A few questions re. restoring a Dynakit mk II

Chris Scott wrote:Hi everyone. I'm restoring a pair of Dynaco mk II amps, and just have a few questions. Though I have built a number of tube guitar amps, I'm not very experienced regarding electronics, so please go easy on me... 8)

My first question is re. the diode - will a 1N4007 be an acceptable retrofit?

Next is the 12 ohm bias resistor - as I have a set of simple octal bias probes, will these not obviate the need for installing them? ...and might I assume a 60% pd figure for a set of JJ E34L's operating in this environment to be reasonable?  

These amps came to me with the original speaker terminal strip having been replaced with a simple set of red/black speaker screw terminals. As the owner has (and will use exclusively) a pr. of 8 ohm speakers, am I wrong in thinking that I would connect the black OT lead to the black (neg) terminal, then the orange (8ohm) secondary to the red terminal (which then connects to eyelet 7 on the pc board) with the 680 ohm resistor bridged across the 2 speaker terminals?

I hope I was clear enough here, and thank you in advance!

Chris  

Chris Scott wrote:Hey Peter thanks so much for responding!

Re the bias setting question - using a probe is no big deal for me, as the adjustment pot is accessible from the top of the units - I just wanted to confirm that I could omit the 12 ohm resistor and take my readings from "upstairs" if you will...

I knew I could omit the tube rectification, but as I had a few anos 5U4's and there's a place to put 'em, we'd go with glass right across the board.

...and I feel a bit stupid for not realizing beforehand that the 680 ohm was for the nfb circuit....the mere fact that I'm working for the first time on a tube hi-fi amp rather than a big ol' nasty distortion generator like a guitar amp has somehow confused me a bit...

Thanks again mate!

Chris Scott wrote:Ok cool, I will leave the 12 ohm in...just to be clear, the bias probes are not meant to be installed permanently - not sure if you knew this - and I'm curious - can one use the probes with the 12 ohm resistor installed? - they're basically just extensions with a 1 ohm resistor from the cathode to ground, so I'm wondering if the 12 ohm resistor would interfere with the reading taken with them?...

...my ignorance is indeed quite obvious here, so thanks in advance for your patience. 8)

PeterCapo wrote:I am not exactly sure what these bias probes are, to which you refer.  I *think* it sounds like you plug your bias probes into the octal sockets for the power tubes, then plug the power tubes into the bias probes, and then set the bias, is this correct?  I think I have seen these kinds of things while looking around online.

I’d want to know what (as in voltage or current) and where (as in which corresponding tube pins) your bias probes are supposed to measure, so that you’d know what value to adjust for.  What is the "60% pd figure" to which you referred, please?  I think you are referring to power dissipation, is this correct?  Dynaco's original specification for biasing the Mark II's pair of EL34/6CA7 power tubes is 130mA (130mA combined total for both power tubes, or 65mA for each power tube).  You could calculate the power dissipation from there.

It drives EL34 hard, and some feel today's current production EL34 can't take it.  If this is a concern for you, an alternative is the Electro Harmonix 6CA7 (big plates). You can also run 6550/KT88 with an adjustment to the bias as described in the manual.  If you lower the bias current to go easier on the power tubes, the distortion rises, but, again, there is debate on how much this matters.

Dynaco used their Biaset scheme (where you retain the 12Ω resistor), which was very simple to use.  Instructions were included in the original Dynaco manual.



My suggestion, if you have not already done so, would be to obtain a copy of the original manual and, if possible, the corresponding pictorial wiring diagrams for your particular Mark II.  This should help with all phases of your restoration.  The tricky thing is that I know of at least three or four different original variations of the Mark II.  Adding to the confusion is that I have seen Mark II manuals accompanied by the wrong pictorial wiring diagrams.

Any chance you can post a series of photos of your Mark II top and inside?  Might be helpful in figuring out what you have.

Try this for starters, but we should still try to identify your particular Mark II: http://www.tubes4hifi.com/Dynaco%20MK2%20manual.pdf

peterh wrote:The 12 ohm resistor has a history behind :

Bias is supposed to be adjusted by the user, and the user may not have an calibrated good voltmeter.
Here dynaco came with a brilliant idea : using a standard drybattery cell as referens, any meter
could be used to adjust the bias, only make it as large as teh voltage of a battery cell ( 1,56V )

Today measuring voltages with precision is a nobrainer with any cheap DMDM. But not 1958!