Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: aajeffcote on August 05, 2021, 03:38:51 pm

Title: Twin Reverb Vibrato
Post by: aajeffcote on August 05, 2021, 03:38:51 pm

Just finished a Twin Reverb AB763 build. I tried the bias vary vib and the bias current swing scared me away. So I put the roach in. It works good, sounds good but the intensity control does nothing from 1 to 7, so all the adjustment happens very quickly from 7 to 10. Its a new CTS 50K RA pot. Used the ground buss across the back of pots grounded at the jacks. What can I do aboit this?

Title: Re: Twin Reverb Vibrato
Post by: Raybob on August 05, 2021, 05:44:48 pm

Double check all component values and all wiring dealing with v5.  Something isn't right.

Title: Re: Twin Reverb Vibrato
Post by: PRR on August 05, 2021, 11:03:17 pm

Bias current swing is how it works. Why is that scary?

Title: Re: Twin Reverb Vibrato
Post by: sluckey on August 06, 2021, 05:24:20 am

You may be interested in this thread...

     https://el34world.com/Forum/index.php?topic=24334.msg261229#msg261229

And here's a link to my web page with just the important info...

     http://sluckeyamps.com/trem/trem.htm

Title: Re: Twin Reverb Vibrato
Post by: aajeffcote on August 09, 2021, 09:28:23 pm

Swing was maybe 35 - 107.  Dont think the tubes would take that very long. I tend to use vibrato and reverb a lot.
I went over every component, checked values, voltages and schematic. Finally pulled that 50K RA pot to check it. I had put a 250KA in for the bias vary and did not swap it⁸ out. Fixed that today now the vib is apparent at about 4. Also slowed it down swapping the .01 caps for .02s.
Could be stronger but it sounds good.

Title: Re: Twin Reverb Vibrato
Post by: aajeffcote on August 09, 2021, 09:38:28 pm

OK, thank you. I have to try the Tremonator now.
I did try a boost with just a 1meg resistor and an .o1 cap. Cap to the volume input and 1meg to pin 1 V1. Too much boost but it really gets a good lead tone.

Title: Re: Twin Reverb Vibrato
Post by: PRR on August 09, 2021, 11:19:53 pm

Quote from: aajeffcote on August 09, 2021, 09:28:23 pm

Swing was maybe 35 - 107.  Dont think the tubes would take that very long.

Average of 37 and 107 is 72. Seems like a happy value? Tubes don't blow-up in an instant, or even a trem-a-lo cycle.

Title: Re: Twin Reverb Vibrato
Post by: aajeffcote on August 10, 2021, 09:15:33 am

So that is an acceptable swing? OK, I do prefer the bias vary trem over what have now. But first, the Tremonator. Then the best trem wins.

Title: Re: Twin Reverb Vibrato
Post by: pdf64 on August 10, 2021, 10:54:38 am

Quote

Swing was maybe 35 - 107

Quote from: aajeffcote on August 10, 2021, 09:15:33 am

So that is an acceptable swing?

You'll need to be more specific about exactly what those numbers are referring to.
No one here was looking over your shoulder when you made those measurements.

Title: Re: Twin Reverb Vibrato
Post by: aajeffcote on August 10, 2021, 10:33:52 pm

Yes. Bias current. The problem was my lack of a thorough check of every component.
Problem solved, thank you.