Rebuild of some really old MKIV's

Dear all,
When I first bought the old MKIV’s I was very confident…. a couple of weeks later I felt I just might have overestimated
my skills….. but then after all, it’s all about learning, isn’t it ?

My first problem about the wiring of the output transformer (faded colours) was kindly resolved by Kevin and Bob.
This amp is now ready for testing (after a triple & quadruple check of the wirings has been done)

Ready to start the second amp…..

This 2nd amp is probably of later manufacture as the power transformer has no colour coded “cloth isolated” wiring,
but rather plastic (PVC?) isolated wiring with different colours. It might also have been assembled by some other person
in the 1960’s, since the wiring has been done in a slightly different way.
The transformer is 230V of which there is no mention in the assembly manual. I guess there has been a “addendum” concerning
the 230V tranny, but I do not have that description. It might be fun to obtain a copy....

#1 I rewired exactly as it was. Noticing that there are two wires twisted together (white/black and yellow, as far as the faded
colours allow me to describe. It was there, so I left it there…. Same situation occurs with the 230V version of the VTA-ST-120
I built last year, which gives me confidence that it is the right way)

But in the second amp, I notice that these two (otherwise coloured) wires have been soldered to the GND of the 5-lug strip……
and there I have a question….. should this be done that way ?

Actual wiring of the PWT :

White/red  fuse holder
White/orange  on/off switch
Orange (1)  pin #4 of V1 (rectifier)
Orange (2)  pin #6 of V1
Black (1) (cloth isolation…..!)  pin #2 of V1
Black (2) (cloth isolation…)  pin #8 of V1
Light green  pin #2 of V2 (EL-34)
Dark green  pin #7 of V2
Orange/blue  GND
Black (pvc)  GND
Green/yellow  GND
Pink (!)  lug 4 of 5-lug strip
Of course I can copy this wiring, I just wonder if it is OK to have 3 PWT leads to GND……

Pictures to show both wirings (the new #1 and the still old #2) in the following link :

https://s861.photobucket.com/user/MarcVBelgium/slideshow/MARKIV-REBUILD---challenging


Thanks in advance for some good advice

BTW, I've been on the internet all day, could find some schematics (Dynakitparts and Triodeelectronics) for the PA-135

It seems though that both amplifiers have a PB-085 transformer....cannot find anything on these....

I have a feeling that I'm in trouble...... :-(

Marc,
To the best of my knowledge...the Dynaco MK IV 230 volt amplifier included a dual voltage 115/230 primary power transformer. The lead colors on your amplifier photo # 2 do not correspond with information I have including a sample of this transformer I have here on the shelf. All of the secondary lead colors should match the PA-135 (120 vac) version exactly.

The only difference is on the primary side...These (4) leads should be Black...Violet...Violet/White...Black/White.

Is it possible that this transformer is an after market replacement and not an original Dynaco product?

The wiring at the ground lug looks a bit odd.....Did you powered up this unit before starting the restoration?

Suggest you remove the transformer and check all the secondary voltages....Should be the same secondary voltages as the PA-135 plus some volts since you will not have any load. This will allow you to identify each lead regardless of the lead colors. A variac would be useful allowing you to test these voltages at let's say 10% of full voltage...Safety first.

Good luck...

Kevin

Kevin, thanks for your reply.

Black...violet....Violet/white....Black/white..... none of these occur on the transformer.
Getting 230V are wires white/red and white/orange.....
So, I'm in way over my head here.... (my skills are limited....I can follow instructions, but I
am not able enough to diagnose electronic problems...)

Both amplifiers have the same "PB-085" transformer, #1 has the cloth isolation, #2 has the
different colours (pvc isolation), none of which correspond with the original.. (not even the same as
PB-085 in amplifier #1). I guess these are aftermarket, can't find anything about "PB-085" on the
net, so I have no clue... Or, did a former owner had it rewired ???

Only one of the amps was powered up during a few seconds and, strangely, it was this one..(the least dirty of the two,
and the one with the red capacitor on the PC-board...)

Here's a link of the two, when I got them.. the most dirty looking is now the finished #1. I just wanted to save them
and give them a second life...:-))

https://s861.photobucket.com/user/MarcVBelgium/slideshow/MKIV-project-2014

As I have gotten this far (all parts except the transformers were bought from Dynakitparts in january 2014
and thus are new, including the chassis)

I will not give up on them..... !!

Just to make sure..... would a custom order of 2 PA-135 @ 230 V be considered ? Although it would cost me considerably,
it would, on the other hand, save me a lot of trouble. And I want these Dynaco's to "shine"

Marc

Marc,
They do look to be Dynaco made transformers...The NOS sample I have here has all PVC leads and is a dual voltage type.

Referring to the original PA-135 lead colors:

Black (cloth) is Yellow

Black (coth) is Yellow

Orange is Red

Orange is Red

L/Green is Green

D/Green is Green

Now this leaves (3) leads. If you look at the MK IV pictorial you will see that these (3) leads are grounded to the chassis either via the ground lug on the (5) lug terminal or via the jumper between terminals (2) & (3) to this common ground lug for the Green/Yellow lead (Center Tap) for the 6.3 vac filament voltage. The amplifier should work if you wire all three of these leads to the ground lug which this builder apparently did. What you don't know is which is the Green/Yellow lead which is typically connected to terminal (2). To correctly identify each lead or more important the Green/Yellow lead you must unsolder these (3) unknown leads from the ground lug....separate each lead and under power measure each lead to either Lug (2) or (7) of socket V2 or V3....you should get about 3 to 4 VAC. I suspect that your Green/Yellow lead matches the original Green/Yellow lead color if you want to just isolate and measure this lead first. trust this all makes sense.....Kevin

Kevin

Thanks for all the trouble you went through. I will follow your advice in the next couple
of days......I will take the time necessary. I am not in a hurry. (I have learned that being
in a hurry is "bad medicine".....)

I will inform you about my findings......

All the best

Marc

Hi Kevin

Thanks to your latest post, I think I have figured it out.....

See picture with comments :

https://2img.net/h/i861.photobucket.com/albums/ab172/MarcVBelgium/img484_zps4f938d8f.jpg


After all the searching, PB-085 is simply the 230V version of the PA-135 and the one in
amp #2 is probably a later version.
I found out that it has 12 leads (pvc) as compared to 13 leads on the older (cloth) version,
where 2 leads are connected to each other.

As I have no powercord attached (dismantling the amp), I will make voltage measurements
when the PWT is mounted into the new chassis. (both transformers are now being painted)

Thanks to you I am quite confident again... :-)))

As I said, I will not give op on them.... far too beautiful amplifiers !

All the best

MarcV; give all us an update on your project. Dale

Hi Dale
As requested.... both are ready to be tested. I will check each and every
wire and solderjoint before putting AC on them.

Next photo's should be those with glowing tubes.... I'll keep my fingers crossed :-)))


https://s861.photobucket.com/user/MarcVBelgium/slideshow/MARKIV-READYTOCHECK

Hi Bob, Kevin, Dale

Today was D-day..... final test of the MKIV's...... Music!

I must admit that I was extremely nervous....what if..... ?? It has been an adventure for me
and I thank Kevin and Bob for their advice.

Just to make sure, I hooked them up on a set of old system800 studio monitors by Tannoy.
These can be had for as little as 150-200 $ 2nd hand, so if I blow them up, that is not disaster...

There was no need to worry however........they sound magnificent. Upon powering them up,
I noticed complete quietness through the speakers..... and when I turned the volume up......
some heavenly sounds filled the room....

Must I say that I am very, very happy...?

Do they sound as good as a ST-120 ? NO !! and they should'nt! They lack the power and low
frequency control of the ST 120, BUT if one likes the vintage tube-sound "at its best", I can
recommend them.

Everything, except the power transformers (230V) and the output transformers , is brand new, including
the chassis, thanks to Dynakitparts.. Transformers where painted, to give them a new appearance.

That's it for now...... I'm going to listen for a few hours :-))))))) and probably hook them up to my main speakers.

https://s861.photobucket.com/user/MarcVBelgium/slideshow/DynacoMKIV-final-test

Marc

I forgot to mention the tubes used :

GZ-34 : SOVTEK
7199 : NOS SYLVANIA, US made
EL-34 : SVETLANA Winged "C"

:-))

Hi Bob

Thanks...!

The day I discovered your website, my life changed :-))))))

Thanks for the support, thanks for this forum

All the best

Marc

Good Job.......... an thanls for the pic....have fun with tubes







Thanks for the site Bob..................

Marc,
The amps look fantastic....You did a nice job restoring these units. I'm pleased to hear they are working well for you.

You may want to consider the "Yellow Sheet" mod to the rectifier tube. This can be found here on the forum. You have a very nice audio system.

Regards,

Kevin @ Dynakit

What is the "Yellow Sheet" mod; I didn't find that @ SEARCH. Dale

Booya...

https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod

Although both amplifiers sound perfectly, I'm still puzzled by one particular situation :

One one amp, I have to turn the bias-pot for about 1/4th turn to get 1,08-1,10 VDC (I read this
voltage in one of the comments by Bob or Kevin on this forum - 1,56V seems too much
for current production tubes)

On the other amp, I have to turn the pot for 3/4th (even a bit more) of the range to get the same result.....Why is this?

As I suspected the power tubes (Svetlana EL-34), I have just changed them with a matched quad brand-new
Electro-Harmonix...... with exactly the same result...... so the power tubes are beyond suspicion.

Where should I check next ?

Thanks

Hi Marc,
Check your B+ voltages & compare from one amp to the other.

Kevin

Hi Kevin,

Just to make sure that I make no mistake, could you
please explain the correct procedure ? B+ is to be measured
where exactly ?

Sorry for my inexperience....

Marc,
I suspect the power transformer secondary voltages may be slightly different. One may measure higher than the other resulting in a unequal bias setting with respect to the rotational postion of the bias pots. You would see this kind of difference when your output tubes are only matched as pairs...but you said these tubes were matched quartets which should not present this type of issue.

Referring to Page ( of the Dynaco MK IV manual you will see a voltage chart. I suggest you measure the AC voltages at socket (V1) of the rectifier tube. Measure pin (4) to ground and pin (6) to ground.....Both should measure around 370 volts AC. Insure that you have all the tubes installed when you make these measurements.

Even with all new components and equally matched output tubes in both amplifiers we cannot expect the bias pot's rotational position to be excatly the same for each amplifier. There are tolerances in the components such as the resistors (bias pots) and output tubes to take into account. I would however expect them to be within 20% of each other assuming you do not find a significant difference in the secondary voltages of the power transformers.

Have you tried swapping the 5AR4 rectifier tubes?

Kevin

Hi Kevin,

Again, thanks...:-))

Swapping the rectifier tubes makes no difference. Settings remain the same on each amplifier.

Tomorrow (21.00 here now) I will open them and measure the voltages.... will keep you posted
on the result..

They DO sound terrific BTW, perfect stereo balance , nice detailed sound....

To be continued...

Marc

Here's a pic of the "yellow sheet" mod ...



Basically, all you're doing is using unused pins on the socket as convenient points to install the diodes. Main thing is to get the polarity of the diodes right.

Hi Kevin, Bob

I'm glad I've mentioned the difference in bias setting on both amps,......
and that I asked you for some advice.

The results :

Amp # 1 (this is the one with the "old style" cloth wiring on the PWT)

V1, pin 4 : 374 Volts AC perfect
pin 6 : 374 Volts AC perfect

But then it comes .... :-/

Amp # 2 (the one with the PVC wires on the PWT) and the one
where I have to turn the bias pot way up (3/4)

V1, pin 4 : 398-400 Volts AC (25 V too high)
pin 6 : 428 !!! Volts AC (54 V too high)

I have no clue as to the history of both amps, but I'm getting more
and more the impression that PWT # 2 has been "rewound" somewhere
during its lifetime (which would explain the discrepancies in the color coding
of the leads)

I guess in # 2 , I would be burning one rectifier after another.....?

Not being a technician, I am surprised that the amp with way too high
voltages, is the one where I have to turn the bias pot way up.....

Is there any solution to this problem ?

Marc

Hi Marc,
These voltages are much too high...and will be a source of problems for this amplifier.

Typically both voltage readings at Pins (4) & (6) should be the same and what you measured on Amp # 2 is a bit strange suggesting this power transformer was wound or (re-wound) incorrectly. I also suspect all of the other secondary voltages are high....

Before throwing this transformer out the window...let's do another voltage test. Same voltage measurements as before at both pins (4) & (6) to Ground @ socket V1. But this time...Remove all of the tubes.

I would expect these voltages now to be measureably higher (no load) upwards of 450 vac.

Let us know what you find.

I would keep this unit open and ready in the event we need to make more measurements.

Kevin