Tube Amps / Music Electronics
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|2/24/1998 6:23 AM|
||Replacement Speaker for Peavey Classic 30|
I need to replace the speaker in my Classic 30 from Peavey. Any suggestions? Anyone know what Ohms the factory speaker is?
|2/24/1998 6:33 PM|
Personally I like the Celestion Vintage 30's, but everybody has different tastes so it's a tough call.
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The Ultimate Tone, Volume III by Kevin O'Connor
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|2/24/1998 8:13 PM|
The best thing to do is to look on the speaker.
their should be a white sticker with the speaker
information on it. ie... s12425, S12825,these are
Scorpion plus 12"speakers 4ohm and 8ohm models. Note
the 4 or 8 in the middle denotes 4 or 8 ohm....
You probally have the Sheffield model speakers in
your amp. you can tell by looking at the speaker
or the data sticker info.... this amp is a 8 or 16
ohm model with a 12" speaker....
|2/25/1998 4:20 AM|
The Classic 30 uses a 16 ohm "Blue Marvel" speaker (probably made by Sheffield). I put in an 8 ohm Celestian Silver Series 60 watt speaker (since they were cheap!) and I've been pleased with the sounds. Any decent speaker would sound better than the stock speaker.
In experimenting around with the amp, I discovered that the 8 ohm speaker tap (the external speaker jack) produces a much more responsive sound than the 16 ohm tap. My theory on this is that the speaker load "robs" some of the signal being sent back to the pre-driver in the negative feedback loop, making for a sound that is more responsive and alive.
I added a jack for the internal speaker on the bottom of the chassis directly under the power switch. The added jack is connected to the 16 ohm tap when used alone. If you don't want to add this jack there is a quick way to connect the internal speaker to the 8 ohm tap: stick a plain old 1/4" phone plug (no connections) into the extension speaker jack. (When something is plugged into the external speaker jack, the internal speaker is connected to the 8 ohm tap in parallel with the ext spkr jack.) This trick will improve the response of the stock speaker as well (but it won't make it a Celestion!)
|2/25/1998 8:35 AM|
Thanks for reminding me about the speaker jack setup. I'm getting the parts together for your tone control mod to my Classic 30, and I'll include the speaker jack.
This 8-ohm speaker substitution is intersting. This would effectively halve the load resistance the tubes see. I don't know what the 30's transformer primary impedance is, but supposing it's the conventional textbook value, halving that would be similar to what was found on the Matchless Spitfire.
I looked at the negative feedback network on the C-30 schematic. It has unusually high series resistor values, an actually is a more complex network of resistors and capacitors with a capacitor feed to the V3A cathode. This is AC feedback, since the capacitor will block DC. This network might just be like a fixed presence boost, and not cut the gain much in general.
|2/25/1998 10:42 AM|
Do you also find that the AC coupled closed loops sound more like a bass boost or hi-freq roll off, compared to a DC coupled FB loop?
I've moved the DC and AC coupled FB loops from the PI/driver to the preamp and back with some different tonal effects too.
I've been experimenting with different combinations of lumped RC Hi pass and Low pass configurations in my FB loops and now I am begining to think of it as an additional overall tone control now.
I think I am safe in saying that a 2:1 mismatch in a tube output circuit is pretty safe thing to do.
It seems like tube circuits are much much more forgiving in this way then say SS devices.
Why don't you unsolder the the OT wires and put a little voltage into the OT, measure everything, do the math and tell us what the real Zed ratio is on that thing.
I have been doing this on every amp I run into lately and I am suprised at how diiferent reality is compared to what the tube books say!!
I for one would be interested as I am suddenly finding more and more uses for odd OTs with different power tubes.
|2/25/1998 12:28 PM|
I haven't done any experimenting with different loop arrangements like you are doing. I guess I just haven't had the time to explore that area yet. I do recall that on various 1950's era audio amplifiers, usually the simpler phono/PA amps like Bogen, that they make use of frequency selective feedback with a rotary switch for various tone settings. I had a cheap Lafayette kit amplifier I built when I was a kid that used the neg. FB loop with a pot as the amp's panel-mounted bass control.
I'm not sure, in looking over the loop on the Classic 30, just what its effect on overall frequency response is. Off the 8-ohm tap there's a 220k paralleled with a .0047 cap, in series with a 100k, to a junction point. From this point is the series combination of 0.2uf and a 10k to ground. Also from the junction, a 22uf/25v cap takes the signal to the cathode of V3 (preamp stage before the split-load inverter) at the top of its unbypassed 1.5k cathode resistor.
In the next week, I plan to have my C-30 taken apart for Steve's tone stack mods. At that time, I may as well measure the turns ratio of the OPT.
Your collection of primary impedance measurements is interesting. Some mass-produced devices are built with compromises, which could account for some of the unusual values you're seeing, such as making an available trans work for a few different amps by the same amp builder. But what puzzles me, until I get to the bottom of it, is that the Matchless was a deliberately designed, low production custom type amp, that had transformers specially wound to Sampson's specs. There must be a shift in harmonic distortion product distribution or something, with the >4k primary impedence.
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