Quad Cap Challenged

Greetings, I am trying to replace the quad cap in a modified Dynaco MK III with sets of series capacitors.  Could someone with a little bit more knowledge of this circuit please explain to me exactly what is the "first capacitor"?  I have seen this referenced a couple of times within this forum.  If possible even a reference to the old quad cap may help.

The "first" capacitor would be the 30uF tied directly to pin #8 of the rectifier. If you download the datasheet for a 5AR4 or GZ-34 you can see what the maximum capacitance that you can safely directly connect to pin #8. That spec is usually in the 50-80uF range which is more than most people around here feel comfortable using.

Are you using series pairs of electrolytics to increase the effective voltage rating of your caps? If so I assume that you will add balancing resistors to make sure the series pairs divide your high voltage equally between them.

I'm with you... I always replace the quad caps with high quality discrete electrolytics. Who knows what lurks is inside those $ilver cans!

here's part of the ST70 schematic, original on top with "quad" cap, and an upgraded cap replacement shown below,
with the "sharing" resistors mentioned to equalize the voltage and current across the bigger caps.
All caps should be rated 350 or 400vdc, and resistors can be 1/2w 270K

Yes, this should do it!  Thanks Beede.  Thanks Roy.

Interesting to me that the SDS board for a Dyna MK III uses 100uf 400vcaps and with 330k 1/2 W "sharing resistors".  Any idea which spec is better for the Dyna MK III?  The rectifier tube was replaced by diodes long ago in this modification.  In fact I prefer it.  Bass to me seems tighter/faster.

tubed1 wrote:Interesting to me that the SDS board for a Dyna MK III uses 100uf 400vcaps and with .1uf 630v "sharing resistors".  Any idea which spec is better for the Dyna MK III?  The rectifier tube was replaced by diodes long ago in this modification.  In fact I prefer it.  Bass to me seems tighter/faster.

As I noted earlier the spec for the maximum capacitive load on pin #8 of the rectifier can be found on the manufacturer's datasheet. I have an effective 47µF on pin #8 which is well below the spec for the 5AR4.

If you are using silicon diode rectification alone you should be running EL-34 or KT-88 outputs to put up with the plate overvoltage--unless the circuit has been otherwise altered. The "yellow sheet" modification uses both: silicon diodes to "pre-rectify" the high voltage ac then a 5AR4 to delay the B+ so the filaments get a chance to do some heating and to provide IR drop to keep the B+ in a range that can be applied to the plate of 6550.

You can get even faster/tighter bass with a solid state regulated power supply. BJT outputs instead of tubes even better yet.

Beede, what exactly is a BJT output?

I'm not sure exactly what the FET is doing in this circuit. it's a mystery to me but I certainly do like how it sounds. Output transformers at turn on and idle run like they are out of a meat locker.

OK Beede, I get it BJT - A bipolar junction transistor (bipolar transistor or BJT) is a type of transistor that uses both electron and hole charge carriers. In contrast, unipolar transistors, such as field-effect transistors, only use one kind of charge carrier. For their operation, BJTs use two junctions between two semiconductor types, n-type and p-type.

BJTs are manufactured in two types, NPN and PNP, and are available as individual components, or fabricated in integrated circuits, often in large numbers. The basic function of a BJT is to amplify current. This allows BJTs to be used as amplifiers or switches, giving them wide applicability in electronic equipment, including computers, televisions, mobile phones, audio amplifiers, industrial control, and radio transmitters.

In this unit there is a JFET. Things have changed since the 80s!

An interesting school of thought regarding losing the rectifier tube and quad cap/power supply upgrades:

The original Dynaco quad capacitor has always been the weakest link in the original
Dynaco design due to its high ESR (series resistance) and marginal voltage ratings
(especially when challenged with 21st century AC line voltages). The high ESR results in
high internal temperatures that severely compromise the amps reliability. These problems
persist even with both NOS and new production “quad” capacitors designs as the high
ESR is due to the physical design of the capacitors.  Besides reliability issues there are audible problems with the original capacitor caused by a rising impedance characteristic that is well within the audio spectrum resulting in loose bass response and dull high frequencies.

On another note, many thanks to Bob for this site and esp. his post regarding a red plating output tube indicating bias circuit problems. (I never would have found the bad output tube resistor without it!)

As for my MK3s......I do agree with you about losing the rectifier tube ...from a sound stand point..
I have been running my old Stock tube amps an preamps on Varics for years.... if not the 3s B+ moves over 5oov......an for the best sound too my ears...the 480-90v .. B+ on the plats .... is tops from a sound stand point....
.....the 6550..KT88 just sound better with grid 2 at the lower volts....as for the caps....I find the Quad cap can sound fine....but my 3 pr of mk3s all run the stock 6AN8 driver board.... if you like the 2 tube drivers...
you may hear a diff...for sure...
all just one mans finding.........





thanks for the site Bob........

For way less $$$ than a quad cap you can use modern electrolytics that will easily fit under the chassis without modification. Using ten 47uF (buck each) 450WVDC radial caps per amp will give you a ~800V, 47-23-23-23 set of caps that will give your MkIII another 50 years of life. Isolate the quad cap and leave it in place if you like that "look".

Full disclosure: I am not a fan of HT or (450-550V) quad caps

Agreed Beede.  At some point in your amplifiers life all owners are faced with a re-cap.  Tube amplifiers can run components and capacitors hard!  High heat does not help.  Even long life caps are only good for 5000 hrs.  With the physical size of capacitors going down and values going up.  It's easy to hide those caps under the chassis.  Even in a MK III.  Boarding those caps/resistors makes everything even more manageable.

some of my radial favorites:

Chemi Con
Rubycon
Panasonic
Nichicon Muse or (FG) Fine Gold for smaller values

FYI:

Here's a shot of my homemade 47-23-23-23µF cap boards. Total cost for two boards was about $35. They easily fit inside the MkIII chassis using the old selenium stack screw hole for mounting--no chassis mods.

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...and

As you can see I left the old quad caps in place to serve the purpose they are best suited for (in my opinion)... filling in the chassis hole that would otherwise be left behind :-)

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Vary nice.......what the B+ on the plates an grid 2...with your cap setup....?
I have had a lot of tube amps over the years......I don't know why the MK3 turn me on...hehe
As far as 2 output tube amps.....EL34..6550s..Kt66...Kt88....go...tuff to beet .....
last pr of 3s I pick up....I gave $100 for the pr....they had not had the cage are bottem removed for 50 year....I let that 1960 air out......it still around here some where..............................life is good
Thanks for any an all input on getting better sound out of tubes......




thanks for the site Bob..................

Yes indeed. A mighty fine looking pair of mono-blocks there Beede. I'm sure they sound wonderful as well. Just curious, you seem to have turned a quad cap into a quint cap. What are you doing with that extra set of series caps? Is that all B voltage?

Hi Tubed1  --- Beede is using ten 47uF at 450WVDC radial caps per amp which gives him about ~800V, broken down as a 47uF-23.5uF-23.5uF-23.5uF set of caps.  

Two 47uF caps-in-series yields 23.5uF.  

Taking four caps, with two-in-series, in parallel with another two-in-series will yield the 47uF section of his board.

I have built several pairs of them and they work well in my Mark IIIs.

Hope this helps.

zx wrote:Vary  nice.......what the B+ on the plates an grid 2...with your cap setup....?
I have had a lot of tube amps over the years......I don't know why the MK3 turn me on...hehe
As far as 2 output tube amps.....EL34..6550s..Kt66...Kt88....go...tuff to beet .....
last pr of 3s I pick up....I gave  $100 for the pr....they had not had the cage are bottem removed for 50 year....I let that 1960 air out......it still around here some where..............................life is good
Thanks for any an all input on getting better sound out of tubes......
thanks for the site Bob..................

The MkIII is a favorite of mine as well. My VTL, Audio Research, Jolida, and Yaqin amps have all come and gone... the MkIII remain. I do regret selling my Heath W-5 with WE350 outputs.

Just in case you missed it, this Heath does look intriguing:

http://www.ebay.com/itm/Dodd-audio-custom-tube-monoblocks-heathkit-w2-AMAZING-/121900232188?hash=item1c61d1edfc:g:rxoAAOSw3xJVeap7