Turning a reel to reel tape deck into an AC4 (First amp build)

ESchap

I like the idea of "recycling" old amplification equipment. I was given a non-working 1964 Philips reel to reel tape recorder by a friend who thought I might be able to salvage some of the valves.

Long story short, inside it had a single ended amplifier which was pretty much an AC4 spec with an EZ80 rectifier, EF86 but using an ECL82 for output. Transformers, mustard caps etc. were all there and of the right spec. There was miles of very nice solid core wire in it too. There was no chassis to speak of, it being built "around" the internal machinery of the tape mechanisms. 

I'd seen posted elsewhere that the current Vox Pathfinder solid state amp can be converted and it does look like an AC4 and very close in dimensions. Picked up one on eBay for £35. Stripped it out and was ready to go.

First job was to "design" the layout, not too difficult in such a simple amp and the chassis is quite roomy, so I chose to follow the Vox placing of the valves with the earth bar in the "middle" to hang everything off. In practice I needed to use 3 double tag strips as well. Next was to cut, drill etc. the holes required. To fit the transformer I had to make it lay down, only having a drill and tin snips that was a complete pain of a job! The chassis doesn't appear very thick but I reckon you could build nuclear submarines with it.  Working in the "panel" controls was also a challenge as the Pathfinder panel has a lot of holes ... mostly the wrong sizes, wrong shape and in the wrong place! 

 Once I'd bodged my way through the drilling, and the chassis was ready, it only took a couple of nights to assemble and solder. I comfort myself with the fact that point to point always looks untidy, but I'm about 80% happy with what I did. Only Issue I had on first start was low output which was quickly rectified when I spotted I'd used a 220k instead of a 22k at the EL84. The amp is very quiet electrically, no hiss, even though I used some old CC resistors.   
 I did make some mods to the original AC4 circuit. First was the addition of a depth control for the Tremolo. Simply put a 250k pot across the 100k resistor. It works pretty well though is a bit over enthusiastic above half and I'll need to modify a bit to get it to where I want. It's also affected strongly by the overall volume of the amp, which I don't really understand.

ICBM advised to move the B+ feed to the output transformer from the second can to the first to allow the EL84 to operate a bit more effectively, introduced a little hum but sounded more lively as a result. Like many others before me I dropped the EF86 screen resistor from 5M6 down to 2M to add a bit more gain and grit. I also added a 100pF "bright" cap, and fixed the input resistor wiring which I’d cocked up … again under ICBM’s tutelage (Cheers!).

The Mains transformer allows me to use an EZ80 rectifier and get perfect "spec" voltages, but I chose to go with an EZ81 as I think the amp and particularly the tremolo sounds better with the slightly increased voltages.

Finished the whole thing off with a control plate made by a local engraver. Couldn't get copper so went for red.   The stock speaker in the pathfinder was not good and 8 ohm and I really needed a 4 ohm.  So went for a Weber "Blue Pup" Alnico which really did open the sound up.  All in, it's a great living room practice amp. It’s hard to define in words, but overall, it does sound “vintage”.  Stays clean most of the way and with the Weber Alnico it is rich and chimy. Driving it to max does darken it to the point that neck pickups sound muddy .. need to find a solution for that, though I believe that the originals were quite dark in tone.  However, bridge pickups sound very good and the drive is smooth and harmonically rich. It takes a Boss Blues driver really well.  It does suffer a little from the inevitable "boxiness" of a small cab and an 8 " speaker.  However, a little applying a little reverb really opens it up. 

stickyfiddle

That really looks fantastic! Great job

Misterg

Saw it on TAG - neat!

ddlooping

Nicely done, ESchap.

ICBM

Having seen this at first hand, I think ESchap is being too modest. Especially as he's not an experienced electronics geek like me . The only tuition I had to provide was for a couple of minor tweaks, he got it basically working himself just from the original schematic.

It's a really nice job and looks very professional from outside - and if you think it's untidy inside, try some original 1950s/60s true point-to-point amps! Having owned a couple of original AC4s, I think this one sounds better too.

ESchap

ICBM said:

Having seen this at first hand, I think ESchap is being too modest. Especially as he's not an experienced electronics geek like me . The only tuition I had to provide was for a couple of minor tweaks, he got it basically working himself just from the original schematic.

It's a really nice job and looks very professional from outside - and if you think it's untidy inside, try some original 1950s/60s true point-to-point amps! Having owned a couple of original AC4s, I think this one sounds better too.

">   you're too kind ICBM!

 

Of course I didn't mention that I fused the house trying to work out the mains transformer wiring and you sorted that for me ... or that I managed to get a full mains blast while turning the chassis over with it plugged in and touching the mains live!  at least I know the RCD in the house works well!

ICBM

It's probably not good for my professional reputation to admit this, but I gave myself a nasty belt the other day - Selmer T'n'B 50, forgot that the standby switch doesn't turn off the HT. I really should have been more careful since fixing that is a standard safety modification I do to them! Familiarity, contempt and all that... now I have a little blood blister on my thumb where the 470V went in :-O.

Cirrus

That looks awesome! Very cool indeed. As for the muddy neck pickup at high volume thing, I suspect you're not the first vox circuit user to have that "problem". My -30 does it as well with humbucker guitars, I guess it's just part of the character. Am curious about the output transformer B+ feed move. Is that just an ac4/ single ended amp thing? What does it do?

ICBM

Cirrus said:

Am curious about the output transformer B+ feed move. Is that just an ac4/ single ended amp thing? What does it do?

The original AC4 circuit has the OT feed connected to the second node of the power supply chain, downstream of the 'choke' resistor. This gives less hum (which is more of an issue with single-endeds than push-pulls), since effectively the supply has been filtered twice. The downside is that it reduces the voltage available to the output valve plate, which reduces the output power of the amp and also means that unless you move the screen grid feed even further down the chain or fit a large screen resistor (neither of which Vox did), the screen will operate at a higher voltage than the plate, which results in it drawing a lot of current when the valve is overdriven and isn't good for it as well as further reducing the power by diverting some of it to the grid where it's useless. Really, it's a hi-fi type design where hum is a problem and maximum power (and certainly what happens when the valve is overdriven) isn't.

If you move the OT feed to the first filter cap, you fix all these problems at once at the expense of a bit more hum, and the higher voltage seems to give a slightly brighter and more dynamic tone and response as well. It makes the amp noticeably more lively, basically.

There are some bigger amps - eg the Selmer T'n'Bs - where it's like this too, with a real choke instead of a resistor but still before the OT feed. These also gain a bit more power by moving the OT feed to before the choke, but it's much less useful or necessary unless you're trying to use it for bass where you need the full power and dynamics.

Cirrus

Cool thanks for that! This thread has got me wanting to go and look for some old gear....

ecc83

ICBM said:

It's probably not good for my professional reputation to admit this, but I gave myself a nasty belt the other day - Selmer T'n'B 50, forgot that the standby switch doesn't turn off the HT. I really should have been more careful since fixing that is a standard safety modification I do to them! Familiarity, contempt and all that... now I have a little blood blister on my thumb where the 470V went in :-O.

Never hurts to remind everyone that the Series Ones and most of the HT range also leave HT present in standby mode. 

It is not easy to get at the HT but do please all be aware!

There SHOULD be a large warning sticker in the amps.

Dave.

ICBM

Never hurts to remind everyone (including professionals who should know better ) that you should never assume what or what not the standby switch turns off without checking!

Quite a few amps turn off the second node of the B+ chain (screen grids and below) leaving the OT and power valve plates live, too. There is a good reason for this since it allows a small trickle current through the valves which prevents cathode poisoning, and it also means less chance of the switch arcing since it isn't switching such a big current... but it does mean you can get a shock from a power valve if you change them 'hot' with the amp on standby, if you're careless enough to wrap your finger round the pins as it comes out.

I know, you're not supposed to change valves with the amp powered up .

Those old Selmers are even worse, all the 'standby' switch does is turn off the HT to the phase inverter - it's purely a mute switch and doesn't even help extend power valve life. It leaves all the rest live, including the lower stages of the preamp.

welshboyo

I'd love to have the knowledge/balls to pull off something like this, truly envious

Flanging_Fred

Looks great. Having a convertered reel-to-reel I know how good it can sound. My Ferrograph must be pretty similar with the EF86 & EL84.