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Vintage threads from the first ten years

Re: Replacement Speaker for Peavey Classic 30

3/7/1998 2:40 AM
Steve A.	Re: Replacement Speaker for Peavey Classic 30 Stephen: Do you have both schems on your site? Not yet... I haven't been able to get my hand scanner to "auto-stitch" full-page schematics (and haven't figured out how to do that manually). I borrowed an old Logitech Page Scanner from work that stopped working, but it doesn't work at home any better than it did it work. Peavy will mail you the schematic and layout for any of their amps for $2.50. Phone: 1-601-483-5365 Fax #: 1-601-486-1361 URL: http://www.peavey.com Steve Ahola

3/7/1998 2:40 AM

Steve A.

Re: Replacement Speaker for Peavey Classic 30

Stephen:

Do you have both schems on your site?

Not yet... I haven't been able to get my hand scanner to "auto-stitch" full-page schematics (and haven't figured out how to do that manually). I borrowed an old Logitech Page Scanner from work that stopped working, but it doesn't work at home any better than it did it work.
Peavy will mail you the schematic and layout for any of their amps for $2.50.
Phone: 1-601-483-5365
Fax #: 1-601-486-1361
URL: http://www.peavey.com

Steve Ahola

2/26/1998 3:17 AM
Steve A.	Bruce: >>>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.<<< Zed? Wasn't he the character that Bobcat Goldthwait played in Police Academy 2, 3 & 4? You know, the guy with the squeaky voice... Tell me what kind of voltage to put where, and what to measure using what formulae. (Am I to check for the ratio of voltage out over voltage in?) I won't even have to plug in my soldering iron because the OT plugs into the pcb with 2 molex connectors. Your comments and Doc's about AC coupled FB loop vs. DC coupled are interesting. It'd seem to me that an AC coupled FB loop would be more effected by a speaker load on the tap than a DC coupled loop since it is AC voltage used by a speaker... Steve Ahola

2/26/1998 3:17 AM

Steve A.

Bruce:

>>>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.<<<

    Zed? Wasn't he the character that Bobcat Goldthwait played in Police Academy 2, 3 & 4? You know, the guy with the squeaky voice...

    Tell me what kind of voltage to put where, and what to measure using what formulae. (Am I to check for the ratio of voltage out over voltage in?) I won't even have to plug in my soldering iron because the OT plugs into the pcb with 2 molex connectors.

    Your comments and Doc's about AC coupled FB loop vs. DC coupled are interesting. It'd seem to me that an AC coupled FB loop would be more effected by a speaker load on the tap than a DC coupled loop since it is AC voltage used by a speaker...

Steve Ahola

2/28/1998 4:22 AM
Jack O.	do the math and tell us what the real Zed ratio is on that thing.<<< Zed? Wasn't he the character that Bobcat Goldthwait played in Police Academy 2, 3 & 4? You know, the guy with the squeaky voice... "Zed's dead" -Bruce Willis in Pulp Fiction

2/26/1998 8:17 AM
art	Hi, I have a couple of (possibly uninformed) comments: (i) isn't the feedback AC coupled only, due to the transformer? (ii) I vaguely recollect reading somewhere from Hammond that the rated frequency range of transformers applies at the rated impedance, at higher impedances it is broader, and at lower impedances, narrower. art

2/27/1998 2:32 AM
Bruce	Hi Art, Yes, the secondary of the OT has an AC signal imposed on it and the signal we are stealing for the feedback loop is AC. But if you move over to the circuit in the preamp where the feedback loop is usually connected, you will find there is a strong DC voltage. EX: The PI/driver tube of the long tail pair we see so often in most popular amps. In that type amp the feedback is connected to the DC voltage of the biasing of the tube due to the voltage dividing effect of the resistors there. You can measure it with your multimeter. If you break that line and insert a capacitor in the feedback loop the the OT is not connected to the DC source. The capacitor is acting like a DC blocking cap, just like in the plate supply. So we loosly call it AC coupled because now only AC can pass from the OT through the cap and on to the tube. If it is not DC coupled then what else can it be called but AC coupled.. If you were to use just a fixed or variable resistance to connect the feedback loop to the OT, it would be connected to the OTsecondaries with some tiny amount of DC voltage. Did that make sense? What I have noticed is that if the feedback is DC connected....no capacitor.... the feedback response curve is fairly flate. It cuts the bass freqs as well as anyting above for the most part. However, if you AC couple the feedback loop to that tube,,,,, with a small capacitor.... then the feedback loop becomes much more reactive and is not effecting the low bass or lower freqs as much because the capacitor is not passing that AC signal you mentioned below some certain frequency. So the frequency of the negative feedback voltage depends on the capacitive reactance and the amount of the negative voltage is more determined by the voltage divider resitances. That can make the whole thing a variable lumped RC circuit. What Doc and I were mulling over was tweaking the feedback loop by AC coupling so as to be a little more selective in what frequency the loop really starts to clamp the driver tube down. Remeber no feedback has the amp running full tilt WFO! The feedback cuts back the drive level and if the feedback is tuned by an RC circuit, you'll get another type of tone control. With a cap in there it makes the amp have more relative bass because what it really is doing is skewing the frequency response to cut more of the mids and highs which makes the driver send less of those signals and more of the bass signals on to the final tubes..... it sure sounds like it has more bass. This is a fun thing to experiment with in any amp. A small capacitance will make the amp VERY bassy sounding and with a great big capacitor, the amp will sound like a normal feedback loop at cutback most all the frequencies from the power section. Just like a tone control is supposed to work. But it isn't a normal tone control. sorry about being so wordy! Pretty late and time to sleep.... And Steve A.... don't you ever sleep? Bruce

2/27/1998 2:32 AM

Bruce

Hi Art,
Yes, the secondary of the OT has an AC signal imposed on it and the signal we are stealing for the feedback loop is AC. But if you move over to the circuit in the preamp where the feedback loop is usually connected, you will find there is a strong DC voltage.
EX:
The PI/driver tube of the long tail pair we see so often in most popular amps.
In that type amp the feedback is connected to the DC voltage of the biasing of the tube due to the voltage dividing effect of the resistors there.
You can measure it with your multimeter.
If you break that line and insert a capacitor in the feedback loop the the OT is not connected to the DC source. The capacitor is acting like a DC blocking cap, just like in the plate supply.
So we loosly call it AC coupled because now only AC can pass from the OT through the cap and on to the tube. If it is not DC coupled then what else can it be called but AC coupled..
If you were to use just a fixed or variable resistance to connect the feedback loop to the OT, it would be connected to the OTsecondaries with some tiny amount of DC voltage. Did that make sense?

What I have noticed is that if the feedback is DC connected....no capacitor.... the feedback response curve is fairly flate. It cuts the bass freqs as well as anyting above for the most part. However, if you AC couple the feedback loop to that tube,,,,, with a small capacitor.... then the feedback loop becomes much more reactive and is not effecting the low bass or lower freqs as much because the capacitor is not passing that AC signal you mentioned below some certain frequency.
So the frequency of the negative feedback voltage depends on the capacitive reactance and the amount of the negative voltage is more determined by the voltage divider resitances.
That can make the whole thing a variable lumped RC circuit.
What Doc and I were mulling over was tweaking the feedback loop by AC coupling so as to be a little more selective in what frequency the loop really starts to clamp the driver tube down. Remeber no feedback has the amp running full tilt WFO! The feedback cuts back the drive level and if the feedback is tuned by an RC circuit, you'll get another type of tone control.
With a cap in there it makes the amp have more relative bass because what it really is doing is skewing the frequency response to cut more of the mids and highs which makes the driver send less of those signals and more of the bass signals on to the final tubes..... it sure sounds like it has more bass.
This is a fun thing to experiment with in any amp.
A small capacitance will make the amp VERY bassy sounding and with a great big capacitor, the amp will sound like a normal feedback loop at cutback most all the frequencies from the power section.
Just like a tone control is supposed to work.
But it isn't a normal tone control.

sorry about being so wordy! Pretty late and time to sleep....
And Steve A.... don't you ever sleep?
Bruce

2/28/1998 4:11 AM
Steve A.	Bruce: And Steve A.... don't you ever sleep? I tried it once but I thought it was just a waste of time! Steve Ahola P.S. I live on the left coast (PST) so there is an hour difference from the times listed on these posts.

2/28/1998 7:30 PM
Bill Harbour	Bruce, Doc and Steve...you guys are amazing. What a learning experience reading your post. Bill Harbour

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