My amp build sounds like a theremin

[Duncan]

Hey guys.

I hate to open this up to the hive mind, but I’m honestly at my whits end.

A while back, I decided to have a go at marrying the brown channel from a Bogner Fish with the power section of a 2204. I sourced the corrected Fish schematic from SLOclone, pulled the 2204 schematic, and set out putting them together.

With some feedback from a reputable builder and some folks over on TGP, I built my wiring diagram and added a couple tweaks (diode clipping on the master, different value of caps, depth/resonance control, etc).

I got everything wired up and it made guitar sounds, albeit much quieter than it should have. Turns out, I had wired the 6.3VAC winding incorrectly.

I fixed the heater wiring and fired it up.

I have a full volume amp now, but now it’s a full volume theremin (with the frequency sweeping with the gain pot). Guitar sounds coming through sound like a very squashed fuzz pedal.

I initially chalked it up to poor lead dress, so I pulled the board out and redid it all, with as much space between leads as possible, run along the chassis. No change.

I also swapped all of the leads to the grids and to the pots with shielded wire. No change.

I also thought maybe it was a ground loop issue, so I moved the star ground to the other side of the chassis, well away from the board/components. No change.

In doing the pop test, I’m getting pops on the grids of the phase inverter, but not on any of the other sockets.

I’ve checked and reflowed all connections, and I have continuity throughout to where it should all being going.

Voltages are very close to where they should be (based on a JCM800), and I’ll be tweaking the dropping resistors once the amp is up (to increase them closer to Bogner specs).

Anyone have experience with this sort of issue? I’m sure it’s a pretty bonehead wiring issue, but fresh perspective would be greatly appreciated.

Through my troubleshooting, I eliminated the switches from the circuit and the depth control. Also, I’ve redone the B+ wiring, so the layout isn’t what’s reflected in the photo.

I don’t have access to a scope, unfortunately.

[sluckey]

Have you swapped the OT primary plate leads?

[Duncan]

Have you swapped the OT primary plate leads?

I have yes, made a terrible sound the other way, so I definitely have those right haha.

[sluckey]

You don't have enough decoupling filter caps. See pic...

[Duncan]

You don't have enough decoupling filter caps. See pic...

Is the theremin effect indicative of insufficient filtering?

[PRR]

> don't have enough decoupling filter caps.

Agree.

It is also hard to accept that you want FIVE gain stages. Even with small pads between and one large cathode resistor. The multiple small cathode caps will cascade into a totally shrill sound (or oscillation).

Less may be more.

> Is the theremin effect indicative of insufficient filtering?

Theremin is hand-wave controlled. Do you have that too?

Unwanted oscillation always includes too much gain. As Sluckey says, your power filtering is not well designed. Missing or misplaced caps. And IMHO far too much gain for any guitar amp.

[Duncan]

> don't have enough decoupling filter caps.

Agree.

It is also hard to accept that you want FIVE gain stages. Even with small pads between and one large cathode resistor. The multiple small cathode caps will cascade into a totally shrill sound (or oscillation).

Less may be more.

Five gain stages might be more than most amps, to be sure, but I’ve always lusted for Jerry Cantrell’s tone from “Dirt” and afterwards (when he was using the Fish). And if I have to build an amp that Reinhold Bogner felt five stages was needed, then that’s where I’m at haha.

> Is the theremin effect indicative of insufficient filtering?

Theremin is hand-wave controlled. Do you have that too?

Unwanted oscillation always includes too much gain. As Sluckey says, your power filtering is not well designed. Missing or misplaced caps. And IMHO far too much gain for any guitar amp.

Nope, the only thing that changes the frequency is the gain control.

[sluckey]

There are only four gain stages in that amp. And if you do as I said on the drawing you will likely fix the "theremin" effect. I'm sure you used the word theremin to describe nasty oscillations. The word oscillation, or screaming, or howling would likely be more appropriate.

[Duncan]

There are only four gain stages in that amp. And if you do as I said on the drawing you will likely fix the "theremin" effect. I'm sure you used the word theremin to describe nasty oscillations. The word oscillation, or screaming, or howling would likely be more appropriate.

Yeah, it’s almost definitely oscillation, but it sounds like a theremin. It’s pretty cool, but definitely not what I want. I’ll shoot a video so you can see what I mean lol.

I’ll make those filtering additions/changes and report back. I have three open sections (was saving one for the FX loop, one for maybe a clean channel down the road).

Thanks!! :)

[TIMBO]

Hey guys,
Looking at both schematic and layout, it arrears the 1m resistor on the input jack is not connected????

[Duncan]

Hey guys,
Looking at both schematic and layout, it arrears the 1m resistor on the input jack is not connected????

I’m the schematic, that’s probably just a photoshop blip. I can confirm that it is connected and connected properly.

[sluckey]

V1 pin 2 has no resistive path to ground. That triode cannot work properly as drawn in the schematic or layout.

[astronomicum]

You may want to consider arc protection and the need for heater elevation on the DC Coupled Cathode Follower, if you have not already done so.

[Duncan]

V1 pin 2 has no resistive path to ground. That triode cannot work properly as drawn in the schematic or layout.

Doesn’t the 33k to 1M to ground provide that?

[sluckey]

V1 pin 2 has no resistive path to ground. That triode cannot work properly as drawn in the schematic or layout.

Doesn’t the 33k to 1M to ground provide that?

The 33K does not connect to the 1M. There is a capacitor between them. Also, your layout shows the tip of the input jack connected to the sleeve (ground). Can't possibly be right.

There are so many errors on your drawings that it would be helpful to post some hi-rez pics of the actual amp.

[PRR]

There are only four gain stages in that amp.

I appreciate the input. Today I drilled 12 holes for 20 bolts and don't know how I miscounted, holes or bolts.

Here is my count. I include the PA driver (V4 LTP) because it gains from 1V to 20V; I neglect the power tubes which have good gain to plate but typically ~~unity gain to loudspeaker. Yes there are other places gain is lost; but like 50 down to 10, not to unity.

Parts-count may not be the judge of a design. My new car/SUV has a better engine than my 1967 Mustang or Cougar, but half the cylinders.

[Duncan]

V1 pin 2 has no resistive path to ground. That triode cannot work properly as drawn in the schematic or layout.

Doesn’t the 33k to 1M to ground provide that?

There are so many errors on your drawings that it would be helpful to post some hi-rez pics of the actual amp.

Hmm okay. How about I take a step back and post the schematic I used to build the preamp section to see if anything doesn’t work there?

[sluckey]

Maybe you could just post the pics I asked for.

[AlNewman]

Today I drilled 12 holes for 20 bolts and don't know how I miscounted, holes or bolts.


My new car/SUV has a better engine than my 1967 Mustang or Cougar.

With all due respect, I could see drilling 12 holes for 20 bolts, but there's no excuse for comparing a new SUV to a 67 Mustang or Cougar.

[nandrewjackson]

Looking at your profile, that is apparently your second build.


Very ambitious 2nd build, as was your 1st.


I'd say +1 for adding filtering as shown.


+1 for show pics of the actual build. More eyes, fresh eyes, might see something you've become blind to.


I've built several high gain, rectifier/SLO/etc preamps. Lead dress can be a big factor, I've had the uncontrollable preamp squeal that lead dress cured.


✔️✔️✔️✝️✝️✝️🎸🎸🎸

[sluckey]

Here is my count. I include the PA driver (V4 LTP) because it gains from 1V to 20V;

Of course that's correct. Many people, including myself, consider the PI as part of the power amp and don't count that when counting gain stages.