UNKNOWN: Dutch EL84 Valve PA head

Unknown amp from the Netherlands….   I really have no idea about this one, and I’ve had no luck searching online.  I can only assume that it's from the Netherlands based on the control labels being Dutch.

I was drawn to it mostly for the look, and the fact that it had 2 x EL84s and an output transformer with regular speaker impedances on the secondaries (and the price was OK).  Apart from these factors, the tag strip running straight down the middle of the chassis was appealing - looked like it might be handy for mods or rebuilding.  Beyond that, the internals are… well, kind of a mess. It looks like someone has been in here before and done some questionable work.

Anyway - despite looking a mess with questionable work, it actually works really well.  I wasn't sure if it would even work, but it sounds great, and has zero hum.   Zero hum, and not a single heater wire is twisted, nor has any effort been made to keep it away from other components / wires. 

ARCHITECTURE

Input jack → EF86 preamp → Volume → 12AX7 gain stage → James‐style bass/treble tone stack → 12AX7 gain stage → Master Volume → 12AX7 floating paraphase inverter → EL84 push-pull output pair.   

The phono input basically skips the EF86 stage and the rest is the same as above - straight into a volume control and forward from there.

Negative feedback from the output transformer, back to the gain stage of the phase inverter.

INPUTS

• mic
• pick-up (phono)

CONTROL PANEL 

• Microfoon
• Pick-up
• Toon
• Toon
• Master volume / power on-off

VALVES

• Tesla EF86 preamp
• 2 x Mullard 12AX7 - phase inverter and gain stages for the tone controls
• 2 x EL84
• EZ80 rectifier, labelled Made in Holland    

OUTPUT TRANSFORMER

• H.STOET'S RADIO
• Primary  7-8k
• Secondaries:  4, 7 and 15 ohm
• 11 watts
• 30hz - 20khz

POWER TRANSFORMER

• Primaries:  120v & 220v
• Secondaries:  270, 0, 270 and 6.3 heaters and an unused pair for 5v   
• 100mA

CHOKE

• 6H
• 100mA
• 150ohms

B+ 

• ~300V @ 220V

 

SCHEMATICS

JAMES EQ RESPONSE (UNMODDED)

Response taken after the 10n cap, before the master volume.  I quite like the sound of it, but I did give it a slight tweak to shift the centre point lower, and reduce the amount of scoop.

MODS

I didn't really have to do anything major circuit wise - just a few tweaks here and there.

• removed the phono input

• moved the input jack from the back to the front

• speaker jack added on the rear

• It still needs a fuse on the AC input, but I'll get around to that shortly.  Probably a new power lead at the same time.

• Removed low value cathode bypass caps

• Adjusted the James EQ a little

• Removed negative feedback

REPAIRS

I did find a couple of things that needed sorting out, some of which changed the tone quite a bit.  

• Bad 100k resistor on the plate of the EF86 and a large 33k dropping resistor in the filter section for the same - I was only getting 35volts on the plate of the EF86.  Mind you, it did sound kind of cool and dirty.

• 2k2 on the cathode of the 12AX7 triode after the James EQ gain stage was creating more noise than I was happy with (of the rushing variety).

• There was a 100p cap from C5 to ground, which I'm assuming may have been put there to try and reduce noise (possibly from the dodgy cathode resistor)

• Random small value cathode bypass caps removed from the 12AX7s either side of the James EQ (6n6 & 4n7) - have no idea why they were even there.  Small values only boost highs.

HEATER VOLTAGE

This amp was originally made for 220V mains, but here in Australia we get ~240V (even though it's technically 230V).  While it's not a big deal for the B+ that small increase can push your heater voltage higher than it should be.  The transformer already ran a bit hot, putting out 6.45V on 220V, while well within the 6.3V +/- 10% specification, it didn't help my situation at all.  Running at 240v it was reading well over 7V and that’s not going to be good for valve life.

To fix this, I added two 1N5408 diodes in series with one side of the heater winding, right after the power transformer.  These drop about 0.7V each, bringing the heater voltage back into a happy place.  The 1N5408s are rated for 3A at 1000V, so they can easily handle the heater current.  Other diodes could work too, but these did the job perfectly in this case.