http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

I finally got the chance to snap some pictures of the amp. To show maximum detail I kept the images fairly large (~350k each)

rather than list out everything, here's a directory containing the pictures. The file names should describe the contents fairly well. I was unable to show the power transistors directly as they're held down to the chassis with a little clip (you can see it in the pictures) The decades have nearly welded that screw in so I didn't want to strip it trying to remove the clip.

http://www.mr2-power.com/photos/hudson/

Now back to diagnosing the annoying hum I've discovered.

At first I was thinking the humming was just AC ripple making its way into the DC rails (bad power supply cap maybe?). But after attaching a mic. to the amp I noticed some odd behavior. When I hum into the mic I notice that in addition to the background hum (Which is quite loud) the amplified signal seems to "pulse". Even more odd is that the higher the signal level (humming louder into the mic) the more rapid the pulsing became. I had the reverb level turned down completely at the time.

I'm not really sure where to begin. I could trace out the circuit schematic to get a better understanding of what's going on.. but I'm not sure it's worth it. I'm doing this as a favor and I don't even know the last time this amp worked (might have been 20 years, who knows).

Any thoughts or suggestions?
-Matt

well, I've gotten the amp to power on (some broken wires in the mains circuit and a blown fuse) but now the amp has a bad hum. Uggh...

I'll try to post some pictures tonight.

I've been asked to see if I can repair an amp for someone. It's a Hudson Electronics "All Solid State Amplifier Model 592". I can't seem to find any information online about this brand of amplifier.

I'm hoping the problem is just in the power supply wiring so a schematic is probably not necessary... But I am curious if anyone knows anything about these amps. Here's what I have been able to observe so far:
-Has three numbered inputs (presumably different gain levels?)
-Controls: Tone, Volume, Reverb, Intensity, Speed
-Reverb tank is mounted to the chassis in the top of the cabinet
-Has an ~8" 10W 8Ohm speaker
-Has two un-labeled 1/4" jacks on the front panel, presumably some sort of preamp output/input like an effects loop(?)

If anyone is curious I can post some pictures.

If anyone knows anything about this amp please share
-Matt

Thanks for the reply!

Is there an easy way to protect the outputs with external circuitry?

I don't have a transformer yet so I will probably do as you suggest and just get a single secondary transformer.

What do you think of these more powerful designs from the same company?
"50W" http://www.futurekit.com/basic_elec/FK660E-2.pdf
"100W" http://www.futurekit.com/basic_elec/FK666E-1.pdf
-Matt

To make a long story short, I picked up this power amp kit for $10 to meet a minimum order requirement .  I wanted to see if I could get any opinions on the quality of the design. It's sold as a 30W amp and takes a single +50V power supply. The kit included a couple of heat sinks for the output transistors but I may upgrade to something larger. Here's a link to the instructions with a schematic. Note: The component values are not shown in the schematic (I guess they want you to buy the kit, since they are listed in the printed version of the document included with the kit).

Here's the document:
http://www.mr2-power.com/FK656E-1.pdf

I can supply specific component values if it would be helpful.

I have a couple of specific questions:
1) Am I giving up anything by using a circuit with a single positive power supply rather than the typical bipolar supplies you see on amps like this?

2) Can someone explain how the power supply design they show in the drawing can result in 50V DC? They say to use a 31-0-31 center tapped transformer... Wouldn't that result in closer to 80V DC after it's rectified?
Edit: Nevermind... I think I understand.. Since they used the center tap as the ground reference the two ends of the secondary are tied together in parallel resulting in a +31V AC voltage potential before being rectified... that would result in somewhere around 45V DC after rectification... In theory I could probably get away with a 35-0-35 transformer.


I plan to build the amp regardless (it was only $10 after all) but I would like to know what I'm in for as far as sound quality and robustness of the design.


Thanks in advance,
-Matt

if the transformer is center tapped then you could use the 15V secondary (don't use the center tap) to generate a 12V DC signal. You would need to rectify the AC voltage to DC and then run it through a 12V regulator to get it down to the proper voltage. What ever you do don't just hook up the 7.5V AC to your fan

It's also likely that the CPU heat sink isn't large enough. Maybe you should post some pictures.
-Matt

This may seem like a dumb question, but here goes... Is there any obvious difference (with regard to the quality/tone of the audio) between an inverted and non-inverted opamp gain stage?

I understand the technical differences such as the fact that non-inverted stages can't have a gain of less than unity and the fact that the output of an inverted opamp stage is out of phase with the input. But other than these technical differences, why would you choose one configuration over another? To me, it seems that setting a high input impedance on a non-inverted stage is easier since the impedance is set by the pull-down resistor rather than the series resistor before the inverted input.

Any thoughts?
-Matt

Take a look at the preamp section of the "Practiceman" design. Here's a link to the schematic:
http://www.generalguitargadgets.com/richardo/practiceamp/practiceman.pdf

If all you want is a clean preamp stage simply look at the top-most gain stage (clean channel, 10x gain). That should give you some ideas.
-Matt

after playing around with the LM386 I decied to search for some of the discontinued chips in the same series. At my local electronics surplus store I picked up two each of : LM380 (2.5W), LM383(7W) and LM388 (1.5W).
Does anyone have any experience with these chips for use in guitar amps?  Any sample schematics?

-Matt

Great, thanks for the information. I like to understand how a circuit works before I build it. So far all of the circuits I've worked with (in distortion pedals) only make use of a single fet for each gain stage in a common source configuration.
-Matt

I have a question for anyone who has built this circuit... Is the schematic 100% correct? There are two crossing nets that do not show a connection on the schematic but I want to make sure they're really not connected...
I'm talking about the crossed nets right above R5 and R8 in the schematic.

So, if the schematic is correct and there is no connection at this point, how does the input signal reach the gates of Q2 and Q4? Or is this an AB style amplification stage where Q1/Q3 amplify the positive half of the input signal and Q2/Q4 amplify the negative half?

Also, if I don't care about having the low-gain input I can just ditch J1 and R2, correct? Will R3 still be required?
-Matt

looks great.  So do you normally house the amp electronics in a metal chassis and then somehow bolt that into whatever cabinet you're going to use (combination or just a head cabinet)? I suppose I need to do some searching for a metal chassis before I plan my cabinet I see a lot of people simply sticking boards down to the bottom of an old amp cab and I'm not sure thats the way to go for me. I could see maybe bolting the transformer down there since it's large and doesn't really need any RF shielding (at least compared to the inputs to the preamp that can tune radio stations if not properly shielded!).

Did you attach your heat sink to the metal chassis directly or is it attached to the wood of the cabinet? I was considering screwing the heat sink down to the wood and just cutting an aperture to allow the IC to bolt to the heat sink.

-Matt

Quote from: joecool85 on June 04, 2008, 07:29:30 AM
Definetely go bigger rather than "might be big enough."  If you read through some of the threads here you will see that a few people have gone with heatsinks that "should be ok" and then fried their chip(s).  Plus big heat sinks look cool.

I will be trying to use the largest sink I can but I'm concerned about how I will fit everything into a cabinet. My plan is to create a combo amp with 2 10" or 12" speakers so trying to find room for a huge heat sink while still keeping a "normal" looking cabinet is proving to be difficult. Rack mount may be the easiest way to go but I'm building this amp for a friend to use for gigs so it's much easier to tote around a single cabinet than a head/cabinet combo.

Speaking of cabinets... what do you guys use? I've seen some amps built in salvaged cabinets but nearly everything else seems to be built as a head only inside either a nice rack mount chassis or a salvaged chassis from some other equipment  (VCRs, Hi-Fi amps, etc...) Does anyone build their own wood cabinets?
-Matt

Thanks for the reply. I have found another heatsink that is much larger than this one. I'm still not sure which route I'll go since I haven't decided on a cabinet for the amp yet.
-Matt