Acrosound Ultra Linear II

[Peter212]

Hello all,
Recently I came into possesion of an ancient Acrosound Ultra Linear II.  It's working, but not quite to the satisfaction due to inability to properly adjust the output tube bias voltage.  The manual calls for negative 36.  I can only get, at best negative 29.  I tried using the recommended sequence of adjusting the trimmer pots, but I am not getting the results.  I checked all resistors and pots, and they're well within the specs. Is there an alternative way of dealing with this bias cicuit?  At negative 29 the tubes get red, more than what I am used to seeing in power tubes.  The amp came with a pair of EL34 and the used pair of 6550.  I was looking for a service manual to see what voltages I should expect at certain points, but no luck.  The assembly manual is not providing any additional info.  Moreover, I am curious why there is a "bias" connectors on the front plate? (J1 and J2)   I am an amateur electronic hobbyist so any info regarding how to adjust this amp will be greatly  appreciated

[sluckey]

The bias jacks provide a convenient place to monitor actual tube current. R19 and R20 are the current sense resistors (typical value would be 1Ω or maybe 10Ω). These resistors have an asterisk that probably references a note on the schematic. What does that note say? Better yet, post the entire schematic.

Which output tubes are you using? 6550s will die quickly with only -29v on the grids. EL34s would likely die more slowly.

[Peter212]

The bias jacks provide a convenient place to monitor actual tube current. R19 and R20 are the current sense resistors (typical value would be 1Ω or maybe 10Ω). These resistors have an asterisk that probably references a note on the schematic. What does that note say? Better yet, post the entire schematic.

Which output tubes are you using? 6550s will die quickly with only -29v on the grids. EL34s would likely die more slowly.

Thanks for the reply
The resistors in question are 4.7 ohms.  The boths pairs of tubes (EL34 and 6550) are quite marginal, perhaps at 60% or less, nonetheless they still work.  I think they wore off prematurely due to improper bias setting. I did not run the amp for more than a minute or two ; the moment the tubes started to glow I would turn it off. 

I am totally unfamilliar with this type of biasing.  Every amp with EL34 I had in my hands had a fixed bias at negative 32 or 36V. 

Here's the complete schematics with parts list. 

Thx
 

[sluckey]

The manual calls for negative 36.  I can only get, at best negative 29.

Where does the manual say to measure that -36V? And what else does the manual say about biasing? Does it mention J1 and J2? What's the complete biasing procedure?

If your manual is in pdf format, just post the manual or just scan the page(s) that deal with biasing.

[Peter212]

The manual calls for negative 36.  I can only get, at best negative 29.

Where does the manual say to measure that -36V? And what else does the manual say about biasing? Does it mention J1 and J2? What's the complete biasing procedure?

If your manual is in pdf format, just post the manual or just scan the page(s) that deal with biasing.

Friend of mine sent me a link with a complete user manual. 
https://audio-circuit.dk/downloads/acrosound/AcroSound-UltraLinear2-pwr-sch.pdf
The one I got was missing few pages. 

This one contains the list of the voltages to be expected.  EL34, pin 5, control grid, negative 36V (see attached)
The biasing procedure is listed on page 8.

 

[sluckey]

OK, now I got the whole picture. The bias procedure is very straight forward and should be easy to perform once the bias supply is putting out enough voltage. Since it's not, a little troubleshooting is in store. Do this...

1. Remove the rectifier tube and don't put it back in until the bias circuit is fixed.

2. Check the AC voltage on the PT orange lead that connects to pin 7 of the pcb. Should be greater than 50VAC. What have you? If OK, proceed to step 3.

3. Check the negative DCV at C2A negative lead. What have you? If less than -50V or -60V, then replace C2A and C2B (they need changing anyway). If voltage is still too low, replace selenium rectifier SR1 with a silicon rectifier diode (1N4007, 1N5408, etc.). Voltage OK now?

Report findings before we move on.

[Peter212]

OK, now I got the whole picture. The bias procedure is very straight forward and should be easy to perform once the bias supply is putting out enough voltage. Since it's not, a little troubleshooting is in store. Do this...

1. Remove the rectifier tube and don't put it back in until the bias circuit is fixed.

2. Check the AC voltage on the PT orange lead that connects to pin 7 of the pcb. Should be greater than 50VAC. What have you? If OK, proceed to step 3.

3. Check the negative DCV at C2A negative lead. What have you? If less than -50V or -60V, then replace C2A and C2B (they need changing anyway). If voltage is still too low, replace selenium rectifier SR1 with a silicon rectifier diode (1N4007, 1N5408, etc.). Voltage OK now?

Report findings before we move on.

Thank you.  I will try these steps tonight after work and report back

[PRR]

Yes, If resistors are OK and you are using a VTVM(DMM), then 30V where we want 36V is C2(ab), which is old enough to buy whiskey in Utah.

This is one branch of the crew which became Dynaco. I had not seen this model before. It may be rare. It is certainly designed "boldly", 28 Watts idle Pdiss in poor EL34s. But they were cheap, 60 Watts was "WOW!" in that day, and 6550 was so very new that it may have missed this boat (or just been too costly for home use).

[Peter212]

Yes, If resistors are OK and you are using a VTVM(DMM), then 30V where we want 36V is C2(ab), which is old enough to buy whiskey in Utah.

This is one branch of the crew which became Dynaco. I had not seen this model before. It may be rare. It is certainly designed "boldly", 28 Watts idle Pdiss in poor EL34s. But they were cheap, 60 Watts was "WOW!" in that day, and 6550 was so very new that it may have missed this boat (or just been too costly for home use).

This is how far I got: I measured 103 vac at the orange wire going into the selenium rectifier, 65V DC on the other end, (quite a drop  ) same thing (65V DC) across the A & B capacitors.
I stopped tracking forward, I had some unexpected visitors ..   at this point I will recheck all the resistors and pots, maybe I missed a step someplace...

[sluckey]

-65V is good. What voltage do you measure on pin 5 of the output tubes now?

[Peter212]

-65V is good. What voltage do you measure on pin 5 of the output tubes now?

The same, negative 29.  I get negative 65 at the R14, (5.6K) and only 34 on the other side of the resistor. By the time it gets to the pot/resistor divider, it drops even further.  I ohmed out the resistors again, and they all check out

[Peter212]

-65V is good. What voltage do you measure on pin 5 of the output tubes now?

I'm beginning to suspect this thing never ran properly.   

[Peter212]

Yes, If resistors are OK and you are using a VTVM(DMM), then 30V where we want 36V is C2(ab), which is old enough to buy whiskey in Utah.

This is one branch of the crew which became Dynaco. I had not seen this model before. It may be rare. It is certainly designed "boldly", 28 Watts idle Pdiss in poor EL34s. But they were cheap, 60 Watts was "WOW!" in that day, and 6550 was so very new that it may have missed this boat (or just been too costly for home use).

Never heard of 6550 until I moved to USA. We had 6CA7 imported from.US and run of the mill EL34

[sluckey]

Try this...

Adjust P2 for maximum voltage on pin 5 of one tube. Now connect one probe to pin 5 of a tube and connect the other probe to pin 5 of the other tube. Adjust P3 for zero volts reading on the meter. Leave P2 and P3 set like this.

Now measure voltage at point A and point B. What have you?

[PRR]

...The same, negative 29.

What is your meter? Make, model? Specs somewhere? (Link?)

Altho even a 1k/V automotive needle-meter would not cause that much drop?

(Some electrician meters have heavy loading to vanish phantom voltages.)

[sluckey]

Replace the selenium rectifier with a silicon rectifier. What are the new voltages? Don't change P2 or P3 settings.

[Peter212]

...The same, negative 29.

What is your meter? Make, model? Specs somewhere? (Link?)

Altho even a 1k/V automotive needle-meter would not cause that much drop?

(Some electrician meters have heavy loading to vanish phantom voltages.)

I have been using Fluke 83 since 2002.  It's accurate enough for what I do.

[Peter212]

Replace the selenium rectifier with a silicon rectifier. What are the new voltages? Don't change P2 or P3 settings.

That's what I think. This thing drops a lot.  It has five plates which should amount to no more than 5-6V drop

[Peter212]

Try this...

Adjust P2 for maximum voltage on pin 5 of one tube. Now connect one probe to pin 5 of a tube and connect the other probe to pin 5 of the other tube. Adjust P3 for zero volts reading on the meter. Leave P2 and P3 set like this.

Now measure voltage at point A and point B. What have you?

Will try these steps and report back
Thanks for your help

[Peter212]

Try this...

Adjust P2 for maximum voltage on pin 5 of one tube. Now connect one probe to pin 5 of a tube and connect the other probe to pin 5 of the other tube. Adjust P3 for zero volts reading on the meter. Leave P2 and P3 set like this.

Now measure voltage at point A and point B. What have you?

Will try these steps and report back
Thanks for your help

 I tried your recommendations and it didn't work.  What I ended up doing was replacing the selenium diode with 1N4005, that got me to 32V. Then, I replaced R15 5.6K with 4.7K 1/2W and it gave me negative 38V max, plenty of margin to set the bias at negative 36.  The amp is working, EL 34 faint glow as expected. 
I appreciate the time you took to help me out.