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Dynaco Mark III Classroom

A 430V secondary could give up to 600V for a few seconds for 117V mains with a GZ34 (as the capacitance is low, and the signal valves often take a few secs longer before starting to load B+, and there may be no other loads until then, and the transformer may provide more than 430-0-430 with no load). At the moment I don't think you know what your mains voltage is ? So I'd suggest a 600V surge may not be sufficient margin. Normal modern capacitor rating has a +10% surge spec (ie. 605V for a 550V nominal), so if they say surge is only 600V then maybe there is little tolerance above 600V.
 
Donstaff said:
Any comment on the first part of my question about the first two can capacitors', Dynakit's 23222-6 vs. their 298906's suitability for the Dynaco Mark III?
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Which cap would give you the most peace of mind when turning on your amp? Which one are they suggesting by their own caution note? Do you know your mains voltage? Mine runs 123VAC, that’s pretty common. Any plans to use SS rectification? The answer to the last two questions could also lead you towards the higher rated cap.

What would I do, I’d likely get the -6, beefing up the power supply is a safe bet.

Bill
 
FWIW... I use 5-watt wire wound & metal oxide resistors in many of the bigger amp power rails. They seem to last longer. :idea:

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Just got a very nice (supposedly working) PAM-1 to go with my "Dynaco Mark III Classroom" project specimen. Until I get the Mark III up and running, how can I safely check out the PAM-1?

Don
 

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with an ST-70 :)

it needs an external power supply, so unless you happen to have either a bench supply, another Dynaco amp, the matching Dynaco supply, or feel like building a functional clone of the Dyna supply, you really can't do any functional testing on it.
 
Notes for the drawing, you just add the 10M ohm resister and .22 uF film cap circuit between the input, ground and the existing 470K resister of each amp.
 
Schmidlapper said:
“Dave Gillespie pointed out in his ST-70 white paper, and also true for the MK III that the PAM 1 2 and 3 preamps had no DC blocking and recommended a DC blocking circuit be added to the input.


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Schmidlapper,
In reference to Dave Gillespie's ST-70 white paper and your attached circuit diagram, I see that on the Dynakit PAM-1 schematic, there is a 0.1mf capacitor and a 62K resister insures with the output signal. Could you explain the exact relationship between the schematic and the DC blocking circuit in your Feb 24 post? IE, what replaces what in the schematic.
 

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I don't have the PAM1 schematic too reference. The blocking circuit added to the inputs of the Dynaco is of no consequence when not needed. Not sure if you are saying your PAM 1 has DC blocking on the outputs?
 
Here is the PAM-1 schematic, very poor image, even in the original manual, but in mower left corner is the output jack and the 62K resistor and 0.1mf capacitor. Hope you can make it out with a magnifying glass.
 

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Yes that is a cap in output signal path. As I was repeating what I read, I can't say I'm familiar with the PAM 1 preamp at all. I will ask what is the source of the schematic? Could it be an update or for an improved version? The DC blocking for the input is still a safety device, I Don't have any PAM preamps but still added it in the event one of my solid state preamps were ever leaking significant DC. Maybe Dave can shed some light on the PAM 1 regarding DC.
 
Yes and I’m likely mixing up the PAM and PAS. It was the PAS 1, 2 and 3's that Dave referred too as not having DC blocking on outputs, fixed later. You can add it or not, it’s another layer of safe guard.
 
Schmidlapper said:
Yes and I’m likely mixing up the PAM and PAS. It was the PAS 1, 2 and 3's that Dave referred too as not having DC blocking on outputs, fixed later. You can add it or not, it’s another layer of safe guard.
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Iirc, the later PAS had the blocking cap output network.
Dave's addition deals with any source feeding it that may have DC bias in the output. It may be redundant if one knows their sources to be DC free.
 
Pio1980 said:
Iirc, the later PAS had the blocking cap output network.
Dave's addition deals with any source feeding it that may have DC bias in the output. It may be redundant if one knows their sources to be DC free.
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The PAS2X and PAS3X both had the blocking cap (1uf just before the 62K resistor on the output).

I don't know for sure if any of the non-X models had this- but to my experience, none of them I have ever seen, did.

When doing a straight-recap of one of these (leaving the original circuit with the tone controls intact), I tend to incorporate a simple solution, on these earlier PAS2 and PAS3 models- adding a small terminal strip near the output RCA jacks, and putting a 1 to 2uf cap in-line with the green and red wires going to the output RCA jacks. This leaves the original 510K resistor on the output jacks to act as a DC ground for the output, while blocking the slight DC offset, coming from the tone feedback circuit, from the preamp output.

Here's a picture of one, that's on my bench, right now- the green caps on the output jacks, are the new output DC-blocking caps.

dyna_pas2_output_dc-blocking_cap.jpg

Regards,
Gordon.
 
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Now that I'm turning my attention back to my Mark III, I've gotten lots of good advice, especially thankful to Retrovert and others, on addressing the fact that this amp was designed for 117 vac vs. the 125-125.5 volts that I get. I am investigating use of 1 watt 2 ohm resistors, a thermistor like the CL-80, or using autotranformer wiring in that amp to knock that voltage down to something it would be happier with. Just wondering if any Dynaco addicts have used any of these methods and what your experiences have been. I'd really appreciate hearing from you.

Don
 
In the "Dynaco PAM-1 Update" thread, also here under Tube Audio, Retrovert suggested the use of a thermistior to control the initial voltage spike at power turn-on of the Dynaco Mark III. He later suggested using a timer like the 555 (bistable with an RC constant) to trigger a solid-state relay (AC relay using a TRIAC, SCR, or something similar) to bypass the thermistor. He added that an even better approach is to hard-wire in a large-wattage resistor (60 V = 1 A × 60 Ω) to limit the voltage to 50% of normal and then switch on a bypass to take the resistor totally out of circuit using a pair of SSRs. Entirely avoids the thermistor.

This sounds like an elligant and clever solution, but it is a little over my head. Does someone have a drawing and schematic showing how this would wired into the power supply portion of the attached schematics?

Thanks,
Don
 

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Is a bucking transformer module beyond your pay scale ? That would operate all your equipment at vintage mains voltage level.
 
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