ab763 bass problem

[lky]

Hi,
I have just finished my new CA with two 6L6.
The sound is fantastic till at 6 of volume.
After 6 of volume, when I play E bass string with the sound of the string there is a hammer blow with the sound of the sting. This effect there is only with the E bass string. If I put bass pot at zero, this effect drecrease.
I have just replaced cables of bass pot with schielded cables but the problem remain.
What do yuo think about?
Thanks

[stokes]

Are you refering to the Bandmaster AB763?Dont know what you mean by your CA reference.Is this a clone or the Fender made amp?From the little info you have given I would suggest increasing the value of you main filter cap as well as the one on the other side of the choke that feeds the screens,to 100uf for the main and 40 or so uf for the screen supply.A common problem in these vintage amps is that they were not designed to be played full out,and when we crank them the stock value caps in these amps arent up to the task and crap out when the amp calls for the extra power needed to support the bass.Also if you are using the Jap or Taiwanese brand caps,I would suggest investing a few extra bucks and get good quality American made Spragues or the German F&T brand,the cheap ones will give you cheap results.If you are refering to a Fender vintage amp it is time for a complete recap any way,and I would give the same advice.Sprague and F&T are the only caps I know of that are not Asian made,even the American sounding Illinois are made in Taiwan.In my experience,any other attempt to cure this bass response would be futile if you dont have a stiff enough power supply.

[lky]

Hi,
thank you for your reply.
My amp is a clone of vibrolux ab763.
The filter cap is a sprague atom of 80 uF and 40 uF after choke.
what do you think about if I reduce the caps value before choke from 80 uF to 40 uF?
Or the only solution isn't play full out?
Thanks
Luca

[stokes]

Reducing the value of the cap will have the opposite effect.Are you using one 80uf or do you have 2 of them stacked?I cant find a schem for the Vibro but the Bandmaster AB763 shows the main filter with 2 70uf's stacked.If you are using 2X 80uf's stacked then you are only getting 40uf.If you have just one 80uf,that should be enough,check that it is grounded properly.Check the bypass caps in your preamp also.A problem with these,or the wrong size could cause a sloppy bass response too.

[lky]

Sorry, my clone is a vibroverb (for schematic see www.ampwares.com/ffg/).
I have 2 80uF's staked. In the preamp (after choke) I have 3 20uF and I'm using 2 7025 instead of 2 12ax7 (classic replace).
Also I have added a pot for bypass tone stake but I think it is not influence the bass response (particulary when it's at zero -> add some overdrive).
Now I check the ground of caps stack.
Thanks
Luca

[stokes]

The 2X80uf is the same as having 40uf.When you connect 2 caps in series the capacitance is halved.The idea of using this arrangement is to get a higher voltage rating,as the voltage rating is the total of the 2 caps.I would suggest using a 100uf of the right voltage rating or 2X220uf/300v caps in the first position and move the 2X80uf you have to the position after the choke,then leave the last 2 20uf's as they are.If you do use the 2X220's in series you must parallel them with 2 220k resistors,as shown in the schem with the 2 70uf's.This is important to be sure the voltage is equally shared by both caps.This should clear up the bass issues you are getting now.If you go with the 220uf/300v's I would suggest using the F&T's,Sprague doesnt make this value.

[lky]

OK.
Tomorrow I change the caps and then I tell you the result.
Thank you

[lky]

I have change the caps but bass response is worse.
I have put back again the 80uF caps because I and I play full out.
When volume is at 10 just I touch the E bass and the A bass string, is how I have tremolo on the first moment. Then the tremolo go away.
This effect is minor when I use second input.
I also changed 6L6 with 5881: also in this case the problem reduce.
Thanks

[stokes]

What did you replace the 2X80uf's with?If it got worse that tells me there is something else wrong.Now you are describing the problem as a tremolo effect?You are describing a tremolo effect,but is it more of a cutting out effect?I had a similar symptom when I was experimenting with a small single ended OT,I over biased it till the tube was drawing more current than the OT was rated for,at first it was a cutting in and out effect,but as I began slowly backing off the bias current it actually started to sound like a tremolo.I am guessing that if you increased the first filter to 100uf,it is possible the stronger bass is causing a similar result.Did you increase the screensupply cap as well?If not it is possible the weaker screen supply is not getting enough power causing a problem.The fact that you say it is minor using the "second input",which is the lower gain input,and the 5881 which is lower gainthan the 6L6, indicates there is a problem in the power supply,or it is biased too hot.

[lky]

I haved replace the 2X80uf's with 2X220uF=110uF -> problem expand.
I haven't tremolo in the amp. I have built amp without tremolo and first channel -> more corrent in circuit for more gain -> Is this the problem?
When I touch E bass or A bass strings for early moments to seem I have a tremolo switch on but I haven't tremolo in the amp.
I see on ww.ax84.com project call california dreamer (fender blackface with 2X6V6). The first cap of power supply is 40uF->2X80uF in series.

[Enzo]

What did you use for an output transformer? I might be suspicious that the output transofrmer is saturating on peaks. That would be at least consistent with the volume level aspect and the bass pot to zero affect. Swapping in the lower gain 5881 for the 6L6 also fits my theory.

[lky]

I used orginal fender output transformer (50W).
Current bias is ~32mA for 6L6 (groove tubes).
Can I reduce bias current? -> Can it reduce saturation of transformer?
Thanks

[stokes]

That 32ma seems okay.And that tranny should be able to handle it.Something aint right,tho.Are either of your power tubes redplating?Is the signal from each side of your PI fairly even?Do you have the same bias voltage on each tube?Could be some low frequency oscillation,maybe in your feedback network.Check all your OT connections are connected properly.Check that the volts are the same at each plate on the power tubes.Make sure your OT center tap is connect to the power supply,and didnt get mixed up and connected to one of your plates.Try switching the plate connections from the OT to each tube.

[Enzo]

Something wrong indeed. I'd be checking for a shorted turn on the OT maybe, or something breaking down, like an arc in the OT or somewhere. The sensitivity to level is a clue of some sort.

I sincerely doubt the filter caps have anything to do with this.

[lky]

I haved measure bias current: 32 mA for each 6L6 (I haved measure whit 1 ohm resistance on pin 1-8 -> ground).
But when I haved measured bias current with shunt method I haved this problem: when I touched the OT brown cable and center tap cable (OT red cable) I can hear from speaker the same oscillations how when I play at full volume E and A bass string.
I didn't hear this effect when I measured between OT blue cable and center tap.
Is the OT broken?
Thanks