6AQ5-Based 5e3 Guitar Amp Build

[Johnrcurry]

I am working on converting a 6AQ5 reel to reel tape recorder circuit to a 5e3-type guitar amp. I have used the 5e3 layout diagram from Triode Electronics as a starting point and redrawn the power tube portion for 6AQ5 tubes instead of 6V6 tubes. I’ve uploaded a copy of my end result (I also redrew the output jack arrangement to match my OT). I've also uploaded the original schematic for the tape recorder. My hope is that some of you experts will take a look and let me know if I’ve screwed something up, or if you see any problems with the plan.

I also have a few general questions about the project that I’d appreciate some help with. . .

First, the data sheet for the 6AQ5 shows pins 1 and 7 being the grid. Can use either 1 or 7, or do I have to connect the two, externally. (i.e. run the wire that would go to pin 5 of a 6V6, to both pins 1 and 7 of the 6AQ5)?

The tape recorder takes its B+ voltage from pin 2 of the 5y3 rectifier tube. All the 5e3 layouts I see take it from pin 8. Does it matter? Does it depend on the power transformer? In my case, should I use pin 2 as the tape recorder did, or should I use pin 8 as the 5e3 designs do?

Thanks for any help you can provide

John

[loudthud]

You are probably safe to just use one pin for the grid of the 6AQ5. On the 5Y3 you can use either pin 2 or 8 (or, if available a CT on the 5V winding). To make the socket compatible with a GZ34 however you should use pin 8. Layout looks ok but you may end up with ground hum. I hope the input and speaker jacks are a type that doesn't ground to the chassis. There are two jumpers near the ground end of the filter caps. Put those on the top of the board so you don't have to remove the board to remove those wires in the event there is a hum problem. Move the ground wire from the end of the 6AQ5 cathode resistor to the filter cap and move the ground wire from the preamp filter cap to the screen filter cap.

[Johnrcurry]

Loudthud,

I really appreciate your input. I will use pin 8 of the rectifier for the B+, as you suggest. Other than moving those two ground wires, is the grounding arrangement OK as far as minimizing hum? Is there a different approach that you would recommend?

Thanks

[loudthud]

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.

I suggest resistors for the 6AQ5 screen grids. Something like 1K 2W flame proof mounted to a terminal strip near the tubes. You can also mount the 1.5K grid stoppers there.

[Johnrcurry]

loudthud,

Once again, thanks for your input. Just to make sure I understand what you're saying. . . .

Your scheme has the center tap of the 6.3 v supply elevated by connecting to the cathodes of the power tubes, instead of grounding. Incidentally, that is how it was connected in the original tape recorder circuit.

The high voltage center tap, the filter caps, basically everything else but the OT is grounded by the input jacks (of if the input jacks are insulated from the chassis, by a wire grounded to the chassis nearby).

The OT is grounded by the speaker-out jacks.

Have I got that right, conceptually?

I've read a lot about grounding and I've come across a host of conflicting advice. Some writers swear by star grounding, and others say there are problems with star grounding and other approaches are better. Do you mind letting me know why you find this approach preferable to star grounding?

Thanks

[loudthud]

Grounding is a complex subject. Kevin O'Connor devotes at least a whole chapter to the subject in one of his books. If I could get permission, I'd use one of his illustrations here to demonstrate what I have tried to do in the layout. I've gotten so obsessive/compulsive about grounds, it's the first thing I do when laying out an amp.

Star grounding can work if you can connect all the wires together at one point. Multiple ground lugs on a transformer bolt isn't a good way of doing that, sadly. Actually, the way grounds were laid out originally would probably work well enough but if the hum is objectionable, a nightmare to fix.

In the real world, every wire and even the chassis has a finite resistance. When current flows through that resistance, a voltage is created. Ground noise is created when current from one circuit creates a voltage that appears as a voltage difference between two grounds. Note that as far as the signal is concerned, B+ and the local filter cap are considered as grounds.

When you try to connect single ended signals between two circuits, the signal current must return thru ground and any difference between the grounds appears as a noisy voltage in series with the signal. Differential circuits such as between the phase inverter and the power tubes or between the power tubes and the output transformer can reject a difference in grounds to a large degree.

Signals in the preamp are very low current and don't cause much ground noise. Unless the gain is very high, they cause no problem. In the speaker circuit of this amp, the high current is enclosed loop and does not flow through ground because the ground wire of the OT goes directly to the speaker jacks. If there was feedback, grounding of the speaker jacks could be an issue. The heater circuit also has high currents but those currents are isolated from ground and don't pose a ground noise problem.

The high voltage supply including the transformer, rectifier and filter cap can be a source of ground noise. Normally, current flows in a loop from the transformer, through the rectifier, through the filter cap and back to the center tap of the transformer in pulses at twice the line frequency. These pulses are normally about 3 to 10 times higher than the DC current drain. The higher the filter capacitance, the bigger and narrower the current pulses. When you ground the center tap at a transformer bolt, the current pulses must still flow to the filter cap and there will be a voltage difference between the ground side of the filter cap and the transformer bolt and chassis.

The philosophy behind the grounding scheme is to keep currents in tight loops with a local star ground for each circuit and have only signal currents flow between circuits. This kind of falls apart on the B+ side but the resistors between caps isolate signal currents and the DC currents are quiet. I call this scheme "A chain of Stars". The other non-ideal aspect is the whole preamp and phase inverter has only one filter cap.

Connecting the heater CT to a positive voltage reduces line frequency hum from transferring to the preamp cathodes.

To answer your question, you seem to have it right on all points.

[Johnrcurry]

Thanks for the detailed explanation. I can see I've got lots of studying to do before I really have a working understanding of these concepts. I'm sure I'll be back with more questions this project progresses

John

[atmars]

Maybe someone can clear one part of this up for me. Regarding speaker jack grounding (with no NFB), is it correct that the ground tap from the OT goes to ground lug of the speaker jack (or jacks), the jacks are isolated from the chassis, and the ground wire does NOT continue to a ground point on the chassis, but stops at the speaker jacks?

[loudthud]

Maybe someone can clear one part of this up for me. Regarding speaker jack grounding (with no NFB), is it correct that the ground tap from the OT goes to ground lug of the speaker jack (or jacks), the jacks are isolated from the chassis, and the ground wire does NOT continue to a ground point on the chassis, but stops at the speaker jacks?

Sorry I wasn't clear on this. You should always ground one side of the OT secondary. There are small leakage currents that can cause really strange things to happen if the speaker wires just float.

[Johnrcurry]

loudthump,

I've done some more studying and have a couple of questions, if you don't mind. . .

From what I understand, it is better to keep the higher current power supply and output stage grounds away from the lower current input stage grounds. With that in mind. . . .

Looking at your drawing, and assuming that the input jacks are not isolated from the chassis, they can provide a ground point for the lower current input stage that is physically far away from the power supply and output stages. If the is right, what I don't follow is the grounding for the power supply and output stages. Should there be a connection between the ground of the first filter cap and the chassis somewhere near the PT? Does the connection between the first filter cap and the rest of the circuit's grounds (ultimately grounding at the input jacks) violate the principle of keeping higher and lower current grounds away from each other?

Finally, is it better to isolate the output jacks and run a wire back to the first filter cap ground, or let them ground through the chassis?

Thanks for your patience.

John

[loudthud]

Yes I was thinking of grounded SwitchCraft type input jacks. There is a ground path all along the board to the first filter cap. If you added a ground at the 1st filter cap, then there would be a ground loop that may cause hum. If big noisy currents are flowing in that wire, there is hum between signal ground and the power cord's safety ground so you won't hear the hum until you connect something like an external reverb unit with a grounded power cord. There have been a couple of threads like that recently.

A common entry point for hum is when the input jacks aren't grounded to the ground side of the 1st tube's cathode resistor. If you have two paths back to a transformer bolt, there may be a small hum voltage that is in series with the guitar's signal.

[Johnrcurry]

It's up and running, but it hisses

I've finished building this amp. I ended up using the grounding scheme drawn out by loudthud (thanks again), except I did not daisy chain the grounds on the four input jacks. I just ran the single ground wire from the eyelet board to the first of the four input jacks. I figured that was enough since all the jacks are grounded to the chassis at that point. If that's wrong, please let me know.

It's very late so I couldn't test it at more than moderate volume, but it seems to sound OK. The normal channel sounds a little dark and muffled. The bright channel sound quite good. I ended up with about 294 volts on the plates of the 6AQ5's. From what I've read, that's a little high, but probably OK. Am I right about that?

The only problem I noted is a hissing. It is effected mostly by the volume control of the bright channel (the middle pot). The amp is quiet with all controls turned down. Turning up the volume control for the normal channel does not bring on any hiss until you hit about 9 or 10, then it gets louder till 12. With the bright channel (middle pot) the hiss starts at about 2 or 3 and gets louder all the way up.

The hiss goes away entirely when the 12ax7 (and only the 12AX7) tube is removed. It is present, but much less pronounced when the 12AY7 tube (and only the 12AY7) is removed.

Because the hiss is effected by the volume controls, and it goes away when the PI tube is removed, I'm thinking it's somewhere in the preamp stage. Any ideas as to where I should look to try to chase it down? Thanks for your help.

[tubeswell]

Hiss is Normal

Hiss is a normal part of the function of amplification. That's why you notice it more as the vol gets higher. (Hi-gain amps are especially prone to it).

You can quieten a hissy amp somewhat by putting in 'over-rated' resistors (i.e. putting in 1W or 2W resistors where the schematic calls for 1/2W etc) especially in at the plate resistor locations, or alternatively by putting in carbon film or metal film resistors instead of carbon comp resistors. (CCs are most prone to hiss - even tho' they provide some vintage mojo if used as plate resistors (but then, lotsa vintage amps were hissy too)

[Johnrcurry]

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.

[tubeswell]

Well then try either going up to 1W carbon film, or try using metal film (if you want to reduce the hiss)