What should I do with my Dynaco MKIII's

[DaveM]

I am a complete noob to tubes. (that has a nice ring to it doesn't it? Tube noob) I was just gifted a pair of Dynakit MKIII's. They have a full set of low hour chinese KT88's in them. Otherwise they are totally stock. One channel won't bias up high enough (runs out at about .96 volts with a target of 1.56) I have found a bias repair kit for about $5 per channel which should get me going again. Likely a good place to start, but I am up for a project. I know of several board upgrade options, but I am looking for opinions and guidance on money best spent to make these things sing.

[kmtang]

If I were you, I would fix it and sell it. Then, take the money to build a very nice one.


Johnny

[Palustris]

"One channel won't bias up high enough "

Go ahead and rebuild the bias supply, but I would guess that the reason the amp won't bias properly is the tube NOT the bias supply.

[stokessd]

A bad bias supply will cause way too high a reading on the biasset pin, not too little. Too little is a weak rectifier, or a weak output tube (or pair)

I don't know if they have gotten better, but I watched fire shoot out of a friend's Citation II in the 1990's due to a chinese KT88 failure. I do not use chinese power tubes as a result.

Sheldon

[DaveM]

"Bias Repair Set for Mark 3Replaces selenium bias rectifier with silicon diode, also includes resistors & capacitors used in bias circuit that are not on the printed circuit board. Parts Included: 1K 2W Resistor, 18K 2W Resistor, .022uF 630V Capacitor, 100uF 100V Capacitors x 2pcs, & 1N4007 Diode. Some of these parts are also included in the Dual Bias Kit on this page.One set required per amplifier"

Looks like this should fix the bias issue. I swapped all the tubes from the good channel to the "bad" and it didn't fix the issue, so I was leaning towards a rebuild, but this little bias repair set seems like a no brainer. But from there....

[Conrad Hoffman]

I had a pair of MkIIIs at one time and they sounded decent. I rebuilt them with better resistors and caps, and added a fan. No doubt you're better off without the selenium rectifier. I can't comment on the Chinese tubes; I had mine back when you could buy new brand name KT88s for $20 a matched pair or thereabouts. I'd at least check the main filter cap sections. Keep 'em or sell 'em? Depends on your commitment to tubes. I found they heated up the room too much and had high operating costs to keep them running at their best. The bass was always a bit muddy at best.

[bear]

The bias supply should be rebuilt as a matter of course if it is still stock with a selenium rectifier - that means it is very old.

The voltage you read on the "bias" test pin is actually the cathode current. It is the voltage across the cathode resistors. CHECK THEM. They are often bad.

When the current is higher the voltage red is higher.

The VALUE of the cathode resistor was selected at a time when VOMs or VTVMs were standard, not DVMs, so it was hard to reliably read voltages much below 1 volt - at least for most kit builders.

You can improve the bass by reducing the cathode resistor value by a factor of 10 or more - but it is a good idea to put a sticker on the chassis to make sure that you don't forget the change and a future buyer has a shot at not killing his/herself trying to bias the amp!

The wattage of the cathode resistors was selected to act as a bit of a fuse in the event of a tube short - not a bad idea since the outputs can and do short, although not very often... it keeps the amp from frying...

So, check and change the cathode resistors... especially if after you fix the bias supply (you can check it now, it ought to be around minus 70-90vdc (iirc) on the high side of the bias adjust pot) it still doesn't appear to bias properly.

It's a good amp to get ur feet wet on - but keep your hands out of it when it is on, you can get a nasty shock or worse. Be very careful when testing voltages, especially on the bottom. NEVER hold the chassis with one hand when testing with the other.

_-_-bear

Oh, Iirc, the Mk II wanted 6550/Kt88 while the MkIII wants 6CA7/EL34 - they won't make full power without the proper tube with proper plate impedance, but they will work ok...

[Bandersnatch]

Originally Posted by bear

_-_-bear

Oh, Iirc, the Mk II wanted 6550/Kt88 while the MkIII wants 6CA7/EL34 - they won't make full power without the proper tube with proper plate impedance, but they will work ok...

The Mk.ii and Mk.iii had what was effectively the same OPT. U-L tapping slightly different is about the size of it, and it was the ii that came with EL34's.
cheers,
Douglas

[tubelab.com]

I don't know if they have gotten better, but I watched fire shoot out of a friend's Citation II in the 1990's due to a chinese KT88 failure. I do not use chinese power tubes as a result.

The 90's vintage Chinese KT88's will remind you that the Chinese invented fireworks! I got a "deal" of 50 of them in the mid 90's. They look well built, but some will red plate at under 30 watts. They will also self destruct without warning. I made guitar amps with them, running them very conservatively at 370 volts in a 35 watt amp. Over half of those tubes failed in the first few months, and one even shattered while playing. Most failed spectacularly with heavy fireworks inside the tube. I still have about 20 of them stashed somewhere. I switched to Sovteks and there were no more fireworks.

The recent vintage Chinese power tubes seem to hold up pretty well. I haven't had any self destruct despite my total ignorance of the maximum ratings. The photo of the Chinese 6L6GC was taken a few years ago. Those tubes had already served a tour of duty in a Fender Bandmaster. I have since used them for several "experiments" where the lives of the tubes are at risk. They are still alive today!

[john curl]

Bear got the numbers reversed. The KT88 goes with the III.