6550 tubes in a Express

[wpaulvogel]

Just need advice. I’m wondering if the output transformer can handle the extra current. I’m running a Pacific 6.6k. I know I have to change the bias supply for more B-. I’m pretty sure I need to lower the grid leak 220k’s to 100k. I don’t want to even attempt it if I risk damaging the transformer. Thanks for any suggestions.

[Roe]

probably not a problem at all. 300ma seems sufficient for 6550s. and they do not draw that much heater current comparred to el34s

[wpaulvogel]

I’m really just concerned about the output transformer. I believe the power transformer has plenty of steam to feed them, I don’t want to exceed the rating of the output transformer.

[pompeiisneaks]

Do you have the specs on the transformer? It looks like 6550's pull about 10W more max current than the EL34's can. (25W vs 35W). I would expect that if you run it less than max volume it will definitely be fine, but if you push it at max and don't know the current capacity of the OT, you may have problems.

~Phil

[martin manning]

6550 is a 42W Pa max tube. That does not matter though, as the power through the OT depends on plate voltage and load impedance, which are unchanged. The thing to be aware of is the idle current. I would bias them on the cool side to avoid overheating and/or early saturation.

[M Fowler]

I ran 6550's in my amp for years, I did lower the 220k/220k power tubes bias resistors down to 150K/150k.

Mark

[pompeiisneaks]

6550 is a 42W Pa max tube. That does not matter though, as the power through the OT depends on plate voltage and load impedance, which are unchanged. The thing to be aware of is the idle current. I would bias them on the cool side to avoid overheating and/or early saturation.

Odd the JJ datasheet I just pulled up said max diss is 35W per tube.. maybe the NOS ones were better? Probably.

~Phil

[martin manning]

Well then, the JJ is not a 6550 :^) EH and Tung-Sol current production are 42W.

[wpaulvogel]

The output transformer is supposed to be exact specs of a 14415. I had them wind me some that are a different number and I got two from R.J. They all measured the same DCR, turns ratio, etc. they are all the same dimensions. Mine have the same color wires as the originals. R.J.’s have different color wires.
Anyway, they are probably about the same. The DCR is 47 ohms from start to CT and 49.1 ohms from CT to finish. I do know the Heyboer transformers have twice the DCR. If I had them I would worry more about the current. I’m thinking the Pacific is rated for about 150mA DC.

[wpaulvogel]

The tubes are Groove Tubes 6550 R rated 6. They are pretty hot, I stuck them in there just for a minute and with the bias pot maxed at about-35 volts, I was seeing about 90mA. I hit a G chord and wow!! SHUT IT DOWN. I did this about a month ago. I left it alone and started the amp with 2 EL84 output. I’m very impressed with it and now back on the 6550 thing.

[redshark]

Has anybody ever run KT66s in an Express?

Does it bias with normal secs or do we need to change resistor in the bias supply?

Thanks!!

[wpaulvogel]

Have to change the bias supply. I stuck some in and they draw too much current. I’ll do them again when I change up for the 6550’s.

[statorvane]

Odd the JJ datasheet I just pulled up said max diss is 35W per tube.. maybe the NOS ones were better? Probably.

The RCA 6550 tube data sheet indicates a maximum plate dissipation of 35 watts, 44 watts triode connected.
http://tdsl.duncanamps.com/dcigna/tubes ... 550-2g.gif
http://tdsl.duncanamps.com/dcigna/tubes ... 550-3g.gif

The GE 6550-A indicates a maximum plate dissipation of 42 watts, triode or pentode connection.
http://www.mif.pg.gda.pl/homepages/fran ... /6550A.pdf

[pompeiisneaks]

Odd the JJ datasheet I just pulled up said max diss is 35W per tube.. maybe the NOS ones were better? Probably.

The RCA 6550 tube data sheet indicates a maximum plate dissipation of 35 watts, 44 watts triode connected.
http://tdsl.duncanamps.com/dcigna/tubes ... 550-2g.gif
http://tdsl.duncanamps.com/dcigna/tubes ... 550-3g.gif

The GE 6550-A indicates a maximum plate dissipation of 42 watts, triode or pentode connection.
http://www.mif.pg.gda.pl/homepages/fran ... /6550A.pdf

Interesting, so even then it depended on the manufacturer.

thanks!

~Phil

[Tony Bones]

35W Pd is a Design Center rating. 42W is Design Maximum.

Supposedly, the first 6550 was released by Tung Sol in 1955 with a Design Center rating of 35W, (though I have not been able to find an original datasheet.) By 1963 the Tung Sol datasheet was rating the tube at 42W Design Maximum.

https://frank.pocnet.net/sheets/127/6/6550.pdf