Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: leevc5 on February 28, 2018, 07:42:33 am

Title: 5F1 - tremolo (oscillation) when volume hits 6?
Post by: leevc5 on February 28, 2018, 07:42:33 am

Another build another mystery, I should probably find another hobby.
So, now this time my build actually fired up and got great sound right out of the gate...that is until I turned the volume up to ~6. It is like I turn on a tremolo switch. Checked with multimeter and it seems to start at B+, where the 5Y3, opt, and first filter capacitor come together. It is not related to input signal strength. A full volume humbucker in is no different than guitar volume off...you can hear it when the volume pot hits that ~6 point. I tried adding filter caps, as was suggested here that solved a similar problem in the past. But alas to no avail.
Any advice will be greatly appreciated.

Title: Re: 5F1 - tremolo (oscillation) when volume hits 6?
Post by: shooter on February 28, 2018, 09:00:24 am

try disconnecting the NFB
try - for testing - a bigger bypass cap on v1
try - for testing - a .001 - .01uf cap from v1B plate to ground

and troubleshooting IS a different hobby than building :icon_biggrin:

Title: Re: 5F1 - tremolo (oscillation) when volume hits 6?
Post by: leevc5 on March 01, 2018, 11:38:10 am

Thank you for the advise. I tried, it still tremolos, I put it in a box and got out a model ship to build. :think1:

Title: Re: 5F1 - tremolo (oscillation) when volume hits 6?
Post by: PRR on March 01, 2018, 08:36:00 pm

Sounds like motorboating. Bad/missing B+ filtering.

> I tried adding filter caps

Better post pictures. Someone may see what your eyes are missing.

Title: Re: 5F1 - tremolo (oscillation) when volume hits 6?
Post by: leevc5 on March 02, 2018, 03:38:55 pm

Okay, so I basically did a bottoms up rebuild and it sounds great. Since I was redoing it I went with this Weber Maggie 6L6 circuit I found. So it is not exactly a 5F1 anymore it is a tweaked 5F1 with a W022772 pwr xfmr, WSE15 opt xfmr, GZ34 rectifier and 6L6 output tube. Watching the temp of the pwr xfmr and so far after and hour it doesn't get above 145F.

Any warnings or suggestions...Will this thing hold together or will something go up in smoke?

Title: Re: 5F1 - tremolo (oscillation) when volume hits 6?
Post by: sluckey on March 02, 2018, 04:08:04 pm

If it smokes, please don't inhale!  :icon_biggrin:

Title: Re: 5F1 - tremolo (oscillation) when volume hits 6?
Post by: leevc5 on March 02, 2018, 07:28:31 pm

B+ @409, Pwr xfmr keeping it's cool, big clean loud 6L6 sound. gonna call it my little 409. The overdrive is not as rich and sweet as a 6V6. Perhaps put it in a 5E3 Chassis with a 5F1 and use little 409  for clean and 5F1 for overdrive... :dontknow: :w2: sorta a tweed boogie.