Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fagus_sylvatica on December 13, 2017, 12:39:39 pm

Title: Tremolo not working AC 10 build
Post by: fagus_sylvatica on December 13, 2017, 12:39:39 pm

Dear all,

I finished my ac 10 build, but I can't get rid of that tremolo not working. Everything is fine on both channels excepted the tremolo not working no matters the trem switch position and speed / intensity pots positions. I have checked the circuit several time without finding an error. I have attached the schematic with some tension I measured, and a picture of the amp as a bonus.

Thank you for any idea !

RG

Title: Re: Tremolo not working AC 10 build
Post by: tubeswell on December 13, 2017, 08:03:46 pm

Check your wiring over again. Especially the socket pins for the trem stage.

Title: Re: Tremolo not working AC 10 build
Post by: shooter on December 13, 2017, 08:35:18 pm

After Tubeswell's suggestion, does the 150vdc you measured wiggle a lot?

Title: Re: Tremolo not working AC 10 build
Post by: PRR on December 13, 2017, 10:05:44 pm

You have the right DC voltages on the trem oscillator.

I'll assume the "150V" is rock-steady, not wobbly (as it should be). That "foot switch" is installed or jumpered.

To oscillate, the C18 C19 C20 string and associated resistors must be right value, wired right.

Given reasonable DC and right C-R-C-R-C-R network, the tube has no good excuse for not wobbling. Unless it is very sick, but it is unlikely to hit nominal DC values if it isn't healthy.

Title: Re: Tremolo not working AC 10 build
Post by: fagus_sylvatica on December 17, 2017, 11:46:35 am

Dear All,

Thank you for helping !

After investigation I found :

• R40 to be 185k instead of 220k (-16 %)
• R 24, R26 and R43  being approx 250 k instead of 330 k (-25%) ; they came from same batch, so maybe an error from retailer ...

Do you see it as a plausible source of non oscillation ?

All best,

Romain

Title: Re: Tremolo not working AC 10 build
Post by: tubeswell on December 17, 2017, 11:46:34 pm

Check the C/R divider filter values in the LFO stage. (C17/R31+R32, C19/R35, C20/R38)

Title: Re: Tremolo not working AC 10 build
Post by: fagus_sylvatica on December 18, 2017, 04:38:55 am

Hi,

The values you asked all lie within a 10% interval  :BangHead:

Cheers,

Romain

Title: Re: Tremolo not working AC 10 build
Post by: tubeswell on December 18, 2017, 08:54:01 am

Are you positively sure? I can't confirm that from the picture you posted. I can't see the cap values - there is not enough resolution in the picture.


(0.01uF = 10nF, and is usually marked as '103'. Similarly, 0.0047uF = 4.7nF, marked as '472')

Title: Re: Tremolo not working AC 10 build
Post by: fagus_sylvatica on December 18, 2017, 09:37:23 am

Hi,

Yes unfortunately checked it 5 times, that's well .01uF Mallory 150M

I'll try to change the 330 k to new ones closer to the right value. And if it doesn't work, I will do without tremolo  :dontknow:

Cheers,

Romain