grid stopper and shunt capacitor in Ampeg V4

http://www.drtube.com/schematics/ampeg/vt22-v4-74.gif

ok first channel, first gainstage, R1, C1.

grid stopper and a shunt capacitor. ok. so then notice the asterisk. They don't appear in all models.

now both of these components would roll off hf. but would there be an audible difference? why would ampeg leave them out?

[quote author="Hangman"]http://www.drtube.com/schematics/ampeg/vt22-v4-74.gif

ok first channel, first gainstage, R1, C1.

grid stopper and a shunt capacitor. ok. so then notice the asterisk. They don't appear in all models.

now both of these components would roll off hf. but would there be an audible difference? why would ampeg leave them out?[/quote]

No. That capacitance is too small to roll off anything; a resistance is even smaller.

Ampeg had problems in many designs with wiring of ground wires in their preamps, and shorting a signal path on HF by controls through ground loops. They could not understand the cause of that problems with hum and oscillations and tried to add components to prevent oscillations, while resoldering of 2 wires and adding 1 resistor eliminates both: a problem with oscillations and, what is more significant, kills the hum totally.

Put 10 kOhm in series with C6 and move a ground wire from a pot and a switch closer to cathode of the 1'st tube (i.e. a ground point on PCB where that wire goes to should be changed).

Next, look for a ground wire that goes from output socket, it should go to the filter cap before going to the preamp. I beat in your version it goes to the preamp like in others, and the cap is grounded by a separate wire. A voltage drop of current pulses through the cap are applied in series with a signal and amplified to an audible level.

do you mean that the capacitance is too small to roll off anything audible?

I'm not sure I understand your other instructions.

when you suggest putting a 10k resistor in series with C6, are you also suggesting putting it in series with C3?
which ground wire are you talking about moving from a pot and switch?

when you say "output socket" do you mean the speaker output jack? or the preamp output jack? perhaps the power tube sockets?

[quote author="Hangman"]do you mean that the capacitance is too small to roll off anything audible?
[/quote]

Yes.

I'm not sure I understand your other instructions.

when you suggest putting a 10k resistor in series with C6, are you also suggesting putting it in series with C3?

Click to expand...

Yes, they have both a pot and a switch on output, so in some position of a switch and a pot anode is connected to ground through a cap and low resistance of a pot, so current flowing through ground creates a voltage drop on HF enough to cause oscillations.

which ground wire are you talking about moving from a pot and switch?

Click to expand...

They have only one ground wire going to PCB (though, on the schematics they go from 2'nd triode to switch then to the pot, then to the 1'st tube, etc...), if your version is close to what I've seen.

when you say "output socket" do you mean the speaker output jack? or the preamp output jack? perhaps the power tube sockets?

Click to expand...

I mean a socket through which the output chassis is connected to a preamp section. Is it separate on your amp?
That amps I've fixed had such a problem, so I decided to share my experience with you.

[quote author="Wavebourn"]

Yes, they have both a pot and a switch on output, so in some position of a switch and a pot anode is connected to ground through a cap and low resistance of a pot, so current flowing through ground creates a voltage drop on HF enough to cause oscillations.


They have only one ground wire going to PCB (though, on the schematics they go from 2'nd triode to switch then to the pot, then to the 1'st tube, etc...), if your version is close to what I've seen.


I mean a socket through which the output chassis is connected to a preamp section. Is it separate on your amp?
That amps I've fixed had such a problem, so I decided to share my experience with you.[/quote]

Ok. I understand the 10k resistor now. that makes sense.

as for the ground... the ground from the preamp board goes to the output tubes pin 8, and then that wire goes to the negative terminal of C17 and then grounded to J6 (the speaker output jack)

there is Also the ground wire from the tone control pcb. that goes to the preamp pcb. There is only one wire, it goes to the preamp board.

does that sound right?

on my vt-22 there is no socket that connects the powertubes to the preamp board, they are connected to the preamp pcb by wires. soldered directly to the board.
perhaps you are thinking of the newer V4 that SLM makes? or confusing it with the SVT?

would it help if I posted a picture of the insides of the amp?

thanks alot for all the advice, I really appreciate your help on this.

Yep, I'm confusing with SVT. They have an electrolytic cap inside of a preamp section to filter B+ voltage, and that cap is incorrectly connected (it should be connected between power and preamp sections, but a ground wire from a power section goes directly to a preamp PCB instead of going to that filtering cap then from it to PCB) so pulses charging the cap are going through a signal ground path.

ok. everything makes sense now. ha ha.

I'll put in those 10k resistors though, see how that goes.

thanks for your help Wavebourn!

are you having problems or just tinkering?

From experience, if it ain't broke, don't fix it.
Or change one thing at a time so you can hear a difference and change it back.

They did some bizarre stuff to get these right.