I'm OK , now that I vented and Blue Moon Kentucky is playing.
Well, I guess all well and fine at the new place!
:thumb:
Well, I guess all well and fine at the new place!
:thumb:
Peavey Mod...Very nice pre.
- Thread starter bluebird
- Start date
Help Support GroupDIY Audio Forum:
[quote author="rafafredd"]Would one need two heater supplies for this thing? I mean, one floating for the cathode follower and another one for the 12ax7?[/quote]
I think you can use one floating heater supply for both tubes.
I think you can use one floating heater supply for both tubes.
NewYorkDave
Well-known member
Not a good idea. The heater-cathode voltage should be fixed. Using a floating heater supply is a sure-fire way to inject hum.
If you're talking about fixing the heater supply at some DC point above ground, that's not "floating" the supply. "Floating" means that it has no ground reference at all.
At any rate, it's hardly necessary unless the cathode of the tube is sitting many volts above ground--say, more than 100V for most small triodes.
If you're talking about fixing the heater supply at some DC point above ground, that's not "floating" the supply. "Floating" means that it has no ground reference at all.
At any rate, it's hardly necessary unless the cathode of the tube is sitting many volts above ground--say, more than 100V for most small triodes.
rafafredd
Well-known member
So, I´m not talking about "floating", I´m talking about "fixing" above a fixed DC point...
NewYorkDave
Well-known member
Right. For a reality check, look at the datasheet. In the case of the 12AU7, peak heater-cathode voltage (either positive or negative) is 200V. So there's no need to "bias up" the heater supply.
SonsOfThunder
Well-known member
Wow, I had no idea that CJ disliked P@t so badly...and here I was thinking I would be on CJ's "good" side! And (British) laws banning his name? I hope I never achieve that distinction on any forum...:razz:
And why would anyone buy a "kit" that uses perfboard? If my choice is to buy a kit, I think Hamptone has the sweet deal, hands down,
Seriously, I have had some experience with him in the past and it was mostly good, though I always felt like I HAD to watch out for my own "interests" because he was certainly watching out for his. He used to have some sketchy details for modding a certain cheepy opto compressor on his site and I'm sure that my inquiry and subsequently figuring out "precisely" what was going on with those mods resulted in him removing all details from the page. That's okay though... :green:
To come back to the actual topic here, I must ask, why or when would you ever need a separate heater supply on a pair of 12A_7?
Peace!
Charlie
And why would anyone buy a "kit" that uses perfboard? If my choice is to buy a kit, I think Hamptone has the sweet deal, hands down,
Seriously, I have had some experience with him in the past and it was mostly good, though I always felt like I HAD to watch out for my own "interests" because he was certainly watching out for his. He used to have some sketchy details for modding a certain cheepy opto compressor on his site and I'm sure that my inquiry and subsequently figuring out "precisely" what was going on with those mods resulted in him removing all details from the page. That's okay though... :green:
To come back to the actual topic here, I must ask, why or when would you ever need a separate heater supply on a pair of 12A_7?
Peace!
Charlie
rafafredd
Well-known member
when cathode is many volts above ground, like in a cathode follower.
[quote author="rafafredd"]So, I´m not talking about "floating", I´m talking about "fixing" above a fixed DC point...[/quote]
My apologies. I mistakenly thought that's what we were talking about. I got floating and biasing confused. (late night, no coffee.)
My apologies. I mistakenly thought that's what we were talking about. I got floating and biasing confused. (late night, no coffee.)
rafafredd
Well-known member
It was my error. I meant to say 6.3vAC ABOVE xxxvDC...
tommypiper
Well-known member
How easy is this mod? Can some of you offer some practical directions, pictures? The Peavey TMP-1 is very cheap and this would be an interesting project, probably for a lot of guys. Especially if it sounds that good afterwards. I have no experience with tube circuits and I would like to try this. Can someone offer a few practical and step by step instructions?
cheers,
t
cheers,
t
skipwave
Well-known member
Sorry to dredge up this thread, but I snatched up one of those $99 TMPs and would love to know what the mod entailed.
From what I can gather:
-change 12at7 for 12au7 in CF.
-change resistor values in 12ax7 common cathode stage?
Did anyone save the schemo that was posted in this thread?
Thanks. :grin:
From what I can gather:
-change 12at7 for 12au7 in CF.
-change resistor values in 12ax7 common cathode stage?
Did anyone save the schemo that was posted in this thread?
Thanks. :grin:
Here's the schemo I have:
http://home.att.net/~giuseppe_poteet/peavey2.gif
I haven't gotten around to doing this yet--soon I hope.
Tom
http://home.att.net/~giuseppe_poteet/peavey2.gif
I haven't gotten around to doing this yet--soon I hope.
Tom
skipwave
Well-known member
Thanks!
skipwave
Well-known member
> parallel cathode follower
This output stage topology is new to me, and I'm trying to understand the rationale behind it. In the PV schemo I don't see where the grid of the second AU7 triode is being fed from. In the MP1 A schemo it's being fed by the cathode of the first stage.
Several comments at the start of this thread saying they like this "simple" cathode follower. Why?
Thanks!
This output stage topology is new to me, and I'm trying to understand the rationale behind it. In the PV schemo I don't see where the grid of the second AU7 triode is being fed from. In the MP1 A schemo it's being fed by the cathode of the first stage.
Several comments at the start of this thread saying they like this "simple" cathode follower. Why?
Thanks!
ciminosound
Well-known member
The AU7 is both halves being used in parallel. Grid 1+2 wired together.
skipwave
Well-known member
I was confused. Raf said MB1, not MP1. It makes sense now, though I'm still curious why folks like this simple CF, instead of a WCF.
I'm going to attempt the mod on my TMP.
Thanks. :green:
I'm going to attempt the mod on my TMP.
Thanks. :green:
ciminosound
Well-known member
Raf was refering to the Pultec MB-1 preamp.
http://www.geocities.com/rafafreddy/DIY/pulteq-mb1/Pulteq-MB1.htm
Simple cathode follower has a lower parts count and some folks like that and think fewer parts sound better, and if you don't need the brute force of a WCF to drive a 600 ohm load, why build it?
Remember what they say about opinions now. :green:
:guinness: :thumb:
http://www.geocities.com/rafafreddy/DIY/pulteq-mb1/Pulteq-MB1.htm
Simple cathode follower has a lower parts count and some folks like that and think fewer parts sound better, and if you don't need the brute force of a WCF to drive a 600 ohm load, why build it?
Remember what they say about opinions now. :green:
:guinness: :thumb:
> curious why folks like this simple CF, instead of a WCF.
The WCF would run half the power current, but when you add the gain stage demand it isn't much less total current.
Output impedance is the same.
Output drive is about the same, though the 2CF needs more power current to do it.
The WCF has poorer power supply rejection (though this may not matter when the 12AX7-b stage runs the same supply).
The WCF needs a high-voltage coupling cap, idling about 350V. (The caps in there may touch 400V at start-up but idle at only ~200V; the wax-paper will last a long time.)
The high-voltage WCF will normally need separate heater supplies top and bottom: here the top cathode is at +223V idle (and tries to go to +400V at start-up!) and may peak over 250V on signal, the bottom cathode stays near ground. Of course if Peafy used the same heater supply for 12AX7 and 12AU7, still the same problem. If the 12AU7 has its own heater supply, the CF is fine. The WCF should have separate top and bottom supplies, which is not possible in a single bottle.
The perfect WCF has darn near zero distortion, which was not the goal here. However if the WCF is not in perfect balance, distortion rises. Here it is not in balance (the grid voltage should be more like 170V instead of 215V) and small changes in bias may cause audible change of tone. The CF is never balanced which means large change of bias gives little change of tone.
If you really nail a perfect WCF it hard-clips both sides. This off-balance CF will clip one side before the other, a more musical way to overload.
The average road-tech does not know what a WCF is, but can recognize a CF and maybe fix what broke.
So for the cost of 2mA extra current you get a simpler quieter cheaper more-stable design.
The WCF would run half the power current, but when you add the gain stage demand it isn't much less total current.
Output impedance is the same.
Output drive is about the same, though the 2CF needs more power current to do it.
The WCF has poorer power supply rejection (though this may not matter when the 12AX7-b stage runs the same supply).
The WCF needs a high-voltage coupling cap, idling about 350V. (The caps in there may touch 400V at start-up but idle at only ~200V; the wax-paper will last a long time.)
The high-voltage WCF will normally need separate heater supplies top and bottom: here the top cathode is at +223V idle (and tries to go to +400V at start-up!) and may peak over 250V on signal, the bottom cathode stays near ground. Of course if Peafy used the same heater supply for 12AX7 and 12AU7, still the same problem. If the 12AU7 has its own heater supply, the CF is fine. The WCF should have separate top and bottom supplies, which is not possible in a single bottle.
The perfect WCF has darn near zero distortion, which was not the goal here. However if the WCF is not in perfect balance, distortion rises. Here it is not in balance (the grid voltage should be more like 170V instead of 215V) and small changes in bias may cause audible change of tone. The CF is never balanced which means large change of bias gives little change of tone.
If you really nail a perfect WCF it hard-clips both sides. This off-balance CF will clip one side before the other, a more musical way to overload.
The average road-tech does not know what a WCF is, but can recognize a CF and maybe fix what broke.
So for the cost of 2mA extra current you get a simpler quieter cheaper more-stable design.
skipwave
Well-known member
Thanks PRR! That's a laundry list of reasons. :grin: :thumb:
I realized where I've seen this type of CF before, in the Altec 1566. The circuits look extremely similar except that the modded Peevee moves the volume pot to before the first stage and adds the negative feedback control.
The volume pot between the input tx and the first stage grid seems like a compromise. There were some very good arguments against this in the One-Bottle thread. I wonder if it's configured that way in the stock pre. I'll open it tonight and check.
I realized where I've seen this type of CF before, in the Altec 1566. The circuits look extremely similar except that the modded Peevee moves the volume pot to before the first stage and adds the negative feedback control.
The volume pot between the input tx and the first stage grid seems like a compromise. There were some very good arguments against this in the One-Bottle thread. I wonder if it's configured that way in the stock pre. I'll open it tonight and check.
hrasco
Member
When I do a search at Musicians Fiend on TMP-1 I get a Nady mic pre. I'm guessing this is just a coincidence and they are not the same or even similar. Is the Peavey TMP discontinued?
Hank
Hank
