: October 23, 2016, 03:28:37 am by kagliostro »
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Offline uki

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Re: favorite old gear to buy for parts
« Reply #357 on: October 23, 2016, 12:42:07 pm »
Hey Franco !!  Thanks for the links man !!

I'm very happy with it, now looks like I finally have all parts for a small amp project !!
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #358 on: November 16, 2016, 05:45:23 pm »
I'm not with the tape recorder preservation society.  :icon_biggrin:  Picked up a V-M 733AV R-R has a couple 6eu7's - 12au7 - 6bq5 - 6ca4 - Ray tube - 600v PT - lamp. All tubes are V-M except 12au7 all very clean. The people that built it must have needed sunglasses. I'm new at this so these parts will wait , till I'm done with a Kalamazoo Model 1 build. The scary part of buying these things , is people plug em in an say it works.  My first donor was a Silvertone R-R I swear it was stored in a chicken barn. Got a 1958 Wollensak T1500 , really just an odd machine it's my least favorite. Not a good conversion amp the OT might be useful.

Offline chrislathan

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Re: favorite old gear to buy for parts
« Reply #359 on: November 16, 2016, 07:10:22 pm »
As an FYI, I am likely to post this monster on eBay will go complete with outboard  preamp section. Unless one of you guys wants it

Offline Paul1453

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Re: favorite old gear to buy for parts
« Reply #360 on: November 16, 2016, 07:23:24 pm »
You will get better at this converting junk into a rocking guitar amp with more experience.

Voice of Music stuff usually has quality transformers at a minimum.

If you are picking these things up for $30-$40 delivered,
you can continue to build up an inventory of quality parts.

They will patiently wait on the shelf for you to gather the
knowledge, skills, and abilities necessary to convert them.
They also usually have quality vintage tubes that are worth much more than the new Chinese or Russian stuff.

The Swollensak, as I call them, is it a PP 12AB5 output circuit?
They are built like a tank, and the ones I got mostly had rebranded Mullard tubes in them.   :icon_biggrin:
V-M tubes are rebranded, if made in Gt. Britain or Holland they are premium tubes.
If this is what you also got, then you have all the major components to make a nice guitar amp.
It is just not a very friendly housing for a guitar amp.
Putting those parts in a nice chassis and housing will create an amp you can be proud of.

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #361 on: November 16, 2016, 11:50:43 pm »
Top price I've paid delivered is about $32. I did just buy a PR-40 power supply for $45. on eBay.
    The Wollensak is a push-pull amp , the PT is like half of a PT and half Widow maker? I recapped it before I looked at the schematic better I'll leave it intact until I learn how to build something with it's parts. There's a few jacks and a couple of caps that are probably usable.
            The V-M has those two 6eu7's yep they're made in Great Britain (need to have em tested). Not much of a collector , my guitars are all pretty cheap , my favorite amp is my H&K Attax 80 lol I do love the sound of these tube amps.

Offline Paul1453

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Re: favorite old gear to buy for parts
« Reply #362 on: November 17, 2016, 12:04:54 am »
2 Mullard 6EU7s can sell for more than you paid on Ebay. 
But if you're like me, you want just add them to your stock of premium tubes.

When you get ready to build your parts in a new chassis,
and use those tubes in your circuits you will be pleased with their sound.  :icon_biggrin:

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #363 on: November 17, 2016, 12:17:31 am »
2 Mullard 6EU7s can sell for more than you paid on Ebay. 
But if you're like me, you want just add them to your stock of premium tubes.

When you get ready to build your parts in a new chassis,
and use those tubes in your circuits you will be pleased with their sound.  :icon_biggrin:
They don't eat or drink my beer ;) The local shop here only has one kind to pick from and they're $34.95 each :)

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #364 on: February 07, 2017, 10:20:05 pm »
A couple of scores to  :icon_biggrin: about. A Silvertone 7072 Reel-Reel $15 Habitat store 12AX7-12AU7-6V6GT-6X5GT all Silvertone tubes. Small Iron

Nice size , like new PT and OT from a Voice of Music VM714 Reel-Reel  ran 3-12A_7's, 1-6v6 & 1-5y3gt ....... $27 eBay  :icon_biggrin:




Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #365 on: February 08, 2017, 03:43:39 am »
Few days ago I got a Reel-Rell Grundig TK16 Recorder for 20€

I'm not so happy because I was hoping in a better OT, however this is what you can recover from one of this recorders (if you find one)

an EF86 (and this may be a good thing)

a pair of EL95 (6aq5) one was used as power tube

an ECC81 (12AT7) and an EM71 (loctal socket Magic Eye)

The PT is small but nice

1 x 6.3v winding with CT @ (at least) 1A (feeds EM71 - ECC81 - 2 x EL95)

1 x 6.3v winding with 100R Hummpot @ (at least) 200mA (feeds V1, the EF86)

I didn't measured the other two windings, but on the TK16 schematic the rectified voltage is

1 x 27v DC @ (at least) 85mA

1 x 270v DC @ (at least) 42mA

OT is small and the spec of the recorder say about around 2.5W output power

Oh, not for me, but if someone is interested on building a Record Turntable the motor is very nice and sturdy



May be I'll be able to recover also the cabinet, but it has large rounded surfaces and I'm not sure it will be possible to cover it with tolex

(at the moment the coating isn't in good shape)

Franco
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #366 on: February 09, 2017, 01:25:33 am »
                 Looking at the speakers these things are driving you can't expect much in the OT.    I've run my little Silvertone tape recorder amp 12AX7-6V6GT-6X4  cranked into the  little OT that was with it. 5k pri - 8ohm The spec'd replacement parts are Merit A2900 , Stancor A3856. both  4watt universal OT's. 2-4 watts is pretty loud thru the right speaker.    :icon_biggrin:

Could of bought a big console stereo 4-6BQ5's  -$75  but my son didn't seem to enthused about loading it up and lugging it home.
        We brought home a big Hammond organ years .... ago it must have weighed 350lbs. My back was bad before that ... too dumb to realize what was in it. Now I'd have to hire someone to fetch one of these monsters home and pay em to haul off the carcass. The smell  :l2: of these old tape recorders is like  :lipsrsealed: I need an intervention.


Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #367 on: February 09, 2017, 04:09:10 am »
I was not expecting a big OT, only, some time ago, I got a Reel-Reel Philips EL3516/42 (that uses an ECL82 as power tube)

that is claimed to be 2.5W output power and the OT on this recorder is near dubled in dimensions (570gr = 1.25lb)

respect the Grundig TK16

So, if you score a Philips EL3516/42 is a good thing  :wink:

Franco






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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #368 on: February 10, 2017, 01:45:16 am »
I was not expecting a big OT, only, some time ago, I got a Reel-Reel Philips EL3516/42 (that uses an ECL82 as power tube)

that is claimed to be 2.5W output power and the OT on this recorder is near dubled in dimensions (570gr = 1.25lb)

respect the Grundig TK16

So, if you score a Philips EL3516/42 is a good thing  :wink:

Franco
Some of these things have an extension speaker jack and have a PA feature. Don't get much European stuff here , see stuff from Germany listed but , it's pretty expensive and shipping. The early 60's Japanese stuff is really , I don't want to use the word cheap. Haven't burned up my Japanese Kalamazoo yet .... knock on wood.  :laugh:

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #369 on: February 16, 2017, 12:34:39 am »
A couple of scores to  :icon_biggrin: about. A Silvertone 7072 Reel-Reel $15 Habitat store 12AX7-12AU7-6V6GT-6X5GT all Silvertone tubes. Small Iron

Nice size , like new PT and OT from a Voice of Music VM714 Reel-Reel  ran 3-12A_7's, 1-6v6 & 1-5y3gt ....... $27 eBay  :icon_biggrin:
I ordered a Sams on eBid for the VM714 PT/OT 14 days ago nothing has arrived. Had to go to Paypal to file a case.

eBay usually takes care of this sort of thing for you. With eBid your on your own  :cussing: Satisfied customers rarely talk about good service , dissatisfied customers , ... well ......  :cussing:

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #370 on: February 17, 2017, 02:02:17 pm »
Getting into the Silvertone model 7072 tape recorder it's an oddball no photofact I see one for a 7074 Didn't want to start a thread for this build until the OT is sorted out 2" x 1-5/8" winding is about 1-1/2" another sub 5watt OT  :wink: 2.5watts maybe.  The PT is pretty small
 The schematic is better than usual lol.  My dilemma is the OT I'm assuming yellow is positive , green is negative , and black is a ground ? ...... haven't seen this before. I'm guessing it's a preamp out? (TRS)

secondary:
yellow to green is 0.9ohms 0.04mH   
yellow to black 9.65ohms resistor
green to black 1.1ohms 0.09mH

Primary
red to blue 432.5ohms  19.70H
Any help would be greatly appreciated

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #371 on: February 17, 2017, 04:32:50 pm »
Ciao Phil

Here a bit more readable image of the OT



Primary

B+ connected to Red

Plate connected to Blue

Secondary

Black connected to ground

Speaker (two speakers in parallel) connected to Yellow (so seems)

Speaker other pin connected to Green

I'm not able to read well the schematic, but seems that secondary has a very low impedance

are you able to measure the Turn Ratio of the OT and compute the impedance ?

With 205V on plates of the power tube I think we can espect around 5.5K of primary impedance

You have 1x 12aU7 +1 x 12AX7 + 1 x 6V6 + 1 x (what seems to be a) 6x4 (is it ?)

So PT has, at least, 1.65A of current for filaments and .... ??? may be 40mA B+

If you use all the tubes you can build .........

An AA764 ?





Franco
« Last Edit: February 17, 2017, 04:34:57 pm by kagliostro »
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Offline HotBluePlates

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Re: favorite old gear to buy for parts
« Reply #372 on: February 17, 2017, 05:18:13 pm »
...  My dilemma is the OT I'm assuming yellow is positive , green is negative , and black is a ground ? ...... haven't seen this before. I'm guessing it's a preamp out? (TRS) ...

Green-to-Black is the full secondary winding (impedance unknown).

Green-to-Yellow is marked 1.6Ω; when nothing is plugged into the Output Jack, the pair of 3.2Ω internal speaker are in parallel from the Green-to-Yellow on the OT secondary (3.2Ω || 3.2Ω = 1.6Ω).  The output jack has a switch to disconnect the internal speakers when something is plugged into the Output Jack.

The Output Jack provides output to (presumably) and external speaker with the OT secondary from Green-to-Black.  There's also another output at that jack which is not entirely clear to me.

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #373 on: February 17, 2017, 05:58:44 pm »
The 7074, also if there is one more tube, is very similar

may be this schematic can help to understand better the 7072



Franco
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #374 on: February 17, 2017, 08:28:10 pm »
I need to learn to read these things better (Green-to-Black is the full secondary winding) I don't have a Variac to check these things out :BangHead:

Wouldn't it be logical to use the whole winding?  It is a TRS jack so maybe they have some weird combination of using internal and external at the same time.  The measurement green to black , is the same on my Silvertone 2230 OT , which is 8ohm.  I'm gonna go with the bigger measurement green to black 1.1ohms. These are resistance measurements. I was trying to find the link to a site where they discussed how to tell if a OT was good or not and they were trying to interpret what 8 ohms and 4 ohms showed for resistance on a VOM. 4 ohm being less than 1 ohm.

Well the PT is good  :icon_biggrin: It's a 6X5GT rectifier.

By the way thank you  :smiley:
« Last Edit: February 17, 2017, 08:30:28 pm by frankenxtein »

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #375 on: February 18, 2017, 12:18:53 am »
The 7074, also if there is one more tube, is very similar

may be this schematic can help to understand better the 7072



Franco
Thanks for sharing that schematic  :worthy1: as soon as I get the Photo fact on the Voice of Music VM-714 , you know where to find it. Left is VM-714 PT , middle is reverb OT , far right VM-733 OT , bottom right is VM-714 OT.

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #376 on: February 18, 2017, 03:41:45 am »
Ciao Phil

Ah, OK, so it is a 6X5

here the spech



---

Quote
.... I don't have a Variac to check these things out

You really don't need a Variac for that purpose, you can simply use a low voltage transformer (also a 6.3v will be fine)

Feed the secondaries of the testing OT with the 6.3v (measure exactly how many volts there are on the secondaries) and then measure the voltage at the primary

Divide the primary voltage for the secondary voltage and you obtain the turn ratio

Give alook here

Quote
http://www.radioremembered.org/outimp.htm

---

Quote
Wouldn't it be logical to use the whole (secondary) winding?

Yes, it will be, but is better if you wait for some other opinion

---

I didn't understand well ..... do you need the Voice of Music VM-714 schematic ?

---

Quote
I was trying to find the link to a site where they discussed how to tell if a OT was good or not ......

is this of any help ?



---

Quote
....... they were trying to interpret what 8 ohms and 4 ohms showed for resistance on a VOM

To use resistance to know which is the impedance value isn't a good idea, a transformer hasn't a fixed impedance, it has a turn ratio

reflected impedance to the primary depends on turns ratio and load impedance connected to secondary

so if a transformer reflects 4K to the primary with 4ohm load connected to secondary, the same transformer reflects 8K to the primary if you

connect an 8ohm load to the secondary (and so on)

Franco


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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #377 on: February 18, 2017, 11:43:35 am »
I'll need to make a rig with my spare 6.3 ....... these computers have great calculators. So doing the odd math  would be easier than doing it with my Abie normal brain.

I've seen it done but , by placing the voltage on the primary. Then reading the secondary?

 It's logical to apply the voltage to the secondary (AC) then read the Primary(DC) 
I'm waiting on some clips , terminal blocks and a better meter so somebody stop me before ....  :rolleyes:

I really love re purposing old tape recorders. It makes you feel nostalgic and evil all at the the same time. Gives you a little revenge , on the old crappy pulleys and belts that dashed my dreams of becoming a hair do sequined suit with a guitar. The Egg shaped sound wobbling ...... :cussing: ....... If I run across a really nice good one I'll save it. These things will have a second life beyond me ...... 
I'd really like to just get costs out of them here to some of the local musicians eventually. Tailor them to the sound they want. I don't have a wind tunnel but we're not going to fly these things .... they are tiny   Maybe build one for my Granddaughter.  Having ballpark idea's of what specs are on them is kinda important seeing what we go through to find them.

Yes I need a Photofact on the VM714 the colors are faded but it looks new. I've got 4 wires coming out the primary side of this thing and wondering what they are. 2 Black 2 yellow. I've got the other side sorted out. 5v, 6.3v, HT ,CT . Then there's always the OT. I ordered Photofact again on eBay and expect it soon. These are really nice transformers (pt 3" tall x  2-5/8" wide x 3-5/8 bell to bell  2" thick lamination 4lbs on the bathroom scale lol. Don't want to mess these up and was hoping to get a current rating on the HT and some idea of what the wattage is on the OT. Could be a nice SE or get a PP OT if the HT current is there. I got those loose , the seller said they came from a VM714 tape machine. 40-50mA is the norm for most everything I've got so far.

I haven't done the neon bulb test on these but I've done that on another set.

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #378 on: February 18, 2017, 04:17:57 pm »
The calculations are very simple

divide the Primary voltage for the Secondary voltage to have the turns ratio

multiply the turns ratio for the turns ratio

multiply the obtained number for your speaker impedance and you obtain the primary impedance (using that speaker)

200 : 6.3 = 31.746

31.746 x 31.746 = 1007.8

1007.8 x 4 = 4031.2

---

Quote
It's logical to apply the voltage to the secondary (AC) then read the Primary(DC)

Errr.... no, my council is to connect the 6.3v to the secondary because this way the voltage you have on the primary is not very small

is a way to have a more readable voltage on the Tester and easier to be used for the calculations

So you are looking for the Voice of Music VM-714 schematic (if I've understand you correctly)



There are

1 x 330v - 0 - 330v AC and the schematic specify 60mA DC (I think about 100mA AC but we are interested in DC mA)

1 x 6.3v with (at least) 300mA

1 x 6.3v with (at least) 1650mA

1 x 5v with (at least) 2A

Franco
« Last Edit: February 18, 2017, 04:28:11 pm by kagliostro »
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #379 on: February 18, 2017, 08:37:23 pm »
 :icon_biggrin: Thank you , I think I understand now , and thanks for the schematic.

I'm hoping the Photofact will be as good as the one I got for the Silvertone 2230.... with a parts list , replacement specs. Maybe that one is an exception , we'll see. I need to build a better prototyping rig lol instead of my 6AQ5 amp It's just right don't want touch that one , it's quiet and clean and pretty loud. I like those 6AQ5 tubes. I've got 3 amps I like the way they are , 1 that's kind of a screamer , and that poor Japanese which will probably end up in the Esteban as that Skylark in the other thread lol.   

Maybe a Verb o Champ will be possible. With the VM714 Iron ? All I need is a tank  :icon_biggrin:

The Silvertone 7072 a Vibro Champ?

The size of these OT's has been a bit of a mystery to me. The one in my 6AQ5 amp is the same size as a Champ OT Which is the same size as the 7072? The only one I've ever got warm is that Japanese OT which is the same size as a champ OT.

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Re: favorite old gear to buy for parts
« Reply #380 on: February 19, 2017, 12:37:57 am »
> my council is to connect the 6.3v to the secondary

Yes, though that leads to HIGH voltage on an OT primary.

In this case, the OTs were never designed for 6V at the secondary. 6V^/4r is 10 Watts, and these are couple-Watt transformers. And 50/60Hz is probably "low" for them. The measurements may be corrupted by transformer distortion.

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #381 on: February 19, 2017, 01:50:40 am »
Thanks for the advice PRR

Which is your  council for this small transformers ?

Thanks

Franco
« Last Edit: February 19, 2017, 01:54:28 am by kagliostro »
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #382 on: February 19, 2017, 07:05:00 am »
I found this on eBay (http://www.ebay.com/itm/162385556809 Olson SC-5f / TT-450 1 - 140VAC 5 Amp Auto Transformer Tested Working)

I was thinking what I could string together to get 1volt ac and thought ....... that never works out to be a bargain. Hopefully this will work.  :dontknow:

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #383 on: February 19, 2017, 08:46:13 am »
If you want a low voltage and don't have a variac, you can use cascaded transformers

120 / 12 > 120 / 12 you have an output around 1.2v

of course other values can be used to obtain an output around 1v

I apologize about the suggestion to connect the 6.3v to the secondaries, I didn't considered the small size of the transformer

Franco
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #384 on: February 19, 2017, 09:37:48 am »
No apology necessary  :smiley:
Everyone here has been so helpful and I don't want to impose on our friendship. I'm having a blast with these amplifiers. I'm sure the next old tape machine I find at the thrift store will be coming home with me. I hope I'm not abusing this thread.
       The new VOM has a 4volt  range so I should be able to get a reading using the methods demonstrated in this video.  https://www.youtube.com/watch?v=Yzo3A-NywSs&t=3s

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #385 on: February 21, 2017, 11:11:48 am »
I don't have a scanner and thought I should share some manufacturer specs on these transformers.

Power transformer for VM-714 used in 750A, 711A
Part # 12558 manufacturer 524839 (on my part)
Pri 117vac .57A
Sec1 720vct .060A
Sec2 5vac 1.8A
Sec3 6.3vac 1.3A
Sec4 6.3vac 1.A

They do not show other replacement PT's

Output transformer fo VM-714  used in  710A, 711A
Part # 12559-1 manufacturer 138852 (on my part)
Pri 5300 ohm
Sec
30 ohm
tap
@ 8 ohm
Replacement parts: Halldorson Z1006 , Merit A-2902 , Ram AU-600 , Stancor A-3849 , Thordason 24S64 , Triad S-55Z


I'm not sure how to interpret that but will update this as soon as my Variac arrives.
That's the way it appears on the parts list. Whether the 30 ohm is the tap or the 8 ohm is the tap. Just learning here so bear with. The following is from the VM-714 manual ....

Power output:
10 watts. Less than 3% distortion at 3db below rated output.Impedance: 8 ohms. Dampening factor: 12.

Output jacks(two)
External Amp. Constant 1 volt output at 10000 ohms from normal tape level.
External Spkr. (dual position) 8 to 16 ohms impedance for single or multiple speakers (may also be used to drive a 600 ohm line: no load necessary) up to 8 volts output at less than 1% distortion.

In summary. These transformers came from a nice tape deck , the amp had frequency response 20-25000 Hz.  Just guessing but the OT is worth much more than I paid for it. The list of replacement output transformers should help somebody in identifying the specs of their finds. I know I will search for those on the eBay  :icon_biggrin: Thank you to all again for helping me , having a blast with this new to me hobby.

Offline PRR

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Re: favorite old gear to buy for parts
« Reply #386 on: February 21, 2017, 02:57:14 pm »
> Whether the 30 ohm is the tap or the 8 ohm is the tap.

"Tap" is always the lower voltage/impedance connection.

LOOK at how this OT is used in the original. The "8" goes to speaker. The "30" actually goes up the 6V6 Cathode. This is NFB. This is why (for undistorted signals) the output does not care 8, 16, or 600 Ohms (or no load). The NFB keeps the output level nearly constant.

The "10 Watts" figure is optimistic. They run the 6V6 near 13W Pdiss. However the happy-load for 360V and 37mA is 10K, not 5K. They are forcing huge bias-shift in the max-output test condition. It may really deliver near 8V on 8 Ohms. We usually let SE amps bias shift but this is extreme. Especially since this is not really a "fixed bias" amp. "-20V" is derived from R25 which is flowing mostly 6V6 current, and will change at maximum output.

Figuring 6W out on a nominal 30 Ohm winding we have almost 14Vrms or 20V peak. The bias on the 6V6 is 20V (the "18V" is meter-loaded), and so there is 6dB NFB. Not a lot. Would be a fine guitar amp just the way it is. However the signal at the 6V6 grid now must be 40V peak. V2b is working hard. V2a adds more gain and then R12 turns that into significant NFB. Sensitivity at V2a grid is near 0.2V.

OMG. Not only "stereo", but Stack/Stagger stereo (on some models).

You have a Germanium diode in there.

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #387 on: February 21, 2017, 05:21:50 pm »
It would have been nice to have gotten the whole 714 deck intact  :wink:

Well it looks like another oddball PT for the too high voltage too low mA pile lol. I was hoping to build a Champ with reverb. Perhaps another OT and that would work. The 714 OT would be good in that Silvertone 7072 saving its tiny OT for another 6AQ5 amp. I really like that 6AQ5 amp.
« Last Edit: February 21, 2017, 09:54:05 pm by frankenxtein »

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #388 on: February 22, 2017, 05:28:59 am »

Ciao Frankenxtein


Quote
Well it looks like another oddball PT for the too high voltage too low mA pile lol
Give a look to the transformer on this thread (that I recovered and is still waiting for a use  :think1:  )

http://el34world.com/Forum/index.php?topic=16097.msg157174#msg157174

Secondary
6.3v @ 1.8A
6.3v @ 0.9A
5v    @ 2A

750v CT @ 50mA AC (375v-0-375v)

375v * 1.4 = 525v DC, not a low voltage as B+ and there is "only" 50mA AC current disposable

Franco


Post Merged

Phil, I'm not able to understand how this mod can operate  :w2: :w2: :dontknow: :dontknow: :w2: :w2:

To me seems an odd schematic  :w2: :w2:



give a look to the different approach of our friend Leevi



Leevi's mod to me has sense (and is full performing for sure)

Franco
« Last Edit: March 17, 2017, 05:03:55 pm by kagliostro »
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #389 on: February 22, 2017, 11:18:15 am »

Ciao Phil
Quote
Well it looks like another oddball PT for the too high voltage too low mA pile lol
Give a look to the transformer on this thread (that I recovered and is still waiting for a use  :think1:  )

http://el34world.com/Forum/index.php?topic=16097.msg157174#msg157174

Secondary
6.3v @ 1.8A
6.3v @ 0.9A
5v    @ 2A

750v CT @ 50mA AC (375v-0-375v)

375v * 1.4 = 525v DC, not a low voltage as B+ and there is "only" 50mA AC current disposable

Franco


Post Merged

Phil, I'm not able to understand how this mod can operate  :w2: :w2: :dontknow: :dontknow: :w2: :w2:

To me seems an odd schematic  :w2: :w2:



give a look to the different approach of our friend Leevi



Leevi's mod to me has sense (and is full performing for sure)

Franco
Wow and I thought I had troubles  :laugh:

I'm not sure what they have going on Down Under with that Champ Reverb (@ Offset Guitars)
Can't see the library , waiting for registration ..... to be activated. That OT listed on his print is a 7k -3.2 ohm 125A3A3 Maybe a 8 ohm load ? That's a revised version of what he started with. Pretty stuff.

If I use a 6L6, 6L6G, 6L6GB, 6L6WGA/5932 and one Preamp tube and a custom built 8k or 10k OT I'd get pretty close. to a Champ?
http://thesubjectmatter.com/calcptcurrent.html

Selected transformer voltage: 325-0-325 ( 330-0-330 mine )

Calculated voltage at first capacitor (B+): 406.25V. Subtract 6V, if using choke.

Calculated filament current (typically the 6.3v secondary): 1.23A

Calculated current: 59.37mA at 8686R calculated load (10% plus factored in). Preamp valves current draw is estimated at typical 12AX7 max dissipation of 1.2W at 330V, i.e. 3.6mA per triode

That's kind of getting away from the "thrifty" 
Well the shelf is looking good for this PT   :BangHead:
I have to buy a tube anyhow?

That 807 is interesting I can get that Tube , socket , cap for about $25

Offline kagliostro

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Re: favorite old gear to buy for parts
« Reply #390 on: February 22, 2017, 12:12:37 pm »
I can get 1625 tube (till few mounts ago) for 5€ each, may be socket for 3.50 or 4.00 € and Topcap for 1.50 - 2.00 € don't remember well

there is less request for the 1625 tube (is a military version of 807) because it has 12.6V windings

---

About the Reverb Champ, I've read the original thread, also the author don't know why it works (he says it works ..... ?????)

http://www.offsetguitars.com/forums/viewtopic.php?t=35226

I prefer a lot Leevi's version


Franco
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Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #391 on: February 22, 2017, 02:25:46 pm »
I'm looking at the few options I have with that 714 PT. I don't understand enough of this stuff to  experiment around too much. I'm reading quite a bit. I'll build something one day with it. I've got stuff to build a breadboard and I'll have a Variac soon. My limited knowledge has me trying to build simple stuff. Compared to the other OT's I have around here that 714's looks big. The custom made OT's run $30 + shipping. Which is probably not bad considering , I don't have a 5Y3 yet lol. I'll be checking out some of the transformers I have around here. Re working a few of these hack jobs I've built. 

PS: Ready to start a thread for the VM714 .... seems I found my stuff here  :icon_biggrin:  http://el34world.com/Forum/index.php?topic=9444.msg86674#msg86674
« Last Edit: February 22, 2017, 09:53:07 pm by frankenxtein »

Offline super&plexi

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Re: favorite old gear to buy for parts
« Reply #392 on: February 26, 2017, 01:40:12 am »
Crap? Maybe Concord RxR heavy as hell.... Barely see tubes.....24-5 bucks chance at salvation/goodwill..........2 bugle boy/mullard  12at7 & 2 of that popular tube people make headphone amps from.....6c7 or something like that. Can't remember exactly which maker of pre amp tube, but know for sure couple of my good stash came from that. Might've been Telefunken's. Point=they were pristine.


 Bonus was markings were untouched, guess owing to fact you really had to disassemble, and crowbar your way in. Seems guitar players always ohing & ahing the chalky amperex markings right off the tube.   CC R's were close tol.


       Multi tier, multi tap switches, tons of metal, even cast frame little 5's.....looked like cute tiny EV's or JBL minis. Not high power, but simply adorable (that's attempt at humor). & you can make single 12" cab out of huge cabinet. Nice heavy metal handle too, although not beneficial for setting head on. ....who cares, it cost 25 bucks
keep on with those scales and that fish is gonna die, if it don't bite you first!

never fried a tranny ..till I built a dim bulb tester. UPDATE-haven't fried anything since learning how to properly build & use one...thanks Uncle Doug, & el34 World

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #393 on: February 26, 2017, 12:30:27 pm »
Crap? Maybe Concord RxR heavy as hell.... Barely see tubes.....24-5 bucks chance at salvation/goodwill..........2 bugle boy/mullard  12at7 & 2 of that popular tube people make headphone amps from.....6c7 or something like that. Can't remember exactly which maker of pre amp tube, but know for sure couple of my good stash came from that. Might've been Telefunken's. Point=they were pristine.


 Bonus was markings were untouched, guess owing to fact you really had to disassemble, and crowbar your way in. Seems guitar players always ohing & ahing the chalky amperex markings right off the tube.   CC R's were close tol.


       Multi tier, multi tap switches, tons of metal, even cast frame little 5's.....looked like cute tiny EV's or JBL minis. Not high power, but simply adorable (that's attempt at humor). & you can make single 12" cab out of huge cabinet. Nice heavy metal handle too, although not beneficial for setting head on. ....who cares, it cost 25 bucks


The 1/4" jacks in the Silvertone 7072 I got for $15 would cost over what I paid.

      Really nice parts sounds like a good score for you. I've yet to find any good speakers , the tubes I have been setting them aside. I need to make an appointment or drop them off to be tested. After I build with some not so rare tubes I have put back the original tubes just to see what they sound like. All three of them  :l2:  Usually it's 12AX7-6aq5 or 6bq5-6x4 and that's fine for me. After I finish my Voice of Frankenxtein. I plan on building an amp with 4 tubes   :icon_biggrin:

      Building from these old machines is more expensive sometimes than buying new parts. It's a heck of a lot harder finding designs other than the original to build with the really high voltages of the PT's . The OT's are usually small sub 5watt with weird wiring , color codes that change from model to model. Then it's all baked in a tiny box ........ if it's well used , I'd imagine the parts are pretty stressed. $10-15 for a Photofact on top of it.  It's cheaper to build with new small iron unless you get  the donor really cheap. I have yet to run out of old crap.

     Hunting certain machines for certain things that you can get specs for , not just schematics. It's challenging , frustrating and fun. None of this is lost on the eBay sellers , trying to make a killing in antique tape recorders. I scan thru there from time to time and find a bargain. lol ..... 

Offline Willabe

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Re: favorite old gear to buy for parts
« Reply #394 on: February 26, 2017, 05:12:49 pm »
OMG. Not only "stereo", but Stack/Stagger stereo (on some models).

??     What is "Stack/Stagger stereo"?

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Re: favorite old gear to buy for parts
« Reply #395 on: February 26, 2017, 05:20:37 pm »
This info from...  http://www.thevoiceofmusic.com/faq_restoration.html#tapeFAQ


Q7-5: I have a switch on my V-M tape recorder with two positions called “Stacked” and “Staggered”. What does it do?

 A7-5: First, a little history. The very first stereo format was on reel to reel tape! V-M Corporation offered a conversion kit for the venerable Model 700 in 1955 - some three years before stereo records and five years before stereo FM broadcasts! The first format? Very simple - take a second half track monaural head, turn it upside down, and mount it to the right of the existing monaural head. Connect it up with another pre-amp and you have the “staggered” stereo tape format! The tapes were recorded with the starting points of the “L” and “R” tracks “staggered” so that both signals would be in sync when picked up by the playback heads. This format is also called “offset”. In 1958, as magnetic head manufacturing capabilities improved, the “stacked” or “in-line” format was invented - and remained the broadcast standard for many years - the so-called “half track stereo” format. The V-M Model 714 was a hit because it had a switch to play both the newer “stacked” and older “staggered” formats. In November of 1959, V-M brought out the Model 720, one of the first on the market to play the new “quarter track” stereo tapes.
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #396 on: March 04, 2017, 08:52:45 pm »
It's been a good day to eBay 37 is the number  :icon_biggrin:

 Snatched up a handful (7)  NOS Single ended OT's for $37 one of them a Merit A-2930 (5k pri  - 3.5ohm sec) 6 others in that lot UTC , Stancor , Thordarson ,  .... Tiny iron :wink: They got me looking at 6y6 tubes  :think1:

Started looking for PT's and ........ well that's a trick finding lower voltage PT's but ..... I'm sure there's a couple here that would work.

Scored a $37 PT from a Stromberg Carlson SR405 Will make a nice PP amp. It ran a couple 6F6 tubes ...

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #397 on: March 08, 2017, 04:26:16 pm »
Found a nice PP OT
35:1
9800 ohm primary blue-red-brown / 8ohms green-black
about 3 + lbs
Iron Size
2.5" wide
3" tall
1-3/8" thick (1.25"minus bells)
2-7/8" bell to bell Winding's

The other side is scratched up and I can't make out any numbers
It has 161230 , under that it has a logo ? Anyone seen that logo?

It was hooked up to 2- 6L6GC's and has that marked on the top. 

Offline PRR

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Re: favorite old gear to buy for parts
« Reply #398 on: March 08, 2017, 11:55:50 pm »
350-450VDC, self-bias, aim at 25W Pdiss per 6L6CG for 450V, get 30W with hi-THD bass, aim lower and get 20W with hi-fi bass.

Offline frankenxtein

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Re: favorite old gear to buy for parts
« Reply #399 on: April 09, 2017, 11:43:18 am »
Got this in a lot of transformers ?  :dontknow:
TF4RX21AH
This number 863780 led me to
https://www.wbparts.com/rfq/5950-00-997-8957.html

 


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