Dynaco MKIII high B+
- Thread starter script56
- Start date
Why not take another approach :
Turn down the variac while reading the B+ until B+ is correct value AND bias 1.56V ( will need some
back and force adjustments as bias will affect B+)
When B+ AND bias is correct, measure the output of the variac using AC scale (ignore the
voltmeter on the variac)
What is the result now ?
Turn down the variac while reading the B+ until B+ is correct value AND bias 1.56V ( will need some
back and force adjustments as bias will affect B+)
When B+ AND bias is correct, measure the output of the variac using AC scale (ignore the
voltmeter on the variac)
What is the result now ?
You can pull the plug out a little and measure the blades or put one probe on the fuse holder and the other on the power switch of the MKIII. With normal B+ and normal 1.56VDC the tubes idle at 140ma which is approx. 33 Watts per tube. So now to get that same 33 Watts of idle dissipation with the high B+ you can't be setting the cathode voltage at the original 1.56VDC. Break out the calculator and do some math, P = V x I. Normal B+ is, according the manual picture, is 475VDC, the normal idle current is 1.56VDC / 11.2 Ohms or 140ma, 475VDC x 70ma = 33 Watts.
Craig
Craig
PakProtector
AK member
Those amps are a fine example of why Horsepower Race amps are not ideal to live with. Their B+ is on the ragged edge of what the tubes will handle. They would do fine with an upright end bell PA060 power iron.
My first tube amps were Mk.iii's. While they work, the B+ should be lower.
cheers,
Douglas
My first tube amps were Mk.iii's. While they work, the B+ should be lower.
cheers,
Douglas
crispycircuit
Super Member
With hi-voltage amps you really need separate bias adjustments for each 6550 to ensure correct current draw for each tube. That would also include a 1 or 10 ohm resistor on pin 8 (to grd.)of output sockets for measuring tube current draw. Dyna's idea of sharing a cathode resistor is not good with modern tubes. Find a friend to install a new bias circuit.
if the tubes are purchased as a properly matched pair, the stock adjustments work OK. better methods exist of course but you certainly can run modern tubes with the stock design without problems. None of that will cause high voltages though. Normal current at too much voltage makes for excessive plate dissipation. Poorly matched tubes will have different annoying problems but both tubes won't red plate from it.
The Dyna Mk II (with the same power transformer) used the 5U4GB rectifier tube. That, in place of the 5AR4, would reduce the B+ by at least 30-40v DC or so.
It's not a delayed-startup, indirectly-heated tube like the 5AR4, but it worked fine in the Mk II, with EL34 tubes (which are less rugged than KT88s)...
Regards,
Gordon.
Inside the MkIII or by using a second outlet of the variac or extension. It's importent to measure
the output of the variac with the MkIII connected and powered on to get correct readings.
whoaru99
Epic Member
When you say same red plate on one of the tubes, is it always the same socket where the tube red plates?
PakProtector
AK member
Gordon, on the 5U4 there is but one issue; that tube conducts very much quicker than the IDH load and the filter caps will run to root2*rms secondary unloaded voltage...which is significantly higher than the filter caps are happy with.
cheers,
Douglas
cheers,
Douglas
Yes same side of plate is red. Will clean sockets and
Turn down the variac while reading the B+ until B+ is correct value AND bias 1.56V ( will need some
back and force adjustments as bias will affect B+)
When B+ AND bias is correct, measure the output of the variac using AC scale (ignore the
voltmeter on the variac) -Peterh
Turn down the variac while reading the B+ until B+ is correct value AND bias 1.56V ( will need some
back and force adjustments as bias will affect B+)
When B+ AND bias is correct, measure the output of the variac using AC scale (ignore the
voltmeter on the variac) -Peterh
Shadowdog
Super Member
I asked the if it's a tube in the same socket redplating in post #26 and the question has been asked twice since, but we still don't know anything exact on that very important point!
You may be best to replace all the tube sockets & don't use ceramic as they are usually poorly made imports and they conduct vibration.
Dynakit sells some nice composite ones and use the premium ones.
You may be best to replace all the tube sockets & don't use ceramic as they are usually poorly made imports and they conduct vibration.
Dynakit sells some nice composite ones and use the premium ones.
Yeah, but Dynaco built a LOT of Mk II amps just like that. And they didn't seem to have an inordinate amount of cap problems.
I usually use 600v caps (under the chassis) for the first two stages when rebuilding these- that takes care of the problem in any case.
Adding a CL80 or equivalent inrush limiter to the primary circuit, will also help damp that voltage spike as well, a bit... it's a small amount, but it will help.
Regards,
Gordon.
The Mk II came with the 5U4- with the same power transformer. So, it's rated to handle the extra rectifier heater current.
Regards,
Gordon.
PakProtector
AK member
If the issue seems to be following the socket, I would want to measure actual pin 5 voltage. clip leads to the socket from beneath, and put the meter between them. Positive lead to the suspect. You should see 0 volts between the two. If not...there are now capacitors to suspect of causing problems.
cheers,
Douglas
cheers,
Douglas
