My VOX AC10C1

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Note: This is not about fixing faults or problems with this amplifier. The amplifier as designed works well. What follows is to satisfy curiosity, do things like I would have done them, whether it be right or wrong, and to see if I can improve some of the niggles I experienced. I am not an amplifier designer, builder, or an expert. I am just having fun.

I shall start this discussion with the last pic from my posts at “NAD - VOX AC10C1”

This opened back did not reduce bass response. So much for the bass being a result of the closed back cabinet.
That was where the planned new back panel stalled. I did make up spacers to space out the planned back panel to allow more space behind the speaker, which I plan to replace with a Celestion G10 Creamback. which I ran with a few other amplifiers, it tested out as “not offensive”.

In May 2022 the amplifier decided to go into loud crackling/popping/sizzling mode, and resetting a skew power valve did not fix that. Before I took out the chassis to poke around, I used a mirror and feel to insert new JJ EL84 valves, which seem to have solved this noise problem. Good. So at least that problem was easily solved. I removed the chassis anyway, to fit different pre-amp valves, and to go ahead with some planned alterations.

Here, at last, is the long awaited (?) picture of the amp chassis insides. Rather neat.

Eye-balling the available real estate inside the box, a handwired amplifier, with extras, is possible.

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First order of business should be putting the power valves right side up. The power amp shelf allows this. There is enough space above the power amp bracket to invert the setup and add some heat shielding. This is just a “I think it is better” mod. There are no reported heat damage issues as far as I am aware of, yet. It has been many years since satisfied owners started using this piece of Cheap Chinese Cr##p. Rather nice amp. Yes, seems the valves may pack up.

The original chassis tab with the power amplifier PCB. Who knew a power amplifier can be so simple?

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The power amplifier PCB removed, and it’s chassis mounting studs pressed out. I am of the opinion that the wires are cheap, hard, and was not specified to be handled at all after assembly. The plug for the signal wires from the phase inverter does not want to come off the PCB pins, take care.

New mounting studs, and new valve socket supports done.

Heat shields to follow.

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Not having the relevant plug sets in stock, I had to cut into the power supply wires to lengthen them.

For the lead from the pre-amplifier, I had the relevant item on hand:

A simple heat shield should keep the worst heat off the main PCB and surrounds.

Plan is to replace the 12AX7 pre-amplifier valve with a 12AY7, and the 12AX7 splitter with a 12AT7. I read scary stories on the internet about having to re-bias the valves, using higher rated plate load resistors, and so on. A Fender circuit with a 12AY7 shows the same cathode and plate resistors, so the re-bias might not be necessary. The pre-amplifier side of the 12AX7 has a normal ¼ Watt plate resistor, the cathode follower side is fitted with a fat 1 Watt cathode resistor, but the splitter valve use tiny dots pretending to be surface mount resistors, not easy to modify those.

I should poke some probes into the amplifier to measure voltages. I am not sure if fitting the 12AY7 and 12AT7 will overstress the relevant components.

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I did probe the circuit, after the previous post, but never posted the results. Life...
I checked with the new JJ EL84’s, the original Chinese 12AX7’s, and with a JJ 12AY7 in the pre-amp socket, JJ 12AT7 in the splitter.
With Eskom supplying 226 to 229 VAC, the B+1 ran at 338 V, B+2 at 329 V, B+3 at 309 V.
The JJ EL84 measured:
Pin V DC
1 NC
2 Grid 1 0.16
3 Cathode / Grid 3 10.57
4 Heater 0.04 VAC
5 Heater 6.5 VAC
6 NC
7 Plate 321
8 NC
9 Grid 2 (“B+2b”) 313

The pre-amplifier section probed as follows:

Original Chinese 12AX7
(One half is a gain stage, the second half is a cathode follower to the tone controls)
Pin V DC
Gain Stage
1 Plate 1 124
2 Grid 1 0?
3 Cathode 1 0.89
4 Heater 13.27
5 Heater 0
Cathode Follower
6 Plate 2 190
7 Grid 2 124
8 Cathode 2 122
9 Heater ground 6.5

JJ 12AY7
Pin V DC
Gain Stage
1 Plate 1 103
2 Grid 1 0
3 Cathode 1 1.2
4 Heater 13.1
5 Heater 0
Cathode Follower
6 Plate 2 188
7 Grid 2 102
8 Cathode 2 102
9 Heater ground 6.5

Phase Splitter- original Chinese 12AX7
Pin V DC
1 Plate 1 175
2 Grid 1 29
3 Cathode 1 43
4 Heater 13.3
5 Heater 0
6 Plate 2 173
7 Grid 2 31
8 Cathode 2 41
9 Heater ground 6.6

Phase Splitter 12AT7
Pin V DC
1 Plate 1 155
2 Grid 1 38
3 Cathode 1 57
4 Heater 13.3
5 Heater 0
6 Plate 2 156
7 Grid 2 38
8 Cathode 2 58
9 Heater ground 6.5

The schematc specifies a ¼ Watt plate load resistor for the 12AX7 gain stage valve. If my calculations are correct, the original 12AX7 draws 0.7 mA through that 100 kOhm resistor, and the JJ 12AY7 draws 0.86 mA. My calculations show the ¼ Watt should still last a while. This resistor is not surface mount, it is a normal axial lead resistor. The 10 kOhm resistor above it, handling the cathode follower current as well as the gain side current, is just a tiny surface mount item. Go figure.
The 12AX7 splitter valve drew 0.47 mA, and the JJ 12 AT7 draws 0.6 mA, measured over one plate resistor (a tiny little 100 kOhm surface mount thingie).
The JJ 12AY7 and 12AT7 valves do put an extra load on the power supply system.

The compact cabinet does have it’s drawbacks. A bit of the speaker is shrouded at the top.

I assembled the amplifier with the JJ 12AY7, 12AT7, and EL84 tubes, a new Creamback speaker, with a speaker lead to the back panel jack.

Initial impression was the gain is less, the tone is not as fizzy as before. And a Top Cut control would be nice.

I eventually kicked myself into gear and drilled/sawed the replacement back panel. With that huge chassis plate taking up so much area, “open back” becomes a rather relative term.
The back panel is not sealed against the steel chassis anymore, more of that steel plate is exposed to ambient air, and the cooling intake area is much more than standard. I shall test it for tone like this for a while, then decide if I cut the two extra openings where the corner holes already exists, before final finishing.

What I need to do, is find a way to put vent holes in the top rear to help the PCB and pre-amplifier valves to stay cooler. The standard setup is sure to lead to a “dead” pocket of hot air right in the top rear corner of that chassis. It does get quite warm in use, and some better airflow there can only help.

OK, that was the simple stuff.