Got my first real tuibe amp today(old RCA)

ben62670

So my aunt calls and says she needs a ride. I pick her up and she said my mom was probably at the thrift store. I told her that I was looking for a consol stereo there and she says I have one that I am junking:biggrin: So I go and check it out. My Grandmother bought it at Salvation Army in 1974 for $34.50. Right now it has a lot of extra crap on it. I hope to be able to get some diagrams on it so I can get it running.

chargerman426

So is going to be/is a separate amp or an integrated? Also, looks like you have a tung-sol black glass tube there, heard they are nice tubes.

ben62670

I am thinking maybe intergrated, but for now it will be just an amp.

ben62670

Lots of stuff in there. I want to remove the tuner, tone controls, and all the lighting ciruits.

Big Dawg

That looks like a mono amp - will you start searching for a twin?

Very sweet!

ben62670

It is actually a stereo amp

headrott

Some good tubes in there Ben.

Greg

mhardy6647

Well, that's a radio. Turning it into an amplifier might be best accomplished by stripping the chassis bare and starting afresh, leveraging off the convenient holes for tube sockets (even salvaging the sockets if you wish). The power transformer would also probably be useful. The output transformers would be, of course, better than nothing :-)

Looks like single-ended 6V6. What's the driver; 6EU6 (an RCA proprietary tube, used also by Magnavox - like a 12AX7, but without the center-tapped filaments and different pinout, IIRC)?

If you can get the chassis number, you can look and see if it is in Sams (it should be). I have a pretty prodigious collection of Sams; I might have the schematic if they have it.

The Sams site is: https://www.samswebsite.com/
The lookup function is pretty picky; if you have the numbers, I have a paper Sams index (from 1975!) that's easier to peruse.

Have fun!

ben62670

Thanks M. On the back plate are some numbers. Is that the info I would need.
Ben

Edit:
AC1168 C
RVG 2748495

mhardy6647

yup; post 'em here and I'll have a look if you like.

ben62670

I did, but I just missed ya.

mhardy6647

d'oh! I'll look 'em up tonight or tomorrow.
might be worth checking Tom Bavis' site, too:
http://www.audiophool.cjb.net/Techno.html

Fongolio

Very cool Ben. Dirty like my old Eico was. That'll clean up nicely though.

scottyboy76

does that make you a tuibe.

seriously, great find, since visiting and doing business with jerryj12, i have all the local junk stores on alert for stuff like this.

This has been a revelation to me that this old stuff is capable of sounding the way it does.

mhardy6647

I think "tuibe" is Dutch :-)

I looked for this chassis in Sams online and didn't find it. I will check the paper index tonight.
Further details as events warrant :-)

NJPOLKER

Ben I have one question for you.

Is there something wrong with you?


:eek::eek::eek::eek::eek::eek::eek::eek::eek::eek::eek::eek::eek:

ben62670

Why do you say that?

mhardy6647

maybe he thinks you'll work on it "live" while you're barefoot standing in a puddle of water on the basement floor...?

ben62670

mhardy6647 wrote: »

maybe he thinks you'll work on it "live" while you're barefoot standing in a puddle of water on the basement floor...?

Well it was live. I measured 300-0-300 and tube rectified 325-0-325. Does that make sense?

Edit: The rectifier tube is a 5AS4A

On3s&Z3r0s

That seriously looks like electrocution in a box! But it'd be fun as hell to play with. Enjoy!

txcoastal1

cool toy...but keep the grand kids away

ben62670

I'm only 40! My kids are ove a 1,000 miles away so I think I am safe

Edit:6V6GT output tubes...

dcmeigs

Ben, I think I recognize that radio chassis. I'm pretty sure it's a Magnavox and I'm pretty sure I have one of those carcass around here somewhere, so if you need a part, just holler.

The news is that if I'm correct, that is a mono radio with two single ended 6V6s running two output transformers. It a bi-amped situation where the old radio cabinet had a woofer and a midrange or full range driver. You will probably find a high pass filter on one and a low pass filter on the other. Remember, the first stereo broadcasts were not until the late 50s (and they were AM/FM simulcasts). That looks like it's from a 40s console radio.

The carcass I have did not have the external transformer attached to the back of the chassis (like an afterthought). Instead it had a field coil speaker. For those unfamiliar with old radios, a field coil speaker is a speaker with an output transformer riveted to the basket. The 200 or 300 VDC B+ runs to the transformer and the return wire, reduced by a six or seven volts, returns to the plate of the 6V6. Sounds dangerous? Well it's inside a cabinet so it's not like it's out laying on the carpet. I imagine it did cause a few cat fires back in the day.

So I agree with mhardy that your best best would be to salvage the PT and the OTs and some of the sockets and start from scratch. The other option is to make the rounds of the junk shops and pick up a late 50s Magnavox with a 6V6 or EL84 push pull power amp. Everybody and their brother has done one or two of these on accounta they are well layed out, have a spacious chassis and the finished result is a useful amp that sounds as nice as you want to make it sound. Another benefit is that you will have schematics and the support, guidance and cheers of a large online community.

With that radio, me thinks it's going to be you, your scope, a signal generator and your (very frightened) cat.

Good luck with whatever you decide.

ben62670

I do know for sure that I took it out of an RCA console. It has stereo tape inputs and stereo phono inputs. Also it had two 12" drivers. None of that really matters though. I am planning on scraping the thing for the transformers and building an amp for my puter. The output transformers only have one tap so I am guessing it is for an 8ohm load? I hope that it is not for a 16ohm load. There is just way too much going on in the amp that I do not want or need. Like tone and balance controls. The power transformer is 300-0-300 and when tube rectified it measures 320-0-320vdc. Does this sound right. I know that SS rectification yields an increase of 1.4x the AC voltage.

dcmeigs

OK you have two questions there.

First the OTs. If they have two wires on one side and two on the other you know that it's a single ended design and not a push pull. The difference is fundamental and instead of me rattling on just google and read about it. You probably aleady understand the difference from your solid state work. You also know that you don't know the impedance and that is a problem you want to solve.

Here is how you do it. First identify the primary and secondary windings. In the tube world the tube side is primary and the speaker side is the secondary. Remove it or at least cut the leads. Connect your variac to the primary leads and connect your fluke to the secondays. Now turn up the voltage on the variac until you get one volt on the primaries. The voltage on the variac (secondaries) is your turns ratio; lets say thats 45 volts. Now square that and you have 2025. Now you grab your trusty tube manual (yeah, right) and look up the 6V6. I'm pretty sure you'll find that the load resistance for a 6V6 will be about 8000 ohms at hi-fi voltage levels. So 8000/2050 is about 4 so in my example the transfo would be for a 4 ohm speaker.

The second question is a little more difficult to answer. With IN1004s you know what you will get but with tube rectification it will depend on load. Again, it's in the tube characteristics curve in the tube manual but you can find that on line in PDF form. Just find the voltage from the PT and follow the curves. But there is an easier way. There is an excellent program called PSUII (PSU 2) that will model your power supply to get the answer. Download that and check it out. It's very straightforward and it's in universal use. As in vintage solid state gear, CRC filters are the way it's done. You can fool with the second C but you have limits on the first dictated by the rectifiers limits.

I suggest you keep the rectifier tube and not go solid state for the B+ (plate voltage). Solid state is great for bias supplies but tube rectification provided a time delay and a soft ramp up as the 5U4 (or whatever) warms up. That delay is a good thing.

Now the PT is a 300-0-300 so you will have a B+ as you say in the 320 volt range, again depending on load. BUT it's not 320-0-320, that's solid state thinking. It's 320 B+ and ZERO grounded to the chassis. There is no negative rail.

ben62670

I do want to keep the tube rectifier. I'm not going to need to run a program to calculate the vdc because I have a working PS correct? I know that it is putting out 320-0-320 I was just curious if that sounded like proper voltage from a tube rectifier. BTW I got nailed yesterday by 220vac off my tube pre's transformer. I was taking measurements and thought I was testing the 20vac secondaries so I just grabbed a hold of both of the hots. It was a revolting experience!

dcmeigs

OK, that's right. I type faster than I think. I edited my post so you probably didn't see it but read the last paragraph about the 320 B+ and ZERO chassis ground. There is no negative rail.

You have got to be real careful in the tube world. That 300 volt B+ can put your heart into defibrillation and then there's the whole clean underwear thing to consider. There is a real good sticky on safety over at diyaudio that you should read. It's about 80% stuff you already know but it's the 20% that you haven't thought about that will kill you. Now that you will be working with it you should take a few minutes and read it over.

dcmeigs

Most of the time you will be working on dead circuits, but when it's show time you will need to check voltages and what not. With tube gear you should not take your test leads and start probing around. It's just bad form. The chicken way to do it is to go down to harbor freight when they have those little red VOMs on sale for a buck ninty nine and buy three or four and a bag of insulated alligator clips. Clip on your leads, step back, power up and watch your voltages come up, overshoot and then settle back in. If you have enough meters you can hit B+ and all your plate voltages and you don't have to probe. Live to fu$k up another day is what I say.

ben62670

I know when you double the voltage it is four times as deadly. I'll do a little more testing on the voltages before I start taking it apart. Time for me to get some sleep.
Thanks much.
Ben