Webcor "Music Man" AMP project (for DIY heads)

[Dr ZEE]

This was built based on electronics and chassis of Webcor Music Man reel-to-reel machine.
The amp is 5watt X 2, SE type with preamp stage outputs. Kind of interesting amp, but I'd say mostly cool for guitar and rather questionable for Hi-Fi listening (also depends on who you ask, as usually). I'm looking forward to getting some vintage speakers and then maybe building some sort of experimental speacial speaker/cabs with speaker-combination selector/switch(s)... well, that' just an idea...

Here's the page for those who may be interested: WEBCOR Music Man Two Channel AMP project.
I wish I've made some more photos at the earlier stages of the project... but I really was not sure about whether or not it 'll work out at all, so not much of details, but the schematics and general idea about how it can be done are there.

If any specific questions or what ever, pls, drop me a note.

/respects

 

[arjoll]

As always, Dr Zee, a nicely executed unit! Sweet

Just noticing you've got bass and treble both at 12 o'clock - you do realise that this circuit is quite a traditional one, so is all cut an no boost? At 12 o'clock you've got both a bass and treble cut happening, not sure if this is what you're intending or not. Could help explain some of the lo-fi sound you're getting.

 

[krhall]

Hmm...

Nice amp

Would it be a good idea to install a cooling fan in the top section to prevent the tubes from overheating ????

Just a suggestion

I got an old Grundig TK-5 valve tape recorder here and i had to change 2 valves as they sustained damage from overheating.

Machine works great now but recording is not so good.

Keith

 

[Dr ZEE]

HI, guys, thanks for comments... buncho'appreciated
Cooling fan? Maybe a good idea. Can't be sure if it needed. I would hate to actually "install" a such thing to the amp like this... The fan could be placed behind or on the side of the amp to blow over the cage ... if it gets really hot. As is, the top of the cage gets pretty warm, but you can place and keep your hand on it comfortably. The power transformer gets pretty hot, also you can touch it and hold the finger on it. I've read that some transformers do get hot as 'normal operation'... but I really don't know much about details. Hot transformer is worrysome in my mind.

Bass and treable pots position. heh heh - sharp eye!
Now, that's a part of a puzzle in my case. I am not really in the position to make any definitive comments about performance of a such amp for a few reasons: I'm not sure if the perfomance is determined by the design, or by components condition (they are all original, including all original Webcor red-labled tubes... so are maybe about 40 year old), or by both; I also do not have knowledge nor equipment to do any sort of serious lab test, and even if I could do so, I don't think I could be able to maintain any level of objectivity.

I can tell this about the amp I have here as it is now.
It perfomes very nicely at low-to-moderate 'volume level'. At about between 12 and 2 o'clock of the volume pot position it starts to audibly distort, and the first it distorts high end (seems like).
It seems that the performance also somewhat depends actually on the speakers. Well that's the whole separate issue, I guess...
The strange thing is (or maybe it is "normal" for this design ??? - have no idea) that the bass and trable adjustment actually slightly but still audibly affects the preamp output. And I have found that when using preamp outs with other external power amp/system it's kind of nicer to keep bass and trable pots half way or so 'down'. this is also questionable. It sounded better this way when I was playing CDs, but when I was playing reel-to-reel tape (on ampex machine - outputs to webcord as preamp out - to JVC mini-system as amp/speakers), - It was better to turn bass and trable all the way up.
Now, why do bass and trable pots affect the preamp out? - I have no clue. Any ideas? It must be some sort of "Loop theory" or something.
Also, I can't tell for sure why the pots are in specific positions on the photos, as I don't remeber what exactly I was doing. I was making the photos while not doing anything really, just plug it in for the photo , but the position of the tone pots could be left from what ever I was doing before... could be messing with guitar.
I guess I have to add here also, that the amp is pretty damn loud at about 12 o'clock volume pots position, turning them up over 12 is not really practical for the normal room listening. That's a different story when using it for guitar playing/recording. Over 12 o'clock all the 'fun' for guitar starts and it gets really nasty and dirty at the 3 o'clock to the max area, and there there's no really good or bad bass/trable settings - it all depends on what is desired. Also at about 2-3 o'clock (and "later") the noise and hum starts kicking in ... who said it's a bad thing? ...heh heh
As, I've said, this amp imo is great for guitar... as low power practice amp or for studio recording. I actually was amazed by the sounds I could get out of those 5" small original speakers. This is why I want to try experimenting with different speakers and maybe building some speacial cab/enclosures for them.
Well, overall, I can tell for sure only one thing - it's a nice toy.

thanks again, guys for checking it out and your notes...

/respects

 

[arjoll]

The power transformer gets pretty hot, also you can touch it and hold the finger on it. I've read that some transformers do get hot as 'normal operation'... but I really don't know much about details. Hot transformer is worrysome in my mind.

Not really an ideal situation, the mains transformer shouldn't really be doing that much work. I'd check the current draw. Also what are those rectifiers? Doesn't seem right.

I am not really in the position to make any definitive comments about performance of a such amp for a few reasons: I'm not sure if the perfomance is determined by the design, or by components condition (they are all original, including all original Webcor red-labled tubes... so are maybe about 40 year old), or by both; I also do not have knowledge nor equipment to do any sort of serious lab test, and even if I could do so, I don't think I could be able to maintain any level of objectivity.

What kind of caps etc are in there? Some caps from that era and earlier can be really, really nasty - the smoothing electrolytics, also any hunts moulded caps etc may need replacing. Its not a matter of keeping it 'authentic' - that's in the design and in how you carry things out. With a classic car you'd still change the spark plugs and brake pads. Same thing here - some of the caps will be shot. Far more likely to have probelms than the valves/tubes.

It perfomes very nicely at low-to-moderate 'volume level'. At about between 12 and 2 o'clock of the volume pot position it starts to audibly distort, and the first it distorts high end (seems like). It seems that the performance also somewhat depends actually on the speakers.

What kind of speakers? If the point of distortion is moving then it could be an impedence mismatch - can be more critical with output transformers than with a transistor output stage. Also if you're pushing the signal through passive crossovers etc then it may just be all the amp can deliver - older speaker designs were often more efficient than modern ones; they had to be because you couldn't just bridge some Labs and throw 6000 watts through 1 ohm like you can now!

The strange thing is (or maybe it is "normal" for this design ??? - have no idea) that the bass and trable adjustment actually slightly but still audibly affects the preamp output.

Not unusual, when you look at the circuit you'll see that these controls are very passive - they just bypass or bring into circuit a couple of caps (one in series with the signal for bass cut, one shunting highs to earth for treble cut). This will affect the sound at both 'ends' although I would have thought the 220k and 68k resistors should minimise the effect. This is quite different to modern designs where your tone control will be in the feedback loop of an op-amp isolated from both input and output.

The other thing looking at the ECL86 is that its basically two devices in one - a triode preamp and pentode power amp in one package. You may be getting some coupling between the two stages, although I really am no expert on valves (tubes). There are some experts over at http://www.vintage-radio.net who may know more.

Over 12 o'clock all the 'fun' for guitar starts and it gets really nasty and dirty at the 3 o'clock to the max area, and there there's no really good or bad bass/trable settings - it all depends on what is desired. Also at about 2-3 o'clock (and "later") the noise and hum starts kicking in ... who said it's a bad thing?

No at all for guitar! For hi-fi listening it kinda sucks but use it for what it does well. Change those caps though!

I actually was amazed by the sounds I could get out of those 5" small original speakers. This is why I want to try experimenting with different speakers and maybe building some speacial cab/enclosures for them.

Worth experimenting. What brand of speakers are they? I know the 5" elliptical in the Ferrograph has an amazingly nice, smooth sound for a small speaker.

Have fun. Check those caps though (am I nagging enough?).

Oh - almost forgot - spot check some resistors, especially any that look 'stressed' - they can go high resistance, and also can get noisy.

 

[Dr ZEE]

whew!
Thanks a ton for in depth follow up 'n through
I already have some rather long wish list for parts (I guess I'll have to turn to something like AES ... . Actually it goes a bit against my "rules" ... making things almost exclusively from dead-bodies parts , that is
*********
a note about the speakers I've mentioned. those 5" I was talking about are the original internal speakers from the recorder. They are 8 ohmers.
Just this morning I've received the 'from-e-bay_ box'... got two pairs of older speakers which are maybe from some old organs. a pair of blue 8" Utah(s) from 60s and pair of CTS 10" square magnet ... have no idea from what year nor what unit. The speakers all are 8ohm. Have not tried them yet.
My idea was to bild a four speaker cab, and so I could configure it as: stereo pair speakers A / stereo pair speakers B / or all four speakers mono A1+B2(series) in parallel with A2+B2.... with speacial three position switch.

/respects

 

[Dr ZEE]

ahh... sorry I've missed this first time...

... what are those rectifiers? Doesn't seem right..

the PS rectifier is full wave c-tap kind ...( this type )...
what doesn't seem right there? what am I missing?.... sometimes when I look too much at something I stop seeing it as it is... sort of thing
the diodes there are some older kinds ... silver top-hat kind
look like this:

 

[arjoll]

the PS rectifier is full wave c-tap kind ...( this type )...
what doesn't seem right there? what am I missing?.... sometimes when I look too much at something I stop seeing it as it is... sort of thing
the diodes there are some older kinds ... silver top-hat kind
look like this:

I was meaning the rectifiers themselves - I'm guessing they're probably selenium rectifiers not silion diodes. I think the selenium ones look like 'top hats' but am not sure - I'll look into it. They may be failing, although they normally smell weird (!) when they're dying.

They can be replaced with silicon diodes (e.g. 1N4007) but the voltage will be too high, so usually you'd put a resistor in series to drop it down a bit.

What wasn't right was the heat in the transformer. It shouldn't be any more than just slightly warm - any more and its doing too much work - too much load, wrong voltage/frequency etc. Thats why I thought checking the AC current draw might be a good idea, see if it looks reasonable.

 

[Dr ZEE]

I think the selenium ones look like 'top hats' but am not sure - .

Selenium(s) look rather like concertina
Here: I've found a good photo of selenium and two type of diodes (as a one shot )... on THIS PAGE
*******
now here's a question : how do I decide what current draw is reasonable ?

 

[arjoll]

Selenium(s) look rather like concertina

'doh! Not sure what I was thinking - of course you're right there!

now here's a question : how do I decide what current draw is reasonable ?

What is it with no volume? I'd have thought a few tens of mA, but I'm not an expert on such things. You definitely shouldn't be seeing a few hundred mA.

 

[Dr ZEE]

here's a small 2x8" speaker cab I've built for this amp.

 

[Beck]

here's a small 2x8" speaker cab I've built for this amp.

Nice looking setup, ZEE.

How a studio looks is a big part of the equation. The warmth of real wood is inspirational to me.

~Tim

 

[Dr ZEE]

wood is cool... well, it's warm too , I guess
I get no inspiration lately at all... simply because I'm kind of realizing (and thanks a ton to "damn" analog recording for that ), that I can't really play s*t ... bad attitude and dirty hands that is., ...well, sort of.

Thanks, Tim

/respects