Vox AC4 - Newbie builder questions

[old tube]

Hi, I'm new on here and very impressed with the level of knowledge and helpfulness of the posters on this forum.

I'm new to amp building .. this will be my first. I recently resurrected an old Selmer 100 Watt PA from dead and converted it to 7581a's for bass use. I really enjoyed doing it and gave me the bug. I've no electronics background, but a local amp tech has been kind enough to tutor me a bit so I've got the basics including safety.

I'm about to build an AC4 "clone", wired point to point. Most of the parts are coming from a 1963 Philips reel to reel tape recorder. It has a single ended amplifier consisting of an EZ80 rectifier, EF86, 12AX7 and a PCL92. As well as the original Mullard valves which all test OK, I've got a load of mustard caps and most of the carbon comp resistors needed, all in spec. From what I can find on the internet, the transformers should be OK for the job, though the OT while being 5k, may be a little "light" if it was not over spec for the PCL92. However, nothing ventured, nothing gained!

My questions are simple really.

Firstly, I plan to follow the early '60's JMI drawn schematic. However, when I look at more modern representations of the circuit, some of the shown values seem to vary from the original, cap changes seem in the main seem to have been done to brighten the amp, resistor changes to get more gain from the EF86. The change that surprises me most is the use of 16uF Electrolytic smoothing caps rather than the 32uF originally specified. I am inclined to use the 32uF but any advice would be greatly appreciated.

Secondly, the original had a combined on/off switch and tone control pot and I can't see a fuse in the circuit! I'm inclined to go for a separate tone pot, on/off switch and a mains fuse all mounted in the face plate ... and an "on" light off the heater circuit .... I assume that this would be "best" practice.

Thirdly, what about a standby switch and HT fuse? I'm not to particularly worried about protecting the valves with a switch as the EZ80 is there, but is a fuse a good idea? and where would it go in the circuit?

Many thanks in advance

[M Fowler]

Not sure about the changes to the circuit overtime but that is what usually happens to most circuits. Keeping the 32uf is fine.

As far as a standby switch it is not necessary on this low voltage amp but if it is something you want in your amps then by all means add it. The HT fuse is also a safety thing and is your choice as well. I would not include it myself on this small amp.

Mark

[martin manning]

But if there is no fuse at all, adding one to the power transformer primary circuit is a good idea. Without trying to put too fine a point on it, a 1A is probably ok. Changes over time might be to tweak the tone, but often they are aimed at reducing cost.

[brewdude]

Please forgive my ignorance, but what is the 12AX7 being used for in the above posted schematic? Is that a tremolo circuit?

[martin manning]

Yes, V2b is the phase-shift oscillator and V2a is a DC coupled cathode follower that is sourcing current through V1's cathode resistor (looks like there is a line missing from V2b's plate to V2a's grid). That takes V1 in and out of cut-off to create the tremolo effect.

[M Fowler]

I was thinking the OP said HT fuse not on AC primary side so that is why I made the statement I did. Fuse away no problem if you have room for HT fuses and AC mains fuse.

Mark

[martin manning]

True, Mark. I was just agreeing with him wrt adding a mains fuse.

[old tube]

Many thanks for the advice gentlemen, much appreciated.

I'll probably skip the HT fuse but definitely fuse the mains. If I put an on/off indicator bulb in the Heater circuit, what current do they usually draw? As the PT is an unknown, want to keep the heater circuit draw to a minimum.

Thanks Again.

[martin manning]

A #47 bulb will use 150mA from your heater winding, half as nuch as an ECC83 (12AX7) filament. If you use an LED you can get away with 20mA or so, or you can run it from the mains and not load the transformer at all.

[old tube]

Thanks again, 150mA will take the draw up to a tad over 2amps, which I'm sure will be fine. But I may just go with a 240v solution as that would leave me some headroom to change the rectifier tube to an EZ81 if I need / want a few more B+ volts.

I'll post up progress with the build and perhaps you can help me sort out my stupid mistakes as I go!

Oh, and I made a mistake in my original post, the output valve of the Tape deck was an ECL82 which is half 12AX7, half EL84 in one bottle.

[rp]

I checked the data sheets for both the EZ80 & 81 and, if i got it right, they can handle 50mf for the first cap. How come these effete euro tubes can handle 2.5 times more than a husky American 5Y3 and more than a corn fed 5U4? I might have to start using them more as an alternative to 5Y3s if I want a stiff tubed supply in a little amp.

To the OP, don't fear the lower capacitance. I built both a stock 5f1 and 5c1 (Champs) which would appear terribly under-filtered but sound just fine soft and loose, IMO the under filtering adds to the 3Dness, my 5F1 is real swirly. Mine are PTP and wired real tight and have no hum even into a 12".