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Old 1st March 2013, 12:12 AM   #1
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Location: was Chicago IL, now Long Beach CA
Default More Peavey Classic amps

I picked up another Peavey Classic 50, so that I can compare stock to my experiments every time I make a change. It also lets me measure and observe and compare in real time. And they hold their value, and after replacing the broken pots and new tolex I'll recover more than I spend if/when I decide to sell any of them. Easier to sell the cleaned-up stock one than the modded one.

I also picked up a Peavey Classic 100, same amp as the classic 50 but 8 EL84 instead of 4. I want to compare it, with half its tubes removed (fixed bias) to the stock classic 50 and modded Classic 50 with its hi-fi output transformer and power transformer from another classic 100. These things seem to go for about the same price as a classic 50. For the sound I'm pursuing (little sag, plenty of bass & treble, plenty of EL84 chime) I suspect that instead of bothering to mod the Classic 50, its much easier and just as good to just turn off half of a classic 100's output tubes. A modded classic 100 might be really versatile.

I sent all the way to Korea to have the correct-impedance full-size reverb tank made to replace the short tank jammed into the classic 50 head. Little did I know that the Classic 50 combo has a full-size tank in that vinyl bag at the bottom of the cabinet instead of the short one in the head version; I could have ordered it from Peavey as a stock part.

Last edited by cyclecamper; 1st March 2013 at 12:35 AM.
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Old 1st March 2013, 01:02 AM   #2
Enzo is offline Enzo  United States
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Peavey sells most parts at reasonable prices. One exception to that is reverb pans, they tend to charge more than the market. But they are just Accutronics pans like you can find anywhere. Antique Electronic Supply for example.

The reverb drive in the Classic 50 is the same drive they use in everything else, so any EB reverb pan will work fine. The 8EB2C1B, the 4EB2C1B and the 9EB2C1B.

The 8 and 9 pans have 800 ohm in and 2575 ohm out. The 4 pans have 600 ohms in and 2250 ohm out. This difference does not matter to the amp.
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Old 1st March 2013, 04:32 PM   #3
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If I'd known the big pan was in the combo, I'd have checked that first. And yes, I could have check the other Peavey models with similar drive and seen what tanks they used. It's also quite possible the small pan and big pan are supposed to have somewhat different impedances, to drive the big one harder for example, or make the small one quieter at its output. I took the numbers on the small pan, and went to the Accutronics site, and converted the small-tank impedance numbers to large-tank numbers with exact same impedances, still 3-spring, but long decay. Nobody stocked it; I really searched & searched. Not as a MOD or other brand either. So I called Accutronics and they put me in touch with the Korean factory directly, where they will make whatever you want. So I had them make a big tank with same impedances as the small tank. I also changed to a long-decay, more like the old long-decay Fender 2-spring (but with 3 springs).

Changing the input impedance will change the drive level, and changing the output impedance will change the volume of the reverb. There's no reverb drive level adjustment pot on the Classic 50, but I never hear the spring coils colliding or anything like that. I put metal bottoms on the tanks and will probalby add some sound-absorption inside the tanks too, and mount them on small soft foam blocks, farthe from any transformers on the head model. The tanks are current-driven. Could change the resistor around the op-amp to change the drive level. Can also upgrade the op-amp to one that will handle more current. I have seen Peavey Classsic 50's with blown drive op-amps despite the zeners protectign the input, but people will plug things with insane levels into a guitar amp in the name of experimentation. There's a guy on U-Tube who did a nice job (not sure whether he did it on a classic 30 or 50??), if you can understand the translation. He changed the drive circuit and where it returns the reverb signal to mix with the direct signal.

I'm going to try both tanks at once too, or maybe a switch in back. Being current-driven I should wire them to drive them in series, but the pickup output ends should problably be in parallel. I've always wanted to do some studies of how the drive amp damping factor and the input impedance of the recovery amp might affect the decay time...and determine whether the trasducers at both ends can damp the spring motion at all.

In the combo, they use a big vinyl bag, much like Fender, to prevent feedback. I might glue a piece of tolex on the big tank's can, just to damp that mettalic ring a little. When I made bottoms for the tanks, I could have made the mottom more like another can, and fitted more sound-absorption inside. But it makes little if any difference anyway.

I'm going to add a small treble-cut pot and a cap (or possibly just a sub-mini toggle tone switch if space is tight) on the front-panel, next to and just above the reverb knob. The Peavey Valverb has tone controls for just the reverb output, and it really is nice; heavy long-decay bass reverb with a treble-cut sounds incredible on a baritone guitar.

...but I'm getting a little fanatical in reaction to a mediocre reverb...I should have just listened to a combo's big-tank reverb before I started messing with the head's small-tank reverb.
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Old 1st March 2013, 11:34 PM   #4
Enzo is offline Enzo  United States
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First, if you have the part coming, then enjoy it.


But at an academic level:

Quote:
Changing the input impedance will change the drive level, and changing the output impedance will change the volume of the reverb.
Only potentially. Like changing the tire pressure in your car affects mileage. But if you change the pressure only one pound, you will never know the difference. The roughly 10% difference in output impedance is not going to materially affect the signal level at the output jack. And even if it did, as far as the amp is concerned it would mean the difference between setting the reverb knob at 4.2 instead of 4.1.

Same thing on the drive side. Sure some small difference in current will result, but of course you won't hear springs crashing. That comes from mechanical motion of the amp, not from the drive. That little IC has nowhere near the guys to swing those springs enough to hit anything.

The 8 and 9 pans have the same impedance list, while the 4 pan uses the other. But whether Peavey uses the smaller 8 pan in one model or the larger 4 pan in another, they use the identical reverb circuits in all.

Quote:
It's also quite possible the small pan and big pan are supposed to have somewhat different impedances, to drive the big one harder for example, or make the small one quieter at its output.
With all due respect, that is just a rationalization. looking at the long 9 pans and the short 8 pans with the exact same impedances kills that theory. It is the same size 9 pans and 4 pans that have the different impedances. The different impedances do not follow pan size, they follow the number of springs in the unit. How long the springs are affects the tone, but not really the drive. But the number of springs means a different size of transducer. All the three spring pans, short or long, share one set of impedances, while all the two spring models share the other list.



Here is an experiment you might like:
ON a bazillion old fender amps, you have the two channels, the left channel is Normal, just bare bones. The other channel is the one with the effects - reverb and trem. On any of those, you can take the return cable from the reverb pan, and plug it into the (normally unused) Normal channel input. Now the Normal channel controls - volume and tone - act as the reverb return. SImple, requires no alteration to the amp, and when you tire of it, just plug the reverb back where it came from.

Quote:
I have seen Peavey Classsic 50's with blown drive op-amps despite the zeners protectign the input, but people will plug things with insane levels into a guitar amp in the name of experimentation.
The zeners are there to clamp off the input signal to the IC so it does not exceed the rails, but that isn't what blows up the IC. The problem for the IC is that it directly connects to the outside world. Static, induces voltages, all manner of potentialloy damaging things are out there, and that is what kills the ICs, not just cranking the input sky high.

Quote:
I'm going to try both tanks at once too, or maybe a switch in back. Being current-driven I should wire them to drive them in series, but the pickup output ends should problably be in parallel. I've always wanted to do some studies of how the drive amp damping factor and the input impedance of the recovery amp might affect the decay time...and determine whether the trasducers at both ends can damp the spring motion at all.
You were the one worried about impedance, so watch out. You want to run two transducers in series? OK, but that doubles the impedance the driver sees, and since the transducer IS the feedback loop for the op amp, you will alter the gain of the IC. And on the return end, I don't think parallel is a good idea, any more than mixing any other two signals together by wiring them parallel. You'd want to mix them, I'd think. At least a couple mixing resistors if not separate stages. You don;t want the signals contending on the same line.

DEcay time is a factor of how long the springs take to go still after a drive impulse. I suppose it is possible to run some DC though the transducer to create a braking effect, but I can't imagine it would be good for the health of the poor thing. Not to mention when turning DC on and off, you'd get a good POP. Of course it would be a nice reverberant pop. The impedance at the return end should nave no effect on decay time. Just my opinion, but all the electronic factors would affect the shape of the drive signal, but once the signal hits the springs, I don't see them affecting it.


Tolex glued to the wide flat top surface probably works. Here we run a strip of self-adhesive foam weatherstripping down the length of the top. Fender and some others have done similar. Carmakers do the same thing inside car bodies so that large areas of sheet metal don't make sheet metal noises as you drive along.
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Old 2nd March 2013, 07:33 AM   #5
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I can appreciate the voice of experience when others are willing to share.

Yes, if using two tanks it would probably be ideal to duplicate the receive/recovery preamp so there's one for each tank, and then mix the outputs of both with some resistor network. But if I just throw them in parallel it's not much different than when you turn on two guitar pickups in parallel. There's always a possibility one tank's output may drive the other backwards, injecting signal into the other tank's output and driving the spring "backwards" but the resultant complex reverb might be nice...or not.

I really have no idea why or how people frequently blow the drive op amp but not the receveive/recovery op amp which I'd expect to be much more sensitive to the environment, if in fact the environmnt is the problem as you claim.

I do intend to experiment VERY CAREFULLY to see whether/how those limiter diodes affect the sound of the reverb. It seems to me there is a lot of potential for them to really **** things up as well as protect the op-amp. If it sounds better without them, I'll try to come up with something better. If not, then great.

Yes, I've plugged spring tank outputs into the other channel on a Twin, and quite appreciated the tone controls for the reverb. My old Tapco unit I used in the '70's also had a small graphic EQ on the reverb, a step beyond the tone controls on the Valverb. Come to think of it, I upgraded those Accutronics tanks from 2-apring to 3 but in those days they were manufactured in rural Illinois not far from where I was in college, and a friend working at Accutronics got them for me in exchange for some beer. There was a Maestro guitar preamp back then (with weird plastic levers almost like organ stops) that would let you assign the tremolo to the reverb OUTPUT (not reverb input) instead of to the parallel dry or pre-reverb, and the way the tremolo modulated the reverb output sounded kind of like a poor man's echo, definitely different, like emulating seperate repeats or the echo of a distant room. It was also nicer when the footswitch would turn the send on/off rather than the receive...you could play something wet to sustain and ring on, click dry and play dry over the sustaining drone as it decayed. There's a name for that with an echo...but darn I can't remember the name for switching the send instead of switching the receive...too many years out of the sound business.

Anyway I'm having fun. Even if I blow the op-amp it's cheap and easy to fix. And from everything I hear you saying, getting the exact same stock impedances for the new large tank like I did was probably safest. Whether it was worth paying for shipping one from Korea is another legit question.

I have some little heat-sinks designed to stick onto an op-amp. Would one be likely to help at all? Somewhere I've also got somebody's recommendation for a drop-in op-amp replacement that handles twice the current...which I might resort to if it comes to it.

This weekend I'll try the combo reverb, and check the numbers on it. The combo has a bunch more pots with broken shafts, just like all the others. So far I've been having good luck using the same Peavey printed-circuit-mount pots for replacements but with the newer Peavey knobs which have a white plastic insert around the shaft. Their new-design knobs stay on much better! The small-diameter split-shaft really doesn't work as a retaining spring for the knobs; instead they break, or more often people probably break them trying to re-tension them. They're far too brittle for that to work. But if you fill the slot by gluing in just about anything, and use Peavey's new replacement knobs, the combination is strong and the knob's split insert supplies the retaiing tension instead of the shaft, and it works OK. A previous owner was creative and used extra-long setscrews in setscrew knobs on broken half-shafts, a resourceful temporary fix. I should have remembered to order a bunch of pots from Peavey.

Last edited by cyclecamper; 2nd March 2013 at 07:42 AM.
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Old 2nd March 2013, 09:32 AM   #6
Enzo is offline Enzo  United States
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Quote:
I really have no idea why or how people frequently blow the drive op amp but not the receveive/recovery op amp which I'd expect to be much more sensitive to the environment, if in fact the environmnt is the problem as you claim.
A couple reasons. One is that the return is an input, and sees whatever accosts it as just another signal. Not only that, but there is a series cap between the return op amp and the pan connection. The drive side is directly connected, no caps, no resistors, no nothing. That thing is also trying to drive current into something, so it works harder.

The limiter diodes should not conduct at all until your signal peaks in excess of their voltage. They are 9.1v zeners. So until the signal gets to 9 volts, they should remain transparent.

Mixing the two pans would be the "proper" way of doing it, but the signal coming out of the pans has no power behind it. One cannot reasonably drive the other one. And it certainly won;t hurt anything, as you say, like two guitars pickups in parallel. The two pickups would have a coherent and related signal from each, while the two reverb signals would be unrelated really. It sounds good or it doesn't.
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