Help in biasing output tubes (Scott 340B)

[angel17]

The only experience I have so far in biasing output tubes is that I had been able to do this procedure to a Bob Latino ST-120, Dynaco ST-70 and TAD's 1000 Monoblocks and was successful at it. I have a serviced Scott 340B done by Mike Urban and I just got me another Scott 340B from one of our members (cohibajoe) in Barter Town this one serviced by Terry Dewick and just need the output tubes which I ordered from Jim McShane (Tung-Sol 7591A). I got me a quad of NOS Sylvania 7591A's from one of our members at BT but these I won't risk installing yet. I read some info here as how is done but I got cold feet, biasing the output tubes don't look difficult but do I need to do DC Balance Adjustments? This one looks to be a more difficult one.

I have a digital multimeter and a milliamps meter shown in pictures. If I get to do the biasing myself which of the two I should use? If using the digital multimeter should I set it at DCA 200m and which of the two positives prongs I use.

Your help and advice is greatly appreciated.

Thanks All

 

[Dave451]

Angel, congrats on the 340B. I'm enjoying mine a lot. The bias adjustment is made simpler for this unit because of the top-of-chassis measuring points. You want to set each pair of tubes for 70 mA of current, measured by a current meter at the test points. Do it pretty much as the manual says. You can use either of your meters to set the current. If you use the DMM, use the "DCA" scale set to "200m" (200 milliamperes). Use the lower of the two positive terminals, which is your 250 mA maximum scale. If you use the milliammeter, use clip leads and hook the negative to chassis and the positive to the test points. Adjust the "DC Bias" pot for the appropriate channel that you are measuring to get 70 mA on the meter.

The DC balance is a little more tricky. Since the cathodes of each pair are tied together, this is the control to balance bias between the two tubes in the pairs. The 340B Service Bulletin (on the AK database--get the right one for your serial number) tells you how to do this, but you need a 16 ohm non-inductive load to hook to the speaker terminals, you pull the two phase inverter tubes, hook your DMM set on AC volts (use the 200 VAC scale) across the terminals you are measuring with the 16 ohm load attached, and adjust the appropriate "DC Balance" pot for a minimum reading on the DMM. I'm not sure your DMM is sensitive enough to do this adjustment, though, but you might give it a try. Do one channel, and then the other.

If this doesn't work very well for you, you can set the pots in the middle and let it be. There is another way to do this, but I don't encourage you to do this unless you are comfortable working around high voltages under the chassis of the 340B. Let me know if you're interested in this method and if the "minimum hum method" (as above) in the Service Bulletin doesn't work out for you. Good luck!
Dave

 

[angel17]

Thank you Dave for the input. I installed all the new Tung-Sols 7591a tubes that arrived today and proceed to warm up the receiver for 30 minutes. I set the digital multimeter to "DCA" 200m scale negative clip lead on the chassis and positive lead to the left channel cathode current point and adjusted the bias pot and it only reads up to 60ma did the same with the right channel and it only reads up to 32.5ma.I noticed that in order to get the readings the bias pot has to be turn to the maximum in both channels. To be sure that my digital meter is in working order I did the biasing with the 100 milliamp meter and I got 60ma on the left and 38ma on the right, I turn the the bias pot on both channels but the readings will not go up or down. I listened to the receiver for 30 seconds but it does not sound as loud as my other Scott 340b. What could be the problem?

 

[Dave451]

I found a problem with mine like that. Like yours, It would not read 70 mA when biasing using current measurement. I ended up having a problem with a faulty ground at a chassis rivet that was preventing the tubes from drawing full current. Before chasing that (if it is indeed your problem), try to set the bias by voltage instead of current. Use your DMM on the DCV 2000 mV scale and connect the positive lead to the test points and negative to the chassis. Then, set the bias on each side to 230 mV (0.230 VDC). Ohms law says that with 3.3 ohm cathode resistors like you have, a cathode voltage of 0.231 V should indicate 70 mA being drawn by the pair. (E = IR, so E volts = 0.070 Amps x 3.3 Ohms, = 0.231 VDC). Set it this way and see how it sounds and report where the bias pot is relative to full deflection. I'm running Tung Sol 7591A Russia tubes in mine and love them.

 

[angel17]

Dave, I tried biasing the tubes by voltage as you suggested and I get 403mv in the left channel and 219mv in the right channel and no matter which way I adjusted the bias pots (low,middle or high) the numbers stays the same. I noticed when I increased the volume it has a hum in both channels.. The AK member that sold me the receiver said the receiver was OK. I forgot to mention that the Scott only came with the tuner tubes, one 12at7 and one 6ke8 tube. The remainder tubes two 6u8 and 5 12ax7 tubes I had them as spares. Maybe one of the other tubes is no good?

 

[Dave451]

Well, something (maybe several things) are not right. You may indeed have tube problems. Were you able to test the tubes you installed? I'm wondering about the 6U8 phase inverter/driver tubes, particularly (V3 and V103). It's concerning that you're getting no cathode voltage change when you swing the bias pots through the range. Did the current measurement change when you adjusted the pots? One thing: with the power off can you measure the resistance between each of the test pins and the chassis? It should be close to 3.3 ohms on each side.

 

[angel17]

I took all the tubes from my good Scott 340b and installed them in the one with the bias problem all with the exception of the power tubes and tuner section. I'm still getting the same reading as before, the cathode voltage and current measurements did not change when I adjusted the the bias pots through the range. I don't have the know how to do other measurements so I guess I will wait and take it to be check by Mike Urban of Urban Antique Radio and Vintage Hi-Fi in the near future. Dave, I want to Thank You for your help.

 

[Dave451]

Well, maybe not tubes, then. Were you able to measure resistance between the test points and the chassis? You would use the ohms scale set on the 200 scale. If not, you may have a problem with the bias supply which will require some measurement and work. Good luck!

 

[Pdow]

Well, maybe not tubes, then. Were you able to measure resistance between the test points and the chassis? You would use the ohms scale set on the 200 scale. If not, you may have a problem with the bias supply which will require some measurement and work. Good luck!

So if fixed can we learn what was corrected !

 

[dr*audio]

Maybe you are adjusting the DC balance pots instead of the bias pots. If you are sure you are adjusting the right pots, something is probably wrong in the bias supply. The selenium rectifier should have been replaced. It looks like a little radiator. It's mounted on the bottom, attached to one of the sides, if I remember correctly. If you see that there, it must be replaced with a 1N4007 diode.