Low Bias Reading on Dynaco ST-70

Hi Folks:

I have a problem with biasing my Dynaco ST-70. This is not a new modern version ST-70, but either a 1960/1970s version with upgrades made by a prior owner. I have very little history of this amp.

About a year ago, I installed new power tubes and biased the amp. Going by memory, voltages were around 425-430 V and the current was around 35 to 40 mA. Again this is what I recall. Anyway, I went back to check the bias since it has been around 700 hours on these tubes and I could not get any of the tubes over 330 volts. The current draw was no higher than 17-20 mA. I checked the meter against another working amp and the meter tested within the range expected. I installed a new battery just to make sure. I swapped out the 4 KT-77 with another quad of known good tubes and same result. Both sides are the same results…low voltage & current. I also swapped out the 5AR4 for a new rectifier and the same results. I turned the bias pot full CW, CCW, and etc. and no improvements. I used a different outlet just to eliminate the source and no change. This is not the first time I biased this amp. I done it a handful of times but on different tubes. This time it is a different story.

I have not done any further testing. My initial gut tells me it is my power supply caps. But again this is why I am turning to the experts for guidance.

Just to inform you the prior owner swapped out the quad cap with a dual cap and two individual electrolytic caps. Unfortunately, I cannot tell what values are the two individual caps under the chassis. The above chassis dual cap is rated at 50uF/50uF @ 500 V. It is a CE Distributor brand cap (has the web address on it). One end of this dual cap is connected to the choke, 2 red wires coming from the OT, and a 6,800 ohm carbon resistor (lug 1 on the original schematic). The other half of this dual cap is connected to the choke and there is wire from it to pin 8 of the rectifier socket. I believe this half of the cap is noted as the “half moon/lug 2” on the original ST-70 schematic.
I cannot see the values from my angle of the two individual electrolytic caps under the chassis. One cap has a wire routed to pin 19 on the circuit board and the other cap is wired to pin 20 on the board. There is a resistor that connects the two caps. I believe the cap that goes to pin 19 is noted as the “square/lug 3” on the original schematic, and the cap that goes to pin 20 is the “triangle/lug 4” on the schematic.

I appreciate any input in checking if it is the caps or something else.

Regards

Hello BNR,
Un plug all the tubes and start with the power supply voltage checks. The readings should be within say 8 to 10% of the AC voltages listed in the st70 manual. 360 VAC on pins 4 to ground and 6 to ground on the 5AR4 socket. Make sure the red and yellow wire is properly grounded. If both read the same. If they don't read the same, let me know. If they do read the same, proceed to plugging in the 5AR4 and check the B+ on down the line.
Caution! You now have lethal voltage inside the chassis. Be careful!
Let me know what you find. J.

Hello BNR,

I read through your post.  I believe the way you describe the second 50 mfd. section of the capacitor is not correct.  Check a stock schematic and see if your wiring agrees.  Two 50 mfd. caps on a stock tube rectifier will shorten the life of the tube.  It is a very substantial load to maintain the Voltage output when warming up and in use.  You'll get it going soon! Jbacik gave you a good starting point.

Hello all.
Good point Jim. Always check the wiring with the schematic. It sounds like one of the legs of the high voltage winding's is missing. It may be an open winding or short. I have only seen this happen once in my lifetime. Also always check the solder joints and make sure they are tight BNR. Once the 5AR4 is back in place, pin 8 of the 5AR4 socket should read around 435 VDC. The other side of the choke should read around 415 VDC. If this is not the case, one side of the high voltage winding should read zero or low voltage DC. Maybe we will hear back soon. J.

The Dynaco can simply amaze with some help.  The output transformers that David had made in the 50's and 60's are the key.  With updated power supplies and driver boards the quietness and dynamics from them made me put my Curl and Pass amplifiers away.  

On another note the two tube driver boards for the Mark III I got from India on eBay were of great quality. Don't see them any more. I'd like to get some more of these. I spent a few days with selecting the resistor an capacitor values using a generator, scope and HP distortion analyzer.  I ended up around 30K and 110 or 120 pf.  What was initially specified to start was ok but lacking the sparkle of my other Dynaco amps.  A got a very fast rise with ever so slight ring and .007 THD for the most part only rising to about .35 THD at the top end and bottom.  The square wave response at 20 Hertz is better than solid state amps.  It blew mew away to think this could really be happening in front of me. I rebuilt the two using new chassis's and rebuilt the transformers with new cloth wiring and the same throughout the point to point wiring.

Thank you Jbacik & Jimzim429. I plan to check the voltages sometime this weekend.

As far as the 50uF cap can, I thought the 5AR4 was rated up to 60 uF.

I apologize for the dual posting on the same topic. This post was accidently placed on the Dynaco Photo site by error. I then reposted my question to the Basket forum site. The moderators moved this post from the Photo to the Basket forum.

That's not a problem BNR. 50 uf is fine. I think I have around40 uf for all of my Dynaco's. The issue here is that I assume that this was working when you received it. If not, then check as I posted and see what you find. Check all soldering as well. Other people's soldering may not be the greatest. It should be checked if it was not you. You will see if there is a problem with the cap's once you have it in working condition. We can then start on the simple upgrades that I now use to make these Dynaco's sound the greatest ever. Regards, Jamey.

I wasn't going to reply but I felt I should.  I have a Ford 429 engine that is rated at 375 hp.  An old IH 1066 tractor has a Diesel engine rated around 125 hp.  That means if I place my 429 Ford engine in an old 1066 tractor I can do about 3 times the work all day, right?

A good rectifier tube can work hard, when new.  If you want to believe you need that extra capacitance on this tube because it makes you happy and you have something to brag about, do it.  No one is stopping you.

What I was saying but left some words out not to be "long in tooth" is that in order to prolong the life of the tube we should consider what is actually required to do the job.  Consider that most good designs of all tube amplifiers limit the direct capacitor connection value to about 30 mfd. for this rectifier. There are designs out there that do have 50 or 60 mfd. to the tube before the next resistor or choke but only after a resistor of 50 to 80 Ohms and 10 to 20 Watt in power value is at this tube first!

I am not out here to argue with anyone, only offering my two cents worth to help out.   I believe we "think" we hear differences or believe things like the bass is better because we all like to tinker.

People need to keep from beating up on each other and their equipment.  That some modification are more "macho" and better sounding than others because the values are greater is for the newbie.

Yes, all small block Fords need a Holley 850 double pumper and 302 injected Fox Mustangs need at least 42 lb/hr injectors with a Walbro Gss342 255lph High Pressure fuel pump because the 850 Double Pumper is for "carbed" engines.

Peace.

Hello B&R. Are you still interested? Have you been able to perform the voltage checks?
Regards, Jamey.

Sorry Jbacik for not getting back sooner with an update.  I was pretty busy over the past several days.

Anyway, I think I was able to get the issue resolved.   I confirmed that one of the caps on the power supply was bad.  One of the dual 50/50 uF cap that fed the #8 on the rectifier socket was reading bad on my brother-in-law’s ESR meter.  That meter is pretty amazing.  The other 3 caps read fine within specs.

Before swapping out the caps here are the following voltage reads.  I set the bias pot at the half-way mark.

I used a bucking transformer.  Taking reads off the AC wiring inside the amp (fuse and switch) registered 112.1 V.  All tubes are in the socket.

Power Supply Cap
50 uF @ 500v (connected to the two OP transformer, choke & 6.8K resistor): 338 V

50 uF @ 500v (connected to the choke lead & #8 of the rectifier): 342 V

50 uF @ 450v (connected to the 22K ohm resistor & pin #19 on the circuit board): 238 V

50 uF @ 450v (connected to the 6.8K & 22K ohm resistor & pin #20 on the circuit board): 292 V.

Rectifier Socket
(#2) 324 V
(#4) 347 V AC
(#6) 347 V AC
(# 342 V

Rear EL34 (V7 on the Dynaco Schematic)
(#1) 0.466 V
(#2) & (#7) 6.26 V AC
(#3) 338 V
(#4) 339 V
(#5) -34.0 V
(#6) -34.0 V
(# 0.466 V

Rear EL34 (V2 on the Dynaco Schematic)
(#1) 0.408 V
(#2) & (#7) 6.27 V AC
(#3) 338 V
(#4) 339 V
(#5) -32.8 V
(#6) -32.8 V
(# 0.408 V

Most of the values read too low below spec.

I removed the 3 power supply caps.  If you recall, I do not have a quad cap but 2 individual 50uF caps and a dual 50/50 cap.  I installed a 80/40/30/20 525V quad cap.  I also replaced the 6.8K and 22K resistors with a 6.8K and 22K ohm, 2W, metal oxide resistors.

I also removed the two 10K resistors for the bias supply and replaced it with two 5.1K ohm, 1W, metal oxide resistors.  The two 100uF @ 100v bias supply caps looked pretty new so I did not change it.

Before any tube was plugged in, I connected the amp through a 100W current light bulb limiter.  The light bulb was not noticeable, if at all glowing.  I could not tell, so I plugged in all the tubes.  I set the bias pot to ¼ turn toward the lower setting.

Voltage Reads with New Quad Cap

80 uF (connected to the two OP transformer leads, choke & 6.8K resistor): 414 V
40 uF (connected to the choke lead & #8 of the rectifier): 425 V
30 uF (connected to one end of the 22K ohm resistor & pin #19 on the circuit board): 286 V
20 uF (connected to the 6.8K & 22K ohm resistors & pin #20 on the circuit board): 353 V

Rectifier Socket
(#2) 424 V
(#4) 341 V AC
(#6) 342 V AC
(# 424 V

Rear EL34 (V7 on the Dynaco Schematic)
(#1) 1.095 V
(#2) & (#7) 6.16 V AC
(#3) 414 V
(#4) 414 V
(#5) -34.6 V
(#6) -34.6 V
(# 1.095 V

Rear EL34 (V2 on the Dynaco Schematic)
(#1) 1.143 V
(#2) & (#7) 6.17 V AC
(#3) 411 V
(#4) 414 V
(#5) -35.4 V
(#6) -35.3 V
(# 1.143 V

Values looked a lot better.  I adjusted the bias so it was a bit more balanced between V2 & V7 (1.11 v on the power takeoffs).

I cleaned up and decided to hook it up for listening to music.  After 10 hours of use during the weekend, everything sounded fine.

Regards

Not sure why the "number-sign-with-8" comes up as a face with sunglasses.

Looks like you zeroed in. If you want to increase the brilliance, put a band-pass filter at the input and get yourself some 1.5uf Janzen standard Z caps for the coupling caps and replace the 7199 tubes with 6GH8A tubes and you will not believe how good it sounds afterwards! You will have to use the bandwidth filter before you can place the 1.5uf coupling caps. You can get adapters for the tube replacement tubes or I can tell you how to rewire the circuit. I have the last 1980's production of the 6GH8A tubes if you like. Let me know your thoughts. Regards, Jamey.

Thanks Jamey

I am using a pair of the 6GH8A with adapters. I don’t recall the difference in SQ between the 7199 and the 6GH8As. But I am sold on the latter. You cannot beat the price and the supply that is out there. But buy from a trusted source to be on the safe side.