new champ build aa764

[scole]

the schematic shows a 25/25 on pin 3 and 2/25 on pin 8, you say use 22/25 for both?

[Jag]

IIRC my '72 Vibro Champ and my mid '70's champs had 2uf caps there. I replaced them with 25/25. I think CBS messed around with that cap and thought 2uf was better. I disagree.

Actually, I just looked at the AA764 schematic and it show a 2uf there. I still like 25uf better.

[scole]

thanks jag, im hoping for the same reaction you had with your champ, "oh my god does that sound good" what exactly did you end up with as far as components, did you follow bruces sketch with the choke and the 470 5w and the 22k and the 2k2? what the heck is that anyway? 2200 ohms?

[Paul P]

So, scole, how is your amp doing lately ? I've read this thread with great
interest but it seems to be missing the final chapter . Have you reached
a point where you're entirely happy with your amp ?

Paul P

[Jester]

Over powered Champ, which sound great....

Hmmm I just spotted something that has my attention.
What rectifier is that? I think it says Fender but it looks like a GZ34... or GZ30 which is 5V4GA type tube in a small bottle... both of which should not be in this amp.
If it is a Russian 5Y3GT then that is where all the extra B+ voltage is coming from because the Russian 5Y3GT is not a real 5Y3GT.
Do you have an old or NOS 5Y3GT laying around to try in there?

Wow, I've read through all this post, and found it fascinating. I to am in the process of building a Champ 5F1. I have a weber power tx, which has two output options, 600v and 660v, so I decided to try them both. The readings I have taken show that I too am frying my EH 6V6GT (and have been now for a month - by the way it sound great with a NOS JAN 5751 as the pre-amp - I've also tried a Sovtek 12AX7LPS, but the JAN has a little more bass. The amp is clean and has loads of head room - but wait 'till you see the numbers...)

Here we go. My Weber PT allows a 240v input, which I'm using (I often measure the mains supply and see it reading 250 volts on a fluke, using a scope I can see the sine wave clamps at 240 volts, and has flat crest and trough.) Under no load I can measure +- 335.5volts on low setting and +- 368 volts on the high o/p setting. So I'm saying I have about 7% extra power from the transformer, and 4% from the supply. Then I stick in either my Sovtek 5Y3GT or my Groove Tube GT-5YE and measure my B+ and I see about 455 or 499 volts.

Then I stick in my other tubes, and play a bit. Here are the readings:

6V6GT low high
Plate 395v 433v
Screen Grid 335v 365v
Cathode 21v 23v over a 5w 460ohm resistor

12AX&LPS Low high
Plate 170v 188v
Cathode 1.7v 1.9v
The second plate and cathode are similar

So I can calculate my high supply power plate dissipation power as:

(23/460)*(433-23) = 20.5 watts,

and similarly for the low supply power at 17 watts and I see why the amp is so clean, with so much headroom.

I also have had a look at the ripple on the B+ and see it's about 6 volts at 100hz (rising quickly and decaying slowly), and drops off quickly when I play. So I've installed a choke (as shown in the diagram - in series with the standby switch and before the B+ take off, and because of this reduced surge in current, I thought I'd try doubling the first cap, so have a second 16uF 475V cap - I read that the 5Y3GT can't supply more than 20uF without arcing.) hoping as you suggested to drop the voltage by about 10% but my Mercury Magnetics choke (FTCC) drops the voltage all the way down 276v or 302v. The amp sounds really different, breaks up very early now, and has lots more bass - I even had to down grade the MM OP transformer from the Axiom to the Tone Clone to stop the low resonance in the low 'e's through to low A. Using the scope I can see now that the B+ is a heap more stable, with only 2 volts sine wave ripple, which when strumming now only fluctuates by a couple of volts. (Which is possibly why the bass is now so full.)

With the choke, I now have plate power of 7.7 and 9.5 watts.

I'm thinking I might try the choke in the position shown on a Fender 5E1 (which is where I intended to put it - but since the ripple is so bad, I thought cleaning up the signal before attaching it to the O/P transformer would be a good thing).

The reason I'm writing all this, is I'm a little confused. Why do you say that the Russian 5Y3GT is not a real 5Y3GT. I can't see in anywhere in The Tube Amp Book (by Pittman) a reference to their GT-5y3 being from Russia, but he does say they don't like more than 330v on them. In his book he shows the o/p rising to the same voltage as the transformer - I seem to have a gain of about 35%, which is not helpful.

I realise I too could use Zenner diodes, to drop back down the voltage, or even stick another resistor in series, with the standby switch, to drop the power before hitting B+, but I just thought I'd ask if anyone has an explanation for me.

I've also read in Tube guitar amp essentials, that the power on the preamp is optimal at 220v - so once I've got the main supply and power amp stage sortred, I'll move on to that - anyone got any comments.

thanks
Jester

[scole]

i put the champ on hold, i have a 5f4 to finish for a customer. as for the russian 5y3, wikipedia even mention this problem.

[Jester]

Thanks for the Wipipedia link - that gives me an idea - it says the new replacement 5t3gt has indirect heating of the tube. I've wired it up for direct heating, I wonder if this extra heating is boosting the o/p volts

there's only one way to try..... oh, I suppose asking here would be two ways. But what have the Romans ever done for us?

[Jester]

okay, so disconnecting the direct heating just stopped the 5Y3GT from working, so I'm not sure I understand what Wikipedia mean. So it's back to the forum...

Does anyone have any ideas about how to get the Sovtek or Groove Tube 5Y3GT to run at normal voltages. Are we back to putting in some diodes to drop the supply back down by 35% so that the tube can then boost it back up???

Does anyone know of any 5yGT that don't do this? I did not think the Groove Tube was from Russia - don't they normally mark those with an R or R2 depending on the factory?

[Paul P]

Does anyone know of any 5yGT that don't do this? I did not think the Groove Tube was from Russia - don't they normally mark those with an R or R2 depending on the factory?

Why don't you buy a NOS RCA 5Y3GT ? They're no more expensive than
new tubes, maybe even less, and they'll produce the proper voltage.

Paul P

[Bruce / Mission Amps]

Why don't you buy a NOS RCA 5Y3GT ? They're no more expensive than
new tubes, maybe even less, and they'll produce the proper voltage.

Paul P

A good NOS 5Y3GT has the proper impedance and will give you lower B+.
The Russian 5Y3GT is more like a Euro GZ30 meets 5u4C/5V4... yes I said GZ30 not GZ34.
The NOS American Made 5Y3GTs can be found just about anywhere on the Net anymore for $15.00 to $20.00 and it will run years and years and years.

[Jester]

Paul,

Thanks for the note - I'll try and find a RCA tube, but here in Australia it's a little difficult to find (well at reasonable prices anyway). I normally source from the USA and then pay the shipping and wait for it to arrive....

Bruce
Thanks for clarifying that the Russian valves are not to spec. Do you know of other valves that fall into this category - I have a number of equivalence's tables that show me the difference between US and European spec'd valves, but none that say these valves made in ..... are not to spec at all (or are to a non-standard spec.) Here I am with two imported 5y3's one Russian (Sovtek) and the other I thought was a good American 5y3, being from Groove Tubes and not having a R in the name (or a C etc..). Thinking they were different, and yet they both supply me with far to high a voltage. I now have have to wait another few weeks for the next valve to arrive... I guess I could take up your earlier suggestion and use some diodes to drop the 100 volts, but it doesn't seem right to chop 35% of the power like this.....


Thanks guys.
Jester

[Paul P]

I'll try and find a RCA tube, but here in Australia it's a little difficult to find

I didn't realize you were 'down under'. Maybe you can find a NOS Philips or
something. Tubeswell could probably tell you what sorts of NOS tubes you
can find locally.

Paul P

[Jester]

thanks Paul