Triode ST 70 Bias Expansion

[kingmatt]

I think I have the king of stupid questions to ask, but that has never stopped me before...so...

I have a Triode El. ST 70 kit (EF86 driver board, SDS Labs cap board, SS rectification through a Weber plug-in rectifier) that I'm tinkering with various tube substitutions in. I've got some 6BG6GA tubes in there now and they seem to want to draw quite a bit more current (1.35v across 10 ohms with 435v B+= 28 watts plate dissipation per tube, spec is a max of 20 w) than I'm comfortable with, even with the bias controls turned fully CCW. Granted, I've been told that these tubes are a bit hardier than their spec sheet suggests, but I'd like to get that bias setting down to more reasonable levels.

I've been reviewing the documentation that came with the driver board, and it suggests that I swap out the bias adjustment pots along with the bias resistors to the left and right of the bias test points (I don't have any resistors there, it may have something to do with the Triode kit and associated modifications). What I do have for "bias resistors" is 2 10 ohm resistors going between chassis ground and pin 8 (I think?) of the output tubes on each side.

My question is this: can I get away with swapping those bias resistors out for 20 ohm units as a means of dropping the bias current and skip replacing the bias pots with 20k or 30k ohm units? I've been pricing out parts and really would rather not spend $10 each on new pots when I may or may not like the new tubes and want to change things back. I do realize my bias voltage readings would have to change (1.7v across a 20 ohm resistor for 18w of plate dissipation). Considering the nearest purveyor of electrical bits is roughly 2 hours away, I'd like to know if I can get away with not ordering pots? Am I missing something here? Thanks in advance...

[20to20]

Changing the 10R bias resistors to 20R won't provide any additional range. They are only there to provide a small test point voltage drop to read, not create a biasing voltage at the cathode. Instead of changing the biasing pots you could add 1K, 2K, 5K on up a little at a time in series to the 10K R to ground coming off the pots until you hit the new biasing range you want. Remember you must have a higher (-) negative biasing voltage to tame the current. So don't put the new resistors in on the wrong side of the pot or you will get the opposite effect.

This is assuming you have the stock 2 pot circuit, still.

[kingmatt]

Thanks a ton for the explanation of where I went wrong-I had myself convinced I was smarter than I actually am. This isn't the first time... I will order the proper parts at once.

Changing the 10R bias resistors to 20R won't provide any additional range. They are only there to provide a small test point voltage drop to read, not create a biasing voltage at the cathode. Instead of changing the biasing pots you could add 1K, 2K, 5K on up a little at a time in series to the 10K R to ground coming off the pots until you hit the new biasing range you want. Remember you must have a higher (-) negative biasing voltage to tame the current. So don't put the new resistors in on the wrong side of the pot or you will get the opposite effect.

This is assuming you have the stock 2 pot circuit, still.

[mesherm]

In the bias circuit the adjustment pots are in between two 10k resistors. The maximum bias voltage is just after the diode rectifier so to increase the bias negative voltage lower the resistance of the resistor that has one lead connected to the diode rectifier. Raise the other 10K by the same amount. If you were to change the resistor at the diode to 5k and the resistor at ground to 15K you would get a 26% increase in maximum available bias voltage. Be carefull of some GE 6BG6GA tubes. I had some that would not bias and some that went red plate after about 10 minutes.
The old 6BG6s work good as well as the set of Raytheons I used in this amp.
http://home.earthlink.net/~mikesherman/images/New6BG6GA-amp-2.jpg

[kheper]

Here is the very simple, bias expansion mod from the dynaco doctor:

http://www.dynaco-doctor.com/ST7%20Bias ... %20Mod.pdf

[burnedfingers]

The Dynaco doctor explanation and voltage points only applies to using EL34's in the 70. When using 6BG6GA's and other tubes you will have to change the bias resistor to allow for a different adjustment range or better yet change out the stock pots in favor of 50K pots. I will have to take a look at mine to see what resistor value I used in order to use either EL34's or 6BG6GA's. I ended up using two resistors in series and then I installed a switch to short across one of the resistors in series so that I could use KT88's in the amp.

[burnedfingers]

Caution

Using 6BG6GA's

Use only 6BG6GA's that are Sylvania or ones that you have tested for a length of time and they didn't red plate. There are some for sale on the internet from a gentleman in missouri that have the large 7027 plates in them. These are the only ones to use in your amp.

[burnedfingers]

One has to remember the bias current flow increases as the negative voltage goes toward zero or decreases. You could probably remove both 10K resistors and simply install 50K pots.

[burnedfingers]

In reading this thread I saw a comment on the 10 ohm resistor from cathode to ground. Its only function is to provide an easy means to see what the tube is biased at. The original was a 15.6 0hm and the reading was supposed to be 1.56 volts across it or 100mA for the tube pair. Changing the cathode resistor to a 10 ohm at each tube will not change the bias range at all and reading the voltage on this resistor of .500 volts will mean it has 50mA running thru it. Its simply Ohms law.

Now going to triode mode does up the current draw some from the ult mode so a bias adjustment when going to triode mode may be necessary as not to run the tubes too hard.

On checking MY 70 with 6BG6GA's in it I am seeing roughly -43.4 volts to bias it up to 50 mA per tube. I modified my bias circuit with 50K pots and changed the 10K resistor on the diode side to roughly 5K total using two resistors one of which I short across when using KT88's in it.

[kingmatt]

Thanks everyone for the replies. I changed out the bias resistors as directed and have tons more negative voltage to control bias now. After going through a few questionable NOS tubes, I found a set of Sylvania 6BG6GAs that don't redplate like the Raytheons I had did. I'm running them on the cool side (14w dissipation per tube...any more and I get a very minor streak of red on one of the tubes) and am not totally convinced that these tubes are the way to go. They are approximately as pleasing to the ears as the 6CA7s I had in there, but they lack the overall power and bass.

In any event I've learned a ton about how this part of the amp works and thank everyone who replied for their time. All is well for now until I screw something else up!

[burnedfingers]

Thanks everyone for the replies. I changed out the bias resistors as directed and have tons more negative voltage to control bias now. After going through a few questionable NOS tubes, I found a set of Sylvania 6BG6GAs that don't redplate like the Raytheons I had did. I'm running them on the cool side (14w dissipation per tube...any more and I get a very minor streak of red on one of the tubes) and am not totally convinced that these tubes are the way to go. They are approximately as pleasing to the ears as the 6CA7s I had in there, but they lack the overall power and bass.

In any event I've learned a ton about how this part of the amp works and thank everyone who replied for their time. All is well for now until I screw something else up!

It sounds like you do not have the heavy duty 6BG6GA's that I have talked about on diyaudio.com You need to purchase your 6BG6GA's from
SND Tube Sales. He has the heavy duty ones that have the 7027 inners and do NOT red plate. I run mine at aproximately 485 B+ with 55-60 mA per tube. I have run them much much harder in my Quicksilver mono block amps that I have modified. You get the right ones and they do NOT red plate

[burnedfingers]

http://imageshack.us/photo/my-images/801/6bg6ga.jpg/

Tube on right is not heavy duty. Tube on the right is the one with 7027 inners. It is the one you want.

[Shannon Parks]

My advice is to go back to the 5AR4 rectifier. The higher B+ on the screens is probably your "red plate" problem. Also screen stability could be part of the issue. Checkout Dave Gillespie's great AudioXpress article. You'll need four 100 ohm, 1W resistors. I would further suggest maybe switching out the octal sockets to 807s which would be a bit beefier than the 6BG6s. That is just more of a personal preference than a practical one - the old JAN stuff is just pretty cool.

I'm actually working (err, have been - another backburner type project) on a 1625 ST70. The 807 and 1625 are the heavy-duty JAN-style tubes and the 6BG6s are the cheaper octal variety. Lots of good info here: http://en.wikipedia.org/wiki/807/ATS25

I am using some cheap RCA 6BG6s called Concert Masters in my Clementine ULtra right now. I'll probably revert to my stash of Russian 6L6GAs.

Shannon

[burnedfingers]

Like I said its all in getting the correct 6BG6GA. The older ones were not HD. I have given the supplier in MO and they are $10 on the last ones I bought. You can push the heck out of these and use the SS rectifier WITHOUT a problem. I have run mine at the setting I posted now for years without red plating and or any problems. I did go thru trying the older 6BG6GA's like the one in my picture on the right. Do not try to use these old tubes.

[Shannon Parks]

Interesting: I didn't see any probs on the screens of my 1625s on a test just now going from 400V to 500V at the same dissipation (i.e. the screens stayed at 1.5W when at 23W overall). But running a hotter bias caused the screens to be unstable. More research is needed. While one tube may prove to be more stable than another, if there is an underlying instability we can damp with these tube types, then that's all the better.

Shannon