new champ build aa764

[scole]

it makes a crackling sound when turned all up. 406 volts pin 3, 424 volts 1rst cap node, 26.8 volts across the cathode resistor wich is actually 460 ohms. i think i have 20.2 watts dissipation. i found this by 26.8/460=.058. 406-58=348. 348x.058=20.18. question, should i bring the plate voltage down to 350 as on the schematic?

[MWJB]

"should i bring the plate voltage down to 350 as on the schematic?" you can if you want to...I seriously doubt that the high plate dissipation is causing your crackling, otherwise every SF champ in the world would crackle ... and they don't.

You should only be deducting the 26.8v at the cathode from your 406v at the plate, not 58v.

406 - 26.8 = 379v

379V * .058mA = 22W, subtract 1W for screen current = 21W/55mA.

Check integrity of solder joints/connections. What other voltages do you have? Any pics of the chassis/layout?

[scole]

heres some pics, im not sure if the board shot is close enough. what other voltages would i need?
http://www.flickr.com/photos/10451485@N02/sets/72157601675424588/

[scole]

i was reading angela instruments article on building a champ. he says the best sounding champ hes ever heard was one wich had a superchamp power transformer, and had 407 volts on the plate. im at 406 volts. so do i rebias to get the dissipation down to 14 watts? what should it be for this amp?

[Bruce / Mission Amps]

The guy at Angela is such a nut case salesman that I would be hard pressed to find serious credibility in too much of what he says about the "best" of anything.
However, some of the coolest sounding screamin' monkey Champs are running the power tube at +15 to 18 watts.
They just won't sound like that for months on end.

At +406 volts DC plate voltage, you'd want about 32ma-34ma on that 6V6.
Since you have to subtract the cathode voltage from the plate voltage to determine actual idle dissipation rating the, you have to start somewhere else first.
Try a 510 ohm, two watt resistor and see what the cathode voltage is.
Be prepared to use something a little higher in resistance, but before changing the biasing resistor to far, I'd drop the B+ with a couple (in series) 5 watt, 9v to 15v zener diodes .

[MWJB]

The pics don't seem to show final layout/wiring?

You may as well check screen voltage (6V6) pin 4 and preamp plate & cathode voltages, while you have the meter out.

510 ohms is only going to marginally reduce plate dissipation, I'd go in at 680ohms, this will probably get you in the 15-16W range. As Bruce says, if you want to drop the plate dissipation stick with the 470ohm and zener down the B+.

[scole]

i always panic when i fire up a new amp. when i first put my bias meter on it read almost 60ma. so i started putting resistors in series with the 460 ohm cathode resistor untill i got to my largest one, 250 ohm 5w cement job. then i saw on the meter it says for class a 55 to 60 ma, so i took out the 250. now it looks like the 250 is going back in.

[MWJB]

How does the amp sound its best, with the 460ohm or 710ohms? That's the best way to decide.

[Bruce / Mission Amps]

Scott... of course you do not want to exceed tube specs for very long but, it isn't so much the max plate current as it is the total dissipation of the tube.
A 6V6 is a 12-14 watt tube.
Running them at 17-21 watts, in the long term, is really hard on them.

For best results you really want to keep the tube closer to that 14 watt rating.
Cooling the tube down with a higher bias voltage on the cathode will make it so the tube will clip unevenly if driven hard enough because the positive going grid signal will top out and draw excessive current and saturate... (it really can't go very much further anyhow because it already is nearly saturated in DC current) but the negative one will never bottom out because the cathode positive voltage is so high, it might be able to be driven right into class B cut off.
IMHO,
Lowering the plate and screen voltage, to keep the tube cooler in idle current, will give you better balance and a cleaner tone.
Cleaner tone is critical with a small SE class A amp like this because it is low power and you'll be able to overdrive the power tube anyhow but if it is over-biased with respect to the high plate voltage, it is actually easier to get it to clip with less drive signal, making the amp sound fizzy and distorted at a lower volume level.
Now, I still do not know exactly what your cathode voltage is... so,
710 ohms and 57.5ma of current means you have 41v on the cathode!!
... could that be right?
406v-41v=365v.
365v x 57.5ma = 21 watts!! YIKES!
Get yourself a few 9.1v to 15v, 5 watt zener diodes in there (reverse biased to ground of the center tap in the high voltage secondary) and get this under control and I think you'll have a better sounding amp.
I've run as much as 45v worth of zener diodes in some small SE 6V6 amps and I think they always sound better idling at 12-14 watts with a lower B+.

[MWJB]

"Now, I still do not know exactly what your cathode voltage is... so,
710 ohms and 57.5ma of current means you have 41v on the cathode!!
... could that be right?" Well the cathode voltage will rise some (maybe up to 30v-ish? so will the plate voltage, but the plate current will come down. Let's say scole ends up with 395-400v on the plate once cathode voltage is subtracted, 31/710 = 43.6mA * 400 = 17.4, subtract 1W for screen current = 16.4W +/- 1W...as a rough guesstimate, erring on the high side.

Dropping the plate voltage to 380-385v (before subtracting cathode voltage) with 460ohms & zeners will get you around the 14-15W mark.

[scole]

i think my cathode voltage was 26.8v. or you mean with the 710 ohms of cathode resistor? also would changing to a 6l6 help? by the way i added some more board pics

[Bruce / Mission Amps]

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?

[scole]

your right bruce, it is a sovtec 5y3, its making a jigly sound so i took it out and put in an old rca 5y3. it sounds a little better. i ended up with a 1k cathode resistor, ill check voltages again with the rca 5y3. with the sovtec i have 434 v pin 3, 438 pin 4, 34.5 pin 8, 34.4 across the cathode resistor, i think im right at 14 watts dissipation. the amp is very loud not much breakup with single coils, not bad with humbuckers. i was expecting a little screamer, this thing will hang with a deluxe no problem.

[scole]

ok heres with the rca 5y3, 412v pin 3, 416 pin 4, 32.5 pin 8, the voltage across the cathode resistor is 32.4. now the cathode resistor measures 990 ohms.

[Bruce / Mission Amps]

OK I think the cathode resistor value is way too high.
I really don't think you should be over 510... instead get the plate and screen voltage down.
So, you lowered the plate a little but it's time to get your screen voltage down to around 405 to 410vdc...
What is the value of that first dropping resistor between the first to sections on your filter cap?
I think it needs to be a lot larger value then what you have now.
If it is a 10K make it 15K.
Plus you could run a 2k7 to 5K resistor from the filter cap lead to the screen lug
#4 to drop it down a little.
Many novice builders are under the assumption the screen in a 6V6 draws enough current to put a load on that resistor... typically, at idle it will be one or two ma.
That means 1v to 2v drop for every 1000 ohms of screen resistor value.
Although not critical to get your screen voltage below the plate voltage, you can see it shouldn't be hard to get it down a little lower then the plate voltage for less idle current draw.

[scole]

http://music-electronics-forum.com/attachment.php?attachmentid=704&d=1180230529
i noticed in that schematic the first filter cap is 40uf, i used a jj 40 20 20 20 cap can. should i switch to the 40 to get rid of the hum? im not using a choke but i could put one in if necessary.

[Bruce / Mission Amps]

Yes. I can't remember if I drew that one but the idea was to use the choke input filter in order to lower the B+.
With a choke input, the B+ will only be around 90% of the rectified DC out of the GZ34 rectifier. Or in your case, the SOVTEK 5Y3GT would be OK now.
So if your power transformer is pushing, say 360vac into a rectifier, then the B+ out of the choke at at the first filter cap would be around 324vdc.
Note that there is NO filter cap connected to the rectifier.
This is a very good filter and will reduce AC hum better then a filter cap on the rectifier and no choke.
In your application a simple 3Hy-5Hy choke for a Twin Reverb would probably be OK.

[scole]

i installed a 470k 3w between pin 8 on the rectifier and the first node, i moved to the 40uf cap on the cap can wich is a 40/20/20/20. heres what i have now, 341v pin3, 350v pin 4, 22.1v pin 8, 22.13v across the 460 cathode resistor. looks like the tube is at about 15 watts. the amp is still loud as hell but it overdrives at about 6 on the vol. it doesnt make that crackling sound anynore. the sound looser now not as stiff, the hum, is almost gone. it sounds very fendery but not as beautifull as a real fender. ill try a better tube and maybe a speaker. i have a new jensen 10 inch alnico in now.

[TD_Madden]

I don't know about anyone else, but I'm finding this thread very interesting and educational!

[scole]

thats what its all about t. im wondering, shouldnt i have 350 volts on pin 3 and 341 on pin 4? in other words should the screen voltage be lower than the plate voltage?

[MWJB]

"in other words should the screen voltage be lower than the plate voltage?" It often so on BF/SF champs & Princetons, the 1K 'B+ to screen dropping resistor' doesn't drop enough voltage when you also consider the much lower current draw at the screen.

[scole]

so now the 1k has to go up? to 2k2?

[MWJB]

And/or add a screen grid resistor between power supply rail & pin 4.

Or, if you are building a tolex champ "clone", live with the voltages you have. Yes, it's not the ideal from a design point of view, but most Champ/Princeton owners didn't know they had a problem until they got the DMM out.

Experiment and go with what sounds best.

[scole]

And/or add a screen grid resistor between power supply rail & pin 4..

i added a 2.2k 2w at pin 4, its getting better. now i have pin 3 345v, pin 4 344v, pin 8 21.76v im still at about 15.3 watts dis on the tube. the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away. the overdrive is still doesnt come untill about 9 or 10 on the vol scale if it went to 12 lets say. i want the gain to start sooner and be alot smoother. so do i keep adding resistance to the b+ rail or rebuild with a choke as in bruces sketch?

[Bruce / Mission Amps]

i added a 2.2k 2w at pin 4, its getting better. now i have pin 3 345v, pin 4 344v, pin 8 21.76v im still at about 15.3 watts dis on the tube. the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away. the overdrive is still doesnt come untill about 9 or 10 on the vol scale if it went to 12 lets say. i want the gain to start sooner and be alot smoother. so do i keep adding resistance to the b+ rail or rebuild with a choke as in bruces sketch?Are you still using that high value cathode resistor?
I think most builders would like it if the amp doesn't come on until the volume was up 2/3 or more.
What is the plate voltage of the preamp tube now?

[scole]

the cathode resistor is still the 470 (460) actual, im running the rca 5y3. my plate voltage at v1 is 201.4 on pin 1, 201.6 on pin 6.

[MWJB]

"the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away."

"the overdrive is still doesnt come untill about 9 or 10 on the vol scale"

Neither of these symptoms seem normal to me, I don't think either are related to your screen voltage specifically. Sounds like a wiring/layout issue.

[Bruce / Mission Amps]

"the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away."

"the overdrive is still doesnt come untill about 9 or 10 on the vol scale"

Neither of these symptoms seem normal to me, I don't think either are related to your screen voltage specifically. Sounds like a wiring/layout issue.Yeah me too.

FOLLOW PROPER SAFETY issues so I don't have to keep repeating myself.


<<<Turn the amp off and unplug it form the wall>>>

Unsolder the grounded lug of the volume pot and the ground end of the .005uF cap on the tone control.

No plug your guitar in and turn it's volume control to ZERO.
Turn the amp on and slowly turn the guitar vol up and see if it sounds radically any different.
Get it so it sounds good with the guitar volume and then shut the amp off again.
Solder the volume pot ground back on and try again.
Your volume will drop but you should be able to get it way back up again by turning it up all the way.
Now try grounding the tone cap again.
Either the tubes are dogs, the OT is bad, you are using a mis-marked part, made an error in reading the difference between a 15K resistor and a 1500 ohm resistor (etc.) or something in the wiring is not right.

[scole]

im double checking everything. i noticed something, what is the bypass cap for v1 pin 8. is that 2 uf. i put a 25/25 there.

[Bruce / Mission Amps]

im double checking everything. i noticed something, what is the bypass cap for v1 pin 8. is that 2 uf. i put a 25/25 there.If you are using the bf tone stack then the 22uF/25v cap on both triodes is what you want.

[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]

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:smoke:

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?:D

[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 PA 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.):confused: 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 :)