2 Dynaco MkII's same issue

I recently purchased a pair of 1955/6 MKII from the original owner. stock including original Mullards . APair of PAM-1 with stereo adaptor and power supply ,thought I hit the mother load but that where the fun ends. Decided to do a complete restoration of MKII first. Purchased every part Dynakit parts has to offer minus transformers and chassis. Both amps are built and both amps have the same problem .will not bias. I did Bobs bench test on transformers and they seem good 873/ 885v on red leads , 5.5/5.4v on yellow, 7/6.5v on green and 58 v on r/b-r/y. Output trans resistance all fall under 150 ohms. 5/7 on speaker taps. I am getting very high voltage readings, 545/550 on pins 3-4 and negative 55v on power tubes. 549v pins 2-8 on rectifier tube. And 12.3 v pins 4-5 on 6an8 tube. These amps were in storage since late 70's . Is it possible that Transfomers have failed from sitting. Wish I could figure out how to post pic
Thanks

-55 volts on the grids of the EL34s should be more like -37 to -42 volts.

You are probably running your Mark IIs on higher AC line voltages than were common when the Mark II was built (120-130 volts compared to 110-117 volts in the early 1950s). That raises the numerical values of all the voltages in the amplifier. You are also probably (SHOULD BE!) using a silicon rectifier in the bias circuit instead of the old, stock selenium rectifier. That silicon diode will have less series resistance, and thus less voltage drop than the stock selenium rectifier. All this tends to make the voltages in the bias circuit more negative....i.e., the -55 volts instead of the correct -37 to -42 volts.

With that -55 volts on the grids of the EL34s, the tubes are being run near cutoff; i.e., not passing sufficient current, so the "Biaset" voltage you are measuring will not approach the desired 1.56 volts.

Does the voltage on the grids of the EL34s only vary more negative than -55 volts as you turn the bias adjust pot? That is, only between -55 volts and -63 volts or so?

If so, I suggest changing the fixed resistors in the bias circuit string. Increase the 1000Ω and decrease the 10KΩ so that the total resistance of the string still totals approximately the original 16KΩ (1000Ω plus 10KΩ plus the 5KΩ bias adjust pot). I believe Bob has had some suggestions in the past, as to exactly what values to use. This will bring the voltage closer to what the circuit really needs to see, while not adversely increasing the current flow through the bias windings in the power transformer.

You never told us: do the amplifiers actually amplify sound? Did they, before you tore them down?

Hope this isn't a double post . No amps were never tested . One of them looked like the multi cap had failed. Did not want to chance energizing 60 year old parts. Yes test where done at line voltage. I rechecked on a variac set at 117vac.
Power tube results pins 3&4 should read 460DC. Test 521VDC
Pins 5&6 should read negative 37VDC test ranged from -43/-65vdc with bias pot adjust.
Rectifier tube pins 2&8 should read 470VDC test 526vdc pins 4&6 should read 430VDC test 419VDC
I have installed Dynakitparts Bias circuit kit. 11.2 ohm bias resistor 10 k bias pots. Multi section cap PC boards. As well as tube sockets inputs and outputs .

your B+ voltage will continue to measure high until the tubes are biased correctly and pulling current.
Follow GP49s recommendations! change the bias resistor string until the bias voltage on pins 5&6 gets into the -35 to -40vdc range.

Hi,
The MK II was originally designed for EL34 output tubes. There are a number of
component value differences between the MK II & MK III. The original MK II used a 5K
bias pot as opposed to 10K with the MK III. Bias resistor on the MK II was 12 ohms
whereas the MK III used an 11.2 ohm resistor. The MK III was designed for 6550 or
KT88 tubes.

Do you still have a 50 ohm 10 watt resistor between the quad capacitor & pin ( of
the rectifier tube?

Are you using a 5U4GB rectifier tube?

Assuming you plan to use EL34 tubes...The MK III bias supply resistor values are not
correct. You should start with the original MK III resistor values which can be
found in the MK II manual. Replace the 4.7K 1/2 watt resistor with a 1K value. Also,
replace the 18K 1 watt resistor with a 10k value. On the PC board replace (C1) mica
cap 12 pF 600 Volt (min) rated with a 25 pf. Silver Mica is fine. You may get by
using the 10K bias pot by tweaking the bias supply resistor values..A bit of trail
& error.

You can retain the 11.2 bias resistor but I would set the bias at 1.12 vdc which
will be about 50 mA per tube.

If you can..Post a few photos...

Kevin

So far i have put stock bias pot back, original bias resistor , swapped 4.7k to 1K & 18K to 10K. You mentioned changing C-1 12pf. To 25pf. Do you mean C3 . C1 is 390pf
Thanks for all your help

Hi,
I'm quessing you replaced the original MK II circuit board with the MK III PC-1. I have no
idea what parts you purchased from us here at Dynakit without a name or parts order number.

Assuming this is the case...the only component difference on the MK II vs MK III circuit board
is capacitor C3 which is 25 pF on the MK II and 12 pF on the MK III. As I said before you can use
a silver mica cap here...the 12 pF cap on the MK III was rated 1000 vdc...so let's asssume this
is the same for the MK II. I would think a capacitor rated 600 vdc would work fine here. Ideally,
you may be able to remove & re-use this C-3 cap from your original MK II board.

I think once you change out this components you should be able to safely use the EL34 tubes. I
would not run these tubes at a bias setting of 1.56 vdc....I would suggest a bias setting of
1.20 vdc which equates to 50 mA per tube based on your 12 ohm bias resistor.

Let us know how this works out.

Regards...Kevin

I am about at my wits end. All of the resistors are corrected .correct bias pot , correct bias resistor. I tried both C3 caps from the stock boards no difference, bias voltage will only reach 0.6 vdc and the negative voltage still varies between -50--70 vdc. I guess I will have to order some 25pf caps nowhere local to source them . If that does not fix problem I do not know what to do.
I have also tried several sets of tubes with the same results
Ben B

A picture says a thousand words.

https://dynacotubeaudio.forumotion.com/t1084-how-to-post-a-photo-on-the-dynaco-tube-audio-forum

Babgrip wrote:I am about at my wits end. All of the resistors are corrected .correct bias pot , correct bias resistor.

Now, it's time to change the fixed resistors in the bias string off their original design values. This may be necessary due to the substitution of a modern silicon diode for the original selenium rectifier. Change 1000Ω to 4700Ω and 10KΩ to 5100KΩ. Retain the Mark II's standard 5KΩ bias adjust pot.

Once you get the bias voltage correct (at the grids of the EL34 output tubes), you should be able to adjust the BIASET voltage to correct; this will get quiescent current flowing through the output tubes, which will lower the B+ voltages.

Since I seemed to have purchased pc board. Bias kit. And bias pot. For a MKIII Would it not be easier to install the choke filter change output and rectifier tubes and make amp into a MK III, Instead of chasing this bias problem ?

Ben,
I agree with GP49 with respect to now tweaking the resistor values to achieve
the desired bias range for the EL34 tubes...You should have the 4.7K resistor from
the MK III bias kit. For the 5.1K resistor, assuming you do not have this value, use
(2) of your original 10K 1 watt resistors connecting these in paraellel to achieve
a 5K value. Try this on one of your amps and see if the bias range improves. You
need to get at least 1.20 vdc for the EL34 tubes.

However, it also is your option to turn these MK II amps into MK III amps. It will
require the addition of a choke and new tubes. I would suggest getting both these
amplifiers working correctly before investing more $$ into them.

I stock the 5.1K 1 watt resistors...Send me a PM with your address and I will put
these in the mail to you at no charge.

Kevin

Update I made all of the recommend changes . All I had on hand was a 5.6 k carbon -film 1/2 watt. Fired up Amp #1 bias pot set in the middle and bias settled out at 1.2 vdc. Problem solved . On to amp #2 bias shot to 6vdc then fuse blew before I could turn it off. Should note this amp had previously blown a fuse after about 20 seconds when I first rebuilt amp . Replaced fuse , this time it held but bias would not go below 1.7vdc heard a pop shut amp down fuse was still good . Wondering if I have a short in power trans . This is the amp that had a bad looking quad cap when I bought them.
My thanks to Kevin ,and GP49 for walking me through this.

Second event of the day , got a call from a local who asked me if I was interested in an Dynaco tuner. Turned out to be an unassembled FM-3 in the box complete with tubes traded him a pair of Bose 301 series-1 speakers . Told him it would fetch a fair price but he did not want the hassle of shipping and he loves Bose he's happy and I'm stoked, don't know if I should build it or not.

Ben,
That's good news on at least one of your amps....The other appears to have more
problems than just setting the bias. You should replace the can caps on both units
if you have not already done this. You should also at some point replace your 5.6K
1/2 watt resistor with a 1 watt rated value.

I also have a Dynaco FM-3 unassembled in the box. I hesitate to build this as the
value will drop considerably.



Regards,

Kevin

Pretty sure I bought the can caps from you. I have a spare transformer but the leads are really brittle bought when I was first have issues. Shrink wrapping leads now and will test it. Fingers crossed. Will be buying one from you if this one doesn't check out
Ben

Babgrip wrote:On to amp #2 bias shot to 6vdc then fuse blew before I could turn it off.

If this is the BIASET voltage you are describing:

Remove the output tubes and try again with a new fuse. Check DC voltage at the grid terminals of
the output tubes. Report back.

Did not see your post until after I had removed power Trans
With output tubes pulled I have 574vdc
Bias voltage tubes in 1.57vdc. I switched rectifier with several tubes RCA, Raytheon , Sylvania.
With the Sylvania bias was 1.3 vdc this is with pot full counter clock
All test done at 117AC

If you do indeed have "873/ 885v on red leads , 5.5/5.4v on yellow, 7/6.5v on green and 58 v on r/b-r/y" from the power transformer, it is very probably OK.

The BIASET voltage climbed to 6v and then the fuse blew. That indicates excessive current through one or both output tubes. This could be caused by a shorted tube, which was why the suggestion to pull the output tubes, then replace the fuse and turn the amplifier on again. If the fuse then held, a measurement of the voltage at the grid pins of both output tubes would tell you whether proper negative bias voltage was present; if NOT, even a GOOD tube running WITHOUT the proper bias voltage would draw excessive current.

Bench testing the transformer AC voltages are in spec. Transformer installed and
Output tubes removed bias readings -20.54vdc - -40.11vdc full range of bias pot.
With output tubes IN lowest bias reading with 2 sets of tubes is 1.7vdc
Same 2 sets of tubes in the other amp bias at 1.2vdc with bias pot 3/4 counter clockwise.
It would seem to me tubes are ok. Still something wrong bias circuit of 2nd amp

I'm wondering when I robbed the 25pf caps off the stock boards. one was good and the other one was bad.do you know if they are a part of the bias circuit

Babgrip wrote:I'm wondering when I robbed the 25pf caps off the stock boards. one was good and the other one was bad.do you know if they are a part of the bias circuit

They are not.

Amps are up and running Amp #1 bias resistors 5K pot, 5.6K , 1K Amp #2. 5K bias pot ,5.6k, 4.7k . Both amps running at 1.2vdc. Finally completed my dedicated vinyl system. (Still need to order proper rated carbon comp resistors) MKII's, Pas-2, A25 speakers and AR-XA turntable . Listening to the the Stone's Some Girls
Thanks again for everyone's help, this forum is best.
Ben

That's GREAT. Now, settle down and enjoy the music.

I have to admit, I do not know what resistor values are in my pair of Mk II in my everyday system. Through retubes, capacitor replacements, and some modifications, the bias system has been cooking along just fine on one of them since installation, and on the other, it has been doing the same since I experimentally installed independent bias adjustment. The amplifiers just go on and on, playing music! I haven't even repainted the cages, which are still in scarfed-up black, the way I got them. Too busy listening!