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Im bringing my 80AZs into the new millennium and while changing out the speaker binding post i got a good understanding of zmatic, impedance sw and the peak adjustment pot. My pair also have 4,8,16 OPTs and the OPT leads going to the binding post not the impedance sw. While checking components i discovered that the zmatic control is a dual type with a pot and a on off sw. It appears both of my amps have bad on off sws as i read the same ohm reading at the speaker common lug whither on or off, a little over 1 ohm. Where as the ground lead on the z sw reads .2 ohms to chassis ground. If i am looking at the schematic right with the Z sw in the off position i should read .2 at the speaker common.
Can any of you monikers help with ideas on how to proceed with or without this feature. It seems Fishers engineers were up late coming up with this. To make matters more interesting feedback is involved also. thanks Mike
Follow Ups:
Can someone explain the bias circuit for the output tubes how it passes through some taps on the output tranny. Thinking it may be a dual choke, the 100 show one tap at 13Ω and the othe 14Ω.
What about C14 (500pF), some versions of the amp use this cap and some don't.
Brad
Brad
Mike
That circuit basically works as a gesture to alter feedback for dampening for different speakers.You will notice when the control is fully counter clockwise,you will notice no change in sound when you move the slide switch.I have this circuit on my 100s and my 80az's and I can tell you that they don't bother anything in the circuit.I will check my units but when the switch is fully counter clockwise past the click,it opens the connection just like a normal switch.
You can really make these amps sound incredibly good just with putting a 35uf film cap off the 5v4 and then putting a couple dual 100s for the other filter caps..I then disconnect the volume pot and put a 470k to ground on the grid of the 12at7.Then,you want to rewire the filaments of the 12au7 for a 6cg7 and then of course,put some k40s on the EL37s and the phase splitter and then change the other caps to your cap of choice and enjoy.
Diversity of opinions on any one technical subject,almost always causes confusion for the listener.
Micky thank you for offering your experience with these fine amps. I have taken your advice with the first filter cap with my 400c and now the 80s. Spent the good part of the day working/playing with my pair. Replaced filter caps and wired one of the 80s. Also removed the volume pot I used a 500k resistor though. If I can ask why rewire the 12au7? While at it I broke off the neon lamp shaped like a candle will any neon bulb do. Thank again Mike.
You are on the right track my man.I see the 30uf cde poly or it may be panasonic but cde made it.Change out the paper electrolytic for a poly if it can fit,or a non polar muse cap.You use the 6cg7 is because its more linear and less than half the distortion.Finish the upgrades and listen to it..Then rewire the 12au7 for the 6cg7 and listen,then you tell me. That is all I need to tell you.If you want,you can make adapter sockets but the 6cg7 is just so much better.
Diversity of opinions on any one technical subject,almost always causes confusion for the listener.
Mike im sold on classic vintage gear for one main reason Classic Rock and Roll. These sweet music makers deliver the musical goods with all of the sound a music listener/lover could ask. Will take the swap test and thanks for replying. Mike
Quite possible that there's a oxide film on the switch contacts so that an ohmmeter won't make it conduct - wouldn't be a problem on a power switch, as 117V would instantly burn it away. Anyway, the grounded end of the pot is the same as "off" anyway.
How it works: the negative feedback is a combination of voltage feedback (from the 16 Ohm terminal) and current feedback (the drop across a sample resistor, with three settings for 4, 8, 16). Voltage feedback lowers output impedance, curent feedback raises it, so the variable mix of the two can give any desired output Z. Most modern speakers are designed to work with low source impedance, so you'd probably leave it at zero.
Tom can't thank you enough for your explanation of zmatic. I did not get to looking into the.switch today played with filter caps. Mike
hey-Hey!!!,
the big Heathkit W6m also had a similar FB combination. Neat stuff, but I eliminated it with the rest of the amp/pi tubes and circuitry in favour of a pentode LTP.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
heres a link to views the schematic use the sams 80az link.
http://www.audiophool.cjb.net/hifistuff.html
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