Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: W5FH on February 26, 2023, 04:55:14 pm
-
Hello-
I incorporated the "original" Sluckey "Trem-O-Nator" (single triode section of 12AX7) into a recent amplifier build. The B+ feed rail was around 375 VDC to circuit. The vactrol I used was a PerkinElmer branded VTL5C1. The output cell was connected to ground on one side and other side to wiper of 50K RA intensity pot, positioned across the input to master volume control, and had a 150K resistor to ground. So the closest to ground the vactrol could take the audio path was 150K.
The amp had a beautiful, deep (described as swampy by guitarist) vibrato but after about an hour and a half of use the vibrato depth fell off noticeably. The speed was unaffected it appears, just the depth. After substituting another tube and checking all resistors the vactrol was replaced. The same thing happened. I am at a loss as to what is causing this, the vibrato is still very nice but not near as strong at full intensity. I am suspecting temperature but will say the vactrol is not adjacent to a hot component but it does warm up under the Deluxe Reverb size chassis with 8 tubes total. Any suggestions or solutions appreciated, thanks.
-
Sounds like a vactrol issue. I don't think the circuit is causing the vactrol to fail. Maybe try a different brand of VTL5C1. I'm using a Vactec brand with no problems in two amps. Others have had success with XVive brand. I don't think heat is an issue.
-
Thanks Sluckey-
Two questions please:
#1. I have not tried using an intensity pot wiring arrangement whereby one end of pot is NOT grounded (vactrol resistance cell to intensity pot wiper, top end of intensity pot to audio path). It seems fairly simple to me when LED ground return is lifted and tremolo is off the vactrol resistance cell is presenting maximum resistance (close to 50M) thereby shunting no signal to ground. In this situation the greatest affect of tremolo will be had. Possibly a higher value of intensity pot (250K) may be needed to give a wide enough range of tremolo effect?
#2. I am placing my intensity pot at Master volume pot. The master volume pot is fed from the tone stack treble pot wiper through a voltage divider. There is a slight interaction of treble and master volume as it is, so it appears to me it is best to connect intensity pot top end to master volume wiper to minimize additional interacton. The master volume wiper feeds phase inverter. Would this be your choice?
Sincere thanks!
-
There's a bit of experimenting involved to adapt the LDR side of the VTL5C1 to your particular circuit. Just play around until you are satisfied.