Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Apr Mon 22, 2019 12:00 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 13 posts ] 
Author Message
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Mon 09, 2004 5:57 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1630
Location: Silver Spring MD, USA
Hi all -<P>I have acquired a couple of identical power amplifier chassis which came out of RCA consoles I would guess from the 1950s. Am planning to use them for parts for a couple of guitar amp projects (probably single-ended or p-p 6V6 type amps). <P>Trying to get a fix on the power transformers to determine if I can use them. <P>One of the chassis has some handwritten tube types near the sockets - 5U4, 6J6, two 6K6. There is also a Magnoval socket as well - no tube type indicated. While I am curious as to how the circuit may have been set up, I am more interested in measuring some voltages and getting a general idea of the current potential of the power transformers.<P>As an aside, it appears the phase splitter (I am assuming they are a push-pull amps...) is incorrectly labelled as a "6J5" (triode). There are .02 uF caps going from pins 3 & 4 of the "6J5" socket to the grids (pin 5) of the output tube sockets, making me think the tube was actually a dual triode. On the "6K6" sockets, Pin 3 - plates - go to the output trans so I'm pretty sure they are correctly labelled. <P>The socket labelled "5U4" is definitely a rectifier.<P>Looking at a tube manual and working backward, I've come up with these estimates:<BR> <BR>If the rectifier was indeed a 5U4, I would assume 5v @ 3A at least for the rectifier (I would likely use a 5Y3). Adding the filaments for a "6J6" and two 6K6s, I get 6.3v @ (at least) 1.1 A. Also see a HT of maybe 300vct @ 160 ma (probably closer to 200 given the 'mystery' tube).<P>Questions: <P>1 - Are these reasonable assumptions?<P>2- Am I ok to disconnect the secondary leads and bring the trans up on a variac to measure voltages? I have an ammeter in line with my variac, so if I saw any kind of current draw, I could assume a problem.<P>3- What could the mystery tube be, is this a p-p design, and what was the phase splitter?<P>Thanks for any info. This is my first stab at harvesting hardware for another use and it will be another learning experience.<P>Kevin <P>------------------<BR>


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Mon 09, 2004 6:17 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34103
Location: Maryland 20709, USA
Hi Kevin,<P>First, a 6J5 is an octal, and a 6J6 is a 7-pin miniature. Pretty easy to figure out which is correct. I would vote for the 6J5, based on the 6K6's being octals of comparable vintage, and the fact that pins 3 and 4 on a 7-pin miniature are the filament pins.<P>Pin 4 on a 6J5 is a no-connect, so probably used as a tie point. If this connection derives from pin 8 (cathode), then the circuit would work as a phase inverter, one signal from the plate, the other from the cathode.<P>Don't know what the Magnoval socket is for (or what it is, for that matter). Since you've identified the output tubes and the phase inverter/driver, I would expect it's an input amplifier of some sort. Where is it connected in the circuit?<P>The 6-volt filament winding would probably be rated at 1.5 amps: 0.3 (6J5) + 2 x 0.4 (6K6) + 0.3 (Magnoval) = 1.4.<P>Difficult to rate the HV winding. I think your estimate is quite high. A pair of 6K6's in push-pull draw 72 ma peak, and the rest of the circuitry draws very little. I would be surprised if you could draw 100 ma from the transformer.<P>Best estimate would be to populate the tubes to load down the filament windings, and connect a variable load to the output of the B+ filter (disconnected from the circuitry & with new filter caps). Increase the load until the B+ drops by 5% to 10% (+ IR drop across the filter choke), and you should have a reasonable estimate.<P>Also, look at the filter choke. If it has a standard manufacturer's number, perhaps we can get the current rating for it.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Mon 09, 2004 8:15 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4478
Location: Charleston, W.Va.
Hi Kevin,<BR>The only thing I would add to Leigh's comments is that I would try to take the easy route first: If indeed from an RCA console there should be chassis numbers, probably stamped in black ink. If from the 1950's these might typically be RC-xxx, RS-xxx, or some such number. If the chassis number can be determined this could be cross-referenced against RCA Service Data and/or SAMS Photofacts, and the specs for your transformer as well as the proper tube lineup could be easily found.<P>Also, are you certain that Magnavol socket is for a tube? Could be a connector socket for a tuner/preamp chassis?<P>------------------<BR>Poston


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Mon 09, 2004 9:14 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 30698
Location: Pocasset, Cape Cod, MA
RCA must have paid someone to sit at a desk thinking up these goofy names. So how many pins does it have, and what diameter pin circle, or what tube type would fit it?<P>------------------<BR>


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Tue 10, 2004 1:43 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1630
Location: Silver Spring MD, USA
Thanks guys for the info. Leigh, I made a typo on the tubes - the label is 6J5, not 6J6. I also mistakenly figured 72ma each for the 6K6s, obviously wrong.<P>Poston, may be the Magnoval could be an interconnector, will take a closer look. Will also see if there are any markings to ID it. I bought these as is, so I've not seen the original animal them came out of.<P>FWIW, I'm thinking an amp or amps with (one variety) 5Y3, 12AX7, 6V6 or possibly (second variety) 5Y3, 12AX7, 12AU7, and p-p 6V6s. <P>Kevin<P>------------------<BR>


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Wed 11, 2004 5:17 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Gainesville, Florida
Sounds like a chassis out of a HiFi of some kind. I have converted many tube PA amps for guitar use. Any chassis with vacuum tubes in it can be changed to something else. If it is class-A you should already have enough current handling hardware to do just about anything. Output transformers designed for class-A are larger. 5U4 handles more current than 5Y3 but are interchangable otherwise as long as the power transformer can handle the load. If this chassis is made for 5U4 then 5Y3 can easily substitute. 6K6 looks nearly equal to 6V6 in the tube data reference Im looking at. Probably sound enough alike to stick with the 6K6 output. If its p/p then thats plenty of power thru something like a 25W 12" music speaker. There are tons of guitar amp schematics to emulate from places like <A HREF="http://schematicheaven.com" TARGET=_blank>http://schematicheaven.com</A> for many guitar amps or <A HREF="http://www.angela.com" TARGET=_blank>http://www.angela.com</A> how to section for single ended amp design. The Angela Super Single Ended is a fun project that I built once. Look at the signal path in the guitar amp schematic of your liking and make the changes to the amp you have. I dont know what you have for a preamp because you mention only a 6J5 into two 6K6 tubes. If the 6J5 is the phase inverter for two 6K6 p/p then you will need some kind of a preamp before there will be enough signal for a guitar to have any influence on the output section as you say.


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Wed 11, 2004 9:59 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Gainesville, Florida
Here ya go. Coincidently the forum thread called 'Driver transformer woes' has just what you need, a diagram of a Sears guitar amp single ended. Uses a 6J7 preamp into a 6SF5 driver into a single ended 6V6 output. Just the ticket. Have a look at the second drawing in the site listed there for a good idea of this simple guitar amp configuration.


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Thu 12, 2004 7:09 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2405
Location: Virginia Beach Va
Possible it once had a radio section and used another tube located there when playing records as Leigh mentioned..<P> <P>------------------<BR>


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Thu 12, 2004 11:31 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Gainesville, Florida
To better asess what you have and what you would need let me add that the power amplifier you have is good to go just like it is from what you described. It already has what it needs to power all the tubes you mention with the transformers for tubes lettered on its own chassis. The audio output transformers are probably there on the chassis as well. You will need to power it up the best way you know how, or using a variac which ever comes first, and take some voltage readings. If you have never taken HT voltages before, forget it. But you could locate the cathodes of the output tubes, measure dc to ground, check tube charts to see if it is biased Class-A, AB, B or whatever. If Class-A the current handling capability of the transformers are high. The console you removed these (probably stereo pair of) amps from most likely had the pre-amp in the radio part in a separate chassis. Any other audio input most likely went thru this pre-amp section including a record player. You can use these chassis you have just as they are building a separate chassis for your pre-amp or install another socket in the chassis you have if there is room. Im sure that the power transformer could handle one more 6vac filament as well as the current it may draw. A first stage (pre-amp) is a voltage amplifier anyway not taking much to run it. A simple 6SN7 (or higher mu 6SL7) duo triode would give you two stages to play with or you could add a 6J7 (with grid cap) or 6SJ7 (without grid cap same tube) for a pentode sound as a pre-amp. But remember have fun but dont kill yourself doing it. C-ya


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Fri 13, 2004 8:04 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Gainesville, Florida
BTW (not to beat this into the wall) as a precaution, Im sure you know already, remember not to power-up the amp unless there is a speaker or other suitable load connected (to the output transformer)<BR>Just a thought


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Sat 14, 2004 1:39 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2405
Location: Virginia Beach Va
those 6sn7's work just dandy and there's plenty of them around. Those transformers are fine, the exact hv isnt critical. you can always set up the 6sn7 fills with a 6-0-6 from radio shack or somewhere if it's too much for the original.<BR>you might try 6cg7 if there's a space problem (might be the same as the connector hole?) and you wont have to find a greenlee punch.<P>------------------<BR>


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Tue 17, 2004 5:41 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1630
Location: Silver Spring MD, USA
Many thanks for all the helpful info. Have been thinking about an octal tube for a preamp, and the 6SN7/6SL7 sounds like the ticket.<P>I built the circuit out of Dan Torres' book a few years ago and have never been happy with it - finally came to the conclusion it has way too much filtering for my taste, even has the filaments running on DC. Sounds quite 'hi-fi' to my ears. I have seen the Angela circuits (in fact I bought these chassis from Steve) and have been eyeing them as well as my '56 Princeton as potential circuits to try. <P>Thanks again<BR>Kevin<P>------------------<BR>


Top
 Profile  
 
 Post subject: Power Transformer Testing - Many questions
PostPosted: Feb Tue 17, 2004 11:01 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Gainesville, Florida
The site below has the guitar amp from a Sears radio site. The second diagram is a great single ended amp similar to Fender Champ only with an extra stage. I would add another pot between second and third stage for master control:<BR> <A HREF="http://www.nostalgiaair.org/PagesByModel/729/M0016729.pdf" TARGET=_blank>http://www.nostalgiaair.org/PagesByModel/729/M0016729.pdf</A>


Top
 Profile  
 
Post New Topic Post Reply  [ 13 posts ]  Moderators: Marcc, Norm Leal

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot], lorenz200w and 18 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB