Low Bias for Dynaco ST-70

Hi Folks:

I just noticed that I posted this question on the wrong forum. I used the photo forum, so I am re-posting it here.

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

.

Based on your report-of-symptoms:

The bias supply is via the OEM selenium diode. And, for the record, it is not a matter of if, but when that fails. Further, when it fails it may do so with spectacular results possibly ending in having tube-based equipment banned from your house forever - if you can get over the smell.

I would suggest that you replace the bias diode, and capacitors, add the appropriate resistors, and you will most likely find everything working properly thereafter. Up in the Stickies is the process for doing this - which takes less than 15 minutes once you have obtained the parts. Unless you are getting significant hum, I doubt it is either the filter caps or the 5AR4 rectifier.

Waxing a bit philosophic, repair-by-substitution can be costly and time-consuming. There is this venue as a resource, as well as others. For someone without a lot of repair experience, it can get very frustrating to 'go it alone'.

This past week when I checked the bias there was a hum, but I also had nothing plugged in on the inputs. It could also be related to the speaker wires lying over the AC line. I was in a bit of a rush so wires were crossing each other. A few days later when I re-checked the bias (now swapping out the existing power tubes with a known working quad) the hum was so barely noticeable. This time there was no crossing of speaker wires and the AC line. But again, I had nothing plugged into the RCA jacks. This time the hum was way less audible then the first time.

I should also mention that their is no selenium diode. That was replaced with a diode by the prior owner.

Regards

As far as the two resistors shown on the quad cap (22K between lugs 3 & 4, and 6.8K between lugs 1 &4), should it be replaced with a 1 watt metal oxide resistor or carbon film, etc.? I assume 1 watt is adequate but is 2 watt recommended?

Thanks

.

Thanks Peter.

I am thinking the same way as you and that is to put in a new quad cap.

Since I will be ordering some parts, I am planning to replace the two bias caps since it may not be that much longer before it goes. The two existing caps are rated at 100uF @ 100v. Since the bias doesn't get close to the 100 volt range that the 100 v rating is adequate. Should I stick to the 50uF or 100uF is preferred?

Thanks

.

Thanks Peter

Yes the two 100uF @ 100v were replaced by the prior owner. The problem is when it was changed out. Since I will be placing an order for some materials, the cost for 2 more electrolytic capacitors is worth purchasing.

You want your ST-70 to be healthy. Understood. Simply replacing parts or doubling their ratings could get you there I suppose. Why not do some simple probing to guide your debug effort?

You have a DVM so you could start with the following very basic power supply and bias checks. This assumes you know how to safely probe a powered on, high voltage device.

-DCV on the bias diode, the end without the silver band
-DCV on the wipers of the left and right channel bias pots
-DCV on pin #8 of your 5AR4
-DCV on pin #8 of each EL-34

I suspect you have already made these measurements, if so I missed seeing the voltages reported in the thread. Do you have a clamp on current probe? Harbor Freight sells an adequate unit for a few dollars. This is a good tool to have so you can do a gross failure check by measuring your amp's idle current. As an aside: If you are ordering parts, I recommend any tube amp hobbyist keep a couple 22uF, 500V electrolytics on hand so you can swap out segments of multi-caps during debug.

j beede wrote:You want your ST-70 to be healthy. Understood. Simply replacing parts or doubling their ratings could get you there I suppose. Why not do some simple probing to guide your debug effort?

You have a DVM so you could start with the following very basic power supply and bias checks. This assumes you know how to safely probe a powered on, high voltage device.

-DCV on the bias diode, the end without the silver band
-DCV on the wipers of the left and right channel bias pots
-DCV on pin #8 of your 5AR4
-DCV on pin #8 of each EL-34

I suspect you have already made these measurements, if so I missed seeing the voltages reported in the thread. Do you have a clamp on current probe? Harbor Freight sells an adequate unit for a few dollars. This is a good tool to have so you can do a gross failure check by measuring your amp's idle current. As an aside: If you are ordering parts, I recommend any tube amp hobbyist keep a couple 22uF, 500V electrolytics on hand so you can swap out segments of multi-caps during debug.

Thanks j.

No, I have not taken any measurements from under the chassis.  I am planning to do so this weekend to verify if indeed the power supply is the cause.

Yes, I have a DMM, but not a current probe. I assume the DMM will provide equal results.

Is a rating of 300-350 working voltage (600V overload) adequate for the 22K & 6.8K across the quad cap? I noticed most sites that sell the 1W or 2W metal oxide resistors are rated for that range. Any higher working voltage would require 3W and larger, which seems excessive.

Regards