aa764 Champ first build voltage issues

[dcline]

I finally built my first amp. An aa764 Champ.
When completed, I eagerly fired it up and felt like Frankenstein when it actually worked. And at curtain volumes it sounds pretty good!
But when I turn the volume and treble control up past 3/4's at the same time it breaks up horribly! I had several suggestions on another forum. I tried various caps to filter out the high end... didn't fix the problem... I tried grid-stoppers... didn't fix the problem.... then I blew the OT.... So after getting a new one I checked voltages. They were all way higher then they should be.
So I used a 5 watt 1k resistor before the filter caps and now everything is down with-in tolerance. Only one issue. The 6V6 plate voltage is good a 327 when it should be 325. But the G2 voltage is higher rather than lower at 335v.
So I have two questions here...
Could this be the cause of the my volume, treble problem?
And what would be the best way to get this voltage down to what it should be? There is a 1k and a 10k resistor connected to the G2 suply at the filter caps. Should I change these values? or use another resistor in series with the G2 supply?
Thank you in advance

[Cantplay]

What rectifier are you running?

Can you post a picture of your layout?

John

[dcline]

It's a new Sovtek 5Y3 which I've heard kick out more voltage then the NOS 5Y3's.
Hopefully these pics help...
Thanks again

[rogb]

I'm having trouble following the build.
Can you check the signal path and component values against the layout. You should be able to see if you made an error. It's a nice simple circuit anyway.
regards
Rog

EDIT Had another quick look, can you check your pot wiring against the layout?

[jjman]

To reduce the screen voltage you could change the 1K dropper to 5K and test. Once the screen is set, you can adjust the 10K to the preamp downwards and get that node back where you want it, assuming it moved a little from adjusting the screen node. Or leave it since it may sound good and be close to specs anyway.

I don't think its higher voltage is the direct cause of the issue. What is the cathode voltage on the 6v6 ? Is that cathode resistor's resistance measuring in spec?

[dcline]

Cathode voltage on the 6V6 is 19.3 which I think is pretty good. The schematic I have says it should be 20. There is about .15v on pin 5 which should be 0.
12AX7 voltages are as follows:
Pin 1= 206.9
2=0
3=1.62
4&5=3.42 AC
6=211.8
8=1.49

I've looked over my work for hours checking everything with the schematic. All component values are correct and everything is wired correctly.
Gonna change that resistor and get the G2 voltage down and see what happens.
This is a strange one....
Thanks again

[Phil_S]

I've looked over my work for hours checking everything with the schematic. All component values are correct and everything is wired correctly.

Could be the tube. Have you got a spare to try?
Have you double checked the dropping resistor? Lift one end from the circuit and put your meter on it to be sure it is the correct ohm value. Maybe you have 100 where you need 1000. This sort of error is common even with experienced builders.

When I build, I always put my resistors on the meter immediately before I put them into the circuit. It's worth taking the time to do that. I'm not talking about a production line where your worker doesn't have the time!

[dcline]

Well, didn't have a 5k resistor but had a 4.7k which put all my voltages in tolerance so I'm good there. Plus now G2 is slightly lower than plate like it should be. So voltages on both tubes look good now.

I do the same thing and check the resistance and capacitance of all components when installing them but I did pull the ones I had to just to be sure even though they had the right color codes. Everything looks good.
I'm starting to think it's a capacitance issue. I replaced the pre-amp tube with a new JJ and that helped. I can turn the treble up a lot more before it breaks up now but it still happens.
I had read that these circuits are known for supersonic oscillation. It was suggested to put a .oo5 uF cap across the OT primary. I tried that and that made the issue worse. I could only turn the treble up 1/2 way before it started to break up. Not sure which cap to play with or witch direction to go.
The only thing I haven't re-checked is the pots. I wonder if one of them could be miss-marked from the factory? I took care when putting them in....
Running out of things to check...

[dcline]

Checked all the pots. They seem to be fine. The bass pot is reading a bit higher then 250k at 259k.
I just played through the amp for a bit. It is sounding really good! Has a nice smooth bite to it. The bass and treble are working... it just seems like the highs are going overboard if you crank the treble up when the volume is up all the way. Any suggestions?
Thanks again

[dcline]

After staring at the schematic... I'm gonna assume that lowering the value of the 250pF cap will bring down the high end?
And what about the 100k and 15k resistors in the tone stack. Will changing the value of them alter the high end?
Sorry if I'm thinking out loud here....

[ChrisM]

After staring at the schematic... I'm gonna assume that lowering the value of the 250pF cap will bring down the high end?
And what about the 100k and 15k resistors in the tone stack. Will changing the value of them alter the high end?
Sorry if I'm thinking out loud here....

Compare the Fender TS to the Marshall TS. Duncan's Tone Stack Calculator is great for this.

Simplified....
Lowering 250pF cap will increase treble
Lowering 100K resistor will bring in more lows and mids
Increasing 15K will decrease mid cut

[dcline]

Very cool! Thank you!
Unfortunately I use mac and they don't seem to have a calculator for mac.
Guess I'll just raise the value of the 250pF cap and see what I get.
Thanks again

[Colossal]

Very cool! Thank you!
Unfortunately I use mac and they don't seem to have a calculator for mac.

dcline,

Download Virtual Box for Mac. I keep it for running these simple electronics utilities like Duncan's Tone Stack modeler, Duncan's Power Supply calculator, etc that are PC only.

See here:
https://www.virtualbox.org/wiki/Downloads

Hope you get your Champ sorted out.

[dcline]

Thank you Colossal.
I will do that.

This problem I'm having is driving me crazy to the point that I couldn't sleep last night. I ended up out in my shop at 2 in the morning going through my cap pile. I found a couple ceramic caps that I twisted together to get 500pF.
I tried that and the problem still existed...

[Colossal]

Dcline,

These problems can be very vexing sometimes. It sounds like you have taken voltages throughout but something subtle is still amiss. It might be useful to print out the schematic, take voltages at every grid, cathode, and plate and mark up the schematic. Also, take voltages at every node in the power supply starting from each AC secondary, through the rectifier tube, to each point at an electrolytic cap.

Anytime that I trouble shoot an amp, I do this process first (before making any changes) so that I can visually make sense of the magnitude of the voltages I am seeing throughout the amp versus what is expected. If something is wrong, sometimes one voltage will jump out as clearly not being appropriate for the location. Also, be sure to mark up any part value changes you made to your version of the amp against the stock Fender schematic.

Second, I go through the amp taking AC voltage measurements at each gain stage. I have a signal generator but if you don't, you can go online to a tone generator and pump that tone out your Mac's headphone jack and into the amplifier's input. I then use my multimeter in the AC mode and look to see that the AC signal is getting larger indicating voltage amplification at each plate.

If you would find the exercise with taking voltages and marking up your schematic useful, scan it and post it. Something may present itself as the root cause.