New SS amp in a 8" combo amp -- questions -- not much OT

[Steben]

Yesterday I opened a 20 year old guitar SS combo amp that stuttered, popped and crackled like hell. I stripped it to the bone, leaving only the furby-covered frame with speakers, reverb tank (yes!) and the amp chassis (about 19" wide) and a salvaged transformer (probably 2x25V).

(For the interested: I'm planning on putting the board and knobs on Ebay for parts. It includes a handful of slim push-on knobs,  couple or RC4558 opamps, TDA2030 chips, MN3101 and 3007 BBD (was a chorus)... and a bunch of diodes, trannies, caps, ...I just don't want to lose time on stripping it. PM me if you want to be fast)

The thing I want to do is: make a decent preamp based on my "bluestate" design - maybe with changing the CMOS to a mu-amp or two - that goes through a FX loop to a linear power chip. Before the power amp I would also "re"-include the reverb tank, since it wasn't the tank that was bad, but the pots and electronics. The power amp part and supply can be bought as a pre-soldered kit, so no questions there.
Which leaves us actually to stompbox level (transistors and screwdrivers...)

>The reverb tank is +- 2.5" wide, 2" high and 12" long. One wire in , one wire out. Does this thing have polarity? This is the unit I know nothing about. Is there a will-drive-it-all reverb circuit around?

>The combo has two 8" speakers. Since it was used as a stereo set-up, my question is: should I rebuild it as a "mojo" stereo, or should I connect the speakers in series? In other words: is it worth it?
I tend not to do it. It means more complexity and actually I don't believe that much in the stereo effect of two speakers with only a couple of inches between them...

>I haven't measured the speaker impedance yet, but I think it is 8 or 4 ohms each and I will know for sure soon. I even don't know the power handling, but they were used for guitar, so probably not low. What I do know is that each one was powered with a TDA2030@+/-18V= 15W each?
Since the power chip I want to use (TDA7294) works like many others on both 4(40Wrms) and 8(30Wrms) ohms, I'm thinking of the following. If they are 4 ohms, I place them in series (8ohms). If they are 8 ohms, I place them parallel (4ohms). Funny... But how will they sound in both cases? The same? I guess the speakers in series will suffer less power (safe assumption).

>If I want to make it sound close to an average 12" speaker, what should I do? I know it should be a bit subtle, comparing the response graphs. The 2-2.5kHz resonance differs very little. Difference is for example from 7.5kHz on, an area where 8" tends to have extra response ... Or the 8" bass cut from 100Hz, where 12" cuts from 70Hz...

[MikeH]

Ususally amps have a "send" and "return" for reverb, so it's probably supposed to run in a specific direction.  On one of my SS amps the Red is the Send and the White is Return (if they're color coded on yours).  You could use the Reverb projects from GGG to control your reverb tank:

http://www.generalguitargadgets.com/index.php?option=content&task=view&id=124&Itemid=142

[R.G.]

>The reverb tank is +- 2.5" wide, 2" high and 12" long. One wire in , one wire out. Does this thing have polarity?

Yes. One wire is in, one is out. Usually the in and out are stamped on the unit.

Measure the DC resistance of the two sides. If they are different, it may be possible to guess the in and out ports. Reverb tanks are usually several hundred ohms on the output side. The input side may be from 2-4 ohms up to the same as the output port. This is because the input side may be designed to be driven by a speaker-level (i.e. 8 ohm) output, 600 ohm line output, or 2k ohm opamps/transistor output. The outputs tend to all be about 2k, hence the few-hundred-ohms on the wire of the output.

This is the unit I know nothing about. Is there a will-drive-it-all reverb circuit around?

No, for the reasons I mentioned above.

>The combo has two 8" speakers. Since it was used as a stereo set-up, my question is: should I rebuild it as a "mojo" stereo, or should I connect the speakers in series? In other words: is it worth it?
I tend not to do it. It means more complexity and actually I don't believe that much in the stereo effect of two speakers with only a couple of inches between them...

You are correct. There is little to be gained from stereo with speakers only a few inches apart. I believe that it was done this way so they could get the power of the two power amp chips (TA2030) to add without going to push-pull. Each one probably drove one speaker to 12+W. It's a simple circuit.

>I haven't measured the speaker impedance yet, but I think it is 8 or 4 ohms each and I will know for sure soon. I even don't know the power handling, but they were used for guitar, so probably not low. What I do know is that each one was powered with a TDA2030@+/-18V= 15W each?

Probably.

Since the power chip I want to use (TDA7294) works like many others on both 4(40Wrms) and 8(30Wrms) ohms, I'm thinking of the following. If they are 4 ohms, I place them in series (8ohms). If they are 8 ohms, I place them parallel (4ohms). Funny... But how will they sound in both cases? The same? I guess the speakers in series will suffer less power (safe assumption).

There are a lot of issues here.
Speaker impedance: measure the DC resistance of the speaker; this will nail down the nominal impedance. 4 ohm speakers tend to be 3-3.5 ohms DC, 8 ohms speakers tend to be 6-7 ohms DC.
Speaker power: Speakers put out power depending in the voltage they're driven with. The nominal power is P = (V^2)/Z where V is the RMS voltage across the speaker and Z is the speaker impedance. If you replace Vrms with Vpeak, then we know that Vrms = Vpeak/1.414, so the nominal power is P = (Vpeak^2)/2*Z. The point is that power is limited by the peak voltage available, and in a SS amp, the output voltage is the power supply voltage minus some losses. Since these speakers were driven by +/-18V in the original amp, they are designed to be driven by +/- 18V (minus whatever losses), so they will put out their best power if the new amp puts out +/- 18V on each speaker.

If you don't change the power supply , then you still have +/-18V, and running them in series will put half the voltage across each one, resulting in half the power out they are capable of, no matter what chip amp you use to drive them.

Now that I think of it (my coffee is hitting me now) you could calculate a guess at the impedance of the speakers from the rated power of the amp and the power supply. We know we can't get the full 18V, so maybe +/-16V gets to the speaker, so the output is 16W per speaker if they are perfect. If they were 4 ohms, that would be 16*16/(2*4) = 32W per speaker. That's not within the amp specs, so they're almost certainly 8 ohm speakers.

Drive them in parallel. Or, since the 7294 is only about $5.00, use two 7294s, one per speaker, like the original amp used two 2030's and have more reliability.

>If I want to make it sound close to an average 12" speaker, what should I do? I know it should be a bit subtle, comparing the response graphs. The 2-2.5kHz resonance differs very little. Difference is for example from 7.5kHz on, an area where 8" tends to have extra response ... Or the 8" bass cut from 100Hz, where 12" cuts from 70Hz...

You can cut the treble response easily enough, but you can't boost the bass to be as efficient at moving air as a 12". My suggestion? Either put this in a new wooden cabinet that can hold one or two 12s, or ignore it.

[Steben]

I've seen opamps like TL072 driving a reverb tank. Guess that would be for the 600ohm? A LM386 could be used for the 8ohm input.

Well, 32 Watt output is nice. Good enough for room practice, but useful in small rehearsals. The TDA2030's were screwed to the chassis that acted as heatsink. I guess I could repeat that, given that the power is the same? TDA7294 is DMOS, could it be more efficient? Than it would be safe.

[Steben]

Ok, here we go:

Speakers are DC measured 3.9 and 4.0 ohms. Clear enough...

AC voltage out of transformer is 2 x 13.9V. Rectified that means ... around 19,5V? (x1.41)
Well, if you know the TDA2030's maximum supply voltage is 18V it must have been lower. So I guess the transformer is easily "sucked" down.

Reverb tank measures on both sides around 45ohms. 

[Mark Hammer]

I'm going to differ with Brother Keen.

There is much to be said for stereo amps, even when they aren't "true" stereo.  The original Roland Jazz Chorus was a quasi stereo amp and used one channel (separate speaker and power amp) for dry and a second for the wet/chorus.  One of the reasons the old JCs are loved is because of the richness of tone produced by having wet and dry coming from separate speakers.

A more recent amp, made by SMF I believe, had a separate amp/speaker for reverb and Guitar Player couldn't say enough good things about it.  I'm one of those folks who feels that the reverb doesn't have to be as distorted as your main guitar signal, so having the reverb signal/channel tapped off the input and fed to a second power amp and speaker (with presumably its own custom tonal tailoring and maybe even a little SS delay added on top) strikes me as a damn fine idea to pursue.  Or you can stick a phaser/univibe circuit in there too.  Indeed, if I can ever get myself organized to get back to working on it, I have a cab/chassis with a pair of 8's sitting in the garage fior that very purpose right now.

Since you don't need THAT much power for the reverb, you could salvage one of the TDA2030 chips and dedicate it to the reverb/modulation channel and use the 7294 for the primary channel.

Just a thought.

[petemoore]

Reverb tank measures on both sides around 45ohms.
  You'll have to get a driver which can handle 45ohms well.
  An LM386 can to this, as it can drive a speaker.
  see Stage Center Reverb.
  I'd consider pressing a power mosfet into reverb driver chip service. Maybe there's a reason this isn't done?
  Using a Power Mosfet as source follower [since you have the PS voltage for adequate voltage swing] to drive the reverb input, another transistor to increase gain into the power mosfet.
  Gain recovery of the 45ohm output should be an easy job for say, a gain stage to work with, maybe a buffer.
  Power mosfet can drive a speaker cone coil @15vdc, it should be a fairly simple matter to get one to drive a tank condenser, the only thing I'd know to worry about is driving the tank input with too much voltage swing.
  Perhaps the 386 is preferrable, it's available enough, I think it'd be interesting to compare the 386 and Power Mosfet as reverb drivers, PM's can have a nice tone when played as amp and used as booster or gain driver.

[trevize]

if you want to make a chipamp as power amp i highly suggest you to use a lm1875 in place of the tda2030.

It should be pin compatible, doesn't need the diodes and sounds definitely better (the tda has audible noise,
less power and lower maximum supply voltage etc...)

[dschwartz]

I MUST suggest you to use TDA2050 chips. they sound really good!!

i made a stereo amp with 2 tda2050 driving 2 6" speakers, and for guitar i just split the signal equally for both channels,.. it sound loud and clear.. and it have a kind a compressed sound really nice...i was impressed by the quality of the chips..
i made a tda2030 amp before and man.. they suck!!! ugly distortion and low power.....

for both amps i used the datasheet application circuits... and better yet, tda2050's run up to +-25 volts, so 19-20 volts is quite good!!

also, tda2050 layout is soooooooooo easy!!! i made both in about 1 hour, ready and playing!!! the bass response is superb, my 2x6" shoebox cabinet danced around the table due to bass movement....