Re: Vibrover/6G16 tremolo/bias



"Phil S" <psymonds_no_spam@xxxxxxxxxxx> wrote in
news:coedndjZXMVd8gHeRVn-gA@xxxxxxxxxxx:

>
> "Norman Frech" <normanfrech@xxxxxxxxxxx> wrote in message
> news:rYXmf.4511$ES.67@xxxxxxxxxxx
>>
>> "Doggone" <running@xxxxxxxxxxx> wrote in message
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>>> "Norman Frech" <normanfrech@xxxxxxxxxxx> wrote in
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>>>
>>>>
>>>> "Doggone" <running@xxxxxxxxxxx> wrote in message
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>>>>> "Norman Frech" <normanfrech@xxxxxxxxxxx> wrote in
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>>>>>
>>>>>>
>>>>>> "Norman Frech" <normanfrech@xxxxxxxxxxx> wrote in message
>>>>>> news:UaHmf.3760$Xx3.1553@xxxxxxxxxxx
>>>>>>>
>>>>>>>>
>>>>>>>> It's breaking into oscillation at the trem. peak (max gain). You
>>>>>>>> could
>>>>>>>> try
>>>>>>>> moving the wires a bit (use chopsticks), increase the NFB, add
>>>>>>>> small caps (2-
>>>>>>>> 10pF) between plates and grids of 6L6's or from grid to cathode.
>>>>>>>> The NFB and
>>>>>>>> caps will change the way it sounds. You may like it, you may not.
>>>>>>>>
>>>>>>>
>>>>>>> I changed the feedback resistors from 10k/470 to 820/100. Still
>>>>>>> osciallates
>>>>>>> like a big dog. The PI tail resistor is a 6.8k. Would bumping
>>>>>>> it up
>>>>>>> to 22K help?
>>>>>>>
>>>>>>
>>>>>> Answer to my own question... Change PI to 470/22k. Still chirps.
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>> Did you install grid stopper resistors. There were none on a 6G16
>>>>> but most
>>>>> blackface amps have them. They should be installed as close to the
>>>>> tube as
>>>>> possible.
>>>>>
>>>>> --
>>>>> If it ain't broke, fix it till it is.
>>>>
>>>> I have 1.2k resistors across Pin 1 & Pin 5; wire to Pin 1. 470
>>>> resistors
>>>> across Pin 4 & Pin 6; wire to Pin 6.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>> The feedback resistors you're using didn't match the schematic.
>>> Try 10K and 4700.
>>>
>>> --
>>> If it ain't broke, fix it till it is.
>>
>> I started with the 10K and 4700 and changed them to 820 and 100. The
>> PI section is now like an AB763 with a 12AT7. These changes really had
>> no effect either way.
>>
>> I think I am going to move the 470 ohm grid resistor back to the top of
>> the board and then run shielded cable over to the grid on the tube.
>> What do you think?
>>
>>
>
> Grid stopper should be on the tube socket. Wouldn't hurt to run
> sheilded downstream from it, but I'm not convinced that this is causing
> the problem.
>
> If I understood your original post, you added a pot to the circuit that
> isn't on the schematic? Bypass that pot (or use a fixed R in it's place
> if needed) and see what happens. Just a wild guess, though.
> Phil
>
>
>

Good call Phil.
Norm, to expound on what Phil said, remove anything that's not a vibroverb.
Then starting from a clean slate, check all the grounds, make sure you've got
good tubes and make sure the wires are routed to minimize the chances of
feedback. Once you have a stable amp, you can add the modifications you want
one at the time, testing thoroughly before moving on to the next.
BTW, are you building from a kit, rewiering a RI, or building from scratch?
Which/whose/what chassis are you using? What's the layout like? What
transformers are in there?

--
If it ain't broke, fix it till it is.
.