Altec 1569A Rebuild Question

[sburke]

Hoping that someone on the board may have experience with Altec 1569A mono amps. My question concerns hum after a rebuild. Specifically, I'm experiencing low level hum from the amp on my 8" full range Rat Shack 8 ohm test speaker, located in my shop. It is constant after the amp has warmed up and doesn't change with the gain control.

All solder joints have been reflowed. Chassis grounds removed, cleaned, and tightened. Input terminals are shorted. Except for the 50uf 6v tantalum cap, all caps have been replaced. The power supply caps have been reformed with leakage well below spec. Capacitance is within 10%. All resistors are within 10% or better. All voltages, with the exception of V2 plate are within 15% of spec or better. V2 shows 210vdc on the schematic. Actual is 260vdc. EL34's are all solid at 428 plate, 425 screen. Bias to all EL34's is solid at 30ma or -37vdc.

I'm not using the balanced input transformer and the C1 & C2 shunts are in place. I've swapped all tubes with different tubes as well as between sockets. I've pulled the tubes and added them back in this order: 5u4GB, 2 EL34's, 4 EL34's, 6CG7's. Hum is non-existent with only the 5U4GB's, near non-existent with 2 EL34's, faint with 4 EL34's, and the most with all tubes inserted. The test speaker is hooked up to the 8 ohm and common terminals.

I don't have a scope and have not yet rewired. I hope I don't have to. Now this hum is not real loud. I have to put my ear to within 12" of the speaker to really hear it but it's there nonetheless. Is this just inherent to these amps? Am I being too an*l to expect total silence from this amp? Is anyone familiar with the article in Vacuum Tube Valley, Issue #7 to know whether or not this is addressed? I would like to use these to drive the 416-8B's in my Model 19's but I really don't want to have to live with any hum.

Any experience or advice is greatly appreciated.

Steve

[marcuswilson]

While I have no experience with that particular amp, from my experience I can give you some ideas.
From what you say, it sounds like there is an imbalance in the output stage, causing the power supply ripple to be output to the speaker. This is caused by one pair of output tubes drawing more current than the other pair.
The cure depends what type of bias arrangement is in the amp.
First though, have you had the output tubes checked and matched?

[sburke]

Hi Marcus. Thanks for your response. I should have added in my initial request that the small signal and output tubes are in fact matched within 10% for gM and mA at 250vdc idle. The rectifiers have been checked and verified as ok also.

Steve

[RClark]

sbb--------i have several of those amps and they are real quiet-----BUT i have replaced the powersupply electrolitic caps-------it is true that the caps are too small by hi fi standards but at no load they are more than adequate if up to spec-------you say that you re-formed the caps------i take this to assume that you did not replace them??????????............RC

[marcuswilson]

I would be surprised if the layout was at fault in an Altec amp of that vintage.
I would measure the current through each pair of output tubes and see if they differ. If so, find the cause and fix it. Sometimes there is a leaky coupling capacitor to the grids, or the fixed bias circuitry has out of spec resistors, or if it's cathode bias the resistors have drifted or a bypass electrolytic is leaky, or the tubes are not matched well enough. Many Fender amps have a bias balancing trimpot to equalise the output tube currents. The Williampson has also.

[marcuswilson]

If I had a circuit, my posts could be more specific.

[sburke]

Dave - Thanks for the suggestions. I'll sure try them out and report back. I did consider tube shields for the 6CG7's but haven't tried them yet.
I'll also try the bypass of the filter caps.

Richard - You are correct. The filter caps were not replaced. I reformed them to their rated voltage (500vdc). Leakage is way below spec for their ratings according to my Sprague TO-5 manual.
All other caps, with the exception of the 50uf 6v cap were replaced with new measured caps.

Marcus - Makes sense about checking the current of the output tubes. I checked the current on 1 and found it to be 30mA. I checked bias voltage on all 4. All of them were rock solid at -37vdc. I'll go ahead and verify current of the other 3 but I'll be very surprised if they're off by more than 10%. A schematic with someone's scribbling can be found at this link:

http://www.triodeel.com/al1569a.jpg

Thank you all very much for your comments and suggestions. Since the weekend is upon me, I'll get a chance to tinker some more.

Steve

[Fitts]

Steve,

One last thought, I don't think that the leakage rate of a capacitor can tell you much about the size of the charge it can hold. If you could measure the actual capacitance of your various caps I bet you would find some of them have less than half their rated values.

You may want to replace your caps starting with the power caps until you get the desired results. I suggest you not use any new old stock caps, instead go with fresh new units. I say this because electrolytics tend to dry out over time and I have seen many new old stock caps that were worthless because they had dried out.

Have fun !

[sburke]

Success! Marcus, you nailed it. One tube was drawing 50% less current than the other three. Goes to show that testers have their limitations. All 4 matched within 10% on both transconductance and current at my tester voltage of 250vdc. My how things changed at 425! Found a spare that matched up (current wise) to the other 3 at 425. No hum. Excellent!

Once again, I thank you all for you kind assistance. I hope I can return the favor one day.

Steve

[Fitts]

Hi Steve,

I would like to throw in my 2 cents worth from my experience with tube equipment back in the late sixties. I have always felt that the filter sections of most amps were short on capacitance. I saw very few amps with enough capacitors to totally eliminate hum and generally the bigger the amp, the greater the amount of hum at idle.

I would recommend that you experiment with larger capacitors in the power supply section. Also, bypassing each large capacitor with a .1 mfd cap of the same or greater working voltage can help control the low level hum too. Don't forget to look at the capacitors in the bias circuts and the 6CG7 supplies as well. It may help to increase them too.

Another source of low level hum that I have found is the result of a tube or tubes being too close to the power transformer or the output transformers being too close to the power transformer. When a chassis becomes too crowded the stray magnetic fields from the power transformer can cause hum trouble. You can experiment with magnetic shields made from the sheet steel used to make auto body repairs. It is a good thickness to use, tin cans are too thin to be effective. Try holding a square steel shield close to the side of power transformer that faces the tubes and see what happens. If it reduces the hum, make a perminent shield and mount it to the chassis.

Good luck,