Triode connection in ST-70

[SDS-PAGE]

Ok, I apologize for being lazy. I am pretty sure the topic has been covered else where, but my question is on triode connection for the EL-34. Do I just strap pin 3&4 with a 100 ohm 1W resistor and not connect screen taps on the OPT? I have the original Dynaco OPTs. Any problem with leaving those screen taps unconnected? Any bias adjustment needed? Thanks for you patience. Min

[dcgillespie]

Your understanding is correct. Remove the screen tap leads and tape them off -- the full B+ voltage is still available at these taps, so tape them off well. Then, install the 100 ohm resistors between pins # 3 & 4 on each tube. This process will not change the bias setting as long as the tubes are reinserted back into their original sockets. When necessary, the process of setting the bias remains the same.

Dave

[burnedfingers]

I will have to disagree on the bias answer. My experience is that when in triode mode the bias will increase from my setting of 50mA per tube to 55mA. This is on amplifiers that I have fitted with individual bias adjustment controls. Yes, do check the bias and be prepared to adjust.

I would question as to why you would want to permanently modify the amplifier for triode only mode. If it were me I would install a quad pole double throw switch in the selentium (sp) rectifier position in the chassis and proceed to wire it up to go normal or triode mode.

Also do not flip the switch when the amplifier is powered on.

[SDS-PAGE]

It wasn't my idea to wire my ST-70 in triode mode. I am selling ti and I have a potential buyer that wanted the mode done. I am more than happy with the ST-70 in UL mode.

Thanks for the input. It's clear now. Thanks,

Min

[Slartibartfast]

I would question as to why you would want to permanently modify the amplifier for triode only mode. If it were me I would install a quad pole double throw switch in the selentium (sp) rectifier position in the chassis and proceed to wire it up to go normal or triode mode.

Well if you have very efficient speakers, the 15 watts would be more than enough.

Also do not flip the switch when the amplifier is powered on.

I wouldn't, but, Bob Latino would say otherwise...

[dcgillespie]

I will have to disagree on the bias answer. My experience is that when in triode mode the bias will increase from my setting of 50mA per tube to 55mA. This is on amplifiers that I have fitted with individual bias adjustment controls. Yes, do check the bias and be prepared to adjust.

I understand you base your comments on experience -- however, my experience and theory respectfully disagrees. Since the screens are now being fed from the plate leads of the primary winding as well as the new 100 ohm resistors, the screen voltage is now slightly less than before, which means if anything, current draw slightly decreases -- all else being equal. However, it is always a good idea to check the bias anyway.

Dave

[burnedfingers]

It is possible I have it switched around. I will make some measurements tonight if time permits.

[burnedfingers]

-----------------I wouldn't, but, Bob Latino would say otherwise...

Most of us will throw in a switch that isn't rated for the voltage it is seeing.
That is the main reason I do not recommend flipping the switch when the amp is running. That and the possibility of arching the contacts and the nasty pop thru the speakers.

If Bob wants to flip his switch let him but I surely wouldn't recommend to amyone to do it.

[mesherm]

Actually you can get about 18 watts in triode if you put in a UF4007 diode in series with the 100 ohm resistor. The cathode of the diode has to be pointing toward the screen. This keeps AC from flowing in the screen grid/plate circuit.

http://www.audioasylum.com/forums/DIYHiFi/messages/6928.html

[TomMcNally]

Actually you can get about 18 watts in triode if you put in a UF4007 diode in series with the 100 ohm resistor.

Could I do it with a pair of 5AR4's ?

Yellow_Light_Colorz_PDT_06

[Quad]

Could I do it with a pair of 5AR4's ?

For best results, make sure you use a NOS matched quad
of Mullard 5AR4s (two per channel).

[EWBrown]

I was gonna suggest that a matched pair of 3B28s might do the job quite nicelyYellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_04 :o Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_01

/ed B

[kheper]

Actually you can get about 18 watts in triode if you put in a UF4007 diode in series with the 100 ohm resistor. The cathode of the diode has to be pointing toward the screen. This keeps AC from flowing in the screen grid/plate circuit.

http://www.audioasylum.com/forums/DIYHiFi/messages/6928.html

Here is an triode-strapped 813 with a screen diode, a string of zener clamps to drop the voltage 31V, caps, etc. The resulting sound is "brigther" with the diode array than without.

Screen diodes can be used, as well, in UL amps to prevent the AC back-flow.

http://www-personal.umich.edu/~monk/rig ... -trick.gif

http://www-personal.umich.edu/~monk/amp.html

[sorenj07]

Actually you can get about 18 watts in triode if you put in a UF4007 diode in series with the 100 ohm resistor. The cathode of the diode has to be pointing toward the screen. This keeps AC from flowing in the screen grid/plate circuit.

http://www.audioasylum.com/forums/DIYHiFi/messages/6928.html

You realize that guy quotes Dennis Fraker right? Just thought you should know. I've seen this called a myth, can't find the link right now, and it seemed credible. The main argument (IIRC) was that AC doesn't flow to the screens in the first place.

[kheper]

Actually you can get about 18 watts in triode if you put in a UF4007 diode in series with the 100 ohm resistor. The cathode of the diode has to be pointing toward the screen. This keeps AC from flowing in the screen grid/plate circuit.

http://www.audioasylum.com/forums/DIYHiFi/messages/6928.html

You realize that guy quotes Dennis Fraker right? Just thought you should know. I've seen this called a myth, can't find the link right now, and it seemed credible. The main argument (IIRC) was that AC doesn't flow to the screens in the first place.

I have never heard of Fraker. However, Grimwood knows his onions about tube amps.

http://www.webace.com.au/~electron/tubes/oes.html

"Another and previously unpublished option to creating an operating environment where the Screen Grids will be at a DC potential sufficiently high enough to attract and accelerate electrons towards the Plates but, to maximise power output, not to collect and divert them to earth through the B+ supply, is the humble silicon diode semi-conductor rectifier.

By inserting a standard half-wave silicon rectifier diode in series with the Grid Stopper resistor, an electronic control circuit is created whereby the Screen Grid will be able to be energised at DC potential attracting and accelerating electrons towards the Plate - still electrostatically controlling current flow in the normal way - but blocking the flow of AC current from the Screen Grid back to the DC source - ie "one way traffic"

This works because the current flow in the tube is always from the Cathode to the Anode (Plate). The diode, being a semiconductor, blocks current flow in the reverse direction, thus enabling DC current to feed it in the conventional manner but blocks AC current from passing back through it to a load."