Rebuild of some really old MKIV's

Hi Bob, Kevin,

Thanks for the reply.

@ Bob : I would immediately order two new transformers (which would
solve the problem and make sure that both have identical PWT),
BUT .....230V on a PA 135 transformer would mean custom winding...
would that be possible ? If so, this may be the safest way to go.

@ Kevin : With no load pin #4 rises from 400 V to 418 V
pin #6 rises from 428 V to 455 V

This is way off....... maybe the PWT was rewound with a MKIII PWT as an example,
who knows....

I'm keeping it open and on the bench for further exploration following your guidance :-)))


Marc

Marc,
This confirms both the voltage readings are above normal and unbalanced. There is no easy fix for this.

Looking at your most recent assembled amp photos...From what I can see, this is representitive of the PA-135 power transformer and the lead routing is as the original with all leads exiting just one side of the transformer. Dimensionally, the MK III power transformer would not fit the MK IV chassis.

I suspect this power transformer was re-wound ...whether they used the MK III power transformer specs...we will never know...but the unbalanced high voltage secondary was just a winding mistake.

I have a dual voltage 120/240 vac NOS PA-135 transformer on the shelf. I would need to bench test this to insure it meets specifications.

I will check out this transformer over this weekend if your interested.

Regards,

Kevin

Hi Kevin,

YES, I'm interested..... please check it out for me.

I will contact you via email to make further appointments.

This is the best solution and I cannot give up on these beautiful amps,
so let's put in a new transformer :-)))

Thanks a million times.

Marc

In a couple of weeks I hope I can post my comments on a happy ending to this challenging
project.
The PWT Kevin mentions in earlier posts, seems to meet all specifications, so it
can replace the one that is wrongly rewound...

Can't wait :-))

To be continued.

A happy Dynaco addict....

they look beautiful . nice job . i have refurbished mark 3's, a stereo 35, and an sca 35 my favorite amps are my prized mark fours . you will have many pleasurable hours of listening ahead . enjoy yourself . pete 

The new (NOS) transformer arrived today. Thanks to Kevin.

By the end of the week, I hope to report about a happy ending of this adventure, resulting in two beautiful mono amps.

Marc

The new power transformer has been mounted today.

B+ voltages are 374,5 vac on pin 6 and 376,5 vac on pin 4

While I'm writing this, I am listening to Mark Knopfler & Chet Atkins through the MKIV's.....Very pleasing indeed !

Current tubes :

GZ34 : Sovtek
7199 : RCA (NOS)
EL-34 : Electro Harmonix

Here is a link to the pictures of my work today.

https://s861.photobucket.com/user/MarcVBelgium/slideshow/MK4-PWT-FINAL

HAPPY END and many hours of pleasant listening ahead of me.... (when not using the ST-120)

Thanks for all the help and support.

Marc, thx for sharing your journey with all us! Great job.. just today ordered Bob's ST120. Like you, can't wait for that tube sound. Chet?? go now! May I suggest Elvis' "Dixieland" ? Horns..... Dale

Everything seems ok now with my new babies mk4

I think that on this forum, somewhere, I read that biasing should be at 1,08 vdc
instead of 1, 56 v that was recommended originally. Story goes that new tubes could not withstand this level of bias.....

Could anyone confirm to me the optimum level of bias I'm supposed to set ?

Marc

Hi Marc,
Happy to hear all went well with the replacement power transformer. I see you did install the (2) CL-90 Thermistors I sent you...I guess my projected voltage drop was pretty close based on testing this unit here at 120 vac. Sometimes the 50 HZ can make a difference.....here in the US we are 60 HZ.

I agree with Bob & others here on the Forum that a lower bias setting will benefit both the amp & tubes without much sacrifice of sound quality...The 1.08 vdc bias setting will equate to
40 mA per tube which is within spec most all new production EL-34 tubes and most variants. Just a side note regarding the Dynaco MK IV amplifier. Although virtually one halve the ST-70 with both units sharing the same A-470 output transformer and basic circuit topology....the Rated power of the MK-IV is noted to be 40 WPC where the ST-70 was rated at 35 WPC. The additional 5 watts is due in part to the higher bias setting of this unit whereas at the specified 1.56 vdc setting each output tube was running at nearly 58 mA as opposed the ST-70 which was 50 mA per tube.

Enjoy the amps!

Regards....Kevin