Dynaco MK III modifications

[riverwarrior]

I have recently purchased a pair of Dynaco Mark III amplifiers. As purchased, neither amp was in working condition. Both have previously been modified by converting to metalized film resistors and upgrading the key capacitors (.1 and .22uf poly and 60uf electrolytics). This is all the modifications that have been applied to the first amp. I have replaced the tubes and changed the high voltage can capacitor, and this amp is now functional. The second amp has had further modifications done. An attempt was made to convert to a regulated B+ supply, eliminating the GZ34, and requiring the addition of time delay relays and much additional circuitry. I have the original documentation that describes this upgrade. This modification was performed in 1985, and when plugged in, it didn't work. The amp has sat in a box until I purchased it a few weeks ago. I am debating the prudence of troubleshooting these modifications as opposed to stripping the mods and recreating the original amplifier circuitry. Any input would be greatly appreciated.

Louis

[bear]

The MkIII is not a favorite of mine, although it does have more power than the ST-70. Same circuit though. The stock circuit was never that good. No matter what you do, you are going to need two identical amps to use them in stereo. So, that is a consideration. You might want to post the modded units schematic?

Generally speaking the stock driver board leaves much to be desired. Dyna used a "predistortion" circuit that tried to overcome the frequency response limitations of the output iron - this is what the extra caps are doing in the low level circuit. The goal back in the day was to get ruler flat "20-20k" response. I'd throw that idea out the window in favor of better transient response and less feedback. But doing that is a completely different amp using the same PS and output iron.

A whole lot depends on what you want and what you are willing to do.

_-_-bear

[20to20]

Restore it with the original component values and own a classic. Or mod it and have some bogus amp that you cannot rightfully call a Mk-III.

Changing the caps and resistors with new but same values is not a "mod". It's just modern maintenance.

20

[Original Burnedfingers]

Quote:
The MkIII is not a favorite of mine, although it does have more power than the ST-70. Same circuit though. The stock circuit was never that good. No matter what you do, you are going to need two identical amps to use them in stereo. So, that is a consideration. You might want to post the modded units schematic?

Generally speaking the stock driver board leaves much to be desired. Dyna used a "predistortion" circuit that tried to overcome the frequency response limitations of the output iron - this is what the extra caps are doing in the low level circuit. The goal back in the day was to get ruler flat "20-20k" response. I'd throw that idea out the window in favor of better transient response and less feedback. But doing that is a completely different amp using the same PS and output iron.

A whole lot depends on what you want and what you are willing to do.



I would read what Bear posted.

About the only good thing on any Dynaco is the output transformer/s. The driver circuit while usable back in the 50's leaves a lot to be desired. While some view this and anything made by Dynaco as a classic when put up against something with a decent driver circuit its totally left out. I'd build a decent driver circuit and let the amplifier sing.

[Conrad Hoffman]

I had a pair of these way back when; probably not the pinnacle of tube amps. I replaced all the resistors and smaller caps with big metal films (watch the voltage ratings- I think you need RN70D) and film caps. IMO it sounded worse. Still, it's not that much work to bring them back to stock. I'd do that and confirm that everything works correctly before starting any new mods. You don't want to start a project and discover a bad tranny.

[Defiant]

Norman Koren describes some mods to a Mk III on this page. I have a fully balanced circuit I have devised that would work really well (it's not a Mk III mod; it is designed around Plitron outputs, but it could be used with Mk III iron), but it requires a total of 5 small signal tubes PER CHANNEL! I could trim it down to 4 at the expense of some gain, which I may do. Right now it reaches 70 watts with only 640mV of input, and thats with 12dB of balanced non-global negative feedback.

I would start with Norman Koren's mods first. My bet is that they will yield improved performance over the original Dyna circuit.

[kstagger]

lots of input boards out there, replacing the stock circuit with something better. Then I would replace the power supply capacitors with something like the SDS board. New tubes... and then sit back and enjoy 'em.

The output iron is pretty good - the pair I refurbished certainly produced better sound than my ol' (modified) Dynaco 70.

[riverwarrior]

Thanks for the responses. I do not plan on changing the audio circuitry at this point. The signal path is still original on both, and I plan to get these working before I attempt any signal path modifications. My question lies in the power supply. The article is entitled "Improving the Dyna Mark III Power Supply" by James E. Boak and appeared in "The Audio Amateur" Issue 1/1978. It really comes down to removing the tube rectifier, replacing it with diodes, and using a simple regulator on the B+ supply. I debate the wisdom of proceeding with this modification on the basis that I am not 100% sure that it has been assembled correctly and it may be more worth my time to return it to the same setup as the working amp than to fix it, and modify the working amp.

[Eli Duttman]

The PSU caps. Dyna used were right at the ragged edge of WVDC limit. The lower forward drop of SS diodes makes installation of higher WVDC parts or assemblies necessary.

If you regulate B+, you MUST regulate the bias supply too.

My preference when converting Dyna amps from vacuum to SS B+ rectification is switching to combination bias on the O/P tubes. A parallel network of a 100 Ω resistor and a 470 μF. cap. between the O/P cathodes and ground "eats" the extra Volts SS rectification yields and provides a very convenient test point for the setting of the "idle" current. The RC bias network also allows the single bias trim pot. Dyna provided to perform better, as some compensation for the differences in tubes is provided automatically.

Make the 1st filter cap. small, say 4.7 μF., and replace the OEM choke with a 5 H. part of appropriate current rating. Pile the energy storage up in the reservoir position. A somewhat small 1st filter cap. holds the rail voltage down and also makes for a cooler running power trafo.

[riverwarrior]

Eli,

Why MUST the bias supply be regulated in combination w/ the B+? It makes sense to regulate both, but what mandates it?

I do not understand why using a small filter cap would reduce the steady state voltage, or increase the steady state current.