hi-fi conversion ?

[patrick]

Hay Folks,
I'm looking for ideas on an old hi-fi amp that i'd like to make more guitar friendly.
The amp in question is a 1950's Bertram D. Hunt out of Toronto, the guy I got it from had gone over it and said it was good quality components but the company was probably only around for a couple of years.(common at the time I guess).
The amp works very well right now. It is a Williamson type push-pull with the following; a 12ax7 for the phono (I think) a 6an8, two 6v6G , a 5y3GT, two big alum caps and twoo big transformers.
When I put my guitar through the phono it gives junk junk and more junk(that's good) it's the perfect dirty sound for me. When you go through either the aux or radio inputs the sound is crystal clear and tubey, again perfect. So what's the Problem you ask?
I'll have to do some more description first.You turn it on with the treble pot which I'm good with, but I thought you were supposed to have a warming circuit to save your tubes?(as you can tell I know nothing about this)
The selector pot is the real mess, it has a position for aux, radio, and 7 phono positions (to accomodate the phonos of the time?). I would like to replace the pot with a two way or on/on switch but there is a circuit on the pot that I think is to deal with all the phono settings but ?
There is a volume pot and bass pot both of which seem intimately connected with the selector and treblepots( and not just the grounds)

As I said I'd like to end up with the same two sounds i currently have but on a two way switch instead of that crazy pot selector.
Can the aux and radio be put into one or one of them be removed ?

After reading this I guess what I,m really asking is is there a good book out there you could recommend? O yeah and any ideas would also be great.
Cheers Patrick

[Rob Mercure]

Hey Patrick,

First off "pot" - other than the Rasta's repast - refers to a "potentiometer." A three leg device that serves as a variable voltage divider. Your "selector pot" is a switch! (confused me for second). OK, lemme hit some of your issues as you presented them. First off while having a standby switch is handy for stage use it really isn't necessary for small amplifiers so don't worry if you only have the on/off switch on the treble pot - and why the switch was put on the treble pot I don't know but this was "fashionable" from the late 1940s-early 1960s and I've seen/have many like this.

As the needle in a phonograph moved faster in the groove to reproduce higher frequencies the output level of the phono cartridge got higher - which would have made the treble much too loud. This, and several other reasons, was why there was seven different options of phono "compensation" available as different manufacturers in different countries addressed this uniquely - and it had to be adjusted for different record speeds.

The Aux or Radio inputs aren't a problem but when you play through the phono is there a particular compensation that you like to use? If so you'll have to ensure that this particular compensation is available if you can simplify the input set up. The particular compensation should have some identifying name, perhaps a set of initials, perhap a company name and perhaps a description of the type of phono cartridge (crystal, moving magnet, etc.). If this compensation is a common one it may be easy to duplicate it's action by removing the old switch and creating a new circuit that duplicates the old one - if the compensation is a somewhat unique one you may have to draw a schematic diagram to determine what the component values are and drawing out a switch with 7 different compensations will be a PITA (the rest of the amp is probably fairly easy).

I hope this helps - you really didn't ask very specific questions but please fire back with these and we'll do our best for you.

Rob

[patrick]

conversion

Thanks Rob,
the pot in question is in fact a switch,(was something burning?)
As for the 7 different compensation settings they do not make any difference in sound for guitar so the little circuit is kinda in the way. Here's a bad description;
little circiut- the one end of all these attach to the shielded input leads, the other ends attach to separate lugs on the switch. we have a resistor(i Think)with gray, red, orange, white, with wt to the ground, next same but with a little advil looking thing with 80? over 10%, next the same resistor but reversed in direction and with an advil that says 1000 and 20%, next is a small white domino with 6 dots bk bn bk on the one column and bk bk bn on the other ( both on the same side), next same resistor reversed to the first two and attached to the domino, next same resistor (reversed) with a green capsual thing marked with bk bn grn violet and twisted in with the resistor and finally there is a brown horse pill going from the same lug as the domino to the body of the volume pot that says pinex .005mfd.
So this is why they come up with schematics!
I just thought this might shed some fog on said circuit and was wondering if I could just chop it out, so to speak? With any luck not everyone will be as confused as I after reading this.
Thanks for any thoughts.
Patrick

[Regis]

Seems to me you could bypass all that phono and aux switching shit by disconnecting it and attaching a Fender style 68k/100meg input setup directly to pin two or seven on the 12ax7. Like Rob said, you don't need any of that HiFi input stuff, you want it equitable for guitar.

You'll hafta install a 1/4 inch jack somewhere too.

[bluesbubba]

I have been working on a HiFi amp conversion with a Teeco HiFi Model 612.
I found it at the swapmeet for 5 bucks. It has 2 12AX7s, 2 EL84s and a 5Y3 rectifier tube.

My goal was to make it into guitar amp with the input jack in the front. I had to disconnect all the wires from the selector switch that read: Mike, Phono, Radio, Aux. I then rerouted the connections to the input jack, ground and the volume pot. I removed a lot wires including the pin 6 and capacitor that went along with it. It sounds great! I now need to take the Off/On Volume pot out and replace it with a standard audio pot and install a toggle switch for the off/on.

Can I add a pilot light from the hot lead of the toggle switch as well.

Next will be building a box for the amp and tolexing it.