Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mars-Hall on November 22, 2014, 02:45:56 pm

Title: 72 Quad Reverb "no reverb"
Post by: Mars-Hall on November 22, 2014, 02:45:56 pm: The symptoms are no reverb, however the reverb knob does affect the tone of the amp. It increases treble as you turn it up, until 7 or 8 then it causes feedback and distorts any notes played, in a bad way. Also, the springs can be heard when the tank is knocked.

Here's what I've checked so far:
Replace the driver and pick up tubes with known good.
Checked the return and send wires for continuity.
Check the tank transducers for continuity.

I do plan on changing the 25uf/25v electrolytics on the board. Could a dried cap cause this issue?
Title: Re: 72 Quad Reverb "no reverb"
Post by: Mars-Hall on December 04, 2014, 04:07:53 pm: Since my first post, I've managed to prove out the reverb tank. I replaced the electrolytic caps on the board. With known good tubes, I get no voltage pins 1 or 8 of v3 and voltages "in range" on pins 1, 3, and 6 of v4. From what I can tell there seems to be no burnt resistors on the board. Do I look to replace the reverb transformer?
Title: Re: 72 Quad Reverb "no reverb"
Post by: PRR on December 04, 2014, 05:03:14 pm: There seem to be several "Quad Reverbs". I don't remember 1972, or which ones were sold then. I hope I am looking at a relevant schematic.

> no voltage pins 1 or 8 of v3

When you turn-up Reverb knob, and kick the amp, do you hear thunder?

If so, power-down, disconnect the Blu and Red on the reverb transformer, measure the Ohms across that winding. (With some meters, you may need to short the Grn Blk winding to get a good reading.) Several K-Ohms is reasonable. Meg-Ohms or Overload (no connection) is bad.
Title: Re: 72 Quad Reverb "no reverb"
Post by: Mars-Hall on December 04, 2014, 05:35:25 pm: I get no reading at all across the the blue and red wires of the reverb transformer.
Title: Re: 72 Quad Reverb "no reverb"
Post by: eleventeen on December 05, 2014, 11:17:54 am: If you are saying there is infinite resistance (eg; open ckt) across the blue-red of the rev tranny it is toast. A bit rare but not unheard of.
Title: Re: 72 Quad Reverb "no reverb"
Post by: Mars-Hall on December 05, 2014, 08:23:55 pm: What I mean to say is, the meter reads as if I weren't testing at all. The meter reads the same if I add or remove the leads.
Title: Re: 72 Quad Reverb "no reverb"
Post by: sluckey on December 05, 2014, 08:36:49 pm: The primary of the reverb transformer is open. You need a new one. Hoffman sells them right here.
Title: Re: 72 Quad Reverb "no reverb"
Post by: Mars-Hall on December 05, 2014, 09:15:48 pm: Quote from: sluckey on December 05, 2014, 08:36:49 pm
The primary of the reverb transformer is open. You need a new one. Hoffman sells them right here.

Thank you sluckey. I will source the part from the store here.