Is it possible to use 6550 tubes in Dynaco Mark II amps? These were
originally made for EL 34 tubes, but someone told me that if the bias
was changed to 1.92 mv, 6550 tubes work fine.

????????
Tom

The bias needs to be in the -50 to -70 volts region.

What you are calling "bias" is probably the voltage across the
odd-value cathode resistors in the stock amp. Dike those out _right
now_ and replace them with some rational value so you can easily deduce
the quiescent current through the tubes.

If the bias supply is brought to a value which is appropriate for the
6550 and if the heater draw does not appreciably exceed that of the
EL34 it will function okay. Get out your tube manual and look.

Relying on "what people tell you" can cause no end of trouble.

wrote:

Is it possible to use 6550 tubes in Dynaco Mark II amps? These were
originally made for EL 34 tubes, but someone told me that if the bias
was changed to 1.92 mv, 6550 tubes work fine.

What do you mean by changed to 1.92mV?

This amount of grid bias voltage is quite wrong for any tube.

Anyway, 6550, KT88 could be used to replace EL34
if there is enough heater current available because the 6550/KT88
need 1.8A for the heaters. So two 6550 will need the extra 0.4 amps
but usually this extra amount is OK in most amps designed to power
a couple of EL34 at 1.6 amps each, plus maybe a few other tubes.

KT90 are an even better choice because it has 1.6 amp heaters.
I have even tried them in Quad II amps and got 26 watts instead of 20
watts.
Ppl tell me KT90 give better bass.

What you ** must ** do when changing tube types in amp is to adjust the
bias currents
to be the same as what the manufacturer intended for the original
specified tubes.

So if your dynaco was supposed to have 40mA of cathode current in each
EL34 tube, you have to make sure
the grid bias is adjusted until there is 40 mA in each 6550/KT88/KT90.

These three types of tubes will need a greater negative grid bias applied
than EL34.
There may not be enough adjustment in the adjust pots to get the wanted
cathode current.
BTW, the cathode current consists of the totoal of plate and screen
currents, and is usually
measured by measuring the voltage across a 10 ohm cathode resistor,
so that 0.4Vdc across 10 ohms = 0.04 amps = 40mAdc.
Measurements are all done with zero signal present.

I don't recall what a mkII has for bias adjustment, but there should be a
bias
adjustment for each output tube, if not, then fit them, or if you can't,
have a tech do it.

Patrick Turner.



????????
Tom

"François Yves Le Gal" wrote:

On Thu, 04 Aug 2005 02:25:54 GMT, Patrick Turner
wrote:

usually this extra amount is OK in most amps

I'm not familiar with the MKII, quite a rare beast, but Dyna power
transformers are usually very poorly dimensioned.

I once bought an ST70, and sure, the feeble power tranny
hummed and ran hot.
During a complete re-enginerering exercize to make it into a stereo
integrated amp with all triode input tubes,
I rewound it with a 50% bigger stack of iron, and
that cured the problem.
But despite the heat it had survived for years running almost too hot to
touch,
and perhaps a few more degrees wouldn't have mattered.
The main reason for the rewind is that it was a 110V mains model,
so changing it to accept tyhe 245V we get here was only sensible.

A MkII schematic is at

http://www.geocities.com/vintageaudio/markii.JPG

It clearly shows a common cathode R with a test point to
measure cathode current.
There is just one bias adjust pot for both output tubes.

This is a rotten way to build any tube amp.

I have serviced many amps with common cathode connection
and only one bias supply and nearly all
need premature re-tubing because the tubes have substantially
different bias currents for the one applied grid bias voltage.

What it needs a rebuild so there are two 10k wire wound bias pots
and separate 10 ohm Rk and two test points.
Alternatively the circuit could be used as shown at

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

The above is an integrated circuit, so the preamp could be left out if
desired.
The bias is balanceable, and in my case I use a transistor circuit ( not shown
)
to show the state of balance between the Ik of each output tube.
So if an imbalance of 5mA occurs in the plate currents of the tubes, an led
turns on,
and the owner simply adjusts a pot to make both leds go out, and
he then knows the balance is OK. If he cannot make both leds remain
extinguished
after pot adjustment it means one of the tubes is stuffed, and he needs to
take the amp to an amp doctor.

Patrick Turner.