Galaxie BR20

[TIMBO]

Hi guys, Got this amp as a chassis only. It was built in the late 60's by Soundcraft in Sydney Australia, VADIS was the amps they built. This one was rebadged(sticker over the Vadis logo).
The amp would normally be a combo http://www.ozvalveamps.org/vadis.htm#br20.

A schem drawn to reveal nothing too obscure.

Checked the OT and PT with MM and they look to be OK.
I have not plugged the PT in yet, as it needs a new lead, as well the fuse is on the neutral.
Whilst tracing the circuit two resistors (II 2x10k/1w) between nodes A and B looks slightly toasted, So a closer look at the circuit has a tube lineup of 5Y3GT, 2x6CA7 and 2X12AX7.
That doesn't look right PP 6CA7 will draw 140mAs and the 5Y3GT is only rated at 125mAs 
Now looking at the 6CA7s would be best used in a 40-50watt amp but looking at the OT its smaller than an 18w OT.
Thinking that the BR20 stands for 20w, they cathode bias a pair of 6CA7s down to 20w 
So I can see why the resistor are a bit cooked.

So the one amp fits all has kicked in and produce an amp that will cover all bases (just change the OT)
There is no info on the transformers but looking at the info that I can find, the PT is up to the task for the tube lineup, as it looks to be the same PT in other larger wattage amps.
The 5Y3GT has got me a bit worried pushing the 6CA7s and the OT is too small as well.

My initial thoughts is to use 6V6s in place of the 6CA7s having a smaller current draw.
But I would think that the OT may have the P to P resistance that would suit a pair of 6CA7s. The OT has two secondary taps that I would think would be 8 and 15ohm.

My only question at this stage is would subbing the 6CA7s be OK with 6V6s.Thanks

[kagliostro]

Ciao TIMBO

at http://www.ozvalveamps.org I read

Make: Galaxie badged VADIS
Model: BR 20
Serial #:2111
Inputs: Bass, Rhythm, Accordion
Controls: Volume, Treble, Bass
Valves: 2 x 6L6, 1 x 5Y3, 2 x 12AX7
Transformers: 3258, 4158
Power O/p: 20Watts?

they say 6L6 not 6CA7, may be someone put 6CA7 instead of the correct tube ?

a pair of 6L6 with Va 270v - Vg2 270v - Rk 125R shared - Zout 5k gives an output of 18.5W

and Va 360v - Vg2 270v - Rk 250R shared - Zout 9k gives an output of 24W

The 6CA7 filament consumption is higher than 6L6 (1.2A more for a pair of tubes)

So, it will be better to investigate about B+ and OT Z before to fry them

I think you'll find something near the 6L6 18.5W configuration

Ciao

Franco

[darryl]

Hey Tim, if you stopped buying old amplifiers, you could afford a pair of shoes . . . 

On a more serious note, this thread at AGGH has some information about Vadis amplifiers: http://www.guitargear.net.au/discussion/index.php?topic=15415.0
 

[TIMBO]

Thanks Darryl, Shoes in the mancave is optional. 

[TIMBO]

Hi K, This one has 6CA7 stamped on the chassis.

So with the info thanks to Darryl, running the 6CA7 has had no ill effects.

The chard resistors are possible only add up to 2w may just be a bit light on.

[kagliostro]

Ah, OK, I didn't recognize the 6CA7 was labeled on the chassis


Franco

[TIMBO]

Hi K, I just applied the calculation to the OT on this amp and it worked a treat.
The OT has two secondary taps but only one was used, I assumed that it was a 15ohm as this was a combo and 15ohm seemed to be the norm for these oldies.
Doing the calculations has confirmed that the two secondary's are 8 and 15ohm with a plate-plate resistance of 8000ohms.
I founds the info easy to follow and now I can determine to secondary taps on the PA type amps I have. Thanks

[kagliostro]

Franco

[TIMBO]

Thanks K, After a few fixups I was able to kick start this oldie back to life.
I checked the OT and it has an 8000ohm primary and a 15 and 7.8(8)ohm secondary
With the voltages taken looks like its putting out about 28w and things seem to be OK

[sluckey]

You sure do find a lot of interesting old amps. Stuff I've never heard of before. Thanks for sharing.

The tone stack looks to be a James, but there appears to be a cap missing on the bass pot?

5Y3 seems a bit lite weight for a pair of EL34s?

[DummyLoad]

steve, there is a mid dip with a center f of about 150Hz with a .001uF cap strapped on bass pot 3-2 (james wired proper). without the .001uF on bass pot 3-2, the center f dip shifts up to 180Hz. stack still works without the .001uF, but bass boost/cut response symmetry is lost, as expected. 


--pete

[TIMBO]

Thanks guys, Sluckey I have had a great run of luck buying these quickie amps off ebay, and as usual you don't know what you get till you get it.

This might be another case of building an amp to a price or the available parts.

I have not cranked it yet till I am convinced that the rectifier and OT can handle this circuit.

Can anyone apply their MATH wizardry or suggest a solution if this setup is pushing the boundary's too much.

Also the Ecaps are still in close proximity to there value, but there is a slight bulging at the + end where the relief hole is.
I have replaced one of the filter caps only because it only measured 10uf when it should be 24uf as marked, can this one still be used or it is past its used by date.

Pete, this amp was a combo and may not have been a guitar amp, although it sounds great.

[jazbo8]

What are the actual plate and screen voltages? Using the figures provided by kaglistro earlier, it seems like 20W max not 28W if you are sure the OPT has 15R and 7.8R taps. It's also seems kinda odd they would use 8k OPT for the 6CA7/EL34's...

[TIMBO]

Hey jazbo8, here is the voltage chart ( the plate voltage for the 6CA7 should be in the "3" and screen in the "4")

[jazbo8]

With the voltages that you provided, the 8k load line is way below the knee, that's why you are getting 28W. If you connect the 8R speaker to the "15R" tap, the output power should go up to ~40W. Here is the comparison of the load lines for your reference:

[TIMBO]

Hey mate, the BR(20) is possibly the output being 20w as the OT is smaller than a standard 18w tranny.
There is no info on the OT so only the physical size of the OT is pointing to it being approx. 20w.
Why they decided to use a undersize rect tube for a pair of over sized power tubes pushing an undersized OT got me STUFFED.
This is why I need some sort of clarification to the thought behind this circuit.
Thanks for your help 

[jazbo8]

It is indeed odd... May be that's the OPT they had in the shop when the amps were built... Perhaps you can stick in some 6V6's or 6K6's, as they seem more suitable for the 8k OPT (you may need to lower the screen voltage is bit).

[PRR]

> Why they decided to use a undersize rect tube for a pair of over sized power tubes pushing an undersized OT got me STUFFED.

It is a small/cheap amp. (They maybe made bigger/costlier amps you could buy if you could afford them.)

EL34 has often been about as cheap as any other tube. In the US in the 1960s they were selling much cheaper than the good-old US types of anywhere near the power.

They couldn't find such a good deal on rectifier or OT (or didn't want to threaten sales of a 35-Watt model) so they went with this line-up.

Nothing wrong with pushing 20 Watts out of a couple EL34. I have done that when I wanted long-term no-trouble.

[DummyLoad]

drop in a 5Y3GT and pair of 6V6


happiness & peace of mind.


--pete

[TIMBO]

Thanks guys, These seem to be fairly rare and I think this one has been sitting on a shelf for a very long time as a chassis only and maybe it escaped the demise that many of the others suffered.

I was thinking along the same lines of using 6V6s as I have a nice matched pair of Tung Sol and Groove Tubes.

[TIMBO]

Hi guys, Gave the 6V6s a bash but I had to increase the cathode resistor to get the output down to 12w each.
In doing this it has pushed up the plate voltage to 415v. I know having 415v on the 6V6s is pushing things a bit.

I am confident that the PT has a guts the use the 6CA7s but as said the 5Y3 is light on.

I have looked at other rectifiers that may do a better job, one being a 5R4.
Can anyone suggest any other options for the rectifier so the 6CA7s can be used safely.

[kagliostro]

GZ34 and for safety I'll add a pair of diodes between PT and rectifier





450v - 250mA and only 1.9A filament instead of 2A and can afford 60uF as first cap

Franco

[Willabe]

5V4? Good for 150mA.

A lot of Gibson's used them.


              Brad   

[kagliostro]

Ciao Brad

The 5V4 datasheet say 375v - 175mA





if 375v isn't enoug, the two diodes between PT and rectifier solve the problem


Franco

[Willabe]

The 5V4 datasheet say 375v - 175mA

Another 25mA's, better yet.



                  Brad   

[kagliostro]

Oh Yes !

Franco

[TIMBO]

Thanks for the info guys.
Is there a way to calculate the total amount of current that an amp draws through a rectifier valve. 

[sluckey]

Is there a way to calculate the total amount of current that an amp draws through a rectifier valve.

Do you mean from a design standpoint? Or for an existing amp such as your BR20? If for your BR20, measure the cathode voltage for the output tubes and divide that by the cathode resistor value to get the cathode current. Now repeat this for the little tubes as well. Notice there are actually three resistors in the LTP PI circuit. Now just add up all the cathode currents. This is the total current draw thru the rectifier.

An easier way would be to put a 1Ω resistor between the PT center tap and ground. You could also put this 1Ω between pin 8 of the rectifier tube and node A. Measure the millivolts across this resistor and multiply X 1 to calculate current.

 

[TIMBO]

Thanks Sluckey, Some interesting results......
I have put a 1ohm resistor between pin8 of the rectifier and the first node as this now has a fuse cradle there.

With the 6V6s in it measured 67.7mAs across the resistor and as per the Webber Calculator and the voltages taken with an out put of about 11w/tube and according the calculator the plate current is about 30mAs each, SO am I correct that from these readings that a pair of 6V6s + preamp tubes ARE drawing around 67mAs'

If this is the case then the readings with the 6CA7s in, The current measured across the resistor is 89mAs and as per the calculator with the power tubes bias at 15w/tube the plate current is about 42mAs/tube.

Then this is telling me that the 5Y3GT has more than enough mAs to do the job.

http://www.tedweber.com/webervst/tubes1/calcbias.htm

[jazbo8]

The figures you showed, are valid for idle conditions, but for class AB operation, the peak current will be significantly higher.

[sluckey]

Then this is telling me that the 5Y3GT has more than enough mAs to do the job.

It's rare to see a 5Y3 paired with EL34s. I'd be nervous.

[TIMBO]

Hey Sluckey, I worries me that you are Worried. SO..........

Retired the old filter and cathode Ecaps and after swopping between the two sets of power tubes I'm thinking that the 6V6s are actually sounding best.
There is clean to about 4 on the dial and beyond that some nice breakup then into some real overdrive.

I felt with the 6CA7s to get them into that breakup zone the volume had to be turned up much higher and therefore very loud and I guess putting out more watts than the OT can handle, although the OT was not even getting warm.
There could still be some tube swopping yet.

The head cab is under construction and hopefully get it tolex this weekend.

[TIMBO]

Hi, What a dilemma................

Acquired a 5V4 so I thought I'd see what it would do.

As thought it pushed up the voltages very high and I had to increase the cathode resistor to trim the output.
Its a great bedroom volume at 2 on the dial and ear bleeding moving up the 7 with little breakup.
I had it working for quite awhile and the OT showed no signs of stress and not even warm.
My filter caps are F&T rated at 500v

The POINTS of concerns...
The 5Y3 plate voltage is max 350v, PT is putting out 380v
The 5V4 plate voltage is max 375v.
The 6v6s/5Y3 has 424v on there plates
The 6CA7/5V4 has 462v on there plates
Is the voltage difference between plate and screen too much.

 

[jazbo8]

A part of the problem is the high mains voltage, when the amps were made, the mains voltage was probably lower (230V?) With the higher voltage, everything is thrown out of spec, even if the amp works just fine, the life of the rectifier and output tubes are likely to be shortened.

[TIMBO]

Thanks Jaz, There is another primary tap on the PT that may well be the 240v tap and you are possibly on the mark with the amp wired for 230v. There is no info on the PT so I can only guess, but I will measure the tap resistance and continuity. Also it has been trimmed off close to the winding 

[darryl]

A part of the problem is the high mains voltage, when the amps were made, the mains voltage was probably lower (230V?) With the higher voltage, everything is thrown out of spec, even if the amp works just fine, the life of the rectifier and output tubes are likely to be shortened.

This is unlikely to be the problem, as the history of mains voltage in Australia is the reverse of that in North America.

Mains voltage in Australia prior to 2000 was 240 volts, and equipment built in Australia was designed for 240 volt operation. Good designs would make allowance for local voltage fluctuations above and below this nominal figure. This was a particular problem for rural areas where a SWER ( Single Wire Earth Return ) supply was used. In 2000 the government decreed that the Australian mains power supply was to drop to 230 volts, but fifteen years later, in most parts of Australia the old 240 volt supply still remains unchanged.

[kagliostro]

In Italy we had 220v for civil use / 380v industrial use and this was for many years

(before at home we had 125v for illumination and 220v for appliances, in some regions it was 260v)

now officially is 230v civil / 400v industrial

but at home I measure still now ~220v

Franco

[TIMBO]

Thanks guys, The PT has four primary wires.
Black - 0 (common)
White - 20.3ohms
Yellow - 21.3ohms
Red - 22.3ohms
The active is connected to the yellow and this is making me think that this is the 230v tap.
If the red is a 240v tap is this going the lower the HT as my wall voltage hovers around 245-250v

[TIMBO]

Hi guys, Any thoughts on using 6L6/6L6GC/5V4 combo. Thanks

[jazbo8]

Assuming the PT and the OPT were spec'd for the 6CA7 output tubes, they should also work with 6L6's, you probably want to change the screen voltage and the cathode resistor though.

[TIMBO]

Thanks jazbo, Gave it try with a pair of 6L6s and sounds fine.

The cathode resistor is 470r and using the 5Y3 I'm getting 12w/tube.
With this combo I get 406v on the plates and 375v on the screens, is the screen voltage too low as I have seen the screens just a tad lower than the plates.
I getting this gap because of the 4.7k dropping resistor between nodes A and B, when this is usually the resistor is only a few 100r's.
I guess I could change the 4.7k between A and B to a lower value (270r) and increase the 4.7k between b and C (10k)

When using the Webber Bias calculator and putting in the figures I have it says that the plate current/tube is 30mAs. Is this why the 5Y3 can be used or is there more to it

[sluckey]

the plate current/tube is 30mAs. Is this why the 5Y3 can be used or is there more to it

Yes, that's the reason. Plus the fact that the amp is cathode biased. Now if the amp were fixed biased that 30mA would increase significantly when driven hard and that would likely exceed the current capacity of the 5Y3.

[TIMBO]

Thanks Sluckey, I'm actually liking the 6L6s as they are staying cleaner for longer on the dial and quite loud at higher volume.
I don't think the 24w is worrying the OT too much as it is staying cool.
I have tried a 12AT7 in the PI and this is helping keeping every thing under control.
I'll take another set of voltages for comparison.
Any thoughts on the plate/screen difference.

[sluckey]

Any thoughts on the plate/screen difference.

Yes. It's kinder to the tubes to run a lower screen voltage. Also notice that the node dropping resistor also doubles as a shared screen limiter resistor. If you decrease that to increase screen voltage be sure to add a couple dedicated screen resistors. But, if the amp sounds good, leave it as is. I have a couple old jukebox amps that ran the 6L6s with about 390v on the plates and 300v on the screens. That usually translated into plenty of power and longer time between service calls. It wasn't until that nut Leo Fender started trying to squeeze every watt he could from an amp that you began seeing screens run at or near the plate voltage.

[Willabe]

It wasn't until that nut Leo Fender started trying to squeeze every watt he could from an amp that you began seeing screens run at or near the plate voltage.

         I wish I was 1/2 as crazy and 1/10 as rich!   

[sluckey]

Crazy like a fox! Remember, in the '60s we wanted more POWER. LOOUUDD!!! So loud they began putting mics on the drums!   

[darryl]

It wasn't until that nut Leo Fender started trying to squeeze every watt he could from an amp that you began seeing screens run at or near the plate voltage.

         I wish I was 1/2 as crazy and 1/10 as rich!   

...but not quite as deceased.

[Willabe]

...but not quite as deceased.

Yes, but Leo had a good long life.   

[Willabe]

Crazy like a fox! Remember, in the '60s we wanted more POWER. LOOUUDD!!! So loud they began putting mics on the drums!   

Yes I do but for me it was in 74'/75' when I was in my 1st garage band.

One of the 2 git players bought a brand new Traynor 50w head 2xEL34's with it's 2x12" cab, he was thrilled with it and said with a big ear to ear smile "it's a poor man's Marshall" that thing was LOUD! He already knew some about tube amp circuits, he went on to be a collage physics professor. (The other git man and the bass man still had no name SS starter amps.)
I was just the singer, and couldn't sing yet to save my life, but nobody else wanted to try.   

And the 1st tube amp I bought when I started blowing blues harp was a Dual Showman but with a 2x12" (Jensen alnico) Band Master (?) cab, for $150? That was 1 LOUD harp amp!   

(BTW, somehow, I figured out, and don't know how, but that I blew up 1 of the 12" Jensen's and I threw it in the garbage. I later asked our bass player where I could get a replacement and that I had thrown the Jensen away and he informed me that you can get them re-coned.)


 

[PRR]

> get 406v on the plates and 375v on the screens, is the screen voltage too low

If you can get custom tubes and get your power voltages free, it *may* be optimum to run G2 fairly low and Plate fairly high.

However this leads to high load impedance which affects OT cost and frequency response.

In real life we get G2 power from the big supply we use for the plates. We can drop it, but a simple resistor gives a varying drop because G2 current varies a lot with signal level.

So "audio power" tubes in our sizes are usually tweaked so they "can" find a happy condition with G2 similar to Plate. Maybe 2/3 of Vp with a heavy dropping resistor. Occasionally 101% of Vp when G2 is fed direct and Plate power passes through OT drop. It's all good.

OTOH many radio-transmitter RF tubes and the *BIG* audio bottles (500 Watts/pair) will often be worked with G2 much lower than the Plates, even at the cost of a separate rectifier for G2 (and any little bottles which could not take the high plate voltage).

406/375 seems fine to me, without knowing the load impedance and doing intense curve-fittings.

I had a similar amp which started as a TOO LOUD Ampeg but blew its PT. I re-rigged it for much lower Power and it worked very sweetly. We are past the days when the guitarist has to carry the whole stadium with just his stage-amp.

If you wanted THE MOST that chassis would give, without regard for amp-life, you would go Silicon rectifiers, fixed-bias, expect 1.5 to double the Watts output, but fear for the old parts breaking down on the next power chord. Better to leave it as-is.