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In Reply to: RE: Resistor Questions posted by mark.korda@myfairpoint.net on September 14, 2015 at 07:47:10
A higher than needed wattage resistor might offer "safety factor" for the resistor.....but it offers less protetion for more important items like tubes. It is like using a 30 amp circuit breaker on 20 amp wire. Nice for the breaker but not nice for the wire.
Metal oxides suck. On most vintage schematics all resistors are 1/2 watt unless noted. 510 ohm is a standard resistor value so nothing special needed there.
That preamp isn't such a great one. I kinda hate to see you put boutique parts in it. A normal quality carbon film resistor such as you get from mouser/digikey/allied and orange drop caps are more than good enough. PEC pots are fine for volume control. Save the Percy parts for a high quality design. The PAM circuit isn't going to know the difference. Better to splurge on telefunken tubes for it.
The downfall of most vintage preamps are all the tone control circuits and rotary switches. They also tend to have way too much gain. If you don't have a turntable most preamps these days need to have negative gain!
A preamp is so easy to make from scratch. Phono stages are a different matter. The original Foreplay from Bottlehead is a decent cheap starting point....and WAY better than the PAM-1. Again only saying this because I hate to see you dump money into a loser.
Follow Ups:
Hi Russ57, thanks so much for your help.When you back up a question with an answer that gives a reason or to me a lesson learned, that's the best. The wire comparison was just that. Before you did answer me about the size of resistors I called up Frank VanAlStine and he told me about everything you did. But he didn't give me time to explain everything.A few years ago I was using the cases,knobs, of the PAM-1's to do other projects in. One was a transformer assisted passive preamp. The holes for the jacks in the PAM fit the new gold jacks perfectly. I kept looking at the PAM and wondered what it would perform like if I could remove the tone controls. A guy on Audio Circle, Clyde the Cat had an answer that even Frank thought was right. The Cat drew some lines on the schematic for me to do. If in theory if I did that, put in gold jacks, eliminated the loudness switch and the tape monitor and found a good 500k stepped volume from say Goldpoint, wouldn't I have something. I know a Ferrari can outperform a Ford model T, but I want the fun of restoration of the old,(PAM), and state of the art can be left to guys like Nelson Pass ect... One of Franks statements was if I want a project to do build radio controlled model planes. That just fueled my fire to complete this project, and I'm going to figure out a new selector switch system like the new PAS mod(Latino's I think) seen on Ebay. I bought all the Vishay resistors needed, 1/2 watt yesterday from Mouser for 12 bucks for both units. I want the fun of restoring something exactly as old as me,58. I will try to upload the process mentioned in Audio Circle on the tone control removing. It's there,if not under their vintage it's under tubes or DIY,I'll find it. I made copies with my now gone scanner. With everything all new and even closer in tolorence, powered by Mark 3 octal output plugs, would this not meet the specs stated in the manual? I thought the specs measured pretty good. Cap research for ordering is next then I think after being all stripped down I'll be ready to build. I want you guys to see the tone control issue and see if it can be done with your opinions...thanks...Mark Korda
Consider what we need a preamp to do.
1) It picks between sources. In the past we might have had a phono, tape deck, tuner, etc. So what do you have? If only a CD/DVD why do you need a selector switch?
2) It provides enough voltage gain to allow sources to drive an amp to full power/clipping. For tube amps this was about 2 volts RMS. Many solid state amps need less. Today a CD/DVD player puts out 2 volts RMS easy.
3) It provided impedance matching. This is a little harder to understand. Best I can do. Think of voltage gain as how fast a car can go. Think of low output impedance as a measure of how fast a car/truck can go when pulling a heavy load. A porsche might do 140 MPH but it won't pull a 40 foot boat or motor home up a mountain. Typically tube amps don't need a low output impedance preamp (but it helps) whereas solid state amps do.
So think about what you need. What are your sources? How many do you have? What is their voltage and output impedance? What do your amps need? Are you building only for current amps or anything that happens in the future?
In short if your only source is CD/DVD then they have enough MPH and torque to drive your amps up the steepest mountain at any desired speed. Anything and everthing in the PAM only gets in the way and degrades performance. Better to buy a quality DAC and do volume control on the digital side (or add a pot to input of amps).
If you are a vinyl addict than you do need a preamp/phono stage. I'll wait to hear more before I say more. You have enough to consider, including why others have tried to talk you out of this path.
Hi Russ, I have 2 preamps, I have 2 set ups going. One is a passive switch box with 2 separate volume controls. All gold and silver in the wire and jacks.I built that in 1990 and always believe in the less is more theory about preamps. The other is a Ace Zero Distortion preamp from 1974 that has a rare black faceplate. I saw an article in AudioXpress about a transformer based passive preamp.I made one and used a Dyna PAM-1 chassis. The holes for my jacks were already drilled! At the same time I was wondering about a stripped down version of the PAM using real good parts. A guy named Clyde the Cat sent me back a PAM schematic with what I'd have to do to remove the tone controls from the circuit. Frank VanAlstine wrote in an agreed with the Cat. The article with pictures I took is on the Audio Circle another forum. Russ, I have some PAM-1's and I have the urge to solder again, now that I've been on the wagon for a several months. I think I know why this bug has hit me. It's cable tv. There are 2 guys that oddly have their jacket sleeves ripped off. Danny the (Count) and Rick from American Restoration. One restores old cars and the other everything else. I think my idolization of watching them restore stuff has a lot to do with me getting the PAMs going. I have nothing to loose except getting electrocuted! Keep on writing Russ, yours was a great letter so I wanted to explain my madness.....keep in touch...Mark Korda
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