6AQ5-Based 5e3 Guitar Amp Build

[cminor9]

Tubeswell,

Thanks for your response. I did use carbon film for the 1/2 watt resistors and metal oxide for the 3 watt resistors between the filter caps. Maybe you're right, and the hiss I'm hearing is normal.

Good candidate resistors for a metal film replacement are all cathode resistors for the 12ax7s, and the 68k stoppers on the input jacks. That should help out a lot. When I did that on my 5E3 it made a huge difference reducing hiss. I realize that could be anecdote or a correlation-equals-causation error on my part, but I have read that those are places where CCs can cause trouble. It helped me.

[Johnrcurry]

cminor9,

Thanks for the reply. It's probably worth trying. With the 68K resistors, I mounted them at the tube sockets and used shielded wire from the input jacks, so that will be a bit of a pain to undo and redo. Which reminds me. . . . as i was checking voltages, when I touched the probe from my meter to those pins I heard a radio station through the speaker. That suggests to me that the shielded wire is doing its job.

[mike the dog]

Hi Loudthump,
I used your layout to build a 5E3 clone, using parts from a working Pentron reel to reel. The only change I made was to use the 6X4 rectifier. The amp works, but sounds bad! Has 20% of the volume it should, distorted and muddy. I have double checked myself over and over and everything looks fine. All resistors and caps are new. I need the help of someone smarter than me(thats why I am here)! Any Ideas what I might look for? Any input would be much appreciated!
Thanks-Mike

[loudthud]

I suggest you start a new thread. You can certainly post a link to this one for reference, it can be hard to find otherwise.

The best indicator of a properly functioning amp is voltage readings on all the tube pins. But first lets check the one thing that your symptoms suggest. Set your meter to AC volts and connect it across the speaker terminals. With no guitar plugged in but the volume controls up, you should get a very low reading in the 10s of millivolts or lower. Anything too high could be an ultrasonic oscillation. You can't hear it but it will make the amp sound really bad.

Did you measure the resistors before you installed them? That's always a good idea.

[olddawg]

Have you looked into the various 18 watt Marshall clone schematics/layouts? Might be a better setup with those tubes. The Lite clone is a no brainer.

[mike the dog]

Thanks for taking the time to reply! I checked the voltage at the speaker terminals, .010 with channel 1 at full volume, .003 channel 2 at full volume, and .006 with both at max. That looks higher than 10 millivolts to me. Also, with #1 at full and #2 at 25% it is at .022. I will check the resistors later today. They are new, and seem to be decent quality. What can cause the ultrasonic oscillation? Thanks for the help!
Mike

[d95err]

(irrelevant post)

[mike the dog]

Thanks for your input! I did check all my resistors, and found I used 220k ohms in place of 220 ohms. Sounds pretty decent, not sure if it sounds correct, a little scratchy/buzzy(not good kind) when the bass notes are pushed. So I will keep messing with it. Thanks again for the help!

[bxershrts]

Hello, All,

I am jumping into this thread some years later and hope that it is still going on. I have always wanted to build a 5E3 style amp with 6AQ5's. I am wondering if this will work let me know if this is a viable idea.

Since the 6aq5's need substantially less volts than the 6v6, could 6ax4/6ax5 rectifier be used. Removing the 5v windings from the socket for the 5y3 and using the 6.3v heater wires that the rest of the tubes use ? Bringing the plate voltage down to what I hope would be in acceptable levels then wire the 6aq5's as per schematic/layout as documented above?

My goal is the keep the current op and pt and everything else I have in an exisiting 5e3 build and just rewire with the 6ax4/5 the bring the plate voltage down.

Thought?

[d. spree]

I'm not a fan of connecting the HV center tap to a transformer bolt. I've modified the layout to show how I would do the ground. If the speaker jacks don't ground to the chassis, run a wire to the ground of the 1st filter. If the input jacks don't ground to the chassis, run a wire to a nearby screw in the chassis.

Loudthud, I'm interested in why you don't like the HV center tap grounded to the bolt on the PT. What's the advantage to grounding it to the ground of the first filter? Asking because this may apply to a build I am currently doing. Appreciate your thoughts. I read your earlier response about the HV center tap ground to PT bolt but am not sure I understand? Sorry if i'm being slow.

[loudthud]

Loudthud, I'm interested in why you don't like the HV center tap grounded to the bolt on the PT. What's the advantage to grounding it to the ground of the first filter? Asking because this may apply to a build I am currently doing. Appreciate your thoughts. I read your earlier response about the HV center tap ground to PT bolt but am not sure I understand? Sorry if i'm being slow.

Some people think of grounding as a black art shrouded in mystery. I prefer a more scientific engineering based approach. Go to my profile (click on my name to the left) and look at the messages tab. You will find links to other posts with in depth explanations. Connecting the center tap to a transformer bolt is the first step leading down a path of less than the best results. Even if you can get the amp quiet, hum will come back if you connect to an effect with a power line ground.

In some instances I may advise connecting the center tap to the chassis rather than re-wiring the entire ground in an amp. That's my practical side. Having looked at the ground in most amp kits now on the market, I haven't seen one that I wouldn't change. One exception might be London Power, but Kevin doesn't really produce complete amp kits.

[R.G.]

Loudthud, I'm interested in why you don't like the HV center tap grounded to the bolt on the PT. What's the advantage to grounding it to the ground of the first filter? Asking because this may apply to a build I am currently doing.

Grounding and hum issues all require secret, arcane knowledge known only to a few. There are two parts to the secret.

1. There are no "short circuits"; all wires are really low-value resistors.
2. You must learn to think of where the current goes, not what the voltages are.

The grounding of the HV centertap is *crucial* to low hum. This wire must go ONLY to the (-) terminal of the first filter cap. This is because the wires are actually low value resistors, and big pulse currents flow through them charging the caps through the rectifiers.

The rectifiers stuff current pulses into the top of the caps. There must be an equal and opposite current coming out of the bottom of the caps. These pulses travel through that CT wire, and cause voltages to appear that are different from one end of the wire to the other.

If any part of that wire is connected to signal ground anywhere else, including the chassis, the pulse-induced voltages are strong enough to push around the signal ground. These pulses occur at 2x line frequency, and are heard as a 2x line frequency hum.

The way to minimize this is to leave the pulses to the rectifers and HV return. If you ground the chassis and the circuit after the first cap's (-) terminal, the rectifier pulses cannot wiggle signal ground around, so much less hum results.