Dynaco MK3 Low output!!!!

I bought some mk3's a few weeks ago and have completely restored them with the stock circuit no mod done at all. I have replaced all caps including the cans. I have also bypass the Selenium rectifier with a diode. One amp works perfect the other has very low output. When I turn it up it starts to distorte. All trannis are original. I have checked the output tranni on the non working amp and it tested good. Test where from an article from Bob using an multimeter. I have also checked and reflowed each solder point and the driver boards are new. A little background from me I am experienced in tube amp repair but there is still a lot to learn. And thing I might be missing? Could the output trannie still be bad and check out ok?

BobtheBuilder wrote:I bought some mk3's a few weeks ago and have completely restored them with the stock circuit no mod done at all. I have replaced all caps including the cans. I have also bypass the Selenium rectifier with a diode. One amp works perfect the other has very low output. When I turn it up it starts to distorte. All trannis are original. I have checked the output tranni on the non working amp and it tested good. Test where from an article from Bob using an multimeter. I have also checked and reflowed each solder point and the driver boards are new. A little background from me I am experienced in tube amp repair but there is still a lot to learn. And thing I might be missing? Could the output trannie still be bad and check out ok?

Are the tubes ok ? Easy to test as you have another working unit.
Try both 6550 first ( or kt88).
If it still sounds bad, change back and then try the 6an8 and finally the gz34.

Thanks for answering. All tubes are brand new except the pre its nos. I have swapped them though anyways and no difference. I also swamped output transformers and that did not make a difference. I really becoming puzzled. I checked all wiring with the working amp and then compared it with the schematic and all is correct. I also checked voltages and compared them with the working amp but to no real difference. I am so confused at this point as to what it could be. Bias is perfect. Ill try to post pics tomorrow.

Thanks for working on your spelling and grammar. I appreciate that.

From your description I can think of a few things I would check or re-check. You said that your voltages are good so I will assume they are and that you haven't reversed the polarity of the components (like the 1N4007 you used to replace the Selenium stack) in the bias circuit (a common error) or of any polarized caps. I am assuming you are running a stock MkIII input board.

-Check your voltages versus the values from the MkIII assembly guide (page 14) with a variac providing ~117VAC
-Make sure your negative feedback R (1k) and C (750pF) measure good. Eyelet 7 should measure a couple hundred Ohms to ground.
-Check the temperature of your quad cap, you should be able to touch it without problem.
-Check the temperature of your choke after running the amp for a while. You should be able to touch it with your finger for five seconds without a problem.
-Did you use a non-polarized cap after the phase splitter (between 6AN8 pin#1 and eyelet #1)?
-Did you use a non-polarized cap after the phase splitter (between 6AN8 pin#3 and eyelet #3)?
-Measure the cathode resistor between output tube pin#8 and ground with the tubes out.
-Measure the voltage drop across the cathode resistor with the amp running.

Let us know what you find.
...j

I know you say you rebuilt them...and the implication is that you used all new parts on a new circuit board. Still...check the 1.5 Meg resistor in the 6AN8 amplifier circuit. If it fails open, the symptoms would resemble yours.

Unfortunately I have loaned my variac out. But I will check all resistor values and continuity as well. I was also thinking about swapping input boards to see if the problem moves to the other amp. I brought the boards and added components to them myself. I did so because when I got them one had evidence of heat damage so I replace both. But to clarify the only original parts are all trannis both chassis tube sockets besides the input tube socket they are new everything else is new. I will report back tonight with my findings.

Thanks for all the help!

I will step in here in the 'stranger things have happened' view of this issue.


Check each board exceedingly carefully for any broken/cut/damaged traces
If these are double-sided boards, make sure that the solder has flowed through the eyelet completely.
Make sure that directional components (diodes, some capacitors) are installed correctly.
Look for broken wires - especially at the connection. Once upon a time, wire strippers looked like this:

http://www.futurlec.com/Pictures/HT-108_300.jpg   Badly handled, using this tool would leave a nick in the wire that can break when stressed at the connection. Further to this, in some cases, the insulation would 'bind', holding the wire in place so that it *looked* good. This is very much an issue if you are using solid hook-up wire.
Mis-labeled components - stranger things have happened. Look for _any_ minute differences between Board A and Board B.  
Make sure that the output transformers have been installed correctly... I have seen (NO kidding!) an assembler who wired his OPT right-to-left in the Right Channel and left-to-right in the Left Channel. He argued with me that it had to be that way....

Remember, stranger things have happened.

Best of luck. I have a bet with myself that this will be some sort of silly mechanical/assembly error.

BobtheBuilder wrote:Unfortunately I have loaned my variac out. But I will check all resistor values and continuity as well. I was also thinking about swapping input boards to see if the problem moves to the other amp. I brought the boards and added components to them myself. I did so because when I got them one had evidence of heat damage so I replace both. But to clarify the only original parts are all trannis both chassis tube sockets besides the input tube socket they are new everything else is new. I will report back tonight with my findings.

Thanks for all the help!

Time to install one of the old driver boards. It's not that many cables, and all components
is still on the oldboard i assume.

So tonight I swapped boards and the issue stayed on the amp no change. So I will assume it's not the board. The other board plays just fine on the other amp. That leaves me with sockets I guess. Checked diode and it's correct. Check voutages for the 10th time lol and all good. I cleaned out the sockets and checked continuity and all good. Checked for a short on the phono jack to chassi and there's not one. Checked wiring again and all good.
Guys I'm really at a loss here. It's a fairly simple circuit I should be able to figure it out.

Do you think I should still install the old original board?

BobtheBuilder wrote:So tonight I swapped boards and the issue stayed on the amp no change. So I will assume it's not the board. The other board plays just fine on the other amp. That leaves me with sockets I guess. Checked diode and it's correct. Check voutages for the 10th time lol and all good. I cleaned out the sockets and checked continuity and all good. Checked for a short on the phono jack to chassi and there's not one. Checked wiring again and all good.
Guys I'm really at a loss here. It's a fairly simple circuit I should be able to figure it out.

Do you think I should still install the old original board?

Have you checked everything else in that channel? Cables upstream, downstream, etc?

BobtheBuilder wrote:So tonight I swapped boards and the issue stayed on the amp no change. So I will assume it's not the board. The other board plays just fine on the other amp. That leaves me with sockets I guess. Checked diode and it's correct. Check voutages for the 10th time lol and all good. I cleaned out the sockets and checked continuity and all good. Checked for a short on the phono jack to chassi and there's not one. Checked wiring again and all good.
Guys I'm really at a loss here. It's a fairly simple circuit I should be able to figure it out.

Do you think I should still install the old original board?

No, you seem to already tried this.

How about the output connection ? Is the Anode and screen connections possibly mixed ?

Need to get more eyeballs on this. Put up some detailed pictures of the chassis wiring.

I have checked output and all is well. I will post pics tonight.

Did it work OK before the rebuild? Does anyone know of a safe way to bypass a tube socket? Can he use jumpers with clips from an output tube directly to the bottom of the sockets installed on the amp (this sounds lethal, I'm thinking NO!).

I would be interested in any tests that can be done. When I got the amp I did not power them on. I was worried I would distrtroy something tubes transformers ext...

Just thinking did you connect ground to one side of the output transformer secondary?

I did yes

I'm going to have to be admitted soon on reason of insanity. This is driving me crazy. I tried to post pics but have to wait for a few days.

Just an update I checked voltages from the power transformer. I swapped filter cap and it did not make a difference. I have checked polarity on the diode and all good. I checked the resistors from the input side of the kt88's Could the power transformer cause this? Should I swap the power transformer from the good amp?

So if you get ride of the spaces you guys can view the photos. I am new and cant post links yet. I hope Bob doesn't mind if so then I will remove it immediately.

I had similar symptoms when a chassis screw shorted an input wire. Have you checked input wiring?

I have and I ohmed the input to the output to the kt88s and it's the same on both amps. I also checked continuity to chassi and there is none except for the outer part of the jack of course.

If I'm getting the correct voltages from the filter can and rectifier can I assume the power transformer is good? There also no noises when nothing is playing it's a very quiet amplifier.

Carefully without killing yourself, take down voltage readings at various volume levels on the working one. Then take the same readings at the same levels on the bad one.

Pin 7 and 2 are 6.39 volts ac on both kt88 power tubes and pin 3 and 4 are 2.391 dc on tube close to transformer and on the other pin 3 and 4 it's 3.310 dc these pins are connected to the output transformer. These voltages did not change with loudness from the input. I haven't compared yet from the working amplifier. Today when I got back from my internship (still in collage) I tested the working amp again to make sure nothing has changed and it sounds wonderful I just wish I could get the other one the same.