Poseidon/MarkIII startup problems

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Poseidon/MarkIII startup problems

Postby phatbob808 » Sat Mar 10, 2012 11:31 am

Hello all!

I'm new to the forum and new to tube tech. First off, allow me to thank all of the knowledgeable folks here who have educated me thus far as a learker. I have acquired a pair of Dynaco MarkIII's and, with careful research, have restored/upgraded with Poseidon boards and SDS Labs power supply capacitor boards, and ALL NEW parts. The only remaining original parts are the chassis and transformers.

At first startup, both amps suffer the same symptoms:

(1) I have no bias voltage at front pre-amp socket test point... further inspection shows no voltage at pins 1/8 of output sockets. I read 10ohms through either test point

(2) Voltages don't check out:

Power transformer 2nd's are about 400VAC each leg for HV, and heater voltages are about 3VAC per leg.

Voltage at bias diode look ok at about 56VAC cathode and -76vdc anode.

Output sockets pin 5 or 6 are about -54vdc

Posiedon:
H= -60vdc
F&G= -59.8vdc or so once balanced

So... my questions are as follows:

(1) Could the power tranny voltages be sagging due to the series cl-80 thermistor on the mains? 2-3 volt drop leaves about 115v-116v measured at the primaries.

(2) What might cause the absence of voltage at output tubes pin 1/8?

These conditions occur with or without tubes (rectifiers have yet to be installed) and are currently present in both amps... which leads me to believe that I may have made a mistake, or possibly have overlooked something.

Many thanx in advance for any help with this matter, and best wishes to all,

Bob
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Re: Poseidon/MarkIII startup problems

Postby Ty_Bower » Sat Mar 10, 2012 1:16 pm

phatbob808 wrote:(2) What might cause the absence of voltage at output tubes pin 1/8?

These conditions occur with or without tubes (rectifiers have yet to be installed) and are currently present in both amps... which leads me to believe that I may have made a mistake, or possibly have overlooked something.


It sounds like you took the same exact restoration route as did I with my Mark III. I think you'll be very happy with them once they are up and running. Although I only skimmed through your post quickly, it looks like you've got a good handle on where the negative bias voltages need to be. It is wise that you check the bias supply before you install the rectifiers.

The voltage drop you expect to measure across the 10 ohm cathode test resistor is a result of current flow across that part. With no rectifiers installed, there will be no B+ and consequently no conduction (current flow) through the tube. Under these conditions it is expected that voltage measured at the test point be zero.
Last edited by Ty_Bower on Sat Mar 10, 2012 1:17 pm, edited 1 time in total.
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Postby DeathRex » Sat Mar 10, 2012 1:17 pm

You're not going to have any voltage on the cathode (pins 1 and 8) until you have the tubes in, a nice red glow from the filament, B+, and a high enough bias. With the tubes in, power on, and bias at maximum (-59), put in your multimeter probes to test for cathode voltage, wait about 12 seconds, start turning the bias up (-59 to -55 or so). At -55 to -52 you should start seeing a voltage at the cathode, might just be 0.2VDC. Keep going to about 0.5V, and see how the tubes are doing, no red plates, or bright blue inside the tube. You will see a very slight blue inside the plate structure and maybe on the glass.

Make sure to have a preamp hooked up and speakers/8 ohm dummy load connected. Test the sound. Mark IIIs can get very loud, keep the volume low on your preamp.

If everything is OK turn up the bias to at least 1V, at most 1.4 volts.
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Postby Ty_Bower » Sat Mar 10, 2012 1:20 pm

DeathRex wrote:If everything is OK turn up the bias to at least 1V, at most 1.4 volts.


I think you are thinking of the original Mark III circuit, where the output tubes were biased together as a pair. On Shannon's Poseidon board, each power tube wears its own individual 10 ohm bias test resistor at the cathode. You don't want to bias the tubes so hot that they are pulling 100 ~ 140 mA per tube. KT88 should be biased at roughly half that.
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Postby phatbob808 » Sat Mar 10, 2012 3:15 pm

Thanx for your reply guys... and quick too! That's too funny cuz i just sat down to update my post with respect to my oversight/rookie over-caution. Upon further review of the schematics, I determined the test point would not show a voltage without a rectifier. The good news is that I'm currently monitoring one of said amplifiers at 700mV, and balanced nicely! So much information to digest all at once for a newbie!

Deathrex... I've learned quite a bit from some of your previous posts, and very much appreciate your help today.

Ty... Yes it does resemble your build so much that i have saved links to many of your posts to aid me along the way.

I want to thank both of you for your efforts and for paving the way for guys like myself. I have watched and learned as you have, and hope to pay it forward as you and many others here have done. Also special thanks to Shannon Parks for the collective info I have learned here at DIYTUBE and for the kick-a$$ Poseidon drivers (which look cool too.)

I'm usually pretty sharp, and I'm pretty proud of making it this far for a first timer. Just get worried about potential hazards to my new babies! I'm going to set bias on the other amp later, and hope to post pics too as these were a full-on restoration and look great too. But for now, its beautiful outside and I promised my dog a long walk if he would bear with me until I got the bias problem cleared. C'mon Meathead (Otis the Boston Terrier,) lets celebrate with a long walk.

Thanx again guys, and as always, keep on DIY,

Bob
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Postby phatbob808 » Sat Mar 10, 2012 3:27 pm

Oh yeah, I forgot to mention...Cryoset's Genelex Gold Lion KT88 Russian repro's and some collection of vintage 12AU7's and 12AX7's. Sooo... 70mA, or 700mV on the 10ohm 1/4 watt test point as per Poseidon circuit integration? Also any recommendations on driver tubes???

Thanx,
Bob
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Postby DeathRex » Sat Mar 10, 2012 6:59 pm

I like to run the outputs a bit cooler, like 600mv, just to make them last longer.

As for driver tubes, I wonder if you can rewire the Posiden to use a 6CG7 in place of the 12AU7? I don't know what it is, but I just love them.
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Postby phatbob808 » Sat Mar 10, 2012 8:04 pm

"Cool" Deathrex! Pun intended... I was curious about running them cooler for extended life, thanx for verifying.

I wonder if running them near the 70mA mark for the first dozen hours or so might help with break in, then you could throttle them back to settle in for the long haul. Hmmm??? I'm curious cuz I'm new to tube tech and things like this will run in my mind all day. That's why I'm a DIY... it's an addiction and I'm a sick man! Sometimes a curse cuz I'll second guess myself and feverishly hunt for an answer to the point of obsession.

ANYHOW... I forgot to mention thanx from your previous post noting necessary input and load conditions for start up biasing. Although I failed to mention, I did in fact have a cheap speaker connected to the 8ohm tap, and input was shorted with terminator plugs. I considered getting an RCA plug and soldering a jumper across it, but found some plugs on the phono jacks of a vintage preamp.

It's good to reiterate what may seem obvious to some, so that those who are reading along can be better informed. I did A LOT of reading about these amps, and at some points I had myself turned inside out over it. So it's great to have forums like this and people like yourself, Ty Bower, and the many knowledgeable others here.

So after biasing second amp (took about an hour to stabilize,) I turned off the amps to cool while a set up a spot to run them with signal. If they sound good for a bit, I'll hook 'em up to the Bozaks and hear them sing!

I'll check in later.

Thanx,
Bob
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Postby Shannon Parks » Sun Mar 11, 2012 7:09 am

Nice work, Bob! I would use "80% max dissipation at idle" as the max bias point for any particular tube type. So if your B+ was 440VDC and you were using the KT88s (40W max), you could go up to:
(40/440)*0.80 = 72.7mA.

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Postby Ty_Bower » Sun Mar 11, 2012 9:44 am

DeathRex wrote:As for driver tubes, I wonder if you can rewire the Posiden to use a 6CG7 in place of the 12AU7? I don't know what it is, but I just love them.


The 12BH7 can pretty much just drop right into the Poseidon circuit without any changes. The Uncle Ned (Tubezone) version of the Poseidon build instructions recommends it and specifies a few slightly different parts in the schematic, presumably to further optimize the circuit for this tube.

There's a whole family of tubes which are similar, but none quite exactly the same as the 12AU7. There's a little reading on it here: http://www.diytube.com/phpBB2/viewtopic ... 9&start=10
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Postby phatbob808 » Wed Mar 14, 2012 10:19 am

Thanx Shannon,
that's great info. It seems that 20% headroom is logical; and practical to so many applications. Oh, Ohms law, what can't you do!!!

Ty_Bower and Deathrex,
Interesting mention of 6CG7, and 12BH7 as I have a couple of each. I will research these approaches out of curiosity, but intend to enjoy the circuit as is for now.
I guess what I meant to ask was if anyone had preferred 12AU7 & 12AX7's for the Poseidon/MarkIII, as I have a box of both to choose from... an assortment of manufacturers.

I will be designing a scratch build eventually, but intend to make baby steps so that I may enjoy these pieces for a while. Gives me time to research and afford another project, as well as tidy up on a few others.

Oh yeah, and as for my MarkIII's... They are wonderful, and powerful. I have them teamed up with a pair of Bozak 302A Urbans with a few mods of their own (transient response is amazing with these.) I happened across a B&K Pro-5 pre-amp that was selected for its pre-amp bypass for passive volume control (and it was free $) With this combination, the music not only has clarity and well defined soudstage, but is airy to the effect that a guest asked me if the speakers were connected... lol, he thought the music was coming directly off the amps! LMAO!!! But really, the sound is not "driven" but seems to naturally emanate through the listening space. Airy indeed, at least at comfortable levels. Jack up the volume (reduce attenuation ;) or disengage bypass if you wanna be blown out of your seat!

Well worth the efforts and investments. I am very pleased indeed. Did I mention how thrilled the room mate is? I made him sit down and mirror every step of the build on one of the MarkIII's. He's a recently converted audiophile with no electronics background! A great entry level build and mod experience, even for the beginner!
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