Built A Sch 40 GA-5 Today

[shortfuse]

Well mine is in a aluminum chassis.  I tip my hat to those who have built it in a 6 X 6 PVC J-box it has to be tough in that space.
I tried to use the new voltage chart.  Please comment.
Sounds great, way better than the Firefly thing I did for my 1st build.  Now to finish the face plate and build a head box.

[phsyconoodler]

Nice work! Keep it up!

[EL34]

yeah, nice
That's a cool looking little chassis

And yes, it's a tight squeeze in a SCH40 PVC box.
I had to use an electronic shoehorn

I did not see the link to your valve data file
here it is in case others want to look at it
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10310

Those voltages are way too high
Was the amp running with the valves in it when you measured them?

[Geezer]

Those voltages are way too high
Was the amp running with the valves in it when you measured them?

Yeah, it sure looks like there's no load by those voltages.......??

[shortfuse]

This is pretty bad but I don't remember now, I did it both ways with the tubes in and out.  I did it the first time with the tubes out and thought I may have done it wrong.  Then did it with the tubes in and then my connection timed out and it didn't save the data so I am not 100% sure which data I entered.
I did it with the tubes out on the plexi 50 build but no one ever commented so I was really unsure.
Can I edit it and redo it with the tubes out?  Should a reading be done on the rectifier tube also?
Please excuse my ignorance on this I just have not been too deep into the theory and knowing what the voltages should be.  I have been able to take proven designs that work and accomplish the builds.  My next step is to start to really understand whats going on inside because right now I know just about enough to be really dangerous.
Thanks
Steve

[sluckey]

The fact that the plate voltages were very high and the cathode voltages were zero indicates the tubes were not conducting. A very likely cause would be that the tubes were laying on the bench. There's no way for the amp to make any sound with the voltages you posted.

[EL34]

I didn't even get as far as looking at the cathode. (In a rush as usual)
I saw the plate voltages on the EL84's and that sent up a big red flag
Should have looked at the cathode also since that tells the whole story (many times).



No way to edit the data files yet.
I went on to another project and have not had time to get back to the valve data project.

Just go ahead and create a new file.

Eventually, I will have a way to edit files.

[shortfuse]

No time to mess with it tonight going out of town tomorrow.
So the voltages are to be read with all tubes in?
When I did try I had a hard time with the 5V4 trying to read pins 2 & 4.  Pins 6 & 8 should be DC voltage should they not?  I am going by the Current Diagram chart http://www.el34world.com/charts/images/CurrentDiagram2.gif  Because the only way I could get a reading was on the A/C setting.

Thank You for the replies.  I am still learning.

Steve

[sluckey]

Pins 4 and 6 will read high ac voltage between each other and high ac voltage to ground.

Pins 2 and 8 will read 5VAC between pins and B+ DC to ground.

[shortfuse]

Ok tried it again tonight with tubes in and with info from previous posts.  But on the rectifier tube the only way I got a 5v AC reading was with the tube out and only from pin 2 to ground and with tube in across pins 2 & 8.
All readings are with tubes in.

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10363

Sluckey why does the current flow diagram http://www.el34world.com/charts/images/CurrentDiagram2.gif show the rectifier circuit as DC or am I misunderstanding the flow diagram in my previous post?

[sluckey]

But on the rectifier tube the only way I got a 5v AC reading was with the tube out and only from pin 2 to ground and with tube in across pins 2 & 8.

You should have 5VAC between pins 2 & 8 with tubes in or out. Ignore any ac reading you get from pin 2 or 8 to chassis. Your meter is just confused with that one.

Sluckey why does the current flow diagram http://www.el34world.com/charts/images/CurrentDiagram2.gif show the rectifier circuit as DC or am I misunderstanding the flow diagram in my previous post?

The output of the rectifier diodes (cathodes in this case) is DC. But the diode anodes are connected directly to the PT secondary which is AC. Did you notice the "AC" and "DC" labels next to the diodes?

[shortfuse]

The output of the rectifier diodes (cathodes in this case) is DC. But the diode anodes are connected directly to the PT secondary which is AC. Did you notice the "AC" and "DC" labels next to the diodes?

No I did not see the forest through the trees I got fixated on the colors and did not pay attention to the details.  Funny thing about that library all the info is there you just gotta find it.  Some times it takes a few looks but its there.

Any comment on the voltages.  The EL84 seems pretty hot.

Thank you

[sluckey]

That particular drawing is very busy. Easy to miss a small detail.

Ignoring screen current, your EL84 is dissapating 23.6 watts. Way over max spec. I'd put a 500Ω cathode resistor in it and recalculate.

[shortfuse]

Changed the 250 to 470 and here are the new readings

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10378

Previous Readings with 250 ohm
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10363

[john_t]

Been away for a bit. Just  saw your post. I have to say my voltages are a bit different. Not sure if that's good or bad. I posted them in the chart section. I did up the cathode resister to a 470 OHM 5 watter. and the screen resister to a 3 watt 47 k. I was worried about the jump on the screen R, but no problems so far. also using a 5y3 recto. Any thoughts on this are appreciated.

Thanks john T

[shortfuse]

John I have been buried at work the last few weeks I have a 530 Ohm 5 watt in now.  I took Sluckeys advice, did some more research and purchased a NOS 5Y3 also.  It should be in this week so I will know more when I get it.  Im not 100% sure what the B+ volt drop will be but I will find out soon.  I began to wonder when does the cathode resistor value become over and beyond the call of duty.  Anyway I will post new data when it comes in.  I hate to admit it but I did try it out and it sounded very good at the high dissipation.
Anyone know what the original transformers for the SCH40 GA5 design put out for B+.

[shortfuse]

Here are the new readings with a 5Y3 and 680 cathode resistor  http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10682

[sluckey]

That's about 9w static diss on the EL84 now. You don't really have 13.7v on V3-2 do you? I would have expected a lower plate voltage if you used a NOS 5Y3?

[shortfuse]

So I finally had some time to mess with this amp again.  Found a tube shop here in town with lots of old vintage tubes so I picked up a new old stock Sylvania 5Y3 and the plate voltage did drop a bit as Sluckey said it would unlike the tube I bought from e-bay which was really no different than the 5U4.
I also was intrigued by a recent post where Tubenit talked about lower voltages on the pre-amp tubes so I took it down to 180 1st go round and it sounded better then I lowered it again with 330K's to 150v and thought it sounded very good.  It had a much more usable range and the crunch sounded much better not way over the top as it did when it was about 195 to 200v.
I just tried to enter my voltages again but it tells I need note characters 1-400 I left the notes box with a no in it but still would not let me post voltages.  What am I doing wrong to post them.  I have done it in the past and have 6 files there for this amp?

[Willabe]

shortfuse, that's a great lookin build!

That's very interesting on how you liked the change of tone with a change of B+ to the pre amp plates, like tubenit reported.

One more thing, what kind of lighting did you use to take those gut shot pic's? I just bought a dig. camera and I'm trying to learn how to get nice gut shot's like your's.

Here's a post I started on it, would you please add to it?


        http://www.el34world.com/Forum/index.php?topic=12283.0


                         Thanks,      Brad       

[shortfuse]

shortfuse, that's a great lookin build!

That's very interesting on how you liked the change of tone with a change of B+ to the pre amp plates, like tubenit reported.

One more thing, what kind of lighting did you use to take those gut shot pic's? I just bought a dig. camera and I'm trying to learn how to get nice gut shot's like your's.

Here's a post I started on it, would you please add to it?


        http://www.el34world.com/Forum/index.php?topic=12283.0


                         Thanks,      Brad       

Brad I just had a desk lamp with a boom arm on it shining in.  The camera I used is a regular Kodak digital camera I think its only 2mp.  I have no idea what settings I used I will guess default because I am not a picture guru by any means.  I think I just set the date and let it rip.

[shortfuse]

Here are the voltages I have using both a 470 ohm resistor and a 680 ohm for the cathode on the EL84.
The power transformer gets pretty warm when using the 680 and using the amp cranked up pretty good for a while.
Should using the higher resistor cause the B+ to be higher thus raising the 12XA7 tube plates also?
Is there any other way to lower the B+ with out changing transformers?
Maybe I should try a 6V6?  If I am reading the tube data correctly it seems it takes a whole lot more voltage than the EL84.
The amp just does not sound very good to me.  Maybe I have done something else wrong elsewhere in the amp.  If there voltages from different points in the amp that would help identify something else please let me know.
The pwer tube is a NOS Sylvania 5Y3, the 12AX7 is a Sovtek and the power tube is a Sovtek EL84.
The file using the 470 ohm is here http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11987
The file using the 680 ohm is here http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11989

[sluckey]

Should using the higher resistor cause the B+ to be higher thus raising the 12XA7 tube plates also?

Yes, that's normal for the B+ to rise because the higher value cathode resistor causes the tube to conduct less and that means less load on the B+ rail.

Is there any other way to lower the B+ with out changing transformers?

You can use a dropping resistor or zener between the rectifier and first filter cap. You really have a lot of voltage to drop to make that EL84 happy. A 250-0-250 or 275-0-275 PT would be a much better solution.

Maybe I should try a 6V6?

I'd give this a try. Could be all you need to do. Here's a schematic...

http://www.el34world.com/charts/Schematics/files/gibson/ga5.pdf

[shortfuse]

Still struggling with this amp and voltages.  I have had some help from Sluckey on the side (Thank You Steve) but figured I would post this in case someone else runs into the same things I have.

Not going to get into all the previous things i did as they are listed above.  I have now changed the power tube from the EL84 to a 6V6.  As suggested before to lower the plate voltage I purchased an NOS RCA 5Y3 to replace the original Sovtek 5U4.  Which did not seem to help too much.  While playing around with many cathode resistors when I did get the dissipation close to where it needed to be the amp did not sound good.  So today on the way home from work stopped at a local business here in Orlando that carries many NOS tubes and purchased another NOS 5Y3.  I though maybe I did not get the right one the last time or maybe it was bad as I got it on line from e-bay.  I explained to the owner what I was trying to do and he suggested a Sylvania Military grade 5Y3WGTA date packed 2-59.  It is interesting to me how the 2 NOS tubes differ in their readings and how it affects the rest of the tube values.  So I guess once I get the power section correct I would fall back and set the pre-amp tube plates to where I think they sound best?  From what I can see for every 10 volt change on the plate I get about 1 on the cathode. 
Anyway
All these readings were taken with the same 680 cathode resistor.
I think I may have to get another PT to make this thing work or sound good.  I did try to add notes to the valve data but could not find the place where you type the notes so the notes for each file are here.  Any suggestion besides changing the PT.

This valve data is the NOS RCA 5Y3
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=12425

This Valve data is the Sylvania NOS 5Y3WGTA
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=12426

This valve data is with the Sovtek 5U4 I purchased when I originally built the amp.
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=12427