65 Deluxe Reverb front end EF86 mod.

[weeping6]

Hello,

This post comes in 2 parts, the first is just a description, the second a question. I recently built a 65 Deluxe Reverb with the AB763 circuit. MM trannies, JJ Tesla tubes throughout, and the Weber 12F150 ceramic speaker.

My take is that it sounds absolutely fantastic (granted my own biases are present) but needs a higher power rated speaker which is a "to do". Also, I found I never used the "normal" input since the reverb isn't included in this part of the circuit. So I changed out the tube and bias circuit with that for an EF86 keeping the original tone stack intact. To include the reverb with the EF86 front end I plumbed the output of the EF86 front end into the second input jack of the normal channel so I could chain it in series with the "vibrato" input. I love the volume and tonal variations that are now available with the addition of another gain stage.

That said, there's an apparent problem I built into the modified portion of the circuit. When the volume is high on the EF86 section there is a cloudy hiss-crackling that starts when the guitar is played hard. When I turn the amp off it sounds bit like a deflating tire. My guess is this is some capacitor discharge but all biases seem fine and it's not heard at moderate volumes when playing. It doesn't happen when I'm playing only through the vibrato input (no EF86 circuit) so that tells me it's in the modified additional front end.

I would love to hear any comments or thoughts on the circuit modification and/or thoughts and suggestions on the flat tire sound present at high volumes. Anything is greatly appreciated.

David

[Steve Conner]

Hi,

You could try subbing in another EF86, also try changing the screen bypass cap. If those changes have no effect, I'd suspect a bad plate or screen resistor, or a bad coupling cap between the EF86 and the next stage. If it's not that, maybe it's a dirty eyelet board that conducts a little. And finally as a last resort, maybe it's parasitic oscillations.

I have a similar amp with an EF86 strapped in front of a classic circuit, and I'm loving it too. I made it a channel switcher, one channel is stock and the other has the EF86.

[jrfrond]

It could also be parasitic oscillation due to increased gain and resulting instability. If you kept the rest of the circuit as-is, with the increased gain of the EF86, this is a good possibility. Unfortunately, you need a scope to see it. If nothing else works, this would be a likely suspect.

[6267]

Sounds like parasitics, although I've gotten weird crackles just before blocking distortion. I'd try reducing the coupling cap a bit, and increasing the grid series resistor for a variety of reasons. Experiment with your lead dress. Weird noises on turning off, like "squawks", "squeeks", and "squeals" are often symptoms of oscillation or poor stability due to high gain. Sometimes you might have to bypass the tube with say 150pf at the anode to ground.

[HTH]

try shielded wire for the grid connections - most times thats enough to cure oscillations, even fairly severe one's when I've done high gain amps.

[soundmasterg]

Sounds like its maybe parasitics....if the above suggestions don't work, then you can reduce gain by reducing the plate and screen voltages. Plate voltage should be about 1.3 to 1.4 times what the screen voltage is for optimum operation of a preamp pentode.

Greg

[HTH]

Sounds like its maybe parasitics....if the above suggestions don't work, then you can reduce gain by reducing the plate and screen voltages. Plate voltage should be about 1.3 to 1.4 times what the screen voltage is for optimum operation of a preamp pentode.

Greg

thats interesting, the info I've always gone off states that in typical guitar amp, Ra:Rg2 ratio for a preamp pentode should be at least 1:10 - in the Vox AC4, they used 220k plate and 5M5 grid2.

My amp using an EF184 preamp pentode has the screen supply taken from a 220k plate resistor (B+ side), then padded down via 1M and 470k voltage divider (470k to ground, junction to screen) and a 0.1uF cap from screen to cathode...

* plate 151v
* screen 35v
* cathode 0.68v

[soundmasterg]

Some of the pentode circuits in use, such as the Vox circuit weren't designed properly, though they work and sound good. As an example, if the screen is 100v, the plate should be 130 volts to 140 volts...you get best linearity this way. Tweak with the dropping resistor to get your supply where you want it with your chosen plate resistor, and play with screen resistor values until you get voltages there where you want them. Keep in mind as you change one it changes the others, so its a balancing act.

I added an EF86 to my brother's Silvertone 1484 several years ago, and initially had some parasitic issues until I adjusted the voltages down, thus making the gain go down also. Once it is dialed in, it sounds really nice.

Good luck!

Greg