Non top boost AC-30 in for repair.

[Mark]

I have a grey panel AC-30 to fix. It came in for noise issues but has a few more issues such as a badly burnt screen grid resistor and the vibrato doesn’t work and is causing the hum. I don’t think the caps in this amp are original and if it were mine they would be out like a shot. The zener diode biasing also troubles me as if they go short-circuit the output transformer is a candidate for destruction.

Any thoughts?

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[pdf64]

That black Hunts cap will almost certainly be dead. When replaced, the vibrato will probably work, perhaps for the first time in several decades.
What cathode voltage does that zener string produce? 3 in series should help to mitigate the risk of any shorting. And compared to the regular 50ohm resistor, at least they will maintain proper bias if any of the EL84get weak / fail / otherwise shut off, which may have happened to that end EL84 with the charcoal screen grid resistor.
So it's swings and roundabouts, but on balance I'd revert to the classic 50ohm//220uF cathode circuit, but insert a 1ohm 1/2W current sensing resistor between each cathode and that. To easily facilitate a check on each of the quad, to verify that they're in reasonable balance.
I fit series protection diodes to the rectifier and a HT fuse to the PT's HT winding's CT to 0V connection; depending on which standard are the norm down under, an F330mA should be fine.

Is that device behind the amp, with the green pillars, your dummy load?

[Colossal]

Cool amp, Mark, thanks for posting the photos. Interesting to see the Parmeko power and output transformers with a Woden choke. If you have time, once it's back up and running, could you post rectified voltages running on modern AC mains? I've love to see what that PT is producing. I am presuming it is 280-0-280 unloaded.

Thanks,
Dave

[pdf64]

Winding ratios for the OT, and winding resistances for all 3, would also be really useful info.

[Colossal]

Winding ratios for the OT, and winding resistances for all 3, would also be really useful info.

Pete,

For reference, I kept your comment regarding your Woden powered AC30, which has been very helpful actually:

The Woden in mine puts out 586Vac with the HT unloaded, 560Vac at idle, 560Vac at max 'clean' 25W, 554Vac at fully overdriven squarewave 38W. The HT idles at 317Vdc.
243Vac mains feeding 245V PT primary, heaters 6.41Vac at the EL84s. Measurements taken with calibration checked Fluke 189.

[Mark]

Thanks for the replies. I was wondering what brand of transformers were in the amp, there weren’t any markings on it.

The zener diodes used are 1N5338 which are 5.1v at 5A. Using three of them would put 15.3v on the cathode. The owner was also running the mains on 225v instead of 245v. Which would have been hard on the output stage.

I will be testing the valves today and start measuring voltages etc. I’ll measure the resistance of the output transformer. How do you want the turns ratio of the output transformer measured?

[Mark]

The amp has a rail voltage of 350vdc which is more than I’d like. I suspect the power transformer has blown and been rewound at some stage as there are only the 115vac and 240vdc tappings.

The output valves had failed as indicated by the burnt screen grid resistor, I have removed the 100 ohm screen resistors and fitted 270 ohm metal oxide resistors, I’ll try and give the output valves some chance of survival.

I think I’ll use a 82 ohm cathode resistor as 50 ohm would allow an insane amount of current to flow.

I’ll work from this layout.

http://turretboard.knucklehead.dk/wp-co ... alrev7.pdf

https://drtube.com/schematics/vox/ac301960.gif

[Colossal]

The amp has a rail voltage of 350vdc which is more than I’d like. I suspect the power transformer has blown and been rewound at some stage as there are only the 115vac and 240vdc tappings.

The output valves had failed as indicated by the burnt screen grid resistor, I have removed the 100 ohm screen resistors and fitted 270 ohm metal oxide resistors, I’ll try and give the output valves some chance of survival.

I think I’ll use a 82 ohm cathode resistor as 50 ohm would allow an insane amount of current to flow.

Hi Mark,

Using a shared 82R, which is 328R for a single tube, and assuming there is about a 10V drop across the OT (350V B+ -10V = 340V plate), and the screen voltage is about the same as the plate voltage (340V)...

The EL84 triode curves show the amp would be biased around 13.8V on the cathodes, 42mA per tube. This is 114% of max dissipation or 13.2W per tube.

EL84 Triode.jpg

[Mark]

So the amp would need a cathode resistor in the order of 120 ohms?

[pdf64]

For a HT of 350Vdc, what’s the loading on the HT? eg are all 4 EL84 drawing current, and how much?
As the HT should sag a bit according to the degree of loading.
The PT's HT winding resistance of mine is 120ohms, is yours similar?

For the OT ratios, a simple method is to isolate the amp, remove the link between the OT secondary common and chassis 0V, and apply a few Vac from a low impedance sine wave source (eg the heater supply from another amp). Then measure the applied Vac at the secondary and the resulting plate to plate Vac at the primary.
Don’t bother if you’re not comfortable with doing it.
The OT primary resistance of mine is 194ohms.

[Mark]

The output transformer measured 167 ohms across the primary. From the centre tap, one side measured 82.6 ohms and the other side measures 84.5.
The secondary is very low, the leads measure 0.2 ohms and I’m getting 0.9 across the 16 ohm secondary and 0.6 ohms across the 8 ohm secondary.

[Mark]

I have the tremolo/vibrato channel under control, but I’m wondering about getting a bit more out of the vibrato.

I’m going to use 6P14P valves, they should be able to take the voltage, though I will have to rebias the output stage.

[Mark]

I wonder if someone could run their eyes over this to see if I have made an error or not understand what is happening.

As far as I can see I have 0.7 watt dissipation at idle on the screen grids, and the dissipation of the EL-84 is 6.38 watts at idle. That seems a very cool bias to say the least. I’m actually a little surprised.

Especially when looking at Colossal’s calculations.

Any thoughts?

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[Mark]

I tried a different set of output valves and much the same.

I thought the bright channel sounds very shrill through my 18 watt Marshall cab (which in fact was made with the same dimensions as an AC-30 cab, the cab is loaded with a Celestion Blue and a Celestion H30. I have seen this combination in one of Brian May’s AC-30’s), I thought I’d try and remove the 470pF styro cap (non original part) and put a 1000pF 715 Orange Drop in (that is all I have in that value). I like it, it adds body to the channel while still sounding like a bright channel. I think the 1000pF was used in the Bass AC-30 models.

[Mark]

The UPDATE: The GZ-34 is stuffed, I replaced it and the rail voltage is back up to 354VDC.