36W Hi-Fi Conversion

[PorkChopsMurphy]

Yet another new guy. Been lurking this forum for a while and I'm amazed at the knowledge that's out there. I'm learning alot.
Anyway, I have an old Sherwood S-1000 mono hi-fi amp that I got out of the garbage about 30 years ago. Everything still works. It runs a quad of 7189's and is rated at 36 watts. I see a guitar amp.
Has anyone here ever converted one of these?
The problem I'm having is that all of the schematics I see use cathode bias whereas this one uses a hum balance control. Anyone ever build a Lite with a hum balance? What are my options here as far as the PT goes? Everything else is very straightforward.
Thanks

[Shrapnel]

I'm not sure about layout, but the 7189s are related to the 6BQ5 or EL84. You have much potential for a 36watt guitar amp... especially if there were at least two more tubes (excluding the rectifier)... and I think that isn't a concern on the tube count.

You can always rip out the original wiring of the output section and rewire it to a more "proper" 18/36watt power section when you gut the rest of it... then again.... There's some money potentials on fleabay for a working monobloc (and use the funds for a regular kit.) Options are here, and open if you haven't already gutted it. Either way, your call.

[bentaro_rifferashi]

If i were you i would gut it. It would seem to have everything you need to make a totally brown guitar amp And I think everyone here can see that you're just itching to get started. Its a problem we all seem to share.
I found a Pye p76 which some people will tell you is a fine piece of radio history. We both know that what it really is, is an amp conversion waiting to happen. You are going to end up with a totally unique sounding and looking amp that will melt your face off. now get on with it!

[zaphod_phil]

Fire up the old amp, and measure the voltages on all the EL84 pins (keeping one hand in your pocket). Also measure the AC voltages coming from the PT, and make a note of what all the different connections are, for future reference. Also likewise note the connections on the OT. Like Shrapnel said, 7189s are extra tough EL84s. So it's quite likely the amp is running at higher voltages than a regular 18W or 36W, and you may need to take steps to reduce its B+ voltage.

Then gut that amp and put in a 36W board.

[Alexo]

I would check out the voltages and transformer spec's before making a final decision. I've scored some old mono hifi amps that, on the surface, look to be only a few tweaks away from a guitar amp, but under the hood, the voltages have wound up way too high or too low, and the same often holds true with output transformer impedances. Don't assume that your OT has a 4K primary Z just because it's running a quad of EL84's - I've found that more often than not, old hi-fi amps run a higher primary Z than we want in a guitar amp, sometimes double!

It's can be no big deal to correct for this - just cut the load in half, i.e. connect an 8 ohm speaker to the 16 ohm tap.

But first, make sure the PT voltages are close to what you want, and then measure the OT impedance by hooking a wall wart AC adaptor across the primary, put a load on the secondary, measure the AC that the transformer induces into the secondary, divide the primary AC by the secondary AC to get the turns ratio, square that number, multiply it by the impedance of your speaker and you're good to go!

For instance:

You connect a 24 volt ac adaptor across the primary, ignoring the center-tap.

You measure 25.9 VAC on the primary (it'll never be exactly 24) and .75VAC on the secondary.

25.9/.75= a turns ratio of 34.533

34.533 x 34.533 = an impedance ratio of 1,192 : 1

And if you have an 8 ohm speaker, you can multiply either side by 8, and you'll find that you have a 9,500 to 8 ohm OT.

Having said all that, I've also found old amps that have exactly the right voltages and impedance for an 18 watt - they can make for a pretty good sounding stereo too!