Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alange5 on July 12, 2017, 02:49:11 pm
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I recently posted about a Gibson Hawk GA-25RVT (7591 version) and was coached through some repairs. Now that it's up and running, I'm tweaking it to taste.
Schematic:
http://music-electronics-forum.com/attachments/40393d1472138853-ga25rvt.gif (http://music-electronics-forum.com/attachments/40393d1472138853-ga25rvt.gif)
I'm having some strange interaction on the reverb channel. At 50% volume and reverb, the effect is full and lush. At 100% volume, the reverb is diminished, and needs to be turned up almost 100% to achieve the same depth. Consequently, I can only get full, splashy reverb when the volume is set lower than the reverb control.
Additionally, the reverb is very sensitive to the treble control. When both reverb and treble are set to max, there will occasionally be a low-volume, high-pitched whine which steadily escalates into feedback. The feedback goes away by either turning the treble down, or turning off the reverb at the footswitch.
Another symptom: When the volume control is set to zero and the reverb is turned up, I can still hear my guitar (and reverb) coming through the speakers.
These are very specific scenarios, and normally wouldn't bother me, however this amp is not terribly loud. While experimenting with tube swaps, I found that a 12AX7 in V2 sounds a lot better to my ears than the 12AU7. It's much louder, bigger, fuller, etc. Herein lies the problem: when a 12AX7 is inserted into V2, the runaway reverb squeals start immediately, regardless of reverb setting, and can only be stopped by disengaging the reverb footswitch, or by turning the treble to almost zero.
These problems only apply to the reverb channel. Channel 1 sounds great with the 12AX7. I'd love to be able to run a 12AX7 in V2 and still be able to use the reverb. Pulling V7 does not make the squeal go away, but pulling V3 does. I'm looking at the reverb return circuit, but can't understand what's causing the feedback. It seems like it's coming in too hot into V2B.
Any input is much appreciated.
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> When the volume control is set to zero and the reverb is turned up, I can still hear my guitar (and reverb) coming through the speakers.
Working As Designed. Reverb feed is tapped before the volume control.
If you prefer reverb gain to follow channel gain, try moving the top of R32 over to R33 wiper. However, this may increase your wrap-around sneakage and squeal.
Clearly Gibson struggled with this, and didn't find the best answer.
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If you prefer reverb gain to follow channel gain, try moving the top of R32 over to R33 wiper. However, this may increase your wrap-around sneakage and squeal.
Thanks for the suggestion. I'll give it a try.
Lifting the cathode bypass cap from V2B completely eliminated the squeal with a 12AX7 in V2. I left the cap on V2A (channel 1). By comparison, channel 1 is a lot more aggressive and a bit louder. I may experiment with smaller cap values on V2B to keep the "edge" while eliminating squeal.