A while ago a friend wanted me to take a look at his Dynaco MK IV amp. The power transformer had burned up and he wanted to know if I could replace it. I was able to find a replacement PA-135 thru Triode Electronics. My friend had brought over the other MK IV and his Audible Illusions Modulus 2D preamp. Unfortunately the amp still does not work and the other amp is wired a little differently.
The big problem is that these amps were modded with 8417 and 6DJ8 tubes.
I have not been able to find any schematic or info on how they were wired. The driver board that includes the 6DJ8 tubes says mk 3 on the back and GSI 84 in the corner , but it does not match anything I have seen on the web. The boards include 2 LM 334 adj. current sources and a MUR 420 rectifier diode along with 2 pots.
The input uses a 3N259 bridge rectifier along with some very large caps(330v 750 uf).
Any suggestions would be appreciated.
The big problem is that these amps were modded with 8417 and 6DJ8 tubes.
I have not been able to find any schematic or info on how they were wired. The driver board that includes the 6DJ8 tubes says mk 3 on the back and GSI 84 in the corner , but it does not match anything I have seen on the web. The boards include 2 LM 334 adj. current sources and a MUR 420 rectifier diode along with 2 pots.
The input uses a 3N259 bridge rectifier along with some very large caps(330v 750 uf).
Any suggestions would be appreciated.
see the thread at
http://www.diyaudio.com/forums/showthread.php?postid=1345283#post1345283
for a comment about a similar board from Andy Fuchs, the principal of GSI.
Andy can probably help you out.
http://www.diyaudio.com/forums/showthread.php?postid=1345283#post1345283
for a comment about a similar board from Andy Fuchs, the principal of GSI.
Andy can probably help you out.
8417 versus 6550 bias
When Dyna and others started making amps, they found that several tubes load well right around 4500 ohms. You can swap EL34s 6l6gcs and 6550s and 8417s into 5000 ohm loads with small power and distortion diffs.
The - grid bias for 6l6, 7027, EL34, 6550 is modestly close.
The 8417 is a monster that was maybe the last tube made.
It is designed for Class-AB1,2 mode, where the idle current is low, and the idle current jumps up with the audio signal.
These amps could put out giant peaks, so the big transits came through on orchestral stuff etc...
The EL34, 6550 and 8417 are "High cathode current" tubes that can handle lots of peak power.
Main difference between the 8417 and the others is that the idle bias for the 8417 is about 1/2 of the others.
Normal for a 8417 is:
600V on the plates, 300V on the screen, and -15V grid
Normal load = 4200 ohms for 120 watts
6550 is the same except 5000 ohms load, 100 watts, -31 grid
So you need to be very carful the idle current is around 40 mills
So you can put 6550s back in if the idle bias is at -31 Volts or melted PC board could Haps!
Look at original Dyna Schemo for referance
I re-stored two Altec 340A amps some years ago.
They had 6550s in Class-A 4500 ohms load.
I cleaned the amps up, then brought one up on a Variac. It sounded fine, but I noticed a lot of heat coming from somewhere and I found the 6550s cooking nicely, but not red.
I checked the idle and the 6550s were idling at 42watts each!
Back then, even in Dynas era, they would really smoke big tubes to load the output transformer for better bass.
Also tubes were cheap.
So you can put 6550's back in.
You need to be able to check the voltages and use a signal tracer to see if the driver stage is working. You can check the driver and bias with the big tubes out, helps if you have a Variac.
You check the driver by putting in some signal, then follow the signal to the grids to see that you have equal drive to each grid.
You can check the bias with the big tubes out from grid, pin 5 to ground. It takes some skill, and getting used to working around HV. I have gotten bit pretty well, use one hand rule.
MAin Thang is to be sure of the bias on 8417s as Diff than others
gEo
When Dyna and others started making amps, they found that several tubes load well right around 4500 ohms. You can swap EL34s 6l6gcs and 6550s and 8417s into 5000 ohm loads with small power and distortion diffs.
The - grid bias for 6l6, 7027, EL34, 6550 is modestly close.
The 8417 is a monster that was maybe the last tube made.
It is designed for Class-AB1,2 mode, where the idle current is low, and the idle current jumps up with the audio signal.
These amps could put out giant peaks, so the big transits came through on orchestral stuff etc...
The EL34, 6550 and 8417 are "High cathode current" tubes that can handle lots of peak power.
Main difference between the 8417 and the others is that the idle bias for the 8417 is about 1/2 of the others.
Normal for a 8417 is:
600V on the plates, 300V on the screen, and -15V grid
Normal load = 4200 ohms for 120 watts
6550 is the same except 5000 ohms load, 100 watts, -31 grid
So you need to be very carful the idle current is around 40 mills
So you can put 6550s back in if the idle bias is at -31 Volts or melted PC board could Haps!
Look at original Dyna Schemo for referance
I re-stored two Altec 340A amps some years ago.
They had 6550s in Class-A 4500 ohms load.
I cleaned the amps up, then brought one up on a Variac. It sounded fine, but I noticed a lot of heat coming from somewhere and I found the 6550s cooking nicely, but not red.
I checked the idle and the 6550s were idling at 42watts each!
Back then, even in Dynas era, they would really smoke big tubes to load the output transformer for better bass.
Also tubes were cheap.
So you can put 6550's back in.
You need to be able to check the voltages and use a signal tracer to see if the driver stage is working. You can check the driver and bias with the big tubes out, helps if you have a Variac.
You check the driver by putting in some signal, then follow the signal to the grids to see that you have equal drive to each grid.
You can check the bias with the big tubes out from grid, pin 5 to ground. It takes some skill, and getting used to working around HV. I have gotten bit pretty well, use one hand rule.
MAin Thang is to be sure of the bias on 8417s as Diff than others
gEo
Dyna Stuff
I have a copy of the original schemo for a ST-70
I also have schemo and notes on turning a Pas-3 into a decent line stage, just have to look...
It is actually harder to find schemos online now than five years ago!
You can go to the library and get a copy of the related SAMS Photofact if that company did that.
gEo
I have a copy of the original schemo for a ST-70
I also have schemo and notes on turning a Pas-3 into a decent line stage, just have to look...
It is actually harder to find schemos online now than five years ago!
You can go to the library and get a copy of the related SAMS Photofact if that company did that.
gEo
I was going to say, if those are diodes in there, it might be an Andy Fuchs design... no matter, if it were mine, I'd scuttle the driver board and make my own. It's not hard to better the old Dyna front end, as it had "predistortion" to end up with the required "flat" frequency response out to 20khz that was required back in the day for specs... it just doesn't really sound very good.
If you check the earlier Acro ULII schematic, that is a better amp, better front end, same designer!
But there are even better ways to do it, SE or LTP type input stage than the old Acro and/or the old Dyna, imho.
The PS and output iron + chassis are where the value is, I think.
_-_-bear
If you check the earlier Acro ULII schematic, that is a better amp, better front end, same designer!
But there are even better ways to do it, SE or LTP type input stage than the old Acro and/or the old Dyna, imho.
The PS and output iron + chassis are where the value is, I think.
_-_-bear
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