Eico ST70 odd sounds.

[Randyshone1]

Hi Folks,

I hope you can help pin point this issue I am having with an Eico ST 70. I have attached a wav file of the sounds coming from the speakers.

I have done some trouble shooting.

1. Clean and lube all the pots and switches.

2. Adjust the the balance pots. (I could not set channel 1 below .303, channel 2 is running at .002

3. Performed bias readings to the 7591 power tubes. (All the tubes are running between 425v to 428v plate voltage. Channel 1 tubes are running between 78v and 75v on the cathode. Channel 2 tubes are running between 24v and 26v on the cathode. Things do seem suspect already in this stage.

4. I have had all the tubes (original Mullard/Eico) tested for shorts, leaks, transconductance, and gas. All the tubes passed and rated good.

5. I have also cleaned all the tube pins and sockets.

I don't claim to be knowledgeable or an expert in electronics or tube equipment. I am learning as I go and am doing a ton of heavy reading to learn this stuff.

Oh yes, the output tubes are JJ's 7591s's. When I purchased the Eico it did have the original power tubes and when I got it home and opened it up about 2 months later BINGO! I found JJ's. Long story short. I am stuck with these for now.

Thank you for giving this a read! I value all your input.

Cheers,

Randy

 

[bberkom]

That does not sound good. Has the amp been restored at all? Coupling caps replaced? Power supply rebuilt?

Is the sound in both channels or only one channel?

 

[gadget73]

first thought is leaky coupling caps if the voltages are so different channel to channel. Also, I'd really suspect those readings to begin with. The cathodes of the 7591 tubes are connected to ground through a 10 ohm resistor. You should have about 0.35 volts at that point. 24 and 75 volts is so high the tubes should have melted into a puddle of goo long ago. Either the meter lies, or those 10 ohm resistors are completely out to lunch.

If that should have been 0.75 and 0.25 volts, then the 0.75 volt ones may have a problem that is causing the voltage on the other channel to drag down. If it really is 25 and 75, confirm those resistors. If they burned out it may indicate a deeper problem.

 

[Randyshone1]

That does not sound good. Has the amp been restored at all? Coupling caps replaced? Power supply rebuilt?

Is the sound in both channels or only one channel?

The sounds are coming out of the right channel only. Same channel 7591's are running high. It's also the channel that the balance measurement if reading high.

Thank you for your response.

 

[bberkom]

At minimum I'd replace the coupling caps, which are likely to be an issue, and any electrolytics, including can caps if the amp is all original. Have you tried checking other voltages from the power supply against the schematic?

You could also try swapping left and right channel output tubes to see if the problem is in the amp or the tubes. If the noise follows the tubes to the other channel, it's the tubes. But I bet it's something going on in the amp if it hasn't been rebuilt.

 

[Randyshone1]

That does not sound good. Has the amp been restored at all? Coupling caps replaced? Power supply rebuilt?

Is the sound in both channels or only one channel?

The amp hasn't been restored at all. The power supply and caps are original. The amp was given a clean bill of health when I bought it and now it's starting to make these noises.

 

[Randyshone1]

first thought is leaky coupling caps if the voltages are so different channel to channel. Also, I'd really suspect those readings to begin with. The cathodes of the 7591 tubes are connected to ground through a 10 ohm resistor. You should have about 0.35 volts at that point. 24 and 75 volts is so high the tubes should have melted into a puddle of goo long ago. Either the meter lies, or those 10 ohm resistors are completely out to lunch.

If that should have been 0.75 and 0.25 volts, then the 0.75 volt ones may have a problem that is causing the voltage on the other channel to drag down. If it really is 25 and 75, confirm those resistors. If they burned out it may indicate a deeper problem.

My mistake! I failed to place the decimal points. I apologize.

Here are the actual readings according to my notes for each tube.

Starting from the right side of the amp and going left.

Tube 1 - .428 = Plate
0.71 = Cathode

Tube 2 - .423 = Plate
0.65 = Cathode

Tube 3 - .429 = Plate
0.23 = Cathode

Tube 4 - .428 = Plate
0.21 = Cathode


Balance reading.

Channel 1 = .307
Channel 2 = .008

I hope this is clearer.

Thank you!

 

[bberkom]

It's always possible that it worked fine when you bought it and has developed a new fault if you want to give the seller benefit of the doubt. If you let it sit for even a few months and then fired it up at full AC line voltage, it may have been too much for the old components. I wouldn't even plug it in again until it's checked out.

 

[Randyshone1]

At minimum I'd replace the coupling caps, which are likely to be an issue, and any electrolytics, including can caps if the amp is all original. Have you tried checking other voltages from the power supply against the schematic?

You could also try swapping left and right channel output tubes to see if the problem is in the amp or the tubes. If the noise follows the tubes to the other channel, it's the tubes. But I bet it's something going on in the amp if it hasn't been rebuilt.

I haven't tried testing the voltage from the power supply against the schematic and can get that started this weekend. I am planning to move the tubes around tomorrow and will report what happens. I have been looking to purchase an ESR meter to test the caps and I am also looking for an amp tech here locally if I need to change them out. Techs are a bit hard to find around here. If I can't find one this maybe a big learning lesson.

Thank you for responding! This is appreciated!

 

[gadget73]

ok those numbers make a lot more sense. Confirm the coupling caps and check the parts in the bias supply.

ESR won't tell you anything of true value about coupling capacitors. The fail mode there is electrical leakage. The only way to check it is to feed them their full rated voltage (400-600 volts) and measure the current flowing across the capacitor. If you don't have one of those testers, throwing in $2 worth of caps to see if the problem goes away is an easy way to rule out problems.

 

[bberkom]

I never even test coupling caps that old. They're cheap enough to go ahead and replace them as insurance that they won't take out expensive output tubes. Just my experience.

 

[gadget73]

I usually don't either. I have the tester, its rare that I bother to use it. I thought about getting a fancier / better one, then I realized how little I use the one I already own and couldn't justify spending more on something that would probably get the same level of use.

 

[CJVx]

I run an unrestored ST70 pretty consistently, I did do some initial repair work which changed some leaky bias electrolytics, (had a red plate issue) and some resistors that drifted up on the front end, causing a constant hissing sound, it's been stable since 1999 after those initial fixes BUT...

every year or so I clean out the 7247 sockets with contact cleaner. when they have issues it sounds exactly like your recording. Wiggle the 7247's around a bit and see if the noise changes etc. that has been the only problem area of my amp, all the controls and other tube sockets have never needed any additional maintenance

 

[Shadowdog]

One should replace the Coupling, Bias, & PS caps as they can take out Any or All the Transformers when they go bad. Caps maybe have a 20 yr life span!

The amp will sound better as a bonus also!

Some amps like the HF-81 or 12 etc use disk ceramic coupling caps that don't go bad , so are often left in, but there are probably better ones for improved sonics.