GA 5 Build (schedule 40) using EZ81 in place of 5V4?

[shaun]

Hi All,
I'm about to build the Schedule 40 GA 5, although I aim to put it in a wood combo cabinet w/speaker. From an old defunct Zenith reverb (yes, they put a reverb tank in a stereophonic record player:), I have a nice little chassis and a power trans with 2 secondaries.

At 120v in:
700v    350v  at C.T.
7.2v     3.6v  at C.T.

The power trans does not have a 5v secondary, so I am trying to figure out whether I can use a 6.3v EZ81 in the GA 5 instead of the 5V4 rectifier tube. I don't care about using different sockets, but I'm vague on whether the EZ81 will provide the correct B+ voltage. I know the EZ81 draws 1amp, so that indicates I probably wouldn't overload the trans (don't know the amp/current rating of the trans).

I believe I'll have to damp down the 7.2v with resistors or diodes to run the 6.3 filaments - I could use some advice on whether that would work, and around what amount of resistance might be appropriate. I might need to do the same with the power cap filter system, although that might be within the limits of acceptability according to the schematic - I'm working off the Hoffman schematic.

I'm not so much going for a specific tone - I'm too green to know what I'm doing yet, and any point to point tube amp sounds great to me. But I do want the amp to function without catching fire. Just to have it run nicely would be a big success. God this is fun. Did I say that yet?

My last inquiry concerns whether the indicated output trans (022905) would match up okay using the EZ81 and trans combination I want to use. Thank you for all and any help. It's much appreciated.

[sluckey]

You can use an EZ81 if the transformer filament winding can supply the current. Do you know what tubes were in the Zenith donor reverb unit?

That 7.2v filament winding will probably drop to just right when you put some tubes on it.

That 22905 OT will be fine.

[shaun]

Good to hear - I forgot the V would drop under load. I can't find a schematic for the Zenith reverb unit, but the old tubes are 6.3v - no 5v at all. Looks like I'm golden:).

[PRR]

>> Do you know what tubes were in the Zenith donor reverb unit?
> I can't find a schematic for the Zenith reverb unit, but the old tubes are 6.3v

Answer the question asked. If you know what and how many tubes it was made for, you can estimate the original B+ and 6V ratings, and compare to your proposed plan.

[ALBATROS1234]

You can usually pick up a filament xfrmr for cheap. I have gotten 5 and 6.3v @ 1.5 to 2.5 amps for like 10 bucks on fleabay. Plus shipping is under $20 total then you can use a 5v3 or 5y3 whatever.

To find out if you can use you trans for the ez81 add up the filament amperage from the tubes used in the zenith. If the amps show enuff avail you are golden

[shaun]

PRR, point well taken. There were three tubes in the reverb unit, so now that you've schooled me (much appreciated, seriously), I'll go do the math. Thank you!

And thanks to albatros1234. It never occurred that trans could be found other than from retail suppliers, which gets a bit pricey. I'll look around. And thanks too for the math - I didn't realize it was that simple.

[sluckey]

PRR, point well taken. There were three tubes in the reverb unit, so now that you've schooled me (much appreciated, seriously), I'll go do the math. Thank you!

What were those three tubes? Can't do the math without knowing that.   

[PRR]

> What were those three tubes?

It's his secret. Let him do math.

[shaun]

Urgh. Excuse me.

So, I found the schematics online for the power amp and the tuner, but not for the reverb unit. All the tubes were out of the chassis (plural of chassis...chassii?) when I got the Zenith, but I was told they were all there. By a process of elimination, I feel confident the three tubes are probably: 6BH8 RCA brand; 6CG7 Zenith brand; 6X4 Zenith brand. The last is a mini tube rectifier.

I didn't realize your help extended to actually doing the math - I didn't want to assume:). But your math help would certainly confirm whether I have a clue as to what I'm doing, so thanks again.

From what I can tell, it seems all three tubes draw 0.6 amps, for a total of 1.8 volts for all three. Is it that simple? I like this sort of math.

However, I want to use an EZ81, an EL84, and a 12ax7, the total draw of which seems to be: EZ81 - 1amp; EL84 - 0.76amp; 12ax7 - 0.3amp. Total of 2.06 amps.

So it seems my transformer is shy of around 0.26 amps. I have no idea whether that small amount would cause problems.

[sluckey]

Just do it. If that PT wont tote the load then get one that will.

[shooter]

I like this sort of math.

hang in there you'll be breakin out the slide rule soon 

all three tubes draw 0.6 amps, for a total of 1.8 volts for all three

um... I think you mixed metaphors

[shaun]

Okay. I'll forge ahead and see how hot it gets under load.
Thanks again for your help.

[PRR]

0.26A over may not be a problem.

Alternatively, instead of EZ81 use 6X4. 0.4A lighter, well under original loading.

My concern is that EL84 will want more plate current than any part of 6BH8 (12AX7 hardly counts). OTOH, they don't usually make super low current windings, super-thin wire is not worth the hassle. So yeah, just smoke-test.

[shaun]

Good idea. I'll look into that. Thanks.

[shaun]

Heyo. I've completed the build on the GA 5 (Schedule 40 schematic) using an EZ81 rectifier tube. I'm getting particularly high VDC off pin 3 of the EZ81: with the variac at around 90v going into the amp, the reading on pin 3 is a little over 400VDC. That seems a bit high. I guess that EZ81 is throwing out more power than I need. I put a 250 ohm 5w resistor coming off pin 3 to see if that would tame the VDC, but it seemed to have little effect.

The tube line-up is EZ81, EL84, 12AX7.  Preliminary testing was done sans powers and preamp tubes. I'm tempted to try a 6X4 for the rectifier, but as it's a 7 pin, I'd have to replace the socket.

As the amp stands now, I don't want to crank the variac to 120V in case things start melting. With the variac at 90v, the PT tests at around 290 either side, which seems about right.

Any suggestions? Thank you in advance for what I'm sure will be helpful.

[sluckey]

Preliminary testing was done sans powers and preamp tubes.

That means the power supply had no load so the voltage will be high. Put all the tubes in and recheck. The voltage will decrease.

[PRR]

> I'm getting particularly high VDC ...a little over 400VDC.... testing was done sans power.. tube..

At my house, I get 125VAC with "nothing on".

On Laundry Day it goes to 108V. The dryer alone pulls it down to 118V.

ALL power sources "sag". While _your_ house wires may not sag much (my wire is just too long), the PT and the rectifier inevitably have significant sag at full load. 10%, 20%, maybe more.

"over 400V" is not zero or 800V (oops!). There's no major flaw.

Now plug in "your dryer" (that EL84) and I bet it sags down to the expected zone.

[shaun]

Wow, thanks guys. Much appreciated. I'll do that.

[shaun]

The problem with the Schedule 40 amp schematic is it doesn't list expected voltages for B+ and at the plates, but it seems to me that mine is way high. I have tubes in, all hooked up. At 90V on the variac I'm getting 160v at the plates on V1 and 450v off pin 3 of the rectifier, so if I crank the variac to 120, I expect things to start melting. Or at least the tubes might not last very long.
My sense is that I need to tame the VDC coming off the EZ81 before going to the filter system, but I don't know how.

[shooter]

I have tubes in, all hooked up.

so what are the volts DC on the PA tube, cathode and plate when you're @ 90vac on the variac

[shaun]

Voltage on plate of the EL84 is 300vdc with 90v on the variac. The cathode is at 3.0vdc, or 0.002vac going up to 0.022 with volume and tone turn up full.

[shooter]

3.0vdc, or 0.002vac

confused;
those are both AC AND DC, so DC is solid regardless of volume?

[shooter]

so the ballbark math says the 84's cooking @ 3.6W at 12mA, loafin

[shooter]

since it's loafin;

monitor the cathode ( Vk) volts DC
here's your math so have a slide rule;
Vk / 250 = tube current
while monitoring and calculating, slowly bring the variac up to 100, you want Ik to stay somewhere < 25mA for now.  if you have plenty of room at 100, bump it., still room, ........  IF you get to 120vac on the adjustable AC, and < 25mA Ik... remove adjustable AC and go straight for the wall plug.

get your Ik value, the plate DC and multiply them ie, 320 *.022 = tube power in watts

[shaun]

Thank you! I guess I'm gonna hafta shrug off my lethargy toward math and learn the basics. But your numbers will give me a guide to whether I'm doing the numbers correctly. Much appreciated.
I assume loafin means idling. 

[shooter]

assume

this part of the build ya don't wanna do that;

work the math 20 times with "pencil" guesses;
hmmm I want 11Watts, I got 3.6W, I need more...........turn the dial, check, turn, check...... playin through a 4 X12 dimed