New Member Modding Hi-Fi Amp

[Gollum]

Hello everyone, as the title suggests I'm new, and boy this looks like a site that might suck my time away...

I've been playing guitar now for about 7 years now and I've been itching to get into some more hardcore electrical work for a couple of those years. I've always had a fascination with audio gear as my dad is a real budget audiophile. I've inherited his mind for music and gearhead'edness (if that's a word), and I've also gotten parts of my mom's father's engineering brain. I've been building things since I could crawl and my grandfather spent most of his life revolutionizing power plants, oil refineries, and early telecommunications. If only I had half of his brain...

Anyway, back on topic.

So I'm huge gear nut, who's also kinda on the cheap (half because it's more fun that way). I run a silvertone twin twelve as my main amp which I picked up for $200 in decent shape.

A few weeks ago I picked up a HH Scott stereomaster tube amp (222A model). It's a 4 x EL84 amp that should make around 13/14 watts RMS per channel. It has 4 12AX7's plus two 6GH8 tubes for PI duty. These amps had a strange PI setup that actually has POT for adjusting load between the tubes, so in theory you could match them better. Kinda wierd...

So I fire it up, no go. Upon inspection it had an obviously bad rectifier (cracked!!!). So I'd read on several sites that these amps do well with SS rectifiers, so I figured I'd build one. Wire'd up some diodes and off we went. Fired up that time and I got noise from the speakers, quite a bit of hum actually, with no response from it's input source... hmmm. So I left it until I could open it up.

Opened it up and couldn't find anything wrong. I LOOKED for a good hour or so. I'd been working on it in my bedroom, and as I'm sitting on this very computer now, while in bed I looked over at the bottom panel of the amp (which is off still) and noticed a big brown spot/stain... Cap juice.. ewww (it's even brown like REAL crap...)

Anyway. I've learned a lot even up to this point. I'm reading all that I can and I think I'm absorbing it relatively well. My hopes is to get this amp working and slowly convert it to guitar use. I'd like to keep as many of the original parts as I can, so basically remove all the Hi-Fi junk I don't need and replace cap and resistor values as needed. The plates are run extremely low on this amp (sub 150 on some and sub 200 on all preamp tubes) so I think much of my modding will be getting the gain turned up a bit without inducing any problems. I'm also huge on just using my guitar volume and touch to control an amp, and I've actually considered changing the volume to be a master volume only in the preamp and running the amp at a "fixed gain" and just use the guitar to control the input. Might be a bit crazy. We'll see how loud this thing is once I get it up and running.

What's really frustrating is knowing what caps to buy. Some people seem to be swearing by atoms, but I was really hoping to stay with a twist-lock can but I have no idea what brands sound like what. I'm hoping since it's a filter cap it won't matter a whole bunch.

I'm probably also going to switch back a stock GZ34 rectifier since I'm not 100% the cap spewed it's guts before the SS rectifier. This whole ordeal also makes me wonder if I should just drop $60 to recap the whole filter supply.


So yea, that's basically it. I plan on using this amp in stereo, and it'll be PLENTY loud enough for me, if not TOO loud actually. I'd like to get a decent amount of gain out of it, but the character of the gain is much more important to me. I also plan to build a semi-open back 2 x 10 to go with it. I'm hoping to use a flanger into it with mild settings to get an ADT (automatic double tracking) effect.

I'm not exactly sure what I'm expecting people to respond to this with. But any advice or suggestions would be wonderful. Books to recommend that I can get from the library or used on amazon or something would be great. I've just finished Dave Hunter's book "The Guitar Amp Handbook". It was a good read and I picked up quite a bit of information thanks to his easy to understand style of going through the workings of an amp. Now I need to go back and read some of the tech info pages I never understood...

[Gollum]

I just realized it's kinda wrong of me not to share pictures, that I already have uploaded even.

[Glasgow]

Gollum,

Welcome to the site and the obsession!

Just a few comments:

First of all, the story is great and the pictures are nice, but sit back and think about some focused questions you have about your project. A big wall of text makes my eyes glaze over sometimes.

As far as solid-state rectifiers go, they generally yield a higher B+ than a tube rectifier would, so you should check this and make sure the result is something tolerable.

As far as capacitors go, the most critical aspect is the voltage handling capability. The rated voltage should be higher than any voltages the cap will see. You will discover that for the different types of caps, they each have a range of (capacitance X voltage handling) and your choices for a particular application will be more restricted. For a coupling cap (which is in the signal path) you'll want very high voltage handling, but relatively small capacitance. You could use ceramics or polys here. People could debate the tonal majesty of one of these over the other for the rest of days, but either type would be a safe choice for this application.

Good luck!
Brian

[olddawg]

First of all, before you start tearing up this nice, clean, old Scott, I would recommend that you check on eBay and other sources to see what it is worth stock. Scott amps can be quite desireable and valuable to tube audiophiles. I'm not an expert, but many old Scott amps are worth a thousand dollars or more. You also might consider that a stereo amp is more problematic for conversion. You might be better off to sell this amp and use the proceeds to buy an easier conversion candidate or, even better, buy a good quality amp kit of your choice. If you like EL84s I would suggest an 18 watt Marshall/Vox clone. If you want more power an AC30 clone. You would still have the pleasure of building, learning, understanding, and modding (if you like) a guitar amp with out the headache and appearance of a conversion and the reliability of new parts. BTW, the Scott may have some valuable and desireable old tubes in it. It also may be quite saleable as is as a doner amp as is if you get tired of messing with it. Just search eBay for what Scott transformers, chassis, etc are going for. Seriously, check it out. Selling the amp and getting a kit might be a better path to take. My 2 cents anyway. BTW, I'm restoring an 1484 Silvertone as we speak and use an 18 Watt Marshall clone.

[GibsonLover]

Welcome, and good luck getting that beast working.
If you're going to make a go of this hobby, and it sounds like the hook is set, you'll eventually buy some test gear. I'd get a signal generator (or build one) and get yourself a cheap scope. Those two, aside from a volt meter will go a long way in helping to understand your circuits and look for problems. Swapping parts really comes after the thing is working and you want to trust your ear for fine tuning.
I'd suggest breaking up the circuit to troubleshoot it. Separate the the peamp from the PI and see if the pre is good. Put a generator on the PI and see how the output works. Pull all of the tubes (except one preamp tube - the P/S needs a little load) and see if voltages are reasonable. Look at everything in pieces and see if they make sense.
Don't obsess over capacitor brands. If you ask 10 people, you get 10 answers. Buy what you can afford, for your own reasons, and make sure the voltages are sufficient. For Hi-Fi, you want tight bass response, so going a little larger in the P/S filter is probably a good thing. (Not so much for guitar...)
Good Luck!

[Gollum]

Thanks for the response thus far guys.

When I checked ebay it looked like a PRESTINE 222A like mine would only go for around $300-400. This one might be able to go for around $250-300 once I get it fixed. I hadn't really thought about parting it out, that's something to look into.

I kinda like the idea of modifying a Hi-Fi amp for guitar use because many old Hi-Fi amps are still a dime a dozen and are plentiful, while vintage guitar gear is still going through the roof in many regards.

The fact it's stereo doesn't bother me, and in fact I like it. We'll see how it works out. Obviously I need to get it working first, and THEN mod. No sense in modifying it and then wondering why it's not working.

[tedmich]

Obviously I need to get it working first, and THEN mod. No sense in modifying it and then wondering why it's not working.

that's a very nice clean old amp: I would cut out the iron and build a stereo Marshall 18 watt; one voiced high and one low. Just modding it to be a guitar amp will sound terrible unless you like having to run two TS-9's up front to get a decent sound, IMHO.

[Gollum]

Well that's why when I say "modding to be a guitar amp" I'm not just talking about hooking up jacks and so forth. I'm talking about removing 50% of the signal chain that I don't need, increasing preamp gain, restructuring tone stack etc.

From what I've learned thus far (someone please correct me if I'm wrong, I want to LEARN), is that the main difference that's kinda "fixable" between hi-fi amps and guitar amps, especially from this era, is that the transformer is most likely linear while guitar amps transformers generally aren't. Other than that it's just a collection of pots, caps, and resistors like any other amp.

[olddawg]

Well that's why when I say "modding to be a guitar amp" I'm not just talking about hooking up jacks and so forth. I'm talking about removing 50% of the signal chain that I don't need, increasing preamp gain, restructuring tone stack etc.

From what I've learned thus far (someone please correct me if I'm wrong, I want to LEARN), is that the main difference that's kinda "fixable" between hi-fi amps and guitar amps, especially from this era, is that the transformer is most likely linear while guitar amps transformers generally aren't. Other than that it's just a collection of pots, caps, and resistors like any other amp.

Well, not necessarily so. A lot of the old amplifers used "off the shelf" hifi transformers, usually what was cheapest to source and do the job. I have a Marshall/Vox 18 watt 2 X EL84 clone amp that I made with recycled transformers. It has an old Zenith OT. I blew away a bunch of botique amps with it at a jam session last night. For a PT it doesn't really matter as long as it has the taps and current that you want. OTs are hit an miss. Some sound better than others and it is, of course, very subjective. I would recommend using new cramic tube sockets, shielded sockets for the preamp tubes, new jacks, switches, and all new components. It isn't that expensive and you won't have a bunch of wierd problems stemming from old failing parts. That's why I suggested selling the Scott and use something else. Really, the best way to do it is to buy a clean, new chassis, use only the transformers and hopefully tubes from the the donner amp, make a new turrent board from new parts, and wire it up with new tube sockets, jacks, switches, etc after they have been mounted on the new chassis. It isn't very expensive, and if done right you will have a real guitar amp on a standard chassis which will fit a standard cab and not some cobble POS with never ending issues. I should also add that if you are not familiar with tube electronics there are potentially lealthal voltages present even if the amp has been unplugged. I would suggest that you do some serious reading on safety proceedures and amp servicing methods as well as basic electronics theory.