AA764 Vibro Champ build (power supply and grounding)

peteb

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Whoa… I didn’t even realize the tremolo connects to the cathode until right now

that is a very good point.

that could explain the missing current that shows on the cathode resistor but not on the plate resistor.

maybe it would be a good idea to disconnect the trem until after the basic amp is working the way you want it to.
 

peteb

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If I just pull V2 (tremolo tube) will that suffice?
That will probably be enough.

the trem circuit adds another ground path to the cathode, 250k ohms to ground regardless of intensity pot setting or if the tube is there. It is an area of possible concern.

looking at the schematic, there is a lot going on at that cathode. The tremolo and the NFB loop connection.


personally I would disconnect the trem and the NFB from the cathode until the tube stage settles down. it is always a good idea to eliminate outside variables.
 

MichelB

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With a three year old and a busy job finding time to tinker with the solder iron is tough.

But I've been able to make some time and pulling V2 and the NFB connection gave me normal voltages around both V1 cathode resistors. The second gain stage still gives less voltage drop and I just realise I've forgotten to measure both plate voltages.

Besides this I've reflowed some eyelets and increased power tube cathode resistor from 470 Ohm, to 680 Ohm. This dropped plate dissipation from 13.8 to 11.1 Watt. (Now a tad to low...)

Amp sounds good, but there's still this huge difference between tonestack and without. Without I hear the "Layla" sound. With the stack the amp is fully clean! Is that normal?
 

andrewRneumann

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Amp sounds good, but there's still this huge difference between tonestack and without. Without I hear the "Layla" sound. With the stack the amp is fully clean! Is that normal?

Probably not, but I’m just guessing. Turn up volume and treble to 100% and measure V1B grid resistance to chassis. Should be around 340K. What you get?

Let’s see a photo of the backs of the pots with all resistors and caps visible.
 

MichelB

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351K Ohm, grid V1b to chassis.

20220130_201404.jpg
The volume pot is an incorrect one I thought I ordered a push pull pot, but actually ordered a rotary switch SPST.

A new one in in mail order, I'm planning to rewire after since I'm not proud of this rats nets (especially the ground bus).

Anyway, I've realized that putting bas/volume pot at "5" reduces total gain and haven't tried the amp with both pots dimed. After trying it matters, but gain is still limited. Although digging deep with a pick gives significant distortion, almost like farting out.
 

andrewRneumann

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351K Ohm, grid V1b to chassis.

View attachment 946351
The volume pot is an incorrect one I thought I ordered a push pull pot, but actually ordered a rotary switch SPST.

A new one in in mail order, I'm planning to rewire after since I'm not proud of this rats nets (especially the ground bus).

Anyway, I've realized that putting bas/volume pot at "5" reduces total gain and haven't tried the amp with both pots dimed. After trying it matters, but gain is still limited. Although digging deep with a pick gives significant distortion, almost like farting out.

Hmmm, I don't see anything obviously wrong. I assume you have triple checked all the wiring and values. No chance that the leads from the 0.047u and 0.1u tone caps got swapped? The 100K is indeed 100K?

We need people who have these amps to chime in.
 

peteb

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increased power tube cathode resistor from 470 Ohm, to 680 Ohm. This dropped plate dissipation from 13.8 to 11.1 Watt. (Now a tad to low...)

this could be part of it. Every single fender single ended 6V6 has the 470R. Any reason for varying this? Expect 16W.

Amp sounds good, but there's still this huge difference between tonestack and without. Without I hear the "Layla" sound. With the stack the amp is fully clean! Is that normal?
This is what you are going to sound like. Tele into an AA764.





please share voltages.

plate and cathode voltages on pre amp.

plate cathode and screen voltages on the 6V6
 

MichelB

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this could be part of it. Every single fender single ended 6V6 has the 470R. Any reason for varying this? Expect 16W.
Besides the maximum 12W spec in the 6V6 datasheet ;-)? Nothing... but sound wise the 470 and 680 ohm sounded similar.


Plate voltages on preamp 212V (both V1a and V1b) cathode voltage v1a 1.68v, voltage V1b = 1.58V

Plate voltage on 6V6 = 356V
Cathode voltage = 24.8V

(13.6V drop over prim. OT with 680 Ohm, 16.9V drop over prim. OT with 470 Ohm cathode. Cathode voltage was in that case 21.6V)
 

peteb

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This is not Layla teritory though and I heard much more overdriven sounds on youtube (on aa764 champs, could be non vibro champs though)
Layla is supposed to be a tweed champ.

what sound do you like better? Tone stack engaged or disconnected?

13.6V drop over prim. OT with 680 Ohm, 16.9V drop over prim. OT with 470 Ohm cathode. Cathode voltage was in that case 21.6V)
I think with the 470R cathode resistor your current and power dissipation are in the standard AA764 range.

if the 860R cathode resistor sounds the same as the 470R cathode resistor, then you are probably good either way.
 

Phrygian77

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Did you mod the tonestack? If you changed the mid resistor, to 6.8k for example, it will increase the overall load of the tonestack and reduce gain.
 

MichelB

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Hi,

The there is also a 6.8k Ohm on a switch, but during testing I've been running the 15k Ohm.

All should be fully acc. Schematics.

The amp.is also too noisy to my taste and my 80w solder iron arrived. I will redo all my ground soldering and recheck everyting on the tonestack.
 




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