tube choices

aliveatfive · 1 tube choices on Sat Mar 21, 2009 3:35 pm

aliveatfive

Posts: 4
Join date: 2009-03-21

I constructed a new ST70 about 18 months ago using Roy's board and transformers from Dynakitparts.com. The amp sounds really good. As a matter of fact, I sometimes prefer it to my basement system which is worth about 50k! Associated equipment is a Grounded Grid preamp, Sangean HD tuner, Theta Miles CD player and LS3/5a speakers from Stirling.

I'd like to try output tube rolling in the amp and I have some questions about biasing it. I have a 2 spare sets of new-stock output tubes: Tung-Sol 6550's and Gold Aero KT66's. If I use either of these 2 sets, do I still set the bias to the same value as the EL34's? Do any resistor changes have to be made with the changed output tubes? (I do have a new power xformer with increased amperage rating.) Thanks for any help, in advance.

Bob Latino · 2 Re: tube choices on Sat Mar 21, 2009 8:32 pm

Bob Latino

Admin

Posts: 249
Join date: 2008-11-27
Location: Massachusetts

Hi,

In your VTA ST-70 with the upgraded power transformer from Dynakitparts you may use 6L6, KT66, EL34, KT77, 6550 or KT88 tubes. There are a lot of thoughts as to the correct bias current in an ST-70. Dynaco ran their tubes pretty hard with higher bias voltages than what might be practical by today's standards. My recommendations are to (and this is for a VTA board driven ST-70 not a stock ST-70 with the 7199 driver borad)

Bias the 6L6 or El34 tubes at .40 volts DC per tube (40 milliamps per tube)
Bias the KT66 or KT77 tubes at .45 volts DC per tube (45 milliamps per tube)
Bias the 6550 or KT88 tubes at .50 volts DC per tube (50 milliamps per tube)

* A note on the KT88 and 6550 tubes - Don't run these tubes if you have a stock PA-060 power transformer. They draw maybe 30% more current and will put a strain on the stock power transformer which had virtually no reserve. You could damage the stock power transformer.

You can run the 6L6, EL34, KT66 and KT77 tubes with no resistor changes. You MAY be able to also run the KT88 and 6550 tubes in your amp also with just a rebias change. If the KT88's or 6550 won't bias or are at the very end of the bias pot rotation then you may have to alter R39 on the VTA driver board. In there right now you have a 10K resistor. You have to LOWER the value of this resistor to 7.5 K or 5 K. One trick to do without having to remove the board (R39 is under the lip of the chassis cutout and would be difficult to get out without pulling the driver board) is to solder on the BOTTOM of the board on the two solder connnections of R39 a 27K resistor which will give you 7.3 K total resistance for R39. If that isn't enough then instead solder on ANOTHER 10 K resistor which will give you 5 K total resistance for R39. Don't go any lower than this because if you do then EL34's my not bias now.

Bob

1 tube choices on Sat Mar 21, 2009 3:35 pm

aliveatfive

2 Re: tube choices on Sat Mar 21, 2009 8:32 pm

Bob Latino